Digital Astrophotography Basics Knowledge Q & A
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Digital Astrophotography Basics
How can I find other people with an interest in digital camera astronomical imaging?
Threads related to use of digital cameras are becoming more common on the sci.astro.amateur newsgroup. A more focused forum for discussions on this topic is the digital_astro Yahoo! Group. As of the end of 2002, less than a year and a half after its founding, this community had over 2700 members.
How good can astronomical images taken with a digital camera get?
The fact that digital camera images are appearing with increasing frequency in the Sky and Telescope magazine Gallery pages is a clear indication that in some cases at least, images from digital cameras can compete with film and CCDs. The digital_astro Yahoo! Group has a monthly challenge imaging competition. Many of the entries in these challenges are certainly among the best images group members have produced. A number of images from monthly challenge winners appear throughout this FAQ. A complete gallery of "best-in-group" and runner-up images from the challenges can be found at http://velatron.com/dca/gallery/, and should provide some idea of how good digital camera images can get.
How good should I expect my images to be?
There is no cut-and-dry answer to this question. What you can image, how much you can magnify it, and how much detail you can capture depend on the capabilities of the digital camera you have; prices for digital cameras range from the order of tens of dollars to the order of thousands of dollars. Camera capabilities vary with price. Low end cameras have exposure times limited to under a second, and allow little to no control over exposure settings; mid-range cameras provide significant control over exposure settings, and allow exposure times of several to many seconds; high-range cameras have even higher-quality CCD chips and allow exposure times of multiple minutes.
Just as important is the astronomical equipment you have. Any digital camera, no matter the quality, will be limited to the light that reaches it. As with visual astronomy, the light-gathering ability of your scope, quality of scope and eyepiece optics, and seeing conditions all affect what comes out of the eyepiece.
Even inexpensive digital cameras can be quite capable of taking snapshots of the moon. With most cameras you will be able to capture an images of the brighter planets, but what you can see in those images depends on your equipment. People have also been able to image a variety of deep-space objects, but this requires a scope with an aperture large enough to see them in the first place, and typically needs a camera that provides longer exposure times and some control exposure settings. The use of image stacking helps (and in some cases is essential) for most DSOs, and performing digital image processing techniques after imaging contributes considerably to the quality of the results.
Your mileage will vary depending on a variety of factors. These will be mentioned only briefly here, as they are discussed in greater detail in other documents that this FAQ links to. At a high level, aside from the standard variables that apply to anyone with a telescope (seeing, darkness of viewing site, quality of optics, etc.), the main factors that affect the results of your venture into digital camera astronomical imaging are:
Size of telescope aperture
More light coming in is better, this being less of an issue for lunar imaging, more important for planetary imaging (since a large aperture will capture an equivalent amount of light in a shorter exposure than a small one), and more important still for any attempts at deep sky object imaging.
Whether or not you have a tracking telescope
Without a tracking telescope, longer exposures will start to blur because of the apparent motion of the sky resulting from the Earth's rotation. The degree of blur increases with level of magnification, so that at high magnifications you may be limited to exposures of only a few seconds even if your camera is capable of longer ones.
Maximum exposure time
Virtually any camera will be able to take pictures of the moon. To take pictures of bright planets, exposure times of one to several seconds will be useful. For DSOs, maximum exposure times of 8 seconds will put a few within reach, but 16 seconds or longer is really needed to expand your options beyond a small handful.
Other features of your digital camera
Maximum resolution will be limited to the number of pixels your digital camera has, but resolution is often limited by your astronomical equipment and seeing conditions rather than by the resolution of your camera. More important is the feature set your camera provides, as that will lead to more flexibility in what you can achieve.
Ambient temperature
Since digital camera CCDs are not cooled, ambient temperature can cause noise in the images. (The issue of noise is discussed in greater detail in a separate question.)
Camera mounting
Whether your camera is hand-held, tripod mounted, or coupled to your telescope has a significant effect on what you do with it. Some mounting methods have advantages over others (including stability and alignment), so even for a mounted camera, the method of mounting can make a difference to some extent.
Image processing
The use of digital image processing techniques can significantly improve the images you produce. A good shot of the moon will not need much in the way of image processing--perhaps only a bit of contrast enhancement and a little sharpening. At the other end of the spectrum, for DSOs, stacking and other forms of image enhancement can do wonders.
Technique
This FAQ is intended only to provide a starting point, and in no way provides a recipe for perfect results every time. A willingness to learn from others, willingness to experiment, and patience on your part will go far.
Terminology I've seen terms like afocal photography, prime focus photography, and eyepiece projection photography... What is the difference and which should I do? Afocal photography consists of taking an unmodified camera focused at infinity and taking a picture through the eyepiece. Mounting the camera to the scope itself is helpful with this method, but for short exposure times images can be obtained simply by holding the camera up to the eyepiece. With prime focus photography, the camera lens is removed from the camera, the eyepiece is removed from the telescope, and the scope itself serves as the lens for the camera. Because alignment is more important, to use this method the camera must be mounted to the telescope (which generally is a tracking telescope). Eyepiece projection photography again uses a camera with the lens removed, but this time uses an eyepiece in the optical path. This results in higher focal length, and greater magnification. Magnifying the image results in less light falling on a given area, which translates to a need for longer exposures than prime focus photography given the same target. Also because of the higher magnification tracking precision is more important than with prime focus photography. In the realm of digital cameras, only the high-end professional grade cameras have removable lenses. As a result, afocal photography is the only method available to the majority of digital camera owners. For this reason, the mounting and imaging techniques discussed on this web site are geared toward afocal photography. Although the best results you can get with afocal photography may not be as good as with prime focus or eyepiece projection photography, afocal photography is the easiest method to get started with.
What does vignetting mean?
Light coming out of the eyepiece creates a cone. The light striking a properly-aligned surface. In the context of astronomical imaging, vignetting occurs when this illuminated area does not fully cover the light detecting surface (the CCD chip in a digital camera). Because the image does not completely cover the CCD chip, the result is a circular image that is smaller than the camera's field of view. This effect, which causes a dark area surrounding a smaller circular image, is called vignetting. The cone of light narrows as distance from the surface of the eyepiece increases. Thus, as a camera lens is moved closer to the eyepiece, the cone of light intersected has a larger diameter, resulting in a larger illuminated area reaching the CCD chip. In most cases, to avoid vignetting, the digital camera should be placed as close to the eyepiece as possible. However, when using eyepieces with relatively long eye relief it is actually possible to reduce the field of view by having the camera too close to the eyepiece. In some cases, the best placement for the camera may be a bit farther away from the eyepiece rather than as close as possible. If you have eyepieces with long eye relief, you may have to experiment a bit to find the best distance. Once the camera has been mounted, use of full zoom will often also be helpful in reducing the effects of vignetting.
This is all Greek to me! What does <insert term here> mean?
There's more technical terminology and jargon that can be included in this FAQ. For astrophotography-related terminology, a good concise glossary of terms can be found at Starizona's web site. If you don't find what you are looking for there, a more comprehensive glossary is available at NASA Goddard Space Flight Center's web site. If you are looking for digital camera-related terminology, good glossaries can be found at Radio Shack's web site or ACD Systems' web site.
Digital Camera Equipment What are some good places to get information about digital cameras? Searching individual digital camera manufacturer sites for information can take a while if you don't know what you are looking for. It is definitely easier to search sites geared toward people interested in buying digital cameras. These sites have more information in one place, can help you find products based on your needs, allow product comparisons, provide equipment reviews, and more. There are many such sites on the web. The ones I found myself using the most are imaging-resource.com, dpreview.com and steves-digicams.com. All three have general camera information, in-depth reviews, and discussion forums. The first two also let you do side-by-side camera comparisons, with imaging-resource.com providing the more comprehensive comparisons in terms of camera features. The dcviews.com web site has also been recommended. There are so many cameras out there that you need to decide on what basis you are going to narrow things down. Unless money is no object, the best way to start is to decide on your budget ahead of time. Keep in mind that in addition to the camera, you will also need to spend money on other camera accessories, as well something with which to mount the camera to your scope. That should narrow things down to a more manageable number of choices. Reviews, side-by-side feature comparisons, and budget constraints should help you prune your list from there.
How many megapixels should my digital camera have?
The camera spec that is almost always the first one mentioned is the number of megapixels it has. This refers to the number of millions of pixels on the camera's CCD chip. This certainly is important, but there are quite a few reasons why there is more to a camera than the number of megapixels it has. For a given price, there will always be tradeoffs. A full-featured 2.3 megapixel camera may cost the same as a bare-bones 3.1 megapixel camera, but the extra pixels in the latter may not make up for the limitations that the lack of features causes. Secondly, your ability to resolve detail depends on the equipment you are using (mainly on aperture) and seeing conditions. Thus, it may be these factors, rather than the number of megapixels on your camera's CCD, that limit the resolution in your images. Along similar lines, even modest digital cameras generally have better resolution than computer monitors, so if you only plan on viewing images on your computer or putting them on the web, the images as displayed will not show the full resolution of the image itself. If you plan on printing out images and enlarging them, that's a different situation. But the basic point here is that there's no use in paying more for a "better" camera if the resolution provided by your telescopic or computer setup won't benefit from additional megapixels. In addition, with megapixels as with anything, it's not just quantity that matters, but also quality. Manufacturers can increase number of pixels, but if it's done without increasing the size of the CCD chip, the pixels will by necessity be smaller. The main benefit of a smaller pixel is that it improves resolution... you will in theory be able to resolve smaller features in the subject of your image. However, there are also drawbacks to smaller pixels. Depending on the quality of the CCD chip, smaller pixels can be less sensitive than larger ones. They can also be more prone to noise. This may sound odd given the previous statement, but it isn't necessarily a contradiction. Remember that you want the pixel to brighten due to incoming light, and noise is caused by a pixel that brightens due to energy that has some source other than incoming light (thermal, electrical, cosmic rays, etc.). A pixel may have lower sensitivity to incoming light (which is what you want to detect), but may be more prone to noise because a fixed amount of unwanted energy can cause more brightening in a smaller pixel than in a larger one. Aside from noise issues, smaller pixels can also be more sensitive to blooming. When one pixel becomes fully saturated (sometimes due to noise, but more often due to incoming light from a particularly bright portion of the target), electrical charge can actually leak into neighboring pixels. Because the additional charge on neighboring pixels is due to leakage rather incoming light, those neighboring pixels brighten more than the would from light alone. This phenomenon is referred to as blooming. Lastly, being over-equipped can even be counterproductive when you consider the fact that more pixels also translate to larger image file sizes (other factors such as level of image compression being equal), which means you'll be able to store fewer images on a given memory card. All this is not to say that more pixels are bad, or that smaller pixels are bad. Fewer pixels and larger pixels both reduce resolution in images, so more and smaller can be good. The thing to keep in mind is simply that more is not necessarily better, and that other issues such as CCD quality, sensitivity, and general limitations due to viewing conditions, will all affect what will give you the best results with your equipment. Micro Publishing News published a nice article on the megapixel issue. What features should I look for when buying a digital camera? The first thing you need to do is consider your budget. There are plenty of useful features, but depending on your price range, they may not all be within your reach. You should also consider your intended usage. What else are you going to use your camera for besides astronomical imaging? If the answer is "nothing" then you may want to consider buying a more specialized astronomical CCD camera instead, but most digital camera astrophotographers want (or already have) a digital camera for regular photography as well. I will be focusing on features that are relevant to astronomical imaging, but there may be other features you are interested in for separate reasons. Manual Focus
One of the more important features to look for is manual focus. Fortunately, this feature is available all but the cheapest of of cameras, even those which do not provide fully-manual control over other exposure settings. Cameras that do not have manual focus setting but that do achieve focus through the lens (TTL focus) will still be able to focus properly on large clear targets (the moon, and possibly magnified planetary views) using the built-in autofocus. For dimmer or less well-defined targets, a camera may have trouble identifying the target and focusing on it. Cameras that do not have manual focus and accomplish focusing with a sensor that is mounted on the camera body (not going through the camera lens) will very likely have serious focusing problems. Their line of sight will be aimed at the outside of the scope rather than up at the sky, and will have a close focus rather than focusing at infinity.
Control of Exposure Settings
The next thing to look for is some degree of manual control over exposure settings. Manual control can include one or more of the following: shutter priority (where you manually set the exposure time and the camera sets the aperture automatically), aperture priority (where you manually set the aperture and the camera sets the exposure time automatically), or fully-manual capabilities where you can control both. Of the two priority settings, for astronomical imaging shutter priority is more useful than aperture priority because control over the duration of an exposure allows you to have some degree of control over noise (which increases with longer exposures) and blurring (which can occur as a result of non-ideal seeing conditions as well as due to the earth's rotation depending on your scope's tracking capability). A camera that does not have aperture priority or a fully manual mode will always automatically set its own exposure lengths based on its own internal algorithms. If you have a camera that that does light metering through the lens (TTL light metering), it may still be possible to get good images, but as the aperture-setting algorithms vary from camera to camera, it would be hard to predict results for a given camera without trying it out. If you are considering a camera that does not give any user control over aperture, it would be best to try finding other people who have tried using that particular model camera for astronomical imaging, and ask about their results, before purchasing it. A camera that does not do TTL light metering but instead uses a photocell mounted elsewhere on the body of the camera will not be usable for taking images of the moon. Without metering light through the lens, the photocell will detect only the darkness in front of the camera and will always take exposures that are much longer than what you need for the bright moon, resulting in overexposed images.
Exposure Times
As indicated previously, the moon makes an easy target even for cameras having very limited capabilities. If you have an interest in experimenting with targets other than the moon, exposure time is an important consideration. Be aware that different cameras have a broad range in maximum exposure times--some have maximum exposure times that are 1/2 second or less, while others have maximum times in the multiple seconds, and others up to several minutes. Virtually any camera will be able to take pictures of the moon. To take pictures of bright planets, exposure times of one to several seconds will be useful. For DSOs, maximum exposure times of 8 seconds will put a few within reach, but 16 seconds or longer is really needed to expand your options beyond a small handful. Longer exposure times tend to result in more noise in the images. However, newer cameras have higher quality CCD chips that are much less noisy. In addition, some cameras have built in noise reduction (dark frame subtraction) to reduce noise, and for those that do not, dark frame subtraction can be done as part of digital image processing.
Lens
Another thing to consider is the size of the lens on the digital camera. Recall that the light from an eyepiece illuminates a circular area on the lens, which then gets focused onto the CCD chip in the camera. Once the diameter of that circular area becomes smaller than the diameter of your lens, the focused light will no longer fully illuminate the CCD chip, which results in vignetting. Vignetting can become a problem with any camera, but as a general rule, the larger the lens on a digital camera is, the more easily you will run into the problem of not being able to fully illuminate the lens. At exactly what point that happens depends not just on the camera, but on your entire setup (scope/eyepiece/camera). In doing camera comparisons, if you are leaning toward a camera whose lens size is not toward the smaller end of the spectrum, it is probably worth doing a bit of asking around to see if you can find somebody else with a setup similar to yours who has had experience with that particular camera.
Optical Zoom
While on the subject of vignetting, you will also want to consider the amount of built-in optical zoom a camera has, since taking pictures at full zoom is generally useful if not necessary for avoiding vignetting. However, it is important to note here that more is not always better. Cameras with more zoom capability may have more optical elements in the light path, so a high zoom can cut down on the amount of light that reaches the CCD. And unlike an external zoom lens which is removed when not in use, a built-in zoom lens cannot be removed. Light will therefore always have to pass through all the optical elements in the camera whether you are at full zoom or not. Note that this discussion involves optical zoom, rather than digital zoom. Though digital zoom can help a bit with focusing, it is not useful for imaging. Unlike an optical zoom, a digital zoom enlarges an image at the expense of reducing resolution. This is generally undesirable in astronomical images. Furthermore, if you do want to enlarge an image digitally despite the resolution loss, it can always be done later using image processing software.
Mounting Method
As mentioned earlier, a camera can be hand-held, tripod mounted, or mounted directly to your scope. If you want to use a tripod mounted camera, your camera should have a tripod mounting hole. If you want to couple your camera directly to your scope, it is advisable to look for a camera that has a threaded lens barrel, which will allow you to couple your camera to your scope with a high degree of alignment. Adapters that are made specifically for cameras that do not have threaded lens barrels can be purchased or built, but several of them do require a tripod mounting hole. These can provide respectable results; however, achieving a very good alignment between the camera's optical axis and that of the scope is much more difficult with these systems. Poor alignment can increase the amount of vignetting in your images, it can affect the way light reaches your camera (leading to reduced brightness or non-uniform brightness) and can affect uniformity of focus which in turn impacts the sharpness of your images.
Hands-off Imaging
If you plan on taking images of anything besides a bright moon, you will likely move into exposure times for which camera motion can blur images. Even very small motions can affect the sharpness of an image. Mounting your camera to your scope or to a tripod helps, but even the act of pressing the button to capture an image can cause some motion. As a result, a self-timer is a useful feature to have. With this feature, pressing the button starts a timer that captures the image after a delay, which allows motion and vibrations to die out. Even better than a self timer is a camera that has a remote control as an accessory (either included or sold separately). This provides more control over exactly when you capture an image, as well as more convenience. Also, the question of whether or not vibrations have completely died out is less of an issue since you are pushing a button that is not located on the camera. For a number of cameras on the market, freeware or shareware software is available to remotely control a camera using a computer. As a result, some people have taken to making a laptop part of their imaging equipment.
In-camera Image Processing
Lastly, for astronomical imaging it is useful to have control over image compression and in-camera image processing. Many cameras have several image quality settings that the user can choose from (higher quality is the result of doing less image compression, at the cost of larger image file sizes). Some cameras even allow images to be saved in TIFF format, which uses no compression (though it results in correspondingly large files). Final images can always be compressed later on, but if you plan on doing any image processing, the less compression done on the pre-processed images, the better. In-camera noise reduction, done using dark frame subtraction, is a useful feature to have. Some cameras have the capability to do other types of in-camera image enhancement (e.g., focus sharpening, contrast enhancement, etc.). While these other features are helpful for general snapshot photographers, for astronomical imaging it is useful to be able to turn these features off. As with compression, image enhancement can be done with software later, but if you plan on doing any kind of image stacking/averaging/subtraction, it is best to do these on raw images that have not been image processed in the camera.
Which camera should I buy?
In the lowest price range, cameras that cost under $50 or so will be extremely limited. If you already have one, feel free to play around with it. But if you are considering buying a camera because you have an interest in getting into astronomical imaging, consider investing a bit more if you want to be at all satisfied with your results. Going up to $100-$200, the capabilities of the camera will very likely constrain your range of targets, but you should be able to get decent shots of the moon.
In the range of $300-$600 dollars, you should be able to get a very versatile camera. Two product lines that are popular among members of the digital_astro Yahoo! Group are the Nikon Coolpix line (8x5 and 9x5 series as well as the newer 4x00 and 5x00 series, where the "x"es represent digits) and the Olympus x040 line of cameras. Though these are popular lines, other lines and brands in this price range make fine choices as well. Your ideal choice will depend on your particular needs. Searching the last six months of digital_astro Yahoo! Group message archive is highly recommended, as you will surely be able to find previous threads for advice on camera purchasing by choosing appropriate search keywords (e.g. advice, recommendations, purchasing, buying, and so on).
Among the professional grade cameras, the two most talked about models are the Nikon D100 and the Canon D60. These two can do very long bulb exposures, are extremely capable, but will set you back in the neighborhood of $2,000. For an investment on that level, you should have some professional use for the camera, or be sure that digital astrophotography is going to keep your interest for a while.
As indicated previously, which camera is best for you in particular depends on a variety of things, not the least of which is what you may consider desirable in a camera for from a non-astronomical use perspective. The short answer is that there is that no one camera is the best choice for everyone. Hopefully the general guidance provided here, along with the information in the previous discussion of features, should give you a starting point.
What other equipment or accessories do I need for my digital camera?
Equipment that you should take into account when budgeting for your camera include rechargeable batteries and a charger, a mount or adapter for coupling your digital camera to your scope, additional memory cards for your camera. In doing research for my equipment purchasing, I found thomas-distributing.com to be a good source for rechargeable batteries and chargers, and newegg.com to be a source for inexpensive memory cards.
As mentioned earlier, a remote control is a very useful accessory if one is available for your camera. Use of a remote control can be more convenient than manual operation, and avoids vibrations from manual operation of a camera. Some cameras can be controlled from a laptop, so if you already own a computer laptop, you may want to look into this as well. Some people whose cameras did not support remote control or whose remote controls were limited have gone and jury-rigged their own hands-off imaging devices (such as a bracket that clamps onto a camera and allows a screw to be tightened to press and hold down the shutter button).
Other convenient but non-essential accessories that you may want to consider include an AC adapter and a memory card reader for your computer. Even if you plan on purchasing an AC adapter, you should still invest in a set of rechargeable batteries and a charger, for non-astronomical usage if nothing else. Digital cameras can go through batteries quite quickly. Relying on disposable batteries can become quite an expense, whereas a rechargeable battery that costs less than $3 can be recharged hundreds of times. (In addition to being more expensive, use of disposable batteries needlessly adds environmentally unfriendly chemicals and materials to landfills.)
Which adapter should I use to connect my camera to my telescope?
Some people have built their own perfectly functional adapters from scratch using parts you can get at a home improvement store, while others have assembled higher-precision adapters from off-the-shelf components, including standard (non-digital) camera adapters and step up/down rings. Links to a variety of homegrown digital camera adapters that have been designed by different people can be found on the Homegrown Digital Camera/Telescope Adapter Page.
In addition to homegrown adapters, there are also many different commercial adapters on the market, made expressly for connecting digital cameras to telescopes. There is somewhat of a spread in cost among the commercial products, and there is by no means a clear-cut answer to which one is best. All of them have advantages and disadvantages: some are easier to connect and disconnect than others, some limit the types of eyepiece that can be used with them more than others, achieving proper alignment is easier with some than with others. Further more, not every adapter can be used with any digital camera and/or with any scope. In other words, your equipment (camera, scope, and eyepieces) may affect which ones you are able to use in the first place, and from that set which ones may be best for you. A more complete summary of the various alternatives is too long to include here, but can be found on the Digital Camera/Telescope Adapter Page, which summarizes equipment limitations, advantages and disadvantages, and provides links to vendors.
Some of the adapters are made to connect to specific cameras, but most of them are generic ones which use a standard T-thread connection. Since digital cameras do not have standard T-threads, you will generally need a step-up or step-down ring to go from your camera's thread to a T-thread in order to connect your camera to a generic adapter. In some cases, depending on the design of your camera, you may also need an extender of some sort. Links to several online sources for step up/down rings, extenders, and related items, can be found at the bottom of either of the two adapters pages mentioned above.
What is the best way to get a good deal on a digital camera?
Once you have decided which camera(s) you are interested, you should compare prices for the camera you are interested in. You can compare prices at online retailers using any of a number of price comparison web sites (such as pricegrabber.com, shopper.com, dealtime.com, pricescan.com, and many others). There's quite a bit of overlap in the retailers they cover, but their coverage isn't identical so it doesn't hurt to try a couple. You may be tempted to try buying at a local brick-and-mortar store. Feel free to shop around, but in my experience I found that the costs at some online retailers were as much as 40% less than places like Walmart, whose prices are already discounted below suggested retail prices.
On the other hand, you should resist the temptation to jump at the lowest price you find. Several online camera retailers run shady business practices. Things I read about doing research for my purchase include misrepresentation, bait-and-switch tactics, and canceling orders for customers who were not willing to buy overpriced accessory packages. Since accessory packages are a way to make up for reduced profits on deeply discounted cameras, there's nothing wrong with dealers offering these packages. But I've heard of dealers delaying shipment or outright canceling orders for people who declined to purchase additional items. If you decide to buy a camera online, I would recommend checking resellerratings.com, which provides ratings (both numerical scores and comments from customers describing their experiences) for tons of online retailers.
One additional shady practice you should look out for is the selling of gray market or refurbished items without identifying them as such. Refurbished items are not new and should be identified as refurbs. Gray market items are items that are packaged for sale overseas and somehow find their way into inventories of dealers in the US. The most serious problem with gray market cameras is that manufacturers generally do not honor warrantees on these items
Equipment Alternatives Why do some people use CCD cameras for astronomical imaging instead of digital cameras? Heat generates noise in CCD (charge coupled device) chips. Many deep sky objects require long exposures, during which noise can easily exceed the signal you are trying to capture. CCD cameras for astronomical imaging use Peltier devices (solid-state heat pumps) to cool CCD chips to temperatures at which they can be operated for long-exposure imaging with minimal noise. There are other benefits to how CCD cameras are designed. For example, they can have more sophisticated electronics with functionality geared specifically toward astronomical imaging (e.g. anti-blooming gates, pixel binning), and more robust designs that can better withstand exposure to temperature extremes and humidity.
If a Peltier cooling device can be bought for under $25, why are CCD cameras for astronomical imaging so expensive?
Not all CCD cameras are that expensive. The low range astronomical CCD cameras are less expensive than a good midrange (not even professional grade) digital camera. And if you are a do-it-yourselfer, you can save some money by building your own CCD camera for even less money (see the Cookbook CCD Camera FAQ at http://www.wvi.com/~rberry/cb245faq.htm for more information and pointers to other resources). But the Peltier device is not the only thing that results in higher costs for commercial astronomical CCD cameras. The specialized electronics and more robust designs mentioned in the previous question contribute to the cost, as does the fact that the CCD camera makers don't have the economy of scales that makers of digital cameras (which are sold by the millions) benefit from.
Webcams are cheaper than digital cameras. Why not use one of them for astronomical imaging?
You can! Some people have been quite successful at doing basic imaging with webcams to produce relatively good quality results. However, because the better digital cameras come with features that are not available on webcams, the best that can be achieved with digital cameras exceeds the best that can be done with webcams. This FAQ focuses on digital cameras because that's where my focus was, but some portions of this FAQ (such as the image processing section) apply equally well to webcam imaging. If you are interested in webcam imaging, check out the QuickCam and Unconventional Imaging Astronomy Group (QCUIAG), which has a membership approaching 1000 people (see their web page at http://www.astrabio.demon.co.uk/QCUIAG/.
What about digital video cameras?
Typically, digital video cameras give the user less control over imaging parameters (aperture, shutter speed, etc.). The more common video formats also do more image compression than some of the still image formats, which results in less detail and unwanted compression artifacts. In addition, digital video cameras often don't provide options for formats or compression. That having been said, people are managing to get impressive results for certain types of imaging using digital video cameras. And of course, with some digital cameras able to take short movies and some digital video cameras able to take more traditional still images, commercial products are starting to populate the previously empty area between the two technologies. You can learn more about video astrophotography by joining the videoastro Yahoo! Group. But again, the focus of this web site is biased by my own activities in the use of digital cameras.
Techniques Should I connect my digital camera to a telescope, and if so, how? It's certainly easy to start experimenting simply using a hand-held camera. However, in doing so you may soon find yourself running into several limitations. The first limitation is basic ergonomics. It takes two hands to hold your camera over the eyepiece reasonably well. That's not a problem for snapping shots, but you want to do imaging often, and you like to switch among different eyepieces, you will find that the basic setup task of focusing, which requires holding the camera over the eyepiece and turning the focusing knob on your scope at the same time, becomes a chore. The second issue you run into is exposure time. Although the slight motion of your hands is not a problem when taking 1/500 second exposures of a full moon, if you want to try your hand at taking longer exposures of dim targets, you won't be able to do it with a hand-held camera. With a hand-held camera you also run into alignment problems. If the optical axis of the camera is not parallel with the optical axis of the scope at the eyepiece, the focal plane will intersect the CCD chip in the camera at an angle rather than falling flat on the chip. Depending on the severity of misalignment, this can cause uneven focus with some regions of the image being in focus (where the focal plane crosses the CCD chip) and increasingly out of focus moving away from that line (where the focal plane is in front of or behind the CCD chip). Lastly, if you have a motorized scope or equatorial platform that gives you tracking capability, with a hand-held camera you lose the advantages that tracking gives you. A hand-held camera won't adequately track the scope's motion even if the scope is effective in tracking the sky's (apparent) motion. One benefit of tracking is the ability to take longer exposures without blurring due to motion of your target in the field of view (the Earth's rotation), but even for short exposures tracking has the advantage of being able to take multiple images of a target in the same position, which is useful for certain types of image processing (e.g. image stacking and image averaging). One alternative to a hand-held camera is a tripod-mounted camera, where a tripod is used to hold the camera at the eyepiece. This is definitely better than a hand-held camera, because it allows longer exposures because the camera is not moving in your hands. However, unless you have an equatorial platform on which the tripod can sit, motorized tracking will not be possible so the length of exposures will still be limited due to the motion of objects in your field of view (the Earth's rotation again). In addition, with a tripod the alignment of camera and scope optical axes remains difficult and needs to be repeated each time you reorient your scope to point at a new target. If you want to make a hobby of digital camera astrophotography, it really does make the most sense to use some kind of camera adapter to mount your digital camera to the scope. See the Which adapter... question in under the Digital Camera Equipment questions for more specific information.
What is a good way to focus my digital camera for astrophotography?
A good way to focus your digital camera is as follows: (1) if possible with your camera, manually set your camera focus to infinity (or possibly set to macro if your camera has a macro mode, as discussed in the previous question), (2) zoom in as far as you can using the optical zoom, as well as a digital zoom if your camera has such feature, (3) point the scope at a bright star, the more overhead the better to reduce atmospheric effects on the image, (4) connect your camera to your telescope, (5) adjust the focuser of your scope so that the image of the star is as small as possible, (6) if you used digital zoom for focusing, remember to turn it off before doing your imaging. If the moon is visible and not too low in the sky, it also makes a good focusing target. Note however, that with either of these methods, the low resolution of LCD viewfinders can sometimes make it hard to know when you have the best focus. An image may appear sharp in the camera's LCD display but may appear unfocused when viewed at full resolution. What has happened is not that the full-resolution image got worse, it's that the reduced-resolution image in the LCD display appeared to sharper than it really was. Here are some additional tips that can make it easier to achieve focus using the above procedure:
With some cameras, even when the conditions warrant a fully open aperture, the aperture remains at a default position and does not fully open unless the camera's button is pushed down part way. A not-fully-opened aperture can make a target star appear dimmer on the camera display. In these cases, holding down the button will open the aperture and brighten the image on the display, making it easier to focus using a bright star.
Increase your camera's sensitivity by using the highest ISO setting available for focusing purposes. Again, with some cameras you may need to hold the camera's button down part way to make this setting change "active" and brighten the image you see. In most cases, you will not want to use the highest ISO level for imaging, so remember to switch it back when you are done focusing.
If your digital camera has a video out jack, any of a number of external viewing devices will make it easier to know when you have achieved the best focus. Among the things that people have used are external monitors or small portable televisions. Small LCD screens in the range of 5" to 5.4" sold as portable displays for home video game systems such as the Sony Playstation or Nintendo Gamecube cost more, but are preferred by some due to their more compact size. The InterAct Mobile Monitor is a popular model among digital_astro users. Most external devices that have a video in jack will give you more resolution than your camera's LCD viewfinder.
It is also possible to buy or make a Hartmann mask to help with focusing. A brief article on Hartmann masks can be found here. Ron Wodaski, author of The New CCD Astronomy provides a nice discussion of Hartmann masks in an online sample chapter of his book, available here.
If your camera has a TTL optical viewfinder (a viewfinder that looks through the lens of the camera), focusing using the optical viewfinder can provide better focus than using the LCD viewfinder because of the LCD resolution issue mentioned above. This assumes however, that you have good vision or are wearing glasses or contacts that provide good corrected vision. If you have poor or uncorrected vision, when you focus using a TTL viewfinder, the best focus for your eyes will compensate for your imperfect vision, and the image obtained by the camera will actually not be at its best focus. If your camera does not have a TTL optical viewfinder, the viewfinder won't be of much help in focusing since looking through it will give you a nice view of the outside of your scope. The advantage of the methods described in the earlier paragraphs is that they are not dependent on good or corrected vision in order to obtain good results. Once focused with a given eyepiece, the focus of your camera should not be very significantly affected by switching targets or by zooming in or out on a target, though periodic tweaks can help. The camera generally will, however, have to be refocused after changing eyepieces.
Should I set the camera's focus to infinity, or should I use macro mode?
In theory, you should set the camera's focus to infinity. Although the eyepiece is right in front of the camera, the target as seen through the various lenses appears to be at infinity. When you observe visually, you focus not on the surface of the eyepiece but on the distant target whose light is being focused by the telescope's optics and your eye's lens; The camera works the same way. Most of the people who have tried doing tests to compare focus at infinty vs. inmacro mode have obtained the expected result (better images with focus at infinity). However, a number of people have reported better results by setting using macro mode instead. Given what "correct" result should be, I dug around a bit online to find out why the opposite might be true in some cases. It turns out that some cameras, most notably some of the Coolpix models, do not focus well at infinity when the camera is at full zoom (as is usually the case when doing astronomical imaging). At full zoom, proper focus at infinity is achieved by manually setting the focus at around 30 ft. -- the infinity setting moves the lens too far. In other cameras, including a couple of the earlier Olympus models, the infinity setting was not calibrated correctly. Proper focus at infinity was achieved with focus set at less than infinity, regardless of zoom level. Since the camera's electronic controls determine how the lenses move under various settings, such flaws can be addressed through software. In at least some of these cases, the focusing issues were recognized by the manufacturer because people reported that upgrading the camera's firmware to the latest version fixed the problem. So in other words, there is no one answer to this question that will apply in all cases. Setting focus at infinity should give the best results, but it may not in all cases. The first step should be to upgrade your camera to the latest available version of the firmware if firmware upgrades are possible. (See your manual for instructions on how to check the current version, and how to perform upgrades.) If you are inclined to to a comparison yourself, try both ways and see which gives better results... if it's macro mode, use that instead. For any tests, be sure to use a target where you can reasonably expect the focus of an image to be decent enough to assess (e.g., the moon, or a distant target during the daytime, rather than a planet or a DSO).
The images I am getting are not sharply focused; what could be causing this?
There are several reasons your camera may have difficulty achieving good focus. First of all, as indicated by the focusing procedure outlined above, focusing of the scope must be done while viewing a target through the camera. If you focus the scope while visually looking through the eyepiece and then attach the camera, it won't be properly focused because the lens of your eye and that of the camera are different. Next, if it isn't obvious, what either you or your camera sees through the scope is limited by the quality of the image that the scope produces. Your camera should be able to focus on a target that appears sharp through the scope, but it will not be able to unblur the image of a target that is degraded due to bad seeing, tube currents, or overly high magnification. There are several other possible reasons your camera may not be focusing correctly. First off, with autofocus turned on, there are a couple of reasons why the camera may not focus correctly. One reason is that cameras which use a sensor mounted on the body of the camera for focus rather than TTL (through the lens) focus will focus the body of the telescope a few inches in front of the camera, rather than the target that the scope is pointed at. Even cameras that do do TTL focusing can sometimes have difficulty locking in correctly when the target is small or dim. Therefore, if your camera has manual focus, you should manually set the focus at infinity (or possibly set to macro if your camera has a macro mode, as discussed in the previous question). Another potential issue concerns the method for connecting your camera to your scope. Several of the available methods consist of an adapter that gets inserted in the scope's focuser. Eyepieces are inserted into this adapter, and the camera is connected to the adapter after that. Because eyepieces are inserted in the adapter rather than in the focuser itself, this has the effect of moving the eyepiece farther back along the optical path. Depending on the geometry of your particular scope and the range of travel of the focuser, you may find that you are simply not be able to focus the scope with certain eyepieces because of insufficient focuser travel. A possible remedy for some eyepieces may be to use a Barlow lens (if your mounting method allows this) inserted as usual between the focuser and the eyepiece. A more involved workaround is to move the mirror cell of the scope forward toward the front of the scope (again if your equipment allows for this), thereby moving the focal point back along the optical axis, hopefully to a point the focuser can reach. If you run into this issue, depending on the severity you may end up having to do without certain eyepieces, or simply finding another method for connecting your camera to the scope. Lastly, you may find that you are able to reach a relatively sharp focus with your camera using the previously described procedure, but that when you view your images on your computer later you find that some portions of the image are more sharply focused than others. Assuming the optics themselves are not the problem, this is probably caused by an optical axis alignment problem, i.e., the optical axis of the telescope is not aligned with the optical axis of the camera. Although camera mountings where a threaded camera barrel is attached to the telescope with adapters and step up/down rings ensure a good axial alignment, good alignment is harder to ensure with adapters designed for cameras that do not have threaded barrels, and even harder still with a tripod-mounted or hand-held camera. If your camera is mounted via a threaded barrel and you have repeatedly non-uniformly focused regions in your images, it be a problem with the optical axis of the scope itself, such as an unsquared focuser.
Which ISO setting should I use?
In general, a higher ISO value equates to higher sensitivity to light. With regular film, this is done by modifying the light-detecting medium (the film) itself, which results in grainier images. In digital cameras, the light-detecting is done by the CCD chip, and that doesn't get changed when you change your ISO setting. The increased sensitivity to light is done by increasing the gain on the CCD sensors. This amplifies the signal from incoming light, but also amplifies unwanted signals due to noise. Most people tend to stick with the lowest available ISO setting. However, for dimmer targets, it may make sense to at least try higher settings. Between newer CCD chips which are less sensitive to noise, and in-camera dark frame subtraction, some people find that they can use higher settings and still obtain acceptable images.
How do I avoid noise in my images?
A detailed treatment of noise in digital cameras is beyond the scope of this FAQ. Rather than defining all types and sources of noise here, I will simply use two general categories of random noise and non-random noise. Random noise is noise that may be different in every image. One source of random noise is cosmic rays. There is not a whole lot you can do to reduce cosmic rays. Another source of random noise is random electron motion. Since random electron motion increases with temperature, one way to reduce noise is to reduce temperature. You don't generally have control over ambient conditions outdoors, but one thing that can make a significant difference for longer exposures is to turn off the LCD viewfinder as much as possible during imaging (if your camera allows you to turn it off, that is). Non-random noise has some level of repeatability among images. One source of non-random noise is variations in sensitivity of individual sensors (pixels) on your camera's CCD chip. Ideally, all pixels are identical, but as with any manufactured artifacts, the output of a manufacturing process is not 100% uniform but has some statistical distribution about a target. A sensor whose sensitivity happens to be particularly far from the mean will either be much more (or much less sensitive) to light, and will always appear brighter (or darker) than equally-illuminated neighboring pixels. When this problem becomes severe enough, you can end up with hot pixels, stuck pixels, or dead pixels (see this article if you want to know more about these). Even less extreme variations in sensitivity can cause repeatable variations in your images. There is not much you can do to control this kind of non-random noise, since it's essentially built into the CCD chip in your camera. There are other kinds of non-random noise that you can control. For instance, if your camera-to-telescope coupling does not completely enclose the camera lens barrel, light can reach the lens from the sides of the camera rather than only through the eyepiece as is desired. Any outdoor lighting getting in this way can cause noise, but if ambient lighting is not uniform (for example if there is a street lamp nearby), the effect of that noise on the image will not be uniform, making it more noticeable. Shielding a camera whose lens barrel is not enclosed by the scope mounting is an effective way of dealing with this type of noise. Another tip for avoiding noise has to do with ISO settings. If your camera has multiple ISO settings, be aware that higher ISO settings increase noise. This is because a higher ISO setting is achieved by increasing the gain on the CCD sensors. This amplifies the signal from incoming light, but also amplifies unwanted signals due to noise. If you have control over ISO settings, stick with the lowest one as much as possible (though depending on how dim your target is, you may sometimes want to experiment with higher settings). If you are a do-it-yourselfer, you can also try active cooling of your camera. Things that people have tried include cold packs of various sorts, actuve air cooling using a fan mounted to the camera, and peltier coolers as attempted here and here, for example. You can search the digital_astro Yahoo! Group message archives for more information on cooling methods. You can also do things to eliminate the noise that you do have using various image processing techniques. To summarize, random noise can be dealt with image averaging and image stacking techniques. Non-random noise can be dealt with various frame subtraction techniques (e.g., dark frame subtraction). Non-random noise can be dealt with very effectively depending on how repeatable it is: the more repeatable, the more easily it can be eliminated with image processing. If you do plan on doing any image processing, there are a couple of things to keep in mind that will provide you with images that better lend themselves to image processing later on. These include (1) use high image quality settings (very little or no compression, though no compression can lead to images with very large file sizes), (2) other than in-camera noise reduction using dark frame subtraction, turn off any options that cause image processing to be done in the camera (such as in-camera digital sharpening). The reason for this is that you can always do equivalent image processing later using software, but there are some processing techniques that you may want to perform first that will work better on unprocessed images.
To be continued... ...
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[ 本帖最后由 小海螺 于 2008-11-8 08:46 编辑 ] |
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我都是高中水平,翻译不了这么多的内容!
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