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
B, I, Ha
J, H, Pa
Eric Roberts
The Case for Non-Standard Gains
Xu & 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-/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)
DNs
Pixel
Pixel
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).
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.