I wanted to share the coolest project that I have done with my equipment so far; photometry using PixInsight and RobPer to determine periods/plot light curves for three variable stars. When I was putting this project together I noticed that not many people were taking advantage of the incredible functionality for photometry in PixInsight so I figured it might be helpful to post publicly. Happy to answer questions but on a high level this was my process:
1) Test equipment and determine exposure time. Basically we want to make sure we are not going to get saturated pixels for the target in question
2) Take a bunch of exposures over multiple nights. One of the stars, V0436 UMA, has a very short period (a matter of hours) and BF Ser a bit over a day. Other stars or objects you investigate may even have longer periods
3) Take flats and darks just like you would for your images
4) Calibrate your images (I used CCDStack here but PixInsight also works). Again same as you do for images
5) Run the AperturePhotometry script in PixInsight. Here you are going to want to play around a bit to ensure your aperture correctly fits the star. The AAVSO guide and PixInsight documentation are helpful here. PixInsight also offers a PSF option and an option to use a wavelet model for the background. I played around with these and both produce excellent results. You do need to pick the right comparison catalog for the next steps (Tycho or Gaia) and PixInsight will fetch information from Vizier for your spreadsheet
6) Next you will get a Excel sheet (really CSV file) which you will use to perform ensemble differential aperture photometry. I used Excel for this but I could have easily done everything in R. I used the formulas from the popular AstroimageJ tool (see documentation in paper below) but you could also use those in the AAVSO or other references. Note you want to choose a check star and then 4 or 5 comparison stars for your ensemble. You want to search for your stars on Simbad and VSX to check for signs of variability. Your comparison stars should be in roughly the same area of the image, have roughly the same magnitude, and have roughly the same color index. We also need to calculate measurement uncertainty
7) Now you want to export the measured light curve and uncertainty terms along with Julian date (PixInsight provides) into a CSV to determine period. For periodicity determination I used the RobPer R package. It has five different models for period fitted with four different regression algorithms. Three of the regression choices are "Robust" which means they are less sensitive to outliers than ordinary least squares. Fitting in RobPer requires 1 line of code. For the curves here I used m regression with a huber function + second-degree Fourier model. You can then plot the periodogram (see example) or use some code to determine most likely period. The periods I determined matched VSX almost exactly
8) Now we take the model parameters and make a pretty plot in Excel, R, Tableau, etc...
With a little bit of additional work you could also submit your data to AAVSO.
What about the fact that I used a Chroma G vs scientific V filter? The chart below shows the relationship between flux measured using PixInsight photometry and the Tycho 2 V magnitude.
Note I have listed all exposures in March 28th for convenience but actual collection took place over March 28th, 30th, April 1st, and April 3rd.
RobPer - https://cran.r-proje...bPer/index.html
PixInsight - https://pixinsight.c...Photometry.html
AstroimageJ paper for formulas - https://arxiv.org/abs/1701.04817
The AAVSO Guide to CCD Photometry - https://www.aavso.or...ometryGuide.pdf
All images are also on Astrobin. https://www.astrobin...2bxrrd/?nc=user
Edited by akulapanam, 15 June 2021 - 12:33 AM.