Not every CCD image ever taken is perfect, of course, one that you're proud to show off. Like half of mine from the last all-night session. Well, never mind, you’ll see. The most common problem is from poor tracking by the clock drive. The simplest fix is to take a series of images, keeping only the best. At worst you're only using up some disk space temporarily. An image made of a composite of short integrations, a minute or so in length, is in fact the usual practice among most CCD'ers. This smeared view of the Dumbbell Nebula, M27, suffers poor tracking or lousy polar alignment or both. This is what they made the “delete” key for.

Take the moonlit Pinwheel Galaxy. It was a clear, bright night with the almost Full Moon nearly as close as it gets along the Ecliptic. M51 was well up in the northeast, everything was working well, and I wasn't ready to call it quits quite yet. Here is a raw, 40-second image. The salt and pepper appearance results from hot pixels and thermal noise; this is normal. The donut rings are due to dust on the optics near the chip, made especially noticeable by the bright background illumination of the Moon. OK. And the galaxy itself appears pinched because the chip's pixels aren't square but rectangular. Fortunately, digital image processing can correct all of these. When a number of such processed images (known as calibrated frames) are registered and combined, the results can be quite pleasing.

A meteor? Plane? UFO? It happens more often that you'd think: a streak across the frame compromises the image. But since its just one of a set, there's no need to process it further or use it in the final composite. Sometimes the object remains a mystery. But in this image of the Crab Nebula, M1, the source of the trail moved slowly enough (it is seen exiting a subsequent 40-s image) that "none of the above" fits the bill. It was most likely a satellite. I guess.

Sometimes it all goes perfectly. This is an image of a G2 spectral-type star (it's the brightest in the field) for calibration of my color filters (Ah yes, quad color imaging... That's another story altogether...) in the Hyades near Aldebaran in Taurus--no problem finding it. Hmmm, magnitudes taken here through various filters didn't square with those of two other G-type stars. You guessed it: wrong star. The intended target is just off the lower-left edge.

This bright streak results from a cosmic ray hitting the chip and knocking loose electrons, just as visible-light photons do. Since they are fleeting, random events and do not repeat between frames, discarding it or using a “median combine” procedure can eliminate this cosmetic defect. Finally, here's another easy-to-fix problem, not the “Zebra Nebula.” This is electrical noise from a running hair dryer to remove the condensation from the corrector plate. A flip of the switch to the off position and the stars will be seen on screen again.

A CCD camera is a system of optical, electronic and mechanical, even atmospheric components, each contributing to or detracting from a given image of the night sky. A CCD camera can produce stunning results. In the hands of an advanced amateur, backyard results rival those produced at professional observatories not all that long ago. But for every great shot, there is sure to be a disk full of bloopers that won't be seen in the glossy magazines!

Moondark is written by Doug Miller and published on the web, in the Delmarva Star Gazers' Star Gazer News and in the Delaware Astronomical Society's FOCUS. Please address comments and suggestions to dmiller@udel.edu. This document was last revised on 22 May 1999. All text and images copyright © 1999 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.