This Month Bill Almond
This article along with others will
appear in the Observer's Corner in future releases of the website.
All of my astronomy friends know that I work nights. For
most night workers that's not a big deal but I'm not just your
average night worker. I don't slump into my car at the end of a
shift and drive home down lighted streets with never a glance
at the night sky, except to note that it's a) raining, b) snowing,
c) sleeting or d) hailing and otherwise not caring whether it's
cloudy or clear. Not me. The instant I leave work I check to see
how the sky is. If it's clear I curse having had to work. If it's
cloudy that's OK.
Clear nights on my nights off are pure magic. Like that
Tuesday night earlier in April. Clear, still, good seeing. The
perfect night for CCD imaging. This, I told myself, is going to
be the night I take the perfect tricolour image. And I had in
mind exactly what it would be. As all astronomers know, the best
time to observe any object is when it's at your zenith, high overhead,
seen through the thinnest atmosphere possible. I'd waited for
just such a moment to image one of my favourite objects -- M51.
The perfect night with beautiful conditions and a night off from
work to do nothing else but take images.
Carefully (I didn't want anything to go wrong) the equipment
was set up, the computer was up and running and the CCD was cooling
to minus 40C. M51 was entered on the keypad and the scope slewed
right to it. What a splendid sight in the eyepiece! A quick image
on the computer to make sure M51 was centered on the screen. A
little bit of adjusting and tweaking. A quick check to make sure
the ST4 guider programme was spot on. Find a guide star and lock
on to it. So far so good. Select a red filter, the first of three
in the RGB series. Then the big moment: turn out all the lights
and hit the computer's enter key. Sitting in the dark I listen
to radio's Art Bell and his amusing string of alarmist paranoids
until the exposure is done. Change the filter to green, turn off
the lights and hit enter. Ditto with the blue filter. Ditto with
A couple of days later the three RGB images were sitting
in their raw state in a second computer waiting to be co-added
and registered in Photoshop. My anticipation was running high.
All my careful work would soon be revealed ready to be shown with
pride at the next club Member's Night.
I opened the red image first, subtracted a dark frame, divided
off a flat field and hit "display". The image scrolled
down the screen. "What's this,"I moaned. My beautiful
red image was ruined. The image lay buried under snowy noise.
Quickly, I ran the other two colours through the process and watched
in dismay as each one showed the same defect. I knew at first
glance what had happened. The one thing I hadn't checked before
starting the exposures was the state of the CCD chip. Pushing
down the chip's temperature to minus 40C had caused a thin coating
of frost to form on its surface, an indication that the moisture-absorbing
dessicant housed next to the chip needed to be replaced.
Sadly I deleted the images from my hard drive. My memorable
observation and images would have to be worked on again. Fortunately,
once the moon's vamoosed early in May I'll have a second shot
at it. Next time the gremlins won't have a chance!
For comments / questions Bill can be reached at email@example.com