How can I improve star shapes? - Forums

Topic starter

Jan 28, 2025 · Edited: Jan 28, 2025

I recently acquired a new telescope and while first light was successful, I am a bit concerned about star shapes in the corners of my image especially as it's not a cheap scope and I am using a small sensor by today's standards.

My imaging train is: Ed127 => altair 0.8x flattener => zwo oag-l => filter wheel => 12mm spacers => zwo 1600mm. All together, this gives me a backfocus of 61mm not accounting for filters.

On the altair website, the backfocus for this reducer should be ~61mm for slightly smaller telescopes. Earlier that night, I started with 14.5mm of backfocus but that showed clear signs that the camera was too far from the flattener. These stars look the opposite but not as bad. They look like the camera is too close. At f/6 I would not have thought this mattered since the difference is only 2.5mm. Could these stars be caused by something else like tilt?

I know my processing wasn't the best but this was my first time doing mono so it was quite a challenge.

https://astrob.in/full/1rjna2/0/?real=

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ApogeeAstro

Jan 28, 2025

Hey Arpad, so it looks like most of the stars in the corners lean upper left to lower right, except for the bottom right side. To me, this looks like tilt, not necessarily a back focus issue. If it was back focus, the stars would be radiating from the center, or circling the center. Also try without the reducer/flattener to see if the star shapes stay the same.

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William

Jan 28, 2025

I disagree that it isn't necessarily backfocus error. You can't just look at a single image and tell that. In fact, its almost guaranteed that it is a combination of both tilt AND backfocus error.

I highly suggest you run NINA, and download Abberation Inspector, and then run that. It will tell you exactly how much tilt and backfocus error you have. In my experience, the ES127 with 3" field flattener did NOT produce sharp stars to the corners at 55mm, and did need the backfocus adjusted (i had to shorten mine by 1.5mm!) before I had sharp stars to the corners. YMMV thats why you need to check. You're always gonna have a tiny bit of tilt, its just about deciding how much is acceptable to you. The abberation inspector plugin will be so helpful here for guidance, because it'll tell you just how much tilt there is.

Btw your stars are not that bad. Also, wind and poor tracking can cause elongated stars too, and where you are pointing when you take the image matters as well. It's pretty nuanced but you will figure it out.

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William

Jan 28, 2025

Also you should probably just throw the book out on what the stars "should" be doing when you change backfocus. My 'stars' told me my sensor was too close, and for the longest time I couldn't figure out why I couldn't get sharp stars to the corner, even though I added more backspace (because again, the charts told me my sensor was "too close. but, it was actually the opposite).

It wasn't until I ran abberation inspector that I realized I need to remove backspacing, not add it. And then I had sharp stars in 3.5 corners. I had tilt in one other corner but I didnt care because it was borderline imperceptible after I fixed the backfocus error.

So TLDR just because the downloaded chart seems to tell you one thing, does not mean that it is.

Brian Carter

Jan 28, 2025

I have the same scope, different reducer/flattener. I might consider getting one of yours, your stars don't look too awful. Mine look almost like little crosses in the corner of my APS-C sensor. If you download NINA, there is a tool that will assess your spacing and tell you if you have any tilt problems. It is free and it'll introduce you to the joys of NINA.

I have run this analysis on mine, and I've messed with spacers until I'm satisfied that it is perfect, but those corners are still not perfect. In my case, I've come to see that my reducer-flattener is only a little bit good as a flattener. I am shopping around for a replacement , but in the meantime I have found that using some of the sharpening tools, BlurXterminator or Cosmic Clarity (Seti Astro) will clean up that last little bit that my flattener cannot. My final images are pinpoint to the edges.

I would still rather my gear produce a perfectly flat field. But in the meantime, my images aren't ruined and I don't have to crop my expensive large field.

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William

Jan 28, 2025

Brian Carter:
I have the same scope, different reducer/flattener. I might consider getting one of yours, your stars don't look too awful. Mine look almost like little crosses in the corner of my APS-C sensor. If you download NINA, there is a tool that will assess your spacing and tell you if you have any tilt problems. It is free and it'll introduce you to the joys of NINA.

I have run this analysis on mine, and I've messed with spacers until I'm satisfied that it is perfect, but those corners are still not perfect. In my case, I've come to see that my reducer-flattener is only a little bit good as a flattener. I am shopping around for a replacement , but in the meantime I have found that using some of the sharpening tools, BlurXterminator or Cosmic Clarity (Seti Astro) will clean up that last little bit that my flattener cannot. My final images are pinpoint to the edges.

I would still rather my gear produce a perfectly flat field. But in the meantime, my images aren't ruined and I don't have to crop my expensive large field.

Can I ask which corrector you're using? That should probably not be happening.

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Arpad R

Topic starter

Jan 29, 2025

William:
I disagree that it isn't necessarily backfocus error. You can't just look at a single image and tell that. In fact, its almost guaranteed that it is a combination of both tilt AND backfocus error.

I highly suggest you run NINA, and download Abberation Inspector, and then run that. It will tell you exactly how much tilt and backfocus error you have. In my experience, the ES127 with 3" field flattener did NOT produce sharp stars to the corners at 55mm, and did need the backfocus adjusted (i had to shorten mine by 1.5mm!) before I had sharp stars to the corners. YMMV thats why you need to check. You're always gonna have a tiny bit of tilt, its just about deciding how much is acceptable to you. The abberation inspector plugin will be so helpful here for guidance, because it'll tell you just how much tilt there is.

Btw your stars are not that bad. Also, wind and poor tracking can cause elongated stars too, and where you are pointing when you take the image matters as well. It's pretty nuanced but you will figure it out.

Thanks, I've just transitioned over from Nina so I'll get my laptop out to check for aberrations. It was windy that night but tracking was good and raw subs also show no elongation in the centre of the frame that's why I suspected it was caused by something else. I have a tilt plate on the oag and a bunch of spacers so I'll try those.

Arpad R

Topic starter

Jan 29, 2025

Brian Carter:
I have the same scope, different reducer/flattener. I might consider getting one of yours, your stars don't look too awful. Mine look almost like little crosses in the corner of my APS-C sensor. If you download NINA, there is a tool that will assess your spacing and tell you if you have any tilt problems. It is free and it'll introduce you to the joys of NINA.

I have run this analysis on mine, and I've messed with spacers until I'm satisfied that it is perfect, but those corners are still not perfect. In my case, I've come to see that my reducer-flattener is only a little bit good as a flattener. I am shopping around for a replacement , but in the meantime I have found that using some of the sharpening tools, BlurXterminator or Cosmic Clarity (Seti Astro) will clean up that last little bit that my flattener cannot. My final images are pinpoint to the edges.

I would still rather my gear produce a perfectly flat field. But in the meantime, my images aren't ruined and I don't have to crop my expensive large field.

I've tried cosmic clarity but it didn't fix my stars unfortunately. I may not have the right settings. The denoise was very good though. I have a bunch of flatteners I haven't tried yet but this one is the only one that is also a reducer which is useful for a slightly larger fov.

Brian Carter

Jan 29, 2025

William:
Brian Carter:
I have the same scope, different reducer/flattener. I might consider getting one of yours, your stars don't look too awful. Mine look almost like little crosses in the corner of my APS-C sensor. If you download NINA, there is a tool that will assess your spacing and tell you if you have any tilt problems. It is free and it'll introduce you to the joys of NINA.

I have run this analysis on mine, and I've messed with spacers until I'm satisfied that it is perfect, but those corners are still not perfect. In my case, I've come to see that my reducer-flattener is only a little bit good as a flattener. I am shopping around for a replacement , but in the meantime I have found that using some of the sharpening tools, BlurXterminator or Cosmic Clarity (Seti Astro) will clean up that last little bit that my flattener cannot. My final images are pinpoint to the edges.

I would still rather my gear produce a perfectly flat field. But in the meantime, my images aren't ruined and I don't have to crop my expensive large field.

Can I ask which corrector you're using? That should probably not be happening.

https://starfieldoptics.com/reducers, the 0.8 variety. Any ideas of something better? Not the 3" ES one, I just can't stomach giving them any more money.

Joe DeNisco

Jan 29, 2025

William:
Also you should probably just throw the book out on what the stars "should" be doing when you change backfocus. My 'stars' told me my sensor was too close, and for the longest time I couldn't figure out why I couldn't get sharp stars to the corner, even though I added more backspace (because again, the charts told me my sensor was "too close. but, it was actually the opposite).

It wasn't until I ran abberation inspector that I realized I need to remove backspacing, not add it. And then I had sharp stars in 3.5 corners. I had tilt in one other corner but I didnt care because it was borderline imperceptible after I fixed the backfocus error.

So TLDR just because the downloaded chart seems to tell you one thing, does not mean that it is.

Hi William,
Your post is very interesting to me. I have an ES ED/APO 127CF FC100 which I happen to love. I also have the dedicated ES 3" .7x reducer for it. I have been on a bit of an obsessive rampage over the past month trying to fix elongated stars. Particularly, in the 4 corners of my 2600MCDuo. As I've added spacers (I am up to 57mm, from the top of the M48 adapter, not underneath it), my elongated stars (pointing out to the corners) have gotten better, but not perfect - yet. And of particular note, The guidestars on my 2600MCDuo started out as "I" shaped and are now shaped like American footballs (better). I believe that once I correct this, my images will improve and my guiding will improve as well.
I am interested in why you say, the chart could be wrong?
Thank You so much for your time. And please know, I am in no way being combative. I am new at the Astrophotography part but have been passionate about Astronomy for most of my 61 yrs.
Thanks in advance for your input.
Joe D

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William

Jan 29, 2025

Joe DeNisco:
William:
Also you should probably just throw the book out on what the stars "should" be doing when you change backfocus. My 'stars' told me my sensor was too close, and for the longest time I couldn't figure out why I couldn't get sharp stars to the corner, even though I added more backspace (because again, the charts told me my sensor was "too close. but, it was actually the opposite).

It wasn't until I ran abberation inspector that I realized I need to remove backspacing, not add it. And then I had sharp stars in 3.5 corners. I had tilt in one other corner but I didnt care because it was borderline imperceptible after I fixed the backfocus error.

So TLDR just because the downloaded chart seems to tell you one thing, does not mean that it is.

Hi William,
Your post is very interesting to me. I have an ES ED/APO 127CF FC100 which I happen to love. I also have the dedicated ES 3" .7x reducer for it. I have been on a bit of an obsessive rampage over the past month trying to fix elongated stars. Particularly, in the 4 corners of my 2600MCDuo. As I've added spacers (I am up to 57mm, from the top of the M48 adapter, not underneath it), my elongated stars (pointing out to the corners) have gotten better, but not perfect - yet. And of particular note, The guidestars on my 2600MCDuo started out as "I" shaped and are now shaped like American footballs (better). I believe that once I correct this, my images will improve and my guiding will improve as well.
I am interested in why you say, the chart could be wrong?
Thank You so much for your time. And please know, I am in no way being combative. I am new at the Astrophotography part but have been passionate about Astronomy for most of my 61 yrs.
Thanks in advance for your input.
Joe D

Man brother. I know exactly what you're talking about. I even took my backspacing to 66mm! And it still looked bad.

I didnt take it as combative at all! I encourage the questions and sharing of info, its not like I am objectively right either... I could be wrong. I just remember pulling my darn hair out because that chart kept telling me my sensor was too close, but no matter how far away I put the sensor, my stars looked bad. I want you to know that this reducer gave me a lot of problems, what ended up fixing mine is moving closer, that is the advice that Abberation Inspector gave me, and when I listened to it, I had totally acceptable stars. Not 10000% perfect but close enough. It was good enough that I felt like I didnt need to worry about it anymore. Highly suggest you download NINA and the plugin called Aberration Inspector. It's LIFE CHANGING my friend. This should fix your issue.

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William

Jan 29, 2025

Example image of one of my corners, i was able to get pretty good for 3 of them.

Slight eggs here...

Sometimes its worse than others. A lot of things can cause elongation in stars that can present itself looking like maybe its tilt or backspacing. Wind, poor tracking, long exposures in general, etc. Do your best to eliminate any variables you can.

It can be a real challenge, but you should be able to get it pretty close.

Regarding guide stars, PHD2 can guide on those stars no problem. They have astigmatism due to the 45 degree mirror, and will typically have some abberation no matter how flat your field is behind the reducer. You will never get them perfect.

Hope this helps. Abberration Inspector and NINA are life changing tools that I would suggest to every astrophotographer in my opinion.

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William

Jan 29, 2025 · Edited: Jan 29, 2025

Brian Carter:
William:
Brian Carter:
I have the same scope, different reducer/flattener. I might consider getting one of yours, your stars don't look too awful. Mine look almost like little crosses in the corner of my APS-C sensor. If you download NINA, there is a tool that will assess your spacing and tell you if you have any tilt problems. It is free and it'll introduce you to the joys of NINA.

I have run this analysis on mine, and I've messed with spacers until I'm satisfied that it is perfect, but those corners are still not perfect. In my case, I've come to see that my reducer-flattener is only a little bit good as a flattener. I am shopping around for a replacement , but in the meantime I have found that using some of the sharpening tools, BlurXterminator or Cosmic Clarity (Seti Astro) will clean up that last little bit that my flattener cannot. My final images are pinpoint to the edges.

I would still rather my gear produce a perfectly flat field. But in the meantime, my images aren't ruined and I don't have to crop my expensive large field.

Can I ask which corrector you're using? That should probably not be happening.

https://starfieldoptics.com/reducers, the 0.8 variety. Any ideas of something better? Not the 3" ES one, I just can't stomach giving them any more money.

Thats actually a bit surprising to me. It sounds like you are in a similar position to me, where I didnt get exactly perfect stars in every corner, but I think the result is totally acceptable and I can live with the remaining issues. They are likely due to tilt anyway in my case.

But I am surprised to see that happen because a 0.8x reducer should be more forgiving than the 0.7. It might have something to do with their dual purpose design as a flattener. It is possible that this config makes adapting it to the widest range of scopes more difficult, but that is a total hypothesis with absolutely no evidence to support.

That said, I've done a lot of homework on FF and FFRs, and there are a few that not only have a high reputation, but are known for being full frame compatible. They are pricey, but I believe that they can basically be adapted to any telescope that is between 80-180mm f6-f8 - but will require you to find the correct backspacing. Any changes in both aperture or f/number will impact the amount of backspacing you need. But you can figure that out.

If you are looking at a new FF, I would suggest considering the SVX line from Stellarvue. They are marketed towards SVX scopes, but they work with any of their scopes from 80-180mm so logic tells me that any other scope that falls in that range will also be adaptable. They give guidance on backspacing depending on the model, and you can use that for a starting point to find your ACTUAL optimal backspacing. The SFFX1and SFFX3are both full frame compatible. They are working on an inhouse reducer too that will work exactly the same, also FF compatible. It is not out yet. The ones on their page right now are not inhouse. I can't vouch but they also are probably fine.

If you intend on a reducer, I'd wait for their inhouse one to be done though. Should be any week now.

Other FFs that have been suggested are TEC's Field Flattener, which is $1,000, and another being actually WO's Flat68.

What I really love about the SFFX1 & 3 is the fact that it uses something like 100MM of backspacing in general (it varies, again your aperture and fstop will ultimately determine your true ideal backfocus). Not only does this allow for an extremely high degree of flexibility when it comes to adapting it to many scopes, but also it allows for special combinations of accessories together: like a rotator, oag, and filter wheel all together at the same time, behind the corrector. This is not an easy thing to do, and is impossible on 55mm setups without using a special rotating microfocuser or putting the rotator on the other side of the corrector. Literally not possible. So this flattener is not only FF, not only high quality, not only adaptable, but also allows for these unique combos of accessories, so that's why it is VERY high on my list. I've already confirmed with Vic that their inhouse reducers will function exactly the same way. Any time you reduce, it makes your backfocus being right more critical/sensitive. That is likely why our 0.7x reducers are so touchy and difficult to get right.

That said, you can find that 0.7x reducer from ES for $250-300 used. I've never seen the Stellarvue sold used. New for new though, I think I'd pick the SV for sure.

Finding your BF point will not be that hard. Connect to NINA, run abberation inspector, and it'll tell you what to do from there. It runs autofocuses, measures the stars for error and tilt, and then provides an easy to read report. Enjoy!

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William

Jan 29, 2025

Arpad R:
William:
I disagree that it isn't necessarily backfocus error. You can't just look at a single image and tell that. In fact, its almost guaranteed that it is a combination of both tilt AND backfocus error.

I highly suggest you run NINA, and download Abberation Inspector, and then run that. It will tell you exactly how much tilt and backfocus error you have. In my experience, the ES127 with 3" field flattener did NOT produce sharp stars to the corners at 55mm, and did need the backfocus adjusted (i had to shorten mine by 1.5mm!) before I had sharp stars to the corners. YMMV thats why you need to check. You're always gonna have a tiny bit of tilt, its just about deciding how much is acceptable to you. The abberation inspector plugin will be so helpful here for guidance, because it'll tell you just how much tilt there is.

Btw your stars are not that bad. Also, wind and poor tracking can cause elongated stars too, and where you are pointing when you take the image matters as well. It's pretty nuanced but you will figure it out.

Thanks, I've just transitioned over from Nina so I'll get my laptop out to check for aberrations. It was windy that night but tracking was good and raw subs also show no elongation in the centre of the frame that's why I suspected it was caused by something else. I have a tilt plate on the oag and a bunch of spacers so I'll try those.

I'm happy to help guide you if you get stuck. I agree with your logic.

Any of you can join us here for further help.

https://discord.gg/EESavpV6

William

Jan 29, 2025 · Edited: Jan 29, 2025

Make sure you run an AUTOFOCUS after you do any backspacing changes! Or you will be out of focus (how much depends on how much BS you changed), and that could hide some detail you dont want it to.

Joe DeNisco

Jan 30, 2025

William:
Joe DeNisco:
William:
Also you should probably just throw the book out on what the stars "should" be doing when you change backfocus. My 'stars' told me my sensor was too close, and for the longest time I couldn't figure out why I couldn't get sharp stars to the corner, even though I added more backspace (because again, the charts told me my sensor was "too close. but, it was actually the opposite).

It wasn't until I ran abberation inspector that I realized I need to remove backspacing, not add it. And then I had sharp stars in 3.5 corners. I had tilt in one other corner but I didnt care because it was borderline imperceptible after I fixed the backfocus error.

So TLDR just because the downloaded chart seems to tell you one thing, does not mean that it is.

Hi William,
Your post is very interesting to me. I have an ES ED/APO 127CF FC100 which I happen to love. I also have the dedicated ES 3" .7x reducer for it. I have been on a bit of an obsessive rampage over the past month trying to fix elongated stars. Particularly, in the 4 corners of my 2600MCDuo. As I've added spacers (I am up to 57mm, from the top of the M48 adapter, not underneath it), my elongated stars (pointing out to the corners) have gotten better, but not perfect - yet. And of particular note, The guidestars on my 2600MCDuo started out as "I" shaped and are now shaped like American footballs (better). I believe that once I correct this, my images will improve and my guiding will improve as well.
I am interested in why you say, the chart could be wrong?
Thank You so much for your time. And please know, I am in no way being combative. I am new at the Astrophotography part but have been passionate about Astronomy for most of my 61 yrs.
Thanks in advance for your input.
Joe D

Man brother. I know exactly what you're talking about. I even took my backspacing to 66mm! And it still looked bad.

I didnt take it as combative at all! I encourage the questions and sharing of info, its not like I am objectively right either... I could be wrong. I just remember pulling my darn hair out because that chart kept telling me my sensor was too close, but no matter how far away I put the sensor, my stars looked bad. I want you to know that this reducer gave me a lot of problems, what ended up fixing mine is moving closer, that is the advice that Abberation Inspector gave me, and when I listened to it, I had totally acceptable stars. Not 10000% perfect but close enough. It was good enough that I felt like I didnt need to worry about it anymore. Highly suggest you download NINA and the plugin called Aberration Inspector. It's LIFE CHANGING my friend. This should fix your issue.

Thank you William. I appreciate your advice. I need to try NINA. I just like easy.. And ASI Air is easy. But lacking.
if 58mm doesn’t work, I might go the other way!
Thanks buddy.
Joe D

ApogeeAstro

Jan 30, 2025

Hey Joe, I found a variable back focus spacer from Askar is pretty neat for dialing it in to perfection. Moves in .2mm increments and swings 15mm+/-2mm. I use it with my 2600mc duo.

Concise

William

Jan 30, 2025

Joe DeNisco:
William:
Joe DeNisco:
William:
Also you should probably just throw the book out on what the stars "should" be doing when you change backfocus. My 'stars' told me my sensor was too close, and for the longest time I couldn't figure out why I couldn't get sharp stars to the corner, even though I added more backspace (because again, the charts told me my sensor was "too close. but, it was actually the opposite).

It wasn't until I ran abberation inspector that I realized I need to remove backspacing, not add it. And then I had sharp stars in 3.5 corners. I had tilt in one other corner but I didnt care because it was borderline imperceptible after I fixed the backfocus error.

So TLDR just because the downloaded chart seems to tell you one thing, does not mean that it is.

Hi William,
Your post is very interesting to me. I have an ES ED/APO 127CF FC100 which I happen to love. I also have the dedicated ES 3" .7x reducer for it. I have been on a bit of an obsessive rampage over the past month trying to fix elongated stars. Particularly, in the 4 corners of my 2600MCDuo. As I've added spacers (I am up to 57mm, from the top of the M48 adapter, not underneath it), my elongated stars (pointing out to the corners) have gotten better, but not perfect - yet. And of particular note, The guidestars on my 2600MCDuo started out as "I" shaped and are now shaped like American footballs (better). I believe that once I correct this, my images will improve and my guiding will improve as well.
I am interested in why you say, the chart could be wrong?
Thank You so much for your time. And please know, I am in no way being combative. I am new at the Astrophotography part but have been passionate about Astronomy for most of my 61 yrs.
Thanks in advance for your input.
Joe D

Man brother. I know exactly what you're talking about. I even took my backspacing to 66mm! And it still looked bad.

I didnt take it as combative at all! I encourage the questions and sharing of info, its not like I am objectively right either... I could be wrong. I just remember pulling my darn hair out because that chart kept telling me my sensor was too close, but no matter how far away I put the sensor, my stars looked bad. I want you to know that this reducer gave me a lot of problems, what ended up fixing mine is moving closer, that is the advice that Abberation Inspector gave me, and when I listened to it, I had totally acceptable stars. Not 10000% perfect but close enough. It was good enough that I felt like I didnt need to worry about it anymore. Highly suggest you download NINA and the plugin called Aberration Inspector. It's LIFE CHANGING my friend. This should fix your issue.

Thank you William. I appreciate your advice. I need to try NINA. I just like easy.. And ASI Air is easy. But lacking.
if 58mm doesn’t work, I might go the other way!
Thanks buddy.
Joe D

Hey there no problem. You can still use your ASI air, you could just use Nina quite literally for the plug in and then go back to your usually scheduled program. Best of luck I'm sure you will get it. You have a high chance to be successful

Joe DeNisco

Jan 31, 2025

Hey Joe, I found a variable back focus spacer from Askar is pretty neat for dialing it in to perfection. Moves in .2mm increments and swings 15mm+/-2mm. I use it with my 2600mc duo.

AA, That might be the ticket right there. I’ve seen those but I forgot about them. I’m going check it out!
thank you!
Joe D

Karl Perera

Jan 31, 2025

I've seen stars much worse than this! I fail to see what the actual problem is. I think you should be happy with your results, they are good in my opinion. However, sharpness of stars can be improved by using Cosmic Clarity which is free and quick and does make a big difference. Try it if you aven't already.