Focal Reducer

[Andy67]

Hi guys. I have an Orion XT8 and I'm using a 25mm BST Starguider. I want to view M31 and I've read that I will struggle to get it all in the field of view. I've also read about the use of a focal reducer to increase the field of view and wondered whether any of you have had any experience using this particular Focal reducer? 

Thanks.

[vlaiv]

I don't think that will work particularly well.

I have similar x0.5 focal reducer and effect it has on 1.25" eyepieces that are already close to max field stop for that format is - vignetting (and it also messes up with eye relief - which can be issue as well - blackouts and things like that).

If you want to observe M31 - you really want 2" eyepiece, but even then - 1200mm FL is just too much to observe it whole.

Use this tool to get the idea of what you can see in your scope with particular eyepiece:

https://astronomy.tools/calculators/field_of_view/

Even with one of the best budget wide field 2" eyepieces - Aero ED 40mm - you won't be able to fit M31 in FOV with that scope:

To completely fit M31 in FOV - you want scope with about 700mm or less focal length (maybe 750 - 800mm but that is pushing it - below 700mm you will get nicer framing).

For example - Skywatcher Star Travel 120 (famous ST120) - wide field scope with 35mm version of AeroED eyepiece will give you very nice framing (here you are approaching binocular magnifications - like x17, btw binoculars are very good way to observe M31 as it is very large):

Just one more note on original topic of that focal reducer - it has clear aperture smaller than max field stop of 1.25" eyepiece and that will be "squeezed" (reduced) into even smaller circle - that is why vignetting occurs with it. It is otherwise ok for shorter length eyepieces like 17mm and below - when you try to make them work as wider field eyepieces (but it is often simper to just get wider field EP - like that 25mm BST).

[Andy67]

Great. Thanks for the advise. I won't bother getting it then,  I'll put the money to better use. 😊