[AZ-Observing] NCP magnitude sequence
- From: Brian Skiff <Brian.Skiff@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Sat, 28 Dec 2002 02:35:11 -0700 (MST)
A few weeks ago Steve Coe started a thread about determining better
numbers for sky-quality and visual magnitude limits. One way to get part
of this information is to view a field of stars whose magnitudes are well
determined. One such field is the sequence around the Ring Nebula observed
by Arne Henden (USNO-Flagstaff), which I included in a 'Sky & Telescope'
article a year or two ago. With several seasonal fields like that, one could
have comparisons available all year. Similar data in the fields of several
open and globular clusters are included in (unbashed advertisement coming...)
Chris Luginbuhl's and my observing handbook. (There are of course other
sources.)
Another possibility is to use something near the celestial pole. Although
visible on any night, it has the disadvantage of being relatively low in the
sky, and not necessarily favorably placed with respect to light-pollution
sources at other than fully dark sites. At any single site, however, it would
have the advantage of consistency. You won't see as faint as is possible
overhead, but the point is to assess the sky (not your skills) in some fixed
manner.
As part of calibration of the Guide Star Catalogue, a popular catalogue
of stars in star-chart software, the Space Telescope Science Institute
arranged to have magnitude sequences established near the center of every
6x6-deg or 5x5-deg Schmidt sky survey field for the entire sky. (This is
called the 'Guide Star Photometric Catalogue', GSPC.) The one closest to the
North Celestial Pole is in northernmost Ursa Minor, roughly 18h45m +88.8 near
lambda UMi, and less than 1.5 deg from the Pole. Thus the change in altitude
during the night or from season to season is small, and the field can be used
for sky-quality checks.
There are two series of GSPC calibration data. One was done in the early
1980s with just a handful of stars between mag. 8 and 15 for the original
catalogue. More recently a survey has been made by taking a set of CCD images
centered on the faintest GSPC star and using whatever other stars happen to be
in the CCD frame to extend the sequence to fainter limits. Good quality
photometry in the second batch usually reaches to mag. 16.5-17.
The combined magnitude series is shown below. The stars P001-A to -G are
the original set of stars, which are spread over roughly a half-degree field
(low power in a telescope). The remaining stars (starting from P001-F) are
from the second GSPC, and fall within about a 10'x10' field of the single
large-telescope CCD frame, so can be viewed at high power visually.
I show V magnitudes plus B-V and V-R colors, which can be ignored for
visual use. I have omitted some of the fainter stars due to their having
close companions that would be problematic. Faint stars with magnitude
errors > 0.05 were also dropped, which cut everything fainter than mag. 15.8
except one suspect star. The resulting run of magnitudes is not especially
smooth: you'd like them to occur at say 0.2-mag. intervals over the whole
range. But depending on telescope aperture and sky conditions, there should
be several useful stars to judge limits. At the very least, repeated views
of the sequence will give you a good feeling for how bright different stars
are in your telescope.
Again this isn't about testing your skill but making a measurement of
sky-quality in a consistent way. You'll find that even under the same
conditions your limit will probably shift somewhat fainter on repeated visits
until you've familiarized yourself with the field (after perhaps half a dozen
visits on separate nights), then will stabilize given similar states of the
sky. You can compare the limits at different magnifications, various amounts
of light-pollution or crud in the air, and see how Moon phase affects your
views in a quantitative way.
It should be straightforward to extend this sequence _brighter_, so that
it could be used for naked-eye and binocular checks. The bright end would be
set by Polaris (V=2.02, a small-amplitude variable, but not obvious visually
so the variation can be ignored), delta UMi (V=4.36), and lambda UMi (V=6.4,
also slightly variable and red. A few other mag. 5 and 6 stars are north of
+88 Dec. I wouldn't go any farther afield than this, but instead choose the
remainder from within a degree or so radius from lambda UMi to extend the list
to mag. 10. The Tycho-2 photometry (corrected to standard V) is good enough
for this, although many stars close to the pole have more reliable data from
other sources.
Perhaps someone could make some suitable charts to go with this list
to post at one of the club Web sites showing the stars in both direct and
reverted orientations for telescope with even and odd numbers of reflections.
\Brian
-------------------------------------------------------------------------------
Telescopic magnitude sequence near the North Celestial Pole
version: 27 Dec 2002
position sources (column 's'):
T Tycho-2
G GSC-ACT
g GSC-2.2
Name RA (J2000) Dec s GSC V B-V V-R remarks
P001-A 18 08 11.80 +88 49 22.1 T 4660-0186 8.22 1.07 HD 187138
P001-B 19 14 45.55 +88 46 59.7 T 4660-0230 10.47 0.42 BD+88 119
P001-C 18 06 03.44 +88 42 00.9 T 4660-0130 11.17 0.53
N001-AAAE 18 47 46.20 +88 46 11.0 T 4660-0196 11.68 0.25
P001-D 18 08 53.10 +88 37 31.9 T 4660-0183 11.82 0.51
N001-AAAB 18 49 14.33 +88 41 40.6 G 4660-0052 12.22 0.74
N001-AAAI 18 27 06.65 +88 38 58.3 G 4660-0036 12.26 0.37
P001-E 18 19 44.33 +88 43 38.3 G 4660-0179 12.48 1.27
N001-AACC 18 27 37.39 +88 37 38.2 G 4660-0063 13.06 0.36
P001-F 18 43 02.95 +88 41 34.3 G 4660-0189 13.07 0.60 0.37
P001-G 18 35 15.19 +88 42 35.9 G 4660-0145 13.08 1.40 0.72
N001-AABV 18 38 56.30 +88 40 36.3 G 4660-0164 13.12 0.44
N001-AABZ 18 33 51.30 +88 39 02.6 G 4660-0049 13.17 0.59
N001-AABU 18 33 54.78 +88 40 37.3 G 4660-0226 13.48 0.53
N001-AABH 18 31 28.54 +88 45 04.8 G 4660-0111 13.70 0.38
N001-AABM 18 39 38.54 +88 42 59.7 G 4660-0001 13.94 0.60
N001-AABG 18 51 42.86 +88 46 25.5 G 4660-0153 13.96 0.47
N001-AABI 18 22 52.60 +88 44 25.8 G 4660-0022 14.33 0.41
N001-AABY 18 22 25.53 +88 38 57.3 G 4660-0110 14.64 0.32
N001-AACF 18 53 34.87 +88 38 13.6 g 15.17 0.49
N001-AACH 18 51 00.25 +88 37 13.1 g 15.58 0.44
N001-AACE 18 53 48.86 +88 38 52.5 g 15.67 0.63
N001-AABR 18 47 25.86 +88 42 08.5 g 15.77 0.48
N001-AAAP 18 35 16.65 +88 44 16.0 g 16.80 0.50
--
See message header for info on list archives or unsubscribing, and please
send personal replies to the author, not the list.
Other related posts:
- » [AZ-Observing] NCP magnitude sequence
