[vsnet-chat 5086] UOs on CCD images...

["Berto Monard" <LAGMonar@csir.co.za>]

Hi, here is something off topic and possibly it is improperly timed in
view of present excitements about SN 2002ap, V838 Mon, QZ Ser and IM
Nor...

I have only been doing a few nights of times series since Nov 2001 due
to local bad weather (the rainy season, what do you expect?). They were
rare occasions indeed.

During such observations things show up on images that are not expected
or wanted to be there. I am not refering to ordinary hot pixels or
cosmic rays or trails of meteors or sattelites. Optical reflections
could be blamed in cases but not always. Sometimes it's difficult to
find explanations for what's on there. 

Case 1

During observations of V1025 Cen (RX J1238-38 / IP type) in Dec 2001 I
noticed a comet like object on different images and at different
positions. Despite the 30sec exposure there was no obvious movement
(trailing) on the image.
I initially expected they were possibly geosynchronous (or
geostationary, what do I know about it?) sattelites, shifting slightly
from image to image. B Skiff refered me to T Beresford who is considered
an expert regarding southern sattelite positions and the like. I
forwarded some of the V1025 Cen images to him. He hasn't come back to me
yet. 
What was that object posing at different positions near V1025Cen? It
did not look like it was on a comet/asteroid orbit and it was not moving
during the 30 sec exposure.  Could it have been an optical image
reflection of a brightish star a few arcmin outside the image? I am
afraid I could have wasted some of Tony's time, but then, on later
images the object was not there anymore.....

Case 2

More recently, on the night 3-4/2/02 I was imaging EC 05114-7955, near
the South pole. On image 256, about halfway the time series, there
suddenly was another bright star in the picture. It was round and a
couple of pixels wide, like a fixed bright star would. It measured about
10.7CR, based on the 45s exposure. 
Was it the optical imprint of a burster, possibly of very short
duration and therefore much brighter than magnitude 10? 

By further investigation, the previous recorded image (255) did not
show anything up to the detection limit (18.5CR). However image 257, the
one following the one with the imprint, showed a distinct mark at the
same location but now it was around magnitude 18CR. This would fit in
with the expected evolution of a burst. It looked like a very faint
star, not a pixel defect. 

Considering there were about 17 seconds between the end and start of
consecutive exposures and from the above data and the image time
information, I could state a possible occurrence of a bright 'optical'
burst at the location (2000) 05 07 54 -79 56 at UT 2002 Feb 03 at
22h42m45s (+ or - 0.5 arcmin in position and 30 sec in time). This
explanation is not based on deep and extensive searching but on a
superficial conclusion and with imprecise estimates. Was the event
recorded in other surveys? 

But then! Could it have been a hot pixel, overflowing during that
exposure, with left over marks on the next image? Could it be the
consequence of a malfunction in the pc hardware?  

      
As a note: 

I find those advanced image processing software packages very useful
for automatic differential photometry of large sets of images.
Initially, during processing I used to go for a walk and come back to
see the result graph or to notice the hiccup due to some large image
shifts or another (again) 'exception error' from my pc. But now I tend
to watch the 'movie' and see all these cosmic hits and hot pixels on and
off. Sometimes they hit near one of the measured stars and I can then
adjust for its influence on the result.
 
Sometimes there is more on the image, as was possibly the case above
and I wonder how often such things are picked up and if they are
investigated and reported?

Regards,

Berto Monard
Bronberg Observatory / CBA Pretoria

PS Images will be made available to someone who considers it worthwhile
to investigate further

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