80mm F/7.5 refractor field illumination

[vlaiv]

I just discovered something quite interesting. It turns out that good field illumination on 80mm F/7.5 and shorter is quite hard to achieve.

I was in process of designing focuser for a telescope that I'm 3d printing (or rather assembling out of off the shelf components and 3d printed parts), and I wondered what sort of draw tube width I can go with.

I have constraints on tube width due to mechanical properties of the focuser (how it all fits together), so I decided to check out what sort of field illumination I could get and if I'm able to go with 2" at all or should I stick with 1.25" backend.

In any case, FreeCad / Sketcher is very nice tool to quickly calculate needed quantities.

For example, if we want to use 2" drawtube or 51mm ID, and want to achieve only 38mm illuminated circle (max field stop for 2" eyepieces is around 46-47mm) - we need to limit our drawtube length to only about 100mm - anything longer will reduce illuminated field.

Maximum distance between focal plane and clear opening of drawtube is 204.5mm (blue value) in above diagram. Out of those 204mm - about 104mm goes on 2" diagonal (depends on diagonal model, but most are just a bit higher than 100mm in optical length).

Similar thing happens if I want to use 1.25" backend and want to have it as much illuminated as possible - say 25mm illumination, but want to use drawtube that is less than 2" - maybe 41mm ID one.

We now have 174.5mm of distance - and about 75mm of those will go toward 1.25" diagonal (unless prism is used and I would not at that speed, but in any case, savings won't be that much).

Still leaves only 100mm long draw tube - and that limits focus travel / adjustment range.

What sort of illuminated field would you expect from such scope - like 80mm F/7.5 achromat?