From E-mail sent to the TASS mailing list
Tom Droege wrote:
I am forwarding a message from Elliot Burke in hopes that you all can make sense of the coma problem. Sorry, Arne, perhaps someone can put the pictures where you can get at them. The coma is clearly worse than he expected. I find that the focal plane is 4.2 +/-0.2" behind the rim of the last lens. From the design drawings I expected it to be more like 2.5". All this indicates possible assembly errors. I will work with Elliot and anyone else that wants to work on this problem.
Here is Elliot's message -- it includes two pictures
Thanks for the CD. I downloaded a viewer and looked at an image. It appears to have a great deal of coma, much more than I would expect from the design predictions. Attached are two GIF, a small image of from an actual image. It appears to have 12 pixels or so of coma. A calculated point spread function, including diffraction, from the edge of the field (3° or 21 mm from optical axis). There are perhaps 3 pixels of coma. The symmetry of the pattern indicates that this is probably not an alignment issue. I wonder if the lens element spacings are correct, for each wavelength?? Is the focus where you expected it to be?
First, the calculated coma
Now, the observed coma
Elliot sent a second message with more pictures
I increased the accuracy of the coma calculations a bit, by using more wavelengths, here are the calculated point spread functions at 21 mm image height:
cross section in X-direction through calculated PSF
cross section in Y-direction through calculated PSF
more detailed calculation of PSF pattern
Elliot sent a third message
I'm trying to find a way to make a reasonable tests of the lens. Why don't you send me one, and I'll check the spacers and lenses as best I can.
The good thing is this: the observed symmetry of the coma means that the problem is either a lens radius or an element spacing. If it's a lens radius, and I can measure the as built radius, then probably can calculate new spacer thicknesses that will fix coma. If all lenses were made the same, then this fix will apply to all.
If the problem is spacer thickness, then this will be quickly apparent after disassembly. The fix will also be simple.
Can you get me the drawings the lens barrels, spacers, etc, were made to? I can read AutoCAD or mechanical desktop drawings, or paper....