So, you've done photometry on your night's images. Now what? Submit the data to CBA. Here is how:
- First, choose a comparison star, which will allow you to determine whether any fluctuations in magnitude are unique to the star you are observing. The AAVSO has identified and compiled data on many comparison stars, which can be accessed at https://www.aavso.org/ . Once there, hover over the "observing" tab, hover over "variable star charts", and then select "variable star plotter".
- In the following window type the name of your variable star into name box and choose F as your chart scale. In the box asking to plot a chart or a table of filed photometry, choose to plot a chart. If needed, in the advanced tab you can select the limiting magnitude and orientation. When ready, click on the "plot chart" button.
- A plot should appear with your star marked by cross-hairs in the middle. There should also be other stars in the field with numbers next to them; these are the comparison stars. Choose one that is also in your images from the observatory (this chart has a larger field of view, so not all of them will be in your images). Keep this tab open, as you will need it later.
- Now that a comparison star has been chosen, it is time to work with the data. If you followed the steps in the photometry section of the wiki, then AstroImageJ should have spit out a huge data file (make sure that one of the comparison stars you used is the same as the one you choose in step 3). From this data file, keep the columns labeled "Label", "JD_UTC", "AIRMASS", "Source-Sky_T1", "Source_Error_T1", "Source-Sky_C2", and "Source_Error_C2" (please note that your comparison star chosen in step 3 may be C3 or C4, not C2, so choose the correct columns).
- Source-Sky_T1 is the number of CCD counts for your target star, which we must use to calculate the instrument magnitude of the star (you can do this in excel or a program you have written). This can be done using the following formula: m = -2.5*log(N/t). Here, m is the instrument magnitude, N is the number of photons, and t is the time in seconds. Do not forget to factor in the gain of our CCD, which is 1.5 photons per CCD count. The following webpage has more information if needed: http://image-analysis.readthedocs.io/en/latest/06_photometry_intro/instrumental_mags_and_the_beauty_of_photometry.html. You will likely get a very bright magnitude, which is because we have not factored in the constant. However, note that we do not care about the constant, since it will subtract out in the following steps.
- Next, calculate the error in magnitude using standard error propagation procedures and save it in a separate column.
- Repeat steps 5 and 6 for your comparison star.
- Subtract the instrument magnitude of your comparison star from the instrument magnitude of your variable star (this is where that constant would cancel out) and save it in a separate column. Do the same with the error in magnitude.
- Now it is time to determine the actual magnitude of your variable star. To do this go back to the AAVSO chart plotter. Instead of choosing to plot a chart, select to show a photometry table of the stars in the field. Once again, click "plot chart".
- You should now see a list of all the comparison stars in the chart you plotted along with their magnitudes in the V filter (which we usually use). To calculate the magnitude of your variable star simply add the difference in magnitude of the variable star and comparison star (calculated in step 8) to the magnitude of your comparison star in the photometry table. Do the same for the error and save the data in separate columns.
- Make an ascii (pure text) file, formatted as in the example below. The first column is time, reported as mid-exposure Julian Dates with 5 decimal digits. The source magnitude is the second column, which is the value calculated in step 10, and reported with 3 decimal digits. The third column is the photometric error on the magnitude. The airmass values should be given in the fourth column. The header of the file should include what filter you observed in, and what star you used as the comparison star.
- Sending the Data
- CBA: The data file should be attached to an e-mail and sent to: firstname.lastname@example.org with a subject like: Subject: MU Cam, 12 Jan 2016 (JD 400.514 to 400.657) that allows them to identify the target, date, and covered interval. If this is your first time submitting data, send your data file to Laura and/or Lilia to check and give you the OK to send to the CBA.
- AAVSO: To submit to the AAVSO head to www.AAVSO.org and go to Data > WebObs (Submit/Search Data). From here you can upload your file that was produced by the python code. Make sure you have your correct Observer ID entered. Follow the prompts to submit.
- Once your data has been submitted to both CBA and AAVSO rename the folder containing that nights observations with *_SUBMITTED
- Example: 2016_08_28 => 2016_08_28_SUBMITTED
Sample Data File Edit
# Variable: MU_Cam
# Date: 12-Jan-2016
# Comp star: 135_AAVSO_(V=13.482)
# Check star: 132_AAVSO_(V=13.219)
# Exp time (s): 45
# Filter: V
# Observatory: Michigan State Campus Observatory
# Location East Lansing, MI, USA
# Comments: Clear most of the night.
# JD Var_Mag Var_eMag Airmass
2457400.51433 15.121 0.019 1.255
2457400.51499 15.145 0.020 1.255
2457400.51564 15.147 0.020 1.256
2457400.51629 15.244 0.021 1.256
2457400.51694 15.267 0.022 1.256
2457400.51760 15.252 0.022 1.257
2457400.51825 15.222 0.021 1.257
2457400.51889 15.166 0.020 1.258