Reflector contrast

[Mike_S]

Hi, I can’t find many comparisons online between the contrast difference sueina visual observing through refractors and reflectors. I am not sure if it us more if an issue for AP which I will not be doing. Can anybody help please?

[Demonperformer]

As I understand it, the main thing is that a refractor has no obstruction in the light path, whereas all reflectors have a secondary that stops some of the light getting in and "disturbing" some of the light that does (like newtonians producing spikes around stars) ... but I may be wrong.

[Rob]

2 hours ago, Demonperformer said:

As I understand it, the main thing is that a refractor has no obstruction in the light path, whereas all reflectors have a secondary that stops some of the light getting in and "disturbing" some of the light that does (like newtonians producing spikes around stars) ... but I may be wrong.

You are right here. The reflectors secondary can vary in size. A larger secondary mirror, in say an f5 would favor AP. However you can of course get a longer focal length Reflector (say f6 or more) and have a smaller secondary that does not impact the contrast!.

I had this with my then OO SPX 200. F6. Really gave the Refractors a run for their money that one!

[Mike_S]

So faster scopes suffer from the contrast problem more? Is it filterable?

[Rob]

'Suffer' I feel is a little extreme to describe it!. However you may be able to change the secondary to a smaller version. I would do your home work on this first Orion Optics UK are always happy to discuss / supply also!. That said they are also not the only supplier of mirrors!. Shop around.

Rob

[Rob]

Oh And TS offer them (and advice)

https://www.teleskop-express.de/shop/index.php/cat/c91_Telescope-Making---Optical-parts.html

 

[andrew s]

Yes a central obstruction reduces contrast but so does smaller aperture! 

What counts is the net performance normally measured in term of the modulation transfer function MTF.  In cost terms a large well designed reflector will have a higher MTF than a similarly priced (hence smaller aperture) reflector.

In the real world seeing can dominate. This can have two main effects. 1) shifting the image about and 2) distorting the image. The switch over occurs roughly at the point where the scale of the seeing matches the diameter of the telescope. Telescopes smaller than the seeing scale get shifted images and larger distorted images.

The seeing scale is typically 300mm so that is often why small refractors  are preferred to large reflectors for visual work.

Regards Andrew 

[Mike_S]

Sorry can you explain more about that seeing scale please and the 300mm? That is interesting but it lost me.

[LukeSkywatcher]

Would I be right in thinking a parabolic mirror in a Newt would go some way in compensating for a central obstruction. Parabolic mirror apparently concentrates light better than a spherical mirror. I've never noticed any difference between Newt and Frac. My Fracs are at best half the aperture of my 8se. I also have a 130p Heritage. 

[andrew s]

Seing scale is the size of the "average" cells in the air with differing density and hence refractive index. If your scope is smaller than them you tend to be looking through just one at a time and as they move they shift the image but don't distort it much. In a big scope you are looking through several at a time and this give a more stationary image but blurred.

300mm is the typical size of seeing cells in the UK if my memory is ok but obviously it varies with the seeing conditions.

The above is a gross simplification. Google may do better.

A parabolic mirror is better than a spherical one in a Newtonian configuration but the same size central obstruction will cause the same amount of reduction in MTF in both. 

Regards Andrew 

 

 

[andrew s]

As an aside, adaptive optics like the SX ao unit are based on a shifting image and tilting a glass prism to shift it back. While the big boy pro ao units compensate for a blurring image by measuring and correcting the wavefront error using a deformable mirror at least for a section of the image.

Regards Andrew

[Louis D]

No one has mentioned it that I can find, but there are unobstructed reflectors.  The simplest method is to make an off axis aperture mask and place it between two secondary vanes.  The vanes themselves also contribute to decreasing MTF as well.

If you can get the central obstruction below about 20% by diameter, the contrast decreasing effect can be minimized.  It's still there, but it is harder to distinguish visually.

[andrew s]

5 minutes ago, Louis D said:

No one has mentioned it that I can find, but there are unobstructed reflectors.  The simplest method is to make an off axis aperture mask and place it between two secondary vanes.  The vanes themselves also contribute to decreasing MTF as well.

If you can get the central obstruction below about 20% by diameter, the contrast decreasing effect can be minimized.  It's still there, but it is harder to distinguish visually.

Any real off axis mask will reduce the MTF more than the combined effects of the central obstruction and support vanes as it will have to fit in the gaps between them. Yes it will remove the diffraction spikes from the vanes. 

It also tilts the field wrt the original optic axis which can be an issue at high focal ratios and with Fabry Perot etalons e.g.  solar H alpha filters.

Regards Andrew

[Ricochet]

While the effect of a central obstruction on contrast might be a valid concern when choosing between a reflector and a premium apochromat I'm not sure it is worth worrying about in this case. Previous threads have been discussing a Celestron Astromaster where the chromatic aberration, lack of baffles and reflective not-so-black internals are likely to undo any contrast advantage the scope might have through not having a central obstruction. 

[Louis D]

2 hours ago, andrew s said:

Any real off axis mask will reduce the MTF more than the combined effects of the central obstruction and support vanes as it will have to fit in the gaps between them.

Reduced relative to a normal obstructed newtonian of the same diameter as the off axis mask?  I find that hard to believe.

[newbie alert]

To make it easier on the op... a refractor uses lenses..and if you can imagine the curved lense bringing the light to focus to a point at the ep or camera sensor..a reflector uses mirrors..newtonian,sct are types of reflectors and the distance the light travels within the tube is the focal length..so a big dob (also a newtionion) can have a shorter fl than a shorter in tube length sct,because of the length the light travels

What are you wanting to use it for?

 

[andrew s]

1 hour ago, Louis D said:

Reduced relative to a normal obstructed newtonian of the same diameter as the off axis mask?  I find that hard to believe.

No reduced relative to the original Newtonian I.e. with its central obstruction and vanes. By far the major input to the MTF  is the aperture of the telescope. Adding a mask to a telescope will reduce it's MTF although it may add other positive attributes e.g. removing diffraction spikes.

Regards Andrew