Open Jashcraf opened 4 years ago
Jashcraf
commented
4 years ago @aafaquerk @trentjbrendel @jdberkson @samendo @ahedglen @yqzhang77 @yambaker Design a 2 mirror system that fits our specs. Feel free to discuss what might need to be updated on our spec sheet. Include a drawing of your system, an OPD plot, and a screencap of your merit function to show you are meeting specs.
ahedglen
commented
4 years ago Well I have to say, it is definitely hard to fit a wide field of view in such a small space, as Jaren found. I wasn't able to do much better than Jaren unless I decreased the aperture size or increased the working distance. So for constraining the optics to a space of 80mm x 80mm x 80mm, here is my effort.
I ended up using an aperture diameter of 40 mm for my primary mirror. For an image size of +/- 10 mm, here are my results:
It is interesting how the wavefront and spot size seem to be best at 0.7 field. I don't think I actually understand that. But I do know that field curvature is the main aberration here. So I'm sure adding a tertiary mirror would help reduce that. Or if we let our detector fall into the 2nd Unit, but keep the primary and secondary mirrors in 1U, that would help a lot.
The optimizer wants to drive the secondary mirror's radius of curvature as small as possible, so it is best to set the radius to a set value (I did -20 mm) and vary the other parameters.
yqzhang77
commented
4 years ago Wavelength: 400 nm to 1000 nm.
Aperture: EPD = 65 mm, considering clear aperture margin, chip zone, mounting, and 80 mm by 80 mm volume restriction.
Field of view: Varied until performance hits restrictions. In this case HFOV = 3.4 mm.
Focal length: 333 mm, calculated from GSD = 60 m @ 2000 km. This is equivalent to F/5.
Performance: OPD @ 1 um < 1 wave for all field of view.
Layout:
OPD:
Merit function editor:
Focal lengths are then set to be 462 mm, 650 mm, 929.5 mm, and 1300 mm for various F/#. Corresponding to F/7.1, F/10, F/14.3, and F/20. The performance requirements stay the same. Field of view is adjusted in each case to maintain the same performance. The following figure shows the relation between FOV and F/#, in both degrees and millimeters.
In this case, the diameter of aperture is fixed at the beginning, which is not very ideal. In reality, there are different factors restricting aperture size and focal length. It would be better if the plot above can be expanded in 3D space, with FOV plotted versus both aperture size and focal length, as the following examples shows.
Besides, in this case, the performance criterion is selected to be OPD < 1 wave, which may not be optimal. Spot size may also not work well as criterions because it is related to wavelength (a lot) and aperture size in addition to aberrations. I would consider switching to MTF for future designs.
jdberkson
commented
4 years ago 
Style: Gregorian EPD: 60mm Field: HFOV 3mm real image height (~0.24 deg object angle) EFL: 720mm (F/12) Performance: OPD < 0.5 wave @ 1um @ all FOV
trentjbrendel
commented
4 years ago Design Form: Gregorian Telescope EPD: 75 mm EFL: 800 mm F/#: 10.67 HFOV: 0 mm, 3.536 mm, 5 mm Performance: <1 wave OPD across field
aafaquerk
commented
4 years ago **Summary*** Aperture: 54 mm EFFL: 360 mm TOTR: 80 mm WFNO: 6.7 Wavelengths: F, d, C (Visible) FOV Angles: 0 to 0.8 deg FOV with equal area intermediate field points Performance Criterion: 80% EE within 2x2 pixels (26 microns) of a 13-micron pixel size detector
Lens data:
Design Layout:
Merit function
I am not super proud of this merit function but it kind of worked (maybe!)
It restricts conic constants for both mirrors
Fixes the total track
And gives a target for EFFL.
The aperture is fixed manually to restrict the F/number.
In short, its highly inflexible to explore the design space but does decent job to get rms spot size.
OPD Fan
Spots
PODS_Two_mirror_Aafaque_widefield.zip
Note: The two fields 0.6 and 0.8 degrees fail to meet the performance criterion.
RMS WFE vs Field
Encircled energy
Note: The two fields 0.6 and 0.8 degrees fail to meet the EE performance criterion.
Discussion:
I think this is not the best possible design and a few trade-offs have to be made on aperture size and fabrication complexity of the mirrors (conic constant) vs field of view. In a different version, I tried changing the field and the aperture size to get better performance by losing on collecting area. The performance for fields <0.4 deg seems reasonable to me. Happy to chat about this offline.
Update: The sidel diagram shows how nicely the two mirrors balance each other out (where they can) and we are left with residue field curvature and coma.
I was wondering if a two-mirror off-axis design could balance the comma leaving some astig and field curvature.
Design Description: Obscured Cassegrain, 2 conics
Discussion
The maximum image space field I was able to get was 8mm HFOV. And even then I'm taking some liberties with the spot size. We may need to reconsider our detector size, I'm realizing that for a full-frame sensor, 44mm diagonal is over half our optical space of 80mm. Woops!
Protip: When designing reflective systems & using OPD, use 1um as your wavelength. Then the waves of OPD translate into microns of OPD.