Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
1llusive
1llusive Senior Member • Posts: 1,571
Re: Another Interesting Al Nagler Article

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.

This actually doesn't make sense unless he is talking about magnification with an eyepiece, such as the following. Aperture, focal length, f-ratio:

100mm, 500mm, f/5 <-- "slow"
100mm, 400mm, f/4 <-- "fast"

What he is supposedly saying is that with a higher magnification eyepiece on the "fast" telescope, the image brightness would end up being the same as the "slow" telescope. That would make sense, though results should be better if you let the focal length of the telescope do the work rather than the eyepiece.

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?

-- hide signature --

Best Regards,
Russ

I think this is interesting, but perhaps doesn't apply to long-exposure photography because the discussion/equation ends as soon as that light hits the sensor plane, in contrast to the further manipulation of the image scale done by the eyepiece of a visual observer.

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