UT Jun 29, 2020: Astrometry of Ross 248 and GX And (and pictures of M31)

Michael Richmond
Jun 29, 2020

On the night of Jun 28/29, 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:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ATIK 11000 CCD camera
• R filter, 20-second exposures
• images binned 2x2
• no guiding
• focus in R at 0.630, FWHM = 2.4 pixels
• camera ran at T = -19 C -- too warm to reach -20 C

Notes from the night:

• the sky seemed bright and hazy when I arrived, but no evidence for clouds in photometry and sky was clear at dawn.
• dark -- no Moon at all
• temperature was very warm, and it was humid as well. Temp dropped from 73/22 at 2:40 AM to 67/19 at 4:44 AM
• at 3:26 AM = UT 2020 June 29 07:26, plus or minus 1 minute, I happened to look to the South and saw a third bright object near Jupiter and Saturn. The third object was similar in brightness to the two of them, so at least as bright as Saturn, V = 0.2. This object appeared north of Saturn, roughly as far from it as Jupiter; so Jup-Sat-object made a rough triangle. The position was very roughly RA = 20h 10m, Dec = -15. The object faded in 2-3 seconds and disappeared. I believe I noticed it after it was at its brightest -- I don't recall seeing it rise.

  Satellite? It reminded me of an Iridium flare, but all the old Iridium satellites have left orbit. Could it be a flare from some other satellite? No information about such a satellite in Heavens Above's list for this morning. CalSky.com doesn't provide any good candidates, either.

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 -19 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.

N.B. There was quite a bit of coma in the stellar images in the left-hand (eastern) side of the field. I have been using a box defined as

sr=0 sc=200 nr=1300 nc=1400

to restrict measurements from the pickoff mirror's shadow, but that includes a bunch of stars with coma. I tried using a more narrow box

cr=650 sc=1000 nr=1300 nc=1000

and found that it yielded slightly smaller residuals from the Gaia DR2 catalog positions. In the future, I ought to use the smaller box for all measurements.

(The measurements recorded for tonight are based on the old, wider box, however).

This is my sixth measurement of Ross 248; measurements are shown as red crosses.

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 two series of 50 exposures of the field, using the R-band filter and an exposure time of only 5 seconds. During the second series, especially, the sky brightened considerably as dawn approached.

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.

Combining the images via a median procedure yielded an image rather more pleasant than the one from last week; the stripes were not so pronounced. Maybe last week's conditions just weren't very good. Don't give up on pretty pictures completely.

Last modified 6/29/2020 by MWR.