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True Focal length of NexStar 6SE

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#1 psuaero

psuaero

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Posted 03 July 2020 - 11:52 AM

I thought my 6SE had a focal length of 1500 mm (150 mm aperture and f/10 ratio) but last night I was setting up NINA and ASTAP and one or the other warned me that my focal length was off based on the solved image. Has anyone else had a similar experience with plate solving? The same thing occurred when I had the Celestron f/6.3 focal reducer on the scope.

 

Celestron specifications and Astronomy Tool Field of View Calculator:

  • 6SE w/ f6.3 focal reducer FL=945 mm
  • 6SE without reducer         FL=1500 mm

 

NINA/ASTAP calculated my focal length from the plate solved images as:

  • 6SE w/ f6.3 focal reducer FL=1053 mm
  • 6SE without reducer         FL=1648 mm

 

Equipment and software:

  • Celestron NexStar 6SE
  • Celestron f/6.3 Reducer/Corrector
  • Canon T2i (EOS 550d) - unmodded
  • NINA v1.9
  • ASTAP v0.9


#2 WarmWeatherGuy

WarmWeatherGuy

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Posted 03 July 2020 - 12:52 PM

SCTs change focal length when you turn the focus knob. If you focus without a diagonal you will get a different effective focal length than when you focus with a diagonal because the focus knob has been turned.




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