Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 11/23/2019 - 21:27
Hi I'm learning photometry using a Johnson-Cousins V filter and also have narrow band (6nm) Halpha, O3 and S2 filters that I use for occasional imaging. Are these filters useful/acceptable for photometry?
Peter
Peter has corected that he meant V not U.... however, I am unable to delte this post, I get the following message and I am signed in: You are not authorized to access this page.
Peter,
As an FYI, U band is typically the most difficult of the JC filters for which we can find photometry to include in the typical BVRI sequence.
I have been trying to develop a small list of survey resources that are suitable for U band inclusion in a sequence.
Please email me directly if you are in need of U band values for sequence inclusion and I will see if I can locate values for you to work with. Frankly, I suspect that quite a bit of the time I will not be able to help you out with U band values... however, nothing ventured nothing gained.
Good Observing & Ad Astra,
Tim Crawford, Sequence Team
tcarchcape@yahoo.com
My apologies that was a very bad typo on my part. I should have said V filter not U. I will correct the initial post
One issue is that the passbands of the narrow band astrophotography filters would not correspond to those of scientific filters used in standard photometric systems.
Roy
Thanks Roy,
I can see that as a significant issue but wonder how often astronomers might pursue studies of Halpha, O3 and S2 that could use measurements with those filters. It may be that such studies are invariably carried out by spectroscopy but thouught I'd enquire anyway.
Peter
Hi Peter,
Narrow-band filters are an imaging way of doing spectroscopy. Halpha and Hbeta have been used for decades by the professional community, though at a low level. For example, Hbeta can be used as a luminosity indicator for blue stars and a temperature indicator for solar-type stars. Halpha can be used to discover HII regions in external galaxies, and to find faint planetary nebulae in the Milky Way. The MDW Halpha Survey (https://www.mdwskysurvey.org) is an example of this, and was the subject of a recent Sky and Telescope article).
For doing photometry through narrow band filters, one recent example is from Munari et al.(2015NewA...40...28M) where they used Halpha in conjunction with other filters to monitor the evolution of novae. Mike Joner (2012AAS...22043802J) has developed an Halpha/Hbeta system for studying emission line objects.
CCDs have the advantage that they are 2D imagers. You can use the two dimensions to cover wide areas of sky and thousands of objects simultaneously, but at one passband. Used with a spectrograph, you cover wide wavelengths for one or a handful of objects, but don't get the wide coverage. So narrow band filter, wide field, photometry is a closer approximation to spectroscopy for specific spectral lines. That said, you have to calibrate the technique, either through the use of standard stars, or by using a "continuum" filter that is near the narrow-band wavelength so that the difference between the measurements in the two filters gives you a calibrated strength of the emission line.
So yes, the narrow band filters can be used for science, especially in the study of emission line objects. There are fewer professionals doing this kind of work, and so your selection of targets will be more limited and you will have to search to find researchers. I'd start with Mike Joner, as he is an AAVSO member, and then branch out from there.
Arne
Hello!
I'd add couple of papers that I recall from 1990-s and beginning of 2000-s. There is a series of papers from Pigulski et al. about variable and Be stars in open clusters. They used in their studies Halpha index (based on two Halpha filters with FWHM of 3nm and 20nm) with R-I colour:
https://ui.adsabs.harvard.edu/link_gateway/2001A%26A...376..144P/PUB_HTML
http://articles.adsabs.harvard.edu/pdf/1997AcA....47..365P
They refer also rather nice analysis of using Halpha and Hbeta narrow-band filters (Goderya et al 1994).
https://ui.adsabs.harvard.edu/abs/1994ApJ...426..159G
Great research can be done with narrow band filters, but IMHO usually it is kind of challenging to compare those results with others.
Best wishes
Tõnis
I'm exploring a similar idea for visual observing and have received supportive comments from spectroscopists that it's worth exploring. Most amateurs do not flux-calibrate their spectra / perform simultaneous photometry.
Observers reporting H-beta variation, or even H-alpha on the brightest targets, could alert the spectroscopy community to Be outbursts. This might connect variable observers and amateur spectroscopists in a new way.
One challenge is non-standard bandwidths of commercial emission-line filters as mentioned above. There is also the possibility of continuum brightness drops, which may not show variation in narrowband. For visual observers, the list of targets would be short.
It would probably be better if the AAVSO community created charts specifically for those wavelengths, but I'm not sure there is enough interest at the moment. I'm a visual-only observer, but if anyone would like to try an experiment or collect data feel free to message me. A star like QR Vul could be an ideal starting point.
Andrew
As an example, the HOYS-CAPS project
http://astro.kent.ac.uk/~df/hoyscaps/index.html
invites amateurs to submit images of star formation regions in many filters, inclufing H_alpha (and then the project will perform photometry on those images). As I understand it, the idea is not that you submit a ton on Halpha images only, but you should submit images taken thru different filters of the same field per night (kind of extremely low resolution spectroscopy).
HB