Non-standard Gains for NICMOS Daniela Calzetti (In lieu of Torsten Boeker) TIPS May 15 th 2003.
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Transcript of Non-standard Gains for NICMOS Daniela Calzetti (In lieu of Torsten Boeker) TIPS May 15 th 2003.
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Non-standard Gains for NICMOS
Daniela Calzetti (In lieu of Torsten Boeker)
TIPS May 15th 2003
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NCS/NICMOS Temperatures
Control Loop
Dewar
Eric Roberts
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NCS Compressor Speed
J, H, PB, I, H
Eric Roberts
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The Case for Non-Standard GainsXu & Boeker, ISR, 2003 + Noll
NICMOS implements a single gain (X2), which corresponds to ~ 6 e-/DN (5.4 e-/DN in NIC1 and NIC2, 6.5 e-/DN in NIC3); However, NICMOS was designed with multiple gain capability, which were never fully implemented;
The NICMOS A/D converter has a 16-bit accuracy, and therefore a dynamic range of 65,536; Least significative bit in NICMOS A/D converter is unstable, which adds an extra 1 DN uncertainty (digitization noise) to all NICMOS signals; 1 DN digitization noise should be compared with 4.2 DN of readout noise in NIC3.
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Prompted by the former IDT, we investigated the pros and cons of the other two gains present in the NICMOS design (X4 and X8), evaluating:
Actual overall noise reduction; Requirements for additional calibrations if offered.
Data from Program # 9324 (in SAA-free orbits):
Note:X4 ~ 3 e-/DNX8 ~ 1.5 e-/DN
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Gains X4 and X8 are almost exactly ½ and ¼ of X2. Reset voltage increases (imposes use of NSETVLT/NOPRVLT FS macros in conjunction with the NDETGAIN macros)
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Structure of darks and FFs is unchanged;Noise properties of darks will be different if darks are readout noise-limited (use ‘superdarks’ as a solution).
DN
s
DN
sPixel Pixel
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Effective noise reduction is ~10%-15% (3-4 e-) for both gains (X4 and X8) relative to standard gain.
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‘Advantages’ of non-standard gains: No need for separate calibrations (caveat on darks!); Noise reduction of ~15%; this implies a 0.11 mag ‘extra depth’ in a 1,000 sec exposure with NIC3/F160W; A potential improvement in the discrimination of low-level CRs is also possible (hard to quantify with current datasets).
But… non-exhaustive list of work to do: ‘Activation’ of flight software macros (NDETGAIN and NSETVLT/NOPRVLT); ADCZERO and ADCGAIN science header keywords need to be properly populated (calnica currently cannot work with non-standard gains); Update of linearity correction reference files (calnica); ETC, APT, etc.
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Conclusion:
Main advantage of non-standard gains is a general readnoise reduction of ~10-15%.However, in order to make non-standard gains supported modes, some work on the FS and ground systems is necessary.
Incremental increase in sensitivity does not warrant effort to support non-standard gains. Might be made available on a case-by-case basis.