Mapping the Diffuse Ultraviolet Sky with GALEX
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[edit] Introduction
The objective is to create a high resolution all-sky map of the diffuse UV background in galactic co-ordinates. The GALEX sky background images are all of size 3840 x 3840 pixels with each pixel covering a region of 1.5" of the sky. More information on the sky background images can be found here. This involves binning the sky background data into appropriately sized bins to provide enough spatial resolution for preserving the small scale variations and making the all-sky map. The steps involved are explained below.
[edit] Binning the Sky Background Data
The fd/nd-skybg images are grouped into bins of size 192" to a side. With each pixel covering a region of 1.5", each bin has a size of 128 x 128 pixels. For each bin thus generated, the mean and median pixel intensity values are calculated and stored in ASCII format along with the corresponding positional information (in both detector co-ordinates and galactic co-ordinates) and the number of non-zero pixels averaged. These ASCII files are stored in the following format with each row corresponding to one bin.
"<X> <Y> <GLON> <GLAT> <Mean> <Median> <No.of pixels>"
Also, another ASCII file has been generated for each skybg image which gives the center co-ordinates in both equatorial and galactic co-ordinates along with the corresponding FUV/NUV exposure times in the following format:
"<Filename> <RA> <DEC> <GLON> <GLAT> <FUV_EXPTIME> <NUV_EXPTIME>"
About 61170 images have been run through the binning program and the extracted data has been saved in ASCII format. Apart from the standard bin size of 192" to a side, another dataset has also been obtained with the bin size set to 384" to a side (which corresponds to 256x256 sized bins).
[edit] Generation of the Detector Map
A 2-D surface map of the detector in physical co-ordinates has been generated to measure the detector response. The detector map has a size of 30x30 pixels, with a resolution of 192" to a pixel, as explained above. The frames in which the circular aperture was off-center have been excluded from the dataset used to generate this surface map. A list of these files can be accessed here.
The intensity value for each pixel is computed by normalising the median value for each bin with the corresponding exposure time for the image and taking the mean. The detector map (combined FUV and NUV) so generated for a bin size of 192" is shown below:
The separate FUV and NUV detector images are shown below:
The peak in the middle is more pronounced for the NUV detector map. This can be clearly seen in the cross-sectional plot that is shown below.
The above images were plotted again with the maximum value normalized to 1 to study the percentage difference between the dips and rises. The corresponding cross-sectional plots are given below:
The skybg images cover a region of 1.6 degree each (3840 pixels per side with 1.5"/pixel). We are interested in the central portion which covers 1 deg. square. From the binned detector maps shown above, the central 16 X 16 bins have been extracted. This was calculated as shown below:
To cover a region of 1 deg. we need the central 3600/1.5(=2400) pixels; in the binned detector map (30x30), this means excluding the outer ~7 bins from each side to get the required field of view. These values were normalized to 1 and plotted to give the following detector maps for FUV and NUV.
The NUV detector map has a pronounced central peak compared to the FUV map. It was found out that about 11410 NUV files had this pattern. This is about one-third of the total number of NUV files. A new NUV detector image generated from the remaining ~20000 NUV files will be uploaded soon.
The following images show the detector image, surface plot and the separate FUV/NUV detector maps for a bin size of 384".
