Bruce L. Gary, Hereford Arizona Observatory (G95)
Last updated 2005.01.02

0.15 to 0.30 mag SE, R-Filter, Using Several Tycho or UCAC2 Reference Stars (10 Minutes)

This method is the same as Method #2 except that catalog magnitudes have to be converted to an R-magnitude. As stated before, be sure you have UCAC2 files on your hard disk and configure your planetarium program to support this catalog.

1) Locate up to 4 unsaturated stars in your image that have Tycho magnitudes displayed in your planetarium program. Avoid Tycho stars with Vt > 10.5.
        Note: You may not find any Tycho catalog star in your FOV if it's smaller than ~15 x 25 'arc.
        If you have many images at about the same air mass try those and use the "zero offset" value for analysis of the image of interest.
2) Convert Tycho magnitudes Bt and Vt to R using Arne Henden's conversion equation: R = Vt -0.014 -0.5405 * (Bt - Vt)
3) If you have to use UCAC2 stas, convert UCAC2 V-mag (which I'll refer to as Vu) and J magnitudes to R using:
        R = 0.17 + 0.83 * Vu + 0.14 * J. (Don't use the Vu 0.25 mag correction mentioned elsewhere.) Estimated accuracy = 0.30 mag.
4) Set the photometry aperture so that most of the "light" from the reference stars fall within the signal circle, and check that no interfering stars are in the sky background reference annuli.
5) Select the Photometry Tool (in MaxIm DL click Analyze/Photometry).
6) Set the mode to Object and left-click the asteroid.
7) Set the mode to Reference, and for each Tycho or UCAC2 reference star left-click it and enter the Tycho-based or UCAC2-based R-magnitude for that star.
8) Click View, and save the results to a CSV-file.
9) View the CSV-file (using your favorite utility, such as Total Commander), and read the asteroid's R-magnitude.

Note 1: The more Tycho stars you use the smaller your systematic error. With just one Tycho star you can expect SEc = 0.30 mag. With 4 Tycho stars it should be ~0.15 mag.

Note 2:
This procedure is a lot more work compared with the procedure for V-filter or unfiltered observations, and the results are less accurate. This is due to the fact that star catalogs don't include  R-magnitudes (except for the very brightest, which will always be saturated for CCD users). Converting B and V magnitudes to R involves additional errors, so the R-mags you calculate are less accurate than the catalog V-mags. Here's an example of Tycho-based R magnitudes versus "true" (Landolt-based) R-magnitudes for a region chosen at random (the Landolt Area at RA = 00:55).

Tycho R-mag vs true

Figure 1
. Tycho-based R-mags versus "true" R-mags, where "true" R-mags were determined using an R-filter image that contained 21 Landolt stars for establishing a zero-shift value.

Note 3: I'm unsure about the accuracy of using UCAC2 stars for deriving R-magnitudes. If the V-mags have SE = 0.2 mag, and the R-mags should be even better (since I think they're derived from JHK magnitudes). I did a quick correlation that indicates 0.15 SE for R-mags derived from the catalog Vu and J mag's (using the formula given in step 3, above).

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This site opened:  January 5, 2005 Last Update:  January 5, 2005