Star test with no rings!

[Rustang]

I had my second very short stint outside with my OO 8inch reflector. Had a quick nose at Saturn and jupiter and also thought while I'm out I would do a star test. My issue is, is that there didn't appear to be any rings to go by either side of focus!? 

[John]

What star did you use ?

 

[Rustang]

Not sure of names, it's been pretty cloudy but they were fairly bright and I tried aiming between the clouds 

[John]

Presumably the airy disk of the star expanded either side of sharp focus ?

 

 

[Rustang]

Yes, it went in and out of focus both sides, can't be to far out of colimation as for the first time ever I managed to make out some small colouration detail on Mars tonight which was pretty cool. Still can't understand the lack of rings though on the star test. 

[Rustang]

Don't worry, I think I've got issues with my eyes lol 😂 I can now see rings on the outer edges and it looks spot on. Not sure what was going on before. I'm currently viewing the moon and I'm speechless!! It's good viewing tonight!! 

[Geoff Barnes]

I usually notice that the rings of the expanded airy disc can be quite hard to see when the atmosphere is unstable, making seeing conditions poor. It seems to cause the rings to scintillate and break up into a mushy mess. Given rock steady seeing conditions I find the rings are nice and sharply defined and easy to see.

[Rustang]

It was a bit mushy to start with so I'm guessing that's what's happened. I'm new to star testing. Conditions seem good at the moment though. 

Edited October 6, 2020 by Rustang

[John]

I agree with Geoff - the diffraction rings are not always well defined by any means. Sometimes they are better defined on one side of focus and mushy on the other. Seeing conditions play quite a role in this and it's all to easy to think you have an optical issue when it's really the seeing playing around with you !

[Rustang]

13 minutes ago, John said:

I agree with Geoff - the diffraction rings are not always well defined by any means. Sometimes they are better defined on one side of focus and mushy on the other. Seeing conditions play quite a role in this and it's all to easy to think you have an optical issue when it's really the seeing playing around with you !

It definitely seems that was what was going on and seeing did get better which was when I started to notice the rings and as you say they were definitely more noticeable on the one side. I'm always learning!

 

Thanks guys 👍

[NGC 1502]

If your views when you’re in focus are good that’s all that really matters.  Easy to get hung up trying to see how good your optics are, perfect optics don’t exist so best not to fret about it 🙂

Ed.

[Adam J]

2 hours ago, Rustang said:

I had my second very short stint outside with my OO 8inch reflector. Had a quick nose at Saturn and jupiter and also thought while I'm out I would do a star test. My issue is, is that there didn't appear to be any rings to go by either side of focus!? 

You need a really bright star and a high power eyepeice or the rings will appear too small to make out forcing you further out of focus where they star blending together.  You need to be at about 200x mag...which maybe you where having been viewing planets.

Did you use a filter? Green is normally a good idea.

Poorly defined rings can be a sign of Spherical Aberation or just poor seeing, poorly cooled scope etc.

Adam

[Adam J]

1 hour ago, John said:

I agree with Geoff - the diffraction rings are not always well defined by any means. Sometimes they are better defined on one side of focus and mushy on the other. Seeing conditions play quite a role in this and it's all to easy to think you have an optical issue when it's really the seeing playing around with you !

Better definition to the rings on one side than the other would indicate spherical aberation (under / over correction). But the test is very sesitive so you will almost always see some SA.

Edited October 6, 2020 by Adam J

[John]

18 minutes ago, Adam J said:

Better definition to the rings on one side than the other would indicate spherical aberation (under / over correction). But the test is very sesitive so you will almost always see some SA.

If the test is done under excellent seeing conditions, at appropriate magnifications that might be the case. Do you really want to star the OP worrying about SA just now though, given the circumstances described in this thread ?

 

 

 

[Adam J]

 

59 minutes ago, John said:

If the test is done under excellent seeing conditions, at appropriate magnifications that might be the case. Do you really want to star the OP worrying about SA just now though, given the circumstances described in this thread ?

 

 

 

I dont want him to worry I just want him to be informed and was very careful to say that you will almost always see some SA. I chose to say this on the basis that if the OP is performing a star test he probably has some sort of technical interest in it. Indeed I usually find people are either doing it out of technical interest or because they already suspect something is not right with the scope and want confirmation.

However, I will expand on what I said further.

OP: Dont worry about Spherical Aberation (SA), star testing is very sensitive and so even very very good optics will still produce slightly less defined rings on one side of focus in comparision to the other, judging the degree of any aberation from a star test requires experiance and something to referance what you are seeing against. It was only my hope that you are better informed about what you are seeing and why it is not unexpected. If it was me that knowledge would make me worry less not more. 

3 hours ago, Rustang said:

It was a bit mushy to start with so I'm guessing that's what's happened. I'm new to star testing. Conditions seem good at the moment though. 

If you are inexperianced in star testing then the main things you are looking for are:

1) Are your rings round and complete, not Oval or triangular. (Astigmatisum / Pinching)

2) Are your rings concentric. (Collimation)

I own a copy if this book:

https://www.firstlightoptics.com/books/star-testing-astronomical-telescopes-a-manual-for-optical-evaluation-and-adjustment-second-edition-book.html

and highly recommend you get a copy if you are interested in star testing your scopes, along side making / purcasing an artifitial star, its very technical mind you so you will need to be so minded or it may send you to sleep lol.

From what you are saying your scope is performing well. So please dont worry.

Adam

Edited October 7, 2020 by Adam J

[Rustang]

I was interested in doing the star test to check collimation as the scopes new to me and I don't have a collimation tool at the moment (shouldn't have sold it!) It's an old Orion Optics scope, I'm not expecting the optics to be first class but I'm going to base my view on this point from last nights experience. For the first time ever I saw some faint details on Mars and the moon, well that just blew my mind and I've looked at the moon alot but not through this aperture or decently at x200 mag. I was very happy when I got to bed last night 😊

Edited October 7, 2020 by Rustang