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.
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=U+u1+u2*X+u3*(U-B)
b=B+b1+b2*X+b3*(B-V)
v=V+v1+v2*X+v3*(B-V)
r=R+r1+r2*X+r3*(V-R)
i=I+i1+i2*X+i3*(R-I)
u, b, v, r, and i refer to the
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:
u2=0.53-0.55
b2=0.15-0.19
v2=0.09-0.12
r2=0.02-0.05
i2=0.01-0.02
| 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
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.)