11 replies to this topic

[fyferoni]

Hey there! I've been imaging for a bit now and have a solid grasp on astrophotography with a refractor, but I just recently bit the bullet and picked up a Apertura Carbonstar 150p to level up my images and focal length/aperture.

 

After the usual teething issues with setting up a new rig, I was finally able to get a couple of hours on M101 last night. I noticed however that i'm getting some unsatisfactory star shapes that I'm a bit unsure of the source. I have a couple screengrabs below for reference. Is this a collimation issue? tilt issue? They also feel softer than they should for a newtonian astrograph. I might be off by a mm or two on my backfocus?

 

I collimated prior to imaging last night, but i've never collimated with a cheshire before, so I'm not sure if I just did a poor job of it! I didn't notice any overt flaring on a star test of capella, so I didn't think it was pinched optics.

 

Any feedback or advice would be greatly appreciated! Thanks everyone!

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[f430]

Are the stars like that across the image, or just in the corners?

[fyferoni]

Are the stars like that across the image, or just in the corners?

Looks like this is across the entire image. This screengrab is just above image center.

[f430]

If it's the entire image, I'd think more along the lines of you guiding. What are you using for guiding?

I've included a back focus chart, to show what to expect with that issue. 

Edited by f430, 13 March 2025 - 01:12 PM.

[fyferoni]

If it's the entire image, I'd think more along the lines of you guiding. What are you using for guiding?

I've included a back focus chart, to show what to expect with that issue. 

Back Focus chart.jpg

Oh that's super helpful! The next clear night I'll try to find a dense starfield and see if my images match the charts above.

 

I'm using PHD2 with an ASI120mm-pro on a 50mm SVBony guide scope. Focal length on the guide scope is 205mm I was averaging 0.6 arcseconds the entire time, which I figured was adequate for the focal length (570mm). Maybe I need to run PHD's configuration wizard again...

 

I might also be slightly out of focus due to backlash from the EAF. I only have backlash compensation going OUT in settings. Should I have compensation in both directions?

[f430]

I have my final focus set to go in. Sort of like to take up any slack in the imaging train.

But that was learned a long time ago, and I'm not sure there is any backlash in modern equipment.

[fyferoni]

I have my final focus set to go in. Sort of like to take up any slack in the imaging train.

But that was learned a long time ago, and I'm not sure there is any backlash in modern equipment.

My EAF definitely has backlash/play in the stepper. I think i'll try to add it for both in and out to see if that helps with finding final focus.

 

Also, your chart was super helpful, and I think I am dealing with a backfocus issue! Here's a screengrab of four corners of my image. I think I see a similar pattern in the stars to the 'too far away' example.

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Edited by fyferoni, 13 March 2025 - 03:30 PM.

[f430]

Slight, But I agree, it looks a bit too far.

Can you shorten the distance by a mm or two?

If you can't easily shorten the distance, then, for a test, lengthen the distance by a couple mm and see if your corners get worse. 

John

[Alex McConahay]

I do not think the issue is simple back focus.

 

The back focus that is related to the images posted by f430 in post 4 are related to spacing between a corrector and the sensor plane. 

 

Your aberration image (in post 7) matches neither of those charts. Notice the aberrations in those charts are distributed radially from the center out. That is, hardly noticeable in the center, but progressively worse toward the edges.   

 

It seems to me that your are okay on the top row of three. Not quite as nice in the central row. And very noticeable in the third row at the bottom. That is not a radial distribution from the middle. It is linear from the top down. 

 

This is a tilt issue, not a backfocus issue. 

 

Are you using a corrector of any sort?

 

Alex

[fyferoni]

I do not think the issue is simple back focus.

 

The back focus that is related to the images posted by f430 in post 4 are related to spacing between a corrector and the sensor plane. 

 

Your aberration image (in post 7) matches neither of those charts. Notice the aberrations in those charts are distributed radially from the center out. That is, hardly noticeable in the center, but progressively worse toward the edges.   

 

It seems to me that your are okay on the top row of three. Not quite as nice in the central row. And very noticeable in the third row at the bottom. That is not a radial distribution from the middle. It is linear from the top down. 

 

This is a tilt issue, not a backfocus issue. 

 

Are you using a corrector of any sort?

 

Alex

I'm using the PRCC designed for the Carbonstar and that it comes bundled with --> https://www.highpoin...-corrector-prcc

 

As for my corner mosaic, to me it looks like the top left is exhibiting similar aberration to the bottom left (just in the opposite direction similar to f430's post #4) though I'm definitely a novice at correcting tilt and backfocus issues. I didn't have any issues like this with my AT72edii.

 

If it's a tilt issue, is that typically due to sag in the image train, or a misalignment between the corrector and the image plane? Can poor collimation cause tilt aberration (if the mirror is focusing the light differently across the image plane)?

 

This is new territory for me, so any feedback or pointing me to helpful resources would be greatly appreciated! Thank you!

[Alex McConahay]

>>>>>As for my corner mosaic, to me it looks like the top left is exhibiting similar aberration to the bottom left

 

And I do not see that. But, I am willing to disagree with my original assessment. 

 

We are talking eyeballing it.

 

There is another way to do this......Get the actual numbers of eccentricity from the samples. 

 

One of my programs will tell you the eccentricities of the stars in your image. I forget which one (sorry) but somebody can come along and tell us. Using this, you can rely on the analysis instead of your eyes. 

 

Collimation can caused tilt, but so can loose fittings, sag, and a number of other things. Start by rotating your camera and see if your aberrations change. 

 

Alex

[fyferoni]

>>>>>As for my corner mosaic, to me it looks like the top left is exhibiting similar aberration to the bottom left

 

And I do not see that. But, I am willing to disagree with my original assessment. 

 

We are talking eyeballing it.

 

There is another way to do this......Get the actual numbers of eccentricity from the samples. 

 

One of my programs will tell you the eccentricities of the stars in your image. I forget which one (sorry) but somebody can come along and tell us. Using this, you can rely on the analysis instead of your eyes. 

 

Collimation can caused tilt, but so can loose fittings, sag, and a number of other things. Start by rotating your camera and see if your aberrations change. 

 

Alex

Took a bit of digging, but you can create it via the FWHMEccentricity tool in PI. Here's the two maps that it spit out on my image below!

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