| Photometry | ||
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Measuring the amount of electromagnetic energy emitted from a celestial object is called photometry. The measurements are commonly not done over the entire sensitivity range of the CCD but in a few well-defined broadband windows. The wavebands most commonly used are UBVR and I, ranging from ultraviolet to infrared. I use only BVRI (from Schuler/Astrodon) because my seal-level observation location isn't very friendly to the ultraviolet. The detector I use for photometry is a SBIG ST-9 CCD with a SBIG CW-8 filter wheel (on loan from DAC, Mörbyskolan, Danderyd) together with a JMI NGF-S focuser. For most photometry, the telescope is operated at f/6.7 using a focal reducer.
I don't know yet how "deep" I will be able to go. For 18 mag stars and assuming a target photometric error of 0.1 I estimate I have to make an approx. 60 minute long exposure in B and about half of that in V,R and I to get a good enough S/N ratio. Some amateurs take part in the search for supernovas in other galaxies. The only supernova discoveries by a Swedish amateur (as far as I know) are the four novas discovered by The number of photometric projects an amateur can do is stunning. Performed carefully, photometric data collected by amateurs can be useful to science. In the the field of variable stars, networks like AAVSO, BAA and CBA maintain databases over observations and enable advanced amateurs to contribute data. Unfortunately, there are only few variable star observers in Sweden at present. HG Lindberg is maybe the most active at present. Long-periodic variables like Mira-type stars are easier than the dramatic and fast-changing cataclysmic variables. Even more difficult is the search for transiting exoplanets, requiring a very high level of photometric precision. A nice project is to do photometry on an open star cluster. These are gravitationally bound groups of stars that are about the same distance from earth, age and composition but of varying sizes. Plotting each star in a colour-magnitude diagram (V to B-V) or two-colour diagram (e.g. R-I vs B-V) reveals a lot about the cluster and can be used to derive its age. In the future, I'll get a H-alpha filter as well. Photometry in H-alpha can be used for such diverse projects as estimating the star-formation rate in a galaxy or measuring the temperature of a white dwarf embedded in a planetary nebula. |
