Off axis guiding with small refractor and DSLR

[RoloFanatic]

Hey,

I am wanting to try autoguiding with my telescope but I am very close to the maximum weight limit of my mount and I cant upgrade my mount because I need it to be highly portable. This is what lead me to look into off axis guiding.

Im using a William Optics Zenithstar 73 with the Z73A adjustable flattener on an ioptron skyguider pro and a astromodified canon 4000D. From the reading I have done I require a 55mm spacing behind the flattener to the camera, this is usually achieved by the 44mm distance in the camera body, and an 11mm gap for the connecting t-ring adapter. What I am planning to use for OAG is the TSOAG9 with the TS EOS adapter which gives a total spacing of 11.4 mm if I understand correctly. Is this spacing ok for my DLSR as it is slightly more than the 55mm stated?

Also, I am planning to use the ZWO ASI120mm guide camera, which I know some people have said may not be sensitive enough but I think they are referring to it not being suitable for use with very long focal length SCT scopes. Would this be sensitive enough for use with a 73mm refractor? 

Any help would be great as this stuff is $$$ so I want to try and get it right first time.

 

Thanks!

[newbie alert]

I personally can't see the benefit from using a OAG with your setup, and can see that it might give you more issues than it solves... first of all weight... it's going to add weight no matter how small it is. Second, the 72 is a widefield scope and the light cone the prism picks off is going to be small..I've not used a 120 but I used a qhy5 which i think had the same sensor and even through a 60mm x244mm guidescope sometimes I only had 1 guide star.. Someone I've been recently talking to recently had issues with a 72mm frac and 290 sensor with a OAG,  just couldn't detect a star to guide on..

I'd suggest a mini guidescope, think they're 30mm.. im sure that you can get a 3d printed adapter that fits the top shoe on your dslr... balance is really important for guiding

[Padraic M]

No, I don't see any reason to use an OAG with this setup. OAGs are a complete pain to configure and for a short focal length scope like yours they're normally not used. 

Since you have the WO Zenithstar, why don't you get the WO 32mm guide scope? It looks like it should bolt to the top of the tube rings and it only weighs 240g. The ASI120mm will work just fine. I have used it and the 290mm Mini to guide both an 80mm refractor and a 150mm reflector and no problems whatsoever. 

I make the total weight of this package 3.63kg. Payload of the Skyguider Pro is given as 5kg, but I don't know if that's imaging payload - manufacturers often provide visual payload, and imaging is half of that. So you may be slightly overweight, but the guidescope is providing very little of that weight. With your scope and camera you're already overweight.

Also, the pixel scale ratio between the imaging scope and cam and the guide scope and cam is <4 which is perfect.

[RoloFanatic]

Hmmmm I think maybe I got a bit overexcited when I read about this because it was something new and I liked the idea of not needing a guide scope. 

But from what you're saying it sounds like it would be more of a pain than it's worth, I ll have a look at some mini guidescopes instead and put the OAG on hold.

Thanks for the advice

[Starflyer]

Just my £0.02

I stopped using a separate guidescope over ten years ago after a couple of years of fighting differential flexure and not being able to get round stars on either my newt or small frac.

They're simple to setup in the daytime or at night pointed at the moon, yes you may need a slightly more sensitive camera, but the benefits to me far outweigh an hour of one-time setup time.  Loads of people have no problem with a separate guidescope on a frac but I never cracked it and always had oblong stars to some degree.

My camera, filterwheel and OAG are always stored bolted together, I just swap out a coma corrector for a flatenner to use it between my scopes, so beyond the initial set up there's zero time spent setting up the OAG each session.  I've never had problems finding a guidestar, PHD2 multi-star often guides on a good selection of stars.

Edited February 6, 2022 by Starflyer