Affiliation
American Association of Variable Star Observers (AAVSO)
Tue, 11/15/2016 - 22:09
The AID accepts unfiltered observations with either V or R zeropoints (bands CV & CR). Under what circumstances are these data useful?
Tom:
The CBA monitors many Cataclysmic Variables to identify and study rapid temporal variations. An accurate magnitude measurement is of secondary importance. For the fainter targets, unfiltered imaging provides better SNR than can be obtained with a V filter in the desired cadence. A Clear filter magnitude with V zeropoint is acceptable in this case and permits comparison of data by multiple observers with modest telescopes.
Ken
From Page 21 of the AAVSO CCD Manual: https://www.aavso.org/ccd-photometry-gude
There are typically three cases where unfiltered observations are useful: when the source is known to have a neutral color [B-V = ~ 0] - whereall wavelengths are equally bright (typically in hotter objects like CVs in outburst) , when the object is very faint, and simply detecting the source has great value (as in gamma ray bursts) or where period-determination is the overriding scientific goal. [ ] are my add.
To fill this issue in a bit more here is the start of the paragraph regarding the three cases:
Properly reduced, your observations will relate better to those of other observers when you use
standard photometric filters. The reason is that each CCD chip model has a slightly different spec
-tral response. Without a filter, your observations could possibly still be useful for period analysis,
but the magnitudes you derive may be unphysical and differ greatly from those of other observers.
Not only will the results reflect the qualities of your particular CCD chip, but the fact that you are
imaging the entire spectrum of a star at once means that your observations could be many magni-
tudes brighter than what was seen visually or imaged with a V filter.
Ad Astra,
Tim Crawford
So one would presume that non-cataclysmic red stars with well-established periods would not be suitable...
Hi Tom,
It might not be best practice to use unfiltered photometry to deal with the types of stars that you mention but there are cases where you may be working on a Mira at minimum light and your system only allows a respectable detection without using a standard filter. In some cases, just knowing the nature of the variability has some value.
One area where unfiltered observations can be quite useful is when dealing with transiting exoplanet verification. Especially for the first pass on most targets, the observer is wanting to know that event actually happens at a predicted time and that the transit depth is close to what the survey has predicted. There are cases where the shape of the ingress and egress are an important quantity. In many of these cases unfiltered photometry can be very useful. In fact, unfiltered photometry can often be very useful in cases where having the highest S/N in the data is a big consideration.
In most cases the choice of which filters to use and what system to draw those filters from depends on the specific science goals behind making the observations.
Mike
The recent and ongoing campaign for high-cadence observations of the intermediate polar FO Aqr specifically requested CV (Luminance with V zeropoint) observations, probably because much shorter exposures are needed, keeping the cadence high enough to be useful.
I was just wondering about which spectral types would work best for these observations, and it turns out that FO Aqr is classified in VSX as "pec(e)", or peculiar, with eclipses (I think), so that wasn't very helpful.
As it happns, I just posted a screen shot from VStar showing simultaneous observations by David Lane, in Nova Scotia, and me in Vermont. Dave was using a 14" sct and a Johnson V filter, while I was using a 17" CDK with Luminance/V zero point. The data points line up almost exactly, so whatever color temp the target was expressing at that time, it must have been the "right" one for comparable CV measures. I have no V and B data from that night to get us to a B-V value, but it must have been close to 0. My exposures were just 20 sec, while David's were 60 sec in V. Since the target undergoes very rapid fluctuations, I believe the investigators suggested CV observations just to resolve this fluttering with amateur-size instruments.
Data from VStar attached, with David's V in light green, and my CV data in a darker shade of green.
Clear skies,
Brad Vietje
Newbury, VT
www.nkaf,org