Photometry is done on Landolt fields using the task PHOT in the IRAF package DIGIPHOT/APPHOT. The settings for the program tasks centerpars, datapars, findpars, fitskypars and photpars are given at the bottom of this page.
Three parameters for each of the filters BVRI are determined for photometric nights using Landolt fields:
1. The Zeropoint Correction, Z, as given by the equation
Z = Instrumental Mag - Literature Mag - (Extinction Coeff)*(Airmass)
where Instrumental Mag is the PHOT result obtained with an aperture radius of 20 pixels and Literature Mag is the magnitude reported by Landolt. For a discussion of the Extinction Coefficient, see below.
2. The Color Term Coefficients B-V for the B,V filters and V-I for the R,I
Each coefficient is the slope, as determined by a least squares fit, of the line for a plot of the Z values found for the separate standard stars in the field in the given filter as a function of the quantity (Instrumental Mag in filter 1 - Instrumental Mag in filter 2). E.g.,
3. The Color-Term Corrected Zeropoint Correction, Z', as given by the equation
Z' = Instrumental Mag - Literature Mag - (Extinction Coeff)*(Airmass) - (ColorTerm Coefficient)*(Instrument Mag filter1-Instrument Mag filter2)
It is the y-intercept, as determined by a least squares fit, of the line of the plot described in (2) above, namely the plot of the Z values determined for the separate standard stars in the field in the given filter as a function of the quantity (Instrumental Mag in filter 1 - Instrumental Mag in filter 2).
Extinction Coefficients in each filter can be determined as the slopes, as given by a least squares fit, of the lines for the plots of (Instrumental Mag - Literature Mag) as a function of Airmass. Since typically only one set of Landolt frames are taken each night (one image in each filter), there is not normally a range of airmasses available for this determination and default values are used instead. The current default values are given in the table next to Extinction Coefficients Values in 1.3-m Photometric Standards page [3]. Approximately once a month, a full set (three different airmasses per filter) of Landolts is taken and the Extinction Coefficents calculation performed. The results from these periodic determinations are used to adjust the default values when it is deemed necessary.
| PACKAGE = apphot | |||
| TASK = centerpars | |||
| (calgori= | centroid) | Centering algorithm | |
| (cbox = | 10.) | Centering box width in scale units | |
| (cthresh= | 0.) | Centering threshold in sigma above background | |
| (minsnra= | 1.) | Minimum signal-to-noise ratio for centering algo | |
| (cmaxite= | 10) | Maximum number of iterations for centering algor | |
| (maxshif= | 5.) | Maximum center shift in scale units | |
| (clean = | no) | Symmetry clean before centering ? | |
| (rclean = | 1.) | Cleaning radius in scale units | |
| (rclip = | 2.) | Clipping radius in scale units | |
| (kclean | 3.) | Rejection limit in sigma | |
| (mkcente= | yes) | Mark the computed center on display ? | |
| (mode = | ql) | ||
| PACKAGE = apphot | |||
| TASK = datapars | |||
| (scale = | 1.) | Image scale in units per pixel | |
| (fwhmpsf= | 5.) | FWHM of the PSF in scale units | |
| (emissio= | yes) | Features are positive ? | |
| (sigma = | INDEF) | Standard deviation of background in counts | |
| (datamin= | -500.) | Minimum good data value | |
| (datamax= | 30000.) | Maximum good data value | |
| (noise = | poisson) | Noise model | |
| (ccdread= | ) | CCD readout noise image header keyword | |
| (gain = | ) | CCD gain image header keyword | |
| (readnoi= | 6.5) | CCD readout noise in electrons | |
| (epadu = | 2.3) | Gain in electrons per count | |
| (exposur= | EXPTIME) | Exposure time image header keyword | |
| (airmass= | SECZ) | Airmass image header keyword | |
| (filter = | CCDFLTID) | Filter image header keyword | |
| (obstime= | JD) | Time of observation image header keyword | |
| (itime = | ) | Exposure time | |
| (xairmas= | ) | Airmass | |
| (ifilter= | V) | Filter | |
| (otime = | ) | Time of observation | |
| (mode = | ql) | ||
| PACKAGE = apphot | |||
| TASK = findpars | |||
| (thresho= | 4.) | Threshold in sigma for feature detection | |
| (nsigma = | 1.5) | Width of convolution kernel in sigma | |
| (ratio = | 1.) | Ratio of minor to major axis of Gaussian kernel | |
| (theta = | 0.) | Position angle of major axis of Gaussian kernel | |
| (sharplo= | 0.2) | Lower bound on sharpness for feature detection | |
| (sharphi= | 1.) | Upper bound on sharpness for feature detection | |
| (roundlo= | -1.) | Lower bound on roundness for feature detection | |
| (roundhi= | 1.) | Upper bound on roundness for feature detection | |
| (mkdetec= | no) | Mark detected stars on the display ? | |
| (mode = | ql) | ||
| PACKAGE = apphot | |||
| TASK = fitskypars | |||
| (salgori= | mode) | Sky fitting algorithm | |
| (annulus= | 25.) | Inner radius of sky annulus in scale units | |
| (dannulu= | 7.) | Width of sky annulus in scale units | |
| (skyvalu= | 0.) | User sky value | |
| (smaxite= | 10) | Maximum number of sky fitting iterations | |
| (sloclip= | 0.) | Lower clipping factor in percent | |
| (shiclip= | 0.) | Upper clipping factor in percent | |
| (snrejec= | 50) | Maximum number of sky fitting rejection iteratio | |
| (sloreje= | 3.) | Lower K-sigma rejection limit in sky sigma | |
| (shireje= | 3.) | Upper K-sigma rejection limit in sky sigma | |
| (khist = | 3.) | Half width of histogram in sky sigma | |
| (binsize= | 0.1) | Binsize of histogram in sky sigma | |
| (smooth = | no) | Boxcar smooth the histogram | |
| (rgrow = | 0.) | Region growing radius in scale units | |
| (mksky = | no) | Mark sky annuli on the display | |
| (mode = | ql) | ||
| PACKAGE = apphot | |||
| TASK = photpars | |||
| (weighti= | constant) | Photometric weighting scheme for wphot | |
| (apertur= | 20) | List of aperture radii in scale units | |
| (zmag = | 25.) | Zero point of magnitude scale | |
| (mkapert= | no) | Draw apertures on the display | |
| (mode = | ql) | ||
Links
[1] http://www.ctio.noao.edu/noao/content/13-m-SMARTS-Photometric-Calibrations-BVRI
[2] http://www.astro.yale.edu/smarts/smarts13m/photometry.html
[3] http://www.ctio.noao.edu/noao/content/13-m-photometric-standards