NGC 1342 Secondary Standards
Bruce Gary, Hereford Arizona Observatory; 2012.12.08 

This web page provides a list of B, V, g', r', i', z' magnitudes for 40 stars in the open star cluster NGC 1342 in Perseus. The magnitudes are 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#0) is 03:31:48 +37:20:36 (J2000). The "observing season" for this star cluster is November 19 (when it crosses the meridian at local midnight). My purpose for calibrating this star cluster is to serve as a Fall counterpart to M67, which has a late winter observing season (January 30). M67 has a long tradition of use for establishing a telescope system's "transformation coefficients" (star color sensitivity, mostly), but it has lacked a counterpart for other seasons. I invite photometry observers to use the magnitudes reported on this web page to complement M67 in affording a greater seasonal coverage for establishing and monitoring telescope system star color sensitivity coefficients. (I've calibrated 47 stars in M67 for BVg'r'i'z' bands, and results are given at link.)

Figure 1. Finder chart for the 40 stars with secondary calibrations determined by Bruce Gary at the Hereford Arizona Observatory.  FOV = 23.4 x 17.7 'arc; north up, east left.

Figure 3.  Magnitudes with estimated SE's. Here's a downloadable text file of the above table: link.

Warning, magnitudes for stars 16 and 29 are slightly variable, so don't use them for establishing zero shifts; their colors are reliable so they can be used for evaluating "star color sensitivity."

Rationale for Using an Open Star Cluster for Monitoring a Telescope System's "Star Color Sensitivity"

A typical FOV for amateurs and professionals alike is ~ 15 'arc square.  An observer wanting calibrated stars within such a FOV can expect ~ 5 to 15 BVRcIc Landolt stars fields this small a FOV. The situation for stars with g'r'i'z' calibration is even worse; only 2 or 3 of the Smith et al (2002) stars can be positioned within this small a FOV. Dozens of stars are needed to establish reliable "star color sensitivity" for each band. Therefore, without use of a well-calibrated open star cluster, such as M67 or NGC 1342 (this web page), an observer would be forced to observe many Landolt star fields. Such an observing session might include a half dozen Landolt star fields, each at a different air mass, and the observer would therefore be forced to simultaneously solve for extinction and extinction trend for each filter band in addition to the telescope system photometry constants. I became tired of doing things this way, so have embarked on a project to create a set of secondary standard stars in two or more open star clusters. M67 has been used for this purpose, but until a decade ago there were too few stars calibrated for this purpose. Arne Henden (2000) calibrated 23 M67 stars for BV; I verified these results recently, and have created a web page with my BVg'r'i'z' magnitudes for 47 stars at another web page (link). Since M67 can be used for only about half a year (about late November to mid-May) there remains a need for a couple more more well-calibrated open star clusters that are observable during other seasons. My choice of NGC 1342 should be useable for northern hemisphere observers from about mid-July to mid-April. I may eventually calibrate an open star cluster for use during May to September.

My original plan for meeting all-sky photometry calibration needs was to rely upon the AAVSO's APASS catalog. However, when I found that the APASS magnitudes (DR5) for M67 had serious systematic errors (on the order of 100 mmag), and when I realized that APASS wouldn't include z'-band, I decided to perform the open star cluster calibration described on this and my M67 web pages. My intent was to have calibrated stars for my personal use, for ~ 3 open star clusters, but since a lot of work would be involved it seemed appropriate to share my calibration results with web pages for each star cluster.

I am fully aware of the need to be cautious in accepting someone else's secondary star calibration results. I should therefore state that any use of my M67 and NGC 1342 calibrated stars is done with risk of being erroneous. I'm sure that Arne Henden would discourage anyone from using my secondary standard stars for any purpose. Please keep in mind that my secondary calibrations do not have the benefit of peer review, and they are therefore uncorroborated by any other observer. Keep in mind also that I am an amateur, and anything amateurs do is treated with great suspicion by professional astronomers. If I had a lawyer advising me I'd have to state here that "Any material on this web page is not meant for use for any purpose." You've been warned!

Estimating SE Uncertainties

Magnitude SE uncertainty was estimated by orthogonally adding three components: 1) RMS of Landolt stars with respect to a fit, 2) RMS of NGC 1342 stars with respect to each other, and 3) the difference between two all-sky observing  sessions (2012.11.26 and 2012.12.01). The first of these components has a magnitude dependence, determined manually ("by eye") from the Landolt stars.  The second component was determined from 5 observing cycles made throughout the observing session (covering a range of air masses). The last component was made using the relationship that the average of two estimates with Gaussian uncertainty is typically 1/2 of the difference between the two values. For i'-band the difference was 1 mmag (between the two observing dates), so the i'-band SE is essentially only due to the first two SE components. For r'-band the two observing dates differed by 12 mmag, so the final SE was dominated by the RMS variation of Landolt stars with respect to the fits. For B- and V-bands the differences were 27 and 23 mmag, so these were the dominant SE component for B- and V-band.

The pattern for these observing session differences is monotonically increasing in going from long to short wavelengths; i.e., the differences increase with atmospheric extinction. This suggests that the observing session differences are due to an imperfect solution for atmospheric extinction, which are likely to be proportional to total extinction for the band in question. This is a reasonable interpretation, and it is supported by the excellent agreement between the B-V star colors for both observing sessions: the median difference between B-V star colors for the two observing sessions is 2 mmag! In other words, B-V star colors in the above table can be trusted more than the absolute magnitudes. For g'-r' and r'-i' star colors, median differences between the two observing sessions is 20 and 11 mmag. These star color consistencies are gratifying since the principal objective for this project is to calibrate an open star cluster for the purpose of evaluating (e.g., monitoring) a telescope system's star color sensitivity parameter.

The first SE component, RMS of Landolt stars, is attributed to use of an imperfect "flat field" calibration. My master flat fields were made using the sky at dusk, which has a blue color. In theory, such a flat field should work better for blue stars than red ones. Since I obtain flat fields using a blue illumination source (instead of white) I should expect flat field calibration errors that increase with wavelength (i.e., worst at the reddest filter).

Comparison With Other Catalogs

The Carlsberg Meridian Catalog, Release 14 (CMC14), shows fainter r'-band magnitudes than my all-sky values. The median difference is 30 mmag, with a standard deviation about the average of 21 mmag. The CMC14 has an estimated SE < 25 mmag for stars with r' < 13.

The rest of this web page is under construction ...

Information on the All-Sky Observing and Analysis Procedures Used

My observing procedure is described elsewhere,, as is my hardware: 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 change modeling).

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                                                                               (1)

    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 × StarColor                                                                                                                                            (2)

Similar equations exist for bands B, g', r', i' and z'.

Because Kv' may vary throughout the night it has to be represented using a temporal trend term. I have found it useful to also solve for a 3rd-order fit to residuals that result from use of only a trend term.

My choice of StarColor = 0.57 × (B-V) - 0.39 is motivated by the simplicity of beginning an iteration with a StarColor value that is typically zero for the first iteration. This definition choice is arbitrary and won't affect results provided the same definition is used for all bands.

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 overhead).

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: 

    AAVSO photometry manual:
    Lou Cohen's 2003 tutorial:
    Priscilla Benson's (1990's) CCD transformation equations tutorial:
    Bruce Gary's CD Transformation Equations derived from basic princples:
    Bruce Gary's All-Sky Photometry for Dummies:
    Bruce Gary's All-Sky Photometry for Smarties - v1.0:
    Bruce Gary's All-Sky Photometry for Smarties - v2.0:
    Bruce Gary's Differential Alternative Equations:
    Bruce Gary's Astrophotos home page:
    Bruce Gary's all-sky observing session of 2011.10.28 (BVRcIcg'r'i'z', 60 Landolt stars, 22 SDSS stars):  
    Bruce Gary's 2011 version of all-sky observing and analysis procedure:  
    Bruce Gary resume:


WebMaster: B. GaryNothing on this web page is copyrighted. This site opened:  2012.11.29 Last Update:  2013.02.19