Objective spacers.

[Merlin]

How do we know what thickness to make objective spacers?

I refer here to a four-inch/f.12 Fraunhofer objective. Thanks for any info'.

[Merlin66]

Over on the ATS group recently they recommended a 0.004 (4 thou) spacer for a similar lens. They also noted that the current cellotape is 4 thou!

HTH

[Peter Drew]

Good idea to match the recommended spacings, optical corrections can be affected by straying too far from the original.

[Merlin]

Thanks Chaps, but I recall that the original spacers, as supplied by the maker ( long deceased ), where considerably thicker than cellotape.

I lost the originals, but it's a long story! I made three from thick foil and they don't seem to affect performance, but I'm no expert.

This is an objective in pristine condition, although uncoated. Maybe we should let sleeping dogs lie.

[AndresEsteban]

Good idea to match the recommended spacings, optical corrections can be affected by straying too far from the original.

That's right! I have an old 3-in f/16 OKKK series refractor and after painstacking process came to leave the original spacers as the best ones. You'll notice the image sharpness is at it's peak with the correct sapcer thickness. Also you must check with a bright star to see if you are over or under-correcting the air space.