What is "good" tracking

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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mfratto
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Joined: Fri May 13, 2022 12:06 am

What is "good" tracking

#1

Post by mfratto »

So, I am starting to get serious about tracking and maybe moving beyond EAA. In the screen shot below. does this constitute "good enough"? I am using a Celestron CGEM with a C8 and 6.3 reducer and an ASI183mc (uncooled). These are 15 second exposures. Tracking seems to be wandering off. Not so much that I am getting blobby starts (I tested up to 8 minutes and the stars looked good from a quick review). Should I be concerned about this tracking? Is this what I can expect? I did do a polar alignment using Sharpcap to "Excellent" so I got that going for me.

Really, just looking for expectations of what I can achieve on my rig. It has nothing to do with Sharpcap.
Sharpcap Screen shot.png
Sharpcap Screen shot.png (395.22 KiB) Viewed 648 times
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admin
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Re: What is "good" tracking

#2

Post by admin »

Hi,

there are 25 frames recorded in that live stack so far, and the total drift looks like it is about 8 or 9 pixels, so if it was a steady drift at about 0.3 pixels per frame then that would be all OK. However, you can see that there looks to be a pattern in the drift, perhaps a slight zigzag, generally going downwards but moving between left and right. Even so, the amount moved per frame is probably only 1 or so pixels on average, which is again OK as it will be significantly less that the star sizes you are getting (very likely).

You have really two options to try to improve this

1) guiding - with a separate guide camera and either a guide scope or off-axis guider. PHD2 will handle this nicely and it should keep the whole thing steady.

2) Periodic error correction in the tracking - periodic error in the RA gears is most likely causing the zig-zag nature of the drift (possibly superimposed on an underlying drift caused by slight polar alignment errors or other causes). PEC will probably help but not completely eliminate the drift - I personally have never used it.

The issue would become more pressing if you started using longer sub-exposures - if the movement from one frame to the next is more than about 1-2 pixels then you may start to see star shape issues.

cheers,

Robin
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