Star Size Getting A Small Amount Larger - Forums

I'm posting an uncalibrated .fit file direct from the camera.

I'm taking 5 minute unguided subs with a 10MicronGM1000 on a permanent mount under Bortle 1-2 skies on nights that have been fairly high quality in terms of seeing and transparency. If you zoom in you may notice a small "bump" at 270 degrees on larger stars.

I want to keep the stars small and round. When it gets to the point where it really needs to be worked on, would you:

1. Do a new polar alignment
2. Do a new model (current model is about 5 months old)
3. Employ the guide scope, guide camera and OAG already on the system and autoguide in NINA with PHD?
4. All three

Up to this point, I'd leave well enough alone, now I am wondering what's the simplest way of getting better stars? I know I can process with BlurX and post-process with star reduction, but I want to keep the initial data as good as possible.

Thanks!
Jerry

The issue you are seeing is related to either pinched optics or something obstructing the optical path. Defocus the field outward and you’ll likely find the problem on the out of focus stars.

Dark Matters Astrophotography:
The issue you are seeing is related to either pinched optics or something obstructing the optical path. Defocus the field outward and you’ll likely find the problem on the out of focus stars.

Would pinched optics or an optical path obstruction cause the stars to change size somewhat from sub to sub? That's what I am noticing--one sub the stars seem slightly bloated, the next sub they look good and sharp. Wouldn't the issues you brought up be more constant? I was thinking one of two adjustments; either the focuser on the scope needs more tension when it's not locked, and/or the electronic focuser might be slipping a bit. I'd think that those issues might show more variation than an obstruction (unless it's a moving obstruction) or pinched optics but it's hard to know. The scope is at a remote site.

The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

Dark Matters Astrophotography:
The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

I'm not sure if that was installed. I'll ask next time I speak to Ed or Chris.

I am not sure of what you mean by the dark line artifact, that's not what I was originally seeing. Is the artifact a straight line? Is it tilted or slanted?

I know that seeing there for the past several months has been good, somewhere between average and excellent. Lots of good clear nights with little to no wind. Another thing I like about DSPR is the lack of dew, there's not much most nights. A few nights the dewpoint got up to 95% for a short time, but that's rare, at least so far.

Honestly Jerry, this looks like guide error to me, a short excursion where for most of the sub ,the mount tracks well. If you're only seeing this on a few subs, that's probably it. If it's constant from sub to sub then it has to be something optical as dark matters mentioned. If what you mean by better is rounder and smaller stars, a shorter subs exposure time would help.

Jerry Gerber:
Dark Matters Astrophotography:
The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

I'm not sure if that was installed. I'll ask next time I speak to Ed or Chris.

I am not sure of what you mean by the dark line artifact, that's not what I was originally seeing. Is the artifact a straight line? Is it tilted or slanted?

I know that seeing there for the past several months has been good, somewhere between average and excellent. Lots of good clear nights with little to no wind. Another thing I like about DSPR is the lack of dew, there's not much most nights. A few nights the dewpoint got up to 95% for a short time, but that's rare, at least so far.

Hi Jerry, I think I see the artifact that Dark Matters was pointing out. A lot of your brighter stars have an artifact on the bottom left of them. This might be creating the illusion of the lobe on the left side. Usually something like this is from pinched optics / something in the light path, but I don't have the expertise to say for sure. I think you are using a PHQ. I have seen similar looking stars from other PHQs, especially when its cold out.

-Chris

Zoom:

Jerry Gerber:
Dark Matters Astrophotography:
The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

I'm not sure if that was installed. I'll ask next time I speak to Ed or Chris.

I am not sure of what you mean by the dark line artifact, that's not what I was originally seeing. Is the artifact a straight line? Is it tilted or slanted?

I know that seeing there for the past several months has been good, somewhere between average and excellent. Lots of good clear nights with little to no wind. Another thing I like about DSPR is the lack of dew, there's not much most nights. A few nights the dewpoint got up to 95% for a short time, but that's rare, at least so far.

Hi Jerry, I think I see the artifact that Dark Matters was pointing out. A lot of your brighter stars have an artifact on the bottom left of them. This might be creating the illusion of the lobe on the left side. Usually something like this is from pinched optics / something in the light path, but I don't have the expertise to say for sure. I think you are using a PHQ. I have seen similar looking stars from other PHQs, especially when its cold out.

-Chris

Zoom:

That’s exactly what I saw. Thanks for posting the image. I’m on mobile otherwise would have posted it myself.

Tony Gondola:
Honestly Jerry, this looks like guide error to me, a short excursion where for most of the sub ,the mount tracks well. If you're only seeing this on a few subs, that's probably it. If it's constant from sub to sub then it has to be something optical as dark matters mentioned. If what you mean by better is rounder and smaller stars, a shorter subs exposure time would help.

Hi Tony,

I hope you're right. Reducing the exposure time is easy, dealing with pinched optics not so much. I actually did increase my subs from 180s to 300s, I can bring them back down to 180 and see if that helps.

Jerry

Dark Matters Astrophotography:
Jerry Gerber:
Dark Matters Astrophotography:
The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

I'm not sure if that was installed. I'll ask next time I speak to Ed or Chris.

I am not sure of what you mean by the dark line artifact, that's not what I was originally seeing. Is the artifact a straight line? Is it tilted or slanted?

I know that seeing there for the past several months has been good, somewhere between average and excellent. Lots of good clear nights with little to no wind. Another thing I like about DSPR is the lack of dew, there's not much most nights. A few nights the dewpoint got up to 95% for a short time, but that's rare, at least so far.

Hi Jerry, I think I see the artifact that Dark Matters was pointing out. A lot of your brighter stars have an artifact on the bottom left of them. This might be creating the illusion of the lobe on the left side. Usually something like this is from pinched optics / something in the light path, but I don't have the expertise to say for sure. I think you are using a PHQ. I have seen similar looking stars from other PHQs, especially when its cold out.

-Chris

Zoom:

That’s exactly what I saw. Thanks for posting the image. I’m on mobile otherwise would have posted it myself.

If the issue turns out to be pinched optics, what can I do about it short of replacing the telescope? As Chris has pointed out, it's cold outside now, in the low 30s, so maybe that increases the artifact?

I would do a star test to see of that's even an issue, check both inside and outside focus. If the doughnut isn't round but triangular or misshapen the mirror is being squeezed into an astigmatic shape. It can be fixed by getting to the primary cell and making sure that anything adjustable that's contacting the mirror from the back or sides isn't too tight. being able to just slip a bit of paper between the points at room temperature should insure that you won't have an issue. I know your scope is remote so hopefully, you have someone on site that has the expertise to deal with this if it turns out you have the problem.

Tony Gondola:
I would do a star test to see of that's even an issue, check both inside and outside focus. If the doughnut isn't round but triangular or misshapen the mirror is being squeezed into an astigmatic shape. It can be fixed by getting to the primary cell and making sure that anything adjustable that's contacting the mirror from the back or sides isn't too tight. being able to just slip a bit of paper between the points at room temperature should insure that you won't have an issue. I know your scope is remote so hopefully, you have someone on site that has the expertise to deal with this if it turns out you have the problem.

The scope is an apochromatic quadruplet refractor, no mirrors. I think it's a minor issue, and may be temperature related. By using Pixinsight's various tools and processes it's not hard to repair. I'll still be on the lookout to see if the problem is getting worse or if it's just a small imperfection that I live with and correct with software. Of course I'd love to have a perfect telescope, perfect sky, perfect mount and perfect camera, but that isn't going to happen!

Just a retaining peg in the lens sticking out. Probably cold-related. Same for the remaining effects.

Jerry Gerber:
Dark Matters Astrophotography:
Jerry Gerber:
Dark Matters Astrophotography:
The stars show a dark line effect which is definitely optical. I thought that is what you were taking issue with.

Seeing can shift from sub to sub. I had nights at that site where seeing would rapidly fluctuate. That’s more likely what you are seeing with stars getting higher FWHM from time to time.

The dark line artifact and somewhat triangular star shapes are an optical issue that will become more apparent as the seeing improves.

Did they install a seeing monitor there yet?

I'm not sure if that was installed. I'll ask next time I speak to Ed or Chris.

I am not sure of what you mean by the dark line artifact, that's not what I was originally seeing. Is the artifact a straight line? Is it tilted or slanted?

I know that seeing there for the past several months has been good, somewhere between average and excellent. Lots of good clear nights with little to no wind. Another thing I like about DSPR is the lack of dew, there's not much most nights. A few nights the dewpoint got up to 95% for a short time, but that's rare, at least so far.

Hi Jerry, I think I see the artifact that Dark Matters was pointing out. A lot of your brighter stars have an artifact on the bottom left of them. This might be creating the illusion of the lobe on the left side. Usually something like this is from pinched optics / something in the light path, but I don't have the expertise to say for sure. I think you are using a PHQ. I have seen similar looking stars from other PHQs, especially when its cold out.

-Chris

Zoom:

That’s exactly what I saw. Thanks for posting the image. I’m on mobile otherwise would have posted it myself.

If the issue turns out to be pinched optics, what can I do about it short of replacing the telescope? As Chris has pointed out, it's cold outside now, in the low 30s, so maybe that increases the artifact?

First you’ll want to identify the root cause. The out of focus star test will greatly help in doing so. Start there and see what you find.

Jerry Gerber:
Tony Gondola:
I would do a star test to see of that's even an issue, check both inside and outside focus. If the doughnut isn't round but triangular or misshapen the mirror is being squeezed into an astigmatic shape. It can be fixed by getting to the primary cell and making sure that anything adjustable that's contacting the mirror from the back or sides isn't too tight. being able to just slip a bit of paper between the points at room temperature should insure that you won't have an issue. I know your scope is remote so hopefully, you have someone on site that has the expertise to deal with this if it turns out you have the problem.

The scope is an apochromatic quadruplet refractor, no mirrors. I think it's a minor issue, and may be temperature related. By using Pixinsight's various tools and processes it's not hard to repair. I'll still be on the lookout to see if the problem is getting worse or if it's just a small imperfection that I live with and correct with software. Of course I'd love to have a perfect telescope, perfect sky, perfect mount and perfect camera, but that isn't going to happen!

Absolutely Jerry, it's all about perfecting the rig to give the best it can give. It's the part of this hobby I really enjoy.

I am imaging the same object tonight as last night. same observatory, same scope, mount, camera and filters, and same exposure time. Both artifacts, the tiny bulge at 270 degrees and the small diagonal line that Dark Matters Astrophotography pointed out, are completely gone on every star I've checked so far. HFR isn't great, but the stars are perfectly round.

One difference is temperature, it's 54, but it will cool down as the night progresses.

Jerry Gerber:
I am imaging the same object tonight as last night. same observatory, same scope, mount, camera and filters, and same exposure time. Both artifacts, the tiny bulge at 270 degrees and the small diagonal line that Dark Matters Astrophotography pointed out, are completely gone on every star I've checked so far. HFR isn't great, but the stars are perfectly round.

One difference is temperature, it's 54, but it will cool down as the night progresses.

Then it is likely caused by the temperature. What was the temp when the problem was visible?

Dark Matters Astrophotography:
Jerry Gerber:
I am imaging the same object tonight as last night. same observatory, same scope, mount, camera and filters, and same exposure time. Both artifacts, the tiny bulge at 270 degrees and the small diagonal line that Dark Matters Astrophotography pointed out, are completely gone on every star I've checked so far. HFR isn't great, but the stars are perfectly round.

One difference is temperature, it's 54, but it will cool down as the night progresses.

Then it is likely caused by the temperature. What was the temp when the problem was visible?

I don't remember, but I know it got down into the 30s (F) last night. Currently the temperature is about 58F

If pinched optics are made worse by cold temperatures, is there a any point in loosening the clamps that hold the scope to the 10MicronGM1000 mount by a very small amount? (not too much of course!!!)

Jerry Gerber:
Dark Matters Astrophotography:
Jerry Gerber:
I am imaging the same object tonight as last night. same observatory, same scope, mount, camera and filters, and same exposure time. Both artifacts, the tiny bulge at 270 degrees and the small diagonal line that Dark Matters Astrophotography pointed out, are completely gone on every star I've checked so far. HFR isn't great, but the stars are perfectly round.

One difference is temperature, it's 54, but it will cool down as the night progresses.

Then it is likely caused by the temperature. What was the temp when the problem was visible?

I don't remember, but I know it got down into the 30s (F) last night. Currently the temperature is about 58F

If pinched optics are made worse by cold temperatures, is there a any point in loosening the clamps that hold the scope to the 10MicronGM1000 mount by a very small amount? (not too much of course!!!)

There is likely a retaining ring on the lens itself, although I would not start down that path yet as it also could be a peg obstructing the lens as Andrea mentioned.

I would wait until you have a repro, then defous the scope outward until you have large disks instead of stars, and snap some photos of that. It should highlight the problem well.

Dark Matters Astrophotography:
Jerry Gerber:
Dark Matters Astrophotography:
Jerry Gerber:
I am imaging the same object tonight as last night. same observatory, same scope, mount, camera and filters, and same exposure time. Both artifacts, the tiny bulge at 270 degrees and the small diagonal line that Dark Matters Astrophotography pointed out, are completely gone on every star I've checked so far. HFR isn't great, but the stars are perfectly round.

One difference is temperature, it's 54, but it will cool down as the night progresses.

Then it is likely caused by the temperature. What was the temp when the problem was visible?

I don't remember, but I know it got down into the 30s (F) last night. Currently the temperature is about 58F

If pinched optics are made worse by cold temperatures, is there a any point in loosening the clamps that hold the scope to the 10MicronGM1000 mount by a very small amount? (not too much of course!!!)

There is likely a retaining ring on the lens itself, although I would not start down that path yet as it also could be a peg obstructing the lens as Andrea mentioned.

I would wait until you have a repro, then defous the scope outward until you have large disks instead of stars, and snap some photos of that. It should highlight the problem well.

That sounds like a good plan. If the problem persists or gets worse, that's what I'll do. But for now I am not going to do anything, artifacts are not there now.

Jerry Gerber:

If the issue turns out to be pinched optics, what can I do about it short of replacing the telescope? As Chris has pointed out, it's cold outside now, in the low 30s, so maybe that increases the artifact?

I have an FSQ and had a similar issue with bright stars, except my black line was more obvious. experts suggested it was aperture vignetting and I experimented for months. eventually I removed a M42-M42 spacer that was between the Stock FSQ focuser and my 2600MC camera. I believe the 42mm diameter was slightly restricting the incoming light path causing the aperture vignetting problem. once I removed the spacer the sensor moved closer to the focuser and the issue went away. I will attach a diagram which may help.

FSQ focuser and spacers.pdf

Good news:–It was in the low 30s last night and I did not see the issue return. Stars are looking good.