Optics Quality, Telephoto Lens Vs Telescope, which is better for DSO astrophotography

[Ed Galea]

Hi Everyone. I'm a beginner so have a beginner question, which may have been previously answered, so hope you will forgive me if that is the case. 

I'm trying to determine which would be better for DSO astrophotography, a telephoto/zoom lens or a telescope? I'm just interested in the optics, not other issues such as cost (other issues will mitigate cost issues) or weight.  I simply would like to know, for a given focal length and aperture size (hence f number), which would be better, a telephoto lens or a telescope, or is there no difference (forgetting price and weight)?

By better, I mean which would, (a) produce the better-quality images and (b) require the least amount of time to produce a given quality image. The reason I ask is that a telephoto/zoom lens will have lots of lenses in the OT, for example 11 lenses in 7 groups, while a telescope would have perhaps 4 lenses in 3 groups. With the light going through so much more glass in the telephoto/zoom lens, does this effectively slow-down the lens compared to an equivalent telescope i.e., require longer exposure times, even though the same f number, and at the same time, does it potentially improve the quality of the image? 

For arugments sake, compare a quality 200mm F/2 telephoto/zoom lens with a quality 200mm F/2 telescope.

[vlaiv]

There is no such thing as quality 200mm F/2 telescope.

Telescope will optically be always better than lens. Telescopes are diffraction limited (not always) and lenses are not.

When you start making telescopes very fast - you start lowering their optical quality. Take for example RASA at F/2 - it is no longer diffraction limited telescope - in fact it has at least twice lower resolution than aperture would suggest.

Speed is not reflected in F/number alone. Sometimes F/6 telescope can be faster than F/4 telescope and even F/2 telescope.

Anyway - it is complex stuff and for the question you asked - telescope wins. Problem is - even if you just take two parameters you listed - optical quality and speed - you don't get the full picture.

I would argue that my 8" F/8 telescope wins over any lens in two categories you mentioned - but only over very tiny field of view.  As soon as you start wanting bigger FOV - then my telescope might not necessarily win compared to some lens.

[Alien 13]

There is a lot of glass in a lens compared to a scope although a true APO scope can have 5 or 6 elements but the important bit is the quality of the coatings that help to reduce artifacts and improve transmissibility so going on f/ratio alone is not the complete answer as mentioned earlier (one reason video lenses use T stops rather than f stops).

The choice should realy be made by the target type you want to image and your camera sensor specifications not forgetting of course how portable you want the whole setup to be.

Alan

[vlaiv]

Thing with lenses is that they are often optimized for terrestrial use.

Macro lenses are optimized for very near focus while most lenses are optimized for "medium" distances. There are good portrait lenses (optimized for relatively close but not as close as macro) and so on...

None of them are optimized for infinite focus as scopes are.

With scopes, when you image close targets - you introduce a lot of spherical aberration. Similarly, it is hard to make lens with large focus range and good spherical correction.

Take for example Samyang 135mm F/2 lens. Often used in astrophotography and regarded as very good and sharp lens:

This is MTF at F/8 and 30 lines/mm. 30 lines / mm equates to pixel size of 1000µm / 30 = 33µm.

At F/8 and 135mm of focal length, equivalent telescope would require 2µm pixel size for critical sampling.

At F/2 - this lens is supposed to have same (or similar) performance:

At that F/ratio - diffraction limited telescope of same aperture and focal length would need 0.5µm pixel size for critical sampling - that is x60 smaller pixel size!