True focal length

[John Fisher]

Hi, I I’m having plate solve failures. I have a celestron 9.25 2350 fl,  f10, using a 6.3 reducer and have been entering 1480 as my focal length in asiair. Failure every time. I’ve heard that isn’t necessarily correct? Anyone with the same problem? What’s the fix? Thanks jk

[Bryan]

The focal length of a telescope with a movable mirror, e.g. SCT, will vary somewhat as the mirror moves.

Submit one of your images to astrometry.net online and it will tell you the actual field of view, from which you can calculate the focal length by re-arranging this

Field of view in arc minutes = (width of chip in mm * 3460) / (focal length of optic in mm)

That said, 1480 should be close enough.  You may need to look elsewhere to find the reason for the failure.

Bryan

[Bryan]

I did some more research.  I learned that the change is actually significant. 

 

Roughly, 3.1 mm focal length for every 1 mm change in mirror movement

 

https://www.cloudynights.com/topic/724682-back-focus-of-sct/

 

https://www.cloudynights.com/uploads/monthly_10_2014/post-70772-0-37566700-1413089368.jpeg

 

Bryan

[kenm.h...@gmail.com]

The actual focal length and subsequent focal ratio of an SCT with and without reducers changes with the back focus spacing.

I use a f10, C11 for spectroscopy, but when the spectrograph etc is mounted on the scope, the focal ratio becomes f11!!!!

The attached spreadsheet may help.

Ken

[John Murrell]

The image quality also falls drastically as you move the mirror from the designed focal position see the paper on the Celestron Edge on the Celestron website and various web sites (There was a good S&T article about it but I don't have access to the archive) . Even a fraction of a millimetre makes a significant difference to the off axis quality. The Celestron Edge paper quotes a distance for the optimum rear focal position which I presume is common to other Celestron SCTs.

Making the back focus adjustable over a large range is good for user convenience but bad for imaging quality

John Murrell

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