Show us your star test

[markse68]

Don't know about you but I find star tests very difficult to interpret even referencing Suiter's highly regarded book "star testing telescopes". Of course it doesn't help that the images are pretty unstable due to seeing and tube currents! If they came out text book perfect then I guess it would be easy...

So I thought it might be worth having a thread where we could post our star tests and get opinions on what they might mean? and hopefully learn from it. Would be great to see some excellent examples as well as examples with issues.

I'll start it off with my Tal 150p- a 6" f5 newt. Phone video of Polaris at 214x. I tried to move the focuser an equal amount either side of focus but it was just done by hand. I like this scope and think it performs very well. I'm getting nice detail on the planets when seeing allows and can easily differentiate the 4 Jovian moons by name. I guess that's all that really matters but the star test seems far from symmetrical. Intra-focal there's a thick outer bright ring then a thick dark ring then another thick bright ring, but extra-focal the rings are thinner and it's all a bit mushy and less contrasty. I think it is showing a slight mis-collimation but seems symmetrical so no astigmatism or pinched optics. I've read that seeing can cause differences either side of focus so maybe it's that but it has always looked like this whenever I've done a star test.

Mark

 

[Captain Scarlet]

Great topic Mark. It would be really good if someone who knows their stuff on these lines might monitor this thread and impart wisdom. I’ll certainly post up some of mine.

My one main thought for star-tests for fast Newtonians for collimation purposes, is that one must not try to collimate away the offset in the doughnut to achieve perfect symmetry. That way lies guaranteed miscollimation.

M

[newbie alert]

Only getting the audio Mark...  What you're describing sounds to me like airy disc, where you defocus slightly to get the concentric rings, is that what it's trying to achieve?

[markse68]

2 hours ago, newbie alert said:

Only getting the audio Mark...  What you're describing sounds to me like airy disc, where you defocus slightly to get the concentric rings, is that what it's trying to achieve?

Hi Newbie, yes sorry about that- it was showing on my iphone but not on my pc 🤷‍♂️ Have uploaded it to Flickr and embedded link that hopefully will work now. Yes it’s the diffraction rings of slight out of focus Polaris at high power (214x). Theory states that perfect optics will produce a perfect identical pattern both sides of focus- clearly mine isn’t yet it seems to work pretty well so interested in interpreting what it can tell about my mirror. Thanks for your interest

Mark

[davidc135]

The rings shown in the out of focus Polaris images are Fesnel rings rather than diffraction. I'm definitely not an expert on the subject but the extra focus shadow seems a little larger and appears sooner than the intra focal shadow so, going on that, it seems a touch over-corrected. But that doesn't explain the sharper rings inside focus.

David

[Paz]

It would be useful to have an image or failing that if you can find an image of a star test online that looks as close as possible to what you can see and refer to that. Descriptions in words are difficult to interpret without leaving quite a lot of scope for misinterpretation.

[markse68]

58 minutes ago, Paz said:

It would be useful to have an image or failing that if you can find an image of a star test online that looks as close as possible to what you can see and refer to that. Descriptions in words are difficult to interpret without leaving quite a lot of scope for misinterpretation.

Hi Paz, I posted a video- can’t you see it? I’m sorry i seem to be having issues with videos from my iphone not showing but i can see it on my phone and my pc.

Mark

[Franklin]

1 hour ago, Paz said:

It would be useful to have an image or failing that if you can find an image of a star test online that looks as close as possible to what you can see and refer to that.

There is useful information in the appendix of The Backyard Astronomers Guide on star-testing and of course Harold Suitors book, Star-Testing Astronomical Telescopes, is a recognized authority.

Personally what I've found is that the star test is a VERY sensitive test and even without a perfect star-test the views are still acceptable. As long as you're reasonably collimated and there are no obvious problems, such as pinched optics, then I think you'll be ok. If your optics are a little over or under corrected then there's not much you can do about it anyway, apart from buying a better scope that is.

[markse68]

17 hours ago, davidc135 said:

The rings shown in the out of focus Polaris images are Fesnel rings rather than diffraction. I'm definitely not an expert on the subject but the extra focus shadow seems a little larger and appears sooner than the intra focal shadow so, going on that, it seems a touch over-corrected. But that doesn't explain the sharper rings inside focus.

David

Hi David, I think the Fresnel rings are caused by diffraction from the aperture? Anyway thanks for your input- I know that mirrors can suffer over/under correction quite easily and commonly due to thermal imbalance and it may just be that- it was a sub zero night and although the scope was out for a few hours it may not have reached equilibrium. But i’ve seen the same in other nights so maybe it is baked in. None of the references i’ve seen show the double thick outer ring that i’m seeing which is the most confusing thing for me. Having said that they do somewhat resemble the images in Suiters book for spherical aberration due to under correction with a 33% obstruction (not sure what mine is- will have to check) where the extra focus rings are more blurry than the intra focus. But what you say about the central secondary shadow breaking out earlier extra focus tallies with what Suiter says about over correction too. But he warns “Ironically, the star test for spherical aberration is almost too sensitive. It is so revealing that nearly any telescope fails casual inspection” 

Mark

Edited December 10, 2022 by markse68

[Captain Scarlet]

Re apparent SA. I’ve had a particular phenomenon in the past, on two supposedly extremely well-corrected scopes, one a 105mm LZOS refractor, and the other a 1/10 wave Orion Optics Newt (with 1/10 secondary). It went as follows, on different occasions for each scope.

One night, even after plenty of cool-down (hours) the scope showed terrible SA, untidy and spiky one side, crisp and sharp the other. Thinks to oneself “sell this piece of rubbish”. Then the next night, perfection (relief)! This has happened as I say to each of those two scopes.

The difference between those pairs of successive nights was that the first night saw continuous and fairly rapid falls in temperature throughout the session, whereas the following-day “good nights” were stable of temperature.

The upshot being of course that on the variable-temperature nights the scope’s temperature was never catching up, always distorted therefore showing poor correction.

It was fascinating actually.

Magnus

 

[markse68]

That’s very interesting Magnus and I’ve seen it too- though with my 8” f8 fullerscope the quality of which was unknown. It always showed asymmetry about focus in a star test except one night when it appeared close to perfect and that was the night i first saw the pup! Maybe i should look for a zerodur mirror  

Mark