…continuedGuiding a Telescope for Imaging
Get comfortable, center the guidestar, and take a couple of minutes to practice before opening the shutter.
Astrophotographers generally align their cross hairs north-south and east-west, parallel to the motions of the equatorial mount. This makes it easy to see the corrections to make. However, if you're guiding on a faint star you may prefer to orient the cross hairs 45° to these directions. This way, as soon as the star begins to drift it will move into open space rather than remain hidden behind a cross hair.
If you have a four-button hand controller, hold it in the same orientation as east-west and north-south in the eyepiece so you won't have to think about which button to push. If the north-south buttons work right but the east-west buttons are reversed, turn the paddle over to orient them correctly. In this case you'll have to press the buttons from underneath.
Declination drives are notorious for having a little loose play -- a "dead zone" -- where nothing happens for a couple of seconds whenever you reverse direction. A trick that may help a bit with this problem is to unbalance the scope slightly; this way its weight stays hanging on one side of the dead zone. In some cases, however, imbalance just makes the situation worse. Another trick is to set the polar alignment slightly east or west of the true pole. This creates a very slow declination drift, but it will always be in the same direction, so you'll never have to reverse direction and go across the dead zone. Just don't misalign so much that you get field rotation.
In many telescopes, especially commercial Schmidt-Cassegrains, the optics themselves have a certain amount of play. The primary mirror can sometimes make a sudden shift after the telescope crosses the meridian. This "mirror flop" may be so large that you can't guide it out, and you end up with doubled or tailed stars. To keep this from happening, if it proves to be a problem with your scope, start your exposure after the subject has crossed the meridian or finish before it gets there.
It sounds almost too good to be true: a robotic device that watches the guidestar and adjusts your telescope automatically, with a machine's tireless precision, while you relax or even sleep. It's no pipe dream; this is part of the CCD revolution that's sweeping high-end amateur astronomy. A CCD (charge-coupled device) is a chip that records images electronically and sends them to a computer, which in this case is rigged to drive the right ascension and declination motors. An autoguider can use a modest, low-cost CCD, making this capability affordable at something like the $400 level.
Some CCD cameras themselves (as opposed to autoguiders) have a "track and accumulate" function that allows them to take many short exposures, then stack them by computer, finding the best possible fit, to create one long exposure. This reduces and can even sometimes eliminate the need for guiding. Some advanced CCD cameras now include two CCD chips -- a small one to autoguide the telescope and a bigger, better one to record the image.
We've come a long way from the days when amateur astrophotography was limited to fanatical tinkerers who had their own machine shops, darkrooms, and entire nights to spend attempting to make things work. Astroimaging has opened up to anyone willing to buy the gear and take the time to gain skills using it. And now the day has arrived when, once completely set up and running, you can literally sleep through the whole thing!