Eyepieces and Adapters,     for suggestions on minimum eyepiece set see 8_inch-d.htm#eyepiece

A 2 inch eyepiece tube supplied with the Crayford focuser.
B Custom machined eyepiece tube that is 2 1/4 inch longer than the standard tube. It was copied from the standard tube and the flat for the friction shaft extends almost the full length. The white dot at the end of the tube is a nylon screw that was glued into a taped hole then trimmed off. This was later replaced with a steel set screw after part D once almost fell onto the mirror. We then put the nylon into part D. The nylon adds just enough friction to keep part D from sliding freely.
C 2 inch to 1 1/4 eyepiece adapter supplied with the Crayford focuser.
D Part D started as a 2 inch to 1 1/4 adapter. The flange was machined off so it will slide into the eyepiece tube. It is used to hold an eyepiece when the camera is used for astrophotography. After we bought the shorty Barlow we found tube B was too short so we milled a grove part way down D to slip over the steel set screw in B.
E Part E started as a 2 inch to 1 1/4 adapter. We had it machined for the camera.
F Our favorite 15 mm eyepiece.
G exit_ppl.gif (129007 bytes)Our 30 mm eyepiece. The 30 mm eyepiece used with the Barlow gives about the same magnification as the 15 mm eyepiece. Using the Barlow plus 30 mm combination gives better eye relief when we are wearing glasses or using the camera. Eye relief has to do with the angle the light passes through the exit pupil of an off axis star. If the camera must be farther from the eyepiece the exit pupil of an off axis star will be so far off the eyepiece - camera axis that part of the exit pupil will be clipped on the camera iris. This causes stars near the edge of the field to be dim.
H The shorty Barlow doubles the magnification.
J The camera grating adapter. ( Used for Ronchi test using an artificial star. )
Geometry of the eyepiece

Geometry of the eyepiece
from 'Optics' by Hecht

 

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