calculate the effective aperture of a scope with central Obs

blurblock · Post by **blurblock** » Fri Jan 23, 2004 10:01 pm

Scope like a Reflector or a Catadioptics has a central obstruction, when we want to calculate the effective aperture (that's the aperture size minus the central obstruction) as compared to a refractor, how should we go able doing it?

An example will be the Celestron Advanced Series C8-S Schmidt Cassegrain scope,

it has a central obstruction of 2.5", 10% by area and 31% by diameter. so how should be calculate the aperture? should it be :

a. 8" - 2.5" = 5.5" aperture as compared to a refractor or
b. 31400mm(sq) - 3066.4mm(sq) = 28333.6mm (sq) as compared to 31400mm (sq) in Area
c. 1256mm - 196.25mm = 1059.75mm as compared to 1256mm in perimeter?

Which should be the more accurate estimation?

Post by **weixing** » Fri Jan 23, 2004 10:51 pm

Hi,
This has been discuss before somewhere in the forum.... Anyway, I think the area will be a better estimation, since it determine the light gathering power of a telescope.

By the way, don't worry too much about the central obstruction, as long as you are not making your own telescope. Any decent brand telescope won't give a big central obstruction than necessary.

Anyway, the atmospheric condition will affect your ability to see more than any central obstruction...

Have a nice day.

addy · Post by **addy** » Sat Jan 24, 2004 9:35 am

I guess the rule of thumb is that to use the diameters values (indirectly translates to areas.. an 8" CAT with 2.5" obstruction will approximately perform same as a 5.5" refractor (apo/ed ??)..

blurblock · Post by **blurblock** » Sat Jan 24, 2004 10:16 am

Not Necassary, an 8" SCT with 2.5" has a surface area to capture light similar to a 7.6" Refractor, thus I am quite puzzled which one to use. Diameter values in this case MIGHT not be accurate.

7.6" Refractor will get a surface area of appromixately 28338mm(sq) pretty similar to a 8" with 2.5" Obstruction.

ChaosKnight · Post by **ChaosKnight** » Sat Jan 24, 2004 12:03 pm

Light gathering power of 8" SCT with 2.5" obstruction, proportional to area = (pi x 4^2) - (pi x 1.25^2) = 45.357 in^s

which is same as a refractor of 7.6" diameter.

But because the SCT has larger diameter than the refractor, in terms of resolution, the SCT wins. Assuming optics of similar quality.

addy · Post by **addy** » Sun Jan 25, 2004 6:12 am

Hmm.. I dont remember seeing any mathematical/theoretical formula for calculating the effective aperture of an 8" SCT, but I have often read in reviews at cloudynights or astrogeek using the rule of thumb I mentioned.. I am quoting Markus from one of his replies in the Makscopes yahoogroups...

Quote ----
From: "APM_Telescopes" <apm_telescopes@w...>
Date: Sat Jan 17, 2004 5:14 pm
Subject: Re: [MakScopes] Re: TEC140 or MN86 or Nexstar11GPS
ADVERTISEMENT

we com pared last year in September in the Alps Starparty with excaptional seeing 1=/10 at Planets the 715 Standart, 703 Deluxe and TMB 130/6, we found the rules working, aperture minus secondary diamater is apo of same quality. The TMB have had very little edge over the Maks

best wishes
Markus

--- Unquote

cheers
Addy

blurblock · Post by **blurblock** » Sun Jan 25, 2004 9:22 am

Don't think that is a mathematical formula proposed by chaosknight, more like common sense ....

Post by **weixing** » Sun Jan 25, 2004 10:13 am

Hi,
Check out the below website for effects of the central obstruction. I found that what the website said is quite logical. Take a look if you are interested:
http://perso.club-internet.fr/legault/obstruction.html

Have a nice day and happy Chinese New Year!!