13.1 INTRODUCTION

                 This  chapter  describes  a  number  of  options  useful  in

            photometry  and  spectrophotometry.   Surface  photometry  is not

            discussed  here,  but  in  Chapter  14.    Several   calculations

            important  in  broadband  photometry  can be carried out with the

            option MC.  These include determinations of the average magnitude

            over  an  instrumental  and/or  a  standard  band,  the effective

            wavelength  over  an  instrumental  and/or  standard  band,   the

            absolute  flux,  the  K-correction  over an instrumental and/or a

            standard  band,  and  the  differential  K-correction.   Aperture

            photometry  is  carried  out with option BB.  The construction of

            photometry calibration curves  is  done  with  option  BP.   This

            option will read the character file produced by BB and a standard

            star  file  to  produce  the  calibaration  curve.   Instrumental

            magnitudes  are  then  converted to standard magnitudes with this

            same option.  Note that the format  of  the  standard  star  file

            [MIIPS.PHOTOM]STANDARDS.DAT is scheduled for revision sometime in

            1987.  Photographic plates traced with  a  PDS  microdensitometer

            can  be calibrated with option PD and converted to intensity with

            option CA.  As of this writing, option PD is not yet operational.

                 Several older routines, in use at  Mt.   Stromlo,  are  also

            available.   They  are  options  OK, AF, PM, and ND, which enable

            flux calibrations and spectrophotometry to be  done  on  spectral

            data.   The data input to these four options must be in SAD files

            of LAMBDA (wavelength) bintype.  The user should therefore do all

            flat-fielding,   coincidence  corrections  and  scrunching  (into

            wavelength  bintype)  before  entry  into  this   section.    Sky

            subtraction  can also be done first, or the skies calibrated into

            flux before subtraction.  The option RE (REBIN) exists to convert

            F(lambda)  vs.  lambda to F(nu) vs.  frequency (and vv.).  Option

            AR (ARITHMETIC) can be used to produce magnitude vs.   lambda  or

            frequency.   The data-type (FITS keyword BUNIT:  brightness unit)

            and bintype (FITS  keyword  CTYPE1)  are  described  in  the  SAD

            header,  and  can  be  easily read by using the PH (PRINT HEADER)

            command.  The header information can also be edited  with  option


            Examples of typical header info.  follows:

                KEY     VALUE           COMMENTS        DESCRIPTIVE NOTES

                                                        (not on header)

                CTYPE1= LAMBDA          /metres         : wavelength bintype

                CRPIX1= 1.0             /               : reference pixel

                CRVAL1= 4020.0E-10      /metres         : wavelength of pixel 1

                CDELT1= 0.47E-10        /metres         : increment per pixel

                PIXOR=  CENTRE          /               : flag specifying pixel

                                                        : origin (after 25-6-81)

                BUNIT=  FLAMBDA         /erg/cm**2/sec/A        : brightness

            or  BUNIT=  FNU             /erg/cm**2/sec/Hz       : unit

            Other information is generally created at  time  of  observation.

            You can insert any other info.  for your own convenience using EH

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            and option I (Insert).  Other keywords recognised by  options  in

            this section are:

                RUN=    43              /               : observational run no.

                                                        : not (yet) on PCA data

                DWELL=  500.000         /               : dwell time in secs

                OBJECT= A VERY FAINT SOURCE /           : ident. - user supplied

            Values associated with these keywords appear in printed output at

            various places and are a useful aid to bookeeping.

            13.2 OPTION BP, PHOTOMCAL

            13.2 OPTION BP, PHOTOMCAL

            13.3 OPTION BB, BBEN

            13.4 OPTION BP, PHOTOMTRY

            13.5 OPTION PD, PDCAL

            13.6 OPTION OK, OKECAL

                 A large compendium of flux standards (refs:  Oke 1974, Stone

            1977,  Breger  1976  etc.)  is  stored  in [MIIPS.DOC]OKESTD.FLX.

            These standards are tabulated as ABv  vs.   wavelength  (together

            with  bandpasses), where ABv = -2.5.LOG10[ F(nu) ] - 48.60.  If a

            required standard does not exist in the standard file,  the  user

            must  enter  the values.  Secondary standards (user specific) can

            be created on your area by using PM with the SCANNER filter  set.

            This produces a file SECOND.FLX on your area - with an identifier

            supplied by you.  OK searches SECOND.FLX for a requested standard

            if no match is found in the standard file.

                 An  observed  flux  standard  is  photometered  within   the

            tabulated   bandpasses,  and  the  observed  magnitudes  (OBSMAG)

            differenced against the OKE magnitudes (AB).  A plot of OBSMAG-AB

            points  vs.   wavelength  results,  and  the user can then delete

            spurious points, and fit a smooth curve (poly  or  spline).   The

            final   curve   represents   the   instrumental   correction  (in

            magnitudes) for each pixel, and is applied to  the  observational

            data in AF (ABSFLX).


            a)  Undue  reliance  should  not  be  placed  on  a  single  flux

            calibration standard.  Observing at least 2 standards will enable

            an objective assesment of  instrumental  photometric  performance

            and  stability  to  be  made.   The  error per flux point in some

            standards can be quite high.

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            b) Not all flux standards are calibrated throughout  the  optical

            region - check this before observing.  Only the defined region is

            plotted in OKECAL -  check  the  full  calibration  curve  before


            c)  Calibration  curves  can  be  averaged   (AD   or   AR)   and

            intercompared  by  dividing  (for  example)  or  using LR (Linear

            Regression).   Option  ME  (MERGE)  can  be   used   to   average

            calibration  curves  over  sub-regions of the observed wavelength



                x       :       wavelength in A

                v       :       frequency in Hz

                Fx      :       F(lambda)

                Fv      :       F(nu)

                c       :       speed of light in A/sec

            i) The default output from AF is F(lambda) vs lambda;  the OKE

            magnitudess are converted to:

                OKEMAGx = -2.5 LOG10 Fx

                        = -2.5 LOG10 Fv - 2.5 LOG10 (c/x**2)

                        = ABv + 48.60 -2.5 LOG10 (c/x**2)

            on entry and used in this form thereafter.

            ii) The AIRMAS is calculated from the SAD header info (which must

            therefore  be correct !).  In both OKECAL and ABSFLX the user can


            select extinction coefficients (default slope of lambda4 term  is


            iii) The observed magnitudes are calculated as:

                OBSMAGx = -2.5 LOG10 (RATE)

            where RATE is the average rate (counts/sec/A) in the bandpass.

            iv) The calibration curve is stored as:

                K(I)    = OBSMAGx - EXTCO*AIRMAS - OKEMAGx

                        = -2.5 LOG10 [obs. rate above atmos/F(lambda) ]

            for each pixel, where:

                LAMBDA(I) = start + (I-1)*increment.

            The calibration curve has  the  FITS  label:   BUNIT  =  KLAMBDA.

            These  curves  can be plotted in the normal way using PL, RP, WP,

            QP etc.

            13.7 OPTION AF, ABSFLX

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                 Option AF converts to absolute flux.  Its output  is  either

            F(lambda)  vs.   lambda  or F(nu) vs.  lambda.  AF will also flip

            the data-type for flux calibrated data from F(lambda) <--> F(nu):

            the option will presume a flip is required for already calibrated

            data, and ask you to verify this.  This option  takes  the  final

            (averaged  ?) calibration curve and applies it to the rest of the

            data,  which  has   otherwise   been   prepared   (flat-fielding,

            coincidence  correction,  scrunching etc) in exactly the same way

            as the flux standard(s).

            Select output as either:

                                        1 F(lambda) vs. lambda

                                or      2 F(nu)     vs. lambda

            - although this can be flipped by a further application of AF.


            a) AIRMAS is calculated  from  header  info  -  extinction  coefs

            should be as used in OKECAL.

            b) For 1DPCA data applying a calibration curve appropriate to the

            TOP  array  (say)  to  data  in  the  BOTTOM  array doesn't work.

            Separate calibration curves must be made for each array.   If  no

            standard  is  available,  a  secondary  standard can be made from

            existing data using  PM  (Photometry)  with  the  SCANNER  (50  A

            rectangular bands) filter set.

            c) For 2D data there are 2 observational possibilities:  i) trail

            the standard along the slit and produce a calibration       curve

            for each row - not yet implemented in SPECT, but can be.  ii)  Do

            the objects at a fixed slit position, then sum constant 

                object rows and subtract constant sky rows to produce '1 row'

            data.  This is obviously unsatisfactory for extended objects.  or

            iii) use CD to produce a mask from the calib star  and  then  use

            this   mask  to  extract  (XT)  individual  line  spectra  to  be

            calibrated.  The std may be masked, used in OK to ->  calibration

            and  then  EITHER  the  entire  2D  map calibrated and the "line"

            spectra extracted later OR the  extraction  may  be  done  first.

            (The opp is commutative.)


            for each pixel


                OBSMAGx = -2.5 LOG10[ DATA(I)/(Dwellinc) ]

            corrected mag

                CORMAGx = OBSMAGx - EXTCO*AIRMAS

                        = -2.5 LOG10 [obs RATE above atmosphere]

                OKEMAGx = CORMAGx - K(I)                ( K from OKECAL )

                        = -2.5 LOG10[ Flambda ]

            then output is

                FLAMBDA = 10**[ -0.4*OKEMAGx ]          erg/cm**2/sec/A

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              or        FNU     = FLAMBDA * X**2/c              erg/cm**2/sec/Hz

            13.8 OPTION PM, PHOT2

                This option accepts  as  input  data-types  FLAMBDA  or  FNU.

            Various filter systems:

                        UBVRI           (Buser 1978, Azusienis & Straizys 1969,

                                         Bessell 1979)


                        CMT1T2          (Canterna 1979)

                        STROMGREN uvby  (Crawford & Barnes 1970)

                        SCANNER         50 A rectangular bands

            are compiled in [MIIPS.DOC]PHOTOM.SYS.

                The  filter   transmissions   are   calibrated   at   varying

            wavelengths  appropriate to their bandwidths.  The user selects a

            filter system, and filters within  the  observational  wavelength

            region  are  interpolated  (TSINC)  onto the data grid.  For each

            filter, and each spectral row the following quantities  are  then


                       filter mean Flambda

                           filter mean Fnu

                        effective wavelength

                MAG             -2.5 LOG10[  ] - 48.60 - ZERO

            The zero points for each filter are included in the library,  and

            are  adjusted  to  give  good  correspondence  with  the standard

            system.     To produce a secondary standard (stored in SECOND.FLX

            on  your  area),  use  PM  on  one row of one map, select SCANNER

            system, and give your standard a unique  identifier  when  asked.

            This standard can then be accessed by OKECAL in the normal way.

            FORMULAE    ( @ = Integral sign )

                        Fxdx    =       Fvdv

                      @ FxRxdx  =     @ FvRvdv           : energy in filter

                AVFLAM=     = @ FxRxdx/@ Rxdx        : filter mean Flambda

                AVFNU =     = @ FvRvdv/@ Rvdv

                                = @ FxRxdx/@ Rvdv/dx.dx  : filter mean Fnu

                EFFLAM=      = @ FxRx.x.dx/@ FxRxdx   : effective wavelength

                MAG             =  -2.5 LOG10[  ] -48.60 - ZERO

            13.9 OPTION ND, NDFILT

                 This subroutine selects a neutral density (or other)  filter

            from  those available on disc file.  filters are assumed to cover

            the full wavelength range of the data individual filters are read

            in  at  a  wavelength  spacing  (del)  given  on the file and the

            interpolated into the spacing (xinc) corresponding to  the  data.

            the filter values are then returned in the array fnew which is in

            pixel to pixel correspondence with the data.

            name: name of the filter required

            fold: filter values read from filter.set

            xold: corresponding wavelength values

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                fnew: array containing filter transmission values

                    at   lambda=xst+(i-1)*xinc

            xnew: corresponding wavelength values

            xst: start wavelength corresponding to first pixel

            xinc: inc. wavelength per pixel

            ni: dimension of fnew (and of data)

            file structure  (t1,t2,...: %transmission values)

            #filter name                comments

                        start   delta   nvals

                        t(1)    t(2)    t(3)    .       .       .

                        .       .       .       .       .       .

                        .       .       t(nvals)

            #filter name                comments

                        start   delta   nvals

                        t(1)    t(2)    t(3)    .       .       .

                        .       .       .       .       


            Azusienis & Straizys 1969 Soviet Astron. A.J. 13, 316

            Bessell 1979 PASP 91, 589

            Breger 1976 Ap.J. Suppl. 32, 7

            Buser 1978 AA 62, 411

            Canterna 1979 Dud. Obs. Rep. 14, 489


            Crawford & Barnes 1970

            Oke 1974 Ap.J. Suppl. 27, 21

            Stone 1977