Aug 06, 2013 UT: SN 2013ej through clouds

Michael Richmond
Aug 6, 2013

On the night of Aug 05/06, 2013, I observed SN 2013ej in M74. I barely beat the oncoming clouds, however, and only collected a few poor images.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with BVRI filters
• 30-second unguided (due to clouds) exposures

Notes from the night

• Just as I arrived at the Observatory, clouds approached from the west. I didn't have time to focus, but did get at least 2 images in each filter.
• FWHM varied from good (2.5 pix in B) to bad (3.5 pix in I).
• I used last night's twilight sky flats

SN 2013ej is a Type II supernova in the relatively nearby galaxy M74. It was discovered by the KAIT group some time (a week?) before maximum light. Here's a chart showing the galaxy, the SN, and some reference stars:

The reference stars marked above have magnitudes in AAVSO chart 12459CA, as follows:

 letter      B     sigB       V     sigV      R      sigR     I    sigI
  B        13.012  0.019   12.510  0.019    12.154  0.019   11.834  0.019
  F        13.848  0.026   13.065  0.022    12.622  0.025   12.152  0.027   
  H        14.338  0.029   13.692  0.024    13.329  0.029   12.964  0.030
  I        14.832  0.027   13.912  0.023    13.416  0.026   12.939  0.030
  K        15.192  0.034   14.613  0.027    14.275  0.034   13.915  0.036

There was no way to guide, given the clouds, so I went with 30-sec unguided images. After discarding the terrible images, I was left with 3 so-so images in B, 2 in V, 3 in R, 2 in I. The quality of these is poor, so the measurements will be somewhat unreliable.

Using aperture photometry with a radius of 4 pixels (radius of 7.4 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. I used the AAVSO magnitudes, plus color terms to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

Results from this morning are:

filter  mag         mag_uncert                          Julian Date

B =   12.715   +/-   0.028  (ens  0.005 zp  0.027)    2456510.82830 
V =   12.553   +/-   0.032  (ens  0.027 zp  0.018)    2456510.82669 
R =   12.321   +/-   0.043  (ens  0.006 zp  0.042)    2456510.82135 
I =   12.291   +/-   0.052  (ens  0.034 zp  0.039)    2456510.82505 

The uncertainties here are about equally due to noise in the images and transforming the instrumental magnitudes to the standard scale.

Grab the text file below for all the RIT measurements of SN 2013ej. All these values have been recomputed with the new color terms of UT 2013 Aug 05.

• sn2013ej_mags.out

Last modified 08/06/2013 by MWR.