Star test, need help interpred the result - Forums

Topic starter

12 days ago

Can someone help me interpred this star test? I struggle to find a good source explaining exactly what this means for my specific mirror.
If this star test is even good enough to actually get something out of it.
It doesn’t look to good, so I’m not expeting much.

I have elongated stars to just one side of the frame and I can’t really find the source of that.

I have a 8” f4.5 Newtonian which was handbuilt in the 1960s and was used in an observatory for a long time so it should be pretty good optically.
Somehow I stuggle to get the right quality out of it.

📷 Startest.jpg

How part of my frame looks like, this is an extreme example, it’s not always that bad.

📷 WhatsApp Image 2026-03-12 at 19.44.05.jpeg

andrea tasselli

Frequent contributor

12 days ago

At a pinch (sight unseen), considering the age of the mirror, I'd say you just have very long duration thermal plume interfering. Back then it was customary to have D/6 thick mirrors which never actually cooled down if big enough. Can't say much else as I'd need to see the in-focus Airy disk to make a more precise evaluation.

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Jens

Topic starter

12 days ago

andrea tasselli · Mar 24, 2026, 10:13 AM
At a pinch (sight unseen), considering the age of the mirror, I'd say you just have very long duration thermal plume interfering. Back then it was customary to have D/6 thick mirrors which never actually cooled down if big enough. Can't say much else as I'd need to see the in-focus Airy disk to make a more precise evaluation.

The Mirror is quite thick and it has a metal mirror cell which holds the mirror quite tight.
The tube is also quite thick.
It is probably really hard for it to actually cool down. Never really thought of that but that is a good point.

Wait a second…. when I checked in which direction the starshape streak was pointing to, it was to the top of the tube. Could the starshape be thermal runaway because the mirror cant cool down properly?
The Telescope just has a hard time reaching equilibrium thus creating the starshape. Could heat have such a big impact on star shape?

Here is the mirror cell.
📷 Screenshot 2026-03-24 111625.png

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Jens

Topic starter

12 days ago

Here you can see the thermal effects a bit better. looks more like the whole donut is missshapen rather than distored by heat

Video of the star test

andrea tasselli

Frequent contributor

12 days ago

Jens:
Wait a second…. when I checked in which direction the starshape streak was pointing to, it was to the top of the tube. Could the starshape be thermal runaway because the mirror cant cool down properly?
The Telescope just has a hard time reaching equilibrium thus creating the starshape. Could heat have such a big impact on star shape?

The answer is yes and yes. Looking at the mirror and cell I'm not surprised at all. You might consider active cooling to help it along but given the size of the mirror probably isn't worth the effort. Too bad as the figure looks nice (from the intra-focal image).

Jens

Topic starter

12 days ago

well wow, you’re good. I was working on this issue for a few weeks now with forums and chats etc. the thermal side was never mentioned.
I checked older photos to see if the spike always point up.

lo and behold, they indeed do. I think you just solved my biggest mistery with that scope. thank you so much!!

To me it is very much worth tackling this issue. this scope has sentimental worth to me.

Now one question remains: when I star collimate the scope, the donut can be perfect in the middle, as soon as it moves to the corner of the frame, the donuts start to deform. they don’t stay donuts.

does that point to spherical abberation, maybe the wrong coma corrector or is that normal?

📷 Screenshot 2026-03-24 130240.png

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andrea tasselli

Frequent contributor

12 days ago

Coma correctors don't correct for severely out-of-focus images so that is pretty normal. Fresnel rings are, however, not very sensitive to precise collimation nuances. You need to be much closer and with larger PSF to be able to achieve critical collimation. Best of all would be a visual collimation at high powers.#

I guess that you'd need active cooling to tame that beast, which would consist of fans, Peltier units and thermostats (and thermocouples).

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Tony Gondola

Frequent contributor

Exemplary contributor

11 days ago

One possible solution to the warm air plume is to give it a way to exit the system without entering into the light path. Most Newtonians have tubes that are too small in diameter relative to the mirror so the plume has nowhere to go. It’s a balance between back focus and diameter but it’s worth looking at.

If you install cooling fans it might be productive to blow air across the face of the mirror. This is done to break up the boundary layer of air clinging to the mirror’s surface. This might also help by getting most of the heat energy to exit the tube early.

You can also think about laminar flow. I forget which way is the most effective, up or down the tube but if you can establish a smooth flow, that can sometimes can tame an issue like this.

Of course, you could always go with a truss and boundary fans, eliminating the tube all together but then stray light becomes an issue.

There’s been a lot written on this issue so some research on the subject might be fruitful. I hope you can find a way to keep that old mirror alive!

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Rick Veregin

11 days ago

Jens · Mar 24, 2026, 12:05 PM
well wow, you’re good. I was working on this issue for a few weeks now with forums and chats etc. the thermal side was never mentioned.
I checked older photos to see if the spike always point up.
lo and behold, they indeed do. I think you just solved my biggest mistery with that scope. thank you so much!!

To me it is very much worth tackling this issue. this scope has sentimental worth to me.

Now one question remains: when I star collimate the scope, the donut can be perfect in the middle, as soon as it moves to the corner of the frame, the donuts start to deform. they don’t stay donuts.
does that point to spherical abberation, maybe the wrong coma corrector or is that normal?

📷 Screenshot 2026-03-24 130240.png

You do collimation only viewing on the center of your field, stars at the edge are off-axis, so will not be symmetrical out of focus, but hopefully are when you are in focus. To do it properly get a star at the center of the field, zoom in as much as you can, and then get it just slightly out of focus so you can just see the donut enough to see if it is centered or not. It is not as sensitive if you are at low magnification because then you need to be way out of focus to see a visible donut.

I don’t have a Newtonian so I can’t help you too much on how to collimate as it can get quite complicated as there are a lot of possible adjustments. There are probably threads here on AB you could search and there are lots of videos on Youtube to show you how.

If you want to know your star quality over the full field, you can of course just focus on the centre of the image, and then zoom in on the stars at the edges—are they in focus, are they mishapen, and what direction are they elongated in. Again it can get complicated, from collimation, how inherently optically flat your field is, camera tilt, etc..

For a quanitative look, DeepSkyStacker is a free program for calibration, registration and stacking. It also recently added a star quality feature. So just load your image file(s) along with any calibration files and calibrate and register them. Then a right click on any image in the file list enables you to select star quality. You can then choose either eccentricity or FWHM, and it plots immediately a contour plot. So what you want to see is that the image quality doesn’t drop off at the edges. You have to be a bit careful in interpretation, because bright stars will be bigger, so you will see round spots for a higher FWHM, so focus on the larger areas to judge the quality. The only downside is it doesn’t show the direction of any eccentricity, just the amount. But the FWHM gives you a good idea of what is happening anyway. Very easy to use, and by the way a wonderful program for calibration, registration and stacking, and super fast if you have multiple processors on your computer.

The most recent version of DSS is unfortunately a bit hard to find, as the main site it used to be on is now obsolete, with a very old version. Here is the current most recent fully tested version on Github, the new official site for DSS:

https://github.com/deepskystacker/DSS/releases/tag/6.1.3

Rick

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Jens

Topic starter

11 days ago

Tony Gondola · Mar 24, 2026, 03:08 PM
One possible solution to the warm air plume is to give it a way to exit the system without entering into the light path. Most Newtonians have tubes that are too small in diameter relative to the mirror so the plume has nowhere to go. It’s a balance between back focus and diameter but it’s worth looking at.
If you install cooling fans it might be productive to blow air across the face of the mirror. This is done to break up the boundary layer of air clinging to the mirror’s surface. This might also help by getting most of the heat energy to exit the tube early.
You can also think about laminar flow. I forget which way is the most effective, up or down the tube but if you can establish a smooth flow, that can sometimes can tame an issue like this.
Of course, you could always go with a truss and boundary fans, eliminating the tube all together but then stray light becomes an issue.
There’s been a lot written on this issue so some research on the subject might be fruitful. I hope you can find a way to keep that old mirror alive!

I’ll probably go with the fan method.
as you can see, I have a mirror with a hole in the middle, that way I can channel air through that hole and let it blow across the mirror surface. That’s my plan anyway.
I’ll also let that air blow underneath the cell to cool down the metal.
Do you guys think installing small heatsinks underneath the mirror would work to cool it down faster?

so what you’re saying is installing vents in the middle of the tube in order to let the hot air out before it comes out the front?

It’s an interesting problem to solve, I’ll probably have to do quite a few changes to the scope.

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andrea tasselli

Frequent contributor

11 days ago

Hi Jens,

If you give me the "numbers" of the cell and mirror (that is dimensions of cell and mirror and maybe a few pics to go along with) I can run a simulation and see what's more efficient. In general in a close tube system you have a chimney effect for the warmer air raising when the ambient temperature goes below the mirror surface temperature. To minimize ripple effects you's need a tube 20% larger than the mirror and no baffles. A combination of push-pull fans across the face of the mirror blowing and sucking air across the side of the mirror avoid the formation of large boundary layers which slowly detaches and moves upward along the tube causing the heat plume oscillation you see in the pics but th emain thing here is to avoid to have a heat plume as much as possible. Adding fans blowing at the back of the mirror (but NOT through the hole itself) increase the heat transfer overall and reduces the time it takes to bring it in sync with outside air temperature. Note that you loose a bit of temperature through radiation against the night sky but it isn't a major factor in the thermal transient speed. More advanced options is to have Peltier cells stuck at the back of the mirror (active cooling) so that you get a feed-back loop (with temperature probes and thermostats) and much larger cooling capacity with respect to only fans blowing air on the back of the scope. Also, you have a very large primary cell which burdens the thermals with its own transients.

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Jens

Topic starter

11 days ago

That would be amazing

I have to remeasure my mirror. I’ll text you the specific size as soon as I have time to measure everything

andrea tasselli

Frequent contributor

11 days ago

Great stuff. PM me when you got everything needed and we'll take it from there.

Tony Gondola

Frequent contributor

Exemplary contributor

10 days ago

Jens · Mar 25, 2026, 07:30 AM
Tony Gondola · Mar 24, 2026, 03:08 PM
One possible solution to the warm air plume is to give it a way to exit the system without entering into the light path. Most Newtonians have tubes that are too small in diameter relative to the mirror so the plume has nowhere to go. It’s a balance between back focus and diameter but it’s worth looking at.
If you install cooling fans it might be productive to blow air across the face of the mirror. This is done to break up the boundary layer of air clinging to the mirror’s surface. This might also help by getting most of the heat energy to exit the tube early.
You can also think about laminar flow. I forget which way is the most effective, up or down the tube but if you can establish a smooth flow, that can sometimes can tame an issue like this.
Of course, you could always go with a truss and boundary fans, eliminating the tube all together but then stray light becomes an issue.
There’s been a lot written on this issue so some research on the subject might be fruitful. I hope you can find a way to keep that old mirror alive!
I’ll probably go with the fan method.
as you can see, I have a mirror with a hole in the middle, that way I can channel air through that hole and let it blow across the mirror surface. That’s my plan anyway.
I’ll also let that air blow underneath the cell to cool down the metal.
Do you guys think installing small heatsinks underneath the mirror would work to cool it down faster?

so what you’re saying is installing vents in the middle of the tube in order to let the hot air out before it comes out the front?

It’s an interesting problem to solve, I’ll probably have to do quite a few changes to the scope.

I wouldn’t blow air through the center hole but rather directly from the side and out the opposite side. Think of this as acting like a broom, sweeping the layer of air that’s clinging to the mirror’s surface away.

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Jens

Topic starter

10 days ago

Aren’t there two things to consider?
the first being the actual temperature of the mirror, which has to be the same as the ambient temperature.
The other is that layer that clings onto the mirror surface.

My idea was to blow air up through the hole but with a 3D printed flow diverter which channels the airflow first underneath the mirror to cool it down and the rest will get blow on to the surface like so:

📷 Screenshot 2026-03-25 171844.png
Of course there will be a tube at the sides.

This is of coursse not proven nor tested, just a quick theoretical concept

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Tony Gondola

Frequent contributor

Exemplary contributor

10 days ago

I think large diameter fans at the side blowing directly across would be less turbulent. Then again, there’s a lot of room for experimentation in this area so give it a try. It’s an elegant solution if it works.

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