C-Munipack 1.2 / User's manual
C-Munipack 1.2 / User's manual
The light curve is a graph of intensity of an object (a variable star in most cases), as a function of time. The light curve of the observered object is usually presented in a form of a graph that has time on the independent axis and the intensity in magnitudes on the other axis. Each point in the graph represents single measurement, which in simple cases corresponds to a single input CCD frame.
To make a light curve of an observed object, we need a set of photometry files with cross-reference information (matched photometry files). An user have to the program which stars are variable stars and comparison stars. In case of aperture phometry, he also have to choose an aperture.
The Muniwin program allows an user to apply further corrections and include additional information to a light curve, all of which will be discussed later in the text.
Reduce the source CCD frames (see the chapter "Reduction of CCD frames"). Then click on the Plotting -> Choose stars item in the main menu. A new dialog appears, the dialog shows the reference frame. The detected stars are highlighted. Click on the variable star using left mouse button. A context menu opens. Select the Variable. The star is now drawn in red color now and the label "var" is placed near to it. Using the same procedure, select one comparison star and one or more check stars. I would recommend you to use two check stars. Confirm the selection by the OK button.
Now, we have to choose the aperture. You can image the aperture as a virtual circular pinhole, placed on each star on a frame to measure its brightness. All pixels that are inside the pinhole are included in computation leaving out the background pixels. The best aperture should be big enough to include most of the star's light, on the other hand, the bigger aperture is used the more background is included and the more noisy the result is. Because of this, the photometry process computes the brightness of each star in a set of predefined apertures of radius in the range of 2 and 30 pixels.
To select the best aperture, we can take advantange of a comparison and check stars - providing that they are constant, we can compute the differential magnitudes between each couple of them on each other and then compute the variance or standard deviation from the mean level. For the best aperture, the deviations are minimal.
Open the Plotting menu and select the Choose aperture item. A new dialog appears. The graph shows the standard deviation for each aperture. Find the aperture with the minimal deviation and click on it using the left mouse button. A context menu appears. Select the Select aperture item. The point is drawn in red color now and the label is placed near to it. Confirm the selection by the OK button.
Now we are ready to make a light curve of the variable star. Open the Plotting menu again and select the Plot light-curve item. A new dialog appears. This dialog shows the light curve of the selected variable star. The magnitudes are differential with respect to the selected comparison star.
Press the Save button. Locate the folder where you want to save the results and fill in the name of the output file. Confirm the dialog by the Save.
The light curve is saved to a text-based file. On its first line, the names of the columns are stored. The second line shows the aperture and color filter used. The data are stored on the followin lines. Each line corresponds to a single source frame. In the first column, there is a time of observation (center of exposure) expressed as a julian date. The second column consists of differential instrumental brightness of the variable stars with respect to the comparison star in magnitudes (V-C). The error estimation is stored in the next column. Following columns consist of differential magnitudes of comparison star and check stars and their error estimations.
While the Earth orbits the Sun, the distance between the observed object and the observer changes during a year. Although the amplitude of these changes is negligible with respect to the mean distance, it becomes significant when you are interested in a time interval between two events, like times of minimum of a eclipsing variable star. In this case, the finite speed of light is not unimportant. The value of the heliocentric correction is the time that the light needs to travel from the Earth's actual position to the center of the Earth's orbit. The value may be either positive, when the Earth is nearer to the object than the Sun, or negative in the opposite case.
It is possible to make a light curve with the heliocentric correction applied. Optionally, it is possible to include the correction value as a separate column in the output table. You can enable the corrections in the configuration dialog (Tools -> Preference) in the Plotting -> Light curve section.
When the heliocentric correction is enabled, the object's celestial coordinates must be provided prior to making a light curve. A new item in the Ploting menu appears. There are several ways how to enter the coordinates. If the input CCD frames contain information about observed object, the program can use them. When you have used a catalogue file as a reference file, the program also looks into this file. For known variable stars, the program provides a tools for searching the common variable star catalogues (GCVS, NSV, NSVS and BRKA). An user can also enter the coordinates manually.
The air-mass coefficient characterizes the attenuation of the star's signal after it has travelled through the Earth's atmosphere. While this effect is stongly dependent on the light's wavelength (color), it must be taken account in computation of the color index (temperature) of an observer object. The attenuation also becomes significant when the color index of the variable and the comparison star are not close. The air-mass coefficient is always positive value, it defined as 1.0 when the object is in the zenith and greater than one when the object is somewhere between the horizon and the zenith.
Although the C-Munipack program doesn't perform any corrections to the measured intensity. It can compute the value for coefficient for each measurement and put those values into the output file to a separate column. You can enable this feature in the configuration dialog (Tools -> Preference) in the Plotting -> Light curve section.
When the air-mass computation is enabled, the object's celestial coordinates must be provided prior to making a light curve. See the heliocentric correction section for details.
In addition, the observer's geographic location must be filled in. A new item in the Ploting menu appears. There are several ways how to enter the coordinates. If you process files from the only location, you can put the default location in the configuration dialog (Plotting -> Observer section. If the input CCD frames contain information about the observatory, the program can use them. An user can also enter the coordinates manually.