Or... one could use the extra degree of freedom to reduce lateral color:
Lucas 85mm f15_F_C.len 930bytes 7 downloads
Edited by rflrs, 28 November 2021 - 07:29 PM.
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Posted 05 December 2021 - 10:25 AM
Hello Mike, Myriam & Richard,
Thanks for those new designs !
Of course, I started a week later than I thought. I'm planing everything with C80 because the crown had deep trenches.
It's very different work from a larger mirror, everything happens fast with the small surface.
For the record, I'm sticking with the following design (Myriam's) :
It seems the most straightforward, and I'm not yet literate enough to assess the others.
I'm keeping a log of everything and will post again when everything has flattened enough to start grinding the curves and wedge is acceptable. That way we'll know how much working time was needed to start with a clean slate !
Have a nice day,
Edited by chantepierre, 05 December 2021 - 10:35 AM.
Posted 06 December 2021 - 06:09 AM
The following tests suggest rigour in designing and testing a doublet is not a bad thing but also indicate just how much tolerance there is in small, slow achromats.
I took out my 1980 vintage 3'' F/15.5 Littrow and measured how much R4 differed from a plane and R3 from R2 and R1. Below are Newton's rings between R3 and R2. R4 and plane are the same whilst R3 nd R1 have 2/3 the number of fringes.
R3 and 4 together are relatively concave compared to the Littrow design by around 25 waves which is the difference between R3 of 524 and 520 in your case.
Crown centre thickness is 7mm whilst flint is 5mm.
Spherical aberration in green light looks to be 1/10 wave.
R3 on R2
I used a double pass ke null to measure difference in focal lengths using a green filter and 656nm and 404nm laser pointers (on a diffusing screen). I have some 480nm leds but haven't fixed them up with any power.
What surprised me was just how over-corrected my doublet was. Red light focused shorter than green by 11/2mm whilst the longer violet was 18mm further back. It was hard to be precise with the null positions for a number of reasons e.g spherochromatism, laser speckle, lens decentering and misalignment etc.
Oil (n=1.51) spacing the doublet brought the red up the green focus whilst the violet shortened by 3mm.
R4 was then (cedarwood) oiled to the back of a float glass optical flat which pushed the red light focus1mm further than the green with the violet shortening by 5 more mm to around 10mm further on than the green. (Numbers doubled because of double pass).
I'd need to rig up the 480nm source to be sure that it is well corrected. Re the ref indices of oil and flint I think 3 equal curves and a plane would still be under-corrected.
So, on the one hand it's good practice to keep a closer eye on surface curvature than I did 40 odd years ago but also bear in mind that in actual use CA is almost a non issue in spite of apparent lack of correction.
The advantage of having R3 and R2 slightly different from the point of view of ghosts should be considered unless the objective is oiled which potentially brings a leakage problem.
Thorough is good. Best of fortune with the project. David
Edited by davidc135, 06 December 2021 - 06:21 AM.
Posted 07 December 2021 - 05:09 PM
Posted 07 December 2021 - 10:00 PM
Never having worked flint glass before, I have been really surprised about how tender it was ! Everything I've read about that was nothing compared to how fast it ground ! Incredible.
It works the other way when you polish it...takes forever. It also never really takes
the beautiful polish that pyrex or plate glass does, so keep that in mind.
Posted 21 December 2021 - 02:18 AM
Edited by chantepierre, 21 December 2021 - 02:19 AM.
Posted 24 December 2021 - 03:27 AM
Posted 24 December 2021 - 09:18 PM
I can't see that coatings would alter the design. David
Posted 24 December 2021 - 09:45 PM
The coatings would not alter the design. To do their antireflection job properly, they need to be within a tiny fraction of a wavelength of uniform thickness over the entire surface. The tolerances for the radii of curvature of the surfaces are much greater than that.
Posted 25 December 2021 - 04:21 AM
Posted Yesterday, 08:49 AM
To my surprise, and one of my friend's, grinding those lenses is way harder on our back and arms than grinding a large mirror. My friend has the same problem with a cassegrain secondary.
So this project goes in a bit of a "Yak Shaving" phase while I try to build a small grinding machine. I think I will go for a "sine table" type machine and am gathering documentation on that. If I understood it well, it's able to generate concave and convex curves, which is nice.
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