Southern Eclipsing Binaries Project

Obtaining Times of Minimum

1.

First, obtain a sequence of images

You need a telescope that can track accurately, with a CCD or DSLR camera. To ensure your image timestamps are accurate, you should also synchronise your computer clock to a local NTS time server - Dimension 4 is widely used. Photometric filters are not needed, but data obtained using them can be used in more advanced research. For this reason, we recommend always using a Johnson V filter if you have it. We have a list of target EB systems. To select a suitable target on a given night, download David Motl’s freeware Ephemerides program, which will list eclipses visible from your site. When choosing a night’s target it can be a good idea to observe again any target you have previously observed – this helps make some of the minima data more consistent and reliable (three minima in an observing season is a good rule). When choosing a new target, find one that culminates a bit after midnight (so you’ve got a month or two yet to re-observe it) and the eclipse is at a high altitude and well away from twilight times. It is a good idea to find out a bit about your target first. The AAVSO VSX portal (Pick a star > VSX) is a mine of valuable information and important external links. Has the target been studied much - and if so what’s known? Are there other minima recorded (links to O-C Gateway and Atlas of O-C Diagrams, etc.); Light curves (links to ASAS-3 and CALEB), research papers (SIMBAD, CDS, ADS), catalogues (notably GCVS) and catalogue search, (VizieR) and field images (Aladin). It helps if you have a good software planetarium with access to major star catalogues such as Tycho, GSC and 2MASS (beware, GSC is useless for magnitudes). Check it to see if there are suitable stars in the imager’s field to act as comparison (C) and check (K) stars. C needs to be of a similar magnitude and preferably colour index; so does K but less stringently. On the night, first of all take some “ranging images” to check the integration time you need. Work at  an SNR on the variable (V) and C of 500 or more, preferably >1000. Ensure your maximum ADU is well within the linear part of your CCD’s response curve. If you can’t contain image drift over the integration period, choose a brighter target. Then image V for several hours around the predicted ToM.

2.

Photometry

Calibrate the images in the usual way, and use an aperture photometry package to obtain a light curve of the target (output <HJD of mid-exposure, magnitude, mag error> in TXT or CSV format). Magnitudes can be differential (V-C - we don’t need absolute mags) and certainly not raw. Record the comp star you used. Avoid ensemble photometry. Input that output to a minimum-measuring package (Tonny Vanmunster’s PERANSO is widely used) to estimate the HJD of minimum and its error. Do not use the Kwee-van Woerden method. Polynomial fitting is advised - order 5 is good. Always record the method you used.

3.

Reporting your minima

Make a copy of the Excel template ‘Minima Analysis Form’ (in Downloads) and rename it for your target V, e.g. ‘AO Vel Minima Form’. Enter all the minima for V in it. Preferably, keep it in our Dropbox under Target Folders in the folder for your V (create the folder if not there). We also intend to publish all minima timings by all observers for all targets every year or so in a refereed journal. To make sure your minima are included, add them to the Excel file ‘Minima & LEs <year>’ in the General folder of our Dropbox. Make sure you add in lexical order of constellation abbreviation (…Car, Cen, CrA,…) then lexically of GCVS ID in the constellation (…AO, GK, R, SZ, V0636, WS, … not the convoluted “official” naming sequence), then in HJD order for your target.

Forging ahead

What more can you do with your minima data? Find V’s light elements and period change, and move up to multi-filter imaging to make phased light curves for modelling.
Page author: TJR Last edit: 2016-08-08