Dec 12, 2013 UT: SN 2013ej in M74 close to the Moon

Michael Richmond
Dec 14, 2013

On the night of Dec 11/12, 2013, I observed SN 2013ej in M74. The target was unfortunately less than 10 degrees away from the Moon, which caused big problems.

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 VRI exposures of SN 2013ej -- no more B

Notes from the night

The first-quarter Moon was only 9 degrees away from M74, and it washed out the SN entirely in V. I had to move the dome slit to cover the Moon as much as I could, but it also occulted part of the telescope's aperture. As the telescope tracked (and the dome did NOT track), a larger and larger fraction of the aperture was blocked, so the overall signal dropped and dropped. I moved the dome periodically, but variations of up to 0.7 mag in overall throughput occur in the data. The R-band images also show a strong sky gradient.
• FWHM was fair: 2.5 - 2.7 pixels.
• I took domeflats as usual.
• It was very cold -- temperature was 7 F/-14 C when I closed around midnight. Brrrr.

SN 2013ej in M74

SN 2013ej is a Type II supernova in the relatively nearby galaxy M74. It was discovered by the KAIT group about one 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

I took 30-second guided images in VRI, and on this night managed to guide in B-band, too. After discarding the bad images, I was left with 16, 27, and 26 images in V, R, and I, respectively.

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.

The SN is now so faint that I've stopped making measurements in B. And tonight, due to the Moon, I also made no measurement in V. I coadded (using a median technique) all the good images in each filter to create a single image with higher signal-to-noise. I now measure the SN in co-added images only. Below is a comparison of the results derived from individual images to those derived from the median image:

 median - individual               B         V         R         I
-------------------------------------------------------------------------
Sep 11   UT                      -0.098    -0.016    -0.003    -0.002

Sep 18   UT                       0.059    -0.005     0.009    -0.013 

Sep 19   UT                      -0.078     0.015    -0.007     0.003 

Sep 25   UT                       0.019     0.006     0.009     0.006 

Sep 27   UT                       0.073    -0.022    -0.005    -0.015 

Sep 29   UT                      -0.015     0.019     0.010     0.015 

Oct 02   UT                       0.026    -0.006    -0.002    -0.020 

Oct 03   UT                       0.015    -0.018     0.005     0.002 

Oct 08   UT                      -0.004     0.004     0.002     0.004 

Oct 09   UT                       0.001     0.011    -0.002    -0.011 

Oct 12   UT                       0.005     0.007    -0.004     0.014 

Oct 13   UT                       0.054     0.016    -0.003    -0.003 

Oct 24   UT                       0.056    -0.008    -0.010     0.027 

Oct 29   UT                                -0.016    -0.013    -0.021

Nov 03   UT                                -0.024     0.019     0.005

Nov 11   UT                                +0.154     0.009     0.020

Nov 20   UT                                 0.090    -0.012     0.028 

Nov 21   UT                                           0.041     0.085 


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I will report the results based on the median images below.

Results from this evening are:

filter  mag         mag_uncert                          Julian Date

R =   15.871   +/-   0.130  (ens  0.120 zp  0.049)    2456638.61406 
I =   15.465   +/-   0.156  (ens  0.154 zp  0.026)    2456638.62064 

The uncertainties here are dominated by extracting the instrumental magnitudes.

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

I don't really trust these latest measurements. I need one good night with no clouds and no Moon!

Last modified 12/14/2013 by MWR.