Drift graph question

Discussion of using SharpCap for Deep Sky Imaging
Post Reply
Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Drift graph question

Post by Ss14843 » Sun Sep 01, 2019 6:10 pm

Hello all,

I have a question about the drift graphs the other night. I am using an 8” edge and Sv80st piggyback for guiding (oag issues lately so switch to 80mm for now). I was guiding 30s subs with very good PA according to PhD2. Images were excellent with round stars, though I started to see pixel rain and the first and last subs translated by a significant amount. I have seen this before even using my oag so I don’t want to jump right in and say it’s differential flexure. Any ideas?

What I plan to do is buy the ADM alt/az adapter plate to get my guide and main imaging scopes perfectly aligned on the same target. Will this improve things as long as my PA is perfect and everything is tightened up etc? Here is a pic of the drift. Thanks in advance!

Steve
EDBFB237-0F7B-40D5-966B-330AC1A097BD.jpeg
EDBFB237-0F7B-40D5-966B-330AC1A097BD.jpeg (354.44 KiB) Viewed 244 times

Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Re: Drift graph question

Post by Ss14843 » Sun Sep 01, 2019 6:20 pm

Sorry I posted this in the wrong place!

User avatar
admin
Site Admin
Posts: 3094
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Drift graph question

Post by admin » Sun Sep 01, 2019 8:26 pm

Hi,

Moved to the deep sky imaging section which is where I think it belongs.

As far as I know, perfect alignment of the guide scope and the main scope are unnecessary, but rigid alignment between the two is very important. Do check for things like dangling cables as I have seen that have significant effects on the polar alignment functionality.

Cheers, Robin

Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Re: Drift graph question

Post by Ss14843 » Sun Sep 01, 2019 9:27 pm

Thanks Robin,

Do you see the graph drift like that when polar alignment is off? What is a reasonable amount of drift and rotation with very good to excellent polar alignment? From what I have read, if polar alignment is off even by a small amount and the main ota and guide scope are not perfectly aligned, there will be a different amount of field rotation in the ota vs guide scopes. I am guessing it’s because both scopes may not be aligned exactly the same with the RA axis? Anyways, if I know what is acceptable drift over time (like an hour or two) I can try to dial things in better. I’m using an ASI294MCpro. Thanks in advance,

Steve

Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Re: Drift graph question

Post by Ss14843 » Mon Sep 02, 2019 2:26 am

My question centers about what I’m observing in the drift graphs and posts such as this:

https://www.cloudynights.com/topic/6482 ... ing-scope/

If sharpcap calculates polar alignment errors observed in the guide scope and I make alt/az adjustments to polar align the guide scope, the main ota imager even being off a degree or two from the guide scope will result in RA drift in main ota images (main ota should theoretically be very closely aligned with the RA axis, while a dovetail mounted guide scope could be off a degree or two unless the dovetail mounting were perfect).

Because of this, since my PA error is usually about an arc min, and I know the guide scope is not perfectly aligned with the ota using a dovetail mount, I will see drift. Is this what I am seeing in the drift graph? It’s about 30 pixels in RA and about 10 pixels in Dec over 15 minutes. My thought is to align the guide scope as much as possible with the main ota. Also, if I then polar align perfectly, any drift should be mainly due to differential flexure. If perfectly polar aligned, what is acceptable drift over 15 minutes or so? Should all the green x’s be close to zero with no differential flexure and perfect pa?

Steve

User avatar
admin
Site Admin
Posts: 3094
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Drift graph question

Post by admin » Mon Sep 02, 2019 7:22 pm

Hi,

Firstly the idea that the guide scope needs to be perfectly aligned with either the main scope or the amount for the SharpCap polar alignment to work correctly is a myth. You will get correct polar alignment from SharpCap even if you're guide scope is a degree or so (or even a few degrees) misaligned. Of course there are other things that can lead to incorrect alignment such as any sort of physical shift of the guide scope as you rotate the mount during the alignment procedure.

Once your correctly aligned (the RA axis pointing at the pole), you should get effectively no drift in declination over time – that's what drift alignment relies on if you use that approach to polar alignment. You may get drift in the RA axis over time due to periodic errors in the mounts drive system.

Regardless, if you are guiding with PHD, then I'd expect the guiding to deal with both of these problems and leave you with a drift graph in SharpCap that shows almost no deviation.

Cheers, Robin

Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Re: Drift graph question

Post by Ss14843 » Sun Oct 13, 2019 12:16 am

Hi Robin,

I have had a chance to tun some further testing. I have changed from a CGX mount to a CEM60EC since my previous post. I imaged with 30s subs for 15 minutes with guiding off. The drift graph looked remarkably similar to what I posted with the CGX (no rotation, but a slow and steady drift to about 20-25 pixels in 15 minutes). Stars were all round edge to edge, but the edges of the image began to be “clipped.” This was unguided with polar alignment error less than 1 arc min according to Sharpcap and PhD2 guiding assistant (no guiding, just a quick PA error check).

Next, I repeated the process while utilizing PhD2 guiding. The drift was in the same direction and to the same extent. Guiding was good with a total RMS of about 0.5. Stars were all round edge to edge, but again, the edges of the image were clipped due to the drift.

Obviously with round stars people would suggest just living with the drift. I can’t concede and just accept this. If I imaged for two hours the data would be un-usable.

I’m using two totally different mounts. The newest has an RA encoder to minimize PE so I highly doubt the CGX with horrid backlash and bad PE would produce the results possible with the CEM60EC. Since I am also getting virtually identical drift unguided and guided, I can rule out differential flexure. Finally, regarding PA error... I can pretty much rule that out as well because results similar in Sharpcap and Polemaster as verified in PhD2. So, is there a stacking/alignment setting I may have wrong? Could this possibly be something with the imaging camera?

Thank you in advance!

Steve

Ss14843
Posts: 10
Joined: Wed Sep 05, 2018 1:16 am

Re: Drift graph question

Post by Ss14843 » Sun Oct 13, 2019 12:53 am

I haven’t tested for cone error, but could cone error cause these issues?

Steve

User avatar
admin
Site Admin
Posts: 3094
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Drift graph question

Post by admin » Sun Oct 13, 2019 8:47 pm

Hi,

I can say confidently that cone error won't produce this sort of problem – it doesn't affect tracking, its main detrimental effect is on accuracy of go to movements and alignment.

When you say you get the same results with guiding as without guiding, have you checked the graphs in PHD to ensure that it is actually making corrections and that the mount is responding to them? I'm just struggling to see how you can get the results you describe if your guiding is working as it should.

Cheers, Robin

Post Reply

Who is online

Users browsing this forum: No registered users and 0 guests