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Clear aperture - Refractors vs everything else.


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Hello all

This is probably a very obvious question but I was wondering about telescopes that have central obstructions and how that affects the view in telescopes?

When you compare the aperture of a refractor and... lets say a reflector - is it a direct comparison between the apertures or do you have to take into account the impact of the central obstruction?

Is it as simple as reducing the area of the central obstruction from the reflector and comparing it to a refractor to give a true comparison - or is it more nuanced in that the width of the perimeter defines light gathering power to a different extent and needs to be considered separately?

Many thanks in advance.

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Right that chart is pretty neat I have never seen it. It pretty much explains it.

Only thing it may not explain is scopes with no co will have better contrast too.

However size plays a big part tho I made a YouTube video called 6inch sct  vs 6 inch reflector vs 6 inch refractor. If u Google that with my name first then title u should see it.

Theres a huge difference in size in a 6sct size and weight compared to 6 refractor, and cause of this scopes with co will never go away. In fact it may still sell more once u get to the 5 inch and above.

Joejaguar 

 

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The question is indeed obvious....but maybe you need to be specific about exactly what you're trying to compare. I can hear the cudgels being prepared by the protagonists of both sides.....

I'm not sure at what level to pitch the answer..hope this helps:

If it's light-gathering power, then yes, you just compare the effective areas, not forgetting to factor in the various other losses for both reflector and refractor. For reflectors you lose an extra 5% (therabouts) at each mirror. For refractors the losses can be very low with modern coatings (less than 1% at each surface) plus a few percent through the glass itself. 

If it's resolution the standard answer from A-level physics is that the resolving power is 1.22*wavelength/ aperture diameter (answer in radians) . This applies for both reflectors and refractors alike. It simplifies to 138/ aperture (arcseconds; aperture in millimetres for green light where the eye is most sensitive). This is known as the Airy criterion for partially overlapping interference patterns; there is a Dawes criterion derived empirically which is slightly more stringent at 114/ aperture. 

If it's contrast then you have to consider how the central obstruction degrades the image by scattering the light. This is actually quite hard to do since the way the scattering is done depends on the size and shape of the central obstruction. This is complicated....but the less obstruction the better. There are mathematical methods of assessing how a given obstruction will degrade contrast. It is related to both the area and the pattern of the central obstruction with its spider if fitted. 

 

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2 minutes ago, wookie1965 said:

So a 5" Refractor is as good as a 6" reflector and a 6" Cassegrain.

Thats pretty much what I found when I owned an Intes 6 inch maksutov-newtonan and my ED120 refractor. I was able to try them side by side on a number of occasions and found their performance very similar indeed. The additional aperture of the mak-newt showed deep sky objects a little better although the contrast of the refractor helped get it pretty close on these targets. On double stars, the planets and the moon the two scopes were pretty much neck and neck.

In the end I kept the ED120 because it cooled down quite a bit quicker and was easier to mount steadilly.

I'm not sure that a 6 inch SCT would have quite kept up with these two though :icon_scratch:

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9 hours ago, wookie1965 said:

So a 5" Refractor is as good as a 6" reflector and a 6" Cassegrain.

 

37 minutes ago, wookie1965 said:

I was thinking about getting a 6" reflector like my first scope I will stick with the 5" Refractor and save money thanks John.

........But isn't chromatic aberration another important consideration though ?

My understanding is that reflectors are automatically free of CA because of the surface silvered mirrors, whereas refractors need to be of the much more expensive Apochromatic ED glass to not suffer significantly from it.

A 5" Apo ED is way more expensive than even an 8" Newtonian.

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15 minutes ago, Astro-Geek said:

 

........But isn't chromatic aberration another important consideration though ?

My understanding is that reflectors are automatically free of CA because of the surface silvered mirrors, whereas refractors need to be of the much more expensive Apochromatic ED glass to not suffer significantly from it.

A 5" Apo ED is way more expensive than even an 8" Newtonian.

I have a Tal 100rs and a Meade lxd55 127 emc Achromatic s I have not noticed any CA on either of them.

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2 hours ago, wookie1965 said:

I have a Tal 100rs and a Meade lxd55 127 emc Achromatic s I have not noticed any CA on either of them.

 

I have a Skywatcher Evostar 120 f8 refractor and an Astrotelescopes AR1522 152 mm f/5.9 refractor.

They're both just achromats, so they both exhibit some CA.

The Evostar f8 shows minimal CA because of the focal ratio, and the AR1522, being an f5.9 shows more.

I use a Baader Contrast Booster  filter on the f5.9 which works really well at minimising the CA, but still no match for an Apo.

Someone on here previously posted a very useful CA guide to Achromats spreadsheet that reveals the increase in CA with lower focal ratios.

Chromatic Aberation -ratio-chart-achro.jpg.3c827544042c07bd80a30471d817315b.jpg

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