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

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#26 Jared

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Posted 12 October 2017 - 02:56 PM

I was able to get very good results out of my 10” RC with an 8300 based camera—same size chip as your 1600. I did use one of the tip/tilt plates to get everything square. Collimating the RC is definitely more work than collimating an SCT, but my scope did not need to be adjusted very often even after being loaded into a car and bounced around for an hour or more.

I don’t know why I never experienced poor collimation from the camera’s lever arm. Not everyone does for whatever reason. Maybe I’m less picky? Not likely, but possible.

That being said, the scope is far from perfect, and you should know the trade offs...

At the time Inised mine there was no available matched focal reducer, so f/8 was it. There also was no matches field flattener, though I found I didn’t quite need one with the 4/3” chip. It was borderline, though, and I generally used a flattener from my refractors that gave very good results. Certainly it took the scope out of “borderline”. You are planning on the AP 0.67 reducer. I believe others have used that with good luck.

You mentioned wanting something you won’t have to focus often. The carbon fiber version is not immune from needing to be refocused. Unfortunately, there is no slip built into the dovetail bar, so it’s expansion and contraction physically compressed and stretches the telescope. I was always worried the compression would be uneven with just the one dovetail, so I always kept the top plate attached as well. End result was that I needed to focus a couple or three times a night with typical falling temps of, say, 7*C per night.

Mine was nearly immune from dewing, as you would expect.

I had the Feathertouch focuser and it was very good.

That’s all I can remember about imaging with mine. A couple of my best photographs were made with the 10” version of the scope.

#27 Jared

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Posted 12 October 2017 - 02:59 PM

Mine was a terrible scope visually, by the way. The very large central obstruction made it a very poor performer on the planets and for double stars (more an aesthetic issue for double stars). It did fine for faint fuzzies. If your usage will be mixed photography and visual I ‘d get an Edge instead. If mostly photography, the RC is a valid choice.

#28 telfish

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Posted 12 October 2017 - 03:02 PM

Wade I went the 152 mm refractor route with an APM apo doublet. problem was the blue channel was always bloated compared to the red and green. So I sold it. I just can't justify the costs of a better Triplet in that size though I would love a TEC 140!

 

Spacing is tight with the 8 inch edge. I am experimenting with spacing and the position of the reducer. I may have to replace my current moonlite on the scope with one that allows the reducer to sit inside the body attached to the drawt ube. That way I can save some space and get the reducer close to the correcting lenses.



#29 ChrisPA

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Posted 12 October 2017 - 04:00 PM

Yeah, I mean, I've thought about building a mask with guitar string if I were to get an SCT, but there's something that feels 'dishonest' about it - you're introducing an aberration unnecessarily. (Stars themselves don't have spikes, even if they look prettier with them...) 

 

That Barlow... man, I don't know. It's expensive and seems too experimental to me. It'd be fun to play around with, but having used Barlows for planetary imaging, I just know you loose too much light. I think there's something to be said for a scope's native focal length. Do you have any reviews of that handy? I couldn't find any.

 

I also have the Hotech SCA field flattener, which I've heard works great with these RCs, even if my chip size wouldn't make it necessary. 

 

I've never seen a good review of that Optec Lepus reducer. Quite the contrary, actually. Are you happy with it?

 

I do have an off-axis guider - I haven't gotten it to work yet, but that has to do with having set it up incorrectly (right next to the filter wheel and behind the spacer instead of in front of the spacer) and then starting a series of projects that forced me to not touch my camera. I'll figure it out once I complete the last three projects - I plan on switching it over to the refractor late next week - I'll fix it by then.

 

There are just so many things about SCTs that just don't appeal to me. I mean, only an EdgeHD would even be considered, but the fragile corrector; the movable mirror (even with the Edge's mirror locks...); expensive, difficult to get (and heavy) proprietary reducers (or ones that aren't too well reviewed) and DEW. It gets very dewy in southeastern PA - I usually wake up to dew on my Newt's mirror, but it's only once affected subs. But as for dew heaters: I'm not electronically inclined enough to build my own dew controller and dew controllers cost WAY too much for what they are (a simple resister).

 

As for my refractor: I've kind of put up with dew as I've so far mainly used it as a travel scope for imaging away from home where dew has been less of an issue. It's always fogged up around 4:00 a.m. at the latest though and I do want to start using it at home this month, thus I just got a USB powered camera lens dew heater. It's a test, but it cost $15 and not $150 (plus the dew strap).

 

I'm just not convinced the 8" EdgeHD is for me. I have a friend that images with the TPO 6" and a QSI with off-axis guider. He's got a Feathertouch on it, but if I put a Moonlite on, I'm figuring I'll be able to get it to work. I'm just disappointed to discover that this is still an issue. It seems like it'd be a relatively easy thing to fix with a little effort, and the fact that they did fix it on their bigger scopes (but not the most popular two) just bugs me.



#30 telfish

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Posted 12 October 2017 - 04:15 PM

i use this for a dew controller. Will control 8 amps at 12 volts $6.60 shipped, now that's a bargain. I use it for two dew strips  and it works really well. 

 

https://www.amazon.c...ed light dimmer

 

Only just started on using the Lepus so the jury is out.  The 2 X coma correcting barlow does make the scope an F8 so not much different from a SCT or the RC. I appreciate your concerns though. I will try and find the reviews I read.

 

Give the 8 inch RC a whirl, you can always sell it! There is a market for them.


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#31 ChrisPA

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Posted 12 October 2017 - 04:55 PM

Whoa. How do you plug in the dew strip to that LED light dimmer?



#32 telfish

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Posted 12 October 2017 - 06:06 PM

Whoa. How do you plug in the dew strip to that LED light dimmer?

I feed 12 volts to the input and then use a couple of phono flying sockets from the output to plug the dew straps into.



#33 Craig H

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Posted 12 October 2017 - 08:49 PM

Soooooooooooo.......  to summarize this excellent thread so far,

 

(a) the TCs offered by Orion, OPT, and Astro-Tech are all essentially the same scope and are manufactured by GSO;

(b) the focuser is attached to the mirror on the tube versions of the GSO RC;

© there has been no recent design change to address this;

(d) this is recognized as a design flaw in general as it causes the mirror to shift slightly during imaging but does not seem to affect everyone;

(e) an OAG is an advantage if exposures more than 3 minutes are desired for this scope to compensate for this design flaw;

(f) collimation is a bit of a PITA; but despite (a) - (f)

(g) for pure AP the advantage goes slightly to the RC vs. the EdgeHD 8" but for visual/mixed the Edge is preferred

 

Have I captured the gist adequately?

 

Given the above maybe my best bet is to wait for a used model to come up on AM and see how it goes.  Trouble is I'd plan on pimping it with a flattener and Moonlight focuser, I'd hate to plop the extra money into it and still not be satisfied... undecided.gif

 

Clear skies!

Craig.


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#34 HockeyGuy

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Posted 13 October 2017 - 12:21 AM

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?

Yikes. I've been considering the 8" RC from GSO for a while, I'm glad I found this out beforehand. I'm also interested in imaging at a longer focal length than my current refractor. Something that yields a flat field which goes without saying. The Edge 8 and the 10 Quattro were my other considerations. Recently I read on CN about a new CMOS that should be available soon. It has a smaller FOV than the ASI1600/QHY163M, 12 bit but uses smaller 2.4 um pixels (it's the Sony IMX183 sensor). Using this with my 80mm APO will yield 1.29 arcsec/pix. This sensor is relatively small, but this image scale would suffice my needs for a while.

 

Anyways, I hope more information is poured into this thread. It has been helpful so far!



#35 ChrisPA

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Posted 13 October 2017 - 10:03 AM

In response to telfish:
Thanks for the info on the dew heater - I think I'm going to go in a slightly different direction, but one that's almost just as cheap:

For the strip, this one does not use the RCA phono plug: https://www.amazon.c...uct/B013QHFMDS/
The dial: https://www.amazon.c...uct/B018LWQEDU/
Power source (does 3 amps sound right to you?): https://www.amazon.c...uct/B00DKSI0S8/

Is there anything I'm not thinking about? I'm just not good with bare wires, and I usually wake up to a coating of dew over everything. Bare or kind of bare wires tend to scare me. At any rate, I never would have thought about an LED dial - thanks for the tip!

 

In response to Craig and Adam:

Yeah, I went back and looked at Astrobin and images from people I know who use the 6" and 8" versions - I actually don't see collimation problems anywhere. In fact, images from these (and the 10" version) are just some of the sharpest, most contrast-y images I can find. Of course, I'm not with them in the field struggling to collimate, but it's clearly possible. With the carbon fiber, diffraction spikes, general light weight (16.5 lbs for the carbon fiber version), fixed mirror, lack of dew issues, and the cachet of just having a scope with the same design as the Hubble, I'm still convinced it's the scope for me. Now, I'm going to have to slew 2-3 times a night, but if I get it with a Moonlite as planned, I really think I'll be alright. People have reported problems, but it hasn't been everybody and the focuser is known to be a weak point on this scope. I'm really thinking that upgrade will solve most issues.

 

Anyway, this has been an informative discussion and has given me a lot to think about, probably solved my dew issues with my refractor and pointed me in the direction of a great RC collimation procedure. I really need to spend more time on Cloudy Nights...


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#36 ChrisPA

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Posted 13 October 2017 - 10:19 AM

Adam, I forgot to mention: I have the 183C - I would LOVE to try out a 183M. That sensor is ridiculously sensitive (far more sensitive than my ZWO 1600), but it has amp glow out the wazoo. This can be completely calibrated out provided you take perfect darks and never use bias frames. Color is just not for me, but a 183M is a game-changer. The sensor is significantly smaller than the 1600's so be aware of that. Also, the small pixels are going to be helpful at lower focal lengths (think 80 mm refractor), but you'll be seeing-limited at higher focal lengths. HOWEVER: with a 183M, you'll be able to use software binning at higher focal lengths and end up with 9.6 micron pixels at 2x2 binning (effectively - remember it's software and not hardware binning). With a back-illuminated CMOS sensor and 9.8 micron pixels, you could do the faintest tiny galaxies with 60 second subs (or shorter). 

 

I couldn't really process images until I switched to monochrome, so most of my work with the 183C is pretty awful, but here's a sample. But pay attention to the amount of data. For the M20 (taken through haze), I only gave it about 1.2 hours:

http://www.astrobin.com/277068/ (still a total beginner when I took this)
http://www.astrobin.com/277070/ (subs were too long, so the stars are a bit saturated)
http://www.astrobin.com/300879/ (severe distortion at the edge due to a misaligned focuser)
http://www.astrobin.com/300946/ (taken through haze, heavily cropped)
http://www.astrobin.com/309055/ (dew, haze, transparency problems...)

It's also good for solar system imaging:
http://www.astrobin.com/298034/
http://www.astrobin.com/291163/



#37 telfish

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Posted 13 October 2017 - 11:07 AM

Chris. 3 amps may be OK depending on which brand dew strips you are using and their length. Personally I would go with an 8 amp as that gives you far more overhead.

 

 

See here.

 

https://www.cloudyni...ater-straps-r22


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#38 MrJones

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Posted 13 October 2017 - 11:13 AM

I'm interested in an RC8 and have talked to a few people about these and the GSO RCs in general. There sure are a lot of them being sold so sales might not be likely as the vendors keep selling out of them as is. What I gather is:

 

The main change from early on was to increase baffling, at least partly by extending the central baffle.

 

The smaller ones mostly work ok with the linear bearing Crayfords and not too much on them.

 

As per above the focuser is attached to the mirror cell (NOT mirror) with the tube RCs. The ones with decoupled focuser attachments also require separate focuser collimation, that's kind of the point.

 

It seems to be a consensus that the primary collimation is usually very good, does not need to be adjusted and if anything should be the last thing you change.

 

They can have different mounting rails. Some are Losmandy only, some are Vixen only and some have the reversible Losmandy/Vixen ones. This can possibly affect flexure and some think the top and bottom Losmandy plates are the best for this.

 

Some have 96% reflectance coatings and some have 99%, there is concern that the 99% reflectance coatings can change the figure a little.

 

Some can be bought with no focuser or different focusers so you can avoid paying for the stock GSO focuser.

 

Some come with a finder shoe mounted and some don't.

 

Some come with extension tubes (which will almost certainly be needed) and some do not.

 

Whether you need a FF or not is dependent on camera chip size. A FF is often not needed with smaller chips but is with larger ones.

 

Nice thread keep it going!


Edited by MrJones, 13 October 2017 - 11:52 AM.

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#39 Robert York

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Posted 15 October 2017 - 04:08 PM

I spent hours trying to get mine collimated, and reading about the mirror cell and focuser being attached issue (which I certainly noticed in my efforts). It's bad enough that unless I can separate the mirror cell and focuser, I don't think I'll get good collimation that will stay. I shouldn't have to touch the primary's collimation, but I do, because of the imaging load. Not to mention the terrible focuser.

 

It got me wondering if anyone's had luck finding an affordable dove-tail support of some kind to get this working properly. I'm sure if I sent an e-mail off to ADM accessories, they could make something work, but it'd probably cost more than the entire rest of the scope.



#40 nemo129

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Posted 15 October 2017 - 07:06 PM

For the 10" and up variety of the GSO generic RCs, Moonlite makes a decoupling flange isolation kit for this issue. I know the 8" models are being discussed here, but if you are in the market and your mount can handle a 10" model it is something to consider. 



#41 ChrisPA

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Posted 15 October 2017 - 08:27 PM

Astrotech actually sells a similar product for the 6 and 8" versions - it's not exactly and "isolation ring", but it allows you to collimate the focuser independently of the primary: https://www.astronom...ens_p20075.aspx



#42 telfish

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Posted 15 October 2017 - 08:38 PM

Astrotech actually sells a similar product for the 6 and 8" versions - it's not exactly and "isolation ring", but it allows you to collimate the focuser independently of the primary: https://www.astronom...ens_p20075.aspx

That is a collimation ring. It's not necessary with a moonlie focuser as they contain a means to collimate the focuser on the flange.



#43 Robert York

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Posted 15 October 2017 - 08:42 PM

And neither option structurally separates the mirror cell from the weight of the focuser and imaging train. As the mass shifts while the scope tracks, it will tug on the mirror cell.


Edited by Robert York, 15 October 2017 - 08:42 PM.


#44 telfish

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Posted 15 October 2017 - 08:54 PM

And neither option structurally separates the mirror cell from the weight of the focuser and imaging train. As the mass shifts while the scope tracks, it will tug on the mirror cell.

Moonlite do an isolation flange but only for the 10 inch CF tube scopes.

 

https://focuser.com/products.php



#45 Robert York

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Posted 15 October 2017 - 11:16 PM

Yes, but that requires the design mentioned earlier, where the back structure has an element distinct from the mirror cell. The smaller ones lack that. You'd basically have to build another rear structural element to the scope to hold the focuser up, then in addition, have something like that MoonLite part.

 

Probably not worth it for my $400 6" RC ;)


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#46 ChrisPA

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Posted 11 June 2018 - 08:34 AM

I just wanted to update everyone and let you know that I did get the 8" RC. I wasn't able to swing the Moonlite, so I have been dealing with the stock focuser and IT STINKS. It's gooey, it doesn't move inward with the micro focusing knob (no matter how I adjust the tension), and the main issue is that it only has two knobs for holding in the camera, and they're placed 90° from each other. Even if I drop the camera in using gravity and gently alternate the tightening, I still usually tilt the camera in the focuser. I'm hoping to upgrade soon, but I'll actually be switching back to the Newt after the two projects I'm currently working on.

 

You really notice f/8 - I do have the reducer, but I haven't used it yet because I wanted a higher focal length. But yeah - it's brutally slow if you're used to imaging at f/3.9 and have a ton of light pollution to fight (I image from Bortle 8 skies a few miles outside of Philadelphia next to LED street lights). 40 hours seems like the minimum under my sky conditions at f/8. I also had an issue with flats not working for a long time that I could figure out - it ended up being due to 1.25" filters in the filter wheel - they weren't fully screwed in, so if I changed filters and changed back to take flats, they'd be slightly off. This ruined my subs for two projects - I was able to salvage the 40 hours I put into NGC 3718, but the Draco Triplet seems to be beyond repair. I might try again, but I've resigned myself to throwing that out. 

 

But I'm happy with the scope so far. I do think what I need to do is incorporate that reducer and try to swing a monochrome IMX183 somehow - that would give me the frame of view I'm after and it'd also allow me to bring my color camera back into the mix, which would be nice. I could definitely use the extra sensitivity with this slower speed - I'd love to go back to doing 15-20 hour projects!

 

But here is what I've managed with it so far:

NGC 3718: https://www.astrobin.com/338207/ [Keep in mind, this is broadband under Bortle 8 skies]

Cat's Eye Nebula: https://www.astrobin.com/348611/

 

I also managed to get 6 hours of hazy SII subs on the Eagle Nebula so far - shooting to the south is pretty challenging for me, but this has been one of my top targets for this scope.

Here you can see how I stack up against Hubble: https://www.astrobin.com/350808/

And this is my full frame of view: https://www.astrobin.com/350808/B/

I'm probably going to need 30-40 hours on this one, so it's a long way away from being finished. Here's hoping the weather turns soon - it's been awful since February.
 


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#47 MrJones

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Posted 11 June 2018 - 10:20 AM

Those look great. Same experience with my with my visual only RC8. Surprisingly even better than expected optics, really annoying focuser.


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#48 ChrisPA

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Posted 07 August 2018 - 08:32 PM

Just wanted to update this thread with a few other images I've taken through this scope:

Ring Nebula: https://www.astrobin.com/356092/

Eagle Nebula in SHO: https://www.astrobin.com/359338/

And along with M16, a comparison of the Hubble's 2014 redux of the Pillars of Creation with my own data (as I hinted to above): https://www.astrobin.com/359341/

 

I have three more images in the works (one might not come out and one I only gave about 3 hours to), but I've taken down the scope and am switching back to the Newt for a bit. I'll be taking it out again for galaxy season though. Hopefully I'll have a Moonlite for it by then!


Edited by ChrisPA, 07 August 2018 - 08:33 PM.

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#49 Jason B

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Posted 07 August 2018 - 09:53 PM

Your images are well done, thanks for sharing.

 

I have had my RC8 since they first came out and have been extremely happy with it.  I use mine with the AP-CCD67 on my CGEM mount and a modified Canon.  Focuser has worked fine for me but I do plan on upgrading to a Moonlite or FT at some point.


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#50 Paul Sweeney

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Posted 08 August 2018 - 05:44 AM

How about the Vixen VC200L? I have one, and I would have to warn anyone thinking of getting one to consider the pros and cons carefully. There seem to be sharp quality differences, but some people get excellent results from them. It does have an extremely flat field, is more or less immune to dewing, has nice fat spider vanes for those who like spikes, and can be completely collimated. They are tough to collimate, though, and the optical quality varies so much that I get the impression that they go out the factory door without ever seeing Quality Control. But it is an option to consider...




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