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RC6 Setup Issues and Solutions

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I wanted to start a thread with my experience using my different telescope setups. See my main imaging rig and my secondary rig configurations below.

The main rig works very well with narrowband and RGB imaging (there are some minor back focus and tilt issues I am sorting out but nothing noticeable unless you zoom in on the captured images). Centre on my target and I get a plethora of stars and off I go.

With the RC6 I have been having a range of issues which I will outline below.

  • Overall weight: The RC6 setup with a guidescope is pushing upwards of 9kg, maybe 9.5kg. It's a bit of a stretch for the HEQ5 Pro in my opinion
  • Balancing: Using a guidescope introduces extra weight on one side of the tube and throws the balance out. This is where an OAG is preferable
  • Imaging: As I use mono for my main rig I wanted to setup the RC6 with a colour camera to obtain subs quicker (can get something decent in a night instead of 3-5 with mono) 
  • Guidescope & OAG: I have seen most people use a guidescope with an RC but I think an OAG would be a lot better. I have tried both and I can't seem to get the OAG working at all and when using the guidescope the guiding is awful. This could be due to the weight of the setup or flexure. I am using the ZWO OAG and the Orion 50mm mini guidescope.
  • Guide camera: ASI120MM and have used a PlayerOne Neptune-M.
  • Camera: The choice of camera I have seen is either a DSLR or a OSC Cooled camera. I am very familiar with DSLR's and cooled mono cameras in refractors but not so much in an RC telescope. I captured M42 using my Z61 refractor and 1200D which came out great. I have tried using a 1200D and a PlayerOne Uranus-C camera (which is meant to be pretty good for deep sky stuff without cooling) and again having some trouble getting them working perfectly.


  • I am looking at upgrading my HEQ5 Pro mount next year with the iOptron HEM27 which has a payload capacity of up to 20kg with a counterweight or 13.5kg without so this may solve some of the issues I am having in regards to weight and balance
  • Should I try use the 1200D or stick with the Player One? I am guessing with colour cameras and no filters there would be less visible stars so how do you work out if you are on target and getting crisp focus?There are more people using a DSLR online and not so much with a webcam styled camera
  • Should I use the OAG or Guidescope? I won't be using a field flat so I am guessing I don't need to achieve 55mm back focus? I can attach the OAG with the DSLR but I have trouble focusing and finding stars.
  • Do I need to upgrade the guidecamera to get a larger sensor?
Edited by 04Stefan07
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I have a mobile rig that is based on a RC6 with OSC camera (533MC) and LPF for colour imaging. I use a 70mm guidescope with ASI120MM, all loaded on a harmonic drive mount, the RST135E, with no CW used. This seems to work well, see my latest image of M51 here...


CS, Andy

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The 150mm f8 RC would have a focal length of 1200mm. For most guidescopes you have a focal length of 200 to 250mm. This means with the pixel size of your 120mm, the guider pixel scale is going to be around 1/6th that of the main scope, which is not desirable (assuming main camera has same pixel pitch, which is likely given you mention DSLR, but you don't seem to mention a specific camera used for the RC6).

At most you want to have a 1:4 scale difference between the guide scope and the main scope. Ideally 1:3 so I read. This is why OAGs are so much more popular than guidescopes for slow cassegraines.


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On 18/12/2022 at 03:14, 04Stefan07 said:

Imaging: As I use mono for my main rig I wanted to setup the RC6 with a colour camera to obtain subs quicker (can get something decent in a night instead of 3-5 with mono

I don't see that it's quicker with OSC, yeah you will get an image from the first sub but side by side comparison, but that's not what the threads about

What issues are you having with the oag?  First what version zwo is it?  Or more directly, what size is the prism? A larger prism will pick off a larger section of light

Are you using the supplied extension rings so not to have the focuser too far out and cause flexure... The oag can be used to take up some of this room, you're not adding the oag to the combination that you're using to achieve focus are you? 

The other thing to have in mind with a oag is to get the main imaging camera to focus first and then measure the distance from the prism to the sensor and then have the same distance from the prism to the guide cam sensor, note that the backfocus distance on the guide cam will be different from the imaging camera so see the spec from the manufacturer...  



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  • 2 weeks later...

I use an RC, with the CCDT67 reducer and an ASI294MC Pro. Focal length is 1004mm with the reducer. I have upgraded to a Baader Diamond Steeltrack focuser with a home made stepper motor focuser kit. I autofocus using APT and this works well. 

I mount the guide scope (280mm ZWO scope with an ASI290mini guide cam) on the top rail of the main scope. This way I don't get any balance issues.  

The 290 guidecam has smaller pixels than the 294 main cam, so pixel ratio is around 2.5:1, which works well. I've avoided OAGs because I have enough tech to struggle with already!

Total weight is around 9kg. 

I mount the main and guide scopes on a CEM40, and on nights of low wind this combination will guide at below 0.5arcsec RMS for extended periods.  I have no experience of the HEM27 but would expect it to work equally well. 

Clear skies


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