Telescope Reviews: Secondary Size in a 10" F/6 Cave

I have used as little as a 1.77" and gotten away with it on a 10". Use the "Newt" program to evaluate illumination / vignetting - smaller secondaries require low profile focuser and moving the primary down the tube.

As long as the secondary is large enough minimally to get the outside light cone of the edge of the mirror, it will act just as well as any other. So lets say with f/6 and a 1.375" diagonal, at a minimum the center of the tube to the focuser's image plane would have to be 6*1.375 or 8.25". At that point an exactly centered star would receive 100% illumination, with extremely small circle of illumination and highly visible vignetting.

Two benefits of larger secondaries are wider field illumination of an wide field EP and a larger uniform illumination for a film sized image (inchs instead of fraction of inches). Some find vignetting annoying.

Two benefits of smaller secondaries are better contrast and a diffraction pattern with a brighter center spot and less light in the first diffraction ring.

We go through selling trends of bigger/smaller all the time. In the late 70's/early 80's big secondaries were the rage - my 10" dating from then had a 3". For a while I had two identical 10" newts and I played games with changing secondaries from 1.77" to 3", and asked for opinions of others on a wide range of wide field and narrow field objects, from M8/M42/M45/... to Saturn/Algieba/Epsilon Lyrae/Mars.

I ended up with a 1.83" for a while, then went to a 1.77". I have one friend who did the same and ended up with a 1.5" - he has a unusual telescope focuser to do this. Another 1.83", and another a 2.14". YMMV

(see http://www.telescope-optics.net/obstruction.htm)

Oh, and I also did the various spider route - started with a circular, tried 1-4 vanes.

Circular(s) are the worst, bar none. The physics of it is that it scatters light the most (through 360 degrees), reducing contrast most. 2 or 4 are best to me, although I still have a 3 vane that works just fine and I'm not planning to change for diffraction reasons, but due to a stiffness issue (it oscillates).

Spider diffraction largely depends on the surface area obscured - more area, more diffraction. Circulars are greater area because an arc is longer than a straight line for spanning the gap to the center. As length increases, stiffness demands a thicker / longer material, and area rises. A thin four vane can have the least area for stiffness given adequate tension and least degrees of freedom of movement(fixation). Live with the spikes.

(see http://www.telescope-optics.net/spider.htm)

At least with the spikes, you can "move" them so as to see detail between them.

Much of this issue is overdone. The 20% secondary size rule is "good enough". Most spiders are good enough. Much of this is just fashion trends - hemlines go up, hemlines down.

If you want to obsess, look at the optical configuration / position / stability / quality of components of the OTA.

You'd be surprised how many scopes, classic and fresh from the the box, are under performing due to basic, obvious flaws.