.. C-Munipack - User's manual Copyright 2012 David Motl Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. $Id: glossary.rst,v 1.1.1.1 2012/08/12 16:57:45 dmotl Exp $ Glossary ======== .. glossary:: :sorted: Flexible Image Transport System FITS The Flexible Image Transport System is a file format designed specifically to store scientific images and data. Since its first publication in 1981 in has become the most frequently used file format for storing and manipulating of images that were carried out by means of the astronomy CCD cameras. In the C-Munipack project, the FITS format accepts the FITS images on its input and it uses the format to store the results of operation that produce a CCD image as an output. Bias frame A bias frame is an image obtained from the CCD camera when all light is blocked and the exposure is indefinitely short (in ideal case). It represents the constant bias level, that is preset in the camera's electronics that reads out the image data. For ideal CCD camera, the level should be constant for pixels and the bias could be represented by a single value. In practice, there are small differences between pixels. In the standard calibration scheme, the bias is included in the dark frame and thus it is subtracted during the dark-frame correction. In the advanced calibration scheme, the bias must be subtracted from a scientific frame before the dark-frame calibration is applied. The bias frame is used in the bias-frame correction phase of the reduction process. To reduce noise, a master bias frame is often used instead of a raw bias frame. Master bias frame A master bias frame is a combined CCD frame made from a set of bias frames. By means of the robust mean algorithm, the noise of the resulting image is reduced. The master bias frame is used in the bias-frame correction phase of the reduction process. Bias-frame correction The bias-frame correction subtracts the bias component from a CCD frame. This step is a part of the reduction process of CCD frames. It is used only in the advanced calibration scheme. To apply the correction to a set of frames, a bias frame is required. Dark frame A dark frame is an image obtained from the CCD camera when all light is blocked. The image represents the thermal current accumulated in a CCD device during an exposure. The mean value of the thermal current is proportional to the CCD temperature and exposure duration. For an ideal CCD camera, the thermal current would be the same for all pixels and it could be represented by a single value. In practice, some of the CCD elements are much more sensitive to the thermal current than the rest. These pixels, which appear as a bright dots on a dark frame, are called hot pixels. Depending on the calibration scheme, the dark frame may also contain the bias level. The dark frame is used in the dark-frame correction phase of the reduction process. To reduce noise, a master dark frame is often used instead of a raw dark frame. Scalable dark frame A scalable dark frame is a special case of the dark frame. It is used in the advanced calibration scheme that allows to use a dark frame of exposure duration different from scientific images for the dark-frame correction. The scalable dark frame can be made from a dark frame by subtracting a bias frame. Thus, the scalable dark frame contains only components that are linearly dependent on exposure duration. Master dark frame A master dark frame is a combined CCD frame made from a set of dark frames. By means of the robust mean algorithm, the noise of the resulting image is reduced. The master dark frame is used in the dark-frame correction phase of the reduction process. Dark-frame correction The dark-frame correction subtracts the thermal component from a CCD frame. This step is a part of the reduction process of CCD frames. In the standard calibration scheme, this bias components is subtracted as well, because the bias is included in the dark frame. To apply the correction to a set of frames, a dark frame is required. Flat frame A flat frame is an image obtained from the CCD camera when a telescope is pointed towards a luminous area of uniformly distributed intensity across the field of view. The image represents the spatial distribution of the sensitivity not only of the CCD elements but whole device. It records the response of the entire optical system - the telescope, filters, camera's window, cover glass and CCD chip itself - to a uniform source of light. Most significant effects that affect a flat frame are vignetting and sensitivity variations of CCD elements. The flat frame is used in the flat-frame correction phase of the reduction process. To reduce noise, a master flat frame is often used instead of a raw flat frame. Master flat frame A master flat frame is a combined CCD frame made from a set of flat frames. By means of the robust mean algorithm, the noise of the resulting image is reduced. The master flat frame is used in the flat-frame correction phase of the reduction process. Flat-frame correction The flat-frame correction equalizes sensitivity variations of the whole optical system (telescope + filter + camera). This step is a part of the reduction process of CCD frames. To apply the correction to a set of frames, a flat frame is required. Time correction The time correction fixes the time of observation of source frames. This step of the reduction process is optional and depends on particular situation. For example, you can use the correction to fix the bias of the PC's system clock in regards to the UTC. Photometry The photometry is a process performed during the CCD image reduction. It takes a calibrated CCD image and detects stars on it. For each star, the brightness is determined. The results are saved to a file, which is called the photometry file. Current version of the software uses the aperture photometry method to measure brightness of an object. Aperture photometry The aperture photometry is a method for measurement of brightness of a star on a CCD frame. After image calibration, the frame consists of two components - sky background level and a signal from a star. To measure star's brightness, we have to separate these two components. The algorithm of aperture photometry was developed from a method used for a observations made by means of a photomultiplier. Unlike this, the aperture photometry for digital images carried out by means of a CCD camera uses the digital image processing algorithms. The algorithm uses two apertures - an annular shaped one to measure the background level in vicinity of a star and a circular one to measure the total flux from a star. The background is subtracted from a star's signal. The brightness of a star is defined as a ratio of its signal to a fixed reference value. Such value, expressed in magnitudes, is called instrumental absolute brightness. Matching The matching is the last step in the standard CCD image reduction scheme. It process a set of photometry files of a same view field and finds a relation between individual stars on the frames. For example, the star on position (100, 100) on frame 1 is the same one as star on position (120, 95) on frame 2 etc. Each star is then assigned a unique identifier, which is the same for the same star on all frame in the project. To perform the matching, one frame must be given as a reference frame. The reference frame is usually one frame from a set of input frames, but another frame may be used - see catalog files. Full Width at Half Maximum FWHM Full Width at Half Maximum is used in the C-Munipack software to express a diameter or width of a star-like objects. The width is measured in the middle between background level and the maximum value. Reduction of CCD frames The reduction of CCD frames is a process which takes raw frames carried out by means of a CCD camera and produces a set of photometry files, one file for each input frame. The photometry file consists of a set of stars. For each star, the derived brightness is stored. An unique identifier assigned to each star is used to identify a particular star between frames. Depending on a reduction scheme, the reduction consists of the following three steps: calibration (bias-frame correction, dark-frame correction, flat-frame correction and time correction), photometry and matching. Light curve A light curve is a graph of brightness of a star, as a function of time. The light curve is one of the products of the C-Munipack software. It is used especially in the observation of variable stars. The brightness of a star is expressed in magnitudes, usually as a difference between two objects - a variable star and a comparison star. One or more check stars are used in addition. Absolute intensities are used sometimes, another post-processing step outside the C-Munipack software is required in this case. Depending on a type of the light curve, a user must choose a variable star, a comparison star and optionally one of more check stars. Object's equatorial coordinates or observer's geographical coordinates may be required, too. Track curve A track curve is a graph of spatial offset of CCD frames, as a function of time. Such curve is used to monitor the stability and precision of telescope mount and its clock drive. Reference file A reference file is a photometry file which is used in the matching process. It is either a photometry file for one of source frames or a catalog file of the same field. The matching algorithm takes two photometry files at a time and find corresponding stars on them. To match more than two scientific frames, one frame from the set is chosen as a reference file and all source frames are matched one by one against the reference. Catalog file A catalog file is a file that consists of a list of stars (their positions and magnitudes). Unlike a photometry file, it can contain identification of variable, comparison and check stars. A catalog file can be made by means of the Muniwin user interface and it is used as a reference file in the matching process. In this case, the software restores the selection of stars from a file and an user don't need to select the stars again. It is useful if you repeatedly observe the same field. Air mass coefficient The airmass coefficient characterizes the attenuation of the light when it travels through the atmosphere. Because the atmosphere is not equally transparent for all wavelengths, this effect becomes important when you want to compute the absolute magnitudes for measured objects. The coefficient equals to 1.0 when the object is in the zenith by definition and it is greater than one for altitudes between the zenith and the horizon. It is not defined for objects at or below the horizon. Altitude Altitude is one of the coordinates of the horizontal coordinate system. It is used for computation of the airmass coefficient. To compute the object's altitude at a time of observation, the date and time of observation, object's equatorial coordinates and observer's geographical coordinates must be known. Heliocentric correction The heliocentric correction fixes the variations of the distance between the observer (Earth) and observed object which occur because the Earth orbits around the Sun. This effect is not negligible especially in observation of eclipsing variable stars or exoplanet transitions, when you need to measure the time interval between two events, that occurred few weeks or months apart. The heliocentric correction transforms the times in such a way that they are related to a virtual fixed point placed in one focus of the Earth's orbit (close to the Sun). The heliocentric correction is optionally performed during construction of a light curve. Object's equatorial coordinates are required. Julian date JD The Julian date (JD) is the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon. It is widely used and recommended by International Astronomical Union to express the time of an event, for example a particular observation, minimum of a variable star, etc. You can use the Muniwin program to convert the Gregorian calendar date to Julian date and back. Heliocentric Julian date HJD JD(hel.) The heliocentric Julian date is a time of an event (a particular observation, time of minimum) with the heliocentric correction applied. Time of observation In the C-Munipack software, the time of a particular CCD frame is always related to a center of the exposure.
