This web page illustrates the appearance of a spreadsheet that I use to determine a "photometry zero shift constant" that allows me to convert star intensity measurements to magnitude.

Before using a spreadsheet like this the user must observe a Landolt star field. For the case illustrated here, only one such star field was observed, and it was observed at only one air mass value. The median combined image of several exposures of this star field is used to measure "intensity" for each Landolt star. These values are entered in cells E11..E18 of the following spreadsheet.

Figure 1. Example of an Excel spreadsheet for determining the telescope system's zero shift constant (green, cell C1). As the value of cell C1 is manually changed the average difference value in cell G7 changes. Additional description of this spreadsheet is in the text.

The Landolt knwon magnitudes and B-V values are entered into the spreadsheet in cells C11..D18. An equation for converting intensity to magnitude displays magnitude values in cells F11..F18. The equation has the form:

    Mag = 19.677 - LOG ( INTv / g ) - 0.15 * m - 0.14 * (B-V -0.64) + 0.05 * m * (B-V -0.64)
          where INTv = measured intensity using V-filter, g = exposure time, m = air mass, B-V = star color

Differences between the equation magnitude and Landolt magnitude are displayed in cells G11..G18, and their average value adn RMS difference are displayed in the yellow cells G7 and G8. The value in green cell C1 is adjusted until the average difference (cell G8) is zero.

Other spreadsheets can be used to establish the photometry zero shift constant for the other filters.

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This site opened:  January 4, 2005.  Last Update:  January 4, 2005