Optimum Bias Setting for the MIPS 24um Detector

Principal: dmkelly
Deputy: eyoung
Data Monkey(s): I will probably recruit some help from the Sili monkeys.
Priority: Necessary
Downlink Priority: Normal
Analysis Time: 72h
Last Updated:

Objective

Determine the best detector bias for the MIPS 24um array

Description

Four performance criteria, (responsivity, dark current, read noise, and stim repeatability) will be checked as a function of bias. A total of four bias settings will be tested.

Data Collected

For each of the four bias settings:
# collect 50 10s DCEs of read noise data
sendcmd CEMIPSUR  'AORID_upper,AORID_lower,0,"NO_COADD",9,50,"MIPS"'
# collect photometric data on a calibration star
24umCompactSourcePhotAOT (AORTarget=MIPS-920 calibration star, cycle=2,
  3s DCEs)


Here is the full AOR:
# Read noise and photometry for the nominal bias of 1.5V
sendcmd mips_rn24um_10s50
24umCompactSourcePhotAOT (AORTarget=MIPS-920 calibration star, cycle=2,
  3s DCEs) 
# Set 24um bias voltage to 1.25V by setting VD24SubVlt to 3.75V
sendcmd CELDTPGIF '0,0x97,"MIPS"'
# wait for the detector to settle
wait 600
# Read noise and photometry for 1.25V bias
sendcmd mips_rn24um_10s50
24umCompactSourcePhotAOT (AORTarget=MIPS-920 calibration star, cycle=2,
  3s DCEs) 
# Set 24um bias voltage to 1.00V by setting VD24SubVlt to 4.00V
sendcmd CELDTPGIF '0,0xA1,"MIPS"'
# wait for the detector to settle
wait 600
# Read noise and photometry for 1.0V bias
sendcmd mips_rn24um_10s50
24umCompactSourcePhotAOT (AORTarget=MIPS-920 calibration star, cycle=2,
  3s DCEs) 
# Set 24um bias voltage to 1.75V by setting VD24SubVlt to 3.25V
sendcmd CELDTPGIF '0,0x83,"MIPS"'
# wait for the detector to settle
wait 600
# Read noise and photometry for 1.75V bias
sendcmd mips_rn24um_10s50
24umCompactSourcePhotAOT (AORTarget=MIPS-920 calibration star, cycle=2,
  3s DCEs) 
# Set 24um bias voltage to 1.5V by setting VD24SubVlt to 3.5V
sendcmd CELDTPGIF '0,0x8D,"MIPS"'
# wait for the detector to settle
wait 600

Data Reformatting Requirements

Array Data Desired:

24um array data

Data Reformatting Option:

Special Instructions:
The 50 DCEs of read noise data should be packed into one FITS file. The photometry data should be packed into one FITS file. Data will be collected at four bias voltages, for a total of 8 FITS files.

Task Dependencies

Calibration Dependencies

Output and Deliverable Products

1) Median standard star brightness, averaged over all observations in the
   photometry AOT, tabulated and plotted as a function of array bias
2) Dark level from the read noise data, tabulated and plotted as a 
   function of bias
3) Standard star repeatability
4) Read noise, tabulated and plotted as a function of bias
5) A report listing the optimum bias voltage, along with an
   explanation of how the input criteria led to this selection

Data Analysis

Step by step analysis:
1) Perform photometry on the standard star AOT data
2) Calculate the mean and sigma of the standard star fluxes
3) Use the 24um read noise and dark current tool on the read noise data
4) Plot standard star brightness, dark current, read noise, and source 
   repeatability versus detector bias
5) Use a TBD method to combine the above results into a figure of merit
   for determining the optimum bias setting

Software Requirements

photometry tool for determining standard star fluxes
tool for determining the read noise and dark current of 24um data

Actions Following Analysis

Determine the optimum bias voltages and apply them for use in future campaigns by making changes to irs-mips_pconst_ier.

Failure Modes and Responses

If this test fails to execute or if it gives ambiguous answers, we will use the data obtained during ground testing as the basis for choosing array biases. As we gain further experience with the arrays, we will assess whether the biases we have chosen are working out well or if some minor adjustments are needed.

Additional Notes

   The preferred source for this task is HD 159330, the 24um calibrator that 
   will be used by Chad in campaign H, mips-920.  This star is also observed
   in campaign E, mips-120, stim calibration with celestial sources.
   The 24um array saturates in 1s for a brightness of 5.6 Jy.  This source
   saturates in about 11s and so is perfect for 3s integrations.  It
   will work well for all four bias voltages.

   Source      F24   F70   F160     B24   B70   B160   Vis. window
                    (Jy)                (MJy/sr)
   HD 159330  0.515 0.06   0.01     18.6  5.5    5.8  April 16 -- Nov 23

   In our instrument-level tests of the 24um array, we adopted an optimum
   bias value of 1.5 V.  In a ground-based test environment, the Si:As 24um
   detectors are very well behaved.  Their read noise is low (about 9 DN),
   and the dark current is small (2-3 DN/s).  The detectors provide good
   stability.  Their response grows with increasing bias, and there is no
   breakdown of the detectors until the bias gets well above 1.5 V.  An
   analysis of the optimum bias is very likely to indicate that we would
   be better off with a bias in excess of 1.5 V.

   In space, the detectors will be subject to a harsh radiation environment.
   For the most part, the detector behavior will still be excellent.  But
   over time, the radiation will cause damage, and that damage will lead
   to a slow degradation in the performance of the array.  The damage accrues
   more quickly with increasing detector bias, and the spikiness and
   nonlinearities are worse the higher the bias on the detector.  As a
   result, we expect to use a modest 24um bias during the early part of
   the mission, and we are prepared to reduce the bias later in the mission
   as radiation damage builds up.

   Cosmic ray effects are difficult to account for in ground-level
   testing.  In this IOC task, we will be attempting to understand the
   benefits and disadvantages of running the 24um detector at higher
   or lower bias than the nominal value of 1.5 V.  These results are
   unlikely to change our mind about how to run the array during the
   early part of the mission, but they will provide a reference for
   deciding when and by how much to change the bias on the array later
   in the mission.