INT WFS Photometric Calibration Technical Note 1.0
Determination of Colour Terms and Extinction: Calibration Stars

INT-WFS-IOA-TN-CAL-1.0

Document History
Version Author Date Details
1.0 rgm 1998 Aug 16 first draft
1.1 rgm 1998 Aug 21 added Generic Landolt fields and proposal to determine
colour terms for each chip
1.2 rgm 1999 Mar 07 converted to html
1.3 rgm 2000 Mar 26 added links to Landolt catalogue and Boyle/Shanks dec=0 photometric sequences

To Do List

  1. add link to calibration results page created by Derek Jones and Peter Bunclark
  2. complete the various tables for the spring season

Summary

This note outlines the basic motivation and a plan for the determination of filter color terms and extinction coefficients for all the filters being used in the WFS. The note includes both a set of recommended extreme colour Landolt standard stars and estimated count rates. These count rates are used to estimate exposures times and identify which observations should be carried out using defocused images to prevent saturation. I have erred on the same side and assumed that the timing is good to 0.1sec. To get 1% precision on a single measurement, I have set the minimum exposure to 10seconds. Since a single maximal unsaturated exposure will produce a S/N of almost 1000, I have tried to set the exposure time to be 1/5 the maximum allowable with the additional constraint that I have tried to use the same exposure time for all bands and used powers of two for the exposure increments. Note that even an exposure time of 1/20 of the maximal could produce a S/N>50 or sigma(mag)=0.015 and 0.02 for a colour.

In addition I have created a modified version of the Landolt catalogue that I copied from the the iraf catalogs. I have added two extra records per Landolt source. These contain the UBVRI magnitudes and saturational expsoures for the WFS as defined below.

Notes:

  1. This version only takes into account the exposure time constraints for the priority standard star in each.
  2. Since it takes 3minutes to read out, unless one can window or only read out a single chip, we shall probably only normally observer one standard in twilight.
  3. In survey mode; no standards shall be observed during the night, except during astronomical twilight.
  4. Survey fields shall be calibrated using overlap regions and secondary sequences set up during a photometric night. We shall aim to put a secondary standard in the centre and at the corner of each survey region.

Introduction

Since all the WFS data is to be made public it is important that we calibrate it properly. This means we should attempt to determine both the colour terms for all the filters we use and the filter dependent extinction terms. We may want to determine extinction colour terms in U on a nightly basis. To facilitate this we need to ensure we observe stars with a wide range of colours. It also important that when we observe such star we do not end up with saturated data. This is quite easy to do when dealing with stars that have extreme red or blue colours. U band colour terms are the most critical and hardest to determine. To minimise potential errors we have selected only blue standards stars that have companion normal coloured stars within a few arc mins.

Since the QE curves for each chip are probably different we should put the extreme coloured standards onto each chip and derive the colour terms for each. This is a long term aim. In the short deriving the colour terms on chip 4 is the priority. In practice, differential colour terms could be deduced from analyzing the intra chip and inter chip colour distribution from all the data.

Filter related issues

Due to delays in the delivery of filters:
  • Prior to Dec 98; U(RGO); B(KPNO); V(Harris); R(Harris); I(WFC); Z(WFC)
  • Dec 98; I(SDSS) delivered
  • Mar 98; R(SDSS) delivered

    Estimation of Maximum Exposure Times due to Saturation

    Below I calculate the maximum total intensity per second permitted without the central pixel of the image profile going into the non-linear regime of the CCD. Note, this calculation is carried out in terms of photons a second and a gain of 3.5 e-/ADU. Assumptions:
    1. INT 0.333 arc/pixel: 9.0 pixels per square arc sec
    2. zeropoint data from WFC User Notes Walton, Rey, Irwin, Bunclark 
                          U       B      V      R      I
                   23.8    25.6   25.7  25.6   25.0
      The zeropoint information above is the alpha-lyrae magnitude that gives one detected photon (or electron)
    3. The full well on all devices is 200,000e- and the gain is around 3.5 for standard read out. A safe option would be not to go above 40,000 ADU peak on any chip OR 140,000 electrons. The WFC user notes specify 2% nonlinearity at 55,000ADUs. 
    4.  Assuming a Gaussian profile:

	 FWHM(seeing) = 2.3548 sigma  [2*sqrt(2ln(2)]


    5.  For a 2D Gaussian:

         Total = Peak * 2 * pi * sigma^2 where sigma is in pixels:
    Thus: Peak = 0.88 * Total / (FWHM^2) or Total = 1.14 * Peak * FWHM^2 Thus for sigma = 1 pixel => FWHM = 2.35 pixels = 0.78 arcsec Peak Intensity = 0.16 Total Integrated Intensity Thus assuming a star is centred on the corner of 4 adjacent pixels ie the worst case, we have: ie Total_max = 8.75 e5 e- = 2.5 e5 ADU => S/N= 935

    Recommended Blue Standards

    Below are tables containing the Blue and Red calibration stars. These shall be observed in addition to some generic Landolt fields. Each standard stars has 3 lines: Line 1: Landolt colours and magnitudes Line 2: Time to saturation with WFC: defined as time to reach a peak of 140,000 electrons in 0.78" seeing(FWHM=2.35pixels) Line 3: recommended exposure time per band: assume Z=I Line 4 onwards: name and saturation time for companions standards (incomplete data) Table 1 Blue Standards (with nearby on-chip 'normal' standard stars) p=priority n=number of stars in field p n U-B B-V V-I U B V R I saturation limit(sec) recommended t_exp(sec) ----------------------------------------------------------------------- 0220+132 PG0220+132 2 3 -0.92 -0.13 -0.17 13.9 14.7 14.8 14.7 15.0 95.9 38.2 38.2 38.2 87.5 40 20 20 20 10 PG0220+132A 809 211 94 64 71 PG0220+132B 408 58 22 15 16 0231+051 PG0231+051 1 6 -1.19 -0.33 -0.53 13.2 14.4 14.7 14.9 15.2 50.4 29.0 34.8 45.9 105 20 20 10 10 10 PG0231+051A 84 12 6 4 5 PG0231+051B 2704 150 36 16 11 PG0231+051C 165 28 14 11 13 PG0231+051D 770 56 19 11 11 PG0231+051E 197 31 15 12 14 0353-004 SA95-42 1 6 -1.11 -0.22 -0.30 14.3 15.4 15.6 15.7 15.9 139 73 80 96 200 40 40 40 40 40 0724-003 RUBIN 149 2 7 -0.78 -0.13 -0.11 0729-021 RUBIN 152 1 7 -1.07 -0.19 -0.14 0918+029 PG0918+029 5 0942-029 PG0942-029 5 1047+003 PG1047+003 4 1323-086 PG1323-086 5 1525-071 PG1525-071 3 1528+062 PG1528+062 3 1530+057 PG1530+057 3 1633+099 PG1633+099 5 1657+078 PG1657+078 5 2043-107 MARK-A 1 4 -1.16 -0.12 -0.24 12.4 13.2 13.3 13.4 13.5 24.1 9.6 9.6 11.5 22.0 10 10 10 10 10 2213-006 PG2213-006 1 4 -1.12 -0.09 -0.20 12.9 14.0 14.1 14.2 14.3 38.2 20.0 20.0 24.0 34.8 10 10 10 10 10 2331+055 PG2331+055 3 3 -0.49 -0.07 -0.04 14.6 15.1 15.2 15.2 15.2 183 55.1 55.1 60.0 79.7 20 20 20 20 20 2336+004 PG2336+004 2 3 -0.78 -0.17 -0.11 14.9 15.7 15.9 16.0 16.0 241 96 105 127 220 40 20 20 20 40 -----------------------------------------------------------------------

    Recommended Red Standards

    Table 2 Red Standards p n U-B B-V V-I U B V R I saturation limit(sec) recommended t_exp(sec) ----------------------------------------------------------------------- 0056+0100 SA92-427 2 0.35 0.81 3.27 15.7 15.4 14.9 14.4 11.6 503 73 42 29 3.8 80 20 10 10 10 defocus for VRI 0200+1304 G3-33 1 1.32 1.80 3.10 15.4 14.1 12.3 10.9 9.2 382 22.0 3.8 1.1 0.4 100 10 10 10 10 defocus for VRIZ 0354+0029 SA95-330 1 2.23 2.00 2.27 16.4 14.2 12.2 11.0 9.9 959 24 3.5 1.3 0.8 100 10 10 10 10 defocus for VRIZ 2111-1308 WOLF 918 2 1.15 1.49 2.06 13.5 12.4 10.9 9.9 8.8 66.4 4.6 1.1 0.5 0.3 10 10 10 10 10 defocus BVRIZ 2131-0947 G26-7 1 1.24 1.67 2.99 15.0 13.7 12.0 10.7 9.0 264 15 2.9 1.0 0.3 20 10 10 10 10 defocus BVRIZ

    Recommended Generic Landolt Standard fields

    Table 3 Suggested Generic Standard Field Centres for WFC [suplied by Mike Irwin] sa92-335 00 55 00 00 44 13 J2000 sa95-330 03 54 31 00 29 05 J2000 sa98-670 06 52 12 -00 19 17 J2000 sa101-427 09 57 26 -00 17 16 J2000 sa104-334 12 42 21 -00 40 28 J2000 sa107-602 15 39 19 -00 15 29 J2000 sa110-361 18 42 45 00 08 04 J2000 sa113-163 21 42 35 00 16 46 J2000

    Landolt standards datafiles

    Boyle, Shanks, Croom, 1995 Data from the above with some supplemenatry fields is here [ AAO version ] [local WFCSUR copy]

    Common Optical IR Photometric Standards

    Table 4 Landolt Standards that are also JHK Standards TBD

    Common Spectrophotometric Photometric Standards

    Table 5 Landolt Standards that are also Spectrophotometric standards TBD

    Calibrations Runs

    Aug 98 Test Run

    Test Run: Calibration plan SA111(1940+00) start of night generic standard field 2043-107 MARK-A blue standard (observe at range of air masses) 2111-1308 red standard for colour terms 2131-0947 red standard for colour terms SA114(2240+01) generic standard field in survey field PG2213-006 blue standard (observe at range of air masses) PG0231+051 blue standard (observe at range of air masses) SA94(0300+00) end of night generic standard field Start of night in Astronomical Twilight: SA110 SA111 2043-107 MARK A [needed for extinction analysis] MARK A and PG2213 at LST 2043 SA114 End of night in Astronomical Twilight: PG2213(for extinction if observed earlier at low airmass) PG0231 SA94

    Autumn 98 Season

    Spring 99 Season

    The priorities for the Spring 99 season:

  • check the U focus offset
  • longer exposures in U in Landolt fields since last season the U frames were under exposed
  • determine colour equations for new filters