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are RCs good for AP?

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#1 John Berger

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Posted 07 October 2024 - 11:45 PM

long story short, my newt (C8-N, see Signature) has a pinched primary mirror and a drooping focuser, and maybe a secondary mirror that's twisted a little in the wrong direction (my fault I think), so, it's now a visual instrument only

 

my only astrograph now is my 65PHQ; yeah, it's a nice scope, but I need a tighter FOV for more selections of objects

 

and it needs to be very cheap (like around $700); I looked at the Carbonstar 6 RC; everything about it looks very good

 

are they sharp scopes? any corrector required? is it like petzval refractors; all you have to do is focus and the field is flat already? how is collimating them?

 

any people specifically have a Carbonstar 6 inch? how do you like it?


Edited by John Berger, 07 October 2024 - 11:47 PM.


#2 Rasfahan

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Posted 08 October 2024 - 12:03 AM

There are plenty of threads about these scopes. All the cheap RCs are made by GSO and are alike except for the branding on them. My take: You get what you pay for. There‘s a reason premium RCs cost 10x as much and it‘s mostly not the mirror size. The cheap RCs are very difficult to collimate. When I got mine (8“) it was like hitting a wall compared to an apo. It took me years to learn how to get use out of it and then I decided to no longer deal with all the hassle and got a decent RC instead (from CFF). So, I wouldn‘t really recommend them but at $700 you‘re quite limited in options. OTOH I paid more than that for the collimation tool I use for mine.

You could repair and optimize your Newtonian. For $700 and some work (probably less than learning RC collimation) you‘ll be ahead of the RC by far.
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#3 bobzeq25

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Posted 08 October 2024 - 12:24 AM

I had the GSO 6 RC, pretty similar. This is not a simple question to answer.

The major issue is that they are hard to collimate. My $400 GSO also had a flexible focuser, the $700 Carbonstar appears to have a better one.

One big reason they're hard to collimate is the the hyperbolic mirrors interact. Another is that the focuser is bolted to the primary. If the scope is delivered out of collimation (mine was) you can't just do one than the other because they interact. This has caused some people considerable difficulty.

On the other hand, if your scope is delivered well collimated, it will likely stay that way for a long time.

Bottom line. It's a somewhat risky purchase. Some people really like the scope, some don't. This is somewhat unusual.

Tips.

Try the scope out first. Do not mess with collimation unless you have to. If you do need to collimate, see if you can leave the primary as is, and only collimate the secondary. That often works. Again, do no more collimation than necessary. If collimating the secondary gives you images you like, do not then try to collimate the primary "just because".

F9 is pretty slow. I was most successful when I used a reducer to speed it up. Apertura makes one specifically for the scope, I'd definitely use that.

That's what you need to know. I'll add my personal experience with the $400 version, but it may not apply to the $700 version.

After weeks spent getting the badly miscollinated scope collimated, the flexible focuser threw the collimation off with a change in altitude. At that point, I gave up on the scope.

But, again, many people like these, and the Apertura may well be different.

Below is my sharpest image, I assume it's good enough for you. Click on the CN thumbnail for a better version, and details. Clearly 42 minutes with the DSLR was not enough. I'd only been imaging for about 15 months when I did this, I could do better now.

Here's a pretty great image with the Apertura version. But even he notes both the difficulties collimating, and concerns about the focuser.

https://www.cloudyni.../#entry13671903





get.jpg?insecure

Edited by bobzeq25, 08 October 2024 - 12:39 AM.

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#4 licho52

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Posted 08 October 2024 - 12:50 AM

They are great for AP, but only if you know what you're doing. You won't get much help or advice from people who successfully use them because, based on your query(unable to tame a misbehaving Newtonian), one can surmise that your chances of achieving success with such scope are low.  RCs are harder to deal with than Newtonians by far.


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#5 vidrazor

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Posted 08 October 2024 - 12:57 AM

Spare yourself the headache, get an SCT.


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#6 bobzeq25

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Posted 08 October 2024 - 01:04 AM

Spare yourself the headache, get an SCT.

At your budget, this is the alternative. The C6 SCT.

https://www.celestro...y-cg-5-dovetail

The virtue is that the spherical primary does not need to be collimated, so pretty much anyone can collimate one pretty easily. The flaw is the optics are not as good as the RCs.

The C6 is a safer bet. The RC has upside potential and downside potential. Hard to put numbers on that.

Both need an off axis guider, the C6 won't work at all well without one.
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#7 vidrazor

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Posted 08 October 2024 - 01:31 AM

At your budget, this is the alternative. The C6 SCT.

https://www.celestro...y-cg-5-dovetail

The virtue is that the spherical primary does not need to be collimated, so pretty much anyone can collimate one pretty easily. The flaw is the optics are not as good as the RCs.

The C6 is a safer bet. The RC has upside potential and downside potential. Hard to put numbers on that.

Both need an off axis guider, the C6 won't work at all well without one.

He can find an 8 inch used in that price range. I found one for $800 used, and I've seen them for less. Collimation is much easier, especially using MetaGuide.

 

Ditto on the OAG. Focus motor is absolutely required as well.
 


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#8 Spaceman 56

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Posted 08 October 2024 - 02:01 AM

One big reason they're hard to collimate is the the hyperbolic mirrors interact.

 

Another is that the focuser is bolted to the primary. If the scope is delivered out of collimation (mine was) you can't just do one than the other because they interact.

 

This has caused some people considerable difficulty.

I Read Bobs and other peoples advice, and most agreed on this point. the GSO 6 and 8 inch RCs have this issue.  undecided.gif

 

For this reason (amongst others) I decided to NOT buy a small RC, and to go straight for the Big 10 inch version.  

 

On my 10 inch GSO Truss RC the Focuser has adjustment screws,  the Secondary mirror has adjustment screws, and the Primary also has independent screws. 

 

This means you can collimate the dam thing without dramas, which I did in about 2 hours using a Howie Glator.

 

Unimatrix says Howies are impossible to find, and insanely priced, but 3 came up second hand in the classifieds, and I got the most expensive one for $300 US.

 

this is my first test shot with the 10 inch RC, and I feel its good for 2 hours of Data.  smile.gif

 

Pinwheel Galaxy with an RC10.

 

Unfortunately my Takahashi FSQ-106 now sits unused in a bedroom collecting Dust.  I guess its good for Nebulas and stuff so I will probably keep it.

 

I am very happy with the 10 inch RC and its a keeper.  waytogo.gif 

 

Should add that you need a half decent mount for any long focal length OTA, but none of the premium stuff is available here in NZ, so I bought a CEM-120.

 

Spaceman


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#9 Drothgeb

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Posted 08 October 2024 - 09:50 AM

I Read Bobs and other peoples advice, and most agreed on this point. the GSO 6 and 8 inch RCs have this issue.  undecided.gif

 

For this reason (amongst others) I decided to NOT buy a small RC, and to go straight for the Big 10 inch version.  

 

On my 10 inch GSO Truss RC the Focuser has adjustment screws,  the Secondary mirror has adjustment screws, and the Primary also has independent screws. 

 

This means you can collimate the dam thing without dramas, which I did in about 2 hours using a Howie Glator.

 

Unimatrix says Howies are impossible to find, and insanely priced, but 3 came up second hand in the classifieds, and I got the most expensive one for $300 US.

 

this is my first test shot with the 10 inch RC, and I feel its good for 2 hours of Data.  smile.gif

 

 

 

Unfortunately my Takahashi FSQ-106 now sits unused in a bedroom collecting Dust.  I guess its good for Nebulas and stuff so I will probably keep it.

 

I am very happy with the 10 inch RC and its a keeper.  waytogo.gif

 

Should add that you need a half decent mount for any long focal length OTA, but none of the premium stuff is available here in NZ, so I bought a CEM-120.

 

Spaceman

Good choice to go straight to the truss type scope. Great image too!

 

I tried finding a Howie Glatter, but eventually gave up. I have several of the cheap lasers. They were way off when I bought them, but were pretty easy to correct. 


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#10 bobzeq25

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Posted 08 October 2024 - 10:15 AM

I Read Bobs and other peoples advice, and most agreed on this point. the GSO 6 and 8 inch RCs have this issue.  undecided.gif
 
For this reason (amongst others) I decided to NOT buy a small RC, and to go straight for the Big 10 inch version.  
 
On my 10 inch GSO Truss RC the Focuser has adjustment screws,  the Secondary mirror has adjustment screws, and the Primary also has independent screws. 
 
This means you can collimate the dam thing without dramas, which I did in about 2 hours using a Howie Glator.
 
Unimatrix says Howies are impossible to find, and insanely priced, but 3 came up second hand in the classifieds, and I got the most expensive one for $300 US.
 
this is my first test shot with the 10 inch RC, and I feel its good for 2 hours of Data.  smile.gif
 
 
 
Unfortunately my Takahashi FSQ-106 now sits unused in a bedroom collecting Dust.  I guess its good for Nebulas and stuff so I will probably keep it.
 
I am very happy with the 10 inch RC and its a keeper.  waytogo.gif
 
Should add that you need a half decent mount for any long focal length OTA, but none of the premium stuff is available here in NZ, so I bought a CEM-120.
 
Spaceman


Very nice.

The OP should note. That's a $3200 scope. The mechanical construction is decent, way better than the $700 Apertura.

An underlying reason why the inexpensive RCs are a risky purchase is that the RC optical design is very sensitive to the mechanical construction.

Example. When I collimated my 6RC, I first used a Cheshire, got it mechanically collimated. I knew I needed to additionally collimate optically on stars. When I'd done that I went back to the Cheshire and was not surprised to find the mechanical collimation was off. I had somewhat expected that, the mechical center and the optical center often do not coincide on the inexpensive RCs. Sometimes people get lucky.

Yet another reason why opinions on these are so variable, why "Are RCs good for (amateur) AP?" is not an easy question to answer.


.

Edited by bobzeq25, 08 October 2024 - 10:22 AM.

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#11 Oort Cloud

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Posted 08 October 2024 - 11:31 AM

At your budget, this is the alternative. The C6 SCT.

https://www.celestro...y-cg-5-dovetail

The virtue is that the spherical primary does not need to be collimated, so pretty much anyone can collimate one pretty easily. The flaw is the optics are not as good as the RCs.

The C6 is a safer bet. The RC has upside potential and downside potential. Hard to put numbers on that.

Both need an off axis guider, the C6 won't work at all well without one.


I have a C6, and now also an EdgeHD 800 because the C6 was so difficult to image with mostly because of the tiny light cone provided by the small rear baffle. I do not recommend trying to image with one, other than planetary and lunar, which they excel at.
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#12 bbasiaga

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Posted 08 October 2024 - 12:27 PM

You can get a collimation plate for the 8" and 6" RCs to decouple the two adjustments. I'm considering an 8", as my C11 is giving me fits for imaging. Inconsistent AF, can't guide for crap even on a premium mount. Ther is clearly a reason they went to the Edge design for imaging.

You'll also probably need a better external focuser for the SCT or RC. Those will cost more than the scope. The Primaluca and Moonlight options are both $500-800 depending on options.

Not a cheap hobby....
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#13 John Berger

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Posted 08 October 2024 - 01:14 PM

okay, so I think the conclusion is, if I want longer focal length, I need an SCT, because RCs are not easy to handle

 

Edge 8 is a bit expensive; looks like I need to save up

 

I wish they had an Edge 6

 

how does the EAF work on the Edge? focuser looks strange.


Edited by John Berger, 08 October 2024 - 01:16 PM.


#14 72Nova

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Posted 08 October 2024 - 01:28 PM

I have the 6” Carbonstar RC with their dedicated reducer and I absolutely love this scope.  It’s as light as my 90mm refractor and gives me 950mm focal length for under $1k.  It arrived well collimated and I haven’t had to adjust collimation in the 5 months I’ve had it.

 

With the reducer, this scope has been very easy to handle for me.  It’s compact, lightweight, EAF ready, and it holds collimation very well.  I do use an OAG with the CarbonStar RC6 and honestly, setting up the OAG was the hardest part for me.  

 

Here is an image of Wolf Rayet 134 that I recently completed with it:

 

[/URL]">http://get.jpg?insecure

 

 


Edited by 72Nova, 08 October 2024 - 01:47 PM.

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#15 John Berger

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Posted 08 October 2024 - 01:48 PM

I have the 6” Carbonstar RC with their dedicated reducer and I absolutely love this scope.  It’s as light as my 90mm refractor and gives me 950mm focal length for under $1k.  It arrived well collimated and I haven’t had to adjust collimation in the 5 months I’ve had it.

 

With the reducer, this scope has been very easy to handle for me.  It’s compact, lightweight, EAF ready, it holds focus through the night, and it holds collimation very well.  I do use an OAG with the CarbonStar RC6 and honestly, setting up the OAG was the hardest part for me.  

 

Here is an image of Wolf Rayet 134 that I recently completed with it:

 

[/URL]">http://get.jpg?insecure

well that's good to hear

 

how do the stars look without the reducer? better or worse?



#16 72Nova

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Posted 08 October 2024 - 01:53 PM

The stars look very similar without the reducer but it’s more challenging to use at the native focal length.

 

Here is M13 with 2600mc camera with no reducer

 

[/URL]">http://get.jpg?insecure


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#17 Oort Cloud

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Posted 08 October 2024 - 02:11 PM

You can get a collimation plate for the 8" and 6" RCs to decouple the two adjustments. I'm considering an 8", as my C11 is giving me fits for imaging. Inconsistent AF, can't guide for crap even on a premium mount. Ther is clearly a reason they went to the Edge design for imaging.

You'll also probably need a better external focuser for the SCT or RC. Those will cost more than the scope. The Primaluca and Moonlight options are both $500-800 depending on options.

Not a cheap hobby....

PM me...I just worked out the AF issues on mine over the past week. There are a few tricks to making it work well, but I'm getting hyperbolas now with an R² of .99 or 1.00, every time.
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#18 John Berger

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Posted 08 October 2024 - 02:28 PM

The stars look very similar without the reducer but it’s more challenging to use at the native focal length.

 

Here is M13 with 2600mc camera with no reducer

 

[/URL]">http://get.jpg?insecure

thank you, now I feel like buying the RC6 again


Edited by John Berger, 08 October 2024 - 02:29 PM.

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#19 Oort Cloud

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Posted 08 October 2024 - 02:47 PM

thank you, now I feel like buying the RC6 again


I've been wanting one for a while, but now that my EdgeHD is starting to show promise (first time it's been precisely collimated since I bought it about 3 years ago, hasn't gotten much use though) - I'm reconsidering. I definitely want a 6" Newtonian at 600mm. I thought a 6" RC would make a good replacement for the EdgeHD 800 at 1,200mm vs 1484mm. Although, I could still do a RC6 to replace the reducer for the EdgeHD, and keep the EdgeHD for the really small stuff at f/10 (2,032mm).
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#20 bobzeq25

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Posted 08 October 2024 - 06:00 PM

okay, so I think the conclusion is, if I want longer focal length, I need an SCT, because RCs are not easy to handle
 
Edge 8 is a bit expensive; looks like I need to save up
 
I wish they had an Edge 6
 
how does the EAF work on the Edge? focuser looks strange.

I have the Celestron focuser on my RASA 8. Works fine. Can't see how the generic EAF would be any better.

https://www.celestro...cts/focus-motor
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#21 vidrazor

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Posted 08 October 2024 - 08:09 PM

I have the Celestron focuser on my RASA 8. Works fine. Can't see how the generic EAF would be any better.
https://www.celestro...cts/focus-motor

Because they can be cheaper. ;) I have two of these, one on my 102, and one on my C5. Work great.
 


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#22 David R.

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Posted 08 October 2024 - 08:55 PM

are they sharp scopes? any corrector required? is it like petzval refractors; all you have to do is focus and the field is flat already? how is collimating them?

 

any people specifically have a Carbonstar 6 inch? how do you like it?

Yes - these are very optically excellent scopes with quartz mirrors. I'll also mention that the tube quality and general fit and finish on this version of the GSO is excellent. 

 

Regarding flat field - it is a fairly flat field natively. In order to get a natively flat field across a larger sensor like the 2600 - the mirror spacing has to be perfect. With the reducer, this is less of an issue. But it should be coma free across the field assuming you have proper optical alignment. 

 

Bob mentioned my post before, I definitely did not receive a well collimated scope. The biggest issue with mine was tilt in the focuser - it doesn't flex, but it is tilted when attached to the extension tubes. I purchased the tilt adapter separately in order to resolve that. I don't really know if the issue is with the focuser itself or some other mechanical issue with the extension tubes. Regardless, I am able to easily correct this with the tilt adapter. 

 

Collimating these scopes is not easy. For every post that someone says "these are a nightmare / hard / whatever" - someone will invariably post that it is easy peasy for them. Again, quoting Bob - "risky" is a good word. The risk is that you will have to collimate it or deal with some other mechanical deficiency. Some people like these kinds of challenges and some people just want to get on with imaging smile.gif I think it is important to consider your personal preferences around these kinds of things. 

 

I will say that the focuser on this version has been quite good so far. I don't like Crayfords in general, but I have had zero issues with the focuser. This has a linear ball bearing version that does not appear to have the mechanical flexure that I have seen with many of my other GSO focusers in the past.  

 

I think these scopes have an immense amount of value for the money.  


Edited by David R., 08 October 2024 - 09:00 PM.

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#23 Robert7980

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Posted 09 October 2024 - 02:56 AM

They can be nearly the ideal imaging scope, most professional observatories use the RC design. 
 

They aren’t for everyone as they aren’t point and shoot like a refactor would be. I would say the difficulty level just depends on if you like to tinker and what your experience is working with precision mechanical instruments. If you are a DIYer and have some patience to learn the procedure for maintaining the optics then there are two scope designs that are nearly ideal one being the RC and the other being the CDK. 
 

The GSO RC scopes are reasonably affordable and are acceptable quality, other manufacturers are superior in construction, but the cost goes up rapidly, and CDKs are mostly cost prohibitive, comparable to a nice used car or new luxury car. So those are dream scopes for most of us. 
 

The price of an RC can and probably will contain hidden costs for accessories and alignment tools, such and collimation tools and upgraded focusers. If you’re careful with these choices it can be done fairly cheaply. 
 

RC’s typically don’t need any additional optics to correct them and are one of the most pure designs available, all you need is the 2 mirrors to get a well corrected large flat feild. So for astrophotography they are a nearly perfect design and why they are used extensively for big professional science instruments like the Hubble. You don’t need a feild flattener, but you can use a reducer to gain extra speed and widen the feild. So it allows more options than other designs. 
 

The alignment process can be tedious, but it’s not difficult. You need to have a basic understanding of how it works, have a couple tools and most importantly follow the procedure. 
 

I have an RC8 and love it. For me personally it fits better than SCT’s and Newts for a few reasons, such as an image train that hangs on the rear cell not off the side and doesn’t have quirky focus design of the SCT. Basically eliminates most of the hassle you can have with mirror scopes, once it’s aligned. It’s extremely flexible so allows for easy changes to the configuration to do everything from DSO to planetary to spectrograph science instrument. 
 

The cost for my setup was more than double the scope itself though, I got a deal on the tube for around $1000 and invested an additional $1400 (At least) in upgrades and tools. So there’s the hidden cost, overall even with the hidden cost they are still competitive against other designs, at least for the GSO scopes. 
 

I would highly recommend the RC for more advanced imagers, those who have already owned SCT’s and Newts and know their way around the hobby. For beginners or those who want point and shoot simplicity I’d recommend going with a quality refactor and pretty much skip any mirror based scope. You already have one so you’re good. 
 

In your budget range it’s a very hard NOPE! The RC will just destroy your motivation without going all in with it. They aren’t extremely expensive, but you have to be willing to throw time and money at them to correct problems and get it working when necessary. 
 

My advice is save your money for now and keep researching for the future. There basically isn’t a good solution in your price range that won’t (Probably) be just endlessly frustrating with mediocre performance at best. Your mileage may vary. 


Edited by Robert7980, 09 October 2024 - 03:24 AM.

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#24 bbasiaga

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Posted 09 October 2024 - 09:58 AM

Would you consider a 120mm or 127mm doublet?  I think those can be had for around $700, plus a little for a reducer/flattener.  

 

Brian


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#25 bobzeq25

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Posted 09 October 2024 - 10:24 AM

They can be nearly the ideal imaging scope, most professional observatories use the RC design. 
 
They aren’t for everyone as they aren’t point and shoot like a refactor would be. I would say the difficulty level just depends on if you like to tinker and what your experience is working with precision mechanical instruments. If you are a DIYer and have some patience to learn the procedure for maintaining the optics then there are two scope designs that are nearly ideal one being the RC and the other being the CDK. 
 
The GSO RC scopes are reasonably affordable and are acceptable quality, other manufacturers are superior in construction, but the cost goes up rapidly, and CDKs are mostly cost prohibitive, comparable to a nice used car or new luxury car. So those are dream scopes for most of us. 
 
The price of an RC can and probably will contain hidden costs for accessories and alignment tools, such and collimation tools and upgraded focusers. If you’re careful with these choices it can be done fairly cheaply. 
 
RC’s typically don’t need any additional optics to correct them and are one of the most pure designs available, all you need is the 2 mirrors to get a well corrected large flat feild. So for astrophotography they are a nearly perfect design and why they are used extensively for big professional science instruments like the Hubble. You don’t need a feild flattener, but you can use a reducer to gain extra speed and widen the feild. So it allows more options than other designs. 
 
The alignment process can be tedious, but it’s not difficult. You need to have a basic understanding of how it works, have a couple tools and most importantly follow the procedure. 
 
I have an RC8 and love it. For me personally it fits better than SCT’s and Newts for a few reasons, such as an image train that hangs on the rear cell not off the side and doesn’t have quirky focus design of the SCT. Basically eliminates most of the hassle you can have with mirror scopes, once it’s aligned. It’s extremely flexible so allows for easy changes to the configuration to do everything from DSO to planetary to spectrograph science instrument. 
 
The cost for my setup was more than double the scope itself though, I got a deal on the tube for around $1000 and invested an additional $1400 (At least) in upgrades and tools. So there’s the hidden cost, overall even with the hidden cost they are still competitive against other designs, at least for the GSO scopes. 
 
I would highly recommend the RC for more advanced imagers, those who have already owned SCT’s and Newts and know their way around the hobby. For beginners or those who want point and shoot simplicity I’d recommend going with a quality refactor and pretty much skip any mirror based scope. You already have one so you’re good. 
 
In your budget range it’s a very hard NOPE! The RC will just destroy your motivation without going all in with it. They aren’t extremely expensive, but you have to be willing to throw time and money at them to correct problems and get it working when necessary. 
 
My advice is save your money for now and keep researching for the future. There basically isn’t a good solution in your price range that won’t (Probably) be just endlessly frustrating with mediocre performance at best. Your mileage may vary. 




Good stuff. I'll note that a major reason the RC design works well in a professional setting is the precision and solidity of the mechanical construction. People often overlook that.

A few companies have tried to make an RC of comparable mechanical quality. They are largely unsuccessful because people won't pay for it. Knaebel is a great example. The picture of a Knaebel RC is shown below. That's the quality that allows the RC design to shine. It's all carbon fiber and titanium, designed by experts using CAD. The focuser moves the secondary, which solves some problems. And it's no longer sold. Few would pay $5000+ for it.

The RC design is superb, if the mechanical construction is. The inexpensive RCs - not so much.

SCTs are justifiably popular because a good SCT can be made inexpensively. Sometimes people wonder why they are no premium SCTs. They would run into the opposite problem, the optical design is not great.

But, on a <1K budget, they definitely come into consideration.

Knaeble-Ritchey-Chretien-RC-300FC-Ti.jpg

Edited by bobzeq25, 09 October 2024 - 10:29 AM.

  • Oort Cloud, Robert7980 and John Berger like this


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