Affiliation
None
Thu, 11/04/2021 - 07:07
I have often been told that differential photometry (even on very dim stars) should always use a V filter. A V filter passes light between about 500nm and 700nm. My CCD can see up to 1000 nm. By placing a V filter in the light track I am losing about 30% of the light emitted by the (sometimes very distant) star. Why not use no filter at all? I do not see the point of a distant star's light traveling half way across the galaxy, only to be dimmed 30% by a filter just millimeters before that light hits my CCD chip. Can anyone explain why a V filter is necessary for differential photometry?
I suppose you could do that for an exoplanet Ed, but then you would not have color data to, for example, discern if a possible exoplanet is really red star that very slightly eclipses a blue star. Also, astronomers may never have figured out stellar evolution without being able to tell what color a star is. Look at an HR diagram.
Also, you would miss why some stars can be variable.
If you favor no filter in order to go deeper, I suppose you would need to worry about stray light like infrared reflected back from the sky that could increase the noise in your measurements. You have some protection in that many camera's sensitivity falls off at UV and IR and peak in the V.
Ray
Hi Ed,
Your Sycamore Canyon Observatory looks great! What equipment do you have?
Unfiltered observations work well for some projects. Take for example the Center for Backyard Astrophysics (CBA) group run by Dr. Joe Patterson of Columbia. He studies cataclysmic variables, often looking for periodic signals that tell him something about the orbital dynamics of these binary systems. CVs are basically colorless stars, or at least you can get overlapping time series from contributing observers and therefore can adjust zeropoints to place the light curves on pretty much the same scale.
However, unfiltered doesn't work for everything. Take for example a red star near the minimum of its cycle. If your camera has a blue sensitive detector, and your buddy has a red sensitive detector, then even if you observe simultaneously, your blue sensitive unfiltered camera will measure the star as fainter than your buddy's system, since you have less effective signal on the red star. You can even be magnitudes apart in value, even if you use the same comp star. Now if this red star is a typical pulsating variable, it gets hotter as it approaches maximum, which means the star gets bluer. If the two of you again measure the star near maximum, you will record a relatively brighter magnitude than your buddy. So even though you are measuring the same star, the amplitude and light curve shape will be different. You may get the same period, but much of the remaining science is lost. If you are both using a V filter, then much of the wavelength-dependent sensitivity is limited by the filter, and your magnitudes and light curve shape will be much more alike. Similarly, if you both use two filters (like B and V), you can transform your data onto the standard Johnson system, and then your measurements will fall on top of one another - even though you are using two very different cameras.
So for most of the AAVSO targets, using a standard filter ensures that your measurements are taken with nearly the same bandpass and sensitivity, and therefore will be far more useful into the future. I agree that a standard filter decreases the throughput, but I'd rather work on somewhat brighter stars and do them right, thereby maximizing the scientific value of the observation that I worked hard to obtain.
Arne