catadioptric OTA mirror obstruction

[bomberbaz]

I feel I should know the answer to this query but I have been thinking about it a bit recently and realised I don't have a grasp that I fully understand.

So can someone clean up a small area of confusion here for me. When these types of scopes are advertised for sale they are frequently quoted as having two levels of obstruction.

Secondary mirror obstruction by diameter or Secondary mirror obstruction by area.  Typically circa 30% and 10%, give or take a few %. 

My slight confusion surrounds which one is the correct one or is it a little of both.  That is to say which one is the actual area of obstruction. Area surely. 

However I also seem to remember reading somewhere that there is a little loss of light gathering around the outer edge due to the corrector lens housing or something like that.

Finally just how much of an effect does the obstruction have on the contrast of the view. I know this last matter is very subjective but I value other peoples opinions otherwise how are we to learn and know what to look for and/or expect.

 

 

[andrew s]

Both are correct. The by area gives a feel for how much light is blocked. The by diameter is related to the impact on resolution. They are related by simple geometry and the size of the telescope and secondary diameters.

Regards Andrew 

Edited June 29, 2020 by andrew s

[Peter Drew]

To answer the second part of the question, the intensity of the Airy disc governs the contrast, refractors, with no central obstruction have the most intense discs.  Central obstructions rob the intensity of the Airy disc and deposits it into the diffraction rings lowering the intensity and of course the contrast.  The larger the obstruction as a percentage of the aperture the lower the contrast.  It's generally accepted that 20% or less obstruction causes no noticeable affect.       🙂

[bomberbaz]

4 minutes ago, Peter Drew said:

To answer the second part of the question, the intensity of the Airy disc governs the contrast, refractors, with no central obstruction have the most intense discs.  Central obstructions rob the intensity of the Airy disc and deposits it into the diffraction rings lowering the intensity and of course the contrast.  The larger the obstruction as a percentage of the aperture the lower the contrast.  It's generally accepted that 20% or less obstruction causes no noticeable affect.       🙂

Brill and thanks peter, but when you say 20%, which one are we using, area or diameter. One great, the other not so good.

See why I get confuddled now 🤣

[andrew s]

1 minute ago, Peter Drew said:

To answer the second part of the question, the intensity of the Airy disc governs the contrast, refractors, with no central obstruction have the most intense discs.  Central obstructions rob the intensity of the Airy disc and deposits it into the diffraction rings lowering the intensity and of course the contrast.  The larger the obstruction as a percentage of the aperture the lower the contrast.  It's generally accepted that 20% or less obstruction causes no noticeable affect.       🙂

...but, for the same money you can get an obstructed SCT or Newtonian with a bigger aperture who's Airy disk is much more intense than the unobstructed refractor. 

Regards Andrew 

PS sorry but it's a hobby horse of mine! 😜

[Peter Drew]

Just now, andrew s said:

...but, for the same money you can get an obstructed SCT or Newtonian with a bigger aperture who's Airy disk is much more intense than the unobstructed refractor. 

Regards Andrew 

PS sorry but it's a hobby horse of mine! 😜

And mine!     

 

2 minutes ago, bomberbaz said:

Brill and thanks peter, but when you say 20%, which one are we using, area or diameter. One great, the other not so good.

See why I get confuddled now 🤣

Diameter.     🙂

[johninderby]

I think too many get hung up on the effect of central obstruction instead of concenrating on how good the views are with any particular scope. 

One factor to consider with say an SCT is how well collimated it is? Refractors are usually well collimated whereas many SCTs aren’t.

Edited June 29, 2020 by johninderby

[bomberbaz]

Ok I think I get it but footies on tv now so will reread it later.  Thanks all. 

[Alkaid]

This is a pretty general statement, but on high contrast detailed observing, (think Moon's terminator & crater shadows, Saturns Rings) the central obstruction does not have much effect.   However, on low contrast objects (separating reds and browns on Jupiter, albedo markings on Mars) then the effect of the obstruction really does take hold.  There's a few old threads on this subject and I think that the general consensus was that if you subtract the diameter of the obstruction from the aperture, you end up with an equivalent scope without obstruction.    

For example, my 8" SCT is "very good" (to me!) on the Moon and allows me to see everything that that aperture should, seeing permitted.  But if I then went for Jupiter and started trying to separate the reds and browns, my scope would behave more like this..... 8" aperture minus 2" obstruction = 6" refractor.

It's an interesting subject.

[andrew s]

@Alkaidhe best discussion on this I know is by Thierry Legault, a world class astro photographer, here http://www.astrophoto.fr/technique.html

Regards Andrew

[vlaiv]

There is quite a bit of difference with effects of CO on imaging and on observing.

For imaging it is not as nearly as important as for observing. We can and do digitally enhance captured images (contrast and deconvolution / sharpening). We can't do that while observing.

In fact, for very technical reasons, I think it is better to image with obstructed aperture than clear one (of the same diameter).

As a final note, I don't think people really understand what sort of contrast we are talking about here as it is frequency dependent contrast and it is only partially related to notion of contrast that we use in everyday life.

[jetstream]

My "best newts" have 19%-21% obstruction but they also have vg optics on vg mirror cells that will cool and collimate easily. I've played around with a truly non scientific experiment, going from 19% to 37% obstruction using various scopes. Up to about 25% all seems well and 30% and up shows contrast loss on the moon, on certain size features.

All optics have .91 Strehl or higher.

[michael.h.f.wilkinson]

I had a slow Newtonian with very good mirrors (1/10 lambda) and small central obstruction (23 % or so) and replaced that with my current main scope: the Celestron C8 with 35% CO. The latter definitely gives me more detail on planets, because although at the same exit pupil the contrast is a bit lower, the actual maximum magnification is higher, so more detail can be seen. At the same magnification, the C8 gives a distinctly brighter, crisper image. The lack of diffraction spikes gives another little advantage to the C8 (and the SN6 I now have for wide-field)

[Louis D]

In my experience, central obstructions can also make it more difficult to spot dim companions next to bright stars, as in the E and F components of the Trapezium, than in an equivalent or even smaller aperture unobstructed system.  The obstruction makes the bright stars bloat so much that you just can't make out the dim companion stars no matter how good the seeing conditions are.

[jetstream]

8 hours ago, michael.h.f.wilkinson said:

I had a slow Newtonian with very good mirrors (1/10 lambda) and small central obstruction (23 % or so)

I'm wondering what the aperture of this scope is?

Edited June 30, 2020 by jetstream

[John]

When I moved from an 8 inch SCT (Celestron) to a fairly ordinary 8 inch dob (GSO made) I was surprised how the much less expensive dob could produce images of the planets and deep sky objects that were as good if not better than the SCT could manage.

My current 12 inch dob (Orion Optics) has a 21% obstruction and high quality mirrors. Good combination I find

I agree with the point made earlier regarding SCT collimation. I've looked though a few other peoples SCT's and found them not in collimation. I have to confess that I didn't feel able to mention it at the time because their owners were enjoying the views. One C8 that I looked through could barely split Epsilon Lyrae.

With a largish central obstruction I think that collimation needs to be accurately maintained and the contrast and sharpness of the image goes down hill sharply even with relatively small amounts of mis-collimation. I recall that Thierry Legault's website illustrates that rather well.

I used to have a 152mm maksutov-newtonian which had a CO of around 18%  and no secondary supports. The images through that were really contrasty and sharp - the closest to an apo refractor that I've seen from a reflecting system.

 

 

[michael.h.f.wilkinson]

10 hours ago, jetstream said:

I'm wondering what the aperture of this scope is?

It was a 6" F/8.