M51 and the Gremlins - Bill Almond

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 Art Bell.

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!