The value in performing a star test

[Chris]

I do try and limit the number of videos a share on SGL as I don't wont to spam the site with the things! However, I thought this one might be worth sharing as a bit of a star testing primer.

If anyone spots any errors please let me know, I can always add a pinned comment with amendments etc, otherwise I'm wondering how many of us star test our scopes considering it can tell us so much? 

 

 

 

 

 

 

Edited December 4, 2022 by Chris

[vlaiv]

I was hoping for some real time shots of star testing done with planetary / guide camera? Maybe that would be worth adding.

Important part of star testing is knowing how to distinguish seeing and tube currents from the rest. Video of actual defocused star in different conditions might help there.

You should also make slightly more distinction between pinched optics and astigmatism.

Astigmatism can be inherent in the optics or caused by improperly supported mirror. That kind can't be fixed with adjustment of the clips. Similarly - pinched optics is more likely to show up as triangle in case of three clips used to hold mirror. Astigmatism due to sagging mirror will be worst when telescope is pointed horizontally (near horizon) and usually happens with larger / heavier mirrors

 

 

[Chris]

18 minutes ago, vlaiv said:

I was hoping for some real time shots of star testing done with planetary / guide camera? Maybe that would be worth adding.

24 minutes ago, vlaiv said:

Important part of star testing is knowing how to distinguish seeing and tube currents from the rest. Video of actual defocused star in different conditions might help there.

 I agree those would have been a real nice touches to have in the video. I guess I tend to be a little time limited uploading videos on a weekly basis e.g. I brushed up and made notes Friday night, recorded Saturday morning, edited Saturday evening/night, and made a thumbnail and uploaded Sunday morning. I was hoping I'd got around the tube current head scratcher by saying make sure your telescope is properly cooled but examples would have been good you're right. 

27 minutes ago, vlaiv said:

You should also make slightly more distinction between pinched optics and astigmatism.

Astigmatism can be inherent in the optics or caused by improperly supported mirror. That kind can't be fixed with adjustment of the clips. Similarly - pinched optics is more likely to show up as triangle in case of three clips used to hold mirror. Astigmatism due to sagging mirror will be worst when telescope is pointed horizontally (near horizon) and usually happens with larger / heavier mirrors

 

Yep good points, I tried adding text to the screen to clarify but more on this would have been good. 

Thanks for taking the time to give your feedback  

 

[Mr Spock]

I always star test. Other methods can get you close, but that last little tweak is what brings out the fine detail on planets and the moon. It also enables you to use higher powers, much like with better quality optics. 
The frequently heard 'you can't get over x200 in the UK' only applies to poor seeing. If your scope has quality optics and is well collimated, when those steady nights come along you can really push the magnification well beyond that.

On a night of good seeing my 12" is razor sharp at x380 (4mm eyepiece) and isn't even breaking into a sweat.

[Chris]

12 minutes ago, Mr Spock said:

I always star test. Other methods can get you close, but that last little tweak is what brings out the fine detail on planets and the moon. It also enables you to use higher powers, much like with better quality optics. 
The frequently heard 'you can't get over x200 in the UK' only applies to poor seeing. If your scope has quality optics and is well collimated, when those steady nights come along you can really push the magnification well beyond that.

On a night of good seeing my 12" is razor sharp at x380 (4mm eyepiece) and isn't even breaking into a sweat.

Good to hear Michael, sharp images at 380x mag is very impressive! 

[Peter Drew]

Star testing is very valuable for determining the presence of broad optical defects such as collimation, spherical aberration, astigmatism and mechanically induced distortions.  The finer points and marginal assessment call for significant experience.     🙂  

[Skyline]

This is a very good book on Star Testing, also available at a well-known  online bookstore.

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

[Astrobits]

FWIW I was once told that Horace Dall ( he of the Dall-Kirkham, Dall null test, Dall tube ( for fluid flow, nothing to do with astronomy ) and probably many other inventions ) would not bother going out to his 15" D-K unless the night would allow the use of a magnification of 1000. I knew Horace and have no reason to doubt this.

Nigel

[Peter Drew]

I remember the 15" D-K,  not easy to get magnifications much less than 1000x with a 15" Cassegrain!.  I wonder where that telescope is these days, it was for a time in the care of Luton AS.    🙂

[Karl Perera]

Interesting discussion this, I must look into this more because I have never ever done this.