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Comparing Appletures

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#1 clamchip

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Posted 04 December 2018 - 12:41 PM

I've always thought when comparing a obstructed telescope to a unobstructed telescope you

simply subtract the the obstruction from the obstructed telescope and you are left with the equivalent

unobstructed telescope.

A example being a C5:

5 inch aperture, minus it's 2 inch obstruction, leaves you with a 3 inch refractor.

And I thought the equivalency is in contrast, the resolution, the ability to see detail, will remain unchanged

meaning the C5 will resolve as a 5 inch aperture.

If you search for this information you get overwhelmed with differing opinions.

The reason I ask is because I really like my C5, but if I had to choose between it and a 3 inch f/15 I think I'd

go with the 3 inch refractor. But I might be more inclined to go with the C5 if it's truly going to perform as

a 5 inch on fine detail. I would rather have the aperture and not need it, than need it and not have it, but I

prefer driving the 3 inch f/15 refractor.

Comparing in the field can show a difference with certain observations like planetary detail and globular

clusters is where I see the C5 move out ahead of the 3 inch refractor, but otherwise it can be difficult to

tell the two apart. 

 

Robert  


Edited by clamchip, 04 December 2018 - 12:58 PM.

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#2 terraclarke

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Posted 04 December 2018 - 01:21 PM

I think a better measure:

 

(D^2 - d^2)^(1/2)


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#3 TOMDEY

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Posted 04 December 2018 - 02:03 PM

Yeah, it's essentially equivalent areas, not diameters. There are other nuances, and avoiding very large central obstructions is highly recommended.  Tom



#4 WoodyEnd

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Posted 04 December 2018 - 03:51 PM

My apples are about 3" in diameter.   wink.gif



#5 davidmcgo

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Posted 04 December 2018 - 04:51 PM

Contrast rule of thumb as I've seen it is to subtract linear central obstruction from the diameter.  So blacks are a little less black and whites a little less white than for an unobstructed scope and about on par with the 3" refractor.

 

Fine resolving of high contrast tracks as function of overall diameter. 

 

Light gathering is subtract area of obstruction from clear area and convert back to equivalent diameter.  However, light gathering can offset some of the contrast reduction since for a given magnification, ie 200x, the image in the obstructed 5" is much brighter so that the eye's response is better.

 

My C5 very much outdoes a 3" refractor on many things including doubles and globulars.  Just don't expect wide fields.

 

Dave



#6 clamchip

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Posted 05 December 2018 - 12:29 PM

I have been doing more digging and found I was looking for the same answer back

in 2009. Its a ruff question to answer: 

https://www.cloudyni...ivalent-of-sct/

I think probably the best thing is side-by-side observations in the field.  I would like

to compare a Celestron C5 with special coatings, a Edmund 3 inch f/15, and 

Edmund 4 inch f/15 on various targets. I might throw in a modern 80mm ED I have.

It may need to wait for spring since it's 23 deg out here. I'm out observing but not

in a scientific capacity, just willy-nilly sky surfing with my 10 inch f4/5 Meade DS-10

being wowed all bundled up.

 

Robert


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#7 DAVIDG

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Posted 05 December 2018 - 02:37 PM

 If your going to do side by side tests, the first things that should be done is to bench test the telescopes. Optical quality is going to be more of  a factor then obstruction size in determine the resolution and contrast of telescope of similar aperture. An unobstructed scope that has poor optical  quality will not beat obstructed one with good optics. So to get any type of valid conclusion you need to compare scopes of equal optical quality.

 

               - Dave 


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#8 rolo

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Posted 05 December 2018 - 04:09 PM

I have been doing more digging and found I was looking for the same answer back

in 2009. Its a ruff question to answer: 

https://www.cloudyni...ivalent-of-sct/

I think probably the best thing is side-by-side observations in the field.  I would like

to compare a Celestron C5 with special coatings, a Edmund 3 inch f/15, and 

Edmund 4 inch f/15 on various targets. I might throw in a modern 80mm ED I have.

It may need to wait for spring since it's 23 deg out here. I'm out observing but not

in a scientific capacity, just willy-nilly sky surfing with my 10 inch f4/5 Meade DS-10

being wowed all bundled up.

 

Robert

I did a side by side with C5 vs a 4" f/15 Unitron and it was no contest. The Unitron 150 easily outperformed the C5 specially on Jupiter. Star testing is a good alternative specially now that the planets are not visible.


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#9 davidc135

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Posted 05 December 2018 - 04:26 PM

Following on from DaveG's post on quality: If the two Edmund refractors are of the same high quality would it be worthwhile obstructing the 4'' and seeing at what point it was overtaken by the smaller scope? Though CA might slightly hamper the 4''.  David


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#10 steve t

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Posted 05 December 2018 - 04:40 PM

FWIW

Below is a Modulation Transfer Function comparison between my 4" (102mm) F/8.5 Newtonian with  a 35% central obstruction and an unobstructed 66mm refractor. 

If I understand it correctly,  what it shows is that on low contrast objects, like Jupiter, a 4" scope with 35% obstruction will perform like a 66mm refractor (primary diameter - obstruction diameter),  but for high contrast, like double stars, it would perform like a 4".

 

Attached Thumbnails

  • 20180817_133132_000.jpg

Edited by steve t, 05 December 2018 - 04:41 PM.

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#11 DAVIDG

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Posted 05 December 2018 - 08:22 PM

 Remember that the 4" will still have much higher resolution and larger light gathering power then the 66mm scope.  Also consider that if both scopes were perfectly figured, that the refractor if made of the of usual BK7 and F2  would still have some level of chromatic aberration while the reflector would not. So there is more going on then just  a change in contrast with obstruction that determines the final image quality. 

 

                    Happy Holidays,

                       - Dave 


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#12 steve t

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Posted 05 December 2018 - 10:05 PM

Hi Dave,

Very good points, and I agree. I should not have mentioned system type (reflector vs refractor vs cat) since that's a whole other can of wormssmile.gif

I was only trying to point out that, with all things being equal,, the MTF supports  the "Objective - Obstruction = Unobstructed" system performance for low contrast objects, but doesn't necessary apply for high contrast objects. 

 

Happy Holidays to you also.

 

Steve T 


Edited by steve t, 05 December 2018 - 10:11 PM.



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