DES Filter Specifications

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DES Filter Specifications

• David Gerdes
• Bruce Bigelow

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Operating Parameters
Parameter Requirement Comments
Beam size at filter Circular, 115mm diameter For normal incidence
Operating focal ratio f/2.9
Operating elevation 2200m (7220 ft.)
Normal operating temp. -7C27C
Survivable temp. range -20C50C
Normal operating humidity 0 (dry nitrogen)
Survivable humidity range 0-100
Operational lifetime 10 years (?) Filters expected to reside in a dry nitrogen environment at least 80(?) of the time
Beam angles of incidence 0-4? for f/2.9 beam (0-12?max range)
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Mechanical Specifications
Parameter Requirement Comments
Substrate dimensions 5801 mm diameter
Substrate material Fused silica n1.47454 (0.365 microns) to 1.44968 (1.06 microns)
Min. clear aperture 570 mm Concentric with substrate diameter
Total thickness 10-13 mm Including laminates
Surface parallelism 1 arcminute or better Relax this? Pan-STARRS required 3 arcmin.
Scratch/dig 60/40 Inspection per MIL-O-13830A, for all substrate surfaces before and after coating. Maybe relax to 80/50, since filters used far from focus, à la Pan-STARRS
Bubble class 0 (0.10 mm2 per 100 cm3 of glass), goal lt0.03 Integral of all inclusion cross sections. Can maybe relax by factor of 2 à la Pan-STARRS
Max bubble size 0.5 mm diameter
Coating durability MIL-C-48497 Moderate abrasion. Excluding tests for saline immersion.
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Mechanical Specifications, contd.
Parameter Requirement Comments
Striae lt30 nm Goal of 20 nm
Pinhole limits None with D gt 60 microns no more than one of the size 30-60 microns over an aperture of 50 mm, nor more than a combined equivalent area of 30 microns per 10 mm region of an aperture. Relax these?
Sealing Filter edges are to be sealed to prevent moisture incursions into the substrate.
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Optical Specifications
Parameter Requirement Comments
Wavefront transmission error lt ?/4 in a 125mm diameter sub-aperture
Optical thickness All filters must have the same optical thickness, 0.10 mm air equivalent
AR coating All air-glass interfaces are to be anti-reflection (AR) coated. Predicted performance of the AR coatings should be provided with the quotation
Radioisotope limits in filter substrate Ult0.8 ppm, Thlt2.5 ppm, Klt0.03 (by weight) Pan-STARRS values, need to recalculate
Radioisotope limits in filter coatings Ult80 ppm, Thlt250 ppm, Klt3 (by weight) Pan-STARRS values, need to recalculate
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Passband Specifications Needed

• Target envelopes
• Out-of-band leak limits
• AR reflectivity requirements

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Pan-STARRS g-band filter