H alpha photometry

Hi Phil,

Our "Filters" webpage (https://www.aavso.org/filters) gives a value of 6563 Angstroms for "HA".

Best regards,
Sara

Above, Sara has apparently misinterpreted your question and answered with the nominal wavelength.  There is probably no standard, for the simple reason that the H-alpha line width depends on a number of factors, including but not limited to stellar mass (leading to a value of the local gravity), magnetic field, rotational velocity, and temperature.  Radial velocity of course results in a systematic shift in the nominal wavelength.  Also, there are a variety of technical approaches to building the actual filters, with different associated costs.

To decide what's best for your application, independent of price, you should list the bandwidths of the stars you plan on observing, and pick the narrowest bandwidth that spans all of them. 

There is a pretty decent overview of existing photometric system (maybe not exhaustive), several if not many have one Halpha filter. Sometimes, another one is used for continuum measurement.

http://ulisse.pd.astro.it/Astro/ADPS/Systems/index.html

More experienced people can comment further, but I suspect that there is not much use of single Halpha filter measurements except mapping Halpha emission in the space. Halpha index (Halpha - some-other-filter) is another story, that can be successfully used e.g. to detect emission line objects. Any Halpha filter minus e.g. Cousins R can be useful here, R for continuum estimate. However, such system has to be calibrated to be really useful.

Common Halpha filters are usually centered on Halpha rest wavelength (in vacuum?) but may have very different effective widths.

Best wishes,
Tõnis

Thank you for your comments.  I have been thinking about how off-the-shelf H-alpha filters might be used for long term monitoring of emission stars by AAVSO observers using differential photometry.   

Suppose a "standard" H-alpha filter, or at least a standard band width, were selected from those currently available from Astrodon or Baader (or others) and a standard color index could be agreed upon, say HA-Rc mentioned by Tonis,  Would this be a useful photometry system for following emission stars (in conjunction with standard BVRI photometry)? 

Of course HA magnitudes would also have to be determined in this system for a comp star (or comp star sequences) in the fields of the targets.

Phil

Hi Phil,

Narrow-band filters are poor man's spectroscopy, but work very well,m especially on extended objects.  The usual method is to use a nearby continuum filter.  Then subtracting the Halpha photometry from the continuum photometry yields the strength of the Halph emission/absorption line.

As you mention, the hard part is calibration.  You can certainly do differential work for a target vs. comp and look at trends, like strengthening of weaking of an emission line in a binary system.  To compare your results with others would require calibration of the comparison star.

Any Halpha filter will work.  The narrower ones give more contrast to the Halpha line; the wider ones handle broad emission features and radial velocity shifts better.  The continuum filter is often around 645nm.  In many cases, you can even use an Rc filter as a continuum filter, though it contains the Halpha line and so biases the result.  Astrodon used to have a 645nm filter.

For references, there is a little bit of information in my book, Astronomical Photometry.  There are papers by Crawford in the 1980's.  Look for Ulisse Munari's paper on observing novae, which includes Halpha photometry as well as using some continuum filters like Stromgren b.  Looking at the MDM Halpha Survey web site and the November S&T article show you a little bit of what is possible with an Halpha filter.

Arne