Finder FOV?

[acey]

What's the true field of view of the SkyWatcher 9x50 finderscope (as fitted on Flextube etc?).

[Merlin66]

Should be around 5.1 degrees. Depends on the eyepiece design, but certainly close to the 5 degree mark.

Ken

[acey]

Hmm, just been googling and found Orion (US) 5.5 degrees, Celestron 5.8 deg.

[Merlin66]

Are they the same spec?

ie 10 x 50, 9 x 50, 8 x 50... some finders don't even quote the correct magnification.

The FOV in degrees is roughly the aperture/ magnification, so a 10 x 50 would be 5 degrees etc

A quick and easy way is to measure the diameter of the bright exit pupil disk with a transparent ruler... the magnifcation = aperture/ exit pupil ie a x 10, 50mm finder would have a exit pupil of 5mm diameter.

The actual FOV is the Apparent FOV of the eyepiece/ magnification, so if you have a Kellner with a 50 degree AFOV and a magnification of x 10 then the FOV = 50/10 = 5 degrees

So, it comes down to the real magnification and the type of eyepiece......

Ken

[acey]

They're all 9x50 and I thought the Orion and Skywatcher were both made by Synta, in which case it would be 5.5 degrees. But of course, quoted FOV and actual FOV often aren't the same thing so maybe 5.5 in theory means 5.1 in practice.

I'd been hoping to avoid measuring it for myself but guess I'll do it from a star transit some time - if I ever get to see any stars....