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Light gathering 8" SCT vs 6" apo

Started by HowardP, 11/11/2003 08:01PM
Posted 11/11/2003 08:01PM Opening Post
I have an 8 inch SCT and am considering a 6 inch apo ... what is the difference in light gathering of the two scopes taking into account the 25-30% obstruction of the SCT?

any thought?

thanks howard
Posted 11/12/2003 11:07AM #1
Specifically in terms of light grasp against and specifically against a C-8XLT a 6" TMB APO will gather 70% as much light. I've factored (published) light loss in the optical train for both scopes at the eye's most sensitive wavelength.

A 6in APO refractor from one of the usual suspects will outperform a well corrected well colimated 8" SCT for the discriminating observer. You must realize though, (IMHO) it takes that discriminating observer and good conditions to see the difference (discriminating observers litter this forum). For some discriminating observers increased contrast and sharp clarity on those great nights makes the $4,000+ premium worth it. You can't see more detail than a larger scope, but often you can discern more detail due to the increased contrast (less light scatter) and slightly sharper image.

Assuming weight, ease of operation and mobility are not issues the real question is...Are you one of those discriminating observers where $4,000+ is worth it?

I've found the following link to essentially reflect my experiences through the eyepiece. and click on "Effects of Central Obstruction" in menu on left.

In addition it is often a bad diagonal and cheap eyepieces that let down mass produced SCTs more than the optics in the telescope itself. To realize the differences between a 6" APO and a 8" SCT you'll need high quaility diagonals and eyepieces.

PS: Regarding the plethora of discriminating observers, please also note you've asked your question on a forum where they'll hotly debate the discernable differences between 6" APO telescopes. ;-)

I used to own a Takahashi FS152. Fabulous views, but the weight and set-up meant I used it 1/3 as often. I now own a 9.25 SCT and a wide field refractor which I use much more often.

I can't find the link, but I once saw an analysis on the effects of a central obstruction that stated the first defraction ring in an 8in SCT was actually inside the airy disk of any refractor under 5in.
Posted 11/12/2003 06:22PM #2

Rather than running all the different numbers to compare an 8" SCT to a 6" APO, I suggest making an effort to go out and see a 6" APO. Only then can you really decide if it is the scope for you. You've used an 8" SCT, so you should have some idea how it performs on the Moon, planets, and DSOs. Make an effort to see how a 6" APO does. No one should buy an expensive telescope sight unseen.

Clear skies, Alan
Posted 11/14/2003 04:59PM #3
For Planetary observing, the general rule of thumb is that for an obstucted scope, subtract the diameter of the obstruction from the aperture and you will have the diameter of the equivilent unobstructed scope.

Thus, an otherwise perfect 8 inch SCT would be equivilent to a perfect APO. This seems to be supported by analysis, transfer functions and that stuff, as well as experience.

Therefore one would expect a 6 inch APO to out perform a great 8 inch SCT.

Achromats like the AR-6 have their own set of problems, false color various other aberations, not applicable to this question.

Posted 11/16/2003 02:00PM #4
Hi Howard,

Haven't read the whole thread, which goes off on the Planetary tangent in some areas.

Here's some other stuff to kick around and think about...

On an SCT

As you've indicated, there's the central obstruction that lowers the actual light gathering of the primary.

Then there are all the other factors folk have indicated, such as "scatter".

There is also up to 1% light loss going through the corrector plate. Folk frequently miss that little bit, but they all add up.

Since there are two mirrors in the SCT, there's compounded light loss. So if you have an older SCT with two 89% reflective mirrors, throughput is only 79% through the mirrors! Newer mirrors are much better.

All "throughput" measurements are by wavelength. A quoted figure like '96% through the lightchain' is often averaged. So that for things like the Veil Nebula using an OIII filter an SCT might result in only really 87% throughput while the refractor might be 93... it really varies by the exact objects you're looking at.

"Contrast" on DSO's in smaller refractors can subjectively make up for "light gathering" in slightly larger SCTs.

My testing was for a 4" APO vs a 5" SCT... subjectively the 4" APO has brighter, better images on nebula and galaxies, imo. Depending upon the seeing and altitude, the little 4" can even trounce on my 8" Nexstar.. For example... I've picked up the Veil in suburban skies in my 4" APO where I couldn't in the 8" Nextstar until it had an hour to cool down.. *sigh* So many factors....

The advantage appears to drop, however, as the apertures increase... on the 8" SCT vs 6" APO it is very, very dependent upon the exact age and models of the SCT we're talking about, and which objects you are looking at.

Best advice is to set up scopes you are curious about side by side and make your own conclusion!

Make sure you use the same diagonal and eyepieces... those two pieces of the light chain can affect light throughput difference by another 10% or more!

For what it is worth, I'm starting to play with a green laser and a laser power meter to try and get some empirical figures put together. But that results in just one set of numbers to consider.

Good Luck