Non-standard Gains for NICMOS Daniela Calzetti (In lieu of Torsten Boeker) TIPS May 15 th 2003

NCS/NICMOS Temperatures Control Loop Dewar Eric Roberts

NCS Compressor Speed J, H, P  B, I, H  Eric Roberts

The Case for Non-Standard Gains Xu & Boeker, ISR, 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 - /DN X8 ~ 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). DNs 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.