Oct 19, 2018: Tests of PG2213 field and (433) Eros astrometry

Michael Richmond
Oct 19, 2018

On the night of Oct 18/19, 2018, Brent Neves and I used the 12-inch telescope to conduct some test observations.

Photometry of the photometric standard field PG2213

One of the tasks we must carry out periodically is a check on the photometric performance of our camera. As described in this lecture, determining the first-order color correction terms is pretty simple: just acquire images of a standard field through multiple filters, and then compare the instrumental photometry to values in the catalogs.

Tonight, we chose the field PG2213-006 as our target.

Field center (J2000) 22:16:28 -00:21:15 


#Full   RAJ2000   DEJ2000  Star          RA2000     DE2000    Vmag     B-V     U-B     V-R     R-I 
        "h:m:s"   "d:m:s"                "h:m:s"   "d:m:s"     mag     mag     mag     mag     mag 

    1  22 16 18  -00 22.2  PG2213-006C  22 16 18  -00 22 15  15.109   0.721   0.177   0.426   0.404
    2  22 16 22  -00 21.8  PG2213-006B  22 16 22  -00 21 49  12.706   0.749   0.297   0.427   0.402
    3  22 16 24  -00 21.4  PG2213-006A  22 16 24  -00 21 27  14.178   0.673   0.100   0.406   0.403
    4  22 16 28  -00 21.2  PG2213-006   22 16 28  -00 21 15  14.124  -0.217  -1.125  -0.092  -0.110
Here's a finding chart.

The main setup was:

Notes from the night:

This evening, we took 15 x 30-second images in V, I, Swann filters. That wasn't enough -- the I-band images, in particular, had very low signal to noise ratios. Even after co-adding all the I-band images, it wasn't enough to make good measurements.

So, next time, use at least 60-second exposures, and take 20 or more of them in each filter.

Astrometry of the minor planet (433) Eros

As practice for measurements of comets in the future, we chose a moving object with well-known properties to be the target for a short run. (433) Eros is an elongated asteroid with an interesting history. It happens to be passing relatively close to the Earth now, at a distance of only about 0.5 AU. That makes it a fast-moving, bright target for astrometry.

Below is an image of the field, centered around 



  04:22:57  +52:09:12  (J2000)

A chart of the field is shown below. The size of the chart is about 25 by 25 arcminutes. I've marked a few stars as references.

I've marked the location of several comparison stars. 



  star        UCAC4              B          V
----------------------------------------------------
   A        UCAC4 711-033429    10.356    9.951
   B        UCAC4 712-032780    12.053   11.153
   C        UCAC4 712-032697    11.830   11.197 

----------------------------------------------------

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

The sky value shows no clouds over this brief, roughly 45-minute observing run.

The number of objects detected -- I required 50 objects for an image to be included in the ensemble.

The FWHM is pretty steady.

The photometric zeropoint didn't vary much.

Using aperture photometry with a radius of 5 pixels (binned 2x2, each pixel is 1.34 arcsec, so a radius of 6.7 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 0.005 mag overall, which is good.

Here are light curves of Eros and the field stars.

Last modified 10/19/2018 by MWR.