Image creep from sub to sub

Sep 22, 2023

This is going to sound stupid. but are you sure you're not dithering on accident? If the scope was drifting during the exposure, you would 100% see elongated stars. If your stars are still pinpoint, it would have to occur between exposures, which leaves dithering as the most obvious suspect.

Helpful Insightful Concise

Topic starter

Sep 22, 2023

I have tried dithering in Sharpcap but it was quite confusing. It does not even stop exposure while dithering. So I'm quite certain it's not Sharpcap that's doing this and I usually make subs back to back so not sure if there is time for dithering to happen. Unless it's really fast and doesn't show up in images.

Maybe I'll look through PHD2 logs for mention of dithering.

Can PHD2 dither by itself somehow?

Frequent contributor

Sep 22, 2023 · Edited: Sep 22, 2023

That is called mount drifting and there isn't much you can do about it, not what your mount anyway. All of mine do too to a smaller or larger extent. Differential flexure (of everything) and non-perfect PA (which is never ever perfect unless is on a concrete pier and even then it would creep over time anyway) is the root cause of it.

Helpful Insightful Concise

Sep 22, 2023

andrea tasselli:
That is called mount drifting and there isn't much you can do about it, not what your mount anyway. All of mine do too to a smaller or larger extent. Differential flexure (of everything) and non-perfect PA (which is never ever perfect unless is on a concrete pier and even then it would creep over time anyway) is the root cause of it.

Surely if his mount was drifting he would see star elongation, no?

Well Written Engaging

Brian Puhl

IOTD Staff

Sep 22, 2023 · Edited: Sep 22, 2023

First thought would have been flexure, but you'd likely see it in your stars.    Does it move every couple frames and not every frame?   Can you attribute the movements to a dither command?

My guess is you're properly dithering, but you don't have a center after drift trigger…. so it's just walking away.      If you don't have that option with current software, then I would agree your move is to go to NINA and run the 'center after drift' trigger.

Also, i just saw your earlier response… sharpcap?   I'd advise against.    Definitely switch to NINA!

Helpful

Frequent contributor

Sep 22, 2023

Mason Steidle:
Surely if his mount was drifting he would see star elongation, no?

No, as you are guiding. It is when you aren't (well, the mount isn't) that the drifting happens.

Sep 22, 2023

andrea tasselli:
Mason Steidle:
Surely if his mount was drifting he would see star elongation, no?

No, as you are guiding. It is when you aren't (well, the mount isn't) that the drifting happens.

It sounds like this is happening while OP is taking his subs, I would imagine that he would be guiding the entire time.

@Dmitrijus Tiazlovas, is there a delay between your subs where you are going unguided, or it the guiding continuous across all of your subframes?

Frequent contributor

Sep 22, 2023

Mason Steidle:
It sounds like this is happening while OP is taking his subs, I would imagine that he would be guiding the entire time.

I don't think so. See below.

Dmitrijus Tiazlovas:
I often have a problem when taking lights that my images move between subs. By that I mean there's a few minute difference between each frame. This movement is not consistent. There is no star trailing or any significant elongation (anything that could not be attributed to optical imperfections). I've been told that it is due to differential flexure but I've improved my guider connection to the point it's difficult to take off let alone move. As for guiding I'm getting decent results ~0.7" average, sometimes dipping below 0.5" RMS.

Minute difference(s) tantamount of few pixels worth in my understanding of the English language and in BETWEEN frames.

Sep 22, 2023

Minute difference(s) tantamount of few pixels worth in my understanding of the English language and in BETWEEN frames.

We agree here. The error is happening between frames, but why would OP stop guiding between frames? Also, he says the movement is inconsistent. If this was drift due to flexure or polar alignment I would expect the drift to be in the same direction each time.

Well Written Insightful Respectful

Topic starter

Sep 22, 2023

Mason Steidle:
@Dmitrijus Tiazlovas, is there a delay between your subs where you are going unguided, or it the guiding continuous across all of your subframes?

I am constantly guiding, it is not connected to my imaging at the moment.

Think I may have made it unclear in terms of magnitude of the creep. It is about an arc minute over an hour, so maybe 2-3 arc seconds each 120s frame. Guess movement could be hidden inside star pixels but it is so much more obvious when blinking.

Frequent contributor

Sep 22, 2023

Mason Steidle:
We agree here. The error is happening between frames, but why would OP stop guiding between frames? Also, he says the movement is inconsistent. If this was drift due to flexure or polar alignment I would expect the drift to be in the same direction each time.

Assuming that you use PHD2 then I think the default, when connected in a server-client configuration, is to stop guiding when the server isn't actually taking any picture although drift actually happens in place and it is the result of cumulative error but it is masked by your RMS error value. And the drift doesn't need to be in the same direction all the time if it is the whole mount adjusting to a different balance of static forces. And the PA error (which might vary if the mount flexures are significant in arcsec sense). Just looking at the PHD2 tracking graph should tell everything you need to know to make your judgment about it.

Topic starter

Sep 22, 2023

andrea tasselli:

So you're saying that if Sharpcap is not taking an exposure but reading the image or doing other things (like processing a live stack) PHD2 could be just taking a break from guiding for a couple seconds?

Well Written Engaging

Frequent contributor

Sep 22, 2023

Dmitrijus Tiazlovas:
andrea tasselli:

So you're saying that if Sharpcap is not taking an exposure but reading the image or doing other things (like processing a live stack) PHD2 could be just taking a break from guiding for a couple seconds?

I don't use SharpCap but my understanding of its operation in NINA is thus.At any rate if experience 1 arcminute drift in an hour PA is the most likely culprit followed by differential refraction (i.e., not use King's Rate), if you shoot below 30 degrees of altitude. Also mirror creep is another potential source of similar issues.

Sep 22, 2023

If your autoguiding is consistent, I think some sort of flexure is propably the most likely culprit. If you are imaging at a low FL especially, the drift rate could be low enough to be undetectable in a single frame.

It doesnt have to be your guide scope flexing either, it could be your imaging train very slowly drooping or your primary mirror slowly shifting. Flexure can come from many places.

The best way to elimate flexure as a factor is to use an OAG.

Helpful Insightful Respectful

Cfreerksen

Sep 23, 2023

Like the last post, likely flexure. I would like to see how the guide scope is mounted. Also since you said it was a RASA it could be flexure of the corrector plate.

Chris

Topic starter

Sep 23, 2023 · Edited: Sep 23, 2023

My guide scope is connected via a losmandy clamp to the rasa plate (on the bottom) so it quite solid. I don't have a photo with RASA but added one with Samyang setup. The part I'm worried about are the rings with the bolts holding guide in place. Maybe should print some solid rings for it instead. Since I do not have any movement at all with a Samyang 135 I'm guessing RASA weight may be a problem causing the equipment to sink or droop a little.

Dan Brown

Sep 23, 2023 · Edited: Sep 23, 2023

If you are using a strain wave mount, without the counterweight, the shifting center of gravity could be your issue. With a normal GEM the scope is balanced with a counterweight. As the ra turns, the center of gravity does not change. Each leg of the tripod is equally loaded. With a strain wave mount you may start the night with 5 kilos to the west and end the night with 5 kilos to the east. The loading on the legs, and mount, changes, it has to flex.
I will never use a strain wave mount since I believe the design is inherently flawed for imaging.
You could try adding a counterweight and see if fixes your issue.
Dan

Helpful Insightful Concise

Frequent contributor

Sep 23, 2023

Dmitrijus Tiazlovas:
My guide scope is connected via a losmandy clamp to the rasa plate (on the bottom) so it quite solid. I don't have a photo with RASA but added one with Samyang setup. The part I'm worried about are the rings with the bolts holding guide in place. Maybe should print some solid rings for it instead

All my guide-scopes are connected similarly (someone in a even less study manner) and I had 7 over the decades and NOT a single one ever flexed, so put your mind to rest; once the bolts are tightened they are not going to shift.