Best DSLR or ILC for Night Sky Imaging?

Started Nov 8, 2017 | Questions thread
kiwi2 Veteran Member • Posts: 4,398
Re: If you want the best, a dedicated cooled CCD cameras are far better than all the DSLRs and ILCs

Trollmannx wrote:

kiwi2 wrote:

1llusive wrote:

As aperture increases on telescopes, we see more detail.


Yes larger apertures on telescopes usually also come with longer focal lengths so you get more magnification and more resolution. Less diffraction due to a greater area of light relative to the edge.

Not quite so - aperture determines light grasp and resolution.

Faster telescopes (lower f/ratios) collect the same amount of light as similarly sized slower telescopes (higher f/ratios). Aperture is the key.

Resolution is diffraction limited. The aperture of the telescope determines resolution, not the focal ratio (f/number).

For visual observing, it still also comes back down to f-ratio to how bright a telescope is for diffuse objects like nebulas and galaxies. A bigger telescope just gives you more magnification for the same brightness relative to its f-ratio. Which is why you get to see more detail in small dim galaxies with the bigger telescope you have.

For visual use f/ratio is irrellevant.

For photographic use take a look at plate scale, extended objects, etendue and point sources of light...

Visual use:

Use different eyepieces with telescopes having similar aperture (we are talking telescopes here so aperture is the diameter of the objective lens or mirror) but different f/ratios to get the same magnification. What happens then?

But visual observers of such deep space objects wouldn't want to use a large f/16 refractor that was built for planetary observation. Deep space subjects would appear too dim in it.


They would rather use big newtonians between f/4 to f/5 like these...

This is more about price and portability than anything else...

So much wrong here I don't know where to start.

The focal length of the telescope determines the magnification. Just as the focal length on a camera lens does. Magnification for a telescope is the telescope's focal length divided by the eyepiece focal length.

So let's run the numbers over a couple of different telescope f-speeds.

Take the 24" f/4 scope from here.

It has a focal length of 2540mm.

A 40mm eyepiece is about the lowest magnification widest field of view available. So 2540 divided by 40 = 63 times magnification. A nice low power that keeps nebula and galaxies looking bright. (ignoring if this too low a magnification for the scope and exit pupil is larger than 8mm and wasting light, for now)

Then take the Great Lick Refractor as another example. A 36" f/19 scope with a focal length of 17,630mm. Drop in that 40mm eyepiece and you get 440 times magnification. The more you magnify something the dimmer it gets. 440 times magnification being the lowest power this telescope could do is still far too much for a lot of deep space objects.

But of course we already knew that just by knowing it was a f/19 scope. Even though it has a larger aperture, its longer focal length drives too much magnification and diffuse objects will look dimmer. Lunar and planets and resolving binary stars is what this telescope would be better at, as they are bright objects and benefit from the higher magnification.

You cannot solely look at aperture alone and ignore the f-ratio when choosing a telescope.

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