Astrograph:Fast Newt vs Focal Reducer + RCT

[LAW]

Hi everyone, I've been researching with the intent of upgrading to an astrograph for months and months but I'm still not convinced I know enough to do this decision justice. My original plan was to build a 10" f/4 scope which would be versatile enough for bigger DSOs like M42 but also have the magnification power for galaxies and planetary nebulae.

My question is about the RC design astrographs which seem to be gaining popularity. Andews Comm have the GSO 8" f/8 RC astrograph for the same price as a Carbon Fibre 8" f/4 Newtonian astrograph, so what is the advantage of this design? Would an 8" f/8 RC with a .5x focal reducer perform equally to an 8" f/4 newt?

[RickS]

I don't know of a good quality reducer that would get a RC8 down to f/4. The Astro-Physics CCDT67 is 0.67x and its big brother, the 27TVPH, is 0.75x. They are among the better reducers but won't give you a completely flat field with the RC design. You can also get a flattener which leaves the F ratio more or less unchanged.

With a newt you'll need a corrector unless you're using a very small sensor. Otherwise you'll get bad coma off axis.

Both options will work but the newt is probably better as a fast, medium focal length astrograph and the RC as a slower, longer focal length astrograph.

Cheers,
Rick.

[LAW]

Ah ok, thanks for clearing that up, that makes sense.

[Poita]

Or you could look at an SCT and a hyperstar, that gets you long focal length @ f10, and short focal length at a blistering f1.9 or f2.

[LAW]

Wow, that's cool. I'd never heard of the Hyperstar before, the only problem I can see is my DSLR would block half the aperture. Haha, I just measured it (550D with battery grip) and its nearly 6" wide! 3/4 of an 8" tube. :P

I think I'll stick with Newts for a while then, I should do SOMETHING with the 15 lineal feet of carbon fibre I bought after all.

[Wavytone]

Might be worth your while to consider a Maksutov Newtonian if you want to use a big fat DSLR, as it puts the camera outside the light path.
Intes Micro makes superb ones in various sizes from 6" to 10" aperture at f/6, Orion makes a 190mm f/5.3 and there are others around f/4.8.

If you want to use Hyperstar I'd forget the DSLR and use something like this http://www.sxccd.com/sxvr-h814-progressive-scan which won't block the scopes aperture. The other reason for this is that the image scale of Hyperstar is very small and the pixel spacing of your DSLR is too coarse to be a good match.

[OzEclipse]

I have no direct experience but I'm pretty sure that the low cost 0.5x focal reducers you're referring to only give a very small usable field and don't even do a good job at that. They are designed for use with the smallest ccd's. Basically a waste of money for your purposes.

You should look at :
Boren-Simon PowerNewt Astrographs

http://www.powernewts.com/

They are basically a 6 8 10 or 12 inch f4 newt with a Philip Keller 0.7x field flattener coma reducer.

This is the reducer they use

http://www.astrooptik.com/ Phillip Kellers page

http://www.astrosysteme.at/eng/correctors.html the sales outlet for the correctors

These are expensive ~$1000 but superb correctors. They have a 0.73x reduction so an f5 becomes f3.75 and an f4 becomes f2.9. They have a fully corrected field that is ideal for a KAF8300 field17mm but can be re-spaced to cover an APS-C DSLR sensor.

You can build your own version of a powernewt that is an f4 newt then add the corrector. They need a big diagonal and lots of back focus or you can just put a good quality coma corrector that doesn't reduce or extend the focal length. But don't go into this thinking a cheap 0.5 x will do the job.

All these systems have big central obstructions.

A DSLR is a big obstruction out of an 8 inch SCT aperture if using hyperstar however it is much less when used with an 11 inch SCT. I've seen hyperstar + 8" SCT images that look pretty good. Just try to use the smallest dslr you can or better still one of the small ccd astrocams.

All the other newt options mentioned also have big obstructions - mak newt, boren simon, straight f4 newt and the RC's. they also have diffraction spikes from the spider vanes. As mentioned these all have the camera out of the optical path so that you only have to think about the diagonal + spider diffraction spikes. The power newt web site has good pictures showing diffraction spikes. They are, of course, worse on objects with bright stars in the field or just out of field.

Good luck with your project

Joe

[LAW]

Thanks guys, it's all fascinating reading, I've definitely got some more research to do.

I'm curious about how different systems responded to different f/ratios (actually, as an engineer, I'm pretty curious about everything I touch), but I think I'm happy to stick with relatively wide field imaging with an f/4 or 5 newt for now. I know I'm not even close to stretching the capability of my 8" f/5, so an upgrade would be purely a luxury at this point.