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[vsnet-chat 863] Re: Magnitude Estimates for Current SNe
- Date: Thu, 30 Apr 1998 12:13:25 +0300
- To: observations@aavso.org, 76620.1721@compuserve.com, vsnet-chat@kusastro.kyoto-u.ac.jp, ISN_chat@mbox.queen.it
- From: Berto Monard <lagmonar@csir.co.za>
- Subject: [vsnet-chat 863] Re: Magnitude Estimates for Current SNe
- Disclaimer: The CSIR exercises no editorial control over E-mail messages originating in the organisation and the views in this message are therefore not necessarily those of the CSIR and/or its employees.
- Sender: owner-vsnet-chat@kusastro.kyoto-u.ac.jp
more specifically on SN 1998bn...
Hi to you,
I just reported my most recent result of visual observations on SN 1998bn for tonight as
SN 1998bn 980430.782 135 MLF*
* as measured vs 'd' star (assumed of being 13.2v)
star 'd' was identified in a previous posting as follows:
////my visual magnitude estimates of 13.5 were determined by comparison to star 'd' which I preliminary assume to be 13.2 v.
Star 'd' is positioned 20" S and 1' E of the 99 star shown on the USNO based chart, downloadable from the VSNET database.
It could be that star 'd' is somewhat fainter than 13.2, but hopefully not variable!! as I intend to also link my future magnitude observations to it.
It's my intention to adjust all my 'old' SN 1998bn estimates in future after an update of the sequences of the region, more specifically a calibration of star 'd'. ///
The 99 star above is identical to the 103 star near NGC 4462 on the preliminary AAVSO chart. The latter indicates a H/T magnitude, which is a positive point towards solving the sequence dilemma for the region, however.... the faint end of the sequence on that chart is from GSC, which as you know carries inaccuracies and inconsistencies due to zero-offset, probably some colour effects and other reasons inherent to 'bulk' calibrations. Tonight I did use that chart and made another set of estimates for the SN but now based on the GSC sequence: the result was 14.2 . This means that star 'd' would be of magnitude 13.9 in the GSC sequence.
Last night, before the AAVSO chart, I attempted a calibration of star 'd' vs 'regional' reference standard stars from the GSPC which are situated some two degrees towards the South and found quite a good brightness match of 'd' with GSPC star S506-E which is 13.34 V and B-V = 0.85. Its (2000) position is given as 12 29 18.7 - 24 54 20 . Due to the high colour factor (>0.7) I tend to around the magnitude upwards and consider it 13.4v... which I then endorsed as the magnitude for star 'd'. Based on this the SN shines at 13.7.
So I have three magnitude estimates for SN 1998bn: 13.5, 13.7, 14.2. If I have to choose which one I trust most to stand a comparison to photometric measures I would select 13.7 without any hesitation.
For future reporting I now have three options: to stick to my 'd' star of 13.2 v for continuity , to go GSC and hope someone will come up with proper field calibrations, or to use a modified version of the GSC values (a simple way is to subtract 0.5 from the values).
The point I wish to make is that the choice will be dictated by considerations of uniformity in measuring scales (sequences) amongst observers, rather than actual photometric accuracies. At least for now!
What do you suggest?
Regards,
Berto Monard,
Pretoria
!
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