Hi Bertrand,

The downloaded DSS data ought to match your field of view. If I
resize the Guide window to be wide and not very high, I get an image
that matches this field.

One slight oddity: the images are always aligned north/south, or
at least close to that. So if you are in alt/az mode, and your field
of view is 13' wide in azimuth and 5' high in altitude, then Guide will
fetch an image that is 13' wide in RA and 5' high in declination. (And
the images are usually tilted a few degrees relative to celestial
north.) That's just the way the images were scanned in, and the way
the server distributes them.

"...especially if I want a much bigger size, for instance to display
an image of a large diffuse nebula, or open cluster, or something else."
Ah, yes... that's a real problem, in two ways.

First, the images become huge. When extracting DSS or RealSky
images from the disks, you can use a sampling parameter to tell Guide
to use every other pixel in every other line, or every fourth pixel in
every fourth line, and so on. But there's no way to persuade the
servers to do this, and (at least for some of the surveys), there are
limits as to how large an image you can get. I think 30' on a side is
about the maximum.

Second, you sometimes just can't get exactly the area you want.
DSS consists of plates 6 degrees on a side, with some overlap between
them. But if you ask for a large area straddling an overlap, the
server will have to pick one of the two plates, and the image it
extracts may extend right over the edge of the plate. There is, at
least at present, no way around these problems... until somebody sets
up a server for wide-field, seamless coverage of the sky.

-- Bill