Nikon Full-Frame Astro DSLR

On Tuesday, February 17, 2015 at 11:33:48 AM UTC-5, Chris L Peterson wrote:
On Tue, 17 Feb 2015 08:30:08 -0800 (PST), wsnell01 wrote:

Here's the bottom line. Unless you're looking at something directly
related to numerical aperture, such as the bandwidth of narrowband
interference filters, there's nothing useful you can do knowing just
the focal ratio.

Strawman argument

Every single important metric of telescope
performance utilizes the aperture and the focal length.

You seem to have a mental block here, peterson. If we know any two of aperture, focal length and focal ratio, we can derive the unknown. Aperture gathers more light, focal length provides more resolution up to a point, and low focal ratio contributes gives a better/signal to noise ratio. That better signal-to-noise separates the faint stuff from the background.

If you want to consider focal ratio important, go ahead.

When making untracked astrophotos it most CERTAINLY IS!

It is obviously important according to this, which you do not seem to comprehend:

http://www.stark-labs.com/help/blog/...ioAperture.php

In each case shown, the shorter focal ratio wins, AEBE.

Those of us
who prefer a more rigorous approach to analysis will not.

I would hardly call your approach "rigorous." You seek merely to optimize resolution, without taking other aspects of the image and the process of acquiring the image into consideration.

On Monday, February 16, 2015 at 10:46:05 AM UTC-6, Chris L Peterson wrote:
On Mon, 16 Feb 2015 08:18:55 -0800 (PST), wrote:

You missed the part about NO tracking for either scope. You answered some other question, not the one I asked.

No, I didn't. There simply wasn't enough information to fully use that
information. Depending on the pixel scale, neither scope could have
produced significant trailing, or both could have produced so much
trailing as to make imaging impossible.

The reduced trailing of the short focal length scope is a factor to
consider, but does not by itself make that scope the best choice.

The proper way to decide which scope is best is the way I suggested.

In real terms (i.e. arc seconds) the trailing is the same for either focal ratio. What you see in the final image will be the same because the trailing as a percentage of the comet's size will be the same for any given exposure time.

Since the pixel size was not given, then I can give an example. An F5 scope with a 5 micron pixel CCD will show the same trailing as an F15 scope with a 15 micron pixel CCD. Both systems will have the same resolution. There will probably be an advantage to the 15 micron system because larger pixels tend to have higher signal/noise ratios.

Where the F5 system will no doubt have an advantage is on field of view. The F15 would need to be 3 times bigger for the same real field size (in degrees).

Uncapixel