Off axis guider vs. guide scope

[Dirty Harris]

Dear Guylain, I am looking for some professional advice, from you or anyone else who has used either an off axis guider or guide scope, or both. Which would you recommend? I've heard arguments for both, but the off axis guider seems to help eliminate the diff/flexure that a guide scope has. What has been your experience? We run a Celestron CPC/SCT fork mounted 11inch monster, that does a super job on planetary, but we're hoping to get into some of that deep-sky stuff, and obviously we'll be needing to guide. Hope you or others have some info. Thanks again, Dirty Harris. Colorado, USA.

[jimwc6491]

Ron

 

I just need to get it outside and see if I can see some stars.

I'm sure it will guide if I can lock on a star.

 

tomorrow Ill do the updates.

[Dirty Harris]

Don't give it another thought JIMwc6491, as I said in an earlier post, this forum is remarkable, and the back and forth knowledge being bandied about is very educational. Thanks again to all. Dirty Harris.

[PoppaChris]

I also run a Celestron CPC 1100.  I found that both the Celestron premium OAG with 6.3 reducer and an 80mm x 600mm guide scope work well.  However, I have had a lot of difficulty finding guide stars under light polluted skies when using the OAG.  I don't have this issue with the separate guide scope.  Additionally, I have reversed the roles of the two scopes at times and used the CPC1100 as the guide scope and the 80mm as the imaging scope for shots that are far too wide-field for the CPC even with the focal reducer.

[dts350z]

FYI Orion announced a new guide scope yesterday:

 

dia 60mm

FL 240mm

FR 4

 

10mm helical focus plus 35mm of "slide" focus. T-threads

guide scope rings and a dovetail bar.

 

http://www.telescope.com/Telescopes/Guide-Scopes/Orion-60mm-Multi-Use-Guide-Scope-with-Helical-Focuser/c/1/sc/20/p/114898.uts

[Dirty Harris]

Thanks "PoppaChris" hadn't thought of reversing the two scopes(OTA's) seems like sound advice, may just try that. And to "dts350z" that does look like a good choice for a guider scope, thanks for the link to Orion. I have much to consider, my thanks again to all who contributed. This forum/support is the cat's meow! Or in our terminology, a night of better than average seeing. Dirty Harris, Colorado, USA.

[dts350z]

One thing to consider is how much back focus you have. Is there enough so you'll still be able to achieve focus with an off axis guider in the optical train in front of the camera. If you have an 11 inch SCT perhaps you are also going to want a focal reducer and that will eat up lots of back focus.

 

Also, I CANNOT recommend the orion thin oag. Yes it's been redesigned but it is still essentially junk, IMO. I gave up and went back to guide scope but yes others swear by OAG. 

[astroman133]

Is your CPC1100 equatorially mounted (on a wedge)?  If not you would still have field rotation to deal with.  I do not know about the Celestron driver, but some drivers don't support guiding commands for an alt-az mount.

 

You can eliminate most of any flexure issues that you would get a piggybacked guidescope but it is still more difficult to get good guiding because of the difference in focal length and resolution between the guiding optics and the imaging optics. By that I mean that if the resolution of the guidescope and guide camera are, say, 3.5 arc-seconds per pixel and the resolution of the imaging train is around 0.5 arc-seconds per pixel, the guiding may be perfectly fine, but you may not be happy with the images because the imaging optics have so much greater a resolution.  All the above issues go away with off axis guiding.

 

One issue that crops up, however, is that with the smaller field-of-view is that it can be a challenge to find a guide star.  Having a good way to rotate everything, without loosing focus, will help. And there are the issues that dts350z brought up that relate to the more complicated imaging setup when you throw an external focuser, and a focal reducer into the mix.

 

So you can see that the issues are more involved than just elimination of flexure when you decide to go to off-axis guiding.

[Dirty Harris]

Yes Astroman 133, we are on a wedge and polar aligned, should've included that in my initial inquiry. I've come to find it's the best way to go for most everything even planetary. I would like to thank you and "dts 350z" for the info/advice. You're both spot-on about the focal length of the train, which I do run a Canon T-3I on the backside. And I hadn't given much thought to the arc-seconds issue, but now that I've done the math, it makes great sense, and sound wisdom. I believe I'll start with the Celestron off axis guider, designed pretty much for my SCT. and see how it goes from there. Thanks again for your help and anyone else that cares to chime in. The knowledge base of this forum/support is just phenomenal.  Dirty Harris Colo. USA.

[s3igell]

 

If the resolution of the guidescope and guide camera are, say, 3.5 arc-seconds per pixel and the resolution of the imaging train is around 0.5 arc-seconds per pixel, the guiding may be perfectly fine, but you may not be happy with the images because the imaging optics have so much greater a resolution.  All the above issues go away with off axis guiding.

 

 

Both PHD2 and MetaGuide perform Sub-Pixel Centroid Calculations.  PHD2 claims they can Guide to 1/10th of a Pixel using this functionality.  That is further than I'll give it credit, but even at 1/4th Pixel the "Guiding Resolution" quickly approaches that of the Imaging Resolution.  This means that the common 400mm FL 80mm Achro with a Guide Camera with 3.5um Pixels (1.8as/P) will easily keep up in "Guiding Resolution" with a 2800mm FL CPC1100 and 4.3um T3i  (0.35as/P).

 

One issue that crops up, however, is that with the smaller field-of-view is that it can be a challenge to find a guide star.  Having a good way to rotate everything, without loosing focus, will help. And there are the issues that dts350z brought up that relate to the more complicated imaging setup when you throw an external focuser, and a focal reducer into the mix.

 

Another issue with an OAG on a regular SCT is the VERY Distorted Star Shapes (Heavy Coma) due to taking that Star from the very verge of the Imaging Circle.