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refractor/reflector "equivalence" question

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#1 gsaramet

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Posted 19 January 2025 - 12:02 PM

Hello all,

 

I am having a bit of a controversy with a friend regarding the "equivalence" of refractor / newton telescopes.

 

Our main issue is the secondary mirror obstruction. As far as "speed" is involved, it seems pretty clear to me that the equivalence can be obtained reasonably well by substracting the secondary area from the primary area. However, I have no idea how "separation" works. The Raleigh limit seems to mention only linear values (diameter). 

 

Can anybody comment on this, or it gets enormously complicated optically?

 

Thanks!



#2 terrypaula

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Posted 19 January 2025 - 01:00 PM

I always thought that a 4" refactor was close to an 8" reflector as far as any optical equivalency goes  I might be wrong.



#3 Cotts

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Posted 19 January 2025 - 01:14 PM

Yes it is a complicated topic.

 

For light gathering, obstructed area can be taken into consideration.e.g. A 20 cm lens/mirror has an area of 314 cm^2.   If this was a Newt with a 5 cm secondary mirror (19.6 cm^2) then the resulting area for gathering light would be 294.4 cm^2.  This works out to a 6.25% loss of light gathering power - a completely negligible amount in practical terms.

 

Light gathering comparisons of refractors and Newtonians must also take into consideration the % of light lost by passing through a refractor lens or light lost by inefficient reflectivity of the mirrors in a Newt...... Working out all these percentages is a very complex problem.

 

Resolution is quite a bit easier.  An 8" refractor and an 8" Newt with 25% (by diameter) have the same resolving power for point sources like stars.  Actually the Newt will do a few percent better than the refractor because the central obstruction makes the central spot of the diffraction pattern a wee bit smaller - the light is slightly diverted to the first diffraction ring.  But in practical terms this difference can be ignored.  

 

For contrast transfer the central obstruction reduces the ability of the telescope to discern fine, low contrast detail.  Tiny central obstructions of, say 10% or 15% (by diameter), make no noticeable effect on contrast transfer.  Even a 20% obstruction make so little a difference that it cannot be noticed, even by expert observers.  Once we start to approach 25% and 30% or even 35% like many Schmidt-Cassegrains the unobstructed scope begins to noticeably pull ahead of the Newt.

 

The difficulty is trying to pin down a 'rule of thumb' which declares an 'equivalence' between a refractor of aperture x cm and a Newt of  aperture y cm with a central obstruction of k cm.  There exists a rule of thumb that states that one should simply subtract the central obstruction from the Newt's aperture to find the  'equivalent' refractor.  I feel this 'rule' is far too harsh.  

The MTF graphs below compare the contrast transfer of a 12 inch, 25% obstructed scope( 3inches) with a 10 inch unobstructed scope.  At every spatial frequency the larger scope transfers more contrast to the eye and it also exceeds the resolving power of the smaller one.  But the 'rule of thumb' which simply subtracts the central obstruction says the larger scope's 'equivalent' refractor is 12" - 3" =9".  This is demonstrably false.   A 12-nch Newt with a 3-inch secondary mirror exceeds the capabilities of a 10-inch refractor in every possible scenario - it's not even close. This also applies to any two scopes whose apertures are in the ratio of 6 to 5....

 

There are so many factors other than the above - air currents in the tube, the 'seeing' near your observing site, the seeing further up in the sky, the quality of the optics you are using....user experience, too.

 

10 inch vs 12 inch MTF copy.jpg

 

Dave


Edited by Cotts, 19 January 2025 - 01:14 PM.

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#4 idclimber

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Posted 19 January 2025 - 01:25 PM

Comparing scope A to scope B with two different focal lengths makes little sense to me, especially if one is comparing different apertures. If the focal ratio is the same the scope with the larger clear aperture wins as it collects more photons. Yes to get the correct math you would have to subtract for obstructions. 

 

But if we are being analyzing everything you would also need to calculate how much loss you have through optics and mirrors. This would probably need measuring equipment and specs that none of us have. I do take note that those NASA guys really seem to like mirrors and not all glass. 

 

Since this is the typical argument with a friend and you are seeking a higher authority here at CN, I will attempt to chime in as an "expert". I do so only with the qualifications that my seven year old grandson bestowed on me this weekend when I was showing him some of my astro stuff and trying to setup for a visual view of Saturn and Jupiter. He called me an Astronomer and is suitably impressed as seven year olds can sometimes be. 

 

So taking my new found title as "Astronomer" and coming from someone who started with a 12" SCT and also has a couple refractors that I have learned to love. Refractors are always better. There is no argument on this point, and anyone who disagrees is simply wrong. lol.gif

 

Now can anyone here point me to a Pixinsight plug in that adds those beautiful diffraction spikes. lol.gif



#5 gsaramet

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Posted 19 January 2025 - 02:04 PM

Thank you guys :)

 

Yes it is a complicated topic.

 

 

 

I suspected so :)

 

 

 

 

Since this is the typical argument with a friend and you are seeking a higher authority here at CN

No, it's not exactly that ;) 

 

Discussing the issue I realized that I did the comparison using MY metric, which is speed. (And after I posted I thought that maybe even speed is not that simple). And I realized I know mostly nothing about resolution. And I haven't thought about contrast :)

 

I can think a bit of geometric optics and I can guess a bit about diffraction, but "concepting" a whole scope is much more than I can chew. And I thought, for sure, that question must have been asked before, and some answers must be in existence. 

 

My friend has received the link to the thread the moment I created it, so he can also monitor it.

 

To elaborate a bit more, he wants a good long focal length scope. We are about neighbors, but the needs are a bit different: I image only from my backyard so far, he is much more mobile and goes out for better skies.  And he's oscillating between a good refractor, a very good newt or an EdgeHD. Quite a pain in the behind to compare all those :)

 

By the way, our climate is quite cloud friendly. 



#6 Cotts

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Posted 19 January 2025 - 04:49 PM

Thank you guys smile.gif

 

 

I suspected so smile.gif

 

 

No, it's not exactly that wink.gif

 

Discussing the issue I realized that I did the comparison using MY metric, which is speed. (And after I posted I thought that maybe even speed is not that simple). And I realized I know mostly nothing about resolution. And I haven't thought about contrast smile.gif

 

I can think a bit of geometric optics and I can guess a bit about diffraction, but "concepting" a whole scope is much more than I can chew. And I thought, for sure, that question must have been asked before, and some answers must be in existence. 

 

My friend has received the link to the thread the moment I created it, so he can also monitor it.

 

To elaborate a bit more, he wants a good long focal length scope. We are about neighbors, but the needs are a bit different: I image only from my backyard so far, he is much more mobile and goes out for better skies.  And he's oscillating between a good refractor, a very good newt or an EdgeHD. Quite a pain in the behind to compare all those smile.gif

 

By the way, our climate is quite cloud friendly. 

Best bet for the bolded goal is a Maksutov/Cassegrain.   6 to 8 inches, f/10 to f/15.  These come up on the used market from time to time.  Skywatcher's 180mm is a good one that will not break the bank.  Russian MakCasses from Intes are also fine instruments.  TEC made a great 8-inch MakCass as well.  There are probably more brands I'm forgetting here.  

 

I'd recommend trolling the Classifieds..

 

Dave



#7 idclimber

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Posted 19 January 2025 - 06:06 PM

For focal length and aperture, it is really hard to beat an Edge. I would however look at the 9.25" if he has an APS sized sensor like yourself. Otherwise the alternatives with folded optics like the Mak/Cas. 

 

For me the scope I would like to replace my 12" SCT is a 14" Planewave or AG Optical. 



#8 dally

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Posted 19 January 2025 - 10:05 PM

I have a RC6, 10" newt, had a C8, also have a ED102 APO, 75Q Petzval and my newest the SVX130T. The big reflectors are great to get in close, but the best images come from my refractors. 

 

The best part of a refractor is there's usually no collimation to worry about. I had such a hard time keeping the C8 collimated, I got so frustrated with it and finally sold it. 




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