Newtonian/Ritchey-Chretien Telescope For Astrophotography

I would suggest to start from the Newtonian. The jump in focal length from your current refractor to an RC is too big, and RC is more difficult to collimate than Newtonian.

Hi Graham,

Like the others, I recommend the 800mm Newtonian.  I have a Skywatcher 8-inch Newtonian and it is an excellent astrograph.  The fast focal ratio (f/4) really helps capture faint details quite nicely.

After you have developed some experience with the Newtonian, the RC might be a good next step.  I purchased a 10-inch RC last year and am very pleased with the high resolution images that it yields.  Collimation was a very involved process that required several attempts before I hit the sweet spot.  The experience you get from collimating the Newtonian will prove valuable if you decide to move up to an RC in the future.

Clear skies,
Kevin

You will likely need to purchase a come corrector for the Newt.  The Skywatcher coma corrector hides excellent results.

Clear skies,
Kevin

Hi Graham. I use a WO GTF81 for widefield and an Astrotech 8" RC for the small stuff and couldn't be happier.  I think a newt would fall in between these two scopes and wouldn't give you the desired field of view that an 8" RC would. I also use the 8" RC for visual. Good Luck.

Thanks for the insight Brian, much appreciated.

I think I'll hold off for the meantime and keep doing some research. There isn't much in stock at the moment anyway. If something becomes available then I might look at getting a slightly larger Newtonian Reflector, maybe something in the range of 1200mm focal length or similar.

Thanks again for the responses. Certainly a few things to consider when switching over from a refractor.

Hey Graham, I was in the same boat as yourself. I recently just purchased a 6” 1370mm FL RC specifically for galaxy season as I am using an 80mm refractor with a FL of 520mm with reducer/flattener.

I had a look around, refractors, reflectors and RCs to see what I thought was most suitable for my current mount. With RCs collimation always came up as a pain/problem (I am still to use my RC as I need to finish a few projects on my refractor) but RCs from my research seem to do well with image quality (maybe suffer a little on contrast). Refractors with large apertures and FL are just way too expensive and too heavy. I found that I could get a lighter scope in an RC than the others, less glass. Reflectors seemed like a good option also as you can get some very very fast ones for a fraction of the cost of an equivalent refractor, still too heavy for me though

I will say, I don’t know how difficult it will be to collimate so can’t really add anything on that but am fairly realistic with my expectations……..we’ll see.

CS
Brad

I think you can't really beat a Newt if cheap, simple and fast is what you want.

With the small pixels of the ASI183 aperture is more important than focal length. RCs tend to have huge focal lengths which you will have to reduce (more complexity), and a corrected field about three times as large as your sensor which you won't need.

Many excellent f/5 to f/6 5.5-8" Newts around  at very good prices, unlike hyperbolic reflectors where the 6" models are bit of a hit and miss and the 8" models are a serious investment, better suited as primary scope. With a f/5 Newt you might even get away without a coma corrector, especially if you are just playing around and do not care about the edges of the field. With a f/6 Newt it is even more likely. Not that a coma corrector would hurt, of course.

Also, coma correctors do have distancing requirements (typically 55mm which is standard DSLR backfocus) but are probably the most forgiving kind of corrective optic. Unlike flatteners which are designed for a specific refractor or a specific range of focal lengths, a f/5 coma corrector works with every f/N telescope where N >= 5 (subject to certain physical limitations in focuser diameter and travel). And if the distancing is too close it will simply undercorrect. No extra distortion, just less correction.

I have ordered a 130PDS for similar reasons as you.

Cheers,
Dimitris

I think you are seeing lots of good suggestions and going either path is fine.  Here is my take:

I have the RC8 and ASI183MM Pro; and other scopes and ZWO cameras (I run 2 rigs).  I'd still recommend the RC8.

As you realize it's the combination of pixel scale and focal length that is important.  The optics in the RC are so good you can push the lower end of the recommended arcseconds/pixel image scale; so you should target 0.67 arcsec/pixel (a bit less, slightly oversampled, is ok). Natively RC8 (1600mm and F8 ) and ASI183MM Pro (2.4 microns) give you 0.3 arcsec/pixel which is a bit more than twice oversampling.  So you need to get back a factor of 2 either using a telecompressor or bigger pixel camera or, as mentioned, 2x2 binning.I personally use a telecompressor working at roughly 0.75x (very common recommendation) and ASI1600MM at 3.8 microns meaning 0.67 arcsec/pixel (in practice 0.64 arcsec/pixel actually).  So the effective focal length is actually 1212mm measured.  I have absolutely no issue at that focal length.  My bigger RC10A runs at 1600mm with no problem  because with bigger pixels I'm still running at 0.67 arcsec/pixel.

When I contemplated a Newtonian I worried about my heavy camera train hanging off the side, the longer tube (more sensitive to wind), getting a specialized coma corrector.  The RC camera setup is rigid and on-axis.I initially worried about the RC collimation, but the worry was unfounded. Don't touch the primary and do very minor adjustments on the secondary with an out of focus star, once. I'm pretty sure the Newtonian will get you within 5 percent of the RC in performance, will save you money and is more compelling for visual work.  If these elements are not critical then the RC.

Some other advice:

From Chuck: don't think for a moment you'll disconnect your camera assembly form one telescope to another; it takes time to get these things optimised you don't want to touch it.  Tilt, dust, spacing, convenience, etc.  You won't be doing it (often).

If you are not going to use it much then don't buy new.  I see enough of either the RCs or Newtonians on iceinspace in Oz.  If whatever you buy doesn't work for you then put it back on the market at the same price.

Cheers
Rod