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Astrotech RC8 OK for visual use?

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#1 markgf

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Posted 13 November 2012 - 06:43 AM

Hi All. I'm new to astronomy and trying for the dumb question of the year award.

The Astrotech RC range is optimised for imaging but good optics and compactness (of the 6 and 8 inch models) attract me for visual use, at least initially.

Do these telescopes put the focus point so far behind the optical tube that using them primarily as a visual scope is not practicable?

Thanks...Mark

#2 David Pavlich

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Posted 13 November 2012 - 08:42 AM

Welcome to Cloudy Nights! The scope can be used for visual and you'll have to experiment with spacers to see where it achieves focus. The scope comes with the spacers that you'll need. You will lose a bit of contrast due to the large secondary mirror (central obstruction). But it'll be ok until you try some real low powered eyepieces.

Have you thought about a Schmidt-Cassegrain like an 8" Meade or Celestron? Dimensionally, very similar and a bit more forgiving on the visual side.

David

#3 markgf

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Posted 13 November 2012 - 09:57 PM

Thanks David. Yes, a small Schmidt-cass might be just the shot. About 15% more unobstructed area than a Ritchey-Chretien. Plus I'm nervous about having the diagonal on an RC sticking out 3 inches behind the optical tube, if that really is necessary.

However, assuming a visual back end on an imaging RC isn't too wierd, I'm happy to trade off a little contrast for improved acuity if RC optics are meaningfully better than SC, and if I can get at least 5" of very high quality unobstructed aperture. A 6" RC with a 50% central obstruction is about the same as a 5.2" unobstructed aperture. (An 8" RC has ~7" equivalent unobstructed). Is there more to it for contrast?

The price of small RC tubes is now competitive with small scmidt-cass tubes, hence my investigations.

Maybe a newbie like me wouldn't notice the difference in optics but I'd like to keep an eye to future requirements too. I'll get fairly good eyepeices (~<$500 for 2 eyepieces and a Siebert telecentric barlow) to compliment the good optics of the scope I buy.

I hear what you say about a low power eyepiece seeing the central obstruction. I love low power viewing (I have indulgent friends). Do you know how to work out minimum magnification before kidney beaning occurs? 30~35x?

Cheers...Mark

#4 David Pavlich

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Posted 13 November 2012 - 10:13 PM

Not really sure of the formula for figuring out minimum useable magnification. Another thing to consider is that an RC isn't the most fun scope to collimate. It's a bit more tricky than an SC. Many RC owners buy a Takahashi collimation scope which is used for the secondary alignment. And if you want to do it right, a good Cheshire is also needed to align the primary. And if you do it very well, you should be able to get by without doing an additional collimation using a star.

With an SC, you center a medium bright star, take it slightly out of focus and get the diffraction rings symetrical, that's it. Something to consider.

David

#5 markgf

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Posted 13 November 2012 - 10:24 PM

OK thanks.
I think I should stop trying to push it uphill. If I get a chance in the near future to try an RC visually, I will. Otherwise, I'll just keep it simple.

I don't suppose anyone has side by side compared the Astrotech RC8 with an 8" SC?

Mark

#6 David Pavlich

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Posted 14 November 2012 - 08:38 AM

I just noticed an error in my post...like I haven't done that before. :p I stated that many "SC" owners buy a Tak collimation scope. It should have been "RC". Just wanted to let you know.

David

#7 korborh

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Posted 14 November 2012 - 09:18 AM

However, assuming a visual back end on an imaging RC isn't too wierd, I'm happy to trade off a little contrast for improved acuity if RC optics are meaningfully better than SC, and if I can get at least 5" of very high quality unobstructed aperture.


RC optics are not better. They have astigmatism and field curvature. And they are extremely hard to collimate precisely. I would recommend the EdgeHD 8" for visual instead. It has better (abberation free) view than the RC8. Better contrast on planets. Its also cheaper. Great for DSO and hyperstar imaging if you go that route.

RC8 suffers from poor mechanics and hair pulling collimation difficulties.

#8 markgf

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Posted 14 November 2012 - 06:19 PM

David
Thats OK, I got what you meant.

#9 markgf

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Posted 14 November 2012 - 09:45 PM

Korborh,
Thanks for your recommendation. I'll put it in the mix. :bow:

You know, if I weren't such a sucker for tech, I'd have to consider circling back to a 6" F8 dob. On the other hand, if not RC or SC, there's that 6" rutten Mak I saw the other day...

cheers. Mark

#10 Jared

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Posted 15 November 2012 - 12:15 AM

An 8" RC works fine as a visual scope, and I wouldn't worry about a visual back sticking out two or three inches from the back of the scope. That being said, I'd recommend an 8" SCT instead if you don't plan on doing a fair amount of astrophotography. I think it has a better set of compromises for visual use. I have owned both an 8" Schmidt Cassegrain and a 10" Ritchey in the past. The SCT should have better performance on the planets in particular, assuming both scopes are of equal optical quality.

Advantages for the SCT in visual use: Better contrast, less expensive, no diffraction spikes, available packaged with a variety of mounts including alt-az computerized fork mounts, much easier collimation.

Advantages for the RC in visual use: Faster cooling (in my opinion, based on experience with both designs--this one is not necessarily consensus), no mirror shift, no coma (yielding sharper stars on the edges of the field when compared to a standard SCT), and less susceptibility to dewing due to the lack of a corrector plate. Also, no spherochromatism with a Ritchey which SCT's have to a very minor extent.

Some other minor differences... The Ritchey will be more work to clean since the primary is exposed to the elements rather than just a corrector plate. Getting the Ritchey collimation the first time can be a real pain in the neck, but the scopes do hold collimation quite well. The SCT will likely be a bit lighter for a given aperture (especially compared to the Celestron scopes--Meade's are going to be closer.). Also, most SCT's are f/10 or f/11while the AT Ritcheys are f/8.

Finally, if the selling point of "no coma" on the Ritcheys is a big factor for you--sharper stars across the field of view--then you could also think about one of the new aplanatic SCT's like the Celestron Edge scopes and the MeAde ACF's. The Meade version, like the Ritcheys, is aplanatic, so no coma to speak of. The Celestron scope is both aplanatic and has a nearly flat field as well, something both the Ritchey and ACF designs lack.

I'm on record as being a big fan of the low-cost Ritcheys from Astrotech and others as imaging scopes. I prefer them to the Edge HD's as astrographs (though the gap has narrowed now that Celestron has released their line of dedicated focal reducers). But for exclusively visual use, I'd take an SCT.

#11 markgf

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Posted 15 November 2012 - 08:45 AM

Wow. Thanks Jared. I can't believe the quality of advice I'm getting here. I think I'll shelve the RC option for now as I'm currently on more of a visual rather than imaging journey.

Compactness and optics/dollar is driving my research. A neighbouring thread nominated a 6" SCT OTA on an AT Voyager mount. That might do, though Celestron's 8" edge SCT would indulge the purist in me a bit more if I can find a not too bulky mount for it. Either would be more flexible long term than a dob if imaging is in my future, even though dobs are really inexpensive and somehow elegantly simple in their own way.

I don't feel the need for a go-to at this stage. I hope an unmotorised push-to and/or a basic planetarium program like Stellarium or Sky Safari will see me get around by star hopping. A combo of Stellarium, binoculars and friends with telescopes has got me on my way (a little bit) so far.

Weather permitting, I get reasonable semi-rural skies about 10 steps away from the ramp off my living room deck, so I don't have to go too far from a laptop with software loaded.

Cheers...Mark

#12 David Pavlich

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Posted 15 November 2012 - 10:49 AM

You're making a good decision, Mark. I'm an SC fan and when used within their wheel house, the SC is a terrific scope! Someday, I hope you get to look through a well turned out C14 or M14. It is, if you pardon the expression, an eye opening experience.

David






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