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Does RMS change with guidescope focal length

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#1 jseivert

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Posted 12 July 2018 - 08:33 AM

Just wondering out loud here:

I normally guide with the Orion Mini Guidescope at 162mm.  I've gotten about 1.2" RMS numbers consistently for a long time.  I'm wondering if nothing else were to change except the focal length of the guidescope, say doubled to 324mm, would the RMS value show .6"?   (PHD would have the appropriate numbers)

 

Is RMS similar to a percentage and wouldn't change?  e.g.  8 of 10 is 80% and 80 of 100 is also 80% 

 

Thanks for any insight!!

 

Jeff

 



#2 rgsalinger

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Posted 12 July 2018 - 09:53 AM

The calculation is independent of focal length. When PHD measures the error, it's in Pixels. That gets converted to arc seconds using a simple formula. If you double the focal length then you halve that relationship, leaving yo in the same place.

 

Now, if you are at - say  - more than 3 arc seconds per pixel, well then the guiding might actually improve and the RMS along with it. It's a good idea to keep the image scale of the guide camera reasonably close to the image scale of the telescope. That's one reason why OAG's are popular with long focal length scopes. That way the software has a few pixels to work with which is doesn't when you have 3 arc second seeing and a 3 arc second image scale.



#3 jseivert

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Posted 12 July 2018 - 12:53 PM

Thanks for the info!  I just ran across these posts in a similar thread that helped quite a bit. 

 

https://www.cloudyni...ngth/?p=6490909

 

Jeff




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