Difference between revisions of "CG4 photometry: more detailed instructions"

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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 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 5 px aperture, 8-13 px radius, aperture correction 1.17.
+
FOR MIPS, use 3 px aperture, 3-5 px radius, 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/
 
real reference (e.g, see other combinations): http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook/32/

Revision as of 21:43, 16 June 2010

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:

  1. stick this value in the conversion value under "more settings" (lower left)
  2. turn on background subtraction in more settings (option B)
  3. Apply settings. Close window
  4. change settings to 6, 6-14 px for aperture, annulus
  5. do photometry (click on object, calculate or recalculate values)
  6. write down "source_intensity (sky-included)" because this is the sky minus the scaled background from the annulus
  7. repeat for each object of interest
  8. multiply measured fluxes by aperture correction (band-dependent -- see below)
  9. 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.

FOR MIPS, use 3 px aperture, 3-5 px radius, 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/

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.