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SHARC II Design Decisions

SHARC II Design Decisions. 2001 Feb.-Mar. Decisions to make soon. Backshort spacing Optics scale/complexity Spectroscopic capability? Pointing at mm wavelengths. Bolometer Predictions. Based on recent measurements (“CSO Run 1”), predicted characteristics of median bolometer are:

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SHARC II Design Decisions

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  1. SHARC II Design Decisions 2001 Feb.-Mar.

  2. Decisions to make soon • Backshort spacing • Optics scale/complexity • Spectroscopic capability? • Pointing at mm wavelengths

  3. Bolometer Predictions • Based on recent measurements (“CSO Run 1”), predicted characteristics of median bolometer are: • D = 33 K, R0 = 1300 W • G (nW/K) = 1.49 × T(K)1.96 • C(pJ/K)  10 × T(K) • This is somewhat different (worse) than expected from Calibration Run in Spring 2000. • Other parameters for calculations: • Tbase = 320 mK • RL = 200 MW at 2.0 K • en = 5 nV Hz-1/2 • V(bol) =2.7 × 10-7 cm3 • A = 78.5 m2, W = 5.88 × 10-10 sr

  4. QE and Background Predictions • Detector absorption for SHARC 1.5  35% with no backshort. Assume 90% for SHARC II due to backshort. • Filter transmission for SHARC 1.5 at 350 mm – Relevant integrals are: • Xxx • Yyy • Backgrounds for 5´´ pixels / Serabyn atmosphere are then: • Q = 144 pW (!). Earlier prediction: 75 pW. • NEPelectrical(sky) = 7.4 × 10-16 W Hz-1/2

  5. Predicting NEFD’s • NEFD = C NEPelectrical/[h A integral(T Q dn)] • C = constant related to observing mode • A = telescope area • h = optical coupling efficiency • T = atmospheric transmission • Q(n) = quantum efficiency • X  NEPelectrical/A integral(T Q dn) makes a good figure of merit, all other things held equal.

  6. Effect of reducing QE NEP’s calculated at 0.03 Hz.

  7. Effect of backshort distance Absorption for 3 film resistances: 250, 350, 450 ohms/square. 0 degrees incidence assumed.

  8. Predicted NEFD’s • Assume C = 2. (Observing mode.) • 350 mm: peak QE = 75%; h = 30% (telescope) × 74% (Airy signal in 4 pixels) × 50% (4 pixels)  NEFD = 0.85 Jy Hz-1/2 • 850 mm: peak QE = 20%; h = 50% (telescope) × 63% (Airy signal in 16 pixels) × 25% (16 pixels)  NEFD = 0.57 Jy Hz-1/2. (Detector noise limited by a factor of 4.)

  9. SHARC II as a R=100 spectrograph

  10. Amplifier Gain Adjustment • To reduce 1/f noise effects of second stage amplifier, we need to increase gain of first stage. • 350 mm • Q = 144 pW  Maximum V  10 mV  Maximum first stage gain = 7 V / 10 mV  700. • 0.01 Hz: en(sky) = 21.8 nV Hz-1/2, en(det) = 8.7 nV Hz-1/2 • 1 Hz: en(sky) = 21.8 nV Hz-1/2, en(det) = 7.8 nV Hz-1/2 • No load • Q = 0 pW  Maximum V  14 mV  Maximum first stage gain = 7 V / 14 mV  500. • 0.01 Hz: en(sky) = 0 nV Hz-1/2, en(det) = 67.4 nV Hz-1/2 • 1 Hz: en(sky) = 0 nV Hz-1/2, en(det) = 23.2 nV Hz-1/2

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