back in the days on the blue forum, I have been talking about testing the eyepieces, and here we are.
Th pinhole imaging is an easy tool to check out the optics for dirt and other blemishes, coming from the manufacturing, and from the later use.
The method is trivially easy, it comes from the elementary science of optics.
Take a needle, and make a hole.
In the case of an eyepiece, it is strightforward to take the old Kodak film barrel, and to make a small hole into its bottom,
Then, you will hold the canister with the pinhole in front of the
Now, the next pic shows 2 situations, I have observed on 2 different eyepieces,
The bubble in the glass of one of the lenses comes from missing QC in the manufacturing process of the glass material melting,
and from the missing QC of the final product (eyepiece) - they should have seen it,
The dirt is a residual after not completed cleaning, nicely visualized with the pinhole imaging.
Thank you for reading and looking,
JG