Analysis
We obtained a 4 point dither map (1
cycle) at 24microns with 3 second DCEs in a region
of the sky chosen to avoid bright point sources. The offsets between
positions are 400 arcseconds. At each position of the dither pattern
there will be 16 images, so a total of 64 images will be obtained
each time the task is executed. The images (one for each pointing)
are named:
mips_AOR_6759936_pht_A24_P24_pos1_3s.fits
mips_AOR_6759936_pht_A24_P24_pos2_3s.fits
mips_AOR_6759936_pht_A24_P24_pos3_3s.fits
mips_AOR_6759936_pht_A24_P24_pos4_3s.fits
The region of the sky observed was: zodi_7a[75] with a predicted brightness
at 24microns of 36MJy/strd.
The average of the number of DN/s for 64DCEs: 2126+/-9 DN/s.
We used mips_sloper
turning off the row droop correction, with the PreFlight Dark Image,
then mips_caler with the 24micron latent correction turned off,
and finally mips_enhancer using the
following parameter file:
verbose
= 0
scale
= 1.
input_list
= listof24flats.txt
prefix
= flat24_campaingD1
nXTile
= 1
nYTile
= 1
dark
= 0
flat
= 1
illumination_correction
= 0
NormalizeInput
= 0
SigmaCut
= 3.0
SigmaTolerance
= 0.05
MaxIterations
= 5
StackSTDEV
= 1
OutlierMinPoints
= 5.0
QuartileFactor
= -1
OverlapTolerance
= 0.5
diagonistics
= 0
WriteGeometry
= 0
WriteOutlier
= 1
WriteTiles
= 0
FluxMean
= 0
FluxMedian
= 1
OverWrite
= 1
distortion
= 0
UseBoostFrame
= 0
thus
basically using a 3sigma rejection and MEDIAN combining a total of
64DCEs.
Results
The
24micron Flatfield observed in Campaign D1 is shown in Figure1. Figure2
shows a histogram of the DN/s of the normalized flatfield.
Statistics on the image provides the following:
24micron Flatfield: Number
DCEs= 64; mean= 0.99123; median= 0.99999; sdtdev= 0.06646; skew=-0.27047
The statistics over the array however includes the effect of the illumination pattern of the
24micron array.
Figure1.
24Micron flat created with mips_enhancer displayed from z1=0.57 to
z2=1.13
Figure2.
Histogram of the 24micron flatfield obtained in campaignD1.
Figure3.
Histogram of the ratio of two flatfields produced using the first 8 DCEs of each
pointing (total 32DCES) and the last 8 DCEs of each pointing (total 32DCEs).
A better way to assess the quality of the observed flatfiels is to remove the illumination
pattern of the 24micron array. To do so, we produced two different flatfields, the first one
using the first DCEs of each pointing, and the second one using the last 8DCEs of each pointing.
Then we divided the first flatfield (32DCEs) by the second flatfield (32DCEs). The resulting
ratio of two flatfields provides the following statistics:
Ratio of two 24micron Flats: mean= 0.99957; median= 0.99957; sdtdev= 0.00254; skew= 0.01414
and the histogram is shown in Figure3.
Another way to assess the quality of the observed flatfield is to do differential photometry
of stars observed on different positions on the array. We 24micron First Light Data (MIPS-100)
observations of the star HD053501. We used IRAF to perform photometry on
all the individual DCEs observed as part of MIPS-100. The photometry was
obtained through a number of different apertures with radii of 3, 5,
7,9, and 10 pixels. The background was measured from an annulus with a radius
of 15pixels and a width of 5pixels for the Large Field Photometry and
Small Photometry Modes. The results of the photometry as well
as the centroids (x and y) of the stars are presented in Table1.
The magnitudes are defined as:
mag = 25. - 2.5*log10(DN/s)
The measurement for the combined 74DCEs are given in the first part of Table1. The number
of DN/s for a 5pixel radius aperture is: 159702.8DN/s and for a 7pixel aperture
is: 165919.9DN/s in good agreement with the independent measurement done by Chad Engelbracht
(see analysis of MIPS-100 in campaignD1).
The measuments in Table1 have been grouped according to neighboring positions of the
star on the array. For each of these groupings we have computed the average
magnitude observed through the five different apertures. This allows
a quick comparison of the measured magnitudes at different positions
on the array and comparison with the measurements for the combined 74DCEs.
From this comparison we find that the statistical quality of
the flatfield (for the few pointings done so far across the
array) is of the order of 2-4%.
Table 1. 24 micron photometry of HD053501 as measured on
individual 24micron DCEs flatfielded
with Campaign D1 flat. The star data are from the MIPS-100
task (24 micron First Light).
NOTES: The first column of the table indicates the DCE ordering
within the AOR file.
x(pix) and y(pix) are measured positions of the star, and m_3pix through
m_10pix are
magnitudes measured through the radius indicated in pixels.