Aug 02, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Aug 03, 2016

On the night of Aug 01/02, 2016, I observed SN 2016coj in NGC4125. I also observed the Landolt calibration fields PG1633, Mark A, and PG2213.

The main setup was:

Notes from the night

SN 2016coj is a Type Ia supernova in the relatively nearby galaxy NGC4125. It was discovered by the KAIT group some time before maximum light:

Here's a chart showing the galaxy, the SN, and some reference stars; the chart is about 12 x 12 arcminutes.

NGC 4125 RA = 12:08:05.7 Dec = +65:10:30 (J2000)

The AAVSO sequence team kindly provided photometry for stars near this object. You can see their full photometric sequence on their website. Below, I show only the members of that sequence which fall into my very small field of view -- taken from AAVSO sequence X16288FJI. Note that star "K" is so faint that I may not detect it clearly in B-band.

letter   B     sigB    V     sigV    R     sigR    I    sigI
B      15.198 0.086  14.133 0.052  13.627 0.116  13.155 0.156 

C      13.317 0.093  12.673 0.058  12.316 0.121  11.980 0.161 

J      15.607 0.109  14.956 0.065  14.603 0.136  14.271 0.182  

K      16.573 0.123  15.975 0.082  15.547 0.174  15.147 0.231  


I created a master dark frame for the SN 2016coj images by combining sets of 10 dark frames taken before sunset and after the SN images.

I took sets of 10-13 images in each filter, guiding in all filters. The guiding worked fine with an exposure time of 5 seconds. I discarded any trailed images.

I used focus settings based on relative positions measured for a bright star earlier. At a temperature of T = 24 C, I determined

Normally, the telescope drifts slightly to the South during long exposures. To avoid backlash in the Dec gear, I move to the field from the South. But NGC 4125 is so far to the West that it starts to move "under the pole" during the latter half of my observing runs, and that seems to reverse the telescope's drift. Therefore, before starting the I-band images, I nudged the telescope North, to reset the gears and avoid backlash.

As explained in the notes to Jun 14, 2016, I used the "rotsub" technique to remove the galaxy's light at the position of the SN.

On this night, I used "method 2", which means performing "rotsub" on each individual image, then combining all the resulting images in a passband to make a "master rotsub" image.

Using aperture photometry with a radius of 4 pixels (radius of 5.5 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 interim reference magnitudes above plus color terms which I am currently revising -- so please treat these results as preliminary to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

Note that in the graph below, I combine data calibrated with UCAC4 photometry (first few weeks) with recent data calibrated with AAVSO photometry. That's inconsistent, and I'll re-compute all magnitudes later. Note further that I use only 2 AAVSO stars (B and C) for calibration, for consistency with earlier measurements; I'll use additional stars in my final calculations.

     filter  mag         mag_uncert                          Julian Date

   SN  V =   15.308   +/-   0.041  (ens  0.035 zp  0.022)    2457602.60463 
   SN  R =   15.149   +/-   0.085  (ens  0.028 zp  0.081)    2457602.58785 
   SN  I =   15.016   +/-   0.097  (ens  0.044 zp  0.086)    2457602.62557 

Below is a preliminary light curve, based on RIT Observatory measurements. I also show measurements of SN 2011fe in M101, an ordinary type Ia supernova, shifted arbitrarily.

Last modified 08/03/2016 by MWR.