Table of Contents


ccdsky - lazy (cosmology) sky scaling calculator


ccdsky [parameter=value]


ccdsky is a lazy sky scaling calculator. It will give you the scaling parameters for a FITS file (which uses degrees for angles, and m/s for velocities), to make your typical observer or astronomical software package happy and print out values in units observers can be in awe about. It is also useful to remind yourself how many arcseconds/minutes a given object is at a given distance.

Although you can transform an image/cube, the default only needs to know the distance (in AU, pc, kpc, mpc, z, ...) and unit size (in AU, pc, kpc, mpc, ....) of the object. This will give you the scaling factors for length and velocities.

When the distance is given in z, Wright’s (2006) cosmology calculator is used.


The following parameters are recognized in any order if the keyword is also given:
Optional input image file. Must be in image(5NEMO) format.
Output image file, only required if an input image is given. Will be in image(5NEMO) format, with optionally scaled image values and axes coordinates.
Distance to your object, and optionally a unit. AU, pc, Kpc, Mpc, and Gpc are allowed. You can also specify a dimensionless number ’z’, in which case the special cosmology calculator is used. [default: 1,pc]
Length scale of object, and optionally a unit. km, AU, pc, Kpc, Mpc, and Gpc are allowed. Together with a distance d=, this will convert the length scale in your maps to degrees, required for FITS. [default: 1,AU]
Velocity scale of object, and optionally a unit. The output units will be the one that FITS uses, m/s. [Default: 1,km/s]
Total flux, in, of the source. Based on the distance, it will the compute the HI and H2 (given some reasonable X factor) masses, in units of solar masses. Default: 1
Scale the intensity values in the data itself. Default: 1.
Specify the cosmology model, with up to 3 parameters. Only used when units in d= are ’z’ were specified. The first parameter is the Hubble Constant. Optionally followed by the WM (Omega(matter)) and WV (Omega(vacuum)), for different cosmologies. If only H0 and WM are given, a flat universe (WV=0) is used. See also Wright(2006). WMAP-9 values: H=71,0.27,0.73 (2011?), Planck-1 values: H=67.15,0.317,0.683 (2013) [Default: 71,0.27,0.73]


Viewing a model at a distance 1 pc, where 1 length unit represents 1 AU, the coordinates need to be multiplied by 0.000277785 in order for FITS to view them in the correct angular size of 1 arcsec.
% ccdsky d=1,pc r=1,AU v=1,km/s
d=1 pc
r=1 AU
v=1 km/s
rscale=0.000277785  (1.00003 arcsec)

To find out the radius of 2 pc at the distance of the galactic center:

ccdsky r=2,pc d=8.5,kpc
rscale=0.0134814  (48.5329 arcsec)

Here is an example of creating a small bar, at position angle 30, and observed at RA=6h and DEC=30d:

% ccdgen "" map4 bar 1,10,0.5,30 size=512,512,1 
% ccdsky map4 map4b
% ccdfits in=map4b out=map4b.fits radecvel=t crval=90,30 crpix=256.5,256.5
% # now switch to MIRIAD
% fits in=map4b.fits out=map4b.mir op=xyin
% cgdisp in=map4b.mir device=/xs labtyp=arcmin
and you should see a bar (possibly with a sign error position angle) of about 1 arcmin in length, in an 8 arcmin field. Notice that ccdfits(1NEMO) also has various options to specify a WCS which can override the one set by ccdgen(1NEMO) .


Although the conversion from an observed HI flux to mass is not too controversial (sans optical depth effects), the conversion from CO to a mass is still under debate. We use the value 2e20 cm-2/K km/s.

See Also

ccdgen(1NEMO) , ccdmath(1NEMO) , ccd2vel(1NEMO) , ccdfits(1NEMO) , snapscale(1NEMO) , image(5NEMO) A Cosmology Calculator for the Web (E.Wright) The CosmoCalc website


NEMO/src/image/trans    ccdsky.c - source code


Peter Teuben

Update History

17-Aug-2012    V1.0 Created        PJT
23-aug-2012    V1.1 added sdv=        PJT
28-aug-2012    V1.2 implemented scale=    PJT
28-feb-2013    V2.2 more verbose, added H=    PJT
16-mar-2013    V3.0 added Wright’s cosmology calculator    PJT

Table of Contents