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New guide scope, can't get precise focus

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#1 petercoxphoto

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Posted 07 August 2020 - 06:54 PM

Hi guys -

I replaced my ST80 guidescope with a shorter, faster one from Teleskop Express - this one: https://www.teleskop....php/info/p6383

 

It arrived today and I fitted it with my ASI120MM and was disappointed to find that I just can't get sharp stars with it. They are misshapen and assymetrical. Here's a link to a short video I took showing my focus procedure - I'm sliding the focus barrel in and out during the video by tiny amounts at a time, passing through the point of best focus to the other side and back again. You can see the point of best focus just isn't good at all.

 

https://www.dropbox...._focus.mp4?dl=0

 

I know guidescopes don't tend to be great in terms of image quality, but this seems excessive?

 

Thoughts welcome!

 

 

Cheers,
Peter 



#2 bobzeq25

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Posted 07 August 2020 - 07:03 PM

Are you tracking when you do that?  If not, it's to be expected.

 

For the very best focus, get it close enough to engage autoguiding.  _Then_ focus.

 

I know getting another guidescope is very unappealing.  But you should know this, nevertheless.  It may help you. 

 

This is why people like guidescopes with helical focusers in addition to the sliding tube.  You get focus close with the sliding tube.  Autoguide.  Tweak focus with the helical focuser.  I'm making movements of maybe 10 degrees rotation when doing that.  Or less.


Edited by bobzeq25, 07 August 2020 - 07:08 PM.


#3 petercoxphoto

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Posted 07 August 2020 - 07:08 PM

Hi Bob - yes, I'm tracking while doing that. The problem really seems to be some sort of aberration in the scope itself. I've played with this a lot and simply can't do better than the video. 

 


Cheers,
Peter


Edited by petercoxphoto, 07 August 2020 - 07:09 PM.


#4 KTAZ

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Posted 07 August 2020 - 07:12 PM

That unit has a back focus of 36mm...have you measured the distance to your chip? You might need to back it out a bit if it is too close.



#5 petercoxphoto

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Posted 07 August 2020 - 07:14 PM

It's not a backfocus problem. If you look at the video you'll see I'm passing through the zone of best focus and out the other side. I can get focus, it's just really poor. 


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#6 Readerp

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Posted 07 August 2020 - 08:13 PM

Looks like tilted optics




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