Comparing contrast delivered by different scopes

[RobertI]

There's a lot of observing of Mars, Jupiter and Saturn going on at the moment with different scopes, so I read with interest an article on the Kunming United Optics website about the factors affecting contrast delivered by a scope and hence its ability to see low contrast detail on planets. In summary they stated the following factors affecting contrast, in descending order of importance:

1. Seeing conditions (or air turbulence) - the single most important factor that adversely affects image contrast when seeking planetary detail through a telescope.
2. Optical figure
3. Collimation
4. Optical smoothness
5. Baffling
6. Central obstruction - and only noticeable when central obstruction goes beyond 25% by area. Note that the increase in central obstruction is rated as the smallest contributor adversely affecting contrast.

I note they do not mention glass type as a factor, although later in the article they say chromatic aberration reduces contrast on planets, so perhaps it's not such a good article after all! They also don't mention anti-reflection coatings, but perhaps that's because pretty much all scopes are multi-coated nowadays?

Anyway the interesting takeaway for me is that central obstruction has the least effect of all.

 

 

Edited October 5, 2020 by RobertI

[Chris]

Hi Rob, this is very interesting indeed! Really surprised that central obstruction sits in last place for effecting contrast! RC telescopes have roughly a 25% central obstruction and are pretty much considered imaging scopes only. I now wonder how contrasty these scopes are visually? 

And here's me thinking I can tell the difference between a refractor and a Newtonian contrast wise.

[John]

Interesting stuff. Thanks for the link to the article.

Here is another, from Roger Vine who has created the excellent "Scopeviews" website:

http://scopeviews.co.uk/PlanetScopes.htm

 

[vlaiv]

I'm not sure that order is correct, or that in fact there is distinct order.

Even the matter of contrast is subtle one - at what frequencies there is loss of contrast.

For example, baffling will create contrast issues uniformly across all frequencies, while other things impact particular frequencies.

I'm also sure that that effects of central obstruction can be seen much sooner than at 25% by area (which is 50% by radius / diameter) and general consensus is that it is usually not noticeable below 20-25% by radius.

[andrew s]

6 minutes ago, vlaiv said:

I'm not sure that order is correct, or that in fact there is distinct order.

Even the matter of contrast is subtle one - at what frequencies there is loss of contrast.

For example, baffling will create contrast issues uniformly across all frequencies, while other things impact particular frequencies.

I'm also sure that that effects of central obstruction can be seen much sooner than at 25% by area (which is 50% by radius / diameter) and general consensus is that it is usually not noticeable below 20-25% by radius.

Yes @vlaiv fully agree. It depends on the size of the aberations for each factor. At some frequencies a central obstruction increases the mtf above that of an un aberrated optic. 

Contrast is a subtle thing and depends on the exact nature of the feature and contrast difference.

Regards Andrew 

[johninderby]

And then for visual there is the BIG factor..........how good is your eyesight? 🤔

Which brings up another question, should we separate the question of contrast into visual and camera? 

[vlaiv]

14 minutes ago, johninderby said:

And then for visual there is the BIG factor..........how good is your eyesight? 🤔

Which brings up another question, should we separate the question of contrast into visual and camera? 

Not sure this should go into "equation".

We can then say - ok, let's throw in eyepieces, and let's also talk about how much sleep did you have on the night before. Did you eat your porridge? Are you warm and comfy and all other important questions

Scope will provide certain level of performance at focal plane regardless of eyepiece and observer and two scopes will differ in performance at focal plane. In that sense we talk about ability to deliver certain contrast. Will that contrast be exploited, well, that is another matter.

Sticking best EP in the world into scope/conditions combination that can't deliver and having eagle eye won't help much.

[RobertI]

It’s an interesting list, I am guessing that the order is determined by the ‘typical’ differences between premium and non-premium manufacturing, but just guessing. I think it has been pointed out before that the  most significant difference between a Tak and a Skywatcher frac with the same glass would be the optical figure, which is top of the list (apart from atmosphere) so seems to make sense. I am also wondering if the difference in contrast between a frac and Newt is due to the cumulative effects of central obstruction, poor baffling, less than perfect figure (large mirror) and micro-mis-collimation, most of which are less of an issue in a frac?  
 

Full disclosure: I am not expert and I don’t know what I’m talking about! 🙂

Edited October 5, 2020 by RobertI

[John]

With scopes, I've tended to try and get the best figured optics that I can over the years on the basis that I'd like as much of the light gathered to be focused where it should be and as little as possible going astray through diffraction, CA, SA, poor collimation etc, etc.

There are a host of other factors that will conspire to scrub contrast, sharpness, resolution etc off the image of course but I figure that I might as well have a decent starting point. Occasionally, when things do all come together in a positive way, I get a chance to see what the scope is really capable of. 

Probably an over-simplistic approach but there it is

 

[NGC 1502]

Another vital factor that I don’t think has been mentioned -

How accurate is your focus ?

Ed.

Edited October 5, 2020 by NGC 1502

[vlaiv]

1 hour ago, RobertI said:

It’s an interesting list, I am guessing that the order is determined by the ‘typical’ differences between premium and non-premium manufacturing, but just guessing. I think it has been pointed out before that the  most significant difference between a Tak and a Skywatcher frac with the same glass would be the optical figure, which is top of the list (apart from atmosphere) so seems to make sense. I am also wondering if the difference in contrast between a frac and Newt is due to the cumulative effects of central obstruction, poor baffling, less than perfect figure (large mirror) and micro-mis-collimation, most of which are less of an issue in a frac?  
 

Full disclosure: I am not expert and I don’t know what I’m talking about! 🙂

Measuring telescope optical figure is certainly within your reach.

You only need a couple of things - like camera, a star - artificial or real and software - Maybe filter as well if you are measuring refractor - best to use NB filters like Ha, or OIII or perhaps Baader Solar Continuum as it has peak around 540nm - closest to visual peak wavelength.

Lookup WinRoddier software / Roddier analysis for this and explanation how to do it.

[Peter Drew]

I think that rather haphazard articles like this from well respected sources are partially to blame for much of the confusion regarding contrast and other attributes of optical systems. 

My main point is item 6.  This should read "25% diameter",  25% area would result from a huge central obstruction!

To be meaningful,  I think the apertures, focal lengths and magnifications should be equal, the differences should be more easily apparent followed by discussion to mitigate them.    🙂  

[RobertI]

7 minutes ago, Peter Drew said:

 

I think that rather haphazard articles like this from well respected sources are partially to blame for much of the confusion regarding contrast and other attributes of optical systems. 

 

Oh dear! I’ve just bought a scope made by this company! 😂😂 (I think you were criticising the article rather than the company though). 
 

The article by Roger Vine referenced above by @John was probably more illuminating. 
 

 

Edited October 5, 2020 by RobertI

[RobertI]

1 hour ago, vlaiv said:

Did you eat your porridge?

I think we should explore this aspect some more.......

[johninderby]

Maybe these would help increase contrast? 😁

 

Edited October 5, 2020 by johninderby

[Stephenstargazer]

Prof Ian Morison normally points out that mirrors have a downside, the light not reflected is scattered and some of it must reduce contrast. So there should be advatnages for refractors and prisms vs obstructed reflectors and mirror diagonals. But I can deploy an 8" reflector with long focal length and 'only' a 4" refractor which possibly evens the score on planetary/lunar observation. Our peepers? Well not only carrots but sadly zero alcohol or caffeine makes a serious difference to visual ability 😧. Ah well we must suffer for perfection!!

That aside, Number one on my List by a mile is the sky and seeing - look for as long as you can and get lucky.

 

Edited October 6, 2020 by Stephenstargazer

[andrew s]

5 minutes ago, Stephenstargazer said:

Prof Ian Morison normally points out that mirrors have a downside, the light not reflected is scattered and some of it must reduce contrast. So there should be advatnages for refractors and prisms vs obstructed reflectors and mirror diagonals. But I can deploy an 8" reflector with long focal length and 'only' a 4" refractor which possibly evens the score on planetary/lunar observation. Our peepers? Well not only carrots but sadly zero alcohol or caffeine makes a serious difference to visual ability 😧. Ah well we must suffer for perfection!!

True but glass also scatters light due to impurities, defects and non uniformity. 

Regards Andrew 

[Peter Drew]

This is true.  A 4" APO will have better contrast than a 4" Newtonian, a 6" APO will have better contrast than a 6" Newtonian and so on.  This convinces many that APO's are better than Newtonians.  A 10" Newtonian should have better contrast than either the 4" and 6" APO's, there aren't many 10" APO's about for a comparison!  In reality, the Newtonian is a better apochromat as it is by physics, perfect, the best APO's get close enough to this perfection but at enormous cost.      🙂

[Stephenstargazer]

Glass some but Fluorite none, mirrors plenty !

Even so I fully agree with Peter. I guess 10 - 15" refractors are not easy to make (well).

Edited October 6, 2020 by Stephenstargazer

[jetstream]

On 05/10/2020 at 11:41, vlaiv said:

baffling will create contrast issues uniformly across all frequencies

Refractors are easier to baffle because of the design, newts are not typically baffled as well ie focuser baffle etc. It is amazing what a low obstruction newt can do when stray light is controlled.

There are not many dobs out there that have stray light well under control IMHO- from the mirror cell to the focuser.

Bottom line- a vg scope "grabs the attention" and takes seemingly limitless mag IMHO.

[johninderby]

I’m impressed by how much more magnification my 8”CC will take compared to a Skymax180. Stray light is very well controlled. 👍🏻

[jetstream]

5 minutes ago, johninderby said:

Stray light is very well controlled

Nice looking scope- what is it exactly?

[johninderby]

4 minutes ago, jetstream said:

Nice looking scope- what is it exactly?

A TS 8” Classical Cassegrain.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10753_TS-Optics-8--f-12-Cassegrain-telescope-203-2436-mm-OTA.html

[jetstream]

It looks like a very good scope and won't let stray light bounce around- the spider secondary, everything has excellent attention to detail.

[johninderby]

I’ve had a Skymax180 and a C8 SCT and now settled on the Classical Cassegrain. No dewing, a fixed primary and a crayford focuser although I’ve upgraded to a Baader Steeltrack.