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,
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 .
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
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.
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..".
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.
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.
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
The JPEG image is created converting a FITS format image.