PHAS3332 (Practical Astronomy 3: Field Trip) 2008

REDUCING YOUR PHOTOMETRIC DATA

Identify which files contain the observations that you intend to reduce, along with bias (offset) and flat field frames.

Create a `Starlink Data Format' version of each of your observation and calibration frames by using the Figaro command rdfits. For example:

rdfits n260082.fits n260082 swap=true float=true

Sum the bias frames into one combined bias frame. If you had four bias frames bias1.sdf, bias2.sdf, bias3.sdf and bias4.sdf, you would combine them as follows:

iadd bias1 bias2 biasum
iadd bias3 biasum biasum
iadd bias4 biasum biasum

Then divide the summed bias frame by the number of frames used to produce it, so that you are left with the mean bias level. Continuing the previous example:

icdiv biasum 4.0 biasmean

Subtract the mean bias frame from each of your observation and flat field frames, e.g:

isub rrgemxx biasmean rrgemxx

Combine the flat field frames into an average flat field in a manner similar to that used to combine the bias frames. If you had three flat field frames (from which the bias has already been subtracted), you would do the following:

iadd flat1 flat2 flatsum
iadd flat3 flatsum flatsum
icdiv flatsum 3.0 flatmean

Divide the average flat field frame by the mean of its own count i.e., normalise it. Determine the mean count in the frame with:

istat flatmean 10 500 10 500
Divide the frame by the mean count (represented below by `m') with:
icdiv flatmean m flatmean

Now divide the normalised flat field frame into each observation frame. For example:

idiv rrgemxx flatmean rrgemxx
Note: use a flat field taken with the filter (B or V) which matches that used for the observation frame.



Use the Gaia program to display and analyse your calibrated observation frames. To start analysing frame rrgemxx, you would enter:

gaia rrgemxx.sdf
Once Gaia has started, select the View option from the control panel at the top of the display window. Then select Colors, and select `ramp' from the list of Colormap options, and `neg' from the list of Intensity options. Press the close button, and adjust the Low and High levels to produce the best view of all the stars that you will measure.

Now select Image-Analysis, Aperture photometry, Results in magnitudes. In the window which appears, press the Define object aperture button. Position the cursor over the image of the brightest star which you wish to measure, then press and hold down the left-hand button on the mouse. A circle will appear around the star: adjust the size of this by moving the mouse, until the image of the brightest star just fills the circle. When you release the left-hand mouse button, the circular stellar aperture will be defined. An annular aperture surrounding the stellar aperture will also be defined; this will be used to measure the brightness of the sky background. The stellar and sky apertures are separated from each other by an annulus of `no-mans land'.

Now press the Calculate results button in the aperture photometry window. After a short delay the magnitude of the star, among other measurements, will appear in the window. The magnitude will be in peculiar units, because the frame has not been calibrated for sensitivity. This peculiarity does not matter, because you are chiefly concerned with the difference in magnitude between the target star and a calibration star (the peculiarity of the unit is assumed to arise from a constant offset value, which cancels out when we calculate the difference in magnitudes between two stars measured on the same CCD frame).

Now press the Copy aperture button in the aperture photometry window. Position the cursor over the previously-defined aperture, and click the left-hand mouse button to `lift' the aperture and place it over another star that you wish to measure. Release the mouse button, and obtain the magnitude of this star as before. It is vital to use the same size aperture for all of the stars in a particular frame.

Calculate the difference in magnitude between the target star and the brightest star in the frame. As a consistency check, also calculate the difference in magnitude between two fainter calibration stars in the frame

When you have measured all of the frames that you can, plot the difference in magnitude between the target star and the calibration star, as a function of Julian day. Present the plot with conventional orientation, so that following a curve upward on your plot is correctly interpreted to mean that the target star is getting brighter. Estimate the period of photometric variation of the star, and the epoch of maximum brightness. Use the estimated period and epoch to produce a plot of magnitude difference as a function of photometric phase. Carry out the foregoing procedure for both the B- and V-band data, then difference the light curves to produce a plot of (B-V) colour as a function of photometric phase. Take care to orientate your (B-V) plot so that following a curve upward is correctly interpreted to mean that the target star is becoming more blue in colour.


Stephen J. Boyle
2008 February 6