Photometry from Mosaic data offers several unique challenges. Typically, Mosaic data is taken by dithering the telescope to 5 positions offset by several hundred pixels (e.g., an arcminute or two) in order to assure that each object is observed a minimum of 3 times despite the inter-chip gaps and cosmetic defects. After the initial processing and flat-fielding, these 5 ditherings are geometrically corrected and then stacked to form a single image. However, the photometric accuracy of the stacked images is compromised by the following:

While earlier work by Massey & Slesnick showed that aperture photometry was still possible at the 2% level, our Survey project needs PSF-fitting on these croweded fields. We have therefore decided to side-step these problems by treating the 8 chips and 5 ditherings separately, doing the photometry right and then averaging the photometry.

A critical step in this process is external calibration of all of our northern fields using the Lowell 1.1-m Hall telescope. The time required to obtain adequate calibration with Mosaic is prohibitive: to move a set of standards from chip to chip for all the CCDs in each of the 5 colors requires 2 hrs just in read-out time; a single standard star field observation requires 15 minutes. In addition, using external photometric calibration allows us to utilize good, but not perfectly photometric nights at the 4-m.

Chip-to-Chip dependent Color Terms and Zero-points

Our calibration efforts are on-going, but here are some tenative findings. The photometry was done by Zhenye Mei, an excellent MIT student who participated in the 2001 MIT Field Camp at Lowell Observatory.

The equations that we solved were as follows:

u, b, v, r, and i refer to the instrumental magnitudes, while U, B, V, R, and I refer to standard magnitudes.. Thus, u1, b1, v1, r1, and i1 are the zero-points, u2, b2, v2, r2, and i2 are the extinction terms, and u3, b3, v3, r3, and i3 are the color terms.

We insisted of course that the color terms and zero points were constant from night to night, but allowed them to be different from chip-to-chip; the extinction we insisted was constant for all the chips but varied from night-to-night. "Typical" values for the extinction for two photometric nights were:


Color Terms Zero Points
Chip u3 b3 v3 r3 i3 u1 b1 v1 r1 i1
imt1 -.043 -.122 +.013 -.031 -.080 1.536 -.065 -.185 -.316 +.274
imt2 -.001: -.128 +.004 -.045 -.038 1.523 -.070: -.172 -.324 +.286
imt3 -.090 -.115 +.030 -.024 -.006 1.545 -.075 -.187 -.340 +.278
imt4 -.043 -.113 +.013 -.042 -.041 1.526: -.078 -.188 -.359 +.268
imt5 -.031 -.109 +.017 -.045 -.040 1.525: -.080 -.185 -.357 +.255
imt6 -.074 -.140 -.001 -.068 -.034 1.551 -.060 -.176 -.335 +.253
imt7 -.033 -.113 -.013 -.068 -.016 1.560: -.075: -.165 -.310 +.276
imt8 +.002 -.140 +.005 -.071 -.045 1.516: -.076 -.190 -.332 +.247

The instrumental magnitudes were defined so that a star with with 1 count/sec/image was assigned a value of 25.00. Thus for a very blue star (U-B=-1.0) with U=20.00 observed at the zenith (X=1.00) we would expect
u=20.00+1.54 +.55x1.00 -0.04 x -1.00 = 22.13.
This is 2.87 mag brighter than 25.00, so we would expect about 14 ADU/sec, or 39 e/sec; this can be compared to the "35 e/sec" estimated in the Mosaic Manual .

The coefficients in the above table are good to approximately .005 mag; less certain values are indicated by ":", where the errors are approximately .010 mag. Thus the differences of 0.09 in the color terms for "U" would translate to errors of .2 mag from a U-B=-1 to +1, if ignored, as might be the case if the images were stacked.

The values for "U" are actually for stars with U-B>0; for stars bluer than this, a correction is needed, as star deviate by 0.1mag from this relationship by the time that U-B=-1.0 is reached. (This just says that the color-term is not linear over a large color range.)


Last updated 6 July 2001