Which eyepiece will work for f/2.5 scope?

[DeafStar]

Hi,

I am making an experiment with a good quality round 360/900 (f/2.5) Fresnel lens just for fun. I think it will work quite well under low magnification (5-20x range) for nebulas since nebulas are almost edgeless feature so I don't care too much about seeing any rings from the Fresnel lens if any and I doubt will see any especially at night time. I don't mind to make my own eyepiece if I have to to correct any coma or aberration. I am trying to stick with the basic for now. Any thoughts to share would be grateful. 

Todd

Edited September 15, 2021 by DeafStar
Forgot to say Hi

[Louis D]

At least with a 900mm focal length, field curvature won't be so bad.  It will still have a radius of curvature of about 300mm IIRC, so you can probably get by without a field flattener.

So, you want to try a 45mm to 180mm focal length eyepiece with it?  You do realize how bright the night sky will appear with 18mm to 72mm exit pupil sizes?  You won't be able to make out any nebulosity due to lack of contrast against the bright background.  That, and you eye won't be able to take in the entire exit pupil, so you'll be working with a 140mm to 35mm effective aperture at best.  Seems like a huge waste of effort, but go for it if you already have all the lenses.

Generally, your best bet to take in nebula at low power is night vision gear or other electronically assisted observing.