Quote:
Originally Posted by Satchmo
HI Stefan
I'm afraid without the provenance of the 'melt data' on the glass, giving a designer the figures of the actual measured dispersion and refractive index of the individual pieces , this glass is really just `land fill ' .
If you were going to actually manufacture such an instrument you would just buy new glass with all the required melt data - BK7 is not very expensive compared to the value of such a finished instrument . I'm not trying to be picky - its just one of those cold hard realities of using optical glass. You may be able to fudge the result by sticking to the design and aspherising one surface via null test in the end result ?
I've thought about these all spherical designs , but to be honest polishing and figuring 8 optical surfaces just doesn't seem to make any sense compared to one paraboloid and an off the shelf Wynne corrector ( at F3 ) . Maybe I have missed something !
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Thanks Mark for the comments but I don't think the accuracy of the glass dispersion data for this instrument is as critical as it would be for an apochromat, for instance, where all the power comes from refraction. Here it is important that the two main lenses come from the same melt and I don't think Schott or Ohara glasses deviate too much from their intended specs. Anyway, a few years ago I made a small Companar using "landfill glass" and it works very well. The worse that can happen is that I will be required to make a couple of precision prisms from the offcuts and measure the glass myself and then adjust the curvatures accordingly. And yes, I will null test it but I don't know at this stage how useful such a test will be as the design is not meant to be diffraction limited.
Please don't try to understand why I'm making it because probably has something to do with insanity.
Regarding the corrected Newtonian: I count 8 optical surfaces and at least one of them is not an easy one. Besides, it is such a 20th century thing.
