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Ritchey-Chretiens: How many choices do I *really* have?

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#1 Craig H

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Posted 08 October 2017 - 11:27 AM

Hi all,

 

After narrowing down my next telescope to an RC (most probably one of the 8" varieties), I am trouble finding out (or maybe just understanding) some basic information -- so I'm going to the experts! laugh.gif

 

Can someone explain to me the real choices that one has when trying to decide between the various RC scopes out there?  I am most familiar with Orion, Astro-Tech, and TPO, although I know that Ioptron carries their Photron brand, and also a web search brings up a brand called Levenhuk which I am otherwise not aware of.

 

However, are these all in fact variations of GSO RCs?  If so, are there in fact significant differences between these brands, in terms of optical quality, robustness of focusers, mechanical build, etc., that would allow one to make an informed decision about which scope is best for one's particular needs?  And if one or more of these brands (or others I am not aware of) are in fact built by a different manufacturer than GSO, what are the significant differences?  (Mostly thinking of the mass market, I am vaguely aware that there are high-end manufacturers out there for those with the means but again I'm focused on a portable 8" or 10" size)

 

Thanks everyone for any clarity you can provide.

 

Clear skies!

Craig.


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#2 ChrisPA

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Posted 08 October 2017 - 11:44 AM

They're all made by GSO. The biggest difference between the brandings that I've seen is in the miscellaneous accessories - the types of dovetails included (some are just for mounting the RC on a mount, others are slotted allowing for guide scopes or other imaging refractors to be attached at the top), whether a finder scope is included, one or two finder scope dovetails, Bob's Knobs included or not, etc. Other companies manufacture RCs, but I haven't seen any others in the 6-10 inch variety, and none anywhere near the price of GSO's RCs. I'm hoping to get an 8" carbon RC myself. Whether I buy a TPO, AstroTech or GSO from Highpoint is going to depend on if any of them have it on sale.


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#3 aneeg

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Posted 08 October 2017 - 12:03 PM

The Photron can be installed with a nice motor focuser at an extra cost. I am into this market myself.

 

Arne



#4 rogeriomagellan

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Posted 08 October 2017 - 01:42 PM

Well, I don't understand much about that telescope design. But if you are looking for an 8" aperture tube, as far as I know, you'd have to pick one of the mass-produced RC telescopes. If you decide to go larger like 10", then, you got a few more options, mainly high-end options so I've read, but you'll have to open the wallet and I wouldn't call it portable at all. 

 

http://rcopticalsyst...10military.html

 

The model below is not a Ritchey-Chretien telescope but a Dall-Kirkham design which I thought that could somehow be of your interest:

 

https://www.teleskop...Flat-Field.html

 

I hope this helps.



#5 mclewis1

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Posted 09 October 2017 - 07:55 AM

Gathering solid information before you buy is important (although you certainly can go overboard too). GSO does indeed offer those RCs to many different resellers and for the most part the scopes are the same aside from any bundled accessories. But GSO also builds and offers different quality versions of many of their scopes for different price points. It would be good to ensure that the optical quality is the same between any scopes you might be comparing.

 

Be careful of only looking at anecdotal information that might be dated. Things can a do change over time ... for example GSO has certainly increased the optical and mechanical quality of their RC scopes over the years. I'd pick few different folks who carry the scopes and ask them directly about the scopes you are interested in. Add to that some user experiences and such and you should have a pretty good picture of what you choices are and how they will compare.


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#6 Richard Whalen

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Posted 09 October 2017 - 09:45 AM

While GSO has improved their line of RCs over the years, branded under various names I've been told they may be built to different specs or that certain companies get the pick of the litter for a bit more money. Don't know if it is advertising hype by rebranders. I bought a VRC10T through Mallincam, which far exceeded my expectations. Don't know if I just got lucky, or ? Mine has a very good mirror, nice mechanics and holds collimation well once we spent around 4 hours with it on the bench making very slight tweaks to make sure everything was centered and square.

 

While the stock focuser worked fine, I replaced it with a custom FLI focuser/OAG/flip mirror system my friend had laying around and modified for use with my 450d which allows super fine electronic focus.

 

Your mileage may vary........

 

 

 

 



#7 glend

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Posted 09 October 2017 - 09:51 AM

GSO have a very good reputation for quality mirrors, which is why i bought  a 8 "GSO, which i owned for about 18 monrhs. It was a real disappointment: poor focuser support, near impossible to keep collimated and lack of the focuser collimation ring as standard,  required additional money to be spent to make it useful, etc. Maybe they are better now, but the forums seem filled with bad news about them, just do a search. Optically fine but mechanically poor.


Edited by glend, 09 October 2017 - 09:54 AM.

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#8 George N

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Posted 09 October 2017 - 11:31 AM

Well, I don't understand much about that telescope design. But if you are looking for an 8" aperture tube, as far as I know, you'd have to pick one of the mass-produced RC telescopes. If you decide to go larger like 10", then, you got a few more options, mainly high-end options so I've read, but you'll have to open the wallet and I wouldn't call it portable at all. 

 

http://rcopticalsyst...10military.html

 

The model below is not a Ritchey-Chretien telescope but a Dall-Kirkham design which I thought that could somehow be of your interest:

 

https://www.teleskop...Flat-Field.html

 

I hope this helps.

Optical Guidance Systems is still around making RCs and Classical Cass's in the 10 to 40 inch size. The current price for an OGS 10-inch RC tube assembly is $16,500. See: http://opticalguidancesystems.com/



#9 ChrisPA

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Posted 09 October 2017 - 04:17 PM

GSO have a very good reputation for quality mirrors, which is why i bought  a 8 "GSO, which i owned for about 18 monrhs. It was a real disappointment: poor focuser support, near impossible to keep collimated and lack of the focuser collimation ring as standard,  required additional money to be spent to make it useful, etc. Maybe they are better now, but the forums seem filled with bad news about them, just do a search. Optically fine but mechanically poor.

Early models had a design flaw - the focuser was attached directly to the mirror, so if your camera weighed more than a few grams (and they all do), it'd skew the primary out of collimation. I saw a fix for this once - the user bought a longer dovetail and used risers to attach the camera to the dovetail so no weight was on the focuser. It was a ridiculous workaround, but I believe it worked. I don't know which year they fixed this.


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#10 andysea

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Posted 11 October 2017 - 12:37 PM

I think the new cell design that decouples the focuser from the mirror cell is only available on the truss tube versions.



#11 telfish

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Posted 11 October 2017 - 08:54 PM

I have had two of these an 8 inch and a six inch. Hated both of them. Neither held collimation when slewing from place to place even with a moonlite focuser. They have a design flaw that is difficult to overcome.

 

Some swear by them and some swear at them!


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#12 Xeroid

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Posted 12 October 2017 - 12:03 PM

I'm somewhat confused (not news..) as the Orion site info for the 8" (200mm) aperture Ritchey-Chretien says in part:

 

 

"Unlike Schmidt-Cassegrains, the primary mirror of the 8" Ritchey-Chrétien telescope is fixed in-place to eliminate annoying "image shift" that can wreak havoc during imaging sessions"

 

and this as well:

 

"The Orion 8" RC Astrograph features a machined, dual-speed (10:1) Crayford focuser equipped with a sturdy linear-bearing mechanism for enhanced rigidity and stable support of heavy astrophotography equipment"

 

Sooooo given the above statements, looks as if some of the previously mentioned RC design flaws have been corrected?

 

Can a recent purchaser confirm?



#13 telfish

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Posted 12 October 2017 - 12:15 PM

I'm somewhat confused (not news..) as the Orion site info for the 8" (200mm) aperture Ritchey-Chretien says in part:

 

 

"Unlike Schmidt-Cassegrains, the primary mirror of the 8" Ritchey-Chrétien telescope is fixed in-place to eliminate annoying "image shift" that can wreak havoc during imaging sessions"

 

and this as well:

 

"The Orion 8" RC Astrograph features a machined, dual-speed (10:1) Crayford focuser equipped with a sturdy linear-bearing mechanism for enhanced rigidity and stable support of heavy astrophotography equipment"

 

Sooooo given the above statements, looks as if some of the previously mentioned RC design flaws have been corrected?

 

Can a recent purchaser confirm?

They are drawing a distinction with an SCT scope. In that an sct mirror moves up and down to focus. A RC mirror does not do that, however there is movement because there is a need to move the mirror for collimation. So it tends to move on it's springs as the scope tracks. The weight of the imaging train and the focuser is bearing on those springs.

 

The Focuser is still attached to the mirror and moves with it. This is what causes issues with loss of collimation as you slew.

 

Bigger and far more expensive RC scopes have the focuser and mirror decoupled. They do not suffer from this issue.


Edited by telfish, 12 October 2017 - 12:17 PM.

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#14 ChrisPA

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Posted 12 October 2017 - 12:54 PM

With regard to the focuser being attached to the mirror cell, I believe this has been corrected in the 6" and 8" version, but I don't have any evidence aside from a Facebook comment a few months ago.



#15 Jared

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Posted 12 October 2017 - 01:02 PM

I had the original 10” version where the focuser is attached to the mirror mount. I used it with an ML series camera and filter wheel, so not a super light imaging train. For whatever reason, I never had problems with it losing collimation as I skewed to different parts of the sky. Don’t know why it wasn’t an issue, but it wasn’t.

I know the design is different on the truss tube versions, so people who are worried about this aspect of the design can safely purchase a truss scope. As far as I know the solid tube scopes remain unchanged.

#16 Jared

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Posted 12 October 2017 - 01:10 PM

With regard to the focuser being attached to the mirror cell, I believe this has been corrected in the 6" and 8" version, but I don't have any evidence aside from a Facebook comment a few months ago.


If the photographs on the AT website are correct, all the solid tube scopes have the focuser connected to the mirror cell. Of course, the photographs could all be older. You can at least see that the truss tube focuser are different.

#17 telfish

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Posted 12 October 2017 - 01:23 PM

With regard to the focuser being attached to the mirror cell, I believe this has been corrected in the 6" and 8" version, but I don't have any evidence aside from a Facebook comment a few months ago.

That would need a fundamental redesign of the scopes. I do not believe there is room to incorporate that redesign in the size of the smaller RC scopes.

 

I bought a 6 inch 2 months ago and they were attached.



#18 ChrisPA

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Posted 12 October 2017 - 01:39 PM

Yeah, I went ahead and investigated after I typed - I wrote directly to Astro-Tech and within minutes they confirmed it is still attached.

 

Thus: I was wrong; the focusers are still directly attached to the mirror cell.

 

I'm trying to figure out if I would have problems here - I've got a ZWO ASI 1600, 8 position filter wheel, off-axis guider, etc. Is that too much weight?

 

I really think this is the scope for me - I want a higher focal length imaging scope; I own a small refractor, but it just hasn't grown on me: dew and I just don't get along, and well, I grew up on Hubble Images - images without diffraction spikes just lack something in my opinion, so SCTs just don't really appeal to me at least for DSO imaging. My only other option would be the VMC-200L or VC-200L - I like diffraction spikes, but I don't like them THAT much, plus I'm not convinced that my field would be 100% flat with those scopes. Also the price for their proprietary focal reducers is outrageous. Even the scopes themselves seem overpriced. AND: I really want a carbon scope. I mainly image with a carbon Newtonian and usually go DAYS without refocusing (at least in the summer and winter when temps are relatively stable). I'm not willing to refocus several times a night as I do my imaging indoors with my scope on the roof. I generally visit it once a night to take off the cloak and dust cap and check focus and don't see it again until I put the dust cap and cloak back on the next morning. The 8" GSO carbon fiber RC offers me just about everything I'm looking for... plus a potentially fatal flaw?

 

What do you guys think?


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#19 telfish

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Posted 12 October 2017 - 01:50 PM

Yeah, I went ahead and investigated after I typed - I wrote directly to Astro-Tech and within minutes they confirmed it is still attached.

 

Thus: I was wrong; the focusers are still directly attached to the mirror cell.

 

I'm trying to figure out if I would have problems here - I've got a ZWO ASI 1600, 8 position filter wheel, off-axis guider, etc. Is that too much weight?

 

I really think this is the scope for me - I want a higher focal length imaging scope; I own a small refractor, but it just hasn't grown on me: dew and I just don't get along, and well, I grew up on Hubble Images - images without diffraction spikes just lack something in my opinion, so SCTs just don't really appeal to me at least for DSO imaging. My only other option would be the VMC-200L or VC-200L - I like diffraction spikes, but I don't like them THAT much, plus I'm not convinced that my field would be 100% flat with those scopes. Also the price for their proprietary focal reducers is outrageous. Even the scopes themselves seem overpriced. AND: I really want a carbon scope. I mainly image with a carbon Newtonian and usually go DAYS without refocusing (at least in the summer and winter when temps are relatively stable). I'm not willing to refocus several times a night as I do my imaging indoors with my scope on the roof. I generally visit it once a night to take off the cloak and dust cap and check focus and don't see it again until I put the dust cap and cloak back on the next morning. The 8" GSO carbon fiber RC offers me just about everything I'm looking for... plus a potentially fatal flaw?

 

What do you guys think?

 

 

I would seriously consider an Edge 8 inch with a focal reducer. That makes it a 1500mm F7 scope with a flat field. add a moonlite to it and add autofocus and you are home.

 

I just could not get decent results with the RC. It was a constant fight with collimation, longer exposures which you need with that scope were a nightmare. Your mileage may vary. There are people who are happy with it and are taking great images, so it's possible.

 

Don't forget that with the RC you will need a field flattener, a tilt plate and also if the stock focuser is not up to it a focuser upgrade.

 

You can add defraction spikes in post processing if you really like them!


Edited by telfish, 12 October 2017 - 01:55 PM.

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#20 ChrisPA

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Posted 12 October 2017 - 02:00 PM

Ugh... they never have those focal reducers in stock. Plus, they're way more expensive than the CCDT67, which I was planning on using with the RC8. I might risk it. I REALLY just don't want to deal with dew... I'm also a bit phobic about cracking a corrector plate. I just don't think I'd handle an SCT well. 

 

But hey, it looks like if I did get an 8 Edge, we'd own all the same scopes.



#21 telfish

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Posted 12 October 2017 - 02:07 PM

Ugh... they never have those focal reducers in stock. Plus, they're way more expensive than the CCDT67, which I was planning on using with the RC8. I might risk it. I REALLY just don't want to deal with dew... I'm also a bit phobic about cracking a corrector plate. I just don't think I'd handle an SCT well. 

 

But hey, it looks like if I did get an 8 Edge, we'd own all the same scopes.

I have the Lepus .62 reducer ($200) , works well, plus I can image at 2000 if I wish to. Dew is not an issue with a dew heater. I do not use a dew shield normally though I have one. Have had 2 SCT's never thought I would crack a corrector. I have never dropped one though!

 

My scope choice has been honed through many mistakes! I am really happy with my current lineup. They give me a range of focal lengths and speeds. I really like the CF ES newt. As you said it seldom needs focusing.



#22 barrett_flansburg

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Posted 12 October 2017 - 02:16 PM

Diffraction spikes would be easy enough to add to an SCT. Just put two perpendicular strips of masking tape across the front of the corrector cell. You can even rotate them as desired. 

 

I had a similar bad experience with a 16" Astro Tech RC. Collimation was impossible because the primary mirror slid around inside the mirror cell when the telescope position was changed. I got rid of it and bought a C14 Edge instead, which allows me to image at F11, F7, and F1.95 with Hyperstar. Great for planetary and deep sky. I'm very happy with the C14 Edge. 

 

Barry



#23 telfish

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Posted 12 October 2017 - 02:21 PM

Diffraction spikes would be easy enough to add to an SCT. Just put two perpendicular strips of masking tape across the front of the corrector cell. You can even rotate them as desired. 

 

I had a similar bad experience with a 16" Astro Tech RC. Collimation was impossible because the primary mirror slid around inside the mirror cell when the telescope position was changed. I got rid of it and bought a C14 Edge instead, which allows me to image at F11, F7, and F1.95 with Hyperstar. Great for planetary and deep sky. I'm very happy with the C14 Edge. 

 

Barry

Or this.

 

https://www.youtube....h?v=m_MMXoP8yWY



#24 telfish

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Posted 12 October 2017 - 02:35 PM

Another option would be to get the APM 2 inch comma correcting barlow for your newt. That would give you 1624 mm at F7.9.

 

Reports seem to suggest that it works really well. Don't go for the 2.7X 1.5 inch one. I tried that and though it worked OK it was just a bit too much magnification for the scope.



#25 WadeH237

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Posted 12 October 2017 - 02:38 PM

I would seriously consider an Edge 8 inch with a focal reducer. That makes it a 1500mm F7 scope with a flat field. add a moonlite to it and add autofocus and you are home.
 
I just could not get decent results with the RC. It was a constant fight with collimation, longer exposures which you need with that scope were a nightmare. Your mileage may vary. There are people who are happy with it and are taking great images, so it's possible.
 
Don't forget that with the RC you will need a field flattener, a tilt plate and also if the stock focuser is not up to it a focuser upgrade.
 
You can add defraction spikes in post processing if you really like them!


I have a 6" RC that is great fun to image with, except for the focuser mount issue that's been discussed above.

 

I would love to replicate the 1260mm focal length of the 6", but with very good mechanical quality.  I decided to take a chance on the EdgeHD 8" with a reducer.  It has been a decidedly mixed bag.

 

I started with an Optec Lepus reducer at .62x.  On paper, it should get me to 1255mm, which is perfect.  The reality is that, even with exact spacing between the reducer and the imaging sensor, you need to adjust the main focuser on the SCT to reach focus.  In the process of doing so, you end up increasing the focal length to about 1450mm.  This would not be a huge deal by itself, but when you get the system to reach focus, you've moved the primary mirror enough that it's spacing with regard to the Edge's internal corrector is such that you are no longer getting optimal correction across the entire imaging field.  With this configuration, stars in the corners of my ASI1600 are slightly elongate.  The aberrations are not particularly large, but I really wanted correction across the entire field.

 

I have located and purchased one of the hard-to-find Celestron .7 reducers for the EdgeHD 8.  With correct spacing, it should get me well the well corrected stars that I am looking for, and should get me 1422mm focal length.  I've somehow ended up making a bunch of configuration changes with my mounts, and the weather has not been great, so I've not had a chance to test with the Celestron reducer yet, but I have high hopes that it will work as advertised.

 

In all cases, you should be aware that for good performance, you will need to use an off-axis guider to be able to do exposures longer than about 3 minutes.  Much longer than that with an external guide scope, and differential flexure due to the movement of the primary mirror becomes objectionable - even with the primary mirror locked.  To be sure, this is subjective.  I found that I could live with 3 minute exposures (although I wasn't terribly happy with the slightly elongated stars).  At 5 minutes, I just through the data away as I considered it unusable.  I spend an entire night of testing with swapping out guide scopes and different guiders to prove that the problem was with the OTA and not any guide scope, the mount, or any bracketry that connects everything.

 

And the final complication is that, for the 8" EdgeHD, both the Optec Lepus and Celestron reducers require 105mm of spacing between the reducer and the imaging sensor.  This is precious little to work with once you add an OAG into the mix.  I was able to get it to work with my Teleskop Express Thin OAG.  I also have a Celestron OAG, and it's not even close to fitting into the available space.  I also have an ONAG.  It can (barely) fit with the main sensor, but to get my guider and main camera parfocal, I have to push the main camera back a bit, which takes me out of the 105mm spacing again.  This means that the ONAG is a non-starter with the 8" EdgeHD (it should work with the 9'25" and up Edges, since they have significantly more back focus with their Celestron reducers).  Also, I like to use a camera rotator when I use OAG, so that I can pre-select a guide star and automatically position the OAG to land it on the guide sensor.  There is no way to get a rotator to fit into the available back focus with the OAG on and 8" EdgeHD.

 

I'm certainly not giving up on the EdgeHD.  I just wanted to point out that there are compromises with it, as there are with anything.  It has very good optics and is a great all-around scope, but has some limitations.  I'm going to see how it goes with the OAG with regard to guide star selection.  I've already tested in star-rich fields and demonstrated that I can guide for as long as I want with round stars (I tested up to 30 minutes on a single exposure).  And my mount is good enough that I can go with really long guide exposures if necessary to get a guide star.  60 second or longer guide exposures are no problem.  The limit on guide exposure length is actually that primary mirror movement again.

 

If I find that I really, really need to get a good setup going for that 1260mm focal length, I'll probably end up with a 5" APO refractor at some point in the future...


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