(抛砖引玉)如何来对极轴

这几天我一直在思考如何把咱们的器材版来办好，大概的一个思路是用好咱们手里的器材，无论是高端，还是低端的，怎么把他的潜能发挥到极致，物尽其用，能够看到和拍摄到很精彩的无尽星空，享受星空的乐趣。

话到正题，无论是目视还是拍摄，如果用赤道仪的话，都涉及到一个问题，那就是-----对极轴。极轴对的精准与否，直接影响了目视和拍摄的效果。

极轴对的不好，目视影响还小，如果是摄影，就会产生托尾，即时是采用了自动导星，也只能保证中间位置的星星不会托尾，会产生场旋。
对极轴的方法有很多，比如用极轴镜，漂移法等。

我现在用的赤道仪使星特朗的CG5,采用漂移法，因为在阳台，看不到北极星。具体做法是观察星点的移动方向，调整赤径或者赤纬，使视野里星点调整的方向和刚才星点移动的方向一致。每调整一次，就把星点再调整到视野的边缘而不是中间，因为这样可以很明显的看出移动方向，一次一次的做，直至星点的移动方向向相反方向运动，然后在往回调，调的幅度越来越小，反复的调整观察星点的时间也越来越长，达到2分钟星点不漂移基本上就可以达到目视要求了，总共用了大概10几分钟。调整好以后，用80ED看土星，果然可以看到更多的细节，比如可以很清楚的看到前后光环的夹角，土星的条纹。

仅做抛砖引玉，请大家把自己的绝活亮出来，共同提高，好帖子可以加分的。。。

我住南城区北边光害严重看不到北极星.你的这个办法不错,学习了

总感觉没有极轴参考的漂移会很麻烦，没想到版主两分钟就能搞定，可是够快的。

这几天我一直在思考如何把咱们的器材版来办好，大概的一个思路是用好咱们手里的器材，无论是高端，还是低端的，怎么把他的潜能发挥到极致，物尽其用，能够看到和拍摄到很精彩的无尽星空，享受星空的乐趣。

话到正题 ...
northwolfwu 发表于 2009-5-13 08:17

                               
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楼主翻译下这些文章好了,其中也涉及到漂移法.

Accurate polar alignment

You can use your finderscope (finder) to accurately polar-align your equatorial mount. This is more accurate than a rough alignment using the mechanical scales on your mount or roughly pointing the whole mount towards Polaris.
To use your finder as a polar finder is a three-step process. First, the optical axis of the finder must be aligned to the mechanical polar axis of your equatorial mount. Second, Polaris will be centered in the finder (polar axis pointing towards Polaris). Since Polaris is nearly one degree away from the true north celestial pole (NCP), the last step will offset the finder’s view and the polar axis to get true polar alignment.

The alignment of the optical axis of the finderscope with the mechanical polar axis of the mount can be done either at night with Polaris or (perhaps more easily) during the daytime on a distant building or landmark. If you do align during the day, use the latitude adjustment screws and the tripod to level the polar axis to make it easier to do.

NOTE: Any star can be used to get the finder optical axis and polar axis aligned. Polaris is chosen for convenience and also because it will be used in the second step.

You’ll flip the mount several times and recenter Polaris (or the landmark) in the finderscope each time, successively getting closer to alignment of the finderscope and polar axis.

Working with Polaris, start by setting up your mount as you would for polar alignment. The Dec setting circle should read 90 degrees. Unclamp the RA and rotate the mount until the telescope and finder are all the way to the left or right, Dec axis horizontal. Get Polaris in the field of view of the finder and centered in the crosshairs by moving the mount (using fine adjustment screws). Now move (flip) the mount all the way to the opposite side (180 degrees or 12 hours RA away from the original position). Note the shift of Polaris off the crosshairs. Actually the finderscope and the crosshairs themselves have rotated in a small semicircle around where the polar axis points. You can see where that is by looking through the finderscope as you rotate the mount, watching for the center of motion. Clamp the mount and turn the three setscrews around the finderscope to move the crosshairs over this pivot point. Recenter Polaris by only moving the mount.

                               
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Even with the telescope positioned 180 degrees away around
the mount, the telescope (and finderscope) should still be
pointing at the same object in the sky.

Repeat the flipping-setscrew-recenter Polaris procedure. Each time you go from one side to the other, the off-center distance of the crosshairs from the pivot point will be smaller. After three or four repeats, the crosshairs won’t move when you flip the mount. You will be pointing at Polaris. Your polar finderscope optical axis is now pointing in the same direction as the polar axis.

(If you did this during the day with a landmark, now wait until dark. Set up your mount as you would for polar alignment. The Dec setting circle should read 90 degrees. Use the latitude scale and adjustment screws to center Polaris in the finder.)

                               
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When rotating the scope and finder 180 degrees around the polar axis,
the crosshairs will rotate around where the polar axis is pointing
(this pivot point is the "X" in the right-hand figure). Adjusting the
finder and the equatorial mount until an object remains centered in the
crosshairs during the rotation aligns the finderscope with the
telescope's polar axis.

Steps one and two are done and the polar axis of the telescope is aligned with Polaris, but as any star atlas will reveal, the true pole lies about ¾-degree away towards the last star in the Big Dipper’s handle (Alkaid). To make this final adjustment, the telescope mount will need to be offset from Polaris towards the actual NCP.

Since Polaris makes a complete rotation around the NCP once a day, how far should the mount be moved and in what direction? One easy way is approximation. Guesstimate the direction by using Alkaid. Guesstimate the amount by knowing the field of your finderscope and dividing it by the ¾-degree distance of Polaris from the pole.

Example: On August 1, at 8PM, Alkaid is above and to the left of Polaris in the 10 o’clock position. You have a 6-degree field-of-view finderscope. Starting with Polaris on the crosshairs, use the fine adjustment screws to shift the mount in altitude (latitude) and azimuth up and left by one-eighth finder field (6 divided by 0.75).

                               
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The true North Celestial Pole (NCP) lies less than a degree away from Polaris in the direction of Alkaid, the last star in the handle of the Big Dipper (Ursa Major).

Now use the setting circles to check how close the polar axis alignment is to the NCP. Unclamp the axes and swing the scope’s tube to a bright star of known RA and Dec near the celestial equator. Turn (set) the RA setting circle to this star’s RA. Now move the tube until the RA setting circle reads 2 hours 31 minutes and the Dec circle reads +89 degrees 15 minutes. These are the coordinates of Polaris and the Pole Star should now be in the finder’s crosshairs. If it’s off, once again move the mount in latitude (altitude) and azimuth to center Polaris.

Now you have a polar alignment for your scope within a fraction of a degree of the NCP. This is excellent for visual purposes and short-exposure photographs piggybacking on the main tube. However, guiding corrections and field rotation will still be problems for long-exposure astrophotography, which demands the most precise polar alignment.

NOTE: At the completion of this process, Polaris may very well not appear in the center of your main scope's eyepeice field. This is because the optical axis of the finder and the polar axis are now parallel. But the finder's optical axis and the main tube's optical axis may not be parallel. So centering a star in your finder won't necessarily center it in your eyepiece. To overcome this, once you've achieved polar alignment, you can realign the finder with the main tube optics to return the finder to normal operation with the main scope.

How To Polar Align an EQ Mount

These instructions will show you how to perform an accurate polar alignment for your Skywatcher telescope and refine the alignment with the drift method.
If like me your Skywatcher Explorer didn't come with instructions, and you don't understand the poorly translated Chinese from the website, then hopefully this guide will help you to setup your mount. It is based on a Sky Watcher HEQ5 mount with dual axis motor drive, but the principle should be the same for any equatorial mount.

Why align the mount?

                               
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Skywatcher Explorer 200 on HEQ5 mount


From our perspective, the planets, sun, moon and stars move across the sky from East to West making a complete circle once every 24 hours. In fact it is not the stars that move, but the Earth rotating that causes the appearance of the stars moving.

If you watch the stars for long enough, or set-up a camera with a very long exposure, you will notice that there is a point in the sky that does not appear to move, and all the other stars rotate around this point. This point is the Northern Celestial Pole (NCP) and is True North (or Southern Celestial Pole if you are in the Southern Hemisphere).
By aligning the mount with the NCP, we can counter the effect of the movement of the Earth by rotating in the opposite direction. This enables us to take long exposures on the mount without any star trails in the picture. It also means that when you view an object, such as Jupiter, it stays in your field of view and you do not have to keep moving the telescope.
The NCP is located close to the star Polaris (also called the Pole Star), and Polaris is our starting point for an alignment, but it is not close enough for an accurate alignment.
I find setting it up best to do before you attach the OTA (Optical Tube Assembly - the telescope), and make fine adjustments when the OTA is attached.

Preparing the MountBefore we begin the polar alignment procedure, the mount has to be prepared by aligning the polar scope to the mounts axis. You only need to do this once, or if something comes loose, or just needs recalibrating. You don't need to do it every time you setup the mount.
Aligning the Polar Scope ReticuleThis bit is a lot easier to do during the day. You need to set-up the mount so you can see a distant object, such as a pylon or telegraph pole.
Remove the dust caps from either end of the RA (Right Ascension) axis, lower the counter shaft bar and rotate the DEC (Declination) axis so that you can see through the polar scope.
When you look through the polar scope you should see an overlay of a cross hair or circles with constellations. My polar scope has constellations and circles as shown below. You can see how Polaris is offset by about 3/4 degree from True North.

                               
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I should point out that you cannot actually see the Big Dipper or Cassiopeia through the polar scope whilst looking for Polaris. I spent ages trying to work out why I could not see them until I realised that they are there only as a guide. Looking through this example, the big dipper should be above me and to my left.

                               
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There are three adjustment screws set 120° apart. This photo shows two of them, the other is underneath. Only make small adjustments to the screws, and only adjust two at a time. Do not over tighten or you can crack the reticule.

The Polar Scope Alignment Process:

• Locate a distant object and centre it in the crosshair using the Altitude and Azimuth adjusters.
• WARNING: Do NOT use any telescope device for looking at the sun without appropriate solar filters. Doing so may cause permanent blindness. Do not use the sun for calibration.
• Turn the RA axis through 180°
• If the object has wandered out of the crosshair, the reticule needs to be aligned.
• Use the three alignment screws (see below) to move the reticule half way back to the object
• Re centre the object back in the crosshair
• Turn the RA axis through 180° in the opposite direction
• If the object has not moved the reticule is aligned, if it has moved repeat until you can rotate the axis in either direction with no movement.

Once this adjustment has been made successfully, it should not be necessary to carry out this procedure again.
Calibrating the Index Marker Ring

                               
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Polaris moves around the NCP

Polaris, like all the other stars, appear to rotate around the NCP. Since the NCP is invisible we use Polaris as a visual guide because it is the closest star to the NCP. Since Polaris is moving we must calculate the position of the NCP in relation to Polaris.

Don't Panic! There is nothing you have to calculate, it is all done by the mount. It is essentially a circular slide rule, but it must first be calibrated for your location. The index marker represents a 'zero' for the slide rule. If the zero is not set correctly, the calculated hour angle will not be correct.

                               
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Index Marker Ring

Polaris not only rotates around the NCP once every 24 hours, but it rotates in a bigger circle every 356 days with the Earths tilt, so it appears to 'wobble' through the year.

We need to set a reference 'zero' point where we know the position of Polaris in the sky. You simply need to look up the time and date at which Polaris is at its highest or lowest position in the sky as these are easiest times to calculate. I used the free planetarium software Stellarium to calculate the highest position that Polaris achieves. I did this by playing with the date/time controls to find the highest point in the Month, then going through all the days to find the highest day, then the highest hour and so on.


For my location the highest position Polaris achieves occurs at 00:06 on 17nd of November, and I will be using this date in this document. You should substitute with your calculated date/time.

                               
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Setting Circles

• Unlock the RA axis a rotate it until Polaris is pointing at the 6 o'clock position.
• Lock the RA axis. In this position, Polaris is at its highest point in the sky.
• Note: The polar scope view is inverted, so Polaris' high position occurs at the bottom of the polar scope.
• Unlock the RA setting circle by loosening the set screw. Rotate the RA setting circle so the pointer indicates 'zero' and lock the RA setting circle.
• Now Unlock the RA axis and rotate it so the RA setting circle indicates 0h 06m on the top set of numbers. The bottom numbers are for use in the southern hemisphere. Lock the RA axis.
• Rotate the Date circle so that 17th November is lined up. Remember to substitute your calculated values.
• Unlock the RA axis and rotate it so that the RA setting circle reads Zero. You should find that Polaris is back where it started - at the bottom.
• Loosen the set screw on the Index Marker Ring and adjust it so that it reads your Longitude offset, East or West of the reference meridian for your time zone. (see below)

from the SkyWatcher manual:

The alignment procedure requires that you set the Longitude scale to "Zero". Depending on where you live, "Zero" can be anyplace between the E and the W on longitude scale, so first you need to determine where zero is for your location. Your Zero point is equal to the difference between your actual longitude and the longitude of the central meridian of your time zone. To calculate the longitude of your central meridian, multiply your time zone offset from Greenwich Mean Time (GMT) by 15.
For example, in Waterloo, Ontario, Canada (Eastern Time) the time zone offset is -5 hours. Ignore the sign and simply multiply 5 x 15 = 75.The longitude of the central meridian for the Eastern time zone is 75 degrees west. The actual longitude at the viewing location in Waterloo is 80 degrees 30 minutes West. Ignore the 30 minutes and just use 80 in the equation.
Now it's simple, 80 - 75 = 5. Since 80 is greater than 75 the result is positive 5. That means Waterloo, Ontario is west of its Central Meridian. In this case, the zero point is at the "5" mark on the W side of the scale. If the location was east of its central meridian the equation would yield a negative value. In that case the E side of the scale should be used.

The Polar scope is now calibrated for your longitude and the Index Mark should be on the right hand side of the Polar scope. This procedure should not have to be repeated unless the reticule has been re-aligned or the index ring has come loose.
Polar Alignment

• Unlock and rotate the RA axis so that the Index Marker lines up with the Date Circle Zero. Lock the RA axis.
• Turn the Date Circle so that your calculated "zero" lines up with the Index Marker (see above)
• Unlock and rotate the RA axis so that today's date is lined up with the Index Marker. Lock the RA axis.
• Loosen and move the RA Setting Circle so that the current time is shown against the Pointer. Tighten the set screw.
• Unlock the RA axis and rotate until the Pointer points to zero on the setting circle.
• The reticule is now in the correct position. Use the Altitude and Azimuth adjusters to align Polaris in the small circle on the perimeter of the larger circle.

Improving the Alignment with the Drift MethodThe drift method allows the fine tuning of the polar alignment process. It is easier to perform with an illuminated reticle eyepiece, but at a push you can use the crosshair on the finder scope.

• Start by performing a rough polar alignment.
• Find a bright star close to the meridian, just north of the celestial equator.
• Point the telescope towards this star and centralise it in the crosshair.
• As the star drifts, align the crosshair with the direction of motion of the star.
• Once the star is moving along the crosshair, re-centre the star.
• Watch the movement of the star and adjust the altitude adjustment knob so that the star is half way back to centre. Use the hand controller to re-centre the star and repeat this step until no movement is observed along that axis.
• Repeat the above, this time for the azimuth axis.
• Repeat the procedure for a second star on the opposite horizon (e.g. if your first star was in the West, repeat for a star in the East).

You should now have an accurately aligned polar mount.
Using the Setting Circles

• Point the telescope towards a known star near the object you wish to locate
• Using a star atlas or software, set the RA and DEC setting circles to the location of your known object.
• Without moving the setting circles (!IMPORTANT!) move the telescope until the objects coordinates line up with the setting circles.
• The object should be visible in a WIDE (low power) eyepiece field of view.

Precise Polar Alignment
The above method of polar alignment is limited by the accuracy of your telescope's setting circles and how well the telescope is aligned with the mount.  The following method of polar alignment is independent of these factors and should only be undertaken if long-exposure, guided photography is your ultimate goal.  The declination drift method requires that you monitor the drift of selected stars.  The drift of each star tells you how far away the polar axis is pointing from the true celestial pole and in what direction.  Although declination drift is simple and straight-forward, it requires a great deal of time and patience to complete when first attempted.  The declination drift method should be done after the previously mentioned polar alignment steps have been completed.

To perform the declination drift method, you need to choose two bright stars. One should be near the eastern horizon and one due south near the meridian.  Both stars should be near the celestial equator (i.e., 0° declination).  You will monitor the drift of each star one at a time and in declination only.  While monitoring a star on the meridian, any misalignment in the east-west direction is revealed.  While monitoring a star near the east horizon, any misalignment in the north-south direction is revealed.  As for hardware, you will need an illuminated reticle ocular to help you recognize any drift.  For very close alignment, a Barlow lens is also recommended since it increases the magnification and reveals any drift faster.  When looking due south, insert the diagonal so the eyepiece points straight up. Insert the cross hair ocular and rotate the cross hairs so that one is parallel to the declination axis and the other is parallel to the right ascension axis.  Move your telescope manually in R.A. and DEC to check parallelism.

First, choose your star near where the celestial equator (i.e. at or about 0º in declination) and the meridian meet.  The star should be approximately 1/2 hour of right ascension from the meridian and within five degrees in declination of the celestial equator.  Center the star in the field of your telescope and monitor the drift in declination.

If the star drifts south, the polar axis is too far east.  
If the star drifts north, the polar axis is too far west.  
Using the telescope's azimuth adjustment knobs, make the appropriate adjustments to the polar axis to eliminate any drift.  Once you have eliminated all the drift, move to the star near the eastern horizon.  The star should be 20 degrees above the horizon and within five degrees of the celestial equator.

If the star drifts south, the polar axis is too low.  
If the star drifts north, the polar axis is too high.  
This time, make the appropriate adjustments to the polar axis in altitude to eliminate any drift.  Unfortunately, the latter adjustments interact with the prior adjustments ever so slightly.  So, repeat the process again to improve the accuracy, checking both axes for minimal drift.  Once the drift has been  eliminated, the telescope is very accurately aligned.  You can now do prime focus deep-sky astrophotography for long periods.

NOTE:
If the eastern horizon is blocked, you may choose a star near the western horizon, but you must reverse the polar high/low error directions. Also, if using this method in the southern hemisphere, the direction of drift is reversed for both R.A. and DEC.

Even with a telescope with a clock drive and a nearly perfect alignment, most beginners are surprised to find out that manual guiding may still be needed to achieve pinpoint star images in photographs. Unfortunately, there are uncontrollable factors such as periodic error in the drive gears, flexure of the telescope tube and mount as the telescope changes positions in the sky, and atmospheric refraction that will slightly alter the apparent position of any object.

Polar alignment, as performed by many amateurs, can be very time consuming if you spend a lot of time getting it more precise than is needed for what you intended to do with the telescope. As one becomes more experienced with practice, the polar alignment process will become second nature and will take only a fraction of the time as it did the first time. But remember that when setting up your telescope's equatorial mount, you only need to align it well enough to do the job you want.

不止两分钟吧，是最后达到两分钟才行，前面调整的时间一共多少呢

漂移法对极轴其实并不复杂，关键在于如何根据星点的移动来判断极轴的偏差方向。另外漂移法还需要一只有十字丝的导星目镜，这个目镜DIY起来很简单。稍后我会在这里跟帖详细阐述。关键是要勇于尝试，不要一有人说难就不敢去尝试了。

我还好了，家里条件比你们好，天睛都能看到北极星。。。
我现在基本上就把北极星导到，极轴镜中央了了事，，，

对的,各位大家踊跃翻译上面的文章吧,相信会有启发作用.

我是个菜鸟，不要笑话我。首先声明我说的是简易的，好的够我买几个镜子的，本人现在不拍摄。我的更简单，首先知道我所处的纬度，然后用两根细线从镜筒上吊下来，下面拴着老婆织毛衣用的长织针，再下面放上指北针，使织针和指北针垂直平行，仔细看使织针逆时钟相对指北针偏转几度（大概吧）就OK啦。当然指向目标后还要调几次赤经、纬轴。作为一般观测足够了。好啦，我是班门弄斧。

为楼上的鼓掌!这叫自己的实际经验,值得拿出来和大家分享!!

为楼上的鼓掌!这叫自己的实际经验,值得拿出来和大家分享!!
rjxie 发表于 2009-5-13 10:18

                               
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怪不好意识的，谢谢楼主夸奖

7# dike

中文有岐义阿，

是一次用了2分钟，总共应该用了差不多10多分钟。

我是个菜鸟，不要笑话我。首先声明我说的是简易的，好的够我买几个镜子的，本人现在不拍摄。我的更简单，首先知道我所处的纬度，然后用两根细线从镜筒上吊下来，下面拴着老婆织毛衣用的长织针，再下面放上指北 ...
发烧友 发表于 2009-5-13 10:15

                               
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这个还真没弄明白，能不能请发烧友详细说明一下！非常感谢！

这个还真没弄明白，能不能请发烧友详细说明一下！非常感谢！
kson 发表于 2009-5-13 11:31

                               
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我的理解，这个毛衣针的方向就是赤道仪朝北的方向，由于真正的天文的北极和地理上的北极不一致，因此需要调整一定的角度。

欢迎指正！

4# rjxie

谢谢老大的资料！

第一篇我看了一下，方法挺新颖的，正在边学边翻译中。

其他两篇，要过一段时间了，看看大家能不能支援一下？

DC无视地飘过，继续享受极轴镜快速对好的极轴

我的理解，这个毛衣针的方向就是赤道仪朝北的方向，由于真正的天文的北极和地理上的北极不一致，因此需要调整一定的角度。

欢迎指正！ ...
northwolfwu 发表于 2009-5-13 12:29

                               
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表达不清，谢谢版主的精确解释。