Simultaneous photometry of hundreds of objects in AstroImageJ

This summer, I saw Tonny Vanmunster's  Facebook post: "Phoranso processed nearly 1,000,000 observations in a single run" -- about photometry of thousands of objects using his new Phoranso software. Nearly 1,000 FITS files, each containing over 1,000 variable stars, were analyzed.

Well, it was an impressive result. 'Can I achieve something like this using free software?' I asked myself. For many years, I've been using AstroImageJ -- a program with a not-so-polished interface, yet one that reliably does what it's supposed to.

Moreover, I had a task for it. I had just conducted a test observation session after a long break -- a ~3.5-hour observation of the vicinity of Stephan's Quintet. Not all frames were perfect (guiding was poor, probably due to gusts of wind), but I eventually selected 150 frames with 60-second exposures. Here's one of them:

Using a small Python script, I extracted the coordinates of 864 stars and had AstroImageJ place the apertures on the image:

 

I picked one of the stars as a reference -- after checking it wasn’t variable -- and ran the photometry.

I hadn’t noted the exact time, but the process took about 1.5 hours. As a result, I obtained approximately 130,000 instrumental magnitudes. Some of them were saturated, so I excluded those.

I wrote another Python script that generated a preview HTML file containing all the light curves. It looked like this:

 

 

 

Among hundreds of noisy patterns, I spotted several well-defined light curves that clearly belonged to variable stars. Well, these were known variables. Still, the approach worked -- and now I have a tool for performing simultaneous photometry on all the stars I capture in a single session.

One of them turned out to be quite interesting, and I'm now investigating it more closely.

 

Using ZWO ASI120MM-S planetary camera as a photometric device

Almost all professional photometric studies of variable stars are performed with CCD cameras. Many experienced amateur astronomers use CCD too, however alternative photometric devices, i.e. digital SLR cameras (DSLR), are quite popular too. Modern DSLR cameras use CMOS sensors. There are also plenty of CMOS-based cameras in the market made specifically for amateur astrophotography.
Uncooled CMOS cameras with small sensors are dedicated to planetary imaging, however, they also can be used for entry-level deep-sky imaging.
In the current study, the possibility of using such low-level uncooled CMOS cameras for differential photometry was investigated.
The author tested ZWO ASI120MM-S planetary camera as a photometric device. The camera has a monochromatic 1/3" CMOS sensor (4.8 x 3.6mm) AR0130CS. The pixel size is 3.75 x 3.75 μm, the sensor has 1280 x 960 pixels.  This camera provides 12 bit ADC.
The camera was attached to SkyWatcher 150 f/5 Newtonian on EQ5 motorized mount.
Sharp Capture software was used to gather images. Exposures = 10s, camera gain = 29 (it corresponds 1 e-/ADU accordingly to camera's specification).
There were two test run, a rapidly changing variable XX Cygni of SXPHE type was selected as a target. No filter was used. Preprocessing of images (calibration with flat, dark and bias frames) was performed using IRIS software (http://www.astrosurf.com/buil/iris-software.html). Photometry was done in AstroImageJ (a measurement of fluxes) with subsequent processing data in Excel.
Resulting standardized CV magnitudes were binned by 5 points. The light curves for the variable and a check star are presented in Fig. 1

AAVSO Chart X24817BFV
Comp Star   000-BJV-171
Comp V Mag  10.606
Check Star  000-BJV-173
Check V Mag 11.757

Observing conditions: almost full Moon in the 1st night, however, the sky was very clear; in the 2nd night the transparency was worse, there were haze and sporadic cirrus. This caused bigger random errors in the second data set.

Phase curve built using period and initial epoch from AAVSO VSX database is shown in Fig. 2.

We can see that the observed position of the maximum is shifted slightly from the predicted position. To prove the result comparison with ASAS-SN data was made, it is shown in Fig. 3 (my data marked as "Unfiltered with V zeropoint").

It is seen that positions of maxima are in excellent agreement. Probably the shift is caused by tiny period change.

We can conclude that even such a simple device gives good reproducibility of results and satisfactory precision. The next step will be testing with Johnson V photometric filter.