Project Leader: Peter Williams

Visual observers are invited to participate in monitoring of BL Tel for which ongoing observations are required. BL Tel was the subject of a long-term visual study between 1983 and 2000 with 8 eclipses recorded during that time. Results of those eclipses are summarised in Publications 25 (C99) of the VSS RASNZ (2000). BL Tel is an Algol type system well suited to visual observation and need only be followed during the 3 months centred on the predicted date of mid eclipse.

Project Description

Introduction

BL Tel was the subject of a long-term visual study between 1983 and 2000 with the recording of 8 eclipses during that time. Results of those eclipses are summarised in Publications 25 (C99) of the VSS RASNZ (2000). BL Tel is an Algol type system of mean period near 778 days and a visual range between magnitudes 7 and 10. Eclipses are of duration up to 2 months and ongoing observations of these eclipses are looking for any changes in the behaviour of this massive binary system. Observations are generally required for the 3 months centred on the predicted date of mid eclipse to ensure both the onset and completion of the eclipse are recorded.

Objective

The objective is to produce a complete visual light curve based on daily mean magnitudes and from this to determine the basic parameters of each eclipse – the date of mid eclipse, daily rate of brightness change, overall duration of each eclipse and the amplitude. These results can then be compared with those from the earlier eclipses and to look for any unexpected changes in behaviour.

Observations

Visual observers should obtain magnitude measurements on a daily basis, commencing a week or two prior to the expected onset of eclipse and continue a through to a week or two after the perceived end of the eclipse. BL Tel is suitable for observation with binoculars when at maximum and during the early and late stages of eclipse. Large binoculars or small telescopes will follow it through minimum.

A chart showing suitable comparison stars is included below.

All observations should be included in usual reporting to the AAVSO. Electronic measurements with DSLR cameras and multi-colour photometry is also most welcome and indeed encouraged. These observations can be ongoing between eclipses to monitor the small amplitude semi regular variations and secondary eclipse that are generally beyond the scope of visual observation.

Ephemeris for BL Tel (HJD/UT) based on epoch HJD 2445589.05

Eclipse ephemeris courtesy of the AAVSO Variable Star Index (VSX). https://www.aavso.org/vsx/index.php?view=detail.ephemeris&nolayout=1&oid=36332

Results

Results for the 2011 eclipse appear in the VSS Newsletter – August 2011.