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Collimate an RC vs Newtonian

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#1 unimatrix0

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Posted 02 September 2021 - 02:47 PM

If I have some experience collimating a Newtonian (but I don't own one currently) how difficult it is to collimate a Ritchey–Chrétien ? 
All I read are horror stories, like this telescope is the end of all things and the doom of astrophotography. I have a hard time believing it, it all sounds to me that people bought it for their cheaper price than a Mak or Cass and had no clue how to collimate telescopes in the first place and don't feel like spending time to calibrate it. 

 

Any inputs? 


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#2 bobzeq25

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Posted 02 September 2021 - 03:28 PM

RCs are definitely relatively hard to collimate.  The "horror stories" are not simply a matter of incompetence or laziness.

 

I've (successfully) collimated one.

 

The 2 mirrors interact.  You can't collimate one at a time, you have to go back and forth.

 

On the inexpensive RCs the optical center and the mechanical center are sometimes not the same.  BTDTGTTS.

 

The focuser on some RCs is connected solidly to the primary mirror, which makes things complicated.

 

The RCs are peculiarly sensitive to mechanical quality.   A quality RC is high technology, and looks like it.  They're not cheap.

 

https://nimax-img.de...C-250-FC-Ti.jpg

 

It's a fine optical design for a professional observatory.  Many amateurs would be better off with a Corrected Dall Kirkham.

 

Collimating an RC bears little resemblance to collimating a Newtonian.  Especially, an inexpensive RC.


Edited by bobzeq25, 02 September 2021 - 03:32 PM.

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#3 cuzimthedad

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Posted 02 September 2021 - 04:27 PM

Moving to Cats & Casses



#4 MitchAlsup

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Posted 02 September 2021 - 04:49 PM

If I have some experience collimating a Newtonian (but I don't own one currently) how difficult it is to collimate a Ritchey–Chrétien ? 

Between 3× an 10× harder.

 

With a slightly oversized secondary, as long as the center of the FoV is fully illuminated, you can get the Newt collimated.

 

Not so with an RC. The optical center of the RC secondary has to be within a couple of thou of the optical center of the primary and the optical center of the focusing mechanism. This is the hardest part. Takahashi makes a telescope designed so you can achieve this necessary optical centering. I would not want to do more than tweaking collimation on an RC without this telescope. An RC without the secondary on the optical axis is simply a dreadful scope.

 

After you get the secondary on the optical axis, then collimation is only 2× more difficult than a Newt.

 

Also note: the distance from primary to secondary distance must be achieved or you realize little of the optical benefits of the RC design. The RC is "focused" by moving the secondary to the proper distance from the primary

creating a focal plane at exactly one back-focal-distance. once the "telescope" is focused on this focal plane, then the "focuser" is used to focus EPs, and cameras to this focal plane.


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#5 ram812

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Posted 02 September 2021 - 04:52 PM

If I have some experience collimating a Newtonian (but I don't own one currently) how difficult it is to collimate a Ritchey–Chrétien ?
All I read are horror stories, like this telescope is the end of all things and the doom of astrophotography. I have a hard time believing it, it all sounds to me that people bought it for their cheaper price than a Mak or Cass and had no clue how to collimate telescopes in the first place and don't feel like spending time to calibrate it.

Any inputs?


Depends on who you ask. I've collimated my XT10 in minutes, but I learned on newts years before I bought one (My brother was a newt builder-and teacher- until he passed on) If you are clear headed (I'm not, anymore) you can do an inexpensive RC in less than an hour providing it's not grossly out of collimation. The first RC I bought came perfect out of the box, and requires very little tweaking to maintain. There's a lot of disinformation out there by the anti-RC crowd who either aren't mechanically inclined or are just plain lazy, but I'd venture to say a star used for collimation of both types of scopes is really the best verification of good collimation. That means a tracking mount, tools, and excellent seeing. Which, by the way, is problematic these days gotten together! An artificial star works, but is a pain to set up. So I tend to verify mine on the 1 of 14 or so nights a year when the seeing does get perfect ( And it does) in the winter.

CS, Ralph

Edited by ram812, 02 September 2021 - 09:06 PM.

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#6 jgraham

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Posted 02 September 2021 - 04:59 PM

Wellll... RCs can be challenging to collimate _if_ the primary needs to be adjusted. If the primary is okay it's _much_ easier, even easier than a Newtonian. Vendors like Agena collimate them before shipping and you've got a good chance of getting a good one. I bought mine used and I had to start from scratch; align the secondary, check for on-axis astigmatism, tweak the primary, repeat until it converges. Take tiny steps. It's not hard if you take your time and be patient with it. My RC8 has blossomed into a fantastic imaging platform using a full-frame DLSR (Canon EOS Ra).

Love it!
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#7 xthestreams

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Posted 02 September 2021 - 07:52 PM

If you’re the sort of person who thinks collimating an SCT or CDK is hard/annoying then stay away from RCs. If you’re patient, methodical and have a practiced eye for star shapes/testing, then it’s a hugely rewarding scope. 
 

Even the GSOs, the newer truss models if you want to avoid the focuser/primary sag issues, make a wonderful scope. I have a PW CDK but actually prefer my GSO RCT $ for aperture 


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#8 ram812

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Posted 02 September 2021 - 09:16 PM

Wellll... RCs can be challenging to collimate _if_ the primary needs to be adjusted. If the primary is okay it's _much_ easier, even easier than a Newtonian. Vendors like Agena collimate them before shipping and you've got a good chance of getting a good one. I bought mine used and I had to start from scratch; align the secondary, check for on-axis astigmatism, tweak the primary, repeat until it converges. Take tiny steps. It's not hard if you take your time and be patient with it. My RC8 has blossomed into a fantastic imaging platform using a full-frame DLSR (Canon EOS Ra).

Love it!



This is how it's done, exactly!👍 Learn the scope inside and out in the process, and with careful handling you'll actually enjoy your RC! The only real, annoying issue these little (Mine is an AT6RC) OTA's have is as stated, primary mirror shift. I'm going to be peeling mine apart this winter (Completely) and see if there's a DIY solution to this mirror shift. Usually happens when slewing all over the sky, so I make sure my target(s) of interest are in the same area and check the collimation and mirror shift. I also keep an Allen wrench with a lanyard attached to wrap around my wrist when star collimating so I don't actually drop the thing down the tube😋! Enjoy!

CS, Ralph

#9 bobzeq25

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Posted 02 September 2021 - 10:59 PM

The only real, annoying issue these little (Mine is an AT6RC) OTA's have is as stated, primary mirror shift. I'm going to be peeling mine apart this winter (Completely) and see if there's a DIY solution to this mirror shift. Usually happens when slewing all over the sky, so I make sure my target(s) of interest are in the same area and check the collimation and mirror shift. I also keep an Allen wrench with a lanyard attached to wrap around my wrist when star collimating so I don't actually drop the thing down the tube! Enjoy!

CS, Ralph

The "mirror shift" is at least partly (maybe mostly?) due to to an inexpensive focuser that can shift with altitude.  The amount varies with camera (and filter wheel and OAG, maybe) weight.  After I got my 6RC collimated at one altitude, moving it to another made the collimation unacceptable to me.

 

Some people address the issue with a Moonlight focuser.  Which costs more than the scope originally did. 

 

I got a very nice 3.7 inch rack and pinion focuser.  It came attached to a TS 130mm F7 refractor.  <smile>

 

I'm not alone.  Check out the title to this thread.

 

https://www.cloudyni...collimate-guys/

 

I believe he did.  There are some excellent imagers here who have abandoned the inexpensive RCs.  They're not ignorant, and not lazy.  If you get one that's well collimated, that's fine.  Mine was so far off the spikes were doubled.
 


Edited by bobzeq25, 02 September 2021 - 11:15 PM.

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#10 ram812

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Posted 02 September 2021 - 11:34 PM

The "mirror shift" is at least partly (maybe mostly?) due to to an inexpensive focuser that can shift with altitude. The amount varies with camera (and filter wheel and OAG, maybe) weight. After I got my 6RC collimated at one altitude, moving it to another made the collimation unacceptable to me.

Some people address the issue with a Moonlight focuser. Which costs more than the scope originally did.

I got a very nice 3.7 inch rack and pinion focuser. It came attached to a TS 130mm F7 refractor. <smile>

I'm not alone. Check out the title to this thread.

https://www.cloudyni...collimate-guys/

He did.


Yeah, I almost nixed on the deal, Bob! 😉 I really wanted my RC to compliment the newts and refractors I have in my kit and fell in love with the darn thing. The focuser, like a lot of folks have pointed out (You, too😁, and quite often at that) can be replaced with a focuser like a Moonlight but the cost pushes people away. To me, it's not all focuser that's the killer. I have to use 2 2" extensions + 2 1" extensions on mine to get the 9 3/4" back for the camera to be near the image plane! Lot's of weight and it just seems like I end up fiddling around half my imaging/viewing time going back and forth. Not to mention having to remove all those extensions just to put the focuser back in place for viewing (Yes, I use it for that, too). I could go on and on about pro's and con's but it's not my thread 😁 and I'm a fence-sitter on most subjects these days, anyway.-smile-

CS, Ralph


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