Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
RustierOne
RustierOne Veteran Member • Posts: 4,314
Re: Another Interesting Al Nagler Article

kiwi2 wrote:

RustierOne wrote:

In view of all the recent discussion about how f-ratio affects image brightness, you might enjoy this brief article by Al Nagler (of Televue fame):

The Joys of Low-power Viewing

An interesting quotation from that :
"f/# Considerations

A fast f/# telescope objective will produce the same brightness as a slow telescope with the same aperture and magnification. For visual observing, the fast f/# implies a short f.l., which does give the most field. Visually, therefore, we can say that a "faster" scope has more potential field, but no more image brightness than a slow scope of the same aperture!"

The emphasis (underlining) has been added by myself. It seems that Al's viewpoint and understanding has weight in our discussion since he's a respected optical engineer and telescope user.

Interesting, yeah?

'If at the same magnification' would be the trick. Say your 80mm was a f/8 with a 640mm focal length. The 40mm eyepieces would now give you 16 x magnification and a exit pupil of 5mm. It would be hard to match that really low power and wide field of view you are getting with the f/5 scope.

Yeah, it's interesting how it can work out:

  • 80mm, f/5
  • Focal length = 400mm
  • 25mm eyepiece = 16X
  • Exit pupil = 5mm

Of course these eyepieces might produce different FOVs. But if they both have the same field stop, say 28mm, then the FOV would be different:

  • 80mm, f/5 (400mm FL), then FOV = ~4.0°
  • 80mm, f/8 (640mm FL), then FOV = ~2.5°

Yes, for sure the F/5 'scope does give a nice wide FOV. It's interesting the same power gives a different FOV depending on the FL (i.e. image scale). Of course this is visual. For photography, I'll exit the discussion for more experienced photographers.

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Best Regards,
Russ

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