UT Apr 19, 2020: Astrometry of Wolf 359

Michael Richmond
Apr 21, 2020

On the night of Apr 18/19, 2020, under good conditions, I acquired a set of observations of the nearby star Wolf 359. It's mostly just for fun, as part of a project involving the New Horizons spacecraft, but this might also provide some good test data for one of SoPA's capstone students next year. Note that I acquired a set of measurements just two nights earlier, too.

Wolf 359

As described in the New Horizons Parallax Program,

On April 22 and 23, New Horizons will take images of two of the very nearest stars, Proxima Centauri and Wolf 359. When combined with Earth-based images made on the same dates, the result will be a record-setting parallax measurement yielding 3D images of these stars popping out of their background star fields that the New Horizons project will share with the public.

We can't see Proxima Centauri from Rochester, but we certainly can detect Wolf 359.

The main setup was:

Notes from the night:

The object WAS located at this position in 2000,

  RA = 10:56:28.8  	  Dec = +07:00:52     (J2000)

but since it has such high proper motion (about 4.5 arcsec per year), it is now closer to RA = 10:56:23.6 and Dec = +06:59:58.3

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, which also appear in light curves below. In particular,

  star       UCAC4               B          V         r
 A          485-052029        10.530     10.157    10.122



I ran the camera at -20 C. Nothing out of the ordinary.

The sky value shows no evidence for clouds. The jump in sky value is due to the change in exposure time from 20 to 30 seconds.

Here's a record of the telescope's drift. The big jump occurs at the change from 20-sec to 30-sec exposure times.

The number of objects detected.

I used an aperture with radius 4.0 pixels tonight.

Below is the ensemble zeropoint as a function of time, for 30-second exposures only.

Using aperture photometry with a radius of 4 pixels (binned 2x2, each pixel is 1.24 arcsec, so a radius of 5.0 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 0.007 mag overall.

Here are light curves of the variable and the field stars. All the stars are pretty much constant in brightness over this short run. Again, this is for 30-second exposures only.

I used the AAVSO value for the V-band magnitude of star "A" to shift the ensemble magnitudes to the standard r-band scale (even though I used an "R" filter).

The primary purpose of these images was astrometry: the goal is to measure the position of Wolf 359 from Earth, and from New Horizons. We can then use the long baseline of almost 50 AU to measure the parallax of the star relative to the more distant stars in the field.

I grabbed stars within a 13-arcminute radius of Wolf 359 from the Gaia DR2 dataset, using the Vizier interface (thank you very much, CDS!). I extracted some 40-50 stars from each of my 20 images, and used my star-matching program to compare the lists and find an astrometric solution.

The results were pretty good: roughly 30 stars appeared in both the catalog and image lists, allowing me to compute both linear and quadratic solutions. The typical residuals in the fits were around 0.15 to 0.25 arcseconds in each coordinate; the values were pretty similar for the 20-second and 30-second exposures, and both were similar to the residuals found two nights ago.

When I applied the models to compute the position of Wolf 359, I found a couple of apparently significant features:

Items to determine in the future:

Last modified 4/21/2020 by MWR.