Method#1

PHOTOMETRY FOR DUMMIES: METHOD #1

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

0.25 mag SE, Planetarium Program Use of a Tycho or UCAC2 Reference Star (4 Minutes)

Order the UCAC2 3-CD set from USNO and copy them to your hard drive. Tell your planetarium program to support UCAC2 (easy with TheSky 6.0). Now you're ready for some easy and accurate photometry.

1) Look for a star in your image that's not seriously saturated (i.e., maximum counts <35,000).
2) Use your planetarium program to identify this star.
3) Use the planetarium program to determine the magnitude for this star (V-mag are normally displayed).
    If it's from the Tycho catalog, great. The star's V-mag is likely to have SE = 0.13 mag. Be sure the star isn't saturated and proceed to next step.
    If it's from the UCAC2 catalog, good. If V-mag >10.5, add 0.25 mag. Proceed to next step.
    If it's from the GSC catalog, try another star (unless you're willing to accept SE = 0.44 with the possibility of errors ~1.1). If you use it, subtract 0.33 mag.
4) Measure the star's "intensity" (set the photometry aperture so that most of the star's image falls within the signal circle, and check that no interfering stars are in the sky background reference annulus).
5) Measure the asteroid's intensity (using the same aperture settings).
6) Calculate the asteroid's magnitude: MagAsteroid = MagStar + 2.5 * LOG₁₀ (IntensityStar / IntensityAsteroid)

Note 1: The accuracy of any measurement has two components: 1) SEs, the stochastic standard error (related to noise level), and 2) SEc, estimated calibration uncertainty. Add them together (orthogonally) and you get accuracy. Use the asteroid's SNR to calculate SEs [mag] = 1 / SNR. (MaxIm DL displays SNR in the information window for the star within the aperture circle.) Assuming your "reference star" magnitude was from the UCAC2 catalog, then SEc ~0.2 mag. If it's from Tycho, then SEc ~0.13 mag. Your final "accuracy" SE = SQRT (SEs² + SEc²).

Note 2: If you use a good observing and analysis program, such as MaxIm DL, the above procedure can be simplified using the "Calibrate" tool to force an intensity to display the corresponding magnitude (in an information window) as you move the cursor over stars in the image. Doing this replaces steps 4, 5 and 6, above. I assume that if you're not using MaxIm DL you can still read "intensity" (or flux, etc) using your analysis program (if you can't then you're out of luck for this and all the other procedures on this web page). Assuming TheSky6 and MaxIm DL are both open, and your image of interest is on display, the entire procedure for deriving an asteroid magnitude should take less than 3 minutes.

Note 3: If you're observing unfiltered then your asteroid magnitude (using V-mags for the reference star) can still be considered to be some version of a V-mag. It is only a good version of a V-mag to the extent that the asteroid's color is the same as the reference star's color. Since the advertised accuracy of this method is only 0.25 mag the color issue is unimportant.

Note 4: If your planetarium program is any good the Tycho magnitude that it gives for a star will be an adjusted version of the Tycho Bt and Vt measured magnitudes. If it doesn't make this conversion, you'd
better do it yourself: V = Vt +0.008 - 0.0988 * (Bt - Vt), which is Arne Henden's suggested correcting equation.

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This site opened: January 1, 2005. Last Update: January 2, 2005