[Aavso-photometry] N CYG 05 (V2361 CYG) sequence measures

[Richard Miles]

The Dutch variable star observer Reinder Bouma has made a chart of this nova already, based on Tycho-2 V (J) and TASS-IV (V) magnitudes.  I have carried out simultaneous high-precision V and I photometry of this field and the following results are based on 50 consecutive images taken on JD 2453417.27 - 2453417.30 with twin Takahashi FS60C scopes and Starlight Xpress SXV-H9 CCD cameras using exposures in the range 30-40 sec.

Analysis was performed on sets of 10 images -  5 sets in all.  Airmasses ranged from X=3.8-4.9.  Since the field is relatively crowded, a 15-arcsec radius aperture was employed and offset sky apertures were specially selected to be uncontaminated by faint field stars.  The quoted errors are standard deviations of each of the 5 sets of measures.  Transformation and extinction corrections have been applied.  Absolute photometry was employed relative to an ensemble of four Hipparcos stars (99454, 99571, 99248, 99181) in the same field as the sequence.  Extinction was calculated based on previously-derived exo-atmosphere zeropoints for each system.  The extreme range in airmass of all the stars in the sequence including the Hipparcos stars was 0.13 atmospheres for the first set, increasing to 0.20 atmospheres for the last set as the altitude of the target decreased.  The standard error in the 5 measured zeropoints in V was 0.003, 0.004, 0.003, 0.004 and 0.004 mag.  I expect the absolute error in V to be about 0.010 mag for the 6 brightest stars in the sequence.  Star '102' is from the AAVSO 2005+39 chart.

Star,  V Mag,  Err in V,  V-I Color,  Err in V-I
V 2361:  10.681,  0.014,  2.599,  0.013
'94':  9.384,  0.004,  1.149,  0.003
'99':  9.915,  0.005,  0.364,  0.020
'101':  10.120,  0.007,  1.195,  0.015
'102':  10.329,  0.004,  0.396,  0.029
'103':  10.248,  0.006,  0.545,  0.016
'106':  10.592,  0.006,  1.159,  0.005
'109':  11.223,  0.016,  0.733,  0.042
'112':  11.365,  0.012,  0.661,  0.024
'120':  11.954,  0.024,  0.665,  0.048
'126':  12.470,  0.020,  0.714,  0.040
'134':  13.237,  0.054,  1.07,  0.12

N.B.  The standard errors are given in the case of Stars '126' and '134'.

I have omitted Star '115' as I noticed this evening that I inadvertently measured TYC 3154 3470 (the star midway between the nova and '115') for which I obtained:
Star,  V Mag,  Err in V,  V-I Color,  Err in V-I
TYC 3154 3470 :  11.120,  0.013,  2.440,  0.011
I shall measure Star '115' tomorrow and report the findings shortly.

This is the first time I have utilised the system for generating comparison star sequences.  Note that the system has a cut-off due to faintness at about V=13-14: the exact limit dependent on galactic latitude (star crowding).  The bright cut-off limit is about V=2.

Richard Miles (MXL)
British Astronomical Association - Variable Star Section
Golden Hill Observatory, Dorset UK.