Getting started with Variable Stars, Some thoughts
If you are new to differential photometry, it is a fascinating field where amateur astronomers can make valuable contributions. We have many amateurs and professionals using iTelescope.Net to collect these data. But there are a few things you need to know before beginning.
First, you should go to the AAVSO web site and download their photometry hand book. You don’t have to be a member to get this valuable resource. Another good resource is Brian Warner’s “A Practical Guide to Lightcurve Phototmetry and Analysis.”
Second, you need to use a telescope that has a photometric V-filter and you need to use this filter for all your observations, at least in the beginning. Do not use the regular photographic filters, they are useless for photometry.
Third, you need to have some idea as to what exposure times will yield acceptable signal-to-noise (S/N) ratios. The precision of your photometric results depends on your S/N ratio. For 0.05 precision you need an S/N ratio of 100 or higher. So, how do you find out? There is no substitute for some hands-on practice. If M67 happens to be up, then there is no better practice than imaging M67 where there are a large number of standard stars. Search “M67 photometry.” The “gold standards” are Landolt fields (search on “Landolt fields”). Landolt Fields are accessible for both northern and southern observers. Finally, there are “Henden Fields” (Arne Henden is now director of AAVSO), but they favor northern observers. Take a series of images of 60, 90, 120, and 180 seconds. Take four or five images for each integration time. If you are an AAVSO member, make sure you have the V-filter and VPHOT enabled with your reservation. If not, you can use whatever photometry software you have once you calibrate your images.
The value of imaging standard fields is that you can use the standard stars as both targets and comparison star and try some ensemble photometry. You can compare known magnitude with the results you obtain from you various exposures. And, you can check our accuracy using different comparison stars. For example, if you use comparison stars that are similar in color to the star you are estimating, is accuracy better than if you use comparison stars that are of different colors? What about relatively low S/N ration to high S/N ratio? What happens when you stack the images from each exposure integration and then measure? Stacking can be the key to increasing S/N ratio while keeping integration times reasonable.
Imaging and measuring known stars (so well know that they are primary or secondary standards) builds confidence and skill. Your results should be fairly good if you work in the S/N ratio range of 100 or greater.
What to pick for your first variable? I suggest that you get into the AAVSO data for Mira-class variables. Pick a few that are actively being imaged (recent data) and try your hand at these variables. Most Miras vary slowly and you can compare your results with the results of others to see if you are on the right track.
Dr Ed Wiley