All I own is Meade Plossls for my ETX90.

I have been digging around here and trying to take an elementary deeper dive into understanding FOV and magnification for eyepieces.

From what I think I understand …

The AFOV for the Meade Plossl design is advertised as 52*.

If I use the general relationship AFOV=FS/FL radians (that need converting to degrees) where FS is field stop for 1.25 barrel (27.4mm is what I’m using) and FL (focal length of the eyepiece in mm) … then it would seem that the AFOV for all of my eyepieces (up to the 26mm that came with my scope) is greater than 52* and so the AFOV of these is limited to 52* introduced by the Meade Plossl design.

On the other hand … my 40mm Plossl (excuse me …”Super” Plossl) calculates to around 39* (Meade advertises 44*). So the actual barrel diameter and field stop, not the Plossl design is now the limiting factor to the much narrower AFOV.

This with the greater eye relief of the 40mm (eye relief increases with focal length, right?) makes it hard for me to use this 40mm unit. Like looking through a straw. It sucks. In my opinion, of course.

Furthermore … I’m getting 49* AFOV for the 32mm Plossl which approaches the max AFOV (in seeking lowest magnification) for the Plossl eyepiece design.

And using the relationship TFOV=AFOV/Magnification … I’ll use the 52* and 44* AFOV advertised by Meade for the 32mm and the 40mm … getting around 1.3 TFOV for both.

So the same TFOV for both the 32mm and 40mm just with higher magnification and more desirable (to me) 52* AFOV for the 32mm Plossl.

I’m trying to understand mathematically why I don't like this 40mm.

I don't like it no matter the math and will probably get rid of it and try a 32mm for lowest magnification.

But ... am I generally on the right track here in terms of the analysis?

Thanks for reading.

**Edited by OldCarolina, 01 December 2023 - 07:58 PM.**