This web page provides a list of B, V, g', r', i', z'
magnitudes for 47 stars in M67, based on two all-sky observations
of six Landolt star fields with primary standard stars for B- and
V-bands (Landolt, 2009) and g'r'i'z'-bands (Smith et al, 2002).
The coordinates for the center of this star field (Star#2) is
08:51:23 +11:48:02 (J2000). The "observing season for M67 is
centered on January 30 (when it crosses the meridian at local
midnight). I'm currently preparing a table of 40 stars with
calibrated BVg'r'i'z' magnitudes for an open cluster n Perseus
(NGC 1342), with an observing season centered on November 19; a
web page with these results can be found at link.
Figure 1. Finder chart for the 47 stars with secondary calibrations determined by Bruce Gary at the Hereford Arizona Observatory.
Estimating Total SE for Each Star
As described below, the magnitudes in the Fig. 2 listing (and downloadable text file) have been corrected for small effects determined by treating each Landolt star field as an "unknown" and compiling "errors" from the true magnitudes and those solved for using my analysis procedure with the other Landolt stars. For the first all-sky observing session (2012 Apr 16) these corrections were small for B, V, g', r' and i' (-3, +0, -12, -9, +4 mmag), but for z'-band, which had the poorest SNR, the corrections was significant (-49 mmag). For the second all-sky observing session (2012 May 2) these corrections were small for all bands B, V, g', r', i' and z' (-9, +1, +5, +2, -2, +11 mmag). Each of the corrections from the "Landolt unknown" procedure has an uncertainty, and the orthogonal sum of the correction and associated uncertainty, for each band, can be used as an estimate of that band's systematic error. Total SE for each star will depend on the star's brightness, since stochastic SE varies with star brightness and total SE is the orthogonal sum of stochastic SE and estimated systematic SE. For each band the stochstic SE versus magnitude was modeled such that reduced chi-square was close to 1 (see Fig. 4, upper-right panel). The following listing is an average of the two all-sky observing sessions.
Figure 2. Secondary standards magnitude listing, including total SE, based on two all-sky observing sessions. If systematic offsets for each band were removed the values in the SE columns would be much smaller.
Here's a downloadable text version of the above list: M67GBL v2515
Comparison With Other Catalogues
The above magnitudes are in approximate agreement with Henden
2000 for B- and V-bands (average difference = +15 and +12 mmag). I
converted the Henden 2000 B- and V-band mag's to g'r'i'z' using
conversion equations given by Jester 2005 (http://www.sdss.org/dr6/algorithms/sdssUBVRITransform.html).
The results were compared with my g'r'i'z' masurements and
differences are given in Table 1.
The AAVSO web site has APASS magnitudes (DR5) for BVg'r'i'. My
magnitudes differ from APASS for these bands by amounts shown in
the following table. Apparently the APASS magnitudes are very
The old Priscilla Benson mag's for M67 agree with Henden 2000; my
V-mag's differ from hers by amounts given in the following table.
Brian Skiff has a short catalog of M67 magnitudes for BVRcIc (N =
20), and only 6 stars are included in my Fig. 2 list. My
magnitudes are compared with his in the following table.
Here's a summary of Fig. 2 magniutde comparisons with these
Table 1. Comparison of Fig.2 Magnitudes With
Other Catalogs (Fig. 2 Magnitude Minus Catalog Magnitude)
|Henden 2000, N=23
||+0.015 ± 0.006
(RMSi = 0.027)
|+0.012 ± 0.003
(RMSi = 0.015)
|-0.006 ± 0.006
(RMSi = 0.027)
|+0.019 ± 0.005
(RMSi = 0.023)
|-0.006 ± 0.005
(RMSi = 0.024)
|+0.005 ± 0.006
(RMSi = 0.029)
|APASS (DR5, N=2), N=27
|Priscilla Benson, N=6
|Brian Skiff, N=6
||+0.022 ± 0.012
||+0.014 ± 0.007
||+0.000 ± 0.010
||+0.018 ± 0.005
||-0.004 ± 0.007
||+0.005 ± 0.006
|Suggested Correction to Fig. 2
In this table the italicised entries are based on converting BVRcIc to g'r'i'z' using conversion equations given by Jester 2005 (http://www.sdss.org/dr6/algorithms/sdssUBVRITransform.html). The last row is my suggested correction to the Fig. 2 listing based on my subjective "consensus" of comparisons with other catalogs. I've been "conservative" iwth the g'r'i'z' adjustments because most of the the catalog comparisons use conversion equations. Here's a listing of the "adjusted" magnitudes.
Figure 3. Secondary standards magnitude listing, including total SE, based on two all-sky observing sessions and adjusted for compatibility with other catalogs. The brightness differences between stars (within a band) is much smaller than implied by the total SE values.
Here's a downloadable text version that contains the magnitude
adjustments in the table's last: M67GBLa
Information on the All-Sky Observing and Analysis
My observing procedure is described elsewhere, http://brucegary.net/allsky2011/,
as is my hardware: http://www.brucegary.net/HAO/.
Briefly, I use a 14-inch Meade with a SBIG ST-10XME CCD. The
telescope is housed in a dome, and both are controlled from my
residence office using buried control cables. I use MaxIm DL for
control of the telescope, CCD, focuser and dome. Flat fields are
taken of the sky before sunset. Bias and dark exposures are also
taken before observations. All imaging is unbinned.
The April 16 all-sky observing session consisted of "observing
cycles" of Landolt star fields and M67. All exposure times are 10
seconds, unguided. An "observing cycle" consists of 4 B-band
images, 4 at V-band, 4 at g', 3 at r', 3 at i' and 5 at z'. Two
observing cycles are made whenever a Landolt star field is
acquired (some fields are acquired at different times of the night
for sampling a large range of air mass). On April 16 I observed
the following star fields: L0652, L0558, M67, L0724, L0853, M67,
L0853, L1637 (note that L0652 means the Landolt star field at RA =
06:52, DE ~ 0). After this sequence I observed L1637 for the rest
of the night for the purpose of detecting the presence of
sub-visible cirrus and aerosol patchiness (one events of ~15 mmag
lasting 1/2 hour was found, as was another with 5 - 10 mag). This
6-hour run also provided extinction trend information (that was
useful in constraining subsequent analyses involving extinction
The rationale for including B and V observations when only
g'r'i'z' magnitudes are to be determined has to do with the need
for an accurate star color for each star. There are many more B
and V standard magnitudes (Landolt 2009) than g'r'i'z' magnitudes
(Smith et al 2002) at each of the observed Landolt star fields.
I use an artificial star for all image analysis, which permits
monitoring of extinction variations. I have come to view the
presence of sub-visible cirrus and aerosol patchiness as very
important for all-sky observing, and the only way I know how to
deal with these extinction variations is with the use of an
artificial star (that occupies an unused 0.1% corner of each
image). The artificial star magnitude won't vary during an
observing session, so it allows star magnitude readings to be
converted to star flux.
All photometry readings correct for "flux capture fraction" - the
ratio of flux within the circular photometry circle when the
radius is small (~ 2.5 times FWHM) versus large (~5 times FWHM).
The small aperture is used for exporting to a spreadsheet for
subsequent analysis. The incentive for using a small photometry
aperture is twofold: bettyer SNR for faint Landolt stars, and
reduced interference from nearby stars. This correction is usually
20 or 30 mmag.
The spreadsheet calculates air mass from JD, my site location and
the target coordinates. All star fluxes are processed with
guidance from the following generic magnitude equation:
Magnitude = Z - 2.5 ×
LOG10 ( Flux / g ) - K' ×
AirMass + S × StarColor + S2
× AirMass × StarColor
where Z is a zero-shift constant,
specific to each telescope system and filter (which should remain
the same for many months),
Flux is the star's flux (sum of counts associated with the star). It's called "Intensity" in MaxIm DL,
g is exposure time ("g" is an engineering term meaning "gate time"),
K' is zenith extinction (units of magnitude per air mass),
S is "star color sensitivity." S is specific to each telescope system (and should remain the same for many months),
StarColor can be defined using any two filter bands. B-V is in common use; I use 0.57 × (B-V) - 0.39,
S2 is a second-order term that is usually ignored because it is only important for high air mass and extremely blue or red stars.
This general equation is true for all filter bands (even
unfiltered), though there are different values for the constants
for each filter. For example, the magnitude equation for V-band
(omitting the last term in Eqn 1) is:
V = Zv - 2.5 ×
LOG ( Flux / g ) - Kv' × AirMass + Sv ×
Extinction at each filter band is represented by a zenith value
at mid-observing session time, a temporal trend parameter, and a
3rd order fit to residuals (identified as aerosol patches drifting
Figure 4. Screen capture of a spreadsheet section for V-band fitting of Landolt stars (N=54, total of 173 photometry readings). Lower-left panel shows solution for zenith extinction; upper-left panel shows solution for star color sensitivity; upper-right panel shows residuals of measured magnitude with respect to modeled magnitude versus V-magnitude. Lower-right area ahs slide bars for matching RMS versus magnitude (upper-right) with a stochastic SE model.
"Landolt Unknown" Analysis
A final adjustment was made by treating each Landolt star field
as an "unknown" and processing it using the other Landolt stars to
solve for telescope system photometry constants. Since five
Landolt star fields were used for the April 16 observing session
there were five estimates of "average star field error" for each
filter band. For example, the V-band average errors were +8, -2,
+8, -21 and +5 mmag. Some star fields had more standard stars than
others, and the average of the 54 Landolt stars in all star fields
for V-mag was +0 ± 11 mmag. For B, g', r', i' the average
errors were +3, +12, +9 and -4 mmag. z'-band was less well
behaved, with +49 mmag average error. Apparently there is
something about my analysis procedure for z'-band that was
underestimating star brightness when all Landolt star fields were
involved in the parameter solution. Until I figure this out I have
decided to adopt the correction values called for by the exercise
of treating Landolt star fields as unknown. This is what was done
in obtaining Fig. 2, above. The following table lists the "Landolt
Bias Corrections" for each band based on treating each of the
Landolt star fields as an "unknown."
Table 2. "Landolt Bias Correction" for Apr 16 (Based on
Treating Landolt Star Fields as Unknown)
|Apr 16 Bias [mmag]
||+3 ± 12
||+0 ± 11
||+12 ± 7
||+9 ± 4
||-4 ± 4
||+49 ± 25
|Apr 16 Nr. Comparisons
|May 02 Bias [mmag]
||+9 ± 6
||-1 ± 3
||-5 ± 5
||-2 ± 4
||+2 ± 3
||-11 ± 9
|May 02 Nr. Comparisons
Total SE for each star will depend on the star's brightness,
since stochastic SE varies with star brightness and total SE is
the orthogonal sum of stochastic and estimated systematic SE. I
estimate that the systematic error for each band is the orthogonal
sum of the applied half of the "Landolt Bias Correction"
correction (above table) and the SE of that correction.
Henden, A. 2000, JAAVSO, 29, 35-43.
Landolt, A. U., 2009, AJ, 137, 4186-4269, May.
Smith, J. Allyn, et al, 2009, AJ, 123, 2121-2144.
Skiff, B., 1997, M67 catalog: http://stupendous.rit.edu/tass/catalogs/m67.html
RELATED WEB PAGE LINKS
AAVSO photometry manual: http://www.aavso.org/observing/programs/ccd/manual/4.shtml#2
Lou Cohen's 2003 tutorial: http://www.aavso.org/observing/programs/ccd/ccdcoeff.pdf
Priscilla Benson's (1990's) CCD transformation equations tutorial: http://www.aavso.org/observing/programs/ccd/benson.pdf
Bruce Gary's CD Transformation Equations derived from basic princples: http://reductionism.net.seanic.net/CCD_TE/cte.html
Bruce Gary's All-Sky Photometry for Dummies: http://brucegary.net/dummies/x.htm
Bruce Gary's All-Sky Photometry for Smarties - v1.0: http://brucegary.net/photometry/x.htm
Bruce Gary's All-Sky Photometry for Smarties - v2.0: http://brucegary.net/ASX/x.htm
Bruce Gary's Differential Alternative Equations: http://brucegary.net/DifferentialPhotometry/dp.htm
Bruce Gary's Astrophotos home page: http://reductionism.net.seanic.net/brucelgary/AstroPhotos/x.htm
Bruce Gary's all-sky observing session of 2011.10.28 (BVRcIcg'r'i'z', 60 Landolt stars, 22 SDSS stars): http://brucegary.net/yygem/all-sky/index.htm
Bruce Gary's 2011 version of all-sky observing and analysis procedure: http://brucegary.net/allsky2011/
Bruce Gary resume: http://brucegary.net/resume.html
WebMaster: B. Gary. Nothing on this web page is copyrighted. This site opened: 2012.04.24. Last Update: 2013.02.19