161. V1830 SAGITTARII   (UG)   Range: 12.0-19?
     Position:   18 13 50.59  -27 42 20.4  (J2000)
     Magnitude: 
                                                            
ID      RA  (2000)   DEC       X    Y  N    V      ERR     B-V    ERR 

a) Comparison star sequence

1    18 13 45.5  -27 41 17   -70   65  1  10.342  0.001   0.035  0.001   
2    18 14 06.3  -27 38 31   207  230  2  10.938  0.004   0.629  0.021   
3    18 13 59.7  -27 36 19   119  362  2  11.630  0.016   0.553  0.010   
4    18 13 36.7  -27 37 55  -186  266  3  12.227  0.019   0.561  0.009   
5    18 13 43.3  -27 38 49   -98  213  3  12.366  0.017   0.173  0.008   
6    18 13 57.2  -27 41 34    86   47  3  12.656  0.016   0.520  0.013   
7    18 13 39.0  -27 39 52  -156  149  2  12.998  0.042   0.398  0.010   
8    18 13 52.3  -27 41 41    21   40  2  13.385  0.012   1.589  0.005   
9    18 13 46.4  -27 42 20   -57    1  2  13.589  0.025   1.470  0.004   
10   18 13 54.8  -27 42 07    54   14  2  13.929  0.017   0.602  0.008   
11   18 13 44.0  -27 42 35   -89  -14  2  14.418  0.012   1.258  0.010   
12   18 13 48.6  -27 43 03   -28  -42  2  14.886  0.006   1.643  0.003   
13   18 13 51.4  -27 42 51     9  -30  2  15.436  0.027   1.579  0.010   
14   18 13 50.5  -27 42 29    -2   -8  2  15.809  0.031   1.608  0.025   
15   18 13 49.9  -27 42 38   -11  -17  2  16.002  0.011   0.709  0.012   


b) Wide-colour extension for CCD calibration

5a   18 13 50.6  -27 42 12    -1    9  3  12.402  0.024   1.573  0.015  


Notes:

1. The close companion to V1830 Sgr, located only 8 arcsec to the north,
and here called 5a in the above wide-colour extension, is very red, and 
visually will appear to be about magnitude 12.7, using the transformation
v=V+0.2*(B-V). It should not be used as a comparison star, and is only 
included here because of earlier questions about its magnitude. Vogt's 
magnitude of 12.03V for this star is clearly incorrect.

2. V1830 Sgr was caught in outburst on a DSS SERC-V 4 minute exposure taken
on 18 May 1987. It is located 0.38 arcsec east and 8.01 arcsec south of the 
12.402V red star 5a discussed above. This gives V1830 Sgr the approximate
coordinates, using UCAC1, of 18h 13m 50.59s -27d 42' 20.4" (J2000).

3. The sequence given here is far from ideal as it contains too many red
stars. There are basically two ranges of stars in this sequence, those with
B-V near 0.6 ('normal' colour), and those with B-V near 1.6 (quite red).
Stars in the second range, namely 8 and 9, and 11 to 14, will appear about 
0.2 magnitude fainter visually than those comparison stars with more
'normal' colours.

4. Thanks to Arne Henden for obtaining the raw data for this sequence and
for his perseverance in obtaining photometry in this very crowded field.


Bruce Sumner
19 April 2000