Posts Made By: William McHale

June 7, 2003 05:45 PM Forum: Telescope Making

Folded Refractor Light Loss ???

Posted By William McHale

Well if you design the scope right, the light loss will only be maybe 10% more than you loose on a normal refractor since the latter is usually used with a star diagonal. I wouldn't fold the scope so that the light returned to the back of the scope; makes it more difficult to mount and baffel. I would set it up so that the light was reflected back up through the top of the tube then you could mount it on a Dob mount smile. With a little thought, you can probably get the 6" f/15 down to no more than the length of a 6" f/6 smile


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Bill

June 10, 2003 04:25 AM Forum: Equipment Talk

The truth, finally: Conventional Wisdom

Posted By William McHale

You wrote:
(3) CW: The central obstruction in SCTs (or most other reflectors, for that matter) greatly reduces contrast
Truth: The central obstruction in SCTs slightly reduces contrast. Consider that practically all professional telescopes built in the last 100 years (ground or space-based) have a central obstruction, and if they are Ritchey-Chretiens, which many are, a rather large central obstruction at that!

My response:
I am afraid I must disagree with your basic assertion. It is true that central obstructions are often blamed for far more than they are responsible for, but the fact remains that large central obstructions really can destroy contrast, particularly on the planets. Where the secondary is small, less than 20% the size of the primary) The impact can be all but negligable, but when it gets up to 35-40% (as it does on SCTs) the image quality drops. I always remember a time we were looking at the GRS through a 6" Astro-Physics, a 4.7" Vixen Neo-Achormat and an 8" SCT with what has been claimed as being above average optics. In the 6" the GRS was easily seen with great definition. In the 4.7" the GRS was readily visible but nearly as well defined.. in the 8" SCT it was barely visible.. .it just blended into the background.

Regarding professional telescopes, it should be remembered that they are used almost exclusively for imaging work. Contrast is not nearly as important in this application because much of the contrast can be restored in processing the image.

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Bill

June 25, 2004 04:38 AM Forum: Telescope Making

The problem with lightweight telescopes...

Posted By William McHale

Well I don't think wind is an issue limited to just ultra-light dobs. regarding baffeling.. well I would experiment with baffels before you commit to the shroud. Also make sure a hole is cut in the baffel to let the wind escape through.

On the flip side, in an 8" scope, there seems to be little reason to go out of your way to be ultra light... a simple lighteight configuration will do about as much as you can hope for anyway in scopes that small.

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Bill

November 7, 2006 03:57 PM Forum: Equipment Talk

Is one scope enough

Posted By William McHale

Well, aside from the philosophical issue of how much one actually needs a telescope smile. I would say that in general 2 or 3 scopes are generally a good idea. Granted at the moment, I only am using an AP130.. Ideally though, I think the optimal mix would be something along the lines of a 3-4" refractor (preferably Apo smile) on a mount that can be moved and set up easily for those 30 minute observing sessions. Then I would go with something along the lines of a 6" Refractor or 7"-10" Newtonian/SCT/MCT/MNT. That scope would be for more serious observing and could be easily Equatorially mounted for high power planetary observing. Finally a 12"-20" Dob or 12"-14" SCT for Deep Sky work (well maybe 16" SCT or equatorally mounted larger Newts if you have an observatory smile.

Just my thoughts smile.

One of these days when I fill in those niches, I might even decide that I don't need the AP130 anymore.. I might but don't bet on it smile.

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Bill