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
- turn on background subtraction in more settings (option B)
- apply settings. Close window
- change to 6, 6-14 px for aperture, annulus
- 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 arcseconds. 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.
real reference (e.g, see other combinations): http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook/32/
Had we been able to do this in real time, I'd have created a spreadsheet with the measurements of the same stars from all 5 channels (irac1234mips1) and we'd have looked at the scatter in the values you measured, and compared them to my values. There's no guarantee I have it right!!
HOMEWORK Do the photometry once we solve these problems. Watch your email to see when/exactly what to do. Then we can have you all post your photometry here by a particular deadline, and we'll THEN do the comparison among us all. Sigh.
