Overview
This is an online help for the UVIT Bright Source Warning Tool (BSWT). Please note that BSWT will only help you assess the feasibility of an observation. Please do report to us If you come across any inconsistent results.
Aim of the tool is to inform the user whether the region of the sky around a science target is safe/unsafe for UVIT to take observations. The program scans for stars brighter than the safety threshold and lists out the count rates of these bright stars in all the 10 filters in the FUV and NUV telescopes. This program identifies all the bright stars within 20 arcmin radius of the target object, as the UVIT has a field of view of 29arcmin. 20 armin is considered as the ASTROSAT pointing accuracy is 3 arcmin.
In order to reduce the complexity of the inputs page, we use javascript to show only the relevent input fields. Javascript is also used to check if the entered values are of expected types and within range. For eg: if you leave any of the required inputs blank, or if you enter a wrong data type, the field will turn red. If the number in a field is out of range, it would turn the number red. Javascript works on the client-side, ie on your browser. Checking input values before submitting them helps to reduce data transfer to and from the server, and (more importantly) it can save you several minutes on slow connections. Submission of input values from user and fetching the results are done using asynchronous requests via javascript (AJAX). Therefore, you need a javascript enabled browser to use this tool.
The next two sections describe the backend calculations and the catalogs used, followed by a description of the input parameters.
Backend Calculations
Catalogs are used to scan for bright stars, similar to the GALEX tool for this purpose. A brief description of the catalogs are given in the section below .
Once all stars in the field are identified, the program then computes the expected count rates for these stars, based on their V and (B-V) values. This is done with the help of a look up table generated using the UVIT Exposure Time Calculator (ETC).
The ETC does not provide count rates for any (B-V) value, it is provided for only specific spectral types. We have created a look up table, between V=6 to 20 and (B-V) -0.33 to 1.9, for all filters in the NUV and FUV channels. Thus, the look up table has count rates for a grid of V and (B-V) values, for all the UVIT filters. Using this look up table, the program does a two dimensional interpolation to calculate the count rates for the detected stars, as they have a range in V and (B-V). Presently, all stars brighter than V=6 are considered as bright stars. (In the next version, we will expand the look up table up to V=4 mag)
The safety limit for the UVIT detector is printed out in the output. This parameter is read from a file and the value may be modified later. The count rate for the detected stars in all the filters are checked against the limit. The program states whether the field is safe for observations against each filter. An output of "Field Is OK For Observation ~ ~" means that all the stars in the FoV have count rates less than the safe limit in that filter. An output of "Bright star detected" means that at least one star in the FoV has count rate more than the safe limit in that filter.
The field is also checked for total count limit in the field. The limit used here is also printed out as part of the output. Each field should satisfy both these limits to qualify for observations. The output gives the count rates of all stars in the field, along with the total count rate, for NUV and FUV filters.
Catalogs Used
The following catalogs are used in this tool:
- Tycho 2
- TD-1
- Bright Star Catalog
- GALEX GR7 - All stars in NUV FUV brighter than 15 mag
The count rates for sources in catalogs 1-4 are estimated for all filters in NUV and FUV. The count rates are estimated suing a look up table, generated using UVIT exposure time calculator. In the case of GALEX sources, the GALEX magnitudes are converted to count rates using the conversion given by GALEX. For these sources, the count rates are applicable to the broad band filters.
The output file contains the count rates for all stars in the field from catalogs 1-3 and only bright stars from GALEX.
Coordinate Systems
The BSWT requires a field coordinates to estimate count rates from all the stars within 20' radius around it. For convenience, the web interface accepts field coordinates in Galactic or Equatorial (J2000) coordinate systems. Alternatively, you can enter the source name if you do not have the coordinates so that BSWT can contact a name resolver to get coordinates of the specified source.
NOTE: Backend software accepts the input coordinates only in J2000 Equatorial coordinates. The web interface handles all the coordinate validation, and converts the coordinates to equatorial using WCSTools. You may not find this functionality if you choose to download the software and run it offline.
Galactic Coordinates
Input is expected in a comma separated longitude, latitude format "ddd.dd, +/-ddd.dd" (without the quotes). If you choose to, you can type in the Galactic coordinates in sexagesimal format as well (ddd mm ss.ss, +/-dd mm ss.ss). Client side javascript will ensure that input format is valid. If you do not know the exact coordinates of your source, leave this field blank and type in the source name. BSWT will use a name resolver to figure out the coordinates of your source.
Equatorial Coordinates
Input is expected in sexagesimal format, comma separated RA and declination: "hh mm ss.ss, +/-dd mm ss.ss" (without the quotes). If you choose to, you can type in the coordinates in decimal degrees format as well (ddd.dd, +/-ddd.dd). Client side javascript will ensure that input format is valid. If you do not know the exact coordinates of your source, leave this field blank and type in the source name. BSWT will use a name resolver to figure out the coordinates of your source.
Source Name and Resolver
If you choose to use source name instead of coordinates, please ensure that you click on the button "Resolve" and wait till the tool contacts simbad to fetch the coordinates. A successful request will update the coordinate field and rewrite the source name as per the data from Simbad name resolver service.
Note: Name resolver may return unexpected results if network connection between client and UVIT server or between UVIT server and Name resolver. In such cases, the coordinate field will be populated with "NaN". In such a case, please try again or use the coordinates instead of source name.
Primary Instrument
The look direction of UVIT can vary by a few arc-minutes as per the primary instrument chosen for your planned observations. These changes, though minute, can result in bright objects moving in/out of the field of view. Therefore, the BSWT need to consider this while evaluating the feasibility of an observation.
Versions, Changelog and TODO
Present Version: 1.7.0
Release Date: 24 November, 2016
Versions are denoted using a standard triplet of integers: MAJOR.MINOR.PATCH. The major version will be updated with new releases of the backend software, or inclusion of new components. Any changes in the instrument parameters - from cutoff limits, search radius, calibration results or other updates will increment the minor version. Minor version shall reset to "0" with a change in major version. Finally, the patch gets updated with modifications/bug-fixes in the cgi/scripting components. Patch will be reset to "0" whenever minor or major version gets an update
Changelog
- V 2.0.0, 26th April, 2017
- Search radius is now defined by the primary instrument of the planned observation
- V 1.7.0, 24th November, 2016
- Updated cutoff limits for NUV and FUV channels. Safe limit is now 1500 counts/s
- V 1.6.1, 04th July, 2016
- Fixed a bug related to an input file containing cutoff limits
- V 1.6.0, 27th June, 2016
- Cutoff limits updated for FUV and NUV channels - 250 and 190 counts/s respectively
- V 1.5.0, 30th March, 2016
- Format of a Tycho catalog file was updated to ensure that coordinates are read correctly. This was resulting in wrong output for a few fields.
- Output files (text, pdf) will now show the release date alongside BSWT version
- V 1.4.0, 13th January, 2016
- BSWT version information included in the output file (txt and pdf)
- V 1.3.0, 10th September, 2015
- Equatorial coordinates are now listed in degrees in the header
- V 1.2.0, 04th August, 2015
- The coordinates of bright sources detected are listed in the output
- The Galex catalog is mentioned as GALEX GR7
- V 1.1.0, 03rd August, 2015
- Correction for the FUV GALEX count rates
- V 1.0.1, 14th July, 2015
- Updated informational messages about GR7 sources
- V 1.0.0, 13th July, 2015
- New catalogs included: GALEX-GR7, TD-1 and Bright Star catalog
- V 0.4.0, 13th March, 2015
- Fixed the bug which was resulting in blank output for certain fields
- V 0.3.0, 05th March, 2015
- Included option to output as a pdf file
- V 0.2.0, 04th March, 2015
- Initial public release
TODO
Here is a list of additions which would be incorporated in the future releases:
References
-
The Tycho-2 Catalogue of the 2.5 Million Brightest Stars
Hog E., Fabricius C., Makarov V.V., Urban S., Corbin T., Wycoff G.,Bastian U., Schwekendiek P., and Wicenec A., 2000, A&A, 355, L27 -
Construction and Verification of the Tycho-2 Catalogue
Hog E., Fabricius C., Makarov V.V., Bastian U., Schwekendiek P., Wicenec A., Urban S., Corbin T., and Wycoff G., 2000, A&A, 357, 367
