On the night of May 20/21, 2020, under good conditions, I acquired images for astrometry of two nearby stars. One was Ross 248 , and the other GX And ; the latter is actually a double star, with two components separated by about 35 arcsec moving together.
One of the things I discovered while reducing this night's data is astrometric measurements of stars before and after May 16/17, 2020, cannot be combined , due to effects of an adjustment to the telescope's collimation at that time.
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.
The main setup was:
Notes from the night:
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:
I ran the camera at -20 C. Nothing out of the ordinary.
I took a series of 50 exposures of the field, using the R-band filter and an exposure time of 20 seconds.
Using the same techniques as described for earlier nights, I matched detected stellar positions to the Gaia DR2 catalog; as usual, I used a linear model and included all stars in the catalog.
This is my second measurement of Ross 248; the graph below shows the positions from the first night (red) and this night (green). There appears to be a shift of about 130 mas in RA -- but, after quite a bit of detective work, I've determined that it is an artefact of the collimation procedure I performed on the night of May 16/17, 2020. I saw a jump of roughly the same size and direction in measurements of GX And (described later in this document) and in measurements of Wolf 359, as shown in the page for May 16/17, 2020.
The bottom line is astrometric measurements of stars before and after May 16/17, 2020, cannot be combined , unless one is prepared to try to characterize and remove the offset.
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:
I took a series of 50 exposures of the field, using the R-band filter and an exposure time of only 5 seconds. The field was at an altitude of about 37 degrees.
Using the same techniques as described for earlier nights, I matched detected stellar positions to the Gaia DR2 catalog.
This is my second measurement of GX And, so there isn't very much to see so far. The offset in RA from May 12 to tonight -- about 130 mas -- is not expected from the known proper motion of GX And -- we would expect to see a change of only about 60 mas. This is another piece of evidence that the collimation adjustment on May 16/17, 2020, has caused a systematic jump in RA.
Last modified 5/21/2020 by MWR.