GSO 10" RC for visual (DSO's) ?

casstony · #1 12-03-2010, 06:16 PM

Can anyone say what these scopes are like for visual deep sky use? Also do they reach focus with a 2" diagonal?

At the lower magnifications I normally use (<100x) I'm thinking the 43% central obstruction won't matter much. The RC has a big advantage over a C11 with regard to cooling and light gathering is probably similar given the RC's higher reflectivity coatings.

TrevorW · #2 12-03-2010, 06:39 PM

Primarily they are more designed for astrophotography than visual but in saying that they can be used for visual depends on what you intend to use it mainly for

casstony · #3 12-03-2010, 07:20 PM

While they are specifically designed for imaging, I'm thinking the only reason they're not so great for visual is the large secondary obstruction, but that only becomes noticable at higher power (eg. planetary). Since I mostly do low power DSO's I figure the RC is worth considering alongside the C11.

I'm interested in hearing about actual DSO observing experiences through these scopes.

Wavytone · #4 12-03-2010, 08:35 PM

At 43% you will definitely notice it visually, but it will also depend on what eyepiece you use. I have a Sigma mirror lens with about the same obstruction and it's not really fit for visual use because of this, yet takes fine photos.

The worst problem arises with low-power eyepieces that have a lot of spherical aberration of the exit pupil, which causes the "kidneybean" blackout. Some early Naglers had this issue, later ones don't.

Personally I'd be looking for a scope with a secondary obstruction ratio no higher than 33%, preferably lower.

The Intes Mak-Newtonians would be my pick in your case - fast focal ratio, a small secondary obstruction, and a closed tube.

pgc hunter · #5 12-03-2010, 08:41 PM

That central obstruction is absolutely huge on these RC's, which will rob you of much needed contrast when observing faint DSOs. The spider vanes also appear to be thicker than those of the GSO dobs, which will introduce contrast robbing diffraction into the view, compounding the effects of the huge CO. If you ever decide to observe planets with these, you'll be losing out big time. The CO of the GSO 10" dobs are around 25% if I recall correctly, compared to 43% for the RC.

If you want to do low power DSO's, it would make more sense to get a dob IMO as you get a shorter focal length which will give a wider FOV, and a much smaller secondary obstruction which will help on contrast of faint objects and planets.

And a 10" dob is much easier on your wallet.

No sense in getting these RCs for visual IMO when you have better options for less money to boot.

casstony · #6 13-03-2010, 12:25 AM

I started out with SCT's, tried a 10" dob, then moved back to SCT's again - I prefer the observing position and the fact that I can store the ota inside my house, whereas the dob had to live in the shed. I also have a great mount now (DM6) and I finally got the Sky Commander working properly tonight - it's a joy to use.

I'm basically after the largest aperture cassegrain that weighs around 30 pounds. I'm not keen on exposed optics that aren't dielectric as I don't want to deal with the maintenance. It'll probably end up being a C11 though I would like to hear of an actual observing experience with the RC10. I had a Meade 10" f/6.3 many moons ago which gave decent views with a 40% secondary obstruction.

AlexN · #7 13-03-2010, 01:03 AM

PM Telecasterguru - He has a GSO 10" RC and as far as I'm aware he's used it for some visual observation... Worth a shot.

Alex.

Paul Haese · #8 18-03-2010, 12:54 PM

Just a couple of things.

From the time I tested my RC this is what I noticed.

Stars were refractor like in that they were like little pinpoints of light. Certainly better than an SCT most of the time. As good as my 18" 0.95 strehl mirror and easily equal to my TSA102 refractor.

Contrast and image brightness was reduced remarkably from a Newtonian of the same size. You can use it for visual work but unless you are doing heaps of imaging then it is better to get something a bit faster and with less obstruction.

Can I say the vanes are not as Sab has suggested thicker than a Newtonian secondary. He might be thinking of a Vixen scopes. The vanes on the GSO RC's are pretty thin really.

casstony · #9 18-03-2010, 01:49 PM

Thanks Paul. The only thing I find surprising in your observations is the reduced brightness compared to a newt; I had thought the 99% coatings(98% system transmission) would be at least as bright as the same sized newt with ~ 92% reflectivity (85% system transmission). Perhaps the RC does not have true dielectric coatings?

Thanks for your suggestion Alex; I did PM Telecasterguru but he appears to be off-planet at the moment.

Paul Haese · #10 19-03-2010, 09:09 AM

Tony that is mainly because you will be hard pressed to find an f8 Newtonian these days. The coatings on the RC's are most likely what is advertised, but it is the different focal ratio's that make the difference.

casstony · #11 19-03-2010, 09:45 AM

Paul, I assumed the brigtness comparison would have been done at the same magnification, so the RC probably is brighter than the same sized newt or SCT at the same magnification. While contrast should be poorer in the RC, perception of contrast difference may not have been accurate when using different magnifications for the two instruments.