14 replies to this topic

[raging_scotsman]

Hey everyone,

 

I'm shooting at F/10 with my Edge HD 8 and a mono IMX571 camera. I'm seeing trailing/elongated stars in short exposures of 5-10 seconds. Guiding is fine, error is around 1 arc second max, better most of the time. Collimation is good, did it an hour ago. Focus isn't perfect but I'm just troubleshooting. Here is a 10 second exposure:

 

 

I rotated the camera and got a similar result. With my EAF and filter wheel I can only rotate about 30 degrees for now, but this was the result:

 

 

Doesn't seem any different to me, I had wondered if I would see the direction of elongation change and maybe that would suggest sensor tilt. I didn't notice any tilt with my refractor that was obvious, and I just switched to the big scope.

 

I wouldn't expect tracking issues with good guiding and such short exposures. Mount is an EQ6-R. Only other thing I can think of right now is primary mirror misalignment or maybe the dew heater ring I installed is too tight?

 

Any ideas or suggestions?

 

Thanks!

Edited by raging_scotsman, 25 March 2025 - 01:20 AM.

[Tapio]

When imaging at 2000mm fl max 1" guiding error might be too much.
What is your guiding setup?
There should be OAG.

Any reason you are not using a reducer?

Edited by Tapio, 25 March 2025 - 04:59 AM.

[17.5Dob]

Your image scale is 0.38"/pixel. Your guiding needs to be well below that.

[ngc7319_20]

Images in the bottom right of the first image are OK -- pretty much round.  While the other corners show varying degrees of elongation.  I suspect a collimation problem.  Or tilt somewhere in the camera set up.  Maybe some of both. 

 

First thing I do to check collimation is put in a 5mm eyepiece and look at star shapes.  If you want round stars, then adjust the collimation looking at the stars.  You also need to make sure the camera ends up where the eyepiece was (not offset).

[raging_scotsman]

When imaging at 2000mm fl max 1" guiding error might be too much.
What is your guiding setup?
There should be OAG.

Any reason you are not using a reducer?

I use an OAG. I just haven’t got the reducer yet, I an planning to. I was curious if F/10 was doable. I haven’t imaged with this scope before, just trying it out now. I will try to improve guiding for sure.

Edited by raging_scotsman, 25 March 2025 - 07:55 AM.

[raging_scotsman]

Images in the bottom right of the first image are OK -- pretty much round. While the other corners show varying degrees of elongation. I suspect a collimation problem. Or tilt somewhere in the camera set up. Maybe some of both.

First thing I do to check collimation is put in a 5mm eyepiece and look at star shapes. If you want round stars, then adjust the collimation looking at the stars. You also need to make sure the camera ends up where the eyepiece was (not offset).

I used an 8.8mm eyepiece last night (highest power I’ve got handy) and spent quite a bit of time getting collimation dialled in. It looked pretty bang on.

I believe I’m within a mm or 2 of the recommended back focus, and there should be a bit of tolerance with this scope. I will double check it.

The imaging train seems solid. No compression rings, everything threaded, and not too long of a tube. I wonder about tilt as well.

[raging_scotsman]

Your image scale is 0.38"/pixel. Your guiding needs to be well below that.

I wasn’t blaming the guiding because the trailing is about the same with a 90 second exposure as with a 10 second. I am meaning to pick up the reducer for this scope to increase that, though I think that gets it in the 0.6-0.7 range. I am still guiding worse than that at the moment.

[KGoodwin]

The guiding is definitely not good enough, but that will probably just result in excessive FWHM rather than the eccentricity issue you’re seeing varying across the frame which is probably something else. If it were equally eccentric across the whole frame you’d suspect the guiding as the root cause.

[raging_scotsman]

The guiding is definitely not good enough, but that will probably just result in excessive FWHM rather than the eccentricity issue you’re seeing varying across the frame which is probably something else. If it were equally eccentric across the whole frame you’d suspect the guiding as the root cause.

I hadn't noticed it last night on my smaller laptop screen but someone else also pointed out that the elongation isn't consistent across the frame. I figured guiding would be more random error than a straight/consistent smudge every time. No cable snags, balance is decent (not perfect)... I was reading a thread yesterday about tilt and how good focus should travel across the sensor roughly in a line if tilt is present and focused is changed - maybe next clear night I will take several exposures through one side of focus to the other and see if I can see a moving zone of better quality. I don't know if that would be flexure or tilt, perhaps those could look similar; but I can't see how there could be much flexure in the short tube between the SCT back and the filter wheel with such a short tube.

[Scott Badger]

What is your seeing? For guiding to be ‘good enough’ it needs to be 1/4 or better of your seeing. So, if your total rms is 1”, that’s fine for 4” seeing, but if your seeing is 2”, say, than guiding should be 0.5” or better (though some say just 1/3 of seeing or better).

Did you look at the dec and ra guiding error? If there’s a large difference between the two, you can get oblong stars even if total rms looks good.

Cheers,
Scott

Edited by Scott Badger, 25 March 2025 - 02:32 PM.

[raging_scotsman]

What is your seeing? For guiding to be ‘good enough’ it needs to be 1/4 or better of your seeing. So, if your total rms is 1”, that’s fine for 4” seeing, but if your seeing is 2”, say, than guiding should be 0.5” or better (though some say just 1/3 of seeing or better).

Did you look at the dec and ra guiding error? If there’s a large difference between the two, you can get oblong stars even if total rms looks good.

Cheers,
Scott

I have no objective way to measure my seeing, other than maybe HFR or FWHM in NINA, but I don’t tend to think about it and just image whenever the sky is clear. I could have been chasing the seeing rather than guiding on stars.

From watching the graph in PHD I don’t remember any huge differences between RA and DEC. They were fairly equal other than the odd spike here and there.

[jonnybravo0311]

Take the images you captured and run them through ASTAP for tilt analysis. Looking at your sample exposures, the stars are very slightly oblong, but they don't appear to be uniformly oblong throughout the frame. In both examples, the stars seem to be better shaped the closer you get to the bottom right. If you have PI, you can run them through the FWHMEccentricity script. If you don't have either ASTAP or PI, upload the unaltered fits files to something like Google Drive and provide the link, and I'll take a look for you.

[Scott Badger]

I have no objective way to measure my seeing, other than maybe HFR or FWHM in NINA, but I don’t tend to think about it and just image whenever the sky is clear. I could have been chasing the seeing rather than guiding on stars.

From watching the graph in PHD I don’t remember any huge differences between RA and DEC. They were fairly equal other than the odd spike here and there.

What I call my seeing is simply what I get for fwhm's on a 4 sec exposure (assuming the scope is focused and collimated) which happens to be the same as the exposure I use for platesolving.

 

Cheers,

Scott

[raging_scotsman]

Take the images you captured and run them through ASTAP for tilt analysis. Looking at your sample exposures, the stars are very slightly oblong, but they don't appear to be uniformly oblong throughout the frame. In both examples, the stars seem to be better shaped the closer you get to the bottom right. If you have PI, you can run them through the FWHMEccentricity script. If you don't have either ASTAP or PI, upload the unaltered fits files to something like Google Drive and provide the link, and I'll take a look for you.

I have ASTAP but I was just using it as a platesolver in NINA, I didn’t know it had these features. I will give this a try when I get some time. If I don’t have any luck maybe I will take you up on your offer. Thanks!

[raging_scotsman]

What I call my seeing is simply what I get for fwhm's on a 4 sec exposure (assuming the scope is focused and collimated) which happens to be the same as the exposure I use for platesolving.

Cheers,
Scott

Fair enough. That’s probably the best I could measure it myself.