Photometry, first steps

As the CFHT pointings (see my queue update) are not designed to hit one, but many targets, I was forced to use the pointing information to determine which *.fits file a given Stripe82 object was located. Further to the problem, the MegaPrime camera as 36 CCDs in its almost 1 square degree field of view. In order to determine on which of these 36 CCD a given Stripe 82 quasar would land, I used the following formulae:

Parameters:

CRVAL1 – RA (in deg) of pointing centre
CRVAL2 – Dec (in deg) of pointing centre

RA/Dec – quasar RA/Dec

CRPIX1, CRPIX2, CD1_1, CD2_2 – constants given by the CCD layout.

Calculation:
x=CRPIX1 + (RA – CRVAL1)/CD1_1
y=CRPIX2 + (Dec- CRVAL2)/CD2_2

If I calculate the (x,y) (in pix) using the constants from all 36 CCDs, only one CCD will satisfy: 0 < x < 2048,0 < y < 4612. That is the CCD on which the quasar is located. Labeled *.fits[ccd##] x y

With chip number and (x,y) coordinates in hand, I can begin photometry. Using Phot, in $/noao/digiphot/apphot, requires the use of 4 parameter files: datapars, centerpars, photpars, fitskypars. Notable parameters: datapars.ccdread, gain, airmass, filter, exptime, photpars.apertur, zmag, fitskypars.annulus,dannulus

photpars.aperture allows you to choose a number of apertures from which to sample the object, showing the flattening of photons gained with ever increasing apertures.

1572573p.fits[ccd08]
1690.088 3079.221 (estimated x=1606, 2963)

aper     sum(counts)     area(pixels)     flux(counts)     mag     merr
2.00     7505.262         12.87262         4447.409         20.171 0.017
3.00     14969.63         28.55213         8187.152         19.508 0.013
4.00     23573.49         50.56833         11561.13         19.133 0.012
5.00     33010.21         78.8496           14279.73         18.904 0.011
6.00     43157.03         113.218           16262.43         18.763 0.011
7.00     54404.07         154.346           17739.63         18.669 0.012
9.00     79897.76         254.7832         19374.78         18.573 0.014
12.00  128163.3          452.8017         20601.61         18.506 0.017
15.00   188872.4         706.9811         20931.22         18.489 0.021

It’s clear the magnitude measured here is levelling off, the best mag_err is at an aperture of 6 pixels. The question now is: how do I convert this magnitude to an an actual magnitude. Looking at the CFHT image header I’m given:

PHOT_C = 26.4850 / Elixir zero point – measured for camera run
PHOT_CS = 0.0098 / Elixir zero point – scatter
PHOT_NS = 56 / Elixir zero point – N stars
PHOT_NM = 18 / Elixir zero point – N images
PHOT_C0 = 26.4600 / Elixir zero point – nominal
PHOT_X = 0.1480 / Elixir zero point – color term
PHOT_K = -0.1500 / Elixir zero point – airmass term
PHOT_C1 = ‘g_SDSS ‘ / Elixir zero point – color 1
PHOT_C2 = ‘r_SDSS ‘ / Elixir zero point – color 2
COMMENT Photometric Analysis is incomplete for this image.
COMMENT MAG_SAT and MAG_LIM cannot be determined.
COMMENT Formula for Photometry, based on keywords given in this header:
COMMENT m = -2.5*log(DN) + 2.5*log(EXPTIME)
COMMENT M = m + PHOT_C + PHOT_K*(AIRMASS – 1) + PHOT_X*(PHOT_C1 – PHOT_C2)

It seems te formulae at the bottom are the key.

 

 

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