DEEP SKY
IMAGING
The images used for testing the limiting magnitude were made in an urban region (in
the neighbourhood of Toulouse) with a naked-eye limiting magnitude of
2.5. The images are taken with an Audine camera mounted on a
190 mm F/D=4 flat-field telescope (central obstruction 0.3).
The equipment set up in a residence in a suburb of Toulouse
(Ramonville Saint-Agne).
The final result is an addition of 4 images, each consisting of the median of
7 single images exposed for 2 minutes. This procedure is a must if we want to eliminate
the parasitic effects of a particularly bright sky background due to light pollution.
The observed region is the field of galaxy NGC 677.
On the left the Audine image (binning 1x1). On the right the same
field from the first version of the Palomar Observatory Sky Survey.
On the left, the Audine image. On the right the same field from the
second edition of the Palomar Observatory Sky Survey. The Audine image shows stars
fainter than magnitude 20, despite of a very unfavourable observation site.
Image acquired without using a shutter, this poses no problem when observing
deep sky objects !
The next 4 images show the variation of the limiting magnitude as a function of
the number of combined images.
Top left, a single exposure of 120 seconds. Top right, median
composite of 3 exposures of 2 minutes each. Bottom left, median composite of 5
exposures of 2 minutes each. Bottom right, a composite of 7 exposures of 2 minutes
each.
