Image Drift - Help needed

[A320Flyer]

Hi folks,

I have noticed that there is a significant drift between the first and last images in an imaging session, even though I am guiding. I want to try an minimise this as much as possible.

For background information, for this particular session, imaging setup was:

ZWO ASI 1600 MM Pro, imaging through a SW ED72 on a HEQ5 Pro mount with belt mod. Guiding with a ZWO ASI 120mono via a SW 50ED Guide scope.

Image acquistion via MaximDL6, guiding via MaximDL6 using EQMod Pulse Guiding.

Guiding RMS was between 0.5 and 0.9 arcsec/px with a small dither of 1.0 pixels between each 300sec exposure and a settle criteria of 0.25 pixels and 15secs delay.

My  imaging camera is fairly accurately aligned with the long side of the chip in RA.

As you can see from the attached images, stars a pretty round but there is a significant drift in RA between Image1 and Image51.

As I understand things, MaximDL stops guiding between exposures to allow download of the image and then recommences guiding. With the above settle criteria, this is about 25secs between exposures.

Polar Aligned using Sharpcap with a PA error of less than 10sec.

Any suggestions or help would be very much appreciated.

Cheers

Bill

 

Edited December 3, 2020 by A320Flyer

[endless-sky]

Can be many things. Polar alignment error would get you drift in DEC. Interrupting guiding would also allow for drift, including RA periodic error, where the mount accelerates/decelerates the sidereal rate due to gear imperfections.

Then there is the infamous differential flexure - which I experienced and had to fight first handed, with a guide scope. Differential flexure can not be completely eliminated (with a guide scope setup), but only limited enough that it doesn't show in your image scale with your normal exposure times. I got it tight enough that I can comfortably shoot 600s subs with same star shape as a 60s sub. However, it's still there. If I blink several 600s subs without star aligning them first, the same star will eventually move quite a few pixels.

You also mention dithering. This itself will cause movement between images - that's its main purpose, anyway: to create a random movement in RA/DEC of however many pixels the user deems necessary. Using a DSLR, I usually dither 10-15 pixels (in the imaging camera) and obviously the last picture will never have the stars in the same positions as the first picture. But that it's true for every dithered frame.

Edited December 3, 2020 by endlessky

[vlaiv]

Yep, differential flexure - not showing in single subs, but it does show during the course of imaging session.

When I was using guide scope, I had drifts like 20-30px in couple of hours due to this. For each sub this is less than one pixel, but over time it adds up.

One way to look at it - it's sort of natural dither and if it's not causing too much issues - just don't worry about it (but do keep in mind it's there - so you don't get surprised)

[Seelive]

Try stacking the images without aligning them. You should then be able to see if the drift is linear, periodic or just plain non-linear (if the difference between images if small try alternate images or every 3 etc). It might enable you to home-in on a cause.

[A320Flyer]

When I play the images through PI Blink, there is a consistent movement from left to right (RA), no stutters, no discernible up/down movement.

If PE was an issue, it would have "corrected" itself on each worm cycle. 

Dithering is random on both axes, so I would not have thought it would not have manifested itself as a consistent left/right movement.

My PA is pretty good as there is very little drift on DEC.

This particular target/imaging session was almost at Zenith, so I'm thinking DF would be minimal?

I have my mount East-heavy as this gives me much better guiding RMS and dither recovery. Could I have it TOO East-heavy? 

I'm using MaximDL multi-star guiding so I would have though at the start of each image, any RA drift would have been corrected as the guiding kicked back in?

I might try single-star guiding with/without Dither to see if that reveals anything.

Thanks for all your suggestions.

Bill

 

[MarsG76]

+1 for flexture.... but the cause might also be the guide star which you're guiding on vs polar alignment accuracy. How far is the guide star from the object being imaged? if it's far enough, coupled with a polar alignment that is out enough, you might get that effect... it might look like it's drifting in RA but over a longer time it will show up as arcs... 

[A320Flyer]

Next clear night I'll make sure my guide scope is accurately aligned.

[david_taurus83]

Yep, flexure somewhere. You might think everything is bolted down solid but its tricky to pin point it. I switched to an OAG and it cured it.

If you've got a big enough FOV and you can still frame your target then I'd leave it as it provides a natural dither. But if you have a small sensor and take long subs it can be quite annoying.

https://stargazerslounge.com/topic/364129-whats-causing-this-drift/

[ollypenrice]

16 hours ago, A320Flyer said:

Next clear night I'll make sure my guide scope is accurately aligned.

It won't solve the problem. My guidescopes aren't aligned at all yet I shoot 15 minute subs at high res and 30 minute subs, on occasion, in widefield. I have no problem with either. Adjustable guide rings were not invented to let you align your guidescope. They were invented for precisely the opposite reason - to allow you to search off-axis for a guide star when that was necessary (for manual guiding or with old, insensitive guide cams.) Because they are a source of flexure I don't use them, preferring to bolt the guide scope down hard while pointing in more or less the right direction.

I am always suspicious of the claims of software, so I wouldn't be prepared to believe what Sharpcap says about your PA without testing it.  A drift test would soon confirm or refute your PA's claimed accuracy.

Olly

[gorann]

I find this thread interesting and also confusing. Apparently Bill's "problem" is that there is a rather small but steady drift between each image, not enough to make the stars elongated, so it is not really a problem. Still it is interesting and I see the same thing from time to time but rarely worry about it.

I assume the argument for differential flexure is that gravitation and the movement of the mount slowly pulls the guidescope in one and the same direction during the session (since all movement between subs is in the same direction).

Could not also poor polar alignment be the cause, or will that cause field rotation rather than a steady movement?

What about a poorly leveled mount?

In any case, Olly has a good point about guide scope rings. I never had any and always bolt my guidescope as steady as possible to the main scope (or use OAG when possible, which is on everything except my RASA). Why do they sell those rings?

Edited December 4, 2020 by gorann

[vlaiv]

23 minutes ago, gorann said:

I assume the argument for differential flexure is that gravitation and the movement of the mount slowly pulls the guidescope in one and the same direction during the session (since all movement between subs is in the same direction).

Could not also poor polar alignment be the cause, or will that cause field rotation rather than a steady movement?

What about a poorly leveled mount?

Above sort of behavior is very consistent with differential flexure. Poor polar alignment won't do that - even if guide scope is somewhat misaligned with main scope.

Indeed it is due to gravity, but most people blame rings and telescope / guide scope fixtures for this. I would rather look at camera / scope connection. If it is not threaded, but simple compression ring - this is where I would look first. Either on main camera or on guide camera. Here, cable snag can also play a part more than on OTA/mount connection.

[tomato]

Here is how much sag I  encountered on a large dual rig after forgetting to tighten the bracing plate across the top of the two scopes. The snaps show how much the target moved relative to the other over 3.5 hrs, and the sag was discernible on each 120 sec sub.