Photometry from online Digitized Sky Survey plates

Abstract

Online Digitized Sky Survey (DSS) material is often used to obtain information on newly discovered variable stars for older epochs (e.g. Nova progenitors, flare stars, etc.). We present here the results of an investigation of photometry on online DSS material in small fields calibrated by CCD sequences. We compared different source extraction mechanisms and found that even down near to the sensitivity limit, despite the H-compression used for the online material, photometry with an accuracy better than 0.1 mag rms is possible on DSS-II. Our investigation shows that the accuracy depends strongly on the source extraction method. The SuperCOSMOS scans, although retrieved with a higher spatial resolution, do not give us better results. The methods and parameters presented here allow the user to obtain good plate photometry in small fields down to the Schmidt plate survey limits with a few bright CCD calibrators, which may be calibrated with amateur-size telescopes. Especially for the events mentioned above, new field photometry for calibration purposes mostly exists, but the progenitors were not measured photometrically before. Also, the follow-up whether stellar concentrations are newly detected clusters or similar work may be done without using midsize telescopes. The calibration presented here is a 'local' one for small fields. We show that the method presented here gives higher accuracies than 'global' calibrations of surveys (e.g. Guide Star Catalogue-II (GSC-II), SuperCOSMOS and the US Naval Observatory Astrometry Catalog B).

Digital Access to a Sky Century at Harvard: Initial Photometry and Astrometry

“…The functional form of this particular calibration curve has been determined by Bacher et al (2005), in terms of four parameters which are in principle measurable from the image, or may be fitted for. The Bacher function enables very precise calibrations using only a limited number of reference stars, which need not extend much fainter than the magnitude at which there cease to be saturated pixels in the cores of the stars.…”

“…In order to calibrate the instrumental magnitudes we need a fitting algorithm: in the case of fixed aperture there is a known functional form (Bacher et al 2005), but for ISO and image size there appears to be no universal function. The literature contains many approaches, ranging from polynomials and splines (e.g., Tinney et al 1993).…”

“…This was (and still is; for example, Berdnikov et al 2007) performed by eye. Interpolating between closely (Bacher et al 2005), and isophotal magnitudes (red points) fit with the non-parametric rlowess algorithm. Residuals from the two fits are plotted in the middle (aper) and bottom (iso) panels; in each case two rms values are displayed: before and after removal of 3σ outliers.…”

A survey for variable young stars with small telescopes: First results from HOYS-CAPS

“…1); ii) Using a least-square optimization find the best fitting eight parameters for the function f (m i ) that minimize m r − f (m i ). Equation 1 shows the parameterisation of f (m i ), where P4(m i ) denotes a 4 th order polynomial and the other term is a photocurve function proposed by Bacher et al (2005) and Moffat (1969); iii) Determine the calibrated magnitudes for all stars using m = f (m i ); iv) Plot the difference m − m r against the calibrated magnitudes to check if the calibration has been successful (see bottom panel in Fig. 1…”

Comparative performance of selected variability detection techniques in photometric time series data

“…The digitized images were processed with VaST following the procedure described by Sokolovsky et al (2014a). APASS B-band photometry of 1600 UCAC4 stars in the magnitude range B=10-16 is used to calibrate the instrumental magnitude scale using the relation between aperture photographic and photoelectric magnitudes proposed by Bacher, Kimeswenger & Teutsch (2005). We identify 23 variable stars including five not previously known (Table 4, Fig.…”