RC Optical Systems & new Astrographs

Mike --

I don't own an RC, but I have been considering one. I've turned up a few things which may be of interest:

1) The 50% obstruction and short back-focus are only "features" in that they allow the primary mirror to be slower (and are therefore the leading contributors to the lower price).

2) The light loss with a 50% obstruction is not terribly significant. As Peter mentioned, it takes a 12" scope down to about a 10 1/2" (12² - 6² = 10.4²).

3) The contrast loss shouldn't be significant either, at least photographically. A 50% obstruction will throw more light into the first and second diffraction rings, but with most CCDs the Airy disk and the first diffraction ring will be smaller than a pixel anyway, so the difference (mostly) won't be recorded. (The "mostly" comes in because some stars might be right near a pixel boundary, and will therefore bloat very slightly where they might not have with a smaller central obstruction.)

4) And even visually, with larger apertures, the contrast loss will often be masked by the seeing. I'm not considering one of the astrographs (as I'm purely visual), but even a "normal" RC has a 40% obstruction, which *sounds* significant. But I calculated that in a 16" aperture, the entire first diffraction ring will normally be within my seeing FWHM (which is usually > 1 arc-second), and therefore *I think* I'd only notice a contrast loss about once or twice a year (ie: when seeing was sub-0.75 arc seconds).

5) There are other ways to save money. Ireland has rather unique conditions. For starters, we don't suffer the rapid temperature declines that drier, Western states do (where many of the RCs are used). You might be able to get an aluminum-tubed "traditional" RC for close to the same money as a carbon-fiber (or Invar) trussed astrographic RC. (Just a thought -- like I said, I'm purely visual so the carbon-fiber or Invar do me no good at all -- but it also means I don't have any experience maintaining focus even in Ireland's slow temp declines. Someone else on these boards might be able to provide more info there....)

6) Lastly, I think the mount is more important than the tube. If you are considering going this route, go ahead and get a Paramount or AP1200 first. Stick a toilet-paper tube with clingfilm on it if you have to.

-- Jeff.

Lastly, I think the mount is more important than the tube. If you are considering going this route, go ahead and get a Paramount or AP1200 first. Stick a toilet-paper tube with clingfilm on it if you have to.