Posts Made By: Roland Christen

Super nice job. Wish it was bigger.

Rolando

It's the Rice effect. Rice burners also tend to put those 10 foot long exhaust tubes on the back which make a terrible racket that can be heard by cops 10 blocks away. Why do they do this? Hondas, Nissans, Mazdas, they all do it.

Beautiful pictures. That is an area I need to revisit. I haven't been up there for 15 years.

Rolando

>>AP 0.75 reducer (2.25")
Optec TCF-S focuser (3.5 - 4.1")
ST-10 XME CFW8a 2"nosepiece (1.75") >>

The AP0.75 reducer screws into the back of the focuser in lieu of the 2.7"-2" visual adapter, so you lose no back focus at all from this device. Normally the unreduced beam comes to focus approximately 7 inches behind the fully retracted drawtube. With the reducer in place, this becomes 7" x 0.75 = 5.25 inches. You do have to screw the 2.7"-2" adapter back into the rear of the reducer, so you lose a small amount there.

Therefore you have the following distances:

2.7"-2" visual adapter = 0.75"
Optec focuser = ?
ST10 with 2" nosepiece = 1.75"

So, for the ? distance you have approximately 2.5" distance left over. This may not be enough. If you want to automate your focuser, you can add a motor to the focuser pinion quite easily. We sell a mounting bracket (Robobracket) for the Robofocus system, and it works quite well.

Rolando

What is it, a Cassegrain or Newt?

Nowadays you can make a flat field coma corrected astrograph from just about any primary optical design. It used to be the thinking that you needed a hyperbolic RC design to tame coma, and that a Newt, classical Cass, or D-K had way too much coma. However, with enough correcting power near the focal plane, one can achieve flat field performance with any of the older simpler designs.

Here is the caveat: With an RC (and apo refractors) you get complete elimination of coma inherently. This means that the optics are not dependent on very careful spacing of critical high-internal power compensating elements. As you go away from this, and you begin to compensate for coma with corrective elements, you begin to get very critical requirements for spacing and exact squaring of the optical components and their mechanical assemblies.

In other words, optical designs with inherent zero coma - RCs, Mak-Cass and Apo refractor - are very forgiving, relatively speaking, for the flattening/correcting optics near the film plane. You can be off a few millimeters plus/minus and still have excellent edge performance. Newts and classical Cassegrains are next up and require much tighter specs on exact spacing. D-Ks require even more control, and may need fancy zero expansion optics and carbon fiber tubes to work over a typical temperature range. Even a 1mm difference in placement of the corrective optic can produce quite noticeable off-axis coma/astigmatism and field curvature.

The way that coma is controlled in coma-producing optics (classical Cass, D-K) is to make their primaries long focus with small secondary powers. This then requires large secondary obstructions for any given focal ratios. For some this is not a big deal, and of course in imaging it is not considered a disadvantage if the aperture is fairly large.

Rolando

Wow, that's cool!

Rolando

Lots of nice detail.

Roland

I like it! 8)

Rolando

>>that means i can take a 410 lb chick along for a ride. >>

I would rather take four 103 lb chicks along

Rolando

>>Is there much variation in field curvature for refractors of a given focal length?>>

No. The field curvature is approximately 1/3 of the focal length. However, the effects of field curvature are aperture dependent. If one 1000mm scope is 100mm aperture and another is 130mm, then the 100mm aperture will show less off-axis aberration.