On the night of Nov 20/21, 2020, under good conditions (until the clouds came early), I acquired images of the eclipsing variable star RT And; one of my capstone students will be analyzing the data later this semester. Tonight, I checked to see if 20-second exposures in B-band could yield good signal-to-noise; the answer is "yes."
I also acquired images of Ross 248 and GX And for our continuing study of their parallax and proper motion. I took both "short" (2-second) and "long" (4-second) exposures of GX And, and found that they produced exactly the same position; but the uncertainty was a bit smaller in the "long" exposures, even though the central pixel(s) might be slightly saturated in the target.
You can find some basic information on this star at the SIMBAD's page for it.
The main setup was:
Notes from the night:
The object is at
RA = 23 11 17.9 Dec = +52 59 59.2 (J2000)
A chart of the field based on pictures tonight is shown below. The size of the chart is about 38 x 26 arcminutes.
The stars "A", "B", and "C" appear in the charts and tables for this field made by the AAVSO. see
The stars "A", "B", and "C" appear in the charts and tables for this field made by the AAVSO. see
I used the star labelled A = "102" to shift my measurements to the "B" magnitude scale. It has r = 10.336 according to the APASS catalog.Here's a picture of the TV with the finder's field of view when pointing at RT And:
The dark current was normal this evening.
The sky value shows that the sky was clear until clouds arrived; I cut off the analysis at JD = 174.575, when clouds first arrived.
The number of objects detected.
I used an aperture of 5 pix = 6.3 arcsec for photometry.
Using aperture photometry with a radius of 5 pixels (binned 2x2, each pixel is 1.25 arcsec, so a radius of 6.3 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.
Sigma-vs-mag plots show that the floor was about 0.003 mag with 20-second exposures. Very nice!
The single jump in the photometric solution is due to one image during which I slewed the telescope.
This is one of the stars that a capstone student may study over the next year in a project involving parallax. Ross 248 is a relatively faint red star surrounded by many other stars of similar brightness, so it's a good candidate for high-precision parallax measurements.
These observations involved:
The object is (currently) near position
RA = 23:41:55.27 Dec = +44:10:06.38 (J2000)
A chart of the field is shown below. The size of the chart is about 41 x 27 arcminutes. The noisy area at right (West) is the shadow of the guider's pickoff mirror.
I've marked the location of several comparison stars.
star UCAC4 B V r ------------------------------------------------------------------------- A UCAC4 671-120730 12.617 10.689 B UCAC4 671-120688 C UCAC4 671-120749 10.987 10.663 P kappa And 4.06 4.14 --------------------------------------------------------------------------
I took a photo of the finder TV's screen when pointing to Ross 248; this could be a useful reference for the future:
The sky value shows no sign of clouds.
The number of objects detected.
The FWHM.
Here are the positions I've measured so far. Note the clear motion to the south-east (lower-left).
Like Ross 248, GX And is a nearby (binary) star which will be the target of a parallax project in the coming year. One of the two components is bright -- about mag V = 8 -- so one must use short exposures to prevent it from saturating the detector. That may mean that this system isn't as easy to measure as Ross 248 or some others.
The object is currently close to this position:
RA = 00:18:28.4 Dec = +44:01:31 (J2000)
but it does have a very high proper motion.
A chart of the field is shown below. The size of the chart is about 41 x 27 arcminutes. The noisy area at right (West) is the shadow of the guider's pickoff mirror.
The two components of the GX And binary sit inside the box. I've marked the location of several comparison stars as well.
star UCAC4 B V r ----------------------------------------------------------- A 671-001473 9.939 9.790 B 670-001639 9.413 8.472 C 671-001509 12.712 11.421 11.001 -----------------------------------------------------------
I took a photo of the finder TV's screen when pointing to GX And; this could be a useful reference for the future:
Using the same techniques as described for earlier nights, I matched detected stellar positions to the Gaia DR2 catalog.
I took two sets of images:
The normal exposure time for this target has been 5 seconds, but I noticed that the sharp focus tonight caused the central pixel(s) of RT And to approach the 16-bit limit of the camera, 65535 counts. As a test, I took these two set of images, figuring that the "long" set might suffer from some saturation, but the "short" set would not. Would it make a difference in the derived positions?
The answer was "no, it didn't matter." The position derived from each set was identical, down the sixth digit after the decimal place in both RA and Dec (in decimal degrees, so about 4 mas). However, the "long" exposures included more reference stars (an average 37, versus 31), and better signal for the reference stars as well. As a result, the standard deviation from the mean positions were
My conclusion is that the "long" 5-second exposures I've been using are fine, especially if the seeing or focus isn't great.
The target is clearly moving in the positive RA and Dec directions, as we would expect from its known (large) proper motion.
Last modified 11/21/2020 by MWR.