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Uranium235

AA 6" RC Astrograph?

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Just seen this on the Altair Astro website, and it looks rather interesting, at an interesting price.

http://www.altairastro.com/product.php?productid=16420&cat=268&page=1

I know an RC is pretty slow, but can you use an SCT reducer with it to get it down to about f6(ish)? How about optical performance? And are there any serious drawbacks to using an RC telescope (apart from the native speed)?

One of these reduced to about f6 (913mm focal length) is a tempting prospect, especially at £399. And its just about light enough to go on the CG5 (5.5kg) - it would also come in very handy when galaxy hunting (which is where the ED80 fails a bit).

Or should I stop thinking about it and just stick with the fraccys? :)

Cheers

Rob

Edited by Uranium235

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We were chatting about these on another thread. The 50 percent CO puts me off.

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You mean only half of the field is useable?

Ahh.. just figured it out... central obstruction!

Though I doubt whether that size of obstruction makes any difference if imaging (waiting for Olly to wade in....lol)

Edited by Uranium235

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I'd have a look at Telescope Service's website if you're that interested :)...

FWIW, I had one for a short time (FWIW it wasn't sold on because I didn't like it) and while the build quality was better than it's price indicated, the big issue for me was and still is the lack of dedicated reducer. You can use alternatives (Intes Micro, Astrphysics) but they're either not cheap or have problems fully illuminating large sensors, plus results seem to be a bit hit and miss. Imaging at f9 on galaxies or nebulae felt awfully slow in comparison even to an ED80. John (JGS001) has one and has produced a couple of decent images of open clusters so it's do-able but I'd factor in and read up on alternative reducers.

TBH, if was looking at going down that route again, I'd probably go with a C6 as the accessories are already there, similar price-wise and given a bit of time, probably just as good.

Tony..

Edited by Whippy

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Thanks Tony, I took a look a TS earlier on (in the reducers section), E185 for the .67x reducer. Not sure what effect it will have on the flatness though - but it seems designed for what seems an identical RC scope (GSO RC).

The RC seems appealing in the fact that there wont be any mirror flop and its got a proper focuser.

Bear in mind I would be using this with nothing bigger than the 314L+, so maybe field illumination wont be an issue.

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If it works then go for it, check with Wolfie at TS, he's normally pretty good :). When I bought mine from him, he was recommending the IM reducers which are a bit more expensive.

Tony.

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Cheers, will get in touch with him if I decide to go for it. It looks like they have the one to do the job now:

Astro Physics CCDT67 0,67x Reducer 2' - z.B. für GSO RC

Need to do a bit more research first though, as in collimation difficulty or any other likely pitfalls. But at the moment its looking like its the only way to get a bit more FL without sacraficing image quality, or having to buy a new mount. Also, it would mean less re-focusing between filters (though it never hurts to check).

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As Tony has said, I have a 6"RC and it's great. F/9 is a little slow, but to get the equiv of 5 minutes on my 80ED I need 7.5 minutes on the RC... but I can hit 10 minutes. The collimation I found surprisingly easy, given that I'd never collimated a newt type device before, and despite transporting it 20 miles by car to a society meet and back, it held perfectly. I plan on trying a 0.8x reducer on it if I ever get the chance. If you do go for the RC, you'll need some tubes between the body and the focuser, at least, I do with my SLR.

It's also light enough I can mount it side by side with my ST80 on my HEQ5.

The scope still wasn't collimated properly for this image.

http://stargazerslounge.com/imaging-deep-sky/133998-m51-rc-repro.html

http://stargazerslounge.com/imaging-deep-sky/136916-m3-second-dso-my-rc.html

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Thanks for the demo John, Ive noticed that with the DSLR you seem to get a little bit of coma towards the right side of the frame. But since the 314L chip is much smaller, I dont think that will be a problem.

The desicion whether to get one will be entirely based on whether the 0.67x reducer will work with it becuase that takes it down to f6.04. Which would make it a nats do-dah faster than my reduced ED80 (f6.38), and I think f6 is a speed I can live with (for now).

I originally had grandure plans of having a Quattro, but once I found out its weight it meant buying a new mount. Then it was the plan of reducing a Megrez 90 to just under f5, but that would have meant going backwards in focal length - and I couldnt justify the cost for the medium speed increase (would have worked out to about a grand).

So im hoping this will be a good compromise, and will sit easier with the wife in regard to the cost :)

Edited by Uranium235

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I'm not sure what's causing the coma. An RC is sposed to be coma free. Either I've still not quite got the collimation bang on, or I've got a little focuser slop. I got the GSO variant from TS that Tony mentioned.

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Ahh, I took a look at your blog last night and saw that you mentioned your camera wasnt sitting quite straight at the time of shooting - so thats probably what was wrong.

cheers

Rob

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The RC AT is a fantastic telescope. I was lucky to get one before it was discontinued. The pictures show a lot of details and it is sharp to the edge. It is true that is a bit slow, but it is good for galaxies and clusters, even M31 or M33 since you can make a mosaic. I really do not see any problem with it. It has a large obstruction so it would be similar to a 5" refractor in term of light gathering but at a fraction of the price (compared with a 5" refractor). AR telescopes are designed for astrophotography so they are not very good for observing. For a 6" I won’t recommend a reducer, instead if you want a faster telescope buy an 8" reflector F4 from Astrotech for almost the same price. Compared with an 80mm refractor you still get more light (photos per unit of exposure time). I have compared that before. Long focal length allows you to collect more photons from the object, since there is less background.

Edited by rroesch

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Just a thought, have you got a coma corrector for your 6" newt Rob? I used my ED120 for ages at it's native focal length (900mm) and it was fine but adding in a x0.8 reducer and bringing down the f/l to 770mm didn't make a huge difference in terms of detail. f5 at 750mm would be pretty decent in my book and you still get diffraction spikes, just like the RC :). The other alternative if you've got the money: Intes-Micro Astrographs - Telescopes UK: Telescopes & Telescope Accessories in your only London shop .

Tony..

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RC design does not need a coma corrector. However, you need to have a good collimation

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Just a thought, have you got a coma corrector for your 6" newt Rob? I used my ED120 for ages at it's native focal length (900mm) and it was fine but adding in a x0.8 reducer and bringing down the f/l to 770mm didn't make a huge difference in terms of detail. f5 at 750mm would be pretty decent in my book and you still get diffraction spikes, just like the RC :D. The other alternative if you've got the money: Intes-Micro Astrographs - Telescopes UK: Telescopes & Telescope Accessories in your only London shop .

Tony..

I have the MPCC for the newt and ive got it to work with the 314L+, but its a matter of contrast. Its great to run at f5, but the contrast is nowhere near that of the ED80, and so I find myself flicking back and forth between them. The RC6 seems to have much better contrast than a bog standard 6" newt, but from what ive now read - the reducers available might not do the job properly ;)

Though I wish the mount could take both my scopes at the same time, Ive got a modded 1000d sitting idle too. Actually, that might be the key..... sell the CG5-GT, get an EQ6 and bung everything I have on it?

Would have to do the maths, but it depends on whether I could get £250 for selling the CG5. That would only make the NEQ6 £80 more (£680) than buying the RC and its reducer (about £600).

Damn.... talked myself into that one.... lol :) Seems the logical choice though.

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If it's an isssue of contrast (and TBH I can't think why), then the RC would be worse than the newt because of it's huge CO. Even then, that normally apllies to visual, I haven't come across that for imaging and I can't think why it would.

I've only imaged with refractors (my RC got used maybe three or four times) so I wouldn't know if you have to process objects differently with data from different scopes.

I think your thought process about upgrading your mount is possibly more sound though, even going to a HEQ5 would be a decent improvement. The only alternative I can think of scope wise would be an Intes Micro MN56, but even then I don't think your mount would cope and it'd only work with your Atik, DSLR's a no-no.

Tony..

Edited by Whippy

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I'm not sure on the contrast issue either. I've used mine for lunar mosaics and planetary... It worked ok. Although I never completed my 2x barrowed mosaic (I forgot to lock the focusser and it slipped after the first 40 panes and I didn't notice till I'd got to 50, and the clouds were moving in :).

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In principle contrast ought to be 'addable' in processing, I'd have thought. You can usually award yourself more than you actually want in Ps so there should be some spare.

I would only use a slow scope with a CCD camera since the DSLR images which rival CCD, as a few do, seem to come from very fast astrographs. Thermal noise limits exposure length.

There was a guy getting nice results from a 6 inch RC on UKAI a couple of years ago. My own feeling is that I would only fancy one as a way of getting a long FL on a light mount. If I were going to reduce it I think there are more straightforward solutions than a complex RC. Yes, you know I'm going to say refractor!

Small point, also; RCs do not have perfectly flat fields but Yves showed me an image from his 8 inch on a full format chip and it wasn't bad at all. I don't know how flat the 6 inch is.

In GWR's original instruments the photoplates were slightly curved.

Olly

Edited by ollypenrice

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Olly, I bought mine for the longer focal length on me heq5, to use as well as my C80ED. So far it's working nicely with my dslr, I worked out that the equiv exposure to 5mins on the frac is 7.5mins with the RC. Of course some more clear nights would be nice.

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