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Can you do a star drift alignment on the sun?

6 replies to this topic

#1 DelVento

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Posted 09 October 2023 - 10:38 AM

Perhaps using a sunspot?

 

Has anybody attempted to do this? I guess I can just try (and I probably will), but if anybody has tried already and could say "forget it, the problem is xxx" or "works, but be careful about yyy" that would be a much better headstart.

 

Thanks

 



#2 SteveInNZ

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Posted 09 October 2023 - 02:24 PM

Yes. It works well because you have time on your side.

If you polar align as well as you can and then point at the Sun, any drift is either due to polar misalignment or the tracking rate. Even if you're tracking at sidereal rate, that error is minimal.

So if you point at the Sun and then keep it centered by only adjusting the Alt and Az, you are doing a continual drift alignment during the partial phases.

The longer you go, the better the alignment, but it doesn't take long to get the Sun to stay in the frame.

Try it and judge for yourself.

 

Steve.


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#3 aroughroad

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Posted 30 January 2024 - 08:51 PM

Yes. It works well because you have time on your side.

If you polar align as well as you can and then point at the Sun, any drift is either due to polar misalignment or the tracking rate. Even if you're tracking at sidereal rate, that error is minimal.

So if you point at the Sun and then keep it centered by only adjusting the Alt and Az, you are doing a continual drift alignment during the partial phases.

The longer you go, the better the alignment, but it doesn't take long to get the Sun to stay in the frame.

Try it and judge for yourself.

 

Steve.

I can get mediocre results doing this but am not sure whether I'm making the right adjustments.  My lens is on a star tracker that tracks at the solar (not sidereal) rate, and my altitude is set using the star trackers degree markers (not super precise but within a degree).  I then use a geared head to move the camera/lens to get the sun in the frame.  Once it's in there (and the sun is on the eastern side of the meridian), what does upward/downward/leftward/rightward drift of the sun in the frame indicate as far as corrections I need to make?  And is it the case that I will never have to make any corrections on the geared head?

 

Thanks!



#4 SteveInNZ

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Posted 31 January 2024 - 03:55 AM

The typical order of items from the bottom up, are tripod, wedge, tracker, ballhead, camera.

 

People use geared heads in place of the wedge or instead of a balhead or they may use a ballhead as a wedge. So whatever you have between the tripod and the tracker (ie the wedge) is what you want to adjust to keep the Sun centered and compensate for drift. Whatever you have on top of the tracker is what you use to center it initially.

 

Steve.



#5 aroughroad

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Posted 31 January 2024 - 10:43 AM

The typical order of items from the bottom up, are tripod, wedge, tracker, ballhead, camera.

 

People use geared heads in place of the wedge or instead of a balhead or they may use a ballhead as a wedge. So whatever you have between the tripod and the tracker (ie the wedge) is what you want to adjust to keep the Sun centered and compensate for drift. Whatever you have on top of the tracker is what you use to center it initially.

 

Steve.

Got it - so once the sun is initially centered in the frame, adjust the alt/az knobs of the wedge and don't touch the geared head.  But do you know which adjustments to make?  If the sun is drifting down in the frame, what should i adjust?  if it's drifting up in the frame, what should i adjust? etc. etc.



#6 SkipW

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Posted 31 January 2024 - 01:06 PM

Got it - so once the sun is initially centered in the frame, adjust the alt/az knobs of the wedge and don't touch the geared head.  But do you know which adjustments to make?  If the sun is drifting down in the frame, what should i adjust?  if it's drifting up in the frame, what should i adjust? etc. etc.

There's the rub...

 

If the object you're using to drift align on (the sun, in this case) is due east and close to the horizon, it will drift to the north if the polar axis elevation angle is too high, to the south if PA angle is too low (I think... I'm visualizing this in my head). Reverse for objects low to the west. It will not (well, hardly at all) be sensitive to PA error in azimuth.

 

If the object is close to the meridian, it will drift to the south if the PA azimuth is too far east, and to the north if too far west (again, I think). Here, PA elevation error has no (or little) effect.

 

Midway between these points, the drift direction will be a mix of these direction errors. 

 

Which direction is up? This depends entirely on the orientation of the camera in the OTA. Easiest to work with is to rotate the camera so that the top of the frame is north (or crosshairs aligned N-S-E-W if you're trying this visually, but beware of complications - flipped image and rotation - if you're using a diagonal).

 

The main problem trying to do this with the sun on the morning of the eclipse is that getting the elevation angle close to correct by measuring its angle is already easy compared to setting the azimuth correctly, and you have to wait until near midday before you can use this method where it's most sensitive to azimuth error.


Edited by SkipW, 31 January 2024 - 01:11 PM.


#7 SteveInNZ

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Posted 31 January 2024 - 01:11 PM

Adjust the Alt or Az as needed to keep it in the center. It's an iterative thing. As you get closer, you will wait longer to see drift and your adjustments will get smaller.

 

Steve





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