Non-standard Gains for NICMOS

Daniela Calzetti (In lieu of Torsten Boeker)

TIPS May 15th 2003

NCS/NICMOS Temperatures

Control Loop

Dewar

Eric Roberts

NCS Compressor Speed

J, H, PB, I, H

Eric Roberts

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.

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

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)

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

Effective noise reduction is ~10%-15% (3-4 e-) for both gains (X4 and X8) relative to standard gain.

‘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.

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.