Simulation of UV Sky
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[edit] Introduction
Although studies of the ultraviolet (UV) sky began in 1946 with observations of the Sun (ref) from captured V2 rockets with observations of UV radiation from stars beginning in 1957 (Byram et al. 1957 AJ, 62, 9), it was not until recently that a sensitive all-sky survey in the ultraviolet has been completed. The Galaxy Evolution Explorer (GALEX: Martin et al. 2006) has observed more than 80% of the sky in two bands (FUV: 1350 - 1700 A; NUV: 1700: 3200 A) since its launch in 2003 but with its mission expected to conclude in late 2011.
We have used GALEX data to predict the ultraviolet sky at any wavelength from 1300 A to 3200 A. The ultraviolet sky is comprised of a number of different sources (Murthy 2010), each of which with a dependence on different variables. This is a critical input into mission planning for new missions such as the Ultraviolet Imaging Telescope (UVIT) on the upcoming ASTROSAT mission (Agrawal et al. ) which are barred from observing bright areas of the sky due to safety considerations.
We have developed a web-based tool to predict the ultraviolet sky for an arbitrary space mission and have applied it to the UVIT instrument. We provide maps of the sky at different times of the year to guide observers in selecting their targets and in the avoidance of bright objects. This tool may readily be adapted to other instruments and wavelength bands.
[edit] A Web-Based All-Sky UV Predictor --- ASTUS
UV \bg simulator uses available GALEX data, Hipparcos (or any other) stellar catalogue, zodiacal-light and airglow calculator to simulate the image of the sky, specified for different telescope parameters, day of the year, coordinate frame and/or different image parameters.
[edit] Stellar contribution - Hipparcos and Kurucz
Since major contributor to the diffuse sky \bg is the scattering of starlight on the interstellar dust grains, it is important to evaluate this contribution to the total diffuse radiation. The model of the interstellar radiation field (ISRF) that we have used here was first described by Sujatha et al. (2004). In brief, the Hipparcos digital catalogue was used as the source of data: stars, their location, spectral type and brightness. Based on the spectral type of the star, a spectral energy distribution for each star was calculated using the appropriate Kurucz models (Sujatha et al. 2004, Kurucz 1992). Intrinsic UV flux of the star was calculated from the observed visual magnitude by ???
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Formula and code.....
[edit] Galactic background - GALEX database
The primary GALEX data products are images of the given field in two GALEX NUV and FUV bands and a merged point source catalogue (Morrissey et al. 2007), which includes the measures of a star flux and a background at its position. We have downloaded the meadians of the backgrounds from the publicly accessible archival (MAST) database and tabulated them. Since GALEX data includes both arglow and zodiacal light, we had subtracted these values from the downloaded data and tabulated only the derived backgrounds.
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Code and implementation on the webpage - Reks
[edit] Zodiacal light
Zodiacal light, sunlight scattered by interplanetary dust, is an important contributor to the UV background. However, it can be easily estimated since it is a essentially a solar spectrum which can be scaled to the UV. The level of the zodiacal light depends on the time and date of observation and will vary if the observations consist of several exposures over months or years. Thus, the zodiacal light contribution to the UV background has to be known to subtract it from the measured values for correct derivation of the true diffuse background. It contributes only to the near-UV, since the solar flux is negligible below about 2000 \A.
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Code.... for calculation. Presentation on the webpage - Reks.
[edit] Airglow
Airglow is the weak light emission from the Earth's upper atmosphere mostly due to the reaction of atmospheric molecules with solar photons, cosmic rays or atmospheric ions. It is an important contributor to the diffuse UV background for the missions on the low Earth's orbit and is difficult to model due to the strong dependance on the altitude, observation time, zenith angle and solar cycle. It is a strong function of the local time (Sujatha et al. 2009) as well, and its distribution is usually determined empirically. Since UV detectors are extremely sensitive, most UV observations are performed only during the local night. Thus, in ASTUS we have assumed the airglow to be constant at the mean value of 200 phot/cm$^2^/sec/sr/\AA (references?). We are planning in the future to implement the option that allows the users to change this constraint to their own input.
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Code and implementation on the webpage - Reks.
[edit] Software Design and Implementation
[edit] Architecture
[edit] block diagram
[edit] inputs and outputs
[edit] screen shots
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[edit] Application
The nearest UV mission will be the UVIT, a payload on the ASTROSAT spacecraft, to be launched in 2012 by the Indian Space Research Organization (ISRO). UVIT will map a small regions of sky (FOV: $0.3^{\circ}$) to a higher spatial resolution ($1.^{\as}5-1.^{\as}8$) than GALEX ($4.^{\as}5-6.^{\as}0$). As part of the multi-wavelength mission, the UVIT science goals are optimized for combined studies with the X-ray payloads and, as with GALEX, bright regions cannot be observed due to the sensitivity of the intensified CMOS detectors [ref, may be Handbook?], thus one of the important goals in the UVIT science planning is to be able to predict where the UVIT shall not look.
As an example of the application of the UV Sky Predictor, we have constructed all-sky images as seen by UVIT. We also have identified the overbright areas to be avoided and estimated the seasonal variability of the background. All results are presented for five out of all UVIT filters (Fig.~\ref{fig:filters}). We have selected one far-UV filter ($BaF_2$ as it is has the widest bandpass of all FUV filters) and four near-UV filters as the most representative filters, excluding the visible ones. The basic pre-flight characteristics of the selected filters are shown in Table~\ref{tab:filters}.
The effective bandwidth was calculated as the integral of the normalized effective areas. However, it shall be kept in mind that the effective bandwidth may be dependent on the spectral shape of radiation for wide filters. The central, or 'mean', wavelength was calculated as \be \l_{\rm c}=\fr{\int \l A_{\rm norm}(\l) d\l}{\int A_{\rm norm}(\l) d\l}\,, \label{eq:central} \ee where $A_{\rm norm}(\l)$ is the effective area normalized to 1. An exact relationship between $F_{\lambda}$ and $F_{\nu}$, the source intrinsic spectral-energy distribution in energy and frequency units, respectively, is provided by the pivot wavelength of the system, \be \l_{\rm p}=\sqrt{\fr{\int A_{\rm norm}(\l) \l d\l}{\int A_{\rm norm}(\l) d\l/\l}}\,, \label{eq:pivot} \ee Both central and pivot wavelengths are independent of the spectrum of the source. The effect of the source power distribution over a given filter is included in the filter's effective wavelength $\l_{\rm eff}$, \be \l_{\rm eff}= \fr{\int \l A(\l) F(\l) d\l}{\int A(\l) F(\l)d\l}\,, \label{eq:effective} \ee where $F(\l)$ is the source spectrum in ergs/cm$^2$/sec/\AA. This is the mean wavelength of the passband as weighted by the energy distribution of the source over the band. In Table~\ref{tab:filters} we give the effective wavelengths for Vega (except of the BaF$_2$ filter, for which it is the spectrum of A1V star).
\begin{table*} \centering \begin{minipage}{140mm} \caption{Basic characteristics of selected filters.} \begin{tabular}{lccccc} \hline & $BaF_2$ & NUVB15 & NUVB13 &NUVB4 &NUVN2 \\ & (\AA) & (\AA) & (\AA) &(\AA) &(\AA) \\ \hline Wavelength range &1300--1830 &1900--2400&2200--2650&2445--2825&2730--2880 \\ Bandpass $\D \l$ & 378.1 & 281.7 & 270.5 & 282.3 & 89.5 \\ Central $\l_{\rm c}$ & 1549.6 & 2435.5 & 2182.9 & 2428.0 & 2789.9 \\ Pivot $\l_{\rm p}$ & 1544.6 & 2433.6 & 2181.08 & 2616.4 & 2789.7 \\ Effective $\l_{\rm eff}$& 1232.3 & 2433.2 & 2171.0 & 2629.0 & 2791.9 \\ \hline \end{tabular} \end{minipage} \label{tab:filters} \end{table*}
