目前中关村D70的价格已经掉到6000左右，对于nikon一族来说，它无疑是最好的选择。本文内容摘录自外国朋友的网站，以供大家参考。
nikon D70是新型的数码单反相机，可以和佳能的10D和300D媲美。Christian Buil 几个月前买了台10D，而我因为有很多nikon镜头就买了D70。以下文章可以让我们对比它们在天文摄影上的表现。
Nikon D70 和 180mm f/2.8
和135mm f/2.0
The remote IR control command
To trigger the D70 in M-Bulb mode, you do not have a lot of choice. Either you do it manually by pressing the button with the risk of vibration, or you use an Infrared command.
The Nikon capture software does not allow you to control from the PC a Bulb exposure, which is really too, too, too bad.
The Infrared remote command was not available at that time. But we found on the web a software which runs on Palm and use the Palm IR port to trigger any type of IR device: Omniremote software can be downloaded for free as a 15 days trial version or can be bought at a cost of 25$. In addition to Omniremote, you need the hex code to excercise the D70. This is available on other site, natec.com where a guy trained a Nikon IR remote command and put the data for free for any one. I installed both the database and the omniremote software on my Clie NR70V and it worked perfectly. One click to open the shutter and another to close it.
Palm Sony Clié with the omniremote screen to excercise the Nikon D70
Make sure you set-up the time-out of the IR port to its maximum. In the D70 menu, select the 15min value for the IR. I can imagine than Birds catcher also may be upset by this, not easy to go back to the camera every 15 minutes to re-energize the IR receiver when you place your camera in a well hidden place...
Nikon D70 without lens, the IR receiver is really placed for "group picture" - just in front...
The IR receiver being located on the front of the camera, you need to make sure you beam the sequence at the right place ohterwise the receiver will not "see" the command. For long exposure you can shut down the palm to save battery and re-energize it before the end. A clock is quite useful to indicates when the exposure is supposed to end. I finally found a way to program a macro button on omniremote but did not test it on the sky yet. See here for more details.
The digital camera with the lens was directly attached to a NJP equatorial mount for sky images.
"Mode 3" acquisition - the "real raw" image mode
As mentioned on Christian Buil's site, the big drawback of the D70 for star imaging is a powerful medium filter applied on the raw image before internal storage. The trick he found to by-pass this filtering is the following:
- set your camera in "dark subtracted" mode (also called "noise reduction") - at the end of the regular exposure, the D70 will take a dark image at the same duration of the previoulsy acquired image.
- during the dark acquisition, turn off the camera (Nikon said there is no damage to do this). You can see then the green led flashing indicating that an image is being written on the card. That's the raw image, and this image has NOT been median filtered... The difference is astonishing.
Raw image of the Neat comet, 90s exposure: on the left, raw image from Nikon - on the right, real raw image with no Nikon Median filtering by interrupting dark image "on the fly"
It is a pity that Nikon did not included in its firmware a way to disable this feature for scientific or debug usage. Let's lobby to get this in a future version. Contact Nikon Customer service and ask for it !! - Nikon Site.
Image processing
Standard image processing has to be performed to get the best of the images.
A serie of 3 offset have been acquired - transformed into fit images, then averaged
Final offset image, reduced 1:5
Final offset image,1:1 region
Final offset image, reduced 1:5
Final offset image,1:1 region
A serie of 5 dark of 90s each have been acquired - transformed into fit images, then averaged
Final Dark of 5x90s averaged duration - 1:5 reduction factor - the light produced by the output amplifier is well visible but perfectly removed after the dark subtraction
Final Dark of 5x90s averaged duration - 1:1, no reduction factor - hot spots are removed after dark subtraction on the final object image
A serie of 5 flat have been acquired
Flat image acquisition: the camera is directed to a white cardboard illumated by an halogen white light. Here you can see Christian Buil making flat with his canon 10D attached to a Takahashi refractor
Final Flat - 1:5 换算系数
Final Dark of 90s duration - 1:1 area - the exposure is set to be the maximum one with no saturated areas
Refer to the new IRIS tutorial web page to get step-by-step description of the IRIS command. Christian had to make it after my two days of questions as I was trying to remember in which order you have to convert which image and so...
All the signal handling processing are made in the fit domain (fit images ).
For the Registration, you need to reconvert the fit image with the red, blue, green pixels from the bayer matrix into a 48 bits colored image, otherwise the star registration will not be accurate.
Then you need to equilibrate the colors, either by adjusting on a white area or on a dark, sky background, zone. Always revert to the original image when testing new color balance, otherwise the noise will be increased as the color coefficient are multiplying factor for each layer.
Some results
135mm f/2 lens, Cygnus constellation - ISO 400ASA, only one 90s exposure - "real raw mode" - image size reduced by 1:3
1-1 region of the Lyrae planetray nebulae - M57
M57 1:1 area - on the left, with a 135mm lens, one exposure of 90s, on the right, with a 180mm lens, two ewposure of 60s each
135mm f/2 lens, Her constellation, M13 - ISO 400ASA, only one 90s exposure - "real raw mode" - image size reduced by 1:3
1-1 region of the M13 cluster - processed with the "asinh 0.005 2" command in Iris after subsky command.
135mm f/2 lens, gamma Cyg region - ISO 400ASA, two exposure of 60s exposure - "real raw mode" - image size reduced by 1:3 - the red layer has been enhanced to make the red clouds more visible.
Neat 2004-Q4 comet - 5 exposures of 60s with the 180mm lens from a darker area close to Toulouse.
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