Zodiacal Light Calculator: Documentation

Overview

This is a brief introduction to the Zodiacal light as implemented in the Zodiacal Light Calculator. For a more detailed description of the zodiacal light, please see the references cited below.

The zodiacal light is mainly due to sunlight scattered by interplanetary dust grains. Because the Sun is a cool star, there will not be much contribution from the zodiacal light in the UV, particularly in the FUV.

The zodiacal light will effectively be a smooth background over the image plane. The level of the zodiacal light will be dependent on the helioecliptic longitude and β, the angle from the ecliptic.

On-line Calculator

The on-line calculator is a front end to the C program. The only inputs required are the date and the observing direction. The output is the zodiacal light spectrum in units of photons cm-2 s-1 sr-1 Å-1 plotted as a function of wavelength. The spectrum itself can be downloaded in a number of formats from the link provided in the output section.

Implementation

Problem Statement

In order to calculate the zodiacal light, we need:

  1. Sun position
  2. Zodiacal light spectrum
  3. Zodiacal distribution
 
Sun Position

CADS Zodiacal model uses libnova for calculating sun's position

Spatial Dependence

The spatial dependence of the zodiacal light has been tabulated by Leinert et al. and is reproduced here. The heliocentric longitude increases with row number and the β angle increases with column number. The units of the zodiacal light are 10-8 W m-2 sr-1 µm-1 at a wavelength of 500 nm. The scale factor to convert these units into photons cm-2 s-1 sr-1 Å-1 is 252 at 5000 Å; ie, the numbers in the table have to be multiplied by 252.

Zodiacal light as a function of ecliptic coordinates
β 0 5 10 15 20 25 30 45 60 75 90
03140 1610 985 640 275 150 10077
5 2940 1540 945 625 271 150 10077
10 4740 2470 1370 865 590 264 148 10077
15 11500 6780 3440 1860 1110 755 525 251 146 10077
20 6400 4480 2410 1410 910 635 454 237 141 9977
25 3840 2830 1730 1100 749 545 410 223 136 9777
30 2480 1870 1220 845 615 467 365 207 131 9577
35 1650 1270 910 680 510 397 320 193 125 9377
40 1180 940 700 530 416 338 282 179 120 92 77
45 910 730 555 442 356 292 250 166 116 9077
60 505 442 352 292 243 209 183 134 104 8677
75 338 317 269 227 196 172 151 116 93 8277
90 259 251 225 193 166 147 132 104 86 7977
105 212 210 197 170 150 133 119 96 82 77 77
120 188 186 177 154 138 125 113 90 77 7477
135 179 178 166 147 134 122 110 90 77 7377
150 179 178 165 148 137 127 116 96 79 7277
165 196 192 179 165 151 141 131 104 82 7277
180 230 212 195 178 163 148 134 105 83 7277
Spectral Effects

The zodiacal light is reddened but by not more than 20% so a first approximation is to simply use the solar spectrum from Colina et al. This spectrum is shown in Fig. 1 with a normalization described below.

zodiacal light spectrum
Fig.1 Zodiacal light from UV to IR

Although there are indications that the colour (the brightness relative to the Sun) of the zodiacal light is dependent on both wavelength and position, I have assumed that the colour is unity. The Colina et al. [spectrum] has been scaled such that the value at 5000 Å is 252, corresponding to 10-8 W m-2 sr-1 μm-1 m-1 at 5000 Å. Thus the spectrum simply has to be multiplied by the appropriate scale factor from the Table, with a colour correction if desired.

Input/Output

All inputs to the program need to be specified in the parameter file called zodiacal_initparams.txt. This is a plain text/ascii file, and internally documented well. If the parameter file does not exist, the program will generate one with default input values so that users can modify it as per their needs.

The output of the program is the zodiacal light at the specified coordinates and date in units of photons cm-2 s-1 sr-1 Å-1 and should be good to within 20%. More accurate values could be obtained by adding a colour to the spectrum.

References