MIPS-121 IOC Task Cookbook:
MIPS 24 µm Focus Confirmation
Principal: Karl Stapelfeldt
Deputy: Dean Hines
Data Monkey(s): Kate Su
Priority: Critical
Downlink Priority: Expedited
Analysis Time: 72 hours
Last Updated:
Fri Aug 15 2003, 14:19:00
Evaluate the focus of the MIPS 24 microns channel at the initial
post-launch position of the SIRTF secondary mirror.
Two stars will be observed using the photometry AOT.
Each star is being observed as a cluster target, with 4 cluster
positions specified on a grid at pixel positions
0.0,0.0; 0.0,+3.5; +3.5,+3.5; +3.5,+0.0. This subsampling
will allow a subsampled PSF to be constructed, while avoiding
latent images. Three cycles of 3 sec exposures will be taken at
each cluster position, for each star.
Standard Photometry AOT data. Two AORs are used, one for each
of the two stars.
# Please edit this file with care to maintain the
# correct format so that SPOT can still read it.
# Generated by SPOT on: 8/4/2003 9:2:10
HEADER: FILE_VERSION=7.0, STATUS = PROPOSAL
AOT_TYPE: MIPS Photometry
AOR_LABEL: MIPS24-FOCUS-GSTAR
AOR_STATUS: new
MOVING_TARGET: NO
TARGET_TYPE: FIXED CLUSTER - OFFSETS
TARGET_NAME: HD 2151
COORD_SYSTEM: Equatorial J2000
POSITION1: RA_LON=0h25m45.07s, DEC_LAT=-77d15m15.3s, PM_RA=2.22", PM_DEC=0.325"
OFFSET_P2: EAST_ROW_PERP=0.0", NORTH_COL_PARA=8.93"
OFFSET_P3: EAST_ROW_PERP=8.93", NORTH_COL_PARA=8.93"
OFFSET_P4: EAST_ROW_PERP=8.93", NORTH_COL_PARA=0.0"
OFFSETS_IN_ARRAY: YES
OBSERVE_OFFSETS_ONLY: NO
OBJECT_AVOIDANCE: EARTH = YES, OTHERS = YES
MICRON_24: FIELD_SIZE = SMALL, EXPOSURE_TIME = 3, N_CYCLES = 2
SPECIAL: IMPACT = none, LATE_EPHEMERIS = NO,SECOND_LOOK = NO
RESOURCE_EST: TOTAL_DURATION=878.225, SLEW_TIME=50.4, SETTLE_TIME=88.625, SLEW_OVERHEAD=180.0, SPECIAL_OVERHEAD=0.0, UPLINK_VOLUME=1209, DOWNLINK_VOLUME=10151096, VERSION=S7.5.1
INTEGRATION_TIME: MIPS_24=92.27469,MIPS_70=0.0,MIPS_160=0.0
AOT_TYPE: MIPS Photometry
AOR_LABEL: MIPS24-FOCUS-KSTAR
AOR_STATUS: new
MOVING_TARGET: NO
TARGET_TYPE: FIXED CLUSTER - OFFSETS
TARGET_NAME: HD 53501
COORD_SYSTEM: Equatorial J2000
POSITION1: RA_LON=6h59m50.54s, DEC_LAT=-67d54m59.2s, PM_RA=-0.027", PM_DEC=0.234"
OFFSET_P2: EAST_ROW_PERP=0.0", NORTH_COL_PARA=8.93"
OFFSET_P3: EAST_ROW_PERP=8.93", NORTH_COL_PARA=8.93"
OFFSET_P4: EAST_ROW_PERP=8.93", NORTH_COL_PARA=0.0"
OFFSETS_IN_ARRAY: YES
OBSERVE_OFFSETS_ONLY: NO
OBJECT_AVOIDANCE: EARTH = YES, OTHERS = YES
MICRON_24: FIELD_SIZE = SMALL, EXPOSURE_TIME = 3, N_CYCLES = 2
SPECIAL: IMPACT = none, LATE_EPHEMERIS = NO,SECOND_LOOK = NO
RESOURCE_EST: TOTAL_DURATION=878.225, SLEW_TIME=50.4, SETTLE_TIME=88.625, SLEW_OVERHEAD=180.0, SPECIAL_OVERHEAD=0.0, UPLINK_VOLUME=1209, DOWNLINK_VOLUME=10151096, VERSION=S7.5.1
INTEGRATION_TIME: MIPS_24=92.27469,MIPS_70=0.0,MIPS_160=0.0
Array Data Desired:
24 µm only
Data Reformatting Option:
Ideally, one fits file per cluster position per AOR.
Each such fits file would contain all the DCEs for that cluster position.
Special Instructions:
1) Quick measurement of PSF FWHM to determine if major problems are present
2) Measurement of absolute offset from best focus, delivered to SIRTF
Focus IPT and Focus review board.
Our goal is to measure the offset of the 24 micron images from best
focus, and report this value to the focus IPT. This is difficult
to accomplish because of the large depth of focus of the MIPS 24
micron channel, relative to the likely defocus that will be experienced
during launch. Simple measurements of encircled energy and FWHM
are not sensitive enough to measure the expected defocus. Noise
pixel measurements have, in theory, the needed sensitivity to defocus;
but since they are also very sensitive to PSF modeling errors which
have never been calibrated against MIPS point source images, we
do not have confidence that noise pixel values can be modeled
for MIPS to the required accuracy. The preferred method for focus
measurement is to look at the brightness in the second DARK Airy
ring, comparing this region of the PSF between the observed SIRTF focus
image to a library of defocused model PSFs.
Pre-requisites for the MIPS 24 micron focus analysis is that the
data be bias subtracted, flat fielded, linearized, and undistorted.
These steps have never been performed with MIPS looking at a star
image through the SIRTF CTA; our first opportunity to completely
exercise the calibration pipelines comes during the focus check
itself. It will be crucial to be able to re-derive these calibration
products quickly, should the first version of the calibrated focus
data prove unsatisfactory. For this reason, MIPS requires three
days of analysis time before we can report the instrument focus
position to the project.
The first step will be to measure the FWHM of the stars using Gaussian
profile fitting. A value close to 2.11 pixels should be obtained if
MIPS is within +/- 50 microns of best focus. Thus we can quickly establish
if the MIPS focus is "near" to the optimal value, and exclude the
presence of any major instrument problems. This focus *check* result
will be reported to the PI and project representatives.
The second step is to do focus *determination* through a detailed
analysis of the PSF structure in the second dark Airy ring. This requires
analyzing the MIPS data in the IDP3/IDL software, where the images will
be combined to elimate bad pixels or flat field problems. Once the
best combined image is obtained, we must measure the X,Y pixel position
of the centroid of the star; measure the total counts in the star image;
and measure the background level around the star. These four values are
needed as inputs to the software which compares the observed focus star
to the library of defocused PSFs.
The SIRTF TinyTim PSF modeling tool will be used to produce model
PSFs. When comparing model PSFs to the 24 micron focus image, it is
essential that the model star images have the same subpixel position
as the observed focus star, to better than 0.1 pixel accuracy. TinyTim
model PSFs have been pre-calculated with 10x pixel oversampling.
Bicubic linear interpolation will be used to shift and rescale these
baseline focus library images to match the observed subpixel position
of the focus star. The result will be an operational focus library
tailored appropriate to the star image data being analyzed.
Focus determination is accomplished by reading in the operational
focus library; forming radial profiles of each image in the focus
library; ratioing these radial profiles to that of the in-focus
(Z4=0) model image; then plotting these radial profiles as a family
of curves. We then read in the observed focus star image, extract
its radial profile, ratio it to the radial profile of the model
in-focus image, and overplot the observed radial profile ratio
on the family of cuves from the operational focus library. Finally,
the radial profile ratio of the observed focus star is replotted
twice, adding and subtracting 1 sigma errors in the background level
and random errors in the radial profile, to indicate the range of
possible error in the radial profile ratio. The defocused radial
profile lying closest to the observed radial profile indicates the
current distance from best focus of MIPS. The replotted star
radial profile with +/- 1 sigma errors will extend across radial
profile curves for other focus positions, indicating the uncertainty
in the focus measurement. Note that only the absolute value of the
defocus can be derived using this technique.
Advise MIPS IOC team if flatfields were sufficient to determine
MIPS focus. Iterate with new flatfields, if necessary.
Advise Principal Investigator of image quality and absolute value
determined for focus offset.
Failure to acquire focus star: Repeat task ASAP. Campaign D2 focus
check will uncover any problem in time for a fix to Campaign E focus check.
Unusable flat fields: Focus determination will not succeed, only a
focus check will be provided.