Program: SKYCALQ #begin section overview OVERVIEW ======== SKYCALQ is derived from the task SKYCALC and uses PyQt4 instead of GIPSY's own GUI toolkit Ggi. It was developed as a 'proof of concept' for GIPSY tasks using PyQt4. SKYCALQ calculates for a given position in one of the sky systems, the corresponding position in other sky systems. A description of the supported sky systems and the used rotation matrices can be found in the documentation of module 'celestial' in the 'Kapteyn' package (url: http://www.astro.rug.nl/software/kapteyn/) Program SKYCALQ can be started either on the command line of Hermes where it runs in the context of GIPSY, or on the Unix/Linux command line. In the latter case the name is entered in lowercase letters: 'skycalq'. The program is terminated with option 'Exit' in the File menu. The initial state of the program is a position 0 degrees in Right Ascension and 0 degrees in Declination in the ICRS. All other fields represent this position in a different sky system. One can change a position in each of these fields and the program converts this new position to all other sky systems. Epoch and Equinox: When an equinox or epoch is changed, the starting point is the coordinate listed in the ICRS field. You can change this behaviour by fixing one of the sky systems by selecting the corresponding 'Fix' button. A fixed system will not change its coordinates when an equinox is changed. This is convenient if one wants to explore how a position changes with precession. Epoch of observation: FK4 is a slowly rotating system. One can correct for this rotation if the epoch of observation is known. This is the time when the mean place was correct. However one should be aware that this correction introduces fictitious proper motions in FK5. If you are sure that this cannot be real (e.g. for extragalactic objects for which you know that they don't have proper motions in FK5), then disable the 'Obs. epoch' field by unchecking the button next to the field. Then an epoch independent transformation is applied. SKYCALQ panels: The SKYCALQ display is divided in several parts. The upper part deals with sky systems that are not associated with an Equinox. The left side of the main panel deals with sky systems that are associated with an equinox. Changing the equinox implies a precession. The right part has the same functionality but for another equinox. This makes comparisons between different equinoxes much easier. For accurate conversions from FK4 coordinates to FK5, an epoch of observation is required. From the file menu a GIPSY set or a FITS file can be selected. When the file is suitable for a sky transformation, then the pixel that corresponds to the header values CRPIX1 and CRPIX2 is the default position in the 'Pixel' field, the value of the FITS keyword EQUINOX or EPOCH is put into the 'Equinox 1' field and any DATEOBS value is put into the 'Observation epoch' field. Positions in this input field correspond to the sky system found in the FITS header. One can exclude the observation epoch from the calculations by disabling the button next to the observation epoch's input field. With the epoch of observation, one corrects for the fact that the reference system FK4 is slowly rotating. Ecliptic coordinates: Ecliptic coordinates are coupled to a certain type of equatorial system. For a transformation of an equatorial coordinate to an ecliptic sky system one needs to know the reference system. Then a value of the obliquity of the ecliptic is calculated. This is the only variable in the rotation matrix for transformations between equatorial and ecliptic systems. ICRS has no associated equinox and therefore corresponds to an ecliptic system that is fixed for J2000. FK4 coordinates are catalogue coordinates that are not (yet) corrected for the elliptic terms of aberration. The system with real mean places is called FK4-NO-E (term originates from FITS). This system is the starting point for transformations to and from ecliptic coordinates (this applies to galactic coordinates also). Note that a coordinate (0,0) in an ecliptic system should transform to (0,0) in the corresponding equatorial system. Besides the standard sky- and reference systems, we also added the dynamical J2000 system. Offsets between ICRS, FK5 and dyn. J2000 are responsible for small differences between positions in these systems. The ecliptic system associated with dyn. J2000 can be precessed to another equinox using the IAU 2000 model for precession. #end section overview #begin section input INPUT ===== Position: -First input field: A position is a hexagesimal longitude followed by a space or a comma followed by a hexagesimal latitude. Right Ascensions are entered in hours minutes seconds. Other longitudes and declinations are entered in degrees minutes seconds E.g.: 23 59 59.9 -00 00 00.12 -Second input field: A position is a decimal longitude followed by a space or a comma followed by a decimal latitude. E.g.: 288.03 89.9 A field is cleared when you press CTRL-U on the keyboard. For the output of coordinates one can change the default precision with a selection from the 'PRECISION' menu on the top bar. Epoch, Equinox: For epochs and equinoxes a prefix is required to set the given number in the right context. The table below lists the possible prefixes and their meaning. Prefixes are case-insensitive. Prefix Epoch ============================================================= B Besselian epoch. Example 'B 1950', 'B1983.5', '-B1100' J Julian epoch. Example: 'J2000.7', 'J 2000', '-J100.0' JD Julian date. This number of days (with decimals) that have elapsed since the initial epoch defined as noon Universal Time (UT) Monday, January 1, 4713 BC in the proleptic Julian calendar Example: 'JD2450123.7' MJD The Modified Julian Day (MJD) is the number of days that have elapsed since midnight at the beginning of Wednesday November 17, 1858. In terms of the Julian day: MJD = JD - 2400000.5 Example: 'MJD 24034', 'MJD50123.2' RJD The Reduced Julian Day (RJD): Julian date counted from nearly the same day as the MJD, but lacks the additional offset of 12 hours that MJD has. It therefore starts from the previous noon UT or TT, on Tuesday November 16, 1858. It is defined as: RJD = JD - 2400000 MJD The Modified Julian Day (MJD) is the number of days that have elapsed since midnight at the beginning of Wednesday November 17, 1858. In terms of the Julian day: MJD = JD - 2400000.5 Example: 'MJD 24034', 'MJD50123.2' RJD The Reduced Julian Day (RJD): Julian date counted from nearly the same day as the MJD, but lacks the additional offset of 12 hours that MJD has. It therefore starts from the previous noon UT or TT, on Tuesday November 16, 1858. It is defined as: RJD = JD - 2400000 Example: 'RJD50123.2', 'RJD 23433' F 1) DD/MM/YY Old FITS format Example: '29/11/57' 2) YYYY-MM-DDFITS format Example: 'F2000-01-01' 3) YYYY-MM-DDTHH:MM:SS FITS format with date and time. Example: 'F2002-04-04T09:42:42.1' Examples: J1983.5 B 1983.5 F2008-03-31T8:09 F 2007-01-14T13:18:59.9 #end section input #begin section keywords KEYWORDS ======== EPOBS= Epoch of observation EPOCH1= Epoch for EQUINOX for left column EPOCH2= Epoch for EQUINOX for right column MAP= Name of a FITS file or a GIPSY set's descriptor file Other keywords are for internal program use only. #end section keywords #begin section about Purpose: Calculator for transformation of single positions between sky systems. Category: COORDINATES, UTILITY File: skycalq.src Author: J.P. Terlouw Keywords: See section keywords Version: 0.1 Notes: This task is derived from SKYCALC and uses PyQt4 instead of GIPSY's own GUI toolkit Ggi. A sufficiently recent PyQt4 package is required. It also uses the Python package 'kapteyn', which is not part of the GIPSY distribution and must be installed separately. More information about this package can be found via the link below. Related document: http://www.astro.rug.nl/software/kapteyn/ Updates: Feb 22, 2010: JPT, Version 0.1 #end section about