Best DSLR or ILC for Night Sky Imaging?

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

RustierOne wrote:

1llusive 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.

This actually doesn't make sense unless he is talking about magnification with an eyepiece, such as the following.

Yes, he is talking viewing with an eyepiece. Really an eyepiece is in essence just a small microscope that allows close inspection of the image formed by the telescope objective lens/mirror.

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.

The problem with letting the focal length do the work is that it it's easier to change eyepiece focal length.

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?

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.

Very true. Al's discussion is just about visual observing. After all Televue sells eyepieces, not cameras.

There is a bit of focal plane manipulation with a sensor cropping the FOV or extending the FOV with full-frame sensor.

I don't know how this fits in with all the spirited discussion here. But personally once I have chosen my telescope (with its full aperture) I do adjust the focal length (reducer or Barlow) to get the FOV needed with my sensor size. I don't fuss a lot as to whether my f-ratio is f/10 or f/6.3. What matters to me is:

  • Aperture (diameter) of the telescope's objective mirror/lens
    - what gathers the light
  • Field of view, as set by the adjusted focal length, modified by sensor size

Whether I got the FOV via a f/5 8-inch Newtonian reflector or a f/6.3 8-inch SCT doesn't matter to me.

Having said that, I acknowledge that not all people view things the same as I do. And in one sense it doesn't really matter - we just post the results of our imaging. As they say, YMMV.

I suppose it depends on your uses. For visual, the demands for perfection are less. But for photography, f-ratio definitely matters, and not just for the reasons discussed here. Things like depth of the focus field and for refractors, higher f-ratio telescopes tend to be better corrected with regard to chromatic aberrations (we see this also in lens reviews, where CA disappears upon stopping down). This is one of the reasons, along with focal length, that planetary refracting telescopes have high f-ratios. For the real discerning types, seeing color fringing on a planet or on the moon is a non-starter. It's possible to design a CA-free refracting optic with a fast f-ratio, but it's a lot more expensive. So you tend to see better image quality from refracting lenses and telescopes with higher f-ratios unless you have the money for a Takahashi.

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