Dr Ed Wiley - iTelescope Science Advisor
Amateur Astronomical Research
From time to time I like to mention some of the books I have found valuable in my own research. But first, I should apologize for the term “Amateur Astronomical Research.” Fact is, if you are contributing valuable data and perhaps even publishing a paper (even one with bunches of authors), you are no longer an “amateur” in the usual sense: you are a “self-funded astronomical researcher.” You might not have a degree in astronomy, but just remember, Darwin had a degree in theology, not biology, Newton had a Masters and the Ph.D. as we know it today in the sciences is a German 19th Century invention. Being a scientist is not about degrees, it is about gathering data and either analyzing those data and seeing it through to publication, announcing discoveries and having them verified, or making those data available to other scientists.
OK, you want to do some science, so where to start?
Probably the very best place start is Robert Buchheim’s book, The Sky is Your Laboratory. In 293 pages, Buchheim covers just about every field of astronomical research that is within the reach of the self-funded astronomer. Of particular interest to GRAS observers in the data gathering and analysis area (Chapters 4 and 5) are Project H, CCD photometry of variable stars, Project I, determining asteroid light curves, Project J, extra-solar planet transit, Project L, asteroid astrometry (comets too!). Naturally, my favorite is Project M, measuring visual double stars.
Many of our iTelescope.Net observers are more interested in discovery. This is covered in Chapter 6 where Project O covers asteroid discovery, Project P treats comet discovery and Q and R cover nova and super nova searches.
Along the way Buchheim discusses the basics of photometry (some of the best explanations I have read), measurement error (fundamentally important, measures without error are as useful as those with errors), how to interact with agencies such as the Minor Planet Center, time, catalogs, data mining, literature and a host of other important topics.
Not directly useful to iTelescope.Net observers are the discussions of equipment. Why? Because iTelescope already provides us with just about all the high quality equipment one could ask for.
Dr Ed Wiley
Cataclysmic Variables Catch Your Fancy?
One of our most experienced iTelescope variable star observers, Bill 'Dingo' Dillon from the AAVSO, loves to catch Cataclysmic Variables (CVs) as they go into an outburst. Running a friendly race with David Levy to be first to catch them in action.
CVs are actually binary systems, with a white dwarf primary and a donor secondary. The white dwarf accrues matter from the donor, rich in hydrogen, and this accreted matter forms an accretion disk around the primary. This is an unstable condition and when a sufficient amount of the accretion disc falls into the primary the density and temperature of the hydrogen rises high enough to ignite nuclear fusion, rapidly burning the donor star hydrogen to helium. This causes a spike in magnitude, an outburst.
There are many classes of CVs. Right now AAVSO has an active observing program for several CVs classed as Z Camelopardalis dwarf novae. “Z Cam” binaries have the peculiar characteristic of becoming “stuck” between an outburst maximum and a resting minimum, called a standstill. It’s a 10 – 40 day cycle.
The AAVSO 2010-2011 Z Cam campaign is now on. The campaign contains both known Z Cam pairs such as RX Andromeda and suspected Z Cam pairs such as TW Triangulum. So there are both opportunities to collect data that will help us understand known pairs and the opportunity to help discover the nature of suspected pairs.
To get in on the campaign, visit the AAVSO Cataclysmic Variable Section and look for the link to the Z Campaign link.
Dr Ed Wiley
Eclipsing binaries and comments on Imaging Exoplanets
If you are thinking about jumping in and trying you hand at exoplanets the first thing you need to try are eclipsing binaries. Just like exoplanets, EBs vary in magnitude because a fainter companion is orbiting the primary star in such a way that the transit is in line with Earth, causing magnitude to vary.
The primary difference is, you guessed it, the signal-to-noise (S/N) ratio needed to successfully obtain a light curve. For EBs, S/N can be quite modest, in range of 0.01 magnitude error. This is because the magnitude variation of many EBs can be as much as 1 magnitude or more. And, you are facing only the usual S/N variation.
With exoplanets the variation in magnitude is quite small, on the order of less than a 0.03 magnitude drop or less. This puts a premium on S/N and the average successful exoplanet observer is look at S/N rations on the order of 0.002-0.005 magnitude (2 to 5 mmag). Further, many exoplanets are bright, which means that you have to worry more about phenomena like scintillation noise in addition to the usual sources of noise (including noise caused by measuring aperture size). However, just like EBs, exoplanet work has one saving grace; it demands precision, not accuracy.
In other words, it is not the actual magnitude of the primary that is of prime importance, it is generating an accurate timing of the minimum and the shape of the light curve that are of primary importance.
So, you say: “Ed, how do you image and measure exoplanets.” I answer: “I don’t!” It is just not something I have done. But if you want to give it a try, I can suggest a book about exoplanet observing that has been invaluable to me in understanding sources of error that have proven valuable for regular photometry.
“Exoplanet Observing for Amateurs” is Bruce L. Gary’s outstanding book for the amateur contemplating exoplanet research. Gary takes a very common sense approach to guiding us through the demands of this most demanding field of amateur photometry. Even if you, like me, are not particularly interested in exoplanets, you will benefit greatly from Gary’s discussions of calibration, setting differential photometric apertures, sources of noise, calibration and a host of other topics.
Understanding the demands of this most rigorous program has helped me in my less demanding pursuits of eclipsing binaries and other variables. Better yet, the book is available as a PDF for free and as a hard copy (which I recommend) for a very modest price and also visit Bruce’s web site.
So, you are still interested in exoplanets and are not put off by the rigor of the program. Begin with some easy eclipsing binaries using the AAVSO EB program as a guide. Start with some fairly easy ones that vary in their light curves and work up to the really “hard” ones that vary only slightly. Finding the target and times are easy, a day-by-day ephemeris is available for the program EBs through the AAVSO program and instructions for using the ephemeris are available at the RAS Wiki on the EB page along with several links I hope you will find useful. And, make sure to see if any of our GRAS observers are imaging exoplanets and ask them about their observing program.
Dr Ed Wiley
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