CG4 photometry: more detailed instructions
Follow the instructions on the Units page under "getting the number APT needs" (near the bottom of the page). These are the numbers I got for my SA101 mosaics. Are they right? Are they the same as what you'd use for the online mosaics?
step 1 step 1 step2 step 3 step 4 step 5 -- # for APT chan CDELT1 CDELT2 sq deg/px sr/sq deg sr/px convert to Jy/px i1 0.000169 0.000169 2.8561E-08 0.000304847 8.70674E-12 8.70674E-06 i2 0.000169 0.000169 2.8561E-08 0.000304847 8.70674E-12 8.70674E-06 i3 0.000169 0.000169 2.8561E-08 0.000304847 8.70674E-12 8.70674E-06 i4 0.000169 0.000169 2.8561E-08 0.000304847 8.70674E-12 8.70674E-06 m1 0.0006944 0.0006944 4.82191E-07 0.000304847 1.46995E-10 0.000146995
Then:
- stick this value in the conversion value under "more settings" (lower left)
- turn on background subtraction in more settings (option B)
- Apply settings. Close window
- change settings to 6, 6-14 px for aperture, annulus if doing IRAC (see below for MIPS)
- do photometry (click on object, calculate or recalculate values)
- write down "source_intensity (sky-included)" because this is the sky minus the scaled background from the annulus
- repeat for each object of interest
- multiply measured fluxes by aperture correction (band-dependent -- see below)
- compare these final flux densities to other people's flux densities
Aperture corrections. See photometry pages for explanation of what and why. The aperture corrections are a function of the aperture and annulus you used, compared to what the IRAC team uses to calibrate the instrument. Here are the aperture corrections for a NATIVE PIXEL 3, 3-7 combination; we have half the pixel size, so this corresponds to 6, 6-14 in the resampled mosaics. Different pixel scales require different combinations.
For this combination, the aperture corrections for the four IRAC channels are 1.124, 1.127, 1.143, 1.234. Operationally, this means, using IRAC-1 as an example, that the 6/6-14 combination misses 12.4% of the flux, so take the flux you measure using 6/6-14, and multiply it by 1.124. Do the same thing for the rest of the channels.
FOR MIPS, use 3 px aperture, 3-5 px annulus, aperture correction 2. THIS IS AN ON THE FLY GUESS. STAY TUNED FOR BETTER VALUES.
real reference (e.g, see other combinations): http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook/32/
When we were visiting in June, we created a spreadsheet with the measurements of the same star from several channels done by each person. If we'd had time, we would have done each channel (irac1234mips1). We looked at the scatter in the values you measured, and compared them to my values. For any given source, there's still no guarantee I have it right!!
Our aggregate photometry xls file from June: File:Thephotometry.xls
