Photometry does not require a large telescope or a perfect sky – especially if you do the photometry with a CCD camera. Medium size refractor or even a telephoto lens is good enough to capture valuable data. And large sensor covers a significant field of view, so more variables can be measured in the same frame.
The weather forecast at the end of March was promising, so I have planned a few hours during four subsequent nights to capture photometric data with my travel setup that contains a 90mm aperture refractor and QHY268M APS-C format camera. This setup field of view is considerable 6 square degrees, so even random parts of the sky will contain several variable stars in that area. I decided to patrol four frames at 09h 30m RA and +54 to +62 Dec.
I did not use any specific photometric filter – just a UV/IR cut filter and used comp star V-band magnitudes. That kind of data is referred to as CV “filter” – Clear unfiltered data using V band comparison magnitudes. During each night I decided to make a series of over 100 exposures with 2 minutes of exposure time. QHY268M camera pixels have a significant capacity, so a wide range of star brightnesses could be covered. For a 90mm telescope and with a 2-minute exposure time, stars at about 9.5mag were already oversaturated, so all 10 magnitude and fainter stars could be measured. The measurement accuracy is related to the signal-to-noise ratio – the fainter star has lower SNR, and the measurement accuracy is also lower. Sky background brightness during these sessions varied between 18 and 19 mag/arcsec2 (light pollution plus Moon present), and 16mag stars on the single subframes had SNR over 10. That value indicates that magnitude can be determined with an accuracy of about 0.1mag – and that is still a useful measurement. Both these edge values mean 5-6 magnitudes range, and that is a decent number. Under a darker sky, it would be probably possible to get 1 more magnitude down.
The image above is a 1:1 fragment of the stacked 100 frames (photometry calibrated and annotated with ASTAP software). Limiting magnitude at this stack is about 20mag while limiting magnitude at a single 2 minutes frame is about 17.2mag.
Each captured field of view contains many variable stars. AAVSO variable star index (VSX) founds 30-60 variable stars in such field of view in Ursa Major. Some of them are of course fainter than my limiting magnitude, a few of them are too bright, many are long-period variables that changes cannot be determined during 3-4 hours, and some variables’ amplitude is too low for my setup. But still, around 10 variables for each 6 square degrees area were detected in each measured field.
I did an actual variable search and measurements with Muniwin software. You may read here and here more about what it is and how to do it. Measurements were uploaded to the AAVSO database where possible. Sometimes the star was present in the AAVSO catalog, but the unique ID was not yet assigned, so the data cannot be uploaded. And there are also some findings not yet available in the AAVSO database – after collecting some more data the star can be reported as a new variable.
The first plot is ZTF J092856.58+595604.4 star from the Zwicky Transient Facility Catalog of Periodic Variable Stars published in 2020. It is an eclipsing variable with 5.97 hours period.
The next star is CSS_J093737.9+553411 – also eclipsing variable at 7.37 hours period. Much fainter than the previous target, so the measurement error is larger here.
CSS_J094009.4+572734 star is the pulsating variable of RR Lyrae type with 6.86 hours period.
CSS_J094229.4+581237 is another EW type eclipsing variable – 8.45 hours period.
GSC 04135-00504 – HADS type pulsating variable star (High Amplitude Delta Scuti). The period is 88.5 minutes – it is amazing how such big object like a star can change its brightness in such a short time.
The minimum of the eclipsing variable LINEAR 7373205 is presented below. A little bit above 17mag – that is a little bit extreme measurement for that setup and conditions.
NSVS 2496401 – another EW type eclipsing binary star – 11.0 hours period.
NSVS 2517147 – it is EA type eclipsing binary, so the same type as Algol. 10.7 hours period.
QU UMa – classic pulsating RRAB type variable star. A brightness increase of over 1 magnitude during about 2 hours was captured.
V0447 UMa – pulsating HADS type variable star – period 101.8 minutes.
These are the selection of the variable stars captured and identified during the four sessions described above. You may search for them in the AAVSO database using the identifier provided and read more about their properties. Some of them did not have any observations before – so my measurements were the first ones since they were reported to the AAVSO!
There are also some more variables identified, but not as spectacular and with lower quality data (worse SNR). But there are also a few variables discovered that are not present in the AAVSO database – I need to collect some more data and probably report them to AAVSO as new discoveries 🙂
Clear skies!
