Photometry of ASASSN-14jg

The three images from T31 finally arrived in my image list within VPhot at the AAVSO. Checking the images told me that the 300s and 180s exposures still gave stars that were too bright. The 60s image gives low values of ADU (counts) so I will take 120s exposures in future. The 60s V image is shown. This time I have set up one star as a comparison star and two stars as check stars. All three stars were from the Tycho catalog which includes their V and B magnitude. I intend to measure B magnitudes as well so that I can apply transformation coefficients and want to use the same comparison stars.

The problem becomes apparent - the target supernova only has a signal to noise ratio of 55 - general thinking suggest an SNR of 100 as a minimum value. If I use this image this is the result.

Now the 180 second image.

 

 The SNR of the target supernova is 114 - above the minimum but not by much. The large aperture is too big for the target. However even at this aperture size the aperture is too small for the comparison and check stars so any measurement would be inaccurate.

At 300 seconds the comparison star is saturated - it shows ADUs of 65535 so this will not work.

 So the problem is that I can not find any Tycho catalogue comparison stars(i.e. with available B and V reference values)  that are similar in magnitude to the target. If I could I could reduce the aperture size - getting a larger acceptable SNR and an accurate value from the comparison.

By taking a 120s exposure that will probably meet the requirements with the current comparison.

However by shifting the target off centre perhaps I can find suitable comparisons. That is my next task.

I could simply use the visual magnitudes (unfiltered) from other catalogues as being equivalent to a V filtered value to get an approximate indication of magnitude.

Using  UCAC3 stars as comparison and check stars

with a result

so an estimate of 14.75 mag. Note the check star measured value corresponds to the given value quite accurately.To be continued....................