J and H Band Extinction

Introduction
When doing system calibration or all-sky photometry it is essential to know the extinction coefficients. The following explains three methods of determining the extinction coefficients.

Differential Photometry Method I
This method is used for differential photometry. The comparison star is used as the star for determining the extinction coefficients and is measured through a range of air masses.

Note: Due to the nature of these bands, extinction is very low and for differential photometry can usually be ignored.

System Calibration Method II
This method uses multiple stars with (J-H) ~ 0.00 at significantly different air masses. This method is used when calibrating the system.

All-Sky Photometry Method III
THis method uses multiple stars of any color, but at significantly different air masses. This method is used when doing all-sky photometry.

Objective

To determine extinction coefficients kj and kjh. Determining the extinction coefficients and zero points depends on using the equation for a straight line Y = MX + B where M is the slope and B is the Y intercept (point where line crosses the Y axis when
X = 0.

The key is to get the coefficient equations in the form of Y= MX + B.

Note: Some books use the Gewwk letters epsilon and mu for the color transformation coefficients for J and (J - H) bands. Because the symbols are used in the UBV bands the symbols rho and lambda are used in this paper for the color transformation coefficients for J and (J - H) respectively.

Note: Air Mass X of each observation must be determined. Determining air mass is somewhat involved and covered in another white paper.

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Method I
Differential Photometry

Using the Comparison Star measure J and H magnitude values of the Comparison Star through a wide range of air masses. Note the air mass X of the observation and calculate instrumental magnitudes j and h.

Corrected counts are counts normalized to 1 second integration and unity gain. Sky readings have been subtracted.

j = - 2.5 log10 (J corrected counts) Equation 1

h= - 2.5 log10 (H corrected counts) Equation 2

The values jo and ho are the extra-atmospheric magnitudes.

Note: The Y intercepts are not used in this method and do not represent the zero points.

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Method II
For System Calibration when system rho and lambda are unknown.

Use multiple stars with (J - H)~ 0.00 .

Measure J and H magnitude values of standard stars with (J - H)~ 0.00 at a wide range of air masses.

Note: Corrected counts are counts normalized to 1 second integration and unity gain. Sky readings have been subtracted.

j = - 2.5 log10 (J corrected counts) Equation 1

h= - 2.5 log10 (H corrected counts) Equation 2

Note: The air mass X of the observation and calculate instrumental magnitudes j and h. By doing this the observation time can be short as the stars do not have to travel through a range of air masses. Different stars should be at different air masses.

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Method III
All Sky-Photometry Multiple stars of any color with known system rho and lambda.

Use multiple stars of any color.

Measure J and H magnitude values of standard stars of any color at a wide range of air masses. Note the air mass X of the observation and calculate instrumental magnitudes j and h.

Note: Corrected counts are counts normalized to 1 second integration and unity gain. Sky readings have been subtracted.

Present Page Version as of 1 November 2006

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