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:

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   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.              
  
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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 .

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)

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.

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..".

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.

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.

JPEG FORMAT

The JPEG image is created converting a FITS format image.


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