Compact Lightweight 18" Scopes

While it appears Obsession has the market with their new ultracompact scope, it appears to have compromises like any other scope. While I think the strengths of the 18" ultracompact appear to significantly negate its weaknesses, here's what I would be concerned about:
1. 3 vane spider: I thought I read somewhere that the three vane units produce 6 spikes on the star image or is this not correct?
2. Use of a 3.1" secondary: I can't see how this size would be large enough to manage an 18" short f-ratio mirror without reducing the effective aperture of the primary. I note my 16" F4.5 mirror utilizes a 3.1" secondary which just encompasses the entire primary so I can't understand how it could possibly do the same with a shorter F ratio 18" mirror. I note both night sky scopes and starmaster use 3.5" secondaries with their 18" models which operate at F4.5.
3. Cost: While expensive, I certainly like the design and the latter, in my opinion, makes it worth the price.
Still, I've looked for an alternative and I found a website offering a lightweight design:
http://www.liteboxtelescopes.com/
Anybody have any experience with this design?

Rod Kaufman said:
1. 3 vane spider: I thought I read somewhere that the three vane units produce 6 spikes on the star image or is this not correct?

Yes, there would be 6 spikes, but they are each
half as bright as for 4 vane spider. So total light
going into the spikes is less with 3 vane spider.

2. Use of a 3.1" secondary: I can't see how this size would be large enough to manage an 18" short f-ratio mirror without reducing the effective aperture of the primary. I note my 16" F4.5 mirror utilizes a 3.1" secondary which just encompasses the entire primary so I can't understand how it could possibly do the same with a shorter F ratio 18" mirror. I note both night sky scopes and starmaster use 3.5" secondaries with their 18" models which operate at F4.5.

The upper tube on the 18" is 22" in diameter.
From the pictures, looks like the top of the
focuser is flush with the outer edge of the 22" ring.
So if you put the focus 1" out (above the focuser),
the diagonal is 12" from the focal point. At F4.2 that
needs a 2.85" diagonal to cover it. So looks like they
just squeak by on the diagonal size with the 3.1".

For another idea, check out the 1st place winner (both mechanical and craftmanship) at Stellafane 2007, a beautiful 18-inch F/4 that a healthy person can just pick up and carry around. I saw this scope and it was in deed a beautiful telescope. I did not get a chance to look thru it, but the Springfield Telescope Makers don't give out awards to poor telescopes.

Here's the link: http://stellafane.org/convention/2007/scopes.html

Look down the page for the telescope made by Ross D. Sackett.

George N

Never had any chance to see one of these, but it looks pretty good. I do not like the closed up bottom with only three holes though, unless you were to use a lot of fans for circulation it would take a while to cool down.
The 18" ground board looks way to weak to me and I would think it would be prone to vibration, but who knows?
Lightweight scopes are usually a mixed bag. Hard to make a really solid scope that is also lightweight.
Prices for the scope without mirrors seems OK, but not really cheap, but certainly not expensive compared to some.

Rod Kaufman said:

While it appears Obsession has the market with their new ultracompact scope, it appears to have compromises like any other scope. While I think the strengths of the 18" ultracompact appear to significantly negate its weaknesses, here's what I would be concerned about:

While neither my temperament nor my vehicles are conducive to big dobs (I do use a 12.5-inch truss tube job), suffice to say that Dave Kriege's new 18-inch is almost enough to turn _my_ head.