PMAK V27 Revisited

More than a year ago I registered PMAK V27 (https://www.aavso.org/vsx/index.php?view=detail.top&oid=844387), a variable that I found in my DSLR frames of the T UMi field.

At that time it was classified as NL: (Nova-like, uncertain) based on that it showed long-scale variability and a 'noise' pattern [ASAS-SN data].

Now, as far as new data become available (more ASAS-SN observations, ZTF points, and even a handful of observations posted to the AAVSO database by SFRA observer), and (that is even more important) I gained experience, I decided to work with it some more.

Here is the combined light curve:

DC DFT analysis (VStar) showed a clearly-visible peak corresponding to a period near 2.4 days. After removing the slow trend the peak became even more prominent.

Then I built a phase plot:

Now it looks like a phase plot of a rotating star with spots!

So, the star was reclassified as BY (thanks to Sebastian Otero for the final decision).

Partial Solar Eclipse 21 Jun 2020 from Kyiv

Partial solar eclipse from Kyiv (Osokorky)

Begin (first contact): 8:29 UT+3

End (last contact): 9:14 UT+3

Canon EOS 600D + EF-S 55-2500f/4-5.6 timelapse

Maximum phase: 8:51 UT+3

Photometry of AE UMa

AE Uma is a short-period pulsator (SXPHE variable) with a period of 0.086017069 days (https://aavso.org/vsx/index.php?view=detail.top&oid=37163)
Two years ago I observed it with DSLR camera (Canon EOS 600D) attached to a Sky Watcher 150/750 Newtonian. Now I made observations using cheap uncooled CMOS camera ZWO ASI120MM-S. This camera positioned as a planetary and guiding camera. It has a small 1/3" chip, which is its main drawback. Yet a field of view of the setup (~16'x22') turned to be sufficient for many interesting variables, such as T UMi, RX UMa, AE UMa, etc.
Here is a comparison of data for AE UMa obtained using ASI120MM camera + Baader photometric V filter with the data obtained using Canon EOS 600D camera.
In both cases, data were transformed: for ASI120MM+Vfilter one-filter transformation was performed (using the previously defined Tv_b-v coefficient and an average color index of the variable); for Canon EOS 600D two-filter transformation using green and blue channels was done (again, using previously defined Tv_b-v, Tb_b-v, and Tbv coefficients).
Exposure per point was 45s for ASI120MM (for each point 3 frames by 15s were stacked) and 30s for Canon EOS 600D. So integration times are roughly compatible.
It is seen that the ASI120MM camera shows better photometric performance (lesser scatter).
The data reduction process with ASI120MM is simpler and much faster.
So in cases where the wider field of view is not required, ASI120 is probably a better choice. I plan to use it in parallel to DSLR for photometry.

Tv(b-v) transformation coefficient for Baader Johnson V + ZWO ASI120MM-S

Even having a single photometric filter (Johnson V), it is useful to determine Tv(b-v) transformation coefficient. This allows the transformation of measured V magnitudes if B-V color indices for target variable and comparison stars are known (assuming that B-V index for the variable does not change significantly).
One of the recommended AAVSO's star standard star fields is an open cluster Messier 67.
Observations of M67 were carried out on 25 Mar 2020 using ZWO ASI120MM-S uncooled CMOS camera with Baared Johnson V filter attached. Sky Watcher 150/750 Newtonian was used as an imaging telescope.
500 images with an exposure of 12s were collected. They were calibrated, as usual, aligned by stars and stacked to 5 stacks of 100 frames each.
Measurements were done using AstroImageJ software. 29 stars were measured, results were averaged by 5 stacks.
The resulting V-v on B-V dependency is shown in Fig. 1 (where V is a catalog magnitude, v is an instrumental magnitude, B-V is a color index).

Fig.1

Tv(b-v) transformation coefficient, determined for the slope of an approximation line, turned out to be equal 0.0086+-0.0042. It is rather small, so even untransformed observations with this filter should give reasonable values.

PMAK V31: a rotating variable of BY Draconis type

A new variable of BY Dra type has been discovered! Looking through my DSLR images that were taken last summer and autumn (in the vicinity of MQ Cas) I have detected a star that definitely had bigger scatter then surrounding stars. Further analysis using data from automatic sky surveys has clearly shown the light curve which resembled one for eclipsing binaries, however with a non-stable period. The star has prominent chromospheric activity (it is associated with 2RXP J001017.3+584051 X-ray source). So I have concluded that the star is likely a spotted rotating variable (could be looks like the star in the first figure -- "artistic interpretation"). It has been registered under an alias PMAK V31 in the International Variable Star Index (https://aavso.org/vsx/index.php?view=detail.top&oid=1500075). It is the first star of BY type in my collection of variables.

Red points in the phase plot are my observations (V band), others are data from ASAS-SN and ZTF surveys (g-band).

Observations have been conducted using Canon EOS 600D camera attached to a 150mm f/5 Newtonian, Muniwin software has been used for searching for new variables.

I should mention that my first assumption of the variability type was RS (also spotted, yet giant stars -- I discovered three of them previously, they had very similar light curves). Thanks to Sebastian Otero, who pointed out that the star has a dwarf luminosity, so it is likely BY.

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Переглядаючи кадри, відзняті влітку та восени, знайшов в сузір’ї Кассіопеї, на кутовій відстані приблизно 28 кутових хвилин від змінної MQ Cas (яка була основним об’єктом спостережень) нову змінну типу BY Draconis, яку зареєстрував як PMAK V31 у міжнародній базі змінних (https://aavso.org/vsx/index.php?view=detail.top&oid=1500075)
Ще один тип змінних в моїй колекції!
Змінні типу BY Dra -- зорі, дещо схожі на наше Сонце (тьмяніші), вкриті величезними зоряними плямами, які переважно згруповані в одній півкулі (приблизно так, як на малюнку — художня інтерпретація).

Зоря обертається і, коли на нас дивиться плямистий бік, вона виглядає тьмянішою. Крива блиску схожа на криві затемнюваних зір, однак періоди (інтервали між затемненнями) плямистих зір трохи змінюються з часом (справжні затемнювані змінні строго періодичні), також змінюється амплітуда коливань яскравості. Це пов’язано з еволюцією зоряних плям. PMAK V31 дійсно демонструє невеликі варіації періоду (це видно з аналізу даних автоматичних оглядів неба за різний час).

Ця зоря асоціюється з рентгенівським джерелом 2RXP J001017.3+584051, тобто випромінює помітну кількість рентгену. А це ще одне свідчення “хромосферної активності”, наявності сильних магнітних полів, які призводять до появи зоряних плям.
Спостереження, які привели до знахідки, проводились за допомогою камери Canon EOS 600D та телескопу-рефлектору SkyWatcher 150/750. Пошук змінних на зображеннях здійснювався за допомогою програми Muniwin.

До речі, існує метод оцінки віку сонцеподібних зірок за швидкістю їхнього обертання -- гірохронологія. Грубо кажучи, молоді сонцеподібні зорі мають більше плям і швидше обертаються, а старіші обертаються повільніше і плям на них значно менше [https://www.skyandtelescope.com/astronomy-news/star-spins-show-ages-010820143/] Знання про вік сонцеподібних зір допоможуть створити чітку картину еволюції нашої Галактики.

Measuring characteristics of ZWO ASI120MM-S camera

Using technics from The AAVSO DSLR Observing Manual (https://aavso.org/dslr-observing-manual), I have investigated my ASI120MM-S camera built on a CMOS sensor.
Measurements were performed in 1x1 bin mode. SharpCap was used to capture images, in native 12-bit mode (without 16-bit stretching!). Measurements were done using a central region of images.
A notebook screen covered by several sheets of paper (to reduce light) was used as a constant light source. A room was darkened.

*** UPDATE 2019-11-03 **** Measurements via SharpCap Sensor Analysis tool were made too.

Gain in e^-/ADU

The gain was measured through Average ADU vs ADU Variance dependencies. Here is a plot for Gain29:

A slope of the line gives a gain in e^-/ADU.
A dependency on the gain in e^-/ADU on ZWO gain is shown in the next plot. Note that measured values are in excellent agreement with those reported by the manufacturer (https://astronomy-imaging-camera.com/product/asi120mm-s)

New results obtained via SharpCap Sensor Analysis tool are almost the same.

*** UPDATE 2019-11-03 **** SharpCap Sensor Analysis tool results

Full well

Using measured system gain, it is easy to get the full well:

Full well for ZWO Gain = 0 (3.5 e^-/ADU) was measured as 14.3 ke^-. ZWO reports 13000 in "Product description" or 14.5 ke^- in the corresponding plot. New results obtained via SharpCap Sensor Analysis tool are almost the same.

*** UPDATE 2019-11-03 **** SharpCap Sensor Analysis tool results

Read-out noise (RON)

RON, measured from zero-exposure frames, was found smaller than reported by ZWO and almost equal for different gains: ~2e^-. This is strange. See also new results obtained via SharpCap Sensor Analysis tool.

*** UPDATE 2019-11-03 **** SharpCap Sensor Analysis tool results

These results are in the excellent agreement with those reported by the manufacturer (https://astronomy-imaging-camera.com/product/asi120mm-s)

Dark current

The AAVSO DSLR Observing Manual shows troubles while measuring the dark current of DSLR camera sensors. I have to admit that my results for ASI120MM-S are also quite uncertain. I've tried to estimate dark current from values of mean pixel intensity of dark images (1st way) and through standard deviation of mean pixel intensity. The camera has no cooler/temperature regulator which introduces additional uncertainty. The values of the dark current were estimated as ~1 .. 2 e^-/s/pixel at room temperature. Additional measurements are needed!

Linearity

The sensor was found to be very linear:

Strange behavior at low ADU for another CMOS sensors was mentioned by Mark Blackford for Canon EOS 600D (https://www.aavso.org/comment/47248#comment-47248) and by Christian Buil for cooled ZWO ASI1600MM camera (http://www.astrosurf.com/buil/CMOSvsCCD/index.html). Buil thinks that that behavior is caused by imperfect timing for very short exposures he used. I think it is not an exposure timing problem rather some features of the CMOS sensor itself (autotuning of dark level?). The next two plots were obtained using dimmer light and longer exposures, it is seen than "stranges" arose at the approximately the same ADU level:

*** UPDATE 2019-11-03 **** Table of results obtained via SharpCap Sensor Analysis tool

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.