Monday, April 13, 2020

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.






Tuesday, April 7, 2020

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.