3. Standard stars photometry


Not finished.

This section describes how to obtain instrumental magnitudes for your standards, and set up the transformation from instrumental to standard. The steps are:

3.1. Identify standard stars


This script assumes three things: that you are using Landolt standards, that all your standard frames are aligned, and that there are at least three standard stars in your observed Landolt frame.

id_standard parameters

  • ref_id_std: stk_2078_crop.fits
  • landolt_fld: pg1323
  • scale_min: 0.1
  • scale_max: 10.0
  • rot_min: 0.0
  • rot_max: 5.0
  • do_plots_C: y

The id_standard script allows the automatic identification of standard stars in your observed frame, requiring only minimal information.

Landolt equatorial standards are the only set supported currently by the code. The alignment of your observed standard frames is required since the coordinates of the standard stars will be transformed to the observed system using a single observed frame. The requirement of three standard stars minimum is related to the algorithm used by the script to perform the transformation to the observed coordinates system.

The user selects a reference (aligned) observed standards frame, and informs the script to which Landolt frame it corresponds. The script will then search for the proper scaling, rotation, and x, y translation that allows it to identify which stars in your observed frame are standards.

An image is produced marking the stars in the observed frame that where identified as standards.


An Lframe_obs.coo file is also generated (where Lframe is the name of the Landolt frame processed, given in the landolt_fld keyword onf the input parameters file) containing the Landolt stars information, as well as their x, y coordinates in the observed system:

        ID      x      y       V     BV      UB      VR      RI      VI      x_obs      y_obs
 pg1323-86  211.0  158.3  13.481  -0.14  -0.681  -0.048  -0.078  -0.127   1872.768   1847.208
pg1323-86A  162.5  137.5  13.591  0.393  -0.019   0.252   0.252   0.506   1521.456   1695.408
pg1323-86B  158.1  128.0  13.406  0.761   0.265   0.426   0.407   0.833   1489.735   1626.427

3.2. Aperture photometry on standard stars


Not finished.

aperphot_standard parameters

  • stnd_obs_fields: pg1323, stk_2082_crop.fits, stk_2127_crop.fits, stk_2129_crop.fits, stk_2131_crop.fits
  • aperture: 15
  • annulus_in: 20
  • annulus_out: 25
  • do_plots_D: y

The aperphot_standard automatically performs aperture photometry on your observed standard frames, for the identified stars in one.


We assume that you will work with a single aperture radius value for the standards from all the nights for all the filters.

The selected aperture value should be large enough to contain as much light from your observed standards as possible, but at the same time small enough to minimize contamination from bad pixels and other stars. The default convention is to use an aperture radius that is ~4.5*FWHM of a stellar image. This is, for a FWHM=3 px you’ll use an aperture around 14-15 px.

Assuming you’ve already run the fitstats script, the median FWHM values for your standard frames are stored in the fitstats.dat file.


If you have more than one exposure per filter for your standard frame, at this point you need to select only one. An ideal frame should allow the detection of all the standard stars in it. This means no over-exposed saturated stars, and no under-exposed undetectable stars.

This script reads as many standard .fits files as you input in the stnd_obs_fields keyword in the params_input.dat file. After that, the corresponding the Lframe_obs.coo file for this Landolt frame is read (the script id_standards generates this file). From this file we read the x, y coordinates for each standard star in the observed frame, along with its calibrated (Landolt) photometry.

Circular aperture photometry is performed on each frame for each standard star. Their calculated instrumental magnitudes are corrected for zero airmass.

Final zero airmass magnitudes are stored in the stnd_aperphot.dat file, along with Landolt default colors and magnitudes for each standard star in each observed standard field:

Filt   Stnd_field    ID                 file   exp_t       A       ZA_mag      Col      Mag
   I       pg1323    86   stk_2082_crop.fits    20.0   1.071       15.358   -0.127   13.608
   I       pg1323   86A   stk_2082_crop.fits    20.0   1.071       14.682    0.506   13.085
   I       pg1323   86B   stk_2082_crop.fits    20.0   1.071       14.269    0.833   12.573
   I       pg1323   86C   stk_2082_crop.fits    20.0   1.071       14.956    0.759   13.244

3.3. Define and solve the transformation equations


Not finished.

fit_standard parameters

  • R^2_min: 0.98
  • RMSE_max: 0.05
  • extin_coeffs: U .49, B .27, V .12, I .02
  • do_plots_E: y


The extinction coefficients for your observed filters are assumed to be known.


The V filter is assumed to be present among your observed filters.

This script will obtain instrumental magnitudes for all the observed standard stars, for each selected frame. The transformation equations are used to put these magnitudes on the standard system.