New Photometric Fields for LBC

During commissioning, fields from Landolt (1992) and Galadi-Enriquez, Trullols and Jordi (2000) were observed to determine the photometric coefficients for the LBCs. Both fields contain bright stars, requiring very short exposure times, of order a few seconds. During a recent re-examination of the linearity and gain of the LBCs, there was some question about whether the exposure time written to the header, which is the requested exposure time plus a fixed offset, is correct. This could affect photometric measurements made from very short exposures.

Meanwhile, several surveys have yielded photometric catalogs which cover a large portion of the sky in the optical and near-infrared, e.g. SDSS, Pan-STARRS and UKIDSS; and, in addition, Clem & Landolt (2016) have published UBVRI photometry for relatively faint stars in selected northern hemisphere fields. Since the fall of 2021, we have been observing these fields with the LBCs in order to verify or update the zeropoints and other photometric coefficients.

There are scripts for each of the following fields. The scripts do 6 dithers (3 small dithers and then chip-to-chip dithers) and have exposure times: t_exp = 30 for UBV and t_exp = 20 for RI and t_exp = 30 for grizYUUs and take approximately 28-min (SDSS+UKIDSS) and 25-min (Clem & Landolt).


Field name RA(J2000) DEC(J2000)
sdssY1 16:44:27.68 30:09:31.8
sdssY2 20:44:19.8 01:03:45
sdssY3 23:49:52.8 15:03:28
sdssY4 01:54:04.4 +14:39:54
sdssY5 03:30:11.60 +04:23:25
sdssY7 07:45:00 +25:00:00
sdssY8 08:35:25 +27:10:30
sdssY9 09:32:00 +32:15:00
sdssY10 09:50:00 +34:30:00
sdssY11 10:40:00 +10:00:00
sdssY12 12:00:00 +12:30:00
sdssY13 14:30:00 +10:30:00
sdssY14 15:40:00 +10:00:00

These fields are in the regions of overlap between the SDSS and UKIDSS surveys which include Y, so there is photometry for these at grizY. The magnitude distributions peak around the following values: g=23; r=21; i=20.5; z=19.5; Y=18.5 (19.134 AB); and the exposure times are calculated to give a SNR ~40 at Y and ~70 at griz. I threw in U and Us to fill up the time on the left side and have not set the exptime for these two observations to meet a SNR level.

The OBs will dither 6 times (4 chip-to-chip dithers and 2 smaller dithers around the central position)
At each dither position, they will execute a series of observations as shown in the table below:

filter exptime filter exptime
sdss_r 20 sdss_r 20
sdss_g 20 sdss_i 20
U-Bessel 30 sdss_z 20
U-spec 30 Y-FAN 40

Clem & Landolt Faint Photometric Fields

These fields are from Clem & Landolt, "Faint UBVRI Standard Star Fields at +50 deg Declination", 2016, AJ, 152, 91.

The magnitudes of the most of the stars in these fields range from V=17.5 to 21, though there is a bright tail and a few that are fainter. The GD fields overlap with old Landolt standard star fields.

Field RAJ2000 DEJ2000
  ("h:m:s") ("d:m:s")
GD_2 00:07:33.6 +33:18:51
J005945+440830 00:59:45.0 +44:08:30
GD_279 01:52:02.4 +47:01:05
J031900+434445 03:19:00.0 +43:44:45
J054610+453445 05:46:10.0 +45:34:45
J103613+421100 10:36:13.0 +42:11:00
J125239+444615 12:52:39.0 +44:46:15
J153645+434530 15:36:45.0 +43:45:30
J175954+452015 17:59:54.0 +45:20:15
GD_391 20:29:50.1 +39:15:53
J205245+405115 20:52:45.0 +40:51:15
J234139+453900 23:41:39.0 +45:39:00

The OBs will execute a 6pt dither (4 chip-to-chip dithers and 2 smaller dithers around the central position) and at each position cycle through Bessel filters and take exposures as shown in the table:

filter exptime filter exptime
U 60 V 20
B 30 R 20
V 20 I 30

The blue side will always take longer, and a future version may modify the red exposure times. The exposure times were set by requiring SN>30 for magnitude 21 (Vega) in the bandpass. The magnitude of saturation is close to 16.

-- OlgaKuhn - 14 Dec 2022
Topic revision: r1 - 14 Dec 2022, OlgaKuhn
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