I have an 8" F/15 doublet lens with no coatings in a machined cell.
The lens and cell are both home made, the maker has long since passes away and it was dontated to our local club. If I hold the lens and cell assembly in my hands, and I look down into the sky lens (through R1 first), I can see the intereference pattern of R2 against R3 since the lights in my home are the new cork screw light bulbs.
As I rotate the entire lens/cell assembly or very gently press onto the sky lens retaining ring, the intereference pattern changes drastically. You see about 20+ sets of rings, and the center of the bullseye pattern is rarely centered. In some orientations, the center of the pattern is way over to the side of the lens, others rotations see the circular pattern change to an oval shape with oval rings all the way to the edge.
I removed the lenses for cleaning and found both lenses are really thin for 8" diameter. Both edge thicknesses are about 3/8". The sky lens is double convex with R2 ROC visually shorter than R1. The rear lens, R3 is concave to a point where the center thickness has got to be no more than 1/4", and just eyeballing R4 the lens has an extremely large ROC, just slightly curved.
The lens formulas I have seen for most Franhofer type prescriptions of this aperature typically call for lenses of about double what I see, even without measuring. ( I don't have a sphereometer or a caliper at the moment). A quick and dirty focalt test showed R3 with a ROC of about 37 to 38 inches.
Here are my questions. Should not the intereference pattern I see be fairly stable, with the bullseye fairly centered regardless of cell rotation?
Is it normal to actually see the R2 to R3 pattern just looking down the sky lens? (no coatings on any lens surface).
My guess is the lenses are way too thin and the pattern change I see is due to thin lenses.
Any thoughts?
The lens and cell are both home made, the maker has long since passes away and it was dontated to our local club. If I hold the lens and cell assembly in my hands, and I look down into the sky lens (through R1 first), I can see the intereference pattern of R2 against R3 since the lights in my home are the new cork screw light bulbs.
As I rotate the entire lens/cell assembly or very gently press onto the sky lens retaining ring, the intereference pattern changes drastically. You see about 20+ sets of rings, and the center of the bullseye pattern is rarely centered. In some orientations, the center of the pattern is way over to the side of the lens, others rotations see the circular pattern change to an oval shape with oval rings all the way to the edge.
I removed the lenses for cleaning and found both lenses are really thin for 8" diameter. Both edge thicknesses are about 3/8". The sky lens is double convex with R2 ROC visually shorter than R1. The rear lens, R3 is concave to a point where the center thickness has got to be no more than 1/4", and just eyeballing R4 the lens has an extremely large ROC, just slightly curved.
The lens formulas I have seen for most Franhofer type prescriptions of this aperature typically call for lenses of about double what I see, even without measuring. ( I don't have a sphereometer or a caliper at the moment). A quick and dirty focalt test showed R3 with a ROC of about 37 to 38 inches.
Here are my questions. Should not the intereference pattern I see be fairly stable, with the bullseye fairly centered regardless of cell rotation?
Is it normal to actually see the R2 to R3 pattern just looking down the sky lens? (no coatings on any lens surface).
My guess is the lenses are way too thin and the pattern change I see is due to thin lenses.
Any thoughts?
