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Overview


Given the center coordinates and size of a field, SkyEye will extract the corresponding data from the CD-ROM discs, uncompress the data, and write them to an image file.

To select images the following input data are required:
  • Field name of four characters valid for a filename.
  • Center coordinates:
    • Right ascension: hh mm ss.sss in free format
    • Declination : Sdd mm ss.ss in free format (where S is an optional sign)
  • Width and Height of field in arcminutes.
Enter the requested information, in free format, and SkyEye will create a image for each of your fields. The filenames for the output images will be "ffffppppxxx.tttt" where ffff is the name you provided for the field, pppp is the designation for the plate from which the field was extracted, xxxx is a random number and tttt is the image format extension.

To access the compressed image data from the CD-ROM set of the Digitized Sky Survey, SkyEye use tasks derived from the program
GetImage.

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Extraction Information


The first thing that SkyEye makes when a field is request, is to calculate the extraction informations, i. e. the coordinates in pixel,the plate and CD number etc. The extraction information for your fields are grouped by the CD-ROM discs from which they would be extracted. This information, which will be shown to you on the SkyEye output-page, are used to proceed in the extraction of the images.
You can ask SkyEye just to show you this information without proceeding in the extraction of the images, so you can check the correctness of your request. To do this, switch ON the option "No image will be extracted..." (to suppress actual image extraction).
It is also possible to write this informations in an extraction file, that you can eventually modify and than use as input for SkyEye. Switch ON "No image will be extracted.." and switch ON "The extraction informations ..." (to create the extraction file for your fields)

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Selective Image Extraction


Because there is overlap among the plates of the Digitized Sky Survey, the fields you request may lie on more than one plate. By default, SkyEye is set to extract the image, for each of your requested fields, which lies closest to a plate center. By switching OFF the option "Only the image closest to a plate center will be extracted", you can direct SkyEye to extract images from all plates which contain your field.

To perform selective image extraction:

first
create an extraction file with extraction information for your fields and all the plates on which they lie. To do this switch OFF the option "Only the image closest ..." (to list extractions from all relevant plates), than switch ON the option "No image will be extracted..." (to suppress actual image extraction), finally create an extraction file by switching ON "The extraction informations ..." (to create the extraction file for your fields).
Second, edit the extraction file to include only the extractions for which you wish to create images.
Finally, run GetImage putting your edited extraction file in the apropriate input page of SkyEye.

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Header


In the FITS file format the image data follow an ASCII character written part, called header, containing many informations about the data themself. The GEIS format doesen' t support this header, so if one choses GEIS format, the software creates two files : a text file containing the header (***.hhh) and an image file (***.hhd). Neither the JPEG format supports the header and the informations stored there will be lost.
It is always possible to create just a text file containing the header switching ON the option "Only ... header..".

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AIPS


Switching ON the option "The image header will be AIPS conform", the KEYWORDS, needed by AIPS to create a reference frame on a FITS image, will be added in the header.

    CTYPE1,CTYPE2
    CRVAL1,CRVAL2
    CDELT1,CDELT2
    CRPIX1,CRPIX2
    CROTA1,CROTA2

Thanks to these keywords AIPS will know the center field coordinates of the image and will create a linear reference frame. No field correction is applied, and the coordinates are exact only in the center of the image. The absence of correction is more rilevant at plate edges, but neverthless for images of 20x20 arcmin the error is at most of 1,2 arcsec.
The 15JAN96 AIPS version contains an appropriate task (SKYVE) that performs the regridding of the entire image, i.e. it applies to all points of the image the correction and obtain a new image which is linear and on which a linear reference frame can be used.
This solution is certainly more accurate.

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Preview


For the FITS format a little preview is provided.
It 's a jpeg image, if your Web Browser is not able to show jpeg, you can just see the preview clicking on it, an external viewer will be called.
The JPEG image is created converting a FITS format image.

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Field Coordinates


To insert the requested informations, put the cursor on each box and type the data.

To change all the informations inserted, click on Reset ALL and retype the data.

When you have made your choices click on BEGIN and wait ...

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Single Field


Put the cursor on each box and type your data
  • Name, the name of the output file will be made of the, at most four, caracters you choosed followed by the plate identification name and a random number that will assure the unicity of the name of your file in current Working Directory. Enter here four character valid for a filename.
  • Right ascension of the center of your image
    Hour (integer value between 0 and 24 )
    Minutes (integer value between 0 and 60)
    Seconds (real value between 00.000 and 60.000)
  • Declination of the center of your image
    Degree (integer value between -90 and +90)
    Minutes (integer value between 0 and 60)
    Seconds (real value between 00.00 and 60.00)
  • Width and Heigth of your image
    in arcminutes (real value up to maximum 60)
    Original plates are 14000x14000 pixel, 6,5x6,5 degree, 365x365 cm.
    1 arcminute are 35 pixels. Selecting 20x20 arcmin you have about 700X700 pixel

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Multiple Fields


Put the cursor on the box and type your data
You have to insert a row for each image that you need to get. In the row you must write: First four character of output file name, coordinates, and field size.

The input looks like:

    aaa1 10 12 30.000 20 30 00.00 25 25
    aaa2 12 15 00 -13 22 30.30 25 25
    aaa3 9 38 0 -03 16 42 25 40

For Field details see at Single Field

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Send Local File


You can write, on your computer at home, a file with the data to use to select images from SkyEye. This file contain the first four characters of the name of the output image, the center field coordinates, and the image size (in arcmin).

The input file look like:

    img1 10 12 30.000 20 30 00.00 25 25
    img2 12 15 00 -13 22 30 25 25
    img3 19 38 14.0 -03 16 42 25 25
    i4 7 10 0 15 0 0 12 12
    im5 15 5 10.000 45 9 9 15 15
    im6 4 32 0 -3 18 4.00 12 24

All these rows are written in valid format.
When you have written your file, on your computer, you can send it to SkyEye, inserting the name in this field mask (you can use the Browse button also) and pushing the BEGIN button.

For Field details see at Single Field

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Options


(Click on the box to switch. The default options are already selected.)
  • Equinox
    The right ascension is measured starting from the point of intersection between the ecliptic and the celestial equator ( the position of sun at spring equinox ).Because of precession, nutation etc., this point varies its absolute position in time, so it 's necessary to specify to which spring equinox point our coordinates refer.
  • Format
    Choose between the three image file formats
    FITS and GEIS are scientific formats
    JPEG is a compressed and widely used format.
  • Compression
    Valid only if FITS format is choosed
    The output file can be compressed with "COMPRESS","GZIP"or "ZIP".
    Maybe your browser will decompress it on the fly, while getting it.

(Push radio-buttoms to change )
  • Some image is on more plates
    The same image may stay on more plates, because the plates overlaps each other on the edges.
    Option ON Just one , the better, image will be extracted
    Option OFF All these images will be extracted
  • Preview
    Valid only if FITS format is choosed
    Option ON A little jpeg image will be created
    Option OFF No preview
  • Header
    In the FITS file format the image data follow an ASCII character written part, called header, containing many informations about the data themself. It is possible to create, for each image, just a text file containing the header.
    Option ON No image will be extracted, just a file containing the header will be created
    Option OFF Image file of choosed format will be created
  • AIPS
    Option ON In the image header the keywords needed by the AIPS package will be added
    Option OFF No changes in the header
  • No extraction
    Option ON No image will be extracted but the extraction informations like plate and disk number, image size in pixels and more, will be written on standart output or in a file if the next option is cheked.
    Option OFF The extraction informations are written on the standart output and the imagefile of the choosed format will be created.
  • Redirection of extraction info
    Option ON The extraction informations like plate and disk number, image size in pixels and more, will be written on the file which's name you will write in the input box.
    Option OFF The extraction informations will be written on stdout.
    The advantage of using this option is to controll, before the image extraction is going on, which images on which plate you request. It is possible to take home the "extraction information file", work on it by adding or removing some row and resending it to this server chosing the apropriate input page in SkyEye's first page.

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Objects in the field (from Dira2 Catalogues)


Using the "Append Catalogue Menu" button you can expand the input form to have the menu' to select catalogues from Dira2 database.

The Dira2 Database is a collection of about 300 astronomical catalogues and millions of objects

With the "Catalogue Menu" form you can get the catalogues objects, inside the selected sky regions, present il the Dira2 database.

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Dira2 Database


Menu' for the selection on the Dira2 catalogues. You must enter here the constraints for the selection.

You can select "All Catalogues" or a class of these: (i.e. Radio, Optic, QSO, galaxies ...). Or you can select catalogues by name (max 32 catalogues).

  • Selection by Class: Clic on the relative button to select the class of catalogues that you need. Whith the Class "All catalogues" you can select all the catalogues of the database.

    Inside a class you can get data only from catalogues where the precision of the coordinate is better than a defined value. In the fields Required Coordinates precision" you must enter the precision code as follow:

        R.A.           DEC
    HH MM SS.ccc    sDD PP SS.ccc
     1 23 45 678      1 23 45 678
    
    (i.e. when you want to use catalogues with precision equal or better of one second in RA and one arsec in DEC you must enter 5 and 5) Catalogues with less precision will be skipped.

  • Selection by Name : Click on the button relative to the catalogues that you need. When you select catalogues by name all the selection by classes (if present) will be ignored.
You can have the list of the catalogues currently available in the database with the " Show a brief list of the Catalogues in the DIRA2 database" button.

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