Image sharpness of refractor vs reflector

[Planetarian]

I'm just a beginner, but looking at many photos and videos I have the impression that a refractor telescope can produce a little sharper, more crisp image, than a reflector one. 

Could this be just because some people don't collimate their reflectors well or is there really a difference in sharpness of the two types, let's say, with the same aperture and magnification? (I know there are many other factors like the material and coating of the objectives etc. )

[Pixies]

The obstruction caused by the secondary mirror in a newtonian (and Mak, SCT, etc.) affects the contrast of the image. 

Maths here: http://www.astrophoto.fr/obstruction.html

[johninderby]

Nor quite as simple as the difference in contrast and sharpness between a refractor and reflector as the aperture has a big effect in the quality of the views and of course seeing on any given night. Probaby why many own both types.😁

Edited August 24, 2021 by johninderby

[Sunshine]

Someone here had a funny cartoon about this comparison on another thread, maybe it will pop up again.

[Peter Drew]

Size for size a refractor will give a crisper image than a reflector.  But, refractors are usually restricted to small sizes, typically 6" and less for the reasons always listed so you have to be sure as to what type of observation you wish to make in the main.  A 6" F15 Newtonian would probably give a F15 refractor good competition.

[michael.h.f.wilkinson]

This is one of those "it depends" issues. A top of the line apochromatic refractor should beat a well-figured, well collimated Newtonian reflector of the same aperture, although things will get very close for a slow Newtonian with small central obstruction. I had a 1/10 lambda 6" F/8 with 24% central obstruction that would give many a refractor a serious run for its money. However, a 6" F/8 achromat will show chromatic aberration, and a 6" F/5 achromatic refractor will be very much worse on planets than the aforementioned 6" F/8 (or even F/5) Newtonian. The price of a 6" F/8 apochromatic triplet refractor is roughly 10  times that of an 6" F/8 Newtonian. For the price of the 6" apochromatic triplet you could easily buy a much bigger Newtonian (10" or 12") with quality mirrors, and under good seeing they would be much sharper  on planets.

[Planetarian]

After your comments, as I see, for a portable one the best could be an apo. refractor while for home use a large Newtonian or Dobsonian. For the most contrast/sharpness.

Thanks for the replies!

[Zermelo]

Many have agreed with that conclusion, even if not all of them have had the pennies to act on it 🙂

[paulastro]

Some good comments here, so I have nothing to add to the responses re the original question myself. 

However, I would like to add  that the sharpest of two images isn't necessarily 'better'.  

A view of Saturn with a 12 inch reflector may not be as sharp as the view with a smaller apo refractor, but on a reasonable night of seeing it will have much  better colour rendition and better resolution of detail, and will capture fainter satellites than the smaller apo.

Which is the  'better'  image is always going to be subjective.  I like nice pin sharp star images in an apo, but also I like the more vivid  true colours of a larger reflector - particularly when observing Jupiter,  Saturn, and the brighter nebula.

It would be mischievous of me to suggest that perhaps this is the reason why so many owners of apos often crack on about how sharp their telescopes are when using ridiculously high powers - so I won't suggest it 😁.

As johninderby suggests - buy an apo and a reflector.  Perhaps a 60mm fluorite doublet for £1250 and an eight inch reflector for around £350?  😊.

 

[doublevodka]

This debate has gone on for years and I'm sure will go on for many more 😉

Quality of optics, seeing conditions, accuracy of mount and what you are trying to do can all have an effect

Doing visual, then as big a newtonian as you can get away with

Imaging, APO, SCT or RASA seem to be the most popular, newtonians less so

Personally I like the sentiment "the best telescope is the one you actually get out and use"