Odd erratic star issues

[HaleBopp2007]

Since I completed my first rig (Esprit 80ED, ZWO ASI533MC, NEQ5) I've suffered from erratic star movement in around 30 to 40% of my exposures, making me scrap a lot of my data. I've put the blame on my mount all this time, but judging by how the wind didn't affect the issue (even at very calm wind speeds I still got the issue), I'm starting to have my doubts. Mount related issues shouldn't be related to these kinds of movements, specially at a quite decent FOV of 1.5° by 1.5°... I have several examples of these movements, and even though these are low resolution versions of the actual exposures, you can perfectly notice that the star trail isn't straight, but it has an irregular trail. Can anyone confirm whether this is either related to my mount as I expected or something else? 

[vlaiv]

Yes, I'd say it is indeed mount related.

Mount errors broadly fall into two categories - those that affect RA axis motion and those that affect DEC axis motion.

If your mount is solid and telescope / mount connection is solid and there are no externally induced vibrations (like walking around the telescope while it sits on a flexible surface like wood flooring or similar - or perhaps heavy traffic near by or whatever) then DEC error will be more or less linear for duration of the frame - nice drift in a straight line.

However, RA error will be "circular" or rather very periodic. This periodic error does not need to be, and most often is not perfect circle - sometimes RA axis lags and sometime it trails true position and does this in very "wavy" way. Combination of the two thus result in semi circular features (RA moves back and forth while DEC always in one direction only).

Examine this animation made out of single exposures on HEQ5 mount:

Motion from right to left is DEC motion while up down motion is due to RA periodic error.

You can even see how stars get distorted in some frames because speed at which mount errors compared to true position is enough to accumulate over the course of single frame.

By the way, if you have set of images - you can do the same to confirm motion of the mount - just inspect images one after another without doing alignment of images you would normally do for stacking.

 

[HaleBopp2007]

Oh I do have a time lapse of comet ZTF!

And, yeah it does show that "zig zag" movement

Edited August 6, 2023 by HaleBopp2007

[vlaiv]

2 minutes ago, HaleBopp2007 said:

Oh I do have a time lapse of comet ZTF!

Not ideal because it is aligned on comet - as mount tracks and makes errors - they will affect both stars and comet the same. By aligning on comet - you removed these errors.

[HaleBopp2007]

Actually I looked for it manually and activated sideral tracking speed, so the comet is not really an important factor this time, since I let it skid through the field, and the timelapse itself is rather short too, with a span of around 40 minutes, explaining that the comet doesn't move much

Edited August 6, 2023 by HaleBopp2007

[vlaiv]

5 minutes ago, HaleBopp2007 said:

Actually I looked for it manually and activated sideral tracking speed, so the comet is not really an important factor this time, since I let it skid through the field, and the timelapse itself is rather short too, with a span of around 40 minutes, explaining that the comet doesn't move much

Well, I thought that I saw some star wobble up and down - and yes, you can see some if you pay attention.

That is the phenomena I'm talking about - but to be sure, why don't you take your latest session - one from which you posted crop in initial post - and simply do animation of frames. You can even crop them to reduce size - just make sure you crop every image to same area.

[HaleBopp2007]

Well, I can do that tomorrow since I don't have my ASIAIR currently... I'll look and see if I have any more exposures, but I think I'll have to wait until tomorrow, with my most recent project which was forunately saved by transparent dark skies. I'll see if I can create a time-lapse asap

Edited August 6, 2023 by HaleBopp2007

[HaleBopp2007]

Nonetheless the exposures of the comet are only stretched and cropped after editing (I have the original video with no cropping and it shows the same effect as well). 

Nonetheless, if it IS an error in declination that definitely discards that it's an error of the motor, since it's not active while tracking. It can't be a balance error either since I've made sure the scope could be easily leveled by adding a longer dovetail. So... I can only come up with mount stress, which isn't really solvable unless I get a newer, sturdier mount

Edited August 6, 2023 by HaleBopp2007

[ollypenrice]

1 hour ago, HaleBopp2007 said:

Nonetheless the exposures of the comet are only stretched and cropped after editing (I have the original video with no cropping and it shows the same effect as well). 

Nonetheless, if it IS an error in declination that definitely discards that it's an error of the motor, since it's not active while tracking. It can't be a balance error either since I've made sure the scope could be easily leveled by adding a longer dovetail. So... I can only come up with mount stress, which isn't really solvable unless I get a newer, sturdier mount

The Dec motor is active in autoguiding since it is used to correct errors in Dec. These occur even in a perfectly accurate mount, perfectly polar aligned. (The apparent position of stars moves with atmospheric diffraction as the altitude of the object varies.) The dec axis can also introduce errors by having backlash. Backlash can also affect the RA axis and can be made worse by perfect balance. It is best to be heavy on the east so that the imbalance leaves the payload resting on the pushing side of the mesh. Dec backlash can be addressed by deliberately small polar misalignment and the disabling of the guiding direction not needed to correct this error. (Obviously it is best to correct the mount mechanically, however.)

The focuser is another popular cause of image shift and the hardware holding the gear into the drawtube is another. Screw fit beats push-and-locknut.  Exceptionally, chips can also be lose in cameras.

My own hunch is that you have more than one source of backlash but mostly RA. The most important thing to do is identify the RA and Dec axes on the trailed images.  Could you post a star trailed image with the axes identified?

Olly

 

[vlaiv]

Were you guiding?

If so - what is your guide log like?

[HaleBopp2007]

I wasn't guiding in that image. Only the RA motors were working

[HaleBopp2007]

39 minutes ago, ollypenrice said:

The Dec motor is active in autoguiding since it is used to correct errors in Dec. These occur even in a perfectly accurate mount, perfectly polar aligned. (The apparent position of stars moves with atmospheric diffraction as the altitude of the object varies.) The dec axis can also introduce errors by having backlash. Backlash can also affect the RA axis and can be made worse by perfect balance. It is best to be heavy on the east so that the imbalance leaves the payload resting on the pushing side of the mesh. Dec backlash can be addressed by deliberately small polar misalignment and the disabling of the guiding direction not needed to correct this error. (Obviously it is best to correct the mount mechanically, however.)

The focuser is another popular cause of image shift and the hardware holding the gear into the drawtube is another. Screw fit beats push-and-locknut.  Exceptionally, chips can also be lose in cameras.

My own hunch is that you have more than one source of backlash but mostly RA. The most important thing to do is identify the RA and Dec axes on the trailed images.  Could you post a star trailed image with the axes identified?

Olly

 

Sorry if it's a silly question but, you mean that the telescope is more stable when the eastermost part is slightly heavier? Like- gaining sturdiness? That could be a solution! I will try as soon as I get my equiment back in action, and I'll also send the footage with the axis identified by tomorrow, but hopefully the east imbalance will do the job

Edited August 6, 2023 by HaleBopp2007

[ollypenrice]

57 minutes ago, HaleBopp2007 said:

Sorry if it's a silly question but, you mean that the telescope is more stable when the eastermost part is slightly heavier? Like- gaining sturdiness? That could be a solution! I will try as soon as I get my equiment back in action, and I'll also send the footage with the axis identified by tomorrow, but hopefully the east imbalance will do the job

Not exactly.  The scope carrying part of the mount is driven by a worm gear driving a wheel gear and these can have too much free play in them, meaning the driven part can oscillate relative to the driving part. What tends to happen is that the scope spends some time resting on one side of the backlash then flops across to the other side. In each position it will produce a stellar image., like OO , each O being a stellar image. As it flops between positions it will also create a bridge of light between them. 0-0.

By running east heavy you can encourage it to rest on one side of the backlash so creating only one of the two stellar images.

Olly

[HaleBopp2007]

Hm I don't fully get every concept. I do understand the 0-0 example, since it has happened to me several times, like two brighter replicas of the same star with a bridge between them. But you're saying that if I leave the eastern part slightly heavier, that bridge of light will dissappear? Sorry if I'm getting too confusing, but I'm trying to grasp as much info as I can to get this 100% straight

[ONIKKINEN]

Balancing slightly east heavy could help with this, but most likely this was a minor part of the issue unless you were at the perfect point of balance where your RA axis wobbles between extremes of backlash (very unlikely). You will need to guide to get rid of the RA periodic error and the issues caused by it.

[HaleBopp2007]

I did try that a couple of times but the jump was (and still is) so massive (sometimes around an arcmin)  and sudden that it was impossible to fix with guiding, even when the guidescope's focal length was of more than half of my main scope's focal

[HaleBopp2007]

Just made this video with my data of M8 from a bortle 4 sky, it shows the same zig zag pattern. I'll try to check if the east-heavy weighting solves this tonight

Edited August 7, 2023 by HaleBopp2007

[ONIKKINEN]

29 minutes ago, HaleBopp2007 said:

Just made this video with my data of M8 from a bortle 4 sky, it shows the same zig zag pattern. I'll try to check if the east-heavy weighting solves this tonight

This video shows your RA periodic error in left-right movement and slight DEC drift in vertical movement.

This is definitely not fixable without guiding, balanced or not. I would try to seek an answer as to why guiding had not solved the issue.

So are the basics in check with guiding? Focal length and pixel size correct, calibration at the right place in the sky and so on? Also is backlash adjusted to be tolerable and the mount in good shape otherwise?

Simple RA guiding should not be too difficult if the gear is set up properly so there might be something you have overlooked.

[HaleBopp2007]

Faur enough. I think that the dec issues are because I made a slightly imprecise polar alignment, so that is fixable. If that only leaves me with RA wobble, I think east-heavy weighting and guiding could solve the issue. I'll see if I can finally stop this mess tonight, and increase my already existing excitement (discovered new asiair tools). Final updates in the next 12h

[ONIKKINEN]

2 minutes ago, HaleBopp2007 said:

Faur enough. I think that the dec issues are because I made a slightly imprecise polar alignment, so that is fixable. If that only leaves me with RA wobble, I think east-heavy weighting and guiding could solve the issue. I'll see if I can finally stop this mess tonight, and increase my already existing excitement (discovered new asiair tools). Final updates in the next 12h

The DEC drift here is due to just that. But its not a real issue as it seems very slow here. Realistically even with a 1 arcsecond polar alignment you will still have that drift because there is probably some cone error in the setup. Long story short a polar alignment of a few arc minutes is as good as it needs to be when guiding. No harm in going for more accurate one but also likely no gain either.

[HaleBopp2007]

Apologies for taking some time. Last night was oddly hazy, humid and foggy so I didn't try anything for the sake of my optics. Tonight looks much more promising, however. I'll take a shot and see if the mount issues are still there after I use east-heavy weighting and guiding.

Edited August 8, 2023 by HaleBopp2007

[HaleBopp2007]

Update! I'm photographing NGC 7000 and so far it looks like your tips did the job! I still have to try with other positions, but so far it looks very promising. Thank you all so much for helping! I really needed that!

Edited August 8, 2023 by HaleBopp2007

[HaleBopp2007]

Okay, confirming half-victory over that pesky backlash! I am however getting slightly elongated stars (just a few pixels) while the field is completely still... I've checked that the  alignment corresponds to the RA axis, implying it's a declination error (probably polar alignment mistake) so maybe I have to apply more aggressive guiding, since I don't think making the mount even more east-heavy is something that improves dec errors. The issue is however mostly solved, I just need to polish the procedure

Edited August 9, 2023 by HaleBopp2007

[HaleBopp2007]

Well, I can now confirm that my scope worked flawlesly now! It did the meridian flip (which made a couple of exposures a bit wonky), but the rest came in wonderfully! Even being west heavy helped the issue, too! Thank you all for your help, I'll make sure to send the image as soon as I finish the project. I wish you clear skies!