Photometry without filters

Hi Lou,

I regularly use a Clear filter (which is compatible to no filter) for observations of Cataclysmic variables. If the B-V color is close to 0 then there is no issue with using a clear or no filter.

For magnitude estimations I use the V references of the AAVSO seuqences or UCAC4 magnitudes.

All this is done using LesvePhotometry as analysis program.

Regards,

Josch

I believe AAVSO recommends a standard filter set for all photometry. No filters means you are just relying on the detectors response curve, the stars colors, unusual emission or absorptions, etc. all of which can vary enormously. Such results are frequently inaccurate, can be misleading, of much less scientific value than properly filtered ones!

Although there are some types of observing in which unfiltered photometry is undesirable, it is an excellent choice in others. It really depends on what your scientific objectives are. It is most useful (1) if you want to study an object that varies extremely rapidly or (2) if small zeropoint offsets are acceptable. Otherwise, a filter would be better -- especially if the target is very red.

Cataclysmic variables are very well suited to unfiltered photometry. Some show variability on timescales of seconds, making it important to observe at a fast cadence. Filters require longer exposures, and longer exposures diminish your ability to detect rapid variations. As an example, let's say that you're observing the famous intermediate polar DQ Herculis in order to study (1) the 71-second spin period of the white dwarf and (2) structure in the eclipse light curve. Both objectives demand a fast cadence (less than 30 sec per image), but they don't require an accurate zeropoint. I think that many amateurs would struggle to get an acceptable signal-to-noise ratio with a filter on DQ Her in such a short exposure time, but it would be easy to do so without a filter. In this scenario, the improvement in time resolution by going unfiltered greatly enhances the scientific value of the data, outweighing the comparatively minor drawbacks.

If you're careful to choose relatively blue comparison stars (B-V < ~0.6) for a CV, my experience is that the resulting zeropoint errors are only a couple of tenths of a magnitude at worst, which is typically quite acceptable for CVs. CVs tend to be rather blue, so bluer reference stars should reduce the zeropoint error.

However, I wouldn't recommend unfiltered photometry of very red stars, as the resulting zeropoint error can be massive (potentially several magnitudes in extreme cases). Likewise, there are some projects in which color information is quite important, such as observing pulsations of RR Lyrae stars. In short, unfiltered photometry is fine in some circumstances, but having filters does increase your observing options.

Colin

Thank you all for your answers. I once had a small school project and used a "non-filtered magnitudes to standard magnitudes" method by Brian Warner. I asked for methods in general in the first post in the hope that there would be other methods suggested besides this one. But if anyone wants to look at it, it's in the great photometry book by Brian D. Warner.

Ok. I once had access to several hours of unfiltered image time of the same patch of sky from a meter telescope and wanted to determine the limiting magnitude. I only could acquire a few filtered images from another part in the sky and applied Warner's method, calculated the conversion terms and applied them to the instrumental magnitudes of the very faint point sources from the stacked unfiltered images. Well knowing that I had no true colour information for this part of the sky, I arbitrarily included several tenths of magnitude in the final standard magnitude error bar calculation. Would there have been any possibility of doing better in this situation without having filtered images from the deep observations sky region?

Thanks again,

Lou