UT Jun 21, 2020: Astrometry of Ross 248 and GX And (and picture of M31)

Michael Richmond
Jun 21, 2020

On the night of Jun 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.


Ross 248

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 -17 C, as it was just a bit too warm to reach the usual -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 fifth measurement of Ross 248; the graph below shows the previous nights (red, green, blue, cyan) and tonight (orange).


GX And

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 51 degrees.

Using the same techniques as described for earlier nights, I matched detected stellar positions to the Gaia DR2 catalog.

The target is clearly moving in the positive RA direction, as we would expect from its known (large) proper motion.


M31

I tried taking images of M31, just for fun, as dawn approached. I set the guider running on a reasonably bright star, using a 5-second exposure time. The last motion I made before starting the guider was to push stars North on the finder screen, meaning that the telescope was moving _South_.

The tracking plot, shown below, reveals that the RA guiding was fine, but that the telescope did drift slightly (and steadily) in the Dec direction. The stars were moving North, so the telescope was moving _South_. In order to correct for this drift, the telescope would have to move North -- but that would involve backlash, as the last motion of the gears was in the opposite direction.

Conclusion: just before starting a guided exposure(s), nudge the telescope so that stars slide South on the screen.

I took a set of 50 images, each 20 seconds long. After combining them via a median procedure, the result looks, well, not bad, if one leaves the contrast low:

But when one pushes the contrast to show faint details, one notices two problems:

  1. the flatfielding isn't very good outside a small central region
  2. the vertical stripes show clearly, despite my usual approach to removing them. They aren't really visible on any individual 20-second exposures -- perhaps that's due to the larger noise overwhelming them.

I tried several tweaks to get rid of the vertical stripes, but nothing made much of a difference. Sigh. This may not be a good camera for pretty pictures.


Last modified 6/21/2020 by MWR.