Jim Easterbrook wrote:

Today I had my first go at measuring the focal length of my eyepieces,
using Don Taylor's magnification of a grating technique. I don't possess
a proper Ronchi grating, but made my own 100 lines/inch grating with a
laser printer and some OHP film.

My measured focal lengths were generally quite close to the stated
values except for my 38mm Surplus Shed Erfle, which measured 31mm. Has
anyone else with one of these eyepieces had a go at measuring it?

Yes,

My so-called 8mm is actually 13mm. I also (together with Beri, whose post
you can find in this thread) assembled or bought some eyepieces, notably:
- 6.5mm (bought in one piece, 10 elements!): very good AFOV, 10 elements
steals at least 1 magnitude
- 7mm Kelner: there were scratched, probably last of its kind in Shed.
Since I don't have shorter ERs, I sometimes use this one for the moon.
- 8mm Kelner: FL is more like 13mm, with *big* coma ta the edges. But, new
ones are on the way (for free!!).
- 11.5mm Kelner: Very good Kelner
- 19mm plossl: superbly sharp. AFOV only about 43, but I love it, it's
great complement to 26mm
- 26mm superplossl: AFOV is 69 degrees!!! Not as clear as 19mm, but who
cares if You can see all of M45 or Coathanger at 35x!!!

Regarding focal lengths, I'm also little suspicious about that, but have no
idea how to measure it.

Altogether, I still think SurplusShed stuff is bestbuy for ATM.

Tonci
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http://astro.fdst.hr