going for advanced RC6 setup #EXOS2 #astrophotography


After a lot of thought, I am going to go for a more difficult setup to do mid-narrow field astrophotography and possibly some other experimentation. I'll be trying out the exos2-gt with the PMC-eight system with an RC6 and off-axis guider. I am not an astronomer, but I have enough optics experience that I should be able to handle collimation just fine. I think I would be a little embarrassed if I could not figure it out with my background. So its going to come down to how well this thing can guide at higher focal lengths, and I am super motivated to try it out.

One thing I want to do is write an algorithm to tell the mount to automatically shift through steps using the DSI collimation method when using an artificial star.

Then write another algorithm to compute the deviation from an ideal diffraction pattern outside of focus like in the DSI method.

I could probably write this in labview, but labview can be more of a pain despite it already having a lot of libraries for imaging.

I wish I could go bigger, but I am not "well to do" to say at least.

Bill Black

Hi Mr. Polynomial,
That's an interesting article. Do you think something along the lines of procedure 2 (balancing the image) would work for collimating an SCT?

This sounds like a neat project. I've never used an RC. I understand they can be excellent for astrophotography, but the collimation can be difficult. So something definitely worthwhile in your project, I think.

I'm guessing that one of the biggest hurdles you might face is getting a good guide star image with your off-axis guide camera. Long focal lengths can be difficult to use for getting a good tight guide star image. So you may want to think about putting a focal reducer on your guide camera, if possible (maybe 50%).

From the article, it sounds like you will be shooting 5 sec to 30 sec exposures through the main camera to try to smooth out the seeing conditions. I'm guessing the longer the exposure the better for that purpose. Your guiding exposures will probably be in the 1 sec to 2 sec range. So you won't have the same opportunity with the guider to smooth out the seeing conditions. Using a star field close to the Zenith would help with the seeing. I would stay away from anything below 50 degrees alt.

Location: Dallas, GA
 Celestron C8, AstroTech 80mm APO

Mounts: EXOS II PMC-Eight, LXD75

Cameras: ZWO ASI294MCPro, ASI385

Wes Mcdonald


Well heck.  Why not.  But do yourself a favor and use a guide scope.  Exos2 pmc8 will guide to about 1”, some get better.  It also works better in some areas of the sky.  Exposures of 3 minutes and longer have round stars at focal lengths of 900-1000.  Understand, seeing conditions usually limit your resolution and thus a long focal length is not going to get you much for DSO.  Lucky imaging for planetary will be nice.  At long focal lengths your astrocamera is going to oversample like crazy, and you will likely want to bin the images (DSO).

Can’t wait to see what you come up with.  I have an SCT with pretty good collimation.  It isn’t hard to do with out of focus star images, visually.  Maybe your algorithm will be an advantage.  

How much does that RC weigh?


Wes, Southport NC
EXos2-GT PMC-8, iExos 100
ES ED 127, 10" LX200GPS+wedge, Astro-Tech 8" Newt, ETX-90, 60mm no-name guide scope ~ 260mm FL
Polemaster, Orion ST-80 and SAG, ZWO 290MM, D5300 astro modified
Nina, Bootcamped Mac Mini control computer, RDP to iMAC
110 amp hour lead acid deep discharge battery for field power
Electrical Engineer, Retired


RC6's all weigh like 10-12 lbs. Im looking into off axis guiding and I already have a DSLR with an APS-C sized chip running 4.7 micrometer pixels for imaging. Apparently, this chip size is recommended for the FOV of the RC6.

But as far as the guide camera goes, I have not really decided on that. I have no issues with planning out my nights by setting the OAG angle ahead of time, to make sure there are guide stars, but I am not sure something like an ASI120mm will actually work with that. Its confusing because some people say it wont work, but then say that "it does not work" for them because they have to rotate the OAG.

Other larger pixel guide cams could be like the orion starshoot g3 mono. Its got large pixels. It can get kind of spendy after that, though.

0.5" - 1" RMS is about 3.3 - 6.6 micrometers at the RC6 focal length. With a 4,928 × 3,264 @ 4.7 micrometer pixel size camera, its not too bad for long exposures with that RMS. Its not like an SCT focal length.

As far as the algorithm stuff goes, I hoping that I end up with a tool that simply gives you a number at each step, and when you reach 0 it means perfection. And that you really only have to hit a key for continue while you play with the adjustment screws.

I have seen some people pay outrageous amounts for collimation tools with RC's, but there is no way around adjusting the primary and secondary using something like the DSI method, a star (artificial or not), and correcting the PSF in each region of the image. The other ways are just not as precise.