Sky Simulation Help
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Contents |
[edit] Purpose
The sky simulation software will produce images of the sky as seen by a telescope. Data are read from a catalog containing positions and count rates and are projected onto the instrument focal plane.
[edit] Principles of Operation
The programs are written in ANSI C and may be downloaded, compiled and/or modified as per the GPL. We have also provided a web interface influenced by the needs of our own mission: TAUVEX.
Because we have tailored our programs for UV operation, we have assumed a photon counting instrument. The count rate
The basic principle of operation is to assume a photon counting instrument where individual photons are recorded and, hence, there is a practical limitation on the total number of counts which can be recorded. This is an online version of the UV sky simulation software. Very few might have the patience to download, compile and install the software on their local machines. Most people would prefer to have a quick look at the simulated sky than to study the details. Sort of a first look at what the instrument would see. That is the main motivation behind this online simulation page. We hope this page will be useful for everyone, starting from casual web surfers to busy astronomers. You may download the simulated data as fits files of individual fields or view it online as an animated-gif movie. Be warned that choosing slow scan rates over large angular extent can result in huge output data files. In the case of animated gif movie, it can result in your browser consuming considerable amount of cpu time.
This page is meant to be an online guide to running the simulation. This is also the place to look for what the simulation can do and what it can't. The online simulation tool is a trimmed down version of UV sky simulator, tailored for TAUVEX. However, it does most of the work for you. If you are interested in a more general purpose software, please get it from the Download Area. Given below is a brief description of the input fields. Technical details of the software can be found in the Documentation.
[edit] User Inputs
[edit] Telescope
The online sky simulator program can be used to simulate TAUVEX data. In this mode, many of the options are pre-configured for the user. For eg. Integration time, scan rate, field of view and detector efficiency of TAUVEX will be fed as input to the simulation software. In addition, you will have only the TAUVEX filters to choose from. Available source catalogs will be limited to those in which we have the count rates available in TAUVEX filters. You may, however upload your own sky catalog.
The second option is "User defined", where you have full control over the various input values, including catalogs and filters (supports uploading user files), instrument efficiency etc.
[edit] Observation
Observation specific values like the sky coordinates and duration of observation. These inputs are dependent on the telescope type. For eg. if you opted for TAUVEX, then you cannot choose pointed observation.
[edit] Coordinates
[edit] Scan mode
The starting point and end point of a scan is specified here. The values correspond to the field center - ie, " (field of view)/2 " area around the starting point is imaged in the first frame, exposed for one "frame time". The field is shifted by one "Scan rate" towards the end point, and next frame is taken and it continues till the end point is reached.
The nominal mode of operation for TAUVEX is to scan along constant declination. Therefore, the declination is kept constant. The simulation program expects co-ordinates in J2000.0. However, we have included an option for B1950 epoch also. User inputs will be precessed to J2000.0 before passing it onto the simulation program. The idea of constant declination is not valid here, since lines of constant declination in B1950 will no longer be constant declination in J2000.0. Therefore, we use the precessed value for the starting point as the constant declination and simulate the sky. Choose a custom/user-defined telescope to get more options like constant R.A. scan.
[edit] Scan rate
This is a TAUVEX specific parameter. Basically it is for a scanning instrument, which does not stop and stare at anything. Scan rate is the angular speed of the scan, specified in degrees/second. Given a scan rate, you may also specify a duration for observation instead of end point coordinate.
Datatype: Float/Integer
Unit: degrees/second
Range: 0.0 to 60.0
[edit] Pointed mode
Targeted observation, where the instrument stops and stare at a point in sky. You need to specify the coordinates and the duration of observation.
[edit] Duration
We assume scanrate=0. Events will be gathered for Duration (seconds) to generate a single image.
Datatype: Float/Integer
Unit: Seconds
Range: 10-8 - 604800
[edit] Observation Date
Observation date is used to calculate the contribution from zodiacal light. For TAUVEX, this information is
also used to estimate the stray light contribution (sunlight scattered off the spacecraft body and solar panels).
Range (years): 1900 - 2100
[edit] Instrument
Instrument specific inputs are listed here. There are only two user defined inputs in this section for TAUVEX - jitter and dark count. We use instrument efficiency and aperture to estimate the effective area and thereby the count rate for a given filter. If your filter response file lists effective area as a function wavelength, then you need to set instrument efficiency as 1 and aperture = 0.564 meters (so that physical area = 1 sq.meter)
[edit] Dark Count
The background noise in the detector. Input is counted as so many counts per pixel per second.
Datatype: Float/Integer
Unit: Counts/pixel/second
Range: 0.0 - 50
[edit] Aperture
Diameter of the objective in meters. 20cm for TAUVEX. Set this to 0.564 if you are using user defined telescope and your filter response file contains effective area as a function of wavelength.
Datatype: Float/Integer
Unit: meters
Range: 0.001 - 30.0
[edit] Field of View
The field of view of the telescope. This is 0.9deg for TAUVEX. You may enter any value here for a user defined telescope, but be aware that large fields of view also means more number of sources, more information and thereby larger files. The output file size also depends on the number of pixels in the field of view.
Datatype: Float/Integer
Unit: degrees
Range: 0.01 - 20.0
[edit] Jitter
Average jitter in X,Y positions in pixels. This value decides how much is the possible scatter or the uncertainity in the co-ordinates of a source.
Datatype: Float/Integer
Unit: pixels/second
Range: 0 - 60
[edit] Instrument Efficiency
Efficiency of the instrument. Set this to 1.0 if you are uploading an effective area v/s wavelength file as filter response.
Datatype: Float/Integer
Unit: none
Range: 0.001 - 1
[edit] Pixel size
The size of pixels in your image (in arc seconds). For TAUVEX, the angular resolution is about 10". For quick results, choose a larger pixel size, so that the output image size is small even for larger fields of view. Pixel size is fixed to 3.538 arc-seconds for TAUVEX.
Datatype:Float/Integer
Unit: arc-seconds
Range: 0.01 to 60.0
[edit] Filter
Response function of the filter being used. If you chose TAUVEX, you can choose from one of the filters available on TAUVEX. In this case, we have the source catalog ready in count-rates. If you choose the custom telescope, then we will have to calculate the count rate of sources in the catalog before we can run the sky simulation program. This will take some additional time.
If you need to upload a filter response function, ensure proper values for instrument efficiency and telescope diameter. Set Efficiency to 1.0 and diameter to 1.128 meters (so that physical area = 1) if you are using user defined telescope and your filter response file contains effective area as a function of wavelength.
[edit] Tauvex Filters
TAUVEX as a set of 9 filters, 3 per telescope. These fall into 5 different wavelength ranges, denoted by the names: BBF, SF1, SF2, SF3 and NBF3.
[edit] The Source Catalog
You have a choice here, to select from the available catalogs or to upload your own source file. The catalogs available are:
TAUVEX Specific: Count rates in all TAUVEX filters.
| Name | RA Range (deg.) | Decl. range (deg.) | Description |
|---|---|---|---|
| Hipparcos | 0.00 to 359.98 | -89.78 to 89.57 | Hipparcos catalog |
| IUE QSO | 2.62 to 347.82 | -76.87 to 76.04 | The QSO catalog from IUE |
| TD1 | 0.00 to 359.98 | -89.89 to 89.27 | TD1 UV catalog |
Others
| Name | Wavelength (Å) | RA Range (deg.) | Decl. range (deg.) | Description |
|---|---|---|---|---|
| GALEX ELAISS1_00 | 2267 | 8.77 to 10.51 | -44.62 to -43.37 | GALEX early release data |
| GALEX MISDR1 | 2267 | 346.95 to 348.17 | -0.82 to +0.43 | |
| GALEX NGA_M51 | 2267 | 200.99 to 101.82 | +46.45 to +47.70 | |
| GALEX SV01 | 2267 | 312.21 to 313.45 | +8.10 to +9.34 | |
| GALEX SV03 | 2267 | 219.05 to 220.30 | +4.70 to +5.92 | |
| GALEX SV06 | 2267 | 12.19 to 13.59 | -27.84 to -26.62 | |
| GALEX SV10 | 2267 | 346.94 to 348.17 | -0.81 to +0.41 | |
| GALEX SV11 | 2267 | 346.36 to 347.59 | +0.05 to +1.27 | |
| GALEX SV12 | 2267 | 345.88 to 347.11 | -0.73 to +0.49 | |
| SMC CAT | 1620 | 10.85 to 15.84 | -73.41 to -71.89 | The Small Magellanic Cloud UV catalog |
| TD1 2365 | 2365 | 0.02 to 360.0 | -89.64 to +89.03 | Source catalog from the TD1 satellite |
If you are not satisfied with those catalogs, you may upload your own. The Sky simulation program expects it to be a 3 column ascii file with Right Ascension, Declination and count-rate (per second) respectively as the three columns.
[edit] Uploading a file
If you have a file with three columns - 1st and second for the co-ordinates and the third for photon counts, you can upload it to our server using the file upload option. The points to note here are the following:
- The file should be plain text and should contain only 3 columns. Any other format will result in a bright yellow colored error message on your screen.
- Any line that start with a "#" is considered a comment and will be promptly ignored by the sky simulation program.
- The co-ordinates are expected to be in degrees
For eg. A typical input file would be:
# My first Catalog # # R.A. Decl. Counts/sec 27.527171 -2.490005 4881.500000 28.137863 -2.491009 3249.919922 27.602962 -2.485519 3892.040039 28.923467 -2.484575 2766.169922 29.836426 -2.486221 3331.219971
Any line that begins with a "#" is ignored as a comment.
[edit] Output format
The output of the simulation is normally saved as fits files, one file per frame. Unix/Linux/BSD people can choose to download the tar.gz archive. If you are using windows95/../Vista/7, then a zipped file is perhaps a better option.
Considering the fact the fits is a not-so-popular image format outside the astronomy community, we have made an attempt to generate gif files too. Now that is viewable from any internet browsers. As far as a quick look is concerned, an animated gif movie conveys much more than a set of fits files. However, this package is still experimental. Please also note that generating an animated gif movie from several hundred frames is more time consuming than making individual frames as fits files. Therefore, it may take a while to get a movie running.
[edit] Email Notification
Running a simulation might take a while. The output page will keep refreshing every 30 seconds till the job is done. You may bookmark the page and return anytime to view/download the output. You may also provide your email address, so that the program will notify you when your job is done. Note that we do not check if the email ID is valid or not.
[edit] Open bugs
| ID | Severity | Assigned to | Description |
| 431 | major | reks_AT_iiap.res.in | link telemetry to online simulator |
| 510 | normal | reks_AT_iiap.res.in | link telemetry file |
| 538 | normal | reks_AT_iiap.res.in | can't download files |
