;+ ; NAME: ; EQ2HOR ; ; PURPOSE: ; Convert celestial (ra-dec) coords to local horizon coords (alt-az). ; ; CALLING SEQUENCE: ; ; eq2hor, ra, dec, jd, alt, az, [ha, LAT= , LON= , /WS, OBSNAME= , $ ; /B1950 , PRECESS_= 0, NUTATE_= 0, REFRACT_= 0, $ ; ABERRATION_= 0, ALTITUDE= , /VERBOSE, _EXTRA= ] ; ; DESCRIPTION: ; This is a nice code to calculate horizon (alt,az) coordinates from equatorial ; (ra,dec) coords. It is typically accurate to about 1 arcsecond or better (I ; have checked the output against the publicly available XEPHEM software). It ; performs precession, nutation, aberration, and refraction corrections. The ; perhaps best thing about it is that it can take arrays as inputs, in all ; variables and keywords EXCEPT Lat, lon, and Altitude (the code assumes these ; aren't changing), and uses vector arithmetic in every calculation except ; when calculating the precession matrices. ; ; INPUT VARIABLES: ; RA : Right Ascension of object (J2000) in degrees (FK5); scalar or ; vector. ; Dec : Declination of object (J2000) in degrees (FK5), scalar or vector. ; JD : Julian Date [scalar or vector] ; ; Note: if RA and DEC are arrays, then alt and az will also be arrays. ; If RA and DEC are arrays, JD may be a scalar OR an array of the ; same dimensionality. ; ; OPTIONAL INPUT KEYWORDS: ; lat : north geodetic latitude of location in degrees ; lon : EAST longitude of location in degrees (Specify west longitude ; with a negative sign.) ; /WS : Set this to get the azimuth measured westward from south (not ; East of North). ; obsname: Set this to a valid observatory name to be used by the ; astrolib OBSERVATORY procedure, which will return the latitude ; and longitude to be used by this program. ; /B1950 : Set this if your ra and dec are specified in B1950, FK4 ; coordinates (instead of J2000, FK5) ; precess_ : Set this to 1 to force precession [default], 0 for no ; precession correction ; nutate_ : Set this to 1 to force nutation [default], 0 for no nutation. ; aberration_ : Set this to 1 to force aberration correction [default], ; 0 for no correction. ; refract_ : Set to 1 to force refraction correction [default], 0 for no ; correction. ; altitude: The altitude of the observing location, in meters. [default=0]. ; verbose: Set this for verbose output. The default is verbose=0. ; _extra: This is for setting TEMPERATURE or PRESSURE explicitly, which are ; used by CO_REFRACT to calculate the refraction effect of the ; atmosphere. If you don't set these, the program will make an ; intelligent guess as to what they are (taking into account your ; altitude). See CO_REFRACT for more details. ; ; OUTPUT VARIABLES: (all double precision) ; alt : altitude (in degrees) ; az : azimuth angle (in degrees, measured EAST from NORTH, but see ; keyword WS above.) ; ha : hour angle (in degrees) (optional) ; ; DEPENDENCIES: ; NUTATE, PRECESS, OBSERVATORY, SUNPOS, ADSTRING() (from the astrolib) ; CO_NUTATE, CO_ABERRATION, CO_REFRACT, ALTAZ2HADEC ; ; BASIC STEPS ; Apply refraction correction to find apparent Alt. ; Calculate Local Mean Sidereal Time ; Calculate Local Apparent Sidereal Time ; Do Spherical Trig to find apparent hour angle, declination. ; Calculate Right Ascension from hour angle and local sidereal time. ; Nutation Correction to Ra-Dec ; Aberration correction to Ra-Dec ; Precess Ra-Dec to current equinox. ; ; ;CORRECTIONS I DO NOT MAKE: ; * Deflection of Light by the sun due to GR. (typically milliarcseconds, ; can be arseconds within one degree of the sun) ; * The Effect of Annual Parallax (typically < 1 arcsecond) ; * and more (see below) ; ; TO DO ; * Better Refraction Correction. Need to put in wavelength dependence, ; and integrate through the atmosphere. ; * Topocentric Parallax Correction (will take into account elevation of ; the observatory) ; * Proper Motion (but this will require crazy lookup tables or something). ; * Difference between UTC and UT1 in determining LAST -- is this ; important? ; * Effect of Annual Parallax (is this the same as topocentric Parallax?) ; * Polar Motion ; * Better connection to Julian Date Calculator. ; ; EXAMPLE ; ; Find the position of the open cluster NGC 2264 at the Effelsburg Radio ; Telescope in Germany, on June 11, 2023, at local time 22:00 (METDST). ; The inputs will then be: ; ; Julian Date = 2460107.250 ; Latitude = 50d 31m 36s ; Longitude = 06h 51m 18s ; Altitude = 369 meters ; RA (J2000) = 06h 40m 58.2s ; Dec(J2000) = 09d 53m 44.0s ; ; IDL> eq2hor, ten(6,40,58.2)*15., ten(9,53,44), 2460107.250d, alt, az, $ ; lat=ten(50,31,36), lon=ten(6,51,18), altitude=369.0, /verb, $ ; pres=980.0, temp=283.0 ; ; The program produces this output (because the VERBOSE keyword was set) ; ; Latitude = +50 31 36.0 Longitude = +06 51 18.0 ; Julian Date = 2460107.250000 ; Ra, Dec: 06 40 58.2 +09 53 44.0 (J2000) ; Ra, Dec: 06 42 15.7 +09 52 19.2 (J2023.4422) ; Ra, Dec: 06 42 13.8 +09 52 26.9 (fully corrected) ; LMST = +11 46 42.0 ; LAST = +11 46 41.4 ; Hour Angle = +05 04 27.6 (hh:mm:ss) ; Az, El = 17 42 25.6 +16 25 10.3 (Apparent Coords) ; Az, El = 17 42 25.6 +16 28 22.8 (Observer Coords) ; ; Compare this with the result from XEPHEM: ; Az, El = 17h 42m 25.6s +16d 28m 21s ; ; This 1.8 arcsecond discrepancy in elevation arises primarily from slight ; differences in the way I calculate the refraction correction from XEPHEM, and ; is pretty typical. ; ; AUTHOR: ; Chris O'Dell ; Univ. of Wisconsin-Madison ; Observational Cosmology Laboratory ; Email: odell@cmb.physics.wisc.edu ;- pro eq2hor, ra, dec, jd, alt, az, ha, lat=lat, lon=lon, WS=WS, obsname=obsname,$ B1950 = B1950, verbose=verbose, precess_=precess_, nutate_=nutate_, $ refract_ = refract_, aberration_ = aberration_, $ altitude = altitude, _extra= _extra On_error,2 compile_opt idl2 if N_params() LT 4 then begin print,'Syntax - EQ2HOR, ra, dec, jd, alt, az, [ha, LAT= , LON= , /WS, ' print,' OBSNAME= ,/B1950 , PRECESS_= 0, NUTATE_= 0, REFRACT_= 0 ' print,' ABERRATION_= 0, ALTITUDE= , /VERBOSE, TEMPERATURE=, ' +$ 'PRESSURE = ]' return endif ;******************************************************************************* ; INITIALIZE STUFF ; If no lat or lng entered, use Pine Bluff Observatory values! ; (near Madison, Wisconsin, USA) ; * Feel free to change these to your favorite observatory * if n_elements(lat) eq 0 then lat = 43.0783d ; (btw, this is the declination ; of the zenith) if n_elements(lon) eq 0 then lon = -89.865d if n_elements(altitude) eq 0 then altitude = 0. ; [meters] if keyword_set(obsname) then begin ;override lat,lon, altitude if observatory name has been specified observatory, obsname, obs lat = obs.latitude lon = -1*obs.longitude ; minus sign is because OBSERVATORY uses west ; longitude as positive. altitude = obs.altitude endif if n_elements(precess_) eq 0 then precess_ = 1 if n_elements(nutate_) eq 0 then nutate_ = 1 if n_elements(aberration_) eq 0 then aberration_ = 1 if n_elements(refract_) eq 0 then refract_ = 1 v = keyword_set(verbose) ; conversion factors d2r = !dpi/180. h2r = !dpi/12. h2d = 15.d ra_ = ra ; do this so we don't change ra, dec arrays. dec_ = dec if v then print, 'Latitude = ', adstring(lat), ' Longitude = ', adstring(lon) if v then print, 'Julian Date = ', jd, format='(A,f15.6)' if keyword_set(B1950) then s_now=' (J1950)' else s_now=' (J2000)' if v then print, 'Ra, Dec: ', adstring(ra_,dec_), s_now ;****************************************************************************** ; PRECESS coordinates to current date ; (uses astro lib procedure PRECESS.pro) J_now = (JD - 2451545.)/365.25 + 2000.0 ; compute current equinox if precess_ then begin if keyword_set(B1950) then begin for i=0,n_elements(jd)-1 do begin ra_i = ra_[i] & dec_i = dec_[i] precess, ra_i, dec_i, 1950.0, J_now[i], /FK4 ra_[i] = ra_i & dec_[i] = dec_i endfor endif else begin for i=0,n_elements(jd)-1 do begin ra_i = ra_[i] & dec_i = dec_[i] precess, ra_i, dec_i, 2000.0, J_now[i] ra_[i] = ra_i & dec_[i] = dec_i endfor endelse endif if v then print, 'Ra, Dec: ', adstring(ra_,dec_), ' (J' + $ strcompress(string(J_now),/rem)+')' ;****************************************************************************** ; calculate NUTATION and ABERRATION Corrections to Ra-Dec co_nutate, jd, ra_, dec_, dra1, ddec1, eps=eps, d_psi=d_psi co_aberration, jd, ra_, dec_, dra2, ddec2, eps=eps ; make nutation and aberration corrections ra_ = ra_ + (dra1*nutate_ + dra2*aberration_)/3600. dec_ = dec_ + (ddec1*nutate_ + ddec2*aberration_)/3600. if v then print, 'Ra, Dec: ', adstring(ra_,dec_), ' (fully corrected)' ;************************************************************************************** ;Calculate LOCAL MEAN SIDEREAL TIME ct2lst, lmst, lon, 0, jd ; get LST (in hours) - note:this is independent of ;time zone since giving jd lmst = lmst*h2d ; convert LMST to degrees (btw, this is the RA of the zenith) ; calculate local APPARENT sidereal time LAST = lmst + d_psi *cos(eps)/3600. ; add correction in degrees if v then print, 'LMST = ', adstring(lmst/15.) if v then print, 'LAST = ', adstring(last/15.) ;****************************************************************************** ; Find hour angle (in DEGREES) ha = last - ra_ w = where(ha LT 0) if w[0] ne -1 then ha[w] = ha[w] + 360. ha = ha mod 360. if v then print, 'Hour Angle = ', adstring(ha/15.), ' (hh:mm:ss)' ;****************************************************************************** ; Now do the spherical trig to get APPARENT alt,az. hadec2altaz, ha, dec_, lat, alt, az, WS=WS if v then print,'Az, El = ', adstring(az,alt), ' (Apparent Coords)' ;******************************************************************************************* ; Make Correction for ATMOSPHERIC REFRACTION ; (use this for visible and radio wavelengths; author is unsure about other wavelengths. ; See the comments in CO_REFRACT.pro for more details.) if refract_ then alt = $ co_refract(alt, altitude=altitude, _extra=_extra, /to_observed) if v then print,'Az, El = ', adstring(az,alt), ' (Observer Coords)' end