New Sub Forum needed - Astro scopes for Visual (4 Viewers)

[OPTIC_NUT]

OK, I see:

http://www.laughton.com/paul/rfo/obs/obs.html

There are photos there to show what 'contrast' means in the night sky.
There is quite a profound effect. Mainly on resolution, adjacent interference
of nearby pixels or points. The black/grey level for all the images is almost
identical. So...the "contrast" due to the Strehl has a major effect on usable
resolution close in, but it also isn't the same as the "contrast" of background noise
and grey level you see in looking and photos. And...you really need a decent-sized
mirror to stay out of trouble. And a small obstruction.

The main effect of the Strehl-related contrast would be on the usable power,
the photos seem to suggest. It would be interesting to see daytime land
pictures at different powers. My main interest is in a 90mm mirror telephoto
I'm cleaning and adapting. I was seeing very bright and clean scenes at
about 30x, but at 70x things got an odd blur. Now I understand why.
I call that a resolution effect, mainly. Not overall contrast, but serious,
because it cannot be recovered from with post-processing.

What it would mean in practice is that while good powers for a refractor
are brightness-limited to 1/2 to 3/4 times Diameter (in mm),
good powers for a folded reflector would be in the range of 1/4 to 1/2 dia...
and that would be a bit worse for smaller barrels (more % obstruction, like you said).

I appreciate the resolution-oriented "Strehl Contrast" now.

I have to think of it quite separately from stray light contrast,
which still is a bigger problem for faster barrels, regardless of type.

 

[henry link]

OPTIC_NUT,

The Strehl ratio in that ad is for the primary mirror alone, before subtracting for the central obstruction. Calculating the Strehl with the central obstruction is explained pretty well here:

http://whichtelescope.com/benchmarks.htm

Suppression of stray light depends on the quality of the baffling, not the design or focal ratio of the optics. But, when it comes to baffling MaKs and SCTs are at a disadvantage because, unlike a Newtonians where the secondary (as seen from the eyepiece) is surrounded by the dark telescope tube, the secondary in a CAT is surrounded by the bright light that enters the front of the tube. Complete baffling is only possible if considerable vignetting is tolerated.

Henry

 

[OPTIC_NUT]

Excellent article! We are almost entirely on the same page now..

You're right on the stray light, baffling, and reflectors.
That's quite true inside the telescope. It affects night stray light as well,
which is why we dropped the Newtonian in suburbia...the distant highway lights
really spoiled. It is possible to get around that using an enlarged-diameter hood/can before the barrel.

So the reflector case is reduced to:
---you really need extra diameter compared to a refractor
---you would need extra hooding as well (for suppression) ,
vignetting or a 'tuna can' to avoid vignetting.

I'm a contrast nut, so I prefer longer barrels with extra flocking and irises.
A slow barrel does cost fov.....at low power. The main effect is a massive reduction in cost.

Cost-wise, I'm in the same boat as the folded reflectors. No $1000 tube, no $300 EP.
The folded reflectors are much heavier, but have less vibration on the same tripod from what I've experienced.
The APOS are heavier too, but I have the issue of more rotational inertia. I need a damper.