Help with drift

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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bhalkett
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Joined: Fri Jan 28, 2022 12:18 pm

Help with drift

#1

Post by bhalkett »

I'm very new to astronomy, AP and EAA so please forgive any obvious mistakes. I seem to be getting a lot of drift and field rotation as indicated by the attached cropped image of M31 and the SC drift chart.

Here are the details:
Scope: Celestron 8se with Hyperstar
Mount: Celestron AVX with latest firmware
Camera: ASI533MC Pro
Software: SharpCap 4 Pro, CPWI v2.3.5

I've setup as follows:
1) Level the mount.
2) Polar align with the Celestron Polar Scope.
3) Connect to the mount with CPWI and SC. Verified that tracking is set to sidereal in SC.
4) Performed alignment with CPWI (several stars recommended by CPWI and all in the general area of M31) with All Star Polar Alignment (ASPA) at the end (zero physical adjustment needed). Note that I used ASTAP within SC to plate solve (Sync mount and re-center target) to center each star.
5) Verified align on a couple of stars.
6) Performed a Goto to M31 in CWPI
7) Verified centered on M31 via ASTAP plate solve (solve only)
8) Started Live Stack (30s exposures)

It appears clear that SC is doing a good job via alignment handling the drift and field rotation. It also appears that the AVX mount is not tracking well. I did also verify (to the best of my ability) that everything is using J2000 epoch.

I realize that this isn't a Celestron AVX forum so my question is what from the attached stacked image and SC drift chart can help my diagnose this.

Thanks in advance.
-Brian
Attachments
M31 cropped image
M31 cropped image
M31-Drift-Crop.png (645.02 KiB) Viewed 2632 times
Drif charts
Drif charts
M31-Drift-Chart.PNG (244.38 KiB) Viewed 2632 times
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admin
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Re: Help with drift

#2

Post by admin »

Hi Brian,

I think the drift is definitely the thing to attack here - it's even possible that the rotation might be a side effect of that.

You can use the pixel position tool in SharpCap (after plate solving) to show the celestial directions on the image to work out which direction the drift is happening in - I did this with your attached image and got the following
Capture.JPG
Capture.JPG (61.72 KiB) Viewed 2628 times
I think from your picture that the drift is that the image is moving up and left, which means drift in both RA and Dec I think.

If it was a pure RA drift then you would look at tracking speed, etc, but with Dec drift too the most likely causes would either be something slipping mechanically (focuser slowly settling, cable pulling on camera, etc) or poor polar alignment.

You should be able to use the polar alignment tool in SharpCap to check the polar alignment - with your 533 camera + hyperstar you should be getting an FOV of about 1.5 degrees square, which should work just fine for the polar alignment tool.

cheers,

Robin
bhalkett
Posts: 7
Joined: Fri Jan 28, 2022 12:18 pm

Re: Help with drift

#3

Post by bhalkett »

Hi Robin,

I can say with confidence that a cable was not pulling on the camera. I made sure that both RA & DEC knobs were snug and that the scope was balanced on both. I don't know what you mean by "focuser slowly settling"?

So if my polar alignment was off I don't understand the goto function results in objects being dead centered. I'm very far to left on this learning so I'm just trying to understand this.

The SC polar align process...I am not familiar with it. Do I need a separate guide scope & camera for that? If so I have a 60mm guide scope coming tomorrow.

Thanks in advance!
Brian
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Re: Help with drift

#4

Post by admin »

Hi Brian,

no need for a separate guide scope/camera combo (but you could use one) - all you need is a field of view of about 1 degree, which your main scope/camera should deal with.

As to why the GOTO is correct, it's possible that multi-star align on your mount is compensating for poor polar alignment - that's what it is supposed to do, although I generally find it doesn't do a great job, but maybe in your case it is.

cheers,

Robin
bhalkett
Posts: 7
Joined: Fri Jan 28, 2022 12:18 pm

Re: Help with drift

#5

Post by bhalkett »

I've discovered another interesting piece of information. All of the tracking was excellent until three days ago. Two things happened then. First, I setup a new mini pc and second, I downloaded and installed a new version of SharpCap on that computer. Before the 25th I was running version 4.0.8564.0. On the 25th I downloaded and started using version 4.0.8629.0.

CPWI is the same version and the firmware for the mount is the same. There is a minor version difference with Win 10 Pro but not much. I'm going to keep on investigating this. I can't see how SharpCap or a slightly different could have any effect on the tracking for the mount.
Last edited by bhalkett on Fri Jan 28, 2022 10:54 pm, edited 1 time in total.
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Re: Help with drift

#6

Post by admin »

Hi,

I just checked and between those two versions the only change related to talking to the mount was to slightly delay the update of the UI after the mount is connected, which avoids fetching some information from the mount multiple times. I don't think that's likely to be related to this issue.

cheers,

Robin
bhalkett
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Re: Help with drift

#7

Post by bhalkett »

I rolled ALL the software back to the same versions I had on my laptop, but otherwise did everything else the same. The results...it worked a lot better. Much less drift and field rotation. I'm going to work with this setup until I'm satisfied the item working well then I'll consider upgrades one at a time.
bhalkett
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Joined: Fri Jan 28, 2022 12:18 pm

Re: Help with drift

#8

Post by bhalkett »

I did a Polar Align in SC tonight. Everything looks much better!

@Robin May I ask what the X & Y axis represent in the in the two charts in my first post?
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Re: Help with drift

#9

Post by admin »

Hi,

good to hear that the drift seems to be under control now.

The graphs X/Y in the graphs in your first post show the amount that the image has drifted (in terms of pixels) - X being in the horizontal direction and Y in the vertical. Strictly speaking I think it is showing how the horizontal/vertical offset of the center point of the image has drifted (as the horizontal and vertical offsets can vary accross the image when there is rotation too).

cheers,

Robin
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