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Observations of the binary system V548 Cygni using STELLINA smart telescope

Oct 01 2023 05:00 AM | Tidu in Articles

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Stefano Giardinelli

10 October 2022

Author Note

Thanks to Marco Milletti for his useful ideas and contributions.

Abstract

In the last couple of years I used STELLINA (Vaonis) to take pictures of classical deep sky objects with some nice results:

https://www.cloudynights.com/gallery/album/22291-stellina-vaonis/

Now I have decided that it’s time to start using this fantastic small robot telescope for some scientific purpose. In particular my challenge is to explore the capability to detect faint luminosity variations using photometry. Having as a final goal to observe exoplanet transits, I present this first project consisting in the successful detection of the light curve for the binary stars system V548 Cygni during one eclipse.

Keywords: variable stars, photometry

Introduction

STELLINA by Vaonis is a smart telescope (80mm aperture, 400mm focal length refractor) that appears to be a smooth plastic box about the size of a small suitcase, and even when it's in operation, there is no eyepiece to look through. It connects via Wi-Fi to your smartphone/tablet where you have installed a clever app (Singularity by Vaonis) that controls and manages the observations. It is quite exciting to see the images of your celestial targets, captured by the integrated 64Mpixel camera, appearing and stacking as jpeg’s on the screen of your tablet. At a deeper level, STELLINA is also able to produce FITS images during the observations, while, in the so called “expert mode”, it is possible to set the exposure times and the gain of the camera. There is also an easy way to generate dark files.

The system observed is V548 Cygni, a typical Algol eclipsing binary (EB) consisting of a main-sequence primary (more massive) star of spectral type A1 V and a cooler star of spectral type of F7. The orbital period is 1.805 days. The visual magnitude of the object varies around 9.

Observations

In the night between 5 and 6 October, V548 Cygni happened to have an eclipse with a predicted duration of 5 hours at about 6.5 UTC (universal time), as indicated in https://www.milwaukeeastro.org/variable/binearyEphemeria.asp. The observations started at 21:15 pm local time in Los Alto CA, corresponding to 4.25h UTC. The STELLINA integrated camera was set to take shots of 10 sec exp FITS at a gain of 27db. STELLINA was stopped at 2:30 am (9.5h UTC) after it had produced a total 1028 FITS images during the whole observation period. After that, with the telescope closed, I got 10 dark FITS files with the same camera settings.

Photometry

I chose the free software AstroImageJ to measure the luminosity of V548 Cyg. It is a simple and powerful tool to perform aperture photometry. There are many tutorials available online. I found this one quite complete and clear on YouTube: https://youtu.be/qr2W_fS7XmI . First of all I created a master dark FITS file, using the 10 dark FITS, to be subtracted from the single FITS to produce calibrated images. Then I selected 112/1028 FITS images (one every 3 minutes) to be calibrated and processed with AstroImageJ. To perform astro-photometry it is necessary to find, in the image field, a reference (or comparison) star having well known, constant magnitude. It is very important to locate another constant magnitude star (check star) to monitor the process. I have followed the directions of Yuan-Gui Yang et al. (fig 1).

fig 1

The reference star is HD189234 with a constant visual magnitude of 7.48. When you launch AstroImageJ opening a FITS image you can do plate solving thanks to a link with Astrometry.net, so that you immediately have a mapping of the field stars (fig 2).

fig 2

My target is in the middle of the field, here labeled HD189371, another name for V548 Cygni. The reference star (HD189234) and the check star (HD189370) are on the center top. When performing the aperture photometry, you start entering the known visual magnitude for the reference star (in this case 7.84) and then the software gives you the resulting magnitudes for the target and the check stars (fig 3).

fig 3

In this particular image, we have magnitudes 8.89 for the target and 8.326 for the check. Repeating these steps for the 112 selected FITS images we get all we need to plot the light curves for the target and the check stars.

Results

Let’s start analyzing the data for the check star (fig 4). The light curve is flat, confirming that HD189370 is a constant luminosity star. The standard deviation of the visual magnitude values is 0.030. We can assume this variation to be the statistical experimental error. The fluctuations of this order of magnitude or less, are not significant.

fig 4

Now let’s focus on the target star results (fig 5). Clearly the light curve has the characteristic shape due to a transit. The total time of the eclipse, from star to end, is about 5 hours (from 4.25 to 9.25 UTC) and the minimum magnitude peak is at about 6.25 UTC. These results are perfectly compatible with the predicted ephemeria.

fig 5

Discussion

The objective of this work was to test the ability of STELLINA in detecting the variations of the visual magnitude for a binary eclipsing star. The presented results clearly confirm it. The next step could be the attempt to detect the magnitude variations of a rotating asteroid.