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Auto guide options - OAG or guidescope

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I have just got myself a Celestron C11-A XLT (CG-5) scope and a SkyWatcher EQ6-R PRO SynScan mount. I have an ASIAIR PRO and the ZWO ASI462MC planetary camera. I want to use my Nikon DSLR for deep sky photography. 

Now I’m looking at auto guide options. I’m thinking of getting a ZWO ASI290MM mini and am wondering if I should get a guidescope or an OAG? Which is better and why?

I don’t have the Edge HD C11 so would the view on my scope be good enough for an OAG? If I go for a guidescope what would I need there? I’ve seen 50mm and 60mm scopes but some people have mounted huge guidescopes which would be way out of my budget. Would a smaller guide scope be up to the job?

Any advice would be greatly appreciated. 

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You don't say which Nikon you'll be using, nor do you say whether or not you'll be using the focal reducer, but even if you are using the focal reducer and have a relatively large pixel Nikon you will probably be imaging at well below an arcsecond per pixel. A good EQ6 (they vary) can deliver a guided precision of about 0.5 arcseconds RMS which limits you to an image scale of about 1 arcsecond per pixel. In short that means that your autoguided mount is unlikely to be able to support the resolution at which you are trying to image. That's not the end of the world but it does mean that your mount will need all the help it can get and that means an OAG. An OAG will react to slight mirror movement when a guidescope won't (not the same light cone) and an OAG will be working at the FL of the imaging scope. Also, there will be no differential flexure between guider and imaging system.

Basically you are asking more of your system than it has any real hope of delivering (but it will deliver images none the less) so you need to give it all the help you can. This calculator http://www.12dstring.me.uk/fovcalc.php  will tell you your image scale if you plug in all your kit details. Again, a well tuned, autogiuded EQ6 will deliver about 0.5 arcseconds RMS which limits you to an image precision of about twice that, so 1 arcsec per pixel. Don't panic. Just don't expect to be able to present the final mage at full size.


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Thanks Olly.  I appreciate your well informed and detailed response. OAG it is then. That helps a lot.

I have an unmodded Nikon D5300. I also have a TSOptics f/6.3 Focal Reducer/Field Flattener.  All of this is an upgrade from my C5 on a CG-5 GT mount which I have only used for observing and occasionally photographing the moon.  It will be interesting to use the OAG and camera on both and compare the results.

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Yes, as @ollypenrice indicates OAG is really the only viable option for a SCT, especially with that focal length (I have an 11" and a 14" SCT and OAG on both). My only additional suggestion is that you go for the most light sensitive OAG camera you can get since finding guide stars with an OAG at a long focal length can sometimes be challengeíng, especially if you go for galaxies outside the plane of the Milky Way, where there are not that many stars. So I suggest you put in another 100 pounds and get a Starlight Xpress Lodestar X2 rather than the ZWO camera you suggsted. The Lodestar has huge pixels (8 times the area of the ZWO camera) so more in tune with your focal length and much more light sensitive.

Edited by gorann
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Thanks Göran, I see that you have ZWO ASI cameras as well as the Lodestar for guiding.  Since the Lodestar is not compatible with the ASIAIR PRO what do you use with it instead? 
Cheers Steve

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