2MASS Photometric Calibration Strategy

2MASS PHOTOMETRIC CALIBRATION STRATEGY

version 4.2

R. Cutri (IPAC), J. Elias (NOAO), E. Howard (UMASS), R. Light (IPAC), E. Persson (OCIW), P. Seitzer (UMich), M. Skrutskie (UMASS) and S. Wheelock (IPAC)

Photometric Calibration Requirements
Basic Photometric Calibration Observation and Targets
Proposed Photometric Calibration Strategy
Photometric Calibration Field Selection Criteria
Nightly Photometric Calibration Field Scheduling
Appendix A. Faint Standard Star Sequences
Appendix B. Calibration Criteria
Appendix C. Tertiary Standard Star Calibration
Appendix D. Selection of a New Calibration Field

1. Photometric Calibration Requirements:
- Provide basic photometric zero point trends, extinction and color corrections on a nightly basis.
- Insure photometric spatial uniformity requirements (<4%) for survey are met. Ties measurements made around the sky, and between hemispheres to common calibration network. Latter point is aided in short term by regular calibration using fields near celestial equator.
- Provide photometric quality statistics for each night. Basis for acceptance or rejection of night's data (see Appendix B), and drives feedback to observatory for re-scanning.
- Provide nightly "archive" of sky offset frames for flat-fielding in complex regions.
- Provide long term stability and health checks on system performance, and site characteristics. Some of the parameters that can be monitored over long timescales include the "gain" of the systems (photometric zero points), sensitivity and completeness/reliability (from repeat measurements of calibration fields), and extinction coefficients.
2. Photometric Calibration Observations and Targets
- A "Calibration Scan" will be approximately one degree long, scanned at nominal survey rate. A Calibration Observation will consist of 6 calibration scans repeated in succession, with the telescope cross-stepped ~5" in RA between each repeat.
- Calibration fields will be selected to contain one or more pre-existing photometric standard stars that are in the brightness ranges: 9.5 < Ks < 13, 10 < H < 13.5, 10.5 < J < 14. These ranges correspond to the "flat" region of the sensitivity curves where photometric accuracy is limited by pixelization effects and not source photon noise. The bright limit is set by R2-R1 linear detector response.
- Objective will be to transfer photometric calibration to each star in field so will eventually have a large number of tertiary calibrators per field.
3. Proposed Photometric Calibration Strategy
- Calibration observations should begin and end each night's observations. Data not bracketed by good calibration scans, such as might occur because of a weather induced shutdown, will be considered of lower quality and marked for rescan (see Issues below).
- Observe one calibration field every hour (+/- 10 minutes) during the night, with the exact interval chosen so that the final calibration observation is coincident with morning twilight. For a "typical" 10 hour night, this provides approximately 66 (6x11) measurements of calibrators per night. This number will grow after calibrations are transferred to other stars in field. However, systematics can effect all stars in a scan so that there will never quite be hundreds of independent measurements.
- Emphasize zero point monitoring by multiply observing several calibration fields each night. Also observe at least one equatorial field each night to develop short term tie points between northern and southern surveys.
- Follow at least one standard field from rising to setting to monitor extinction. All stars in field can be used to measure extinction coefficients. This field is referred to as the "Monitor Field."
- Observe red standard(s) periodically to measure and monitor color terms. Handle scheduling of this by working red stars in the normal rotation of equatorial standards.
- If possible, observe a calibration field several times between 1.6 and 2.2 airmasses as it sets in the morning (after aerosals have settled out of the local atmosphere), to obtain a minimum time interval between observations. This will be carried out as special observations periodically, and will not be integrated into the automated calibration plan.
- Issue:
  - Is it possible to extend valid photometric calibration outside period not bracketed by valid calibration observations?
4. Photometric Calibration Field Selection Criteria
- Initially driven by requirement to have an existing standard star in proper brightness range (sec 2.b) within field. Limited to the following standard sequences: UKIRT Faint Standards; NICMOS Standards (Green and Rieke, and Persson) and Faint Red Standards (Persson and Elias et al.). Appendix A contains listing of candidates from these sequences.
  Because the UKIRT and Persson's standards have the most complete available photometry, we select primarily from their lists.
- An agreement was made in March of 96 for 2MASS and DENIS to use a common set of calibration stars, thereby insuring that the surveys are calibrated on the same systems. In practice, we can plan to have a subset of calibrators in common with DENIS, but because their requirements on brightness range and sky coverage are different, it will be difficult to have an exact set of calibrators service both surveys.
- Minimize total number of calibration fields, emphasizing instead high degree of repetition during survey to insure uniformity. However, select enough fields to allow uniform sampling over sky for calibration uniformity, to allow flexible scheduling, and to accommodate rejection of a few fields if they prove unsatisfactory for any reason.
- Select a set of equatorial and +/-30deg declination fields on ~2h centers, if possible. Select fields containing known red standard stars for some of the equatorial fields so that occasional observations of red calibrators will be made to monitor photometric transformations over time.
- Because the candidate calibration star samples have very few stars at +30deg declination in the 20-22hr range, selected a field in that area that can be calibrated over the first few months of observations. See Appendix D for the criteria used to select this field.
- The table below gives a subset of the fields in Appendix A that satisfy the brightness and distribution requirements summarized above. These fields are now being scheduled for observation by the Survey Scheduling software at Mt. Hopkins. Note that these fields should be examined for crowding, etc. The field "type" describes if the field is a zenith or equatorial "monitor or transit" field (MT), or a red standard (R) field. This list is now in use for scheduling the northern 2MASS facility, and will be used for the southern facility in early 1998.
2MASS Calibration Field List
```
Field        RA(J2000)   DEC(J2000) T    J     sJ      H     sH      K     sK      Ks    sKs  Source
------------------------------------------------------------------------------------------------------------
BRI0021-0214 00:24:24.6  -01:58:20   R  11.835 0.008  11.086 0.007  10.552 0.010  10.561 0.008 Persson-4,2MASSpos
S294-D       00:33:15.2  -39:24:10   MT 10.932 0.006  10.657 0.004  10.596 0.005  10.594 0.004 Persson-1
FS4          01:54:37.8  +00:43:02   MT 10.532 0.008  10.297 0.010  10.259 0.003               UKIRT
P247-U       03:32:03.0  +37:20:39   MT 11.934 0.005  11.610 0.004  11.492 0.011  11.503 0.005 Persson-2
S301-D       03:26:54.59 -39:50:51.1 MT 12.153 0.007  11.842 0.005  11.772 0.010  11.788 0.006 Persson-1,2MASSpos
P533-D       03:41:02.4  +06:56:13   MT 11.737 0.009  11.431 0.006  11.337 0.008  11.336 0.005 Persson-1
LHS191       04:26:19.96 +03:36:38.2 R  11.621 0.013  11.058 0.012  10.667 0.020  10.717 0.016 Persson-2,2MASSpos
FS13         05:57:07.6  +00:01:11   MT 10.486 0.005  10.173 0.008  10.128 0.003               UKIRT
S121-E       06:29:29.4  -59:39:31   MT 12.114 0.006  11.838 0.005  11.765 0.009  11.781 0.005 Persson-1
P161-D       07:00:52.0  +48:29:24   MT 11.680 0.006  11.408 0.006  11.356 0.013  11.352 0.006 Persson-3
S312-T       08:25:35.52 -39:05:57.1 MT 11.949 0.006  11.669 0.005  11.608 0.004  11.609 0.004 Persson-1,2MASSpos
LHS2026      08:32:30.4  -01:34:38   R  12.066 0.006  11.497 0.005  11.129 0.007  11.156 0.008 Persson-2,2MASSpos
M67/FS15-17  08:51:15.1  +11:49:19   MT 13.017 0.025  12.720 0.028                12.668 0.035 UKIRT/2MASS(FS16)
P091-D       09:42:58.7  +59:03:43   MT 11.683 0.008  11.338 0.007  11.276 0.011  11.282 0.010 Persson-3
LHS2397a     11:21:49.22 -13:13:08.2 R  11.897 0.008  11.190 0.007  10.691 0.008  10.709 0.010 Persson-2,2MASSpos
S217-D       12:01:45.2  -50:03:10   MT 11.323 0.007  11.002 0.005  10.931 0.003  10.936 0.004 Persson-1
P266-C       12:14:25.4  +35:35:56   MT 11.642 0.009  11.378 0.008  11.324 0.011  11.343 0.008 Persson-3  
S860-D       12:21:39.4  -00:07:13   MT 12.213 0.007  11.917 0.006  11.861 0.005  11.865 0.005 Persson-1
S867-V       14:40:58.0  -00:27:47   MT 12.045 0.008  11.701 0.005  11.622 0.005  11.633 0.005 Persson-1 
S273-E       14:56:52.0  -44:49:14   MT 11.341 0.007  10.924 0.005  10.851 0.004  10.849 0.004 Persson-1
P272-D       14:58:33.2  +37:08:33   MT 11.640 0.008  11.277 0.006  11.210 0.008  11.223 0.007 Persson-3
T868-53850   15:00:26.4  -00:39:29   R  11.589 0.008  10.993 0.008  10.633 0.009  10.657 0.012 Persson-2,2MASSpos
Oph_n9a&b    16:27:13.24 -24:41:33.0 R  17.549 0.022  12.389 0.011   9.527 0.014   9.620 0.018 Persson-2,2MASSpos
P565-C       16:26:42.7  +05:52:20   MT 12.180 0.007  11.895 0.006  11.842 0.007  11.844 0.006 Persson-1
P330-E       16:31:33.6  +30:08:48   MT 11.816 0.007  11.479 0.005  11.419 0.007  11.429 0.006 Persson-3
S279-F       17:48:22.7  -45:25:45   MT 12.477 0.009  12.118 0.006  12.026 0.006  12.031 0.006 Persson-1
P182-E       18:39:33.8  +49:05:38   MT 12.104 0.005  11.764 0.004  11.688 0.006  11.696 0.005 Persson-3
L547         18:51:15.6  -04:16:03   R  11.872 0.010   9.831 0.011   8.888 0.014   8.870 0.016 Persson-2,2MASSpos
S234-E       20:31:19.61 -49:38:58.0 MT 12.464 0.011  12.127 0.008  12.095 0.007  12.070 0.007 Persson-1,2MASSpos
S813-D       20:41:05.2  -05:03:43   MT 11.479 0.005  11.142 0.005  11.082 0.010  11.085 0.005 Persson-2
ABELL2409    22:00:32.50 +20:46:51.5 MT 11.218 0.026  10.987 0.020                10.904 0.209 2MASS
BRI2202-1119 22:05:35.8  -11:04:28   R  11.652 0.010  11.083 0.009  10.721 0.013  10.736 0.011 Persson-4,2MASSpos
S893-D       23:18:10.1  +00:32:55   MT 11.403 0.009  11.120 0.006  11.045 0.006  11.055 0.006 Persson-1
P290-D       23:30:33.5  +38:18:57   MT 11.634 0.005  11.337 0.004  11.257 0.008  11.262 0.006 Persson-2
```
5. Nightly Photometric Calibration Field Scheduling
- Philosophy: Define a dynamic scheduling scheme that accommodates arbitrary observing start times, from normal evening twilight, to middle-of-night starts. Produce a list of calibration fields and schedule for their observation covering start time to morning twilight.
- Initial parameters:
  - Observatory site
  - UT date
  - Evening and morning Nautical Twilight times
  - Start time for observations (Default is ending of evening Twilight excluding dark frame and twilight sky observations)
  - Maximum allowable airmass (zenith angle)
  - List of candidate standard fields
- Alternate hourly calibration observations between two calibration fields. The first field should be selected to be near transit and the second should be a high airmass, rising calibration field at the beginning of the night. When the first of these fields sets, select the next calibration field according to the guidelines below, and alternate observations with the second field. After the second calibration field sets, alternate observations of the third field with calibration fields with airmass nearest 1.0. If necessary, after the third field sets, finish night by observing calibration fields with the minimum airmass each hour.
- Selection Guidelines:
  - Field_1: Calibration field with minimum airmass at start of observations. Observe this field every other calibration observation period until its setting airmass exceeds ~2.2. Can be zenith or equatorial field.
  - Field_2: Field that has the highest rising airmass (<=2.2) approximately one hour after start of observations (i.e. at the second calibration period). Alternate observations of this field with Field_1, until it's setting airmass exceeds ~2.2. Can be zenith or equatorial field.
  - Field_3: Selected for the calibration period when Field_1 exceeds airmass of ~2.2.
    
    If neither Field_1 nor Field_2 were equatorial fields, then Field_3 is the equatorial field with the minimum airmass. Observe every other hour until setting airmass exceeds ~2.2.
    
    Else, Field_3 is the calibration field with the highest rising airmass <=2.2. Observe every other hour until setting airmass exceeds ~2.2.
  - Field_4 through Field_n: After Field_2 airmass exceeds setting airmass of ~2.2, begin alternating observations of Field_3 with calibration fields having the minimum airmass (closest to 1.0) at each calibration period. If Field_3 is the field with minimum airmass at a particular period, then Field_4-n is the first calibration field to the east of Field_3. After Field_3 sets below airmass of >=2.2, complete night with observations of calibration fields with minimum airmass at each calibration period. Repeating a calibration field in consecutive periods is allowable if it still has the minimum airmass.
- Example:
  
  1 October 1997 UT - Mt. Hopkins
  LST at midnight = 00:21
  MST at PM Twilight end = 19:01
  MST at AM Twilight start = 05:26
  Max. allowable Airmass = 2.2
  
  Field_1 = P182-E
  Field_2 = S893-D
  Field_3 = P161-D since Field_2 is equatorial
  Field_4 = P247-U
  
  Calibration Schedule:
```
	 Cal.  UT     MST      Field     X*   Note 
         ---------------------------------------------------
	 1    02:01   19:01    P182-E   1.06  Field_1
	 2    03:03   20:03    S893-D   1.64  Field_2
	 3    04:05   21:05    P182-E   1.22  Field_1
	 4    05:07   22:07    S893-D   1.21  Field_2
	 5    06:09   23:09    P182-E   1.71  Field_1
	 6    07:11   00:11    S893-D   1.21  Field_2
	 7    08:13   01:13    P161-D   2.20  Field_3
	 8    09:15   02:15    S893-D   1.69  Field_2
	 9    10:17   03:17    P161-D   1.38  Field_3
	 10   11:19   04:19    P247-U   1.03  Field_4
	 11   12:21   05:21    P161-D   1.08  Field_3

	 * approximate airmasses
```
- Special considerations:
  Moon avoidance: If a calibration field normally selected using the above guidelines will be within the moon avoidance radius specified by the 2MASS Survey Strategy any time during the night, then that field is removed from consideration as a calibration field for the night.
Appendix A. Faint Standard Star Sequences

1. UKIRT Faint Standards
```
Name  RA(1950)  DEC(1950)    K          J-K          H-K
FS1  00 31 22.7 -12 24 29  12.967 .021  0.462 .011  0.081 .012
FS2  00 52 36.0 +00 26 58  10.466 .003  0.247 .003  0.038 .003
FS3  01 01 46.6 +03 57 34  12.822 .007 -0.222 .011 -0.097 .007
FS4  01 52 03.7 +00 28 20  10.264 .005  0.292 .003  0.040 .007
FS5  01 52 04.7 -07 00 47  12.342 .006 -0.007 .004 -0.002 .004
FS6  02 27 39.2 +05 02 34  13.374 .015 -0.135 .014 -0.069 .012
FS7  02 54 47.2 +00 06 39  10.940 .005  0.165 .012  0.037 .010
FS10 03 46 17.4 -01 07 38  14.919 .072 -0.170 .077 -0.049 .060
FS11 04 50 25.4 -00 19 34  11.278 .018  0.076 .025  0.016 .019
FS12 05 49 34.8 +15 52 37  13.898 .003 -0.217 .014 -0.091 .018
FS13 05 54 33.8 +00 00 53  10.135 .003  0.382 .002  0.047 .005
FS14 07 21 41.2 -00 27 10  14.261 .012 -0.153 .005 -0.079 .020
FS15 08 48 21.9 +11 55 02  12.360 .021  0.418 .008  0.060 .007 M67
FS16 08 48 31.0 +12 00 36  12.631 .008  0.340 .006  0.038 .005 M67
FS17 08 48 35.4 +12 03 26  12.270 .007  0.411 .007  0.073 .003 M67
FS18 08 51 02.1 -00 25 14  10.522 .008  0.292 .003  0.031 .003
FS19 10 31 14.5 -11 26 08  13.796 .025 -0.231 .021 -0.142 .047
FS20 11 05 27.6 -04 53 04  13.473 .017 -0.120 .015 -0.069 .012
FS21 11 34 27.6 +30 04 35  13.132 .004 -0.184 .033 -0.101 .037
FS33 12 54 35.1 +22 18 08  14.240 .016 -0.223 .010 -0.078 .024
FS23 13 39 25.7 +28 44 59  12.374 .000  0.623 .004  0.155 .006
FS24 14 37 33.3 +00 14 36  10.753 .008  0.151 .006  0.019 .004
FS25 15 35 59.9 +00 24 03   9.756 .017  0.475 .003  0.070 .005
FS27 16 38 54.2 +36 26 56  13.123 .018  0.371 .013  0.058 .014
FS28 17 41 32.5 -00 23 44  10.597 .016  0.148 .010  0.047 .005
FS35 18 24 44.5 +04 01 17  11.757 .017  0.474 .008  0.089 .005
FS34 20 39 41.9 -20 15 21  12.989 .011 -0.170 .008 -0.075 .012
FS29 21 49 53.0 +02 09 16  13.346 .024 -0.171 .011 -0.075 .012
FS30 22 39 11.3 +00 56 55  12.015 .020 -0.092 .013 -0.036 .005
FS31 23 09 50.4 +10 30 46  14.039 .010 -0.241 .020 -0.120 .017
FS32 23 13 38.2 -02 06 58  13.664 .012 -0.205 .011 -0.088 .015
```
2. NICMOS Flight Standards (Green and Rieke)
```
Name     RA(1950)   DEC(1950)  Kest   E(B-V)  B-V   V-I   
P225-C  16:35:56.2  +42:31:14  10.57   0.08  0.64  0.71  
P138-C  17:12:42.8  +54:36:44  11.20   0.01  0.48  0.58 
P133-C  13:56:48.9  +52:20:57  10.88   0.00  0.59  0.65
P177-D  15:57:43.2  +47:45:07  12.09   0.01  0.63  0.71  
P272-D  14:56:34.9  +37:20:29  11.27   0.01  0.72  0.78 
P091-D  09:39:21.2  +59:17:27  11.21   0.00  0.72  0.75
P041-C  14:51:42.9  +71:55:26  10.56   0.01  0.62  0.69  
P330-E  16:29:35.8  +30:15:09  11.62   0.01  0.63  0.74 
```
3. Persson's NICMOS Standards Observed as of July 15, 1997

Positions and approximate Ks magnitudes for Eric Persson's faint near-infrared standard star list. The 'n's in column 1 of the first table are Eric's internal identifiers. The 'N's are the numbers of measurements in each passband (J, H, and Ks). The notes in the last column mean:
1 = original 17 stars set up with respect to Elias et al (1982), measured only at the 40-inch telescope at Las Campanas.
2 = additional stars set up with respect to set#1, only at 40-inch
3 = additional stars set up with respect to set#1, only at Palomar 60-inch
4 = additional stars set up with respect to set#1, at both 40- and 60-inch; results combined.
```
		    INFRARED STANDARD STARS

------------------------------------------------------------------

 n     HST     RA(2000)   Dec(2000)   Ksh      N        note
------------------------------------------------------------------

9101  P525-E  00 24 28.3   07 49 02   11.2  16 16 10 17    4
9103  S294-D  00 33 15.2  -39 24 10   10.5  15 16 9 16     1
9104  S754-C  01 03 15.8  -04 20 44   10.6  17 17 8 17     2
9105  P530-D  02 33 32.1   06 25 38   10.9  8 8 7 8        1
9106  S301-D  03 26 53.9  -39 50 38   11.7  11 11 6 11     1

9107  P247-U  03 32 03.0   37 20 40   11.5  16 18  6 18    3
9108  P533-D  03 41 02.4   06 56 13   11.3  9 9 8 9        1
9109  S055-D  04 18 18.9  -69 27 35   11.2  65 65 2 66     2
9111  S361-D  04 49 54.6  -35 11 17   10.9  11 11 1 11     2
9113  S252-D  05 10 25.6  -44 52 46   10.7  16 16 1 16     2

9115  S363-D  05 36 44.8  -34 46 39   11.8  16 16 13 15    1
9116  S840-F  05 42 32.1   00 09 04   11.0  4 4 3 4        4
9118  S842-E  06 22 43.7  -00 36 30   11.2  3 3 2 3        3
9119  S121-E  06 29 29.4  -59 39 31   11.7  16 15 6 16     1
9121  S255-S  06 42 36.5  -45 09 12   11.3  27 27 0 26     2

9122  P161-D  07 00 52.0   48 29 24   11.3  8 8 3 8        3
9123  S427-D  06 59 45.6  -30 13 44   10.4  9 9 3 9        2
9125  S005-D  07 19 38.6  -84 35 06   10.5  9 9 4 9        2
9126  P309-U  07 30 34.5   29 51 12   11.4  2 2 0 2        3
9129  S209-D  08 01 15.4  -50 19 33   10.4  9 9 2 9        2

9131  P035-R  08 25 43.8   73 01 18   10.5  6 6 4 6        3
9132  S312-T  08 25 36.1  -39 05 59   11.6  16 16 14 17    1
9133  S495-E  08 27 12.5  -25 08 01   10.9  7 7 3 7        2
9134  P545-C  08 29 25.1   05 56 08   11.5  11 11 11 9     1
9135  S705-D  08 36 12.5  -10 13 39   12.0  4 4 0 4        2

9136  S165-E  08 54 21.7  -54 48 08   12.1  8 7 3 8        2
9137  S372-S  09 15 50.5  -36 32 34   10.8  7 7 2 7        2 
9138  S852-C  09 41 35.8   00 33 12   10.9  9 9 3 9        4
9139  P091-D  09 42 58.7   59 03 43   11.2  5 5 3 5        3
9140  S262-E  09 45 42.8  -45 49 40   11.0  4 4 0 4        2  

9141  S708-D  09 48 56.4  -10 30 32   10.7  6 6 1 6        2
9142  P212-C  10 06 29.0   41 01 26   11.6  9 9 5 9        3
9143  P550-C  10 33 51.8   04 49 05   12.0  12 12 12 12    1
9144  S264-D  10 47 24.1  -44 34 05   11.2  6 6 2 6        2
9145  P064-D  12 13 12.0   64 28 56   11.6  4 4 3 4        3

9146  S217_D  12 01 45.2  -50 03 10   10.9  17 18 15 18    1
9147  S064-F  12 03 30.2  -69 04 56   11.7  9 9 4 9        2
9148  P266-C  12 14 25.4   35 35 55   11.3  4 4 4 4        3
9149  S860-D  12 21 39.3  -00 07 13   11.8  14 14 12 14    1
9150  S791-C  13 17 29.6  -05 32 37   11.2  7 7 3 7        2

9152  P133-C  13 58 40.2   52 06 24   10.8  4 4 4 4        3
9153  P499-E  14 07 33.9   12 23 51   11.5  7 7 3 7        3
9154  S008-D  14 23 45.5  -84 09 58   10.9  8 8 7 8        2
9155  S867-V  14 40 58.0  -00 27 47   11.6  11 11 10 11    1
9156  P041-C  14 51 57.9   71 43 13   10.5  6 6 5 6        3

9157  S273-E  14 56 51.9  -44 49 14   10.8  16 17 15 17    1
9158  P272-D  14 58 33.1   37 08 33   11.2  6 6 6 6        3
9160  S870-T  15 39 03.5   00 14 54   10.6  6 6 5 6        2
9162  P177-D  15 59 13.6   47 36 40   11.8  3 3 3 3        3
9164  P565-C  16 26 42.7   05 52 20   11.8  8 8 6 7        1

9166  P330-E  16 31 33.6   30 08 48   11.4  9 9 9 9        3
9169  P138-C  17 13 44.5   54 33 21   11.0  8 8 8 8        3
9170  S875-C  17 27 22.2  -00 19 25   10.7  13 13 13 13    4
9172  S279-F  17 48 22.6  -45 25 45   12.0  8 8 8 8        1
9173  S024-D  18 18 46.2  -80 06 58   10.7  8 8 7 8        2

9175  S071-D  18 28 08.9  -69 26 03   11.8  11 11 6 11     2
9177  P182-E  18 39 33.7   49 05 38   11.6  15 15 15 15    3
9178  S808-C  19 01 55.4  -04 29 12   10.5  9 9 4 9        2
9181  S234-E  20 31 20.4  -49 38 58   12.0  5 5 4 5        1
9182  S813-D  20 41 05.1  -05 03 43   11.0  15 15 7 15     2

9183  P576-F  20 52 47.3   06 40 05   11.8  23 23 14 23    4 
9185  S889-E  22 02 05.7  -01 06 02   11.5  18 18  8 18    4
9186  S893-D  23 18 10.0   00 32 56   11.0  7 7 6 7        1
9187  S677-D  23 23 34.4  -15 21 07   11.5  39 39 10 39    2
9188  P290-D  23 30 33.4   38 18 57   11.2  15 15  8 15    3
-----------------------------------------------------------------
```
4. Persson's Faint Red Standards
```
-----------------------------------------------------------------
 star          RA(2000)   Dec(2000)   Ksh     N         note
-----------------------------------------------------------------
BRI0021       00 24 10.8  -01 59 52   10.5  6 6 6 6        4
T832-38078    03 03 47.9   00 44 47   10.9  5 5 5 5        4
LHS191        04 26 06.4   03 36 50   10.7  2 2 2 2        2
IRAS537W      05 40 06.0  -07 27 13   10.0  4 4 3 4        2
IRAS537S      05 40 06.0  -07 27 13   11.0  3 3 3 3        2

LHS2026       08 32 16.6  -01 33 19   11.1  10 10 10 10    2
LHS2397a      11 21 35.5  -13 11 45   10.7  6 6 6 6        2
cskd_12       12 31 30.0  -63 42 41   8.60  8 8 8 8        2
cske_23       12 31 56.0  -63 37 43   7.88  5 5 5 5        2 
T868-53850    15 00 12.5  -00 38 25   10.6  5 5 5 5        2

T868-110639   15 10 03.1  -02 40 06   11.3  5 5 5 5        2
L134          15 53 23.0  -04 39 12   8.89  4 4 4 4        2
Oph_N9        16 27 01.5  -24 41 50   9.62  2 2 2 2        2
L547          18 51 03.6  -04 16 48   8.87  4 4 4 4        2
BRI2202       22 05 21.3  -11 05 48   10.7  4 4 4 4        4      

-----------------------------------------------------------------
```
Appendix B - Photometric Calibration Acceptance Criteria
1. Basic Photometric Transformations:
  - The basic transform to convert instrumental magnitude to calibrated magnitude is assumed to have the form:
    
    Mcal = Minst + c1 - c2*(X-1.0) + [c3*C]
    
    where c1 is the photometric zero point, or offset, c2 is extinction coefficient (mag/airmass), X is airmass (sec z), Minst is instrumental magnitude of star, and Mcal is the calibrated magnitude. An optional color term with coefficient c3 and color C (i.e. J-H, H-Ks, J-Ks) may be included. Currently, the color term, c3, is not included.
  - Issue: For which bands will color terms be necessary? What is plan for measuring color terms?
2. Photometric Zero Points
  - Decouple determination of zero point from measurement of extinction coefficient. Assume we know a priori the extinction slope, c2, and correct calibration data to unit airmass before zero point determination.
  - The photometric Zero Point can be determined in two ways:
    - Constant photometric zero point for night.
      
      c1 = Average(Mcat - Minst')
      
      Minst' = extinction corrected instrumental magnitude = Minst - c2*(X-1.0).
      
      Acceptance criterion: RMS(c1) < 0.04 mag.
    - Zero point varies linearly with time.
      
      Fit c1(t) = A + Bt, after extinction correction.
      
      Acceptance criterion: RMS deviation from fit < 0.04 mag. (TBD) AND $B < 0.1 mag / 24 hours$ (TBD)
    - Issues:
      - What should determining factor be in fitting constant or linearly varying zero point for night?
      - What should maximum allowable change in zero point over night be? (i.e. what is Bmax?).
  - General issues:
    - How to handle color terms?
    - How to deal with nights where c1 differs from recent trends in zero points by large value? Is this grounds for rejecting night?
3. Extinction correction
  - Decouple from zero point determination. Assume a priori value for extinction slope, c2. Measure this slope for each site during telescope commissioning, and early operations, and monitor and refine measurement during life of survey.
    Initial values used at beginning of northern operations are 0.0887 mag/airmass (J), 0.0873 mag/airmass (H) and 0.0763 mag/airmass (Ks). Airmass correction is to unit airmass, not zero airmass.
  - Measure nightly, following one field across sky. Use all stars in the brightness range 9.5 < Ks < 13, 10 < H < 13.5, 10.5 < J < 14 in field to determine extinction slope. Linear regression to determine extinction as a function of airmass. Do not use nightly measured extinction slopes, but add results to running average, and trend of extinction as a function of time.
  - Occasionally following one field as it rises or sets. Measure frequently while in the airmass range 2.2 < X < 1.6, so get a larger change in airmass with minimum change in time. This hopefully minimizes impact of potential transparency variations on extinction measurements.
  - Update weighted mean of extinction coefficients at J, H and Ks determined during the survey. Use long-term average coefficients for extinction correction of calibration and survey measurements.
  - Issues:
    - How often should we allow "canonical" extinction coefficient to change?
    - How to deal with nights where c2 measurement indicates large difference with canonical value?
Photometric Calibration Acceptance Criteria:

A night, or a portion thereof, will be considered to have an acceptable photometric calibration if the following criteria are met:
- There are at least three sequential calibration periods, during which three independent calibration observations are made. The average time between calibration observations is one hour, so the nominal minimum acceptable photometric period is approximately 2 hours.
- During the minimum acceptable photometric period, each of the three calibration observations must be considered of high quality. For a calibration observation to be ranked of high quality, the following criteria must be met:
  - Each of the six individual scans must complete processing without data-quality related errors.
  - Each of the individual scans must have been assigned high quality with respect to photometric and astrometric Q/A. (includes tests for seeing, background, value checking, position reconstruction residual, optical catalog association, etc...)
  - Each scan must have covered >= 90% (TBR) of the nominal position of the calibration tile.
  - The average dispersion in the magnitudes of all stars in the brightness range (9.5 < Ks < 13, 10 < H < 13.5, 10.5 < J < 14) measured in the six individual scans making up each observation must be <= 0.04 mag. Two exceptions to this may be made. The first is if the calibration scan set in question was made at high airmass. The second is if there is evidence that the photometric zero point was constant throughout the night, but individual calibration sets have large internal dispersions due to poor seeing and/or high backgrounds (i.e. reduced sensitivity). In the former case, no photometric quality degradation takes place. In the latter, the photometric quality of the night will be degraded in accordance with the degree to which the internal dispersion threshold is relaxed.
  - Issue: The photometric "quality" of each scan, and each set of six calibrations scans can be measured several ways. 1) The dispersion in the difference between catalog and instrumental magnitudes for all of the calibrator(s) in a single scan (initially one, but eventually many); 2) The dispersion in the instrumental- catalog magnitude differences for the calibrator(s) in all six scans; 3) Dispersion of all high SNR stars in all six scans. What should driving dispersion be for determining if a calibration observation is good? Should all three be tested?
- During the minimum acceptable photometric period, the solutions for photometric zero point must meet one of the following criteria:
  - The dispersion about the constant, mean photometric zero point, c1, is <=0.04 magnitudes, or
  - The dispersion about a linear fit to the J-band photometric zero point with UT time is <= 0.04 magnitudes, and the rate of change in the zero point with time is < 0.2 mag/24 hours (TBD)
  - Issues
    - Any limit on number of photometric periods per night?
    - Can the two criteria above be satisfied by adjacent time periods? (i.e. are abrupt changes in zero point allowable?)
    - Use simulations to determine minimum number of calibration observations to determine zero point to 4%.

General Issues:

Should we shorten "minimum acceptable photometric period?" For example, set the minimum period as the time bracketed by 2 calibration sessions. Alternately, rank scans that fall between two high quality calibration sessions, but not three, as intermediate quality.
How to utilize stars in survey scan overlap regions to monitor zero points on inter-calibration timescales?
How to incorporate frame background values as an indicator of clouds.
Is is possible to confidently extend the useful photometric period outside of time bracketed by calibration observations using overlap regions and background monitoring?
Should we allow for "moderate" quality photometric calibrations? RMS > 0.04 mags, but < TBD? If so, how many levels of "moderate" quality should there be?
How to incorporate/calibrate 3um sky channel information, if it becomes available?
What mountaintop Q/A information will become available?

Appendix C - Tertiary Standard Star Calibration

Each calibration field contains hundreds of stars in addition to the one or more predefined standards for the survey. Because calibration scans take only ~1 minute, under conditions in which the photometric zero point does not change significantly over that time period, it will be possible to calibrate all stars within the scan relative to the local standard independent of extinction corrections.

The practice within the pipeline processing will be to always calibrate the photometry of sources within a calibration scan using the measurements of the calibrators in that scan, only. Thus, each scan will produce a list of internally calibrated stars, and these scans can be continuously combined to produce a (hopefully) highly accurate set of tertiary calibrators in each calibration field.

Appendix D - Selection of a New Calibration Field

There is a gap in the distribution of northern calibration star candidates between 18:30 and 23:30 right ascension. Therefore, we will define a field in that region that will be observed during the first few months of survey operations, and then calibrated internally against the other 2MASS calibration observations.

2MASS Science Team member, Dave Monet, suggested that since we have free reign to select a new field, then a field should be selected that has a moderate star density and a higher than normal concentration of galaxies so that long term stability and uniformity in the extended source processing can be monitored during the life of the survey. Lists of Abell and X-ray selected galaxy clusters were searched for possible candidates (Abell, Corwin and Olowin 1989, ApJSupp, 70, 1; Allen et al. 1992, MNRAS, 259, 67). There are no known nearby, rich clusters of galaxies in the desired region, but two candidates were identified that should contain sufficiently bright galaxies to be detected by 2MASS: Abell 2409 and RXJ2120.2+2258. Abell 2409 has a redshift of z=0.147 and contains 10 galaxies with brighter than 16.8 mag as gauged on the red POSS image. Visual inspection of the X-ray selected RXJ2120.2+2258 cluster reveals that it has very similar characteristics, and its redshift is z=0.143.

Test one degree scans of both cluster cores, as well as an area slightly off the core of Abell 2409 that contains several more prominent galaxies visible on the E POSS print, were made under non-photometric conditions with the 2MASS facility at Mt. Hopkins on 2 June 1997 (UT). As reported by Tom Jarrett at http://spider.ipac.caltech.edu/staff/jarrett/2mass/3chan/std/std.html , eleven galaxies with Ks<13.5 mag were found in the cluster core scans, and 9 in the off-core scan of Abell 2409. Examination of the environment of the two core scans revealed that most of the bright galaxies are badly contaminated by nearby and overlapping stars which would render the extended source measurements of these objects problematic. Therefore, the off-core region of Abell 2409 was selected as the new calibration field.

The center of the new calibration field is listed above in the preliminary table of calibration fields for 2MASS. Lying at a galactic latitude of -27 deg., it has a large enough number of high signal-to-noise ratio stars (95 stars in the range 9 < Ks < 13) to supply excellent photometric statistics, but not so high a star density that confusion will render the point source photometry suspect. This field will be placed in the normal rotation of calibrators for Survey operations, and a calibration will be built up over the next several months while it will be observed.

R. Cutri (IPAC)
Last Update - 24 August 1998

2MASS PHOTOMETRIC CALIBRATION STRATEGY

version 4.2

R. Cutri (IPAC), J. Elias (NOAO), E. Howard (UMASS), R. Light (IPAC), E. Persson (OCIW), P. Seitzer (UMich), M. Skrutskie (UMASS) and S. Wheelock (IPAC)

Contents

1. Photometric Calibration Requirements:

2. Photometric Calibration Observations and Targets

3. Proposed Photometric Calibration Strategy

4. Photometric Calibration Field Selection Criteria

2MASS Calibration Field List

5. Nightly Photometric Calibration Field Scheduling

Appendix A. Faint Standard Star Sequences

1. UKIRT Faint Standards

2. NICMOS Flight Standards (Green and Rieke)

3. Persson's NICMOS Standards Observed as of July 15, 1997

4. Persson's Faint Red Standards

Appendix B - Photometric Calibration Acceptance Criteria

Appendix C - Tertiary Standard Star Calibration

Appendix D - Selection of a New Calibration Field