Why are my stars shaped like flat discs??? - Forums

If I hadn’t seen it, I wouldn’t believe it was possible. A little backstory: For various reasons, I’ve struggled with collimation on my Meade SN8. Finally feeling I got it down, I went out 3 nights ago and got the best collimation I’ve ever had. This resulted in the best lunar closeup I’ve ever had. I’ve shot the Apollo 11 site many times and always failed to resolve the Armstrong crater. After proper collimation, I was able to easily resolve not only Armstrong, but the other two as well.
Leaving everything completely untouched, I went back last night, after seeing the extreme improvement on the moon, and hoped to see the same improvement on Saturn which I imaged a week prior.
I knew my collimation wasn’t perfect. Being a new night with an early start, I wanted to tweak it a bit to get it a little better. When I flipped the camera on, my defocused star looked as it should. It was very close to collimation. After some tweaking and not seeing much improvement, I decided to swap the camera for my quality collimation laser to make sure I wasn’t taking the primary out of alignment. It was slightly off. I fixed it. Then, as I always do, I moved on to my Cheshire. As my laser is pretty accurate, I needed almost no adjustments. Then I put the camera back in and got the horrific image below. The exact same star that looked good 2 minutes prior on the exact same camera now had 2 squished edges. When I bring it to focus, I get the flat discs in the second image. I’ve tried everything I can think of. Nothing changes the shape. Every star in the entire field of view is the same. I can’t imagine what could possibly happen that would result in such an image.
If that’s not confusing enough, I switched from my ASI2600MC deep sky camera to my ASI224MC planetary camera. When I attached it, it just so happened that a perfectly defocused star right there on screen. Surprisingly, it looked almost perfectly collimated. Even more surprisingly, when I brought it to focus, the perfectly round star transformed right back into a flat disc. This makes zero sense to me on so many levels. I ended up cranking the gain so I could lower the exposure time in watch it in real time. Once again, I was completely dumbfounded. It won’t let me add video, but the star looked exactly like a candle flame floating in mid air. Same color, same shape, same behavior. After trying everything I could think of, I sat back down and watched this candle flame dance on the screen for about 20 minutes while I debated setting the rig on fire and quitting the hobby because I can’t afford a new scope. I’m at a complete loss right now.

If its at the corner of the FOV, its normal, the Schmidt-Newtonian is not a completely Coma free system.

That image above of then defocussed star, is far from being even well collimated, it’s a long way off, what makes you think that is “almost perfectly collimated”, as you put it..??
I would start there and get the scope very well collimated before messing with anything else.

If it’s at the corner of the FOV, its normal, the Schmidt-Newtonian is not a completely Coma free system.

It was in this photo. But I tried moving it. Every star in the field of view looks like this regardless of where in the frame it is. I also have a CC. So, I shouldn’t be seeing results like this anyway.

AstroShed:
That image above of then defocussed star, is far from being even well collimated, it’s a long way off, what makes you think that is “almost perfectly collimated”, as you put it..??
I would start there and get the scope very well collimated before messing with anything else.

As I stated, I knew it was “almost perfectly collimated” by doing the defocused star test and seeing all concentric circles. More evidence, as I mentioned, was the fact that I was able to resolve extremely tiny details on the moon that I’ve never resolved before and are only possible with good collimation.
I can’t very well start by getting collimated as the entire point of the post is that I can’t get it collimated. To collimate, in layman’s terms, I have to put the small circle perfectly inside the big circle. How do I that if the shapes aren’t circles?
And the biggest question is how something like this happens anyway. The camera has nothing to do with collimation. So, it makes no sense that the shape changed simply by taking the camera out. Or why a different camera would see a different shape. The mirrors didn’t move. Any camera should see the same thing, right?

I had stars like that on my OAG when I was poorly collimated and didn’t have the starizona corrector for my C8.

Whether it was the better quality corrector (I had been using Celestrons) or better collimation, I’m not sure which fixed it.

Quinn Groessl:
I had stars like that on my OAG when I was poorly collimated and didn’t have the starizona corrector for my C8.

Whether it was the better quality corrector (I had been using Celestrons) or better collimation, I’m not sure which fixed it.

That's what I don't understand. I was collimated. Nothing about the system changed except removing the camera. Everything was great one night. I parked the scope to home and left it. When I came back, I turned the camera on and still had a well collimated defocused star. Because I've had issues in the past, I use multiple tools to double and triple check. So, here’s the progression again: I was looking at a star (Polaris) that was defocused and pretty well collimated. I took out the camera and double and triple checked with a laser and Cheshire. With all being good, I put the camera back in. This is the resulting image. Nothing changed except the camera. Hence my confusion. How does a camera affect collimation??? Sure, I can my the small oval inside the large oval. But my concern is why I have ovals in the first place. Or why one camera has ovals and the other has circles (but neither one have round stars).

Forgetting the cameras for a moment, I can collimate by putting a circle in a circle. But where do I get the circles??? Moving the primary only moves where the ovals are. It doesn’t change them back into circles.

I have experimented with SN’s in the past and I’ve never liked any of the results when I was done. Like you I did everything. In the end I have to agree with @V.M Legary’s comment. SN are not totally coma free whether your using a cc or not.

In the end I went back to my newts and refractors for imaging.

My suggestion is consider a different imaging configuration. I understand you don’t have the money for a new scope but you could sell this one and then move to something that has much more success rates and is easier to utilize like a nice 80mm refractor.

This is probably not what you wanted to hear but I just thought I’d put it out there.

Good luck on whatever you decide.

Jaymz Bondurant:
If it’s at the corner of the FOV, its normal, the Schmidt-Newtonian is not a completely Coma free system.

It was in this photo. But I tried moving it. Every star in the field of view looks like this regardless of where in the frame it is. I also have a CC. So, I shouldn’t be seeing results like this anyway.

Ah, then you're not collimated. Also a corrector or reducer for an SN should be a SN optimized corrector as the optical characteristics of an SN are different from a normal Newtonian.

Jaymz Bondurant:
Quinn Groessl:
I had stars like that on my OAG when I was poorly collimated and didn’t have the starizona corrector for my C8.

Whether it was the better quality corrector (I had been using Celestrons) or better collimation, I’m not sure which fixed it.

That's what I don't understand. I was collimated. Nothing about the system changed except removing the camera. Everything was great one night. I parked the scope to home and left it. When I came back, I turned the camera on and still had a well collimated defocused star. Because I've had issues in the past, I use multiple tools to double and triple check. So, here’s the progression again: I was looking at a star (Polaris) that was defocused and pretty well collimated. I took out the camera and double and triple checked with a laser and Cheshire. With all being good, I put the camera back in. This is the resulting image. Nothing changed except the camera. Hence my confusion. How does a camera affect collimation??? Sure, I can my the small oval inside the large oval. But my concern is why I have ovals in the first place. Or why one camera has ovals and the other has circles (but neither one have round stars).

Forgetting the cameras for a moment, I can collimate by putting a circle in a circle. But where do I get the circles??? Moving the primary only moves where the ovals are. It doesn’t change them back into circles.

Collimating the primary gives you the exterior donut shape, while the secondary collimation gives you the hole's position.

I would highly recommend a Duncan or Tri-Bahtinov mask.

Jaymz Bondurant:
Quinn Groessl:
I had stars like that on my OAG when I was poorly collimated and didn’t have the starizona corrector for my C8.

Whether it was the better quality corrector (I had been using Celestrons) or better collimation, I’m not sure which fixed it.

That's what I don't understand. I was collimated. Nothing about the system changed except removing the camera. Everything was great one night. I parked the scope to home and left it. When I came back, I turned the camera on and still had a well collimated defocused star. Because I've had issues in the past, I use multiple tools to double and triple check. So, here’s the progression again: I was looking at a star (Polaris) that was defocused and pretty well collimated. I took out the camera and double and triple checked with a laser and Cheshire. With all being good, I put the camera back in. This is the resulting image. Nothing changed except the camera. Hence my confusion. How does a camera affect collimation??? Sure, I can my the small oval inside the large oval. But my concern is why I have ovals in the first place. Or why one camera has ovals and the other has circles (but neither one have round stars).

Forgetting the cameras for a moment, I can collimate by putting a circle in a circle. But where do I get the circles??? Moving the primary only moves where the ovals are. It doesn’t change them back into circles.

Easy, a camera effects collimation by having a tilted sensor, it’s pretty common nowadays…so you need to check that, i have a modern QHY268c camera, and the sensor is quite badly tilted which would cause your issues in the type of scope you have there…
see here…my video on correcting tilt in the camera…

https://youtu.be/Gw2oTwb4GjY?si=izYarooeAb3SF4Qb

Dale Penkala:
you could sell this one and then move to something that has much more success rates and is easier to utilize like a nice 80mm refractor.

The only problem with that idea is that I'm much too determined (translation: stubborn) to give it up. There are generally two opinions regarding this scope. One, as you mentioned (and easily the most common), is that it's just too much trouble. But for those who have figured it out (of which there are very few) the consensus is that this scope produces images that are among the very best they have. Is it worth the effort? I'm still trying to determine that.
Let's not forget also that the single best image this specific scope has ever taken (while in my possession) came only 4 nights ago. Going from having everything dialed in to abandoning it in just a matter of days seems a bit harsh.
But there's also a pride factor. My Apollo 11 shot from the other night is going on the wall. Is it the best Apollo 11 shot out there? Of course not. Did the better ones use better equipment? Almost guaranteed. Did any of them have to work as hard to get it? Not likely. I know it sounds crazy to want to work harder. Trust me, I've been eyeballing that new Askar 71F that you just slap a camera on right out of the box. But the feeling of seeing that stacked image of the Apollo 11 site after all the work it took to make it happen was pretty incredible. I'm not sure I'm willing to give that up yet. Especially now that I know it can be done.

Ah, then you're not collimated. Also a corrector or reducer for an SN should be a SN optimized corrector as the optical characteristics of an SN are different from a normal Newtonian.

I've got the proper reducer and it's optimized specifically for the SN. I should clarify that I'm not just starting this journey. It's not a matter of me trying to get it dialed in. It's a matter of me already having it dialed in and now not understanding why it all went away (especially without having touched anything on it). I've imaged DSO without issue. My recent attempts at SSO with a 3x barlow required better collimation. I achieved that and got my shot. Excited at the much improved results on my lunar shot, I went back out to try Saturn and I'm no longer collimated. This is where I'm confused. I'll be the first to admit that I'm far from an expert at collimation. In the most laymen's terms possible, my job is to put the circle inside the other circle. I'm not aware of any step requiring me to first turn the ovals into circles. Hence my confusion. If I can understand why I have ovals instead of circles, I can collimate the scope. Moving the primary mirror doesn't change the shape and I have no idea what would cause it.

I've kinda felt like I'm in the Twilight Zone in that I'm over here absolutely freaking out because I think my scope is destroyed and, in three different forum posts, nobody seems to see anything but only minor issues in the photos I've shared. I only mention this because I'm starting to think that the problem is sooooo minor, it's going under everyone's heads rather than over. Like I've asked for help on an algebra problem but the part I'm stuck on is 2+2. I'm wondering if, because it's algebra, everybody is skipping over the 2+2 because it's something I should already know. Maybe if we rethink it with the idea, "Hey, this guy might be stupid. Explain it to him like he's 5", I'll get it. Lol.

I have clear skies tonight. If I can't figure it out in the next few hours, I fear I have no choice but to tear apart the scope and rebuild it from scratch.

Actually, that kinda explains my dilemma a bit. I'm good enough to tear apart the scope and rebuild it. But I'm not good enough to figure out why my stars are flat. If I can figure that out, I can save myself a lot of trouble.

Collimating the primary gives you the exterior donut shape, while the secondary collimation gives you the hole's position.

I would highly recommend a Duncan or Tri-Bahtinov mask.

I tried collimating the primary during the star test but the shape never changed. That's when I switched to other tools. I do have a Tri-Bahtinov mask. However, I didn't attempt it this time because with what you see in the photos, it was clear the mask was never going to work anyway. Strangely, both my laser and Cheshire agree that I'm in collimation. That's why I'm extra confused. You can make plenty of arguments against lasers (although I didn't spend a significant amount of money to make sure I had a high quality collimated laser). But, to my knowledge, it's hard for a Cheshire to lie. I could be wrong, but looking at the oval shapes in the photos tells me that collimation has to be wildly inaccurate. So, this is the scenario I imagine: As of now, the laser and Cheshire show everything to be centered. I move the primary around until everything is round again. But the movements would be so significant that when I put the Cheshire back in, not only will the two circles not be overlayed, but they're going to be so far apart that they may not even be touching. This would also result in the laser hitting somewhere inside the tube wall rather than back into the focuser. It seems to me that all collimation tools should at least be in the ballpark of agreement. If I've got two tools giving wildly different results from the other two tools, that tells me there's got to be something majorly wrong somewhere in the system.

AstroShed:
Easy, a camera effects collimation by having a tilted sensor, it’s pretty common nowadays…so you need to check that

I will definitely check this out! It could be it. At this point, I'm running out of things to troubleshoot!

Jaymz Bondurant:
Collimating the primary gives you the exterior donut shape, while the secondary collimation gives you the hole's position.

I would highly recommend a Duncan or Tri-Bahtinov mask.

I tried collimating the primary during the star test but the shape never changed. That's when I switched to other tools. I do have a Tri-Bahtinov mask. However, I didn't attempt it this time because with what you see in the photos, it was clear the mask was never going to work anyway. Strangely, both my laser and Cheshire agree that I'm in collimation. That's why I'm extra confused. You can make plenty of arguments against lasers (although I didn't spend a significant amount of money to make sure I had a high quality collimated laser). But, to my knowledge, it's hard for a Cheshire to lie. I could be wrong, but looking at the oval shapes in the photos tells me that collimation has to be wildly inaccurate. So, this is the scenario I imagine: As of now, the laser and Cheshire show everything to be centered. I move the primary around until everything is round again. But the movements would be so significant that when I put the Cheshire back in, not only will the two circles not be overlayed, but they're going to be so far apart that they may not even be touching. This would also result in the laser hitting somewhere inside the tube wall rather than back into the focuser. It seems to me that all collimation tools should at least be in the ballpark of agreement. If I've got two tools giving wildly different results from the other two tools, that tells me there's got to be something majorly wrong somewhere in the system.

Now this is all starting to make sense, camera tilt might be the main culprit since you said that you put the camera back on and the problem began, a deflection of even 1mm in any direction could easily make a collimated system appear uncollimated. My 8HD had the same problem, I always thought my collimation was the problem and my stars kept looking down and to the right until I fixed the tilt issue.

Hope you can figure this out without a teardown of the scope. Another possibility is the focuser-- I don't see that being likely, but it is a possibility.

Now this is all starting to make sense, camera tilt might be the main culprit since you said that you put the camera back on and the problem began, a deflection of even 1mm in any direction could easily make a collimated system appear uncollimated. My 8HD had the same problem, I always thought my collimation was the problem and my stars kept looking down and to the right until I fixed the tilt issue.

Hope you can figure this out without a teardown of the scope. Another possibility is the focuser-- I don't see that being likely, but it is a possibility.

That's the next thing I'm looking into. But my frustration has been that, for everything that seems like it could be the cause, I find some sort of evidence to say that it isn't the cause. In this example, I could point out that a different camera also showed the same flat stars. This would indicate the problem is in the optics. However, the second camera only saw the flawed stores when in focus. Taken out of focus, the star test looked fine. This obviously makes ZERO sense. A flaw is a flaw whether in focus or not. Or so I thought. And this is why I sat there blankly staring at the screen for 20 minutes. The cameras don't agree on what they see. The collimation tools don't agree on what they see. I don't know where to begin because I don't know what tools can be trusted at this point.

Now that I've said all that, I kinda feel like a complete tear down might be necessary. At least that way, whatever problems I may run into tonight, I can eliminate the scope itself as the problem and focus in on camera tilt or whatever else I may need to look at that I don't know about yet.

Update for anyone following along with this drama: I decided to go ahead and strip down the scope. The only noticeable issue was both of the paper gaskets on each side of the corrector plate had come loose and were warped. Two of the ends were protruding into the glass. This could very well explain the aspherical shape during my star test. Any stretching of the warped sides back into place made the opposite two sides warp. I alleviated this by cutting them both in half and putting each half in place. Of course, I made sure the CP was properly centered while in there.

Unfortunately, this minor problem shouldn’t prevent me from getting round stars while in focus. So, I’m still at a loss there.

i did check everything before I tore it apart. All looked good aside from the two pressure gaskets. I’ve got two cameras to test with. Now that it’s all rebuilt, I’m just waiting for some stars to come out.

I don't know what else to do. This is the view through two cameras and an eyepiece. I give up.

The paper baskets are there for a reason, and should be put back in exactly the same place, also the corrector should be marked on the edge of the glass, and marked on the tube, and inserted in the exact same orientation, was this the case…? Also after removing the corrector, I assume you re collimated again…?

AstroShed:
The paper baskets are there for a reason, and should be put back in exactly the same place, also the corrector should be marked on the edge of the glass, and marked on the tube, and inserted in the exact same orientation, was this the case…? Also after removing the corrector, I assume you re collimated again…?

Yes, everything is aligned and centered. Collimation looks good during a star test. Stars in focus do not. Which I don’t understand. There are obviously flaws in the stars. I would assume they would show up defocused as well.

Can you post an image of an entire star field? I can't tell if you are showing the center or out on the edge of the fov. This stuff isn't rocket science. Forget lasers and collimation aids. Get an image of a medium bright star into the CENTER of the FOV and hold it there. Collimate until the rings are PERFECTLY concentric. Also remember that with each adjustment, the star will move and you'll have to re-center. You could also do this visually with a high power ocular and a barlow. If you can not get perfect collimation or nothing seems to change then take a good hard look at the mechanic of your scope. Is the primary canted in it's cell somehow? Is your secondary totally out of alignment? Are you turning the right screws? You mentioned that collimation changed radically from one night to the next. That might be a clue?

Can you post an image of an entire star field? I can't tell if you are showing the center or out on the edge of the fov. This stuff isn't rocket science. Forget lasers and collimation aids. Get an image of a medium bright star into the CENTER of the FOV and hold it there. Collimate until the rings are PERFECTLY concentric. Also remember that with each adjustment, the star will move and you'll have to re-center. You could also do this visually with a high power ocular and a barlow. If you can not get perfect collimation or nothing seems to change then take a good hard look at the mechanic of your scope. Is the primary canted in its cell somehow? Is your secondary totally out of alignment? Are you turning the right screws? You mentioned that collimation changed radically from one night to the next. That might be a clue?

I don’t have the full image handy at the moment. But I can tell you the stars are the same throughout the entire field of view on an APS-C sensor. I’ve tried all the tricks. My star test is concentric. That’s why it’s baffling. I have a good star test and, as it goes into focus, it transforms from round to this shape.

Jaymz Bondurant:
Can you post an image of an entire star field? I can't tell if you are showing the center or out on the edge of the fov. This stuff isn't rocket science. Forget lasers and collimation aids. Get an image of a medium bright star into the CENTER of the FOV and hold it there. Collimate until the rings are PERFECTLY concentric. Also remember that with each adjustment, the star will move and you'll have to re-center. You could also do this visually with a high power ocular and a barlow. If you can not get perfect collimation or nothing seems to change then take a good hard look at the mechanic of your scope. Is the primary canted in its cell somehow? Is your secondary totally out of alignment? Are you turning the right screws? You mentioned that collimation changed radically from one night to the next. That might be a clue?

I don’t have the full image handy at the moment. But I can tell you the stars are the same throughout the entire field of view on an APS-C sensor. I’ve tried all the tricks. My star test is concentric. That’s why it’s baffling. I have a good star test and, as it goes into focus, it transforms from round to this shape.

So when you have the stars out of focus you say you get perfect doughnuts, as you say the collimation is good, but when you bring into focus you end up with flat stars…?? Is this correct..??
if this is the case it’s not a scope with a moving primary mirror when you focus, such as an SCT then something is totally out of line with the focus tube when focusing…