Wednesday, December 7, 2011

Narrowband Imaging III: Guiding

Narrowband filters pass light only near discrete wavelengths. This makes them effective at reducing light pollution, which is mainly spread across a broad range of wavelengths. The downside is that faint nebula don't generate all that much radiation even at the passed wavelengths.  This means that you will probably want to have a total exposure times of several hours using each filter. (Narrowband imaging is not for those who need instant gratification!) You can do this by combining a large number of short exposures or a small number of long exposures.  Generally, the latter is preferred.

Let's assume you want to use modestly long five minute exposures (also known as subs).  It's not unusual for skilled imagers to use exposures as long as 20 minutes to an hour.  There are a number of things that can happen during five minutes to degrade your images; wind, poor polar alignment, and imperfections in your mount's mechanics can all lead to misshapen stars instead of nice round ones.

Your first line of defense is polar alignment. A good but probably not adequate alignment can be obtained using a polar axis scope. A better alignments results from using a software method that is built into many modern mounts.  The best alignment results from using the declination drift method. The drift and software methods require you to use an eyepiece with illuminated cross hairs. The drift method demands time and patience, and can be confusing the first few times you try it. But it's the only real way to get a nearly perfect alignment. With a near-perfect polar alignment you can take exposures of five to ten minutes with minimal defects. The longer the focal length of your imaging telescope the greater the need for an accurate polar alignment.

A great polar alignment virtually eliminates field rotation when using a GEM mount or wedge-mounted fork. There's still a problem of small motions in right ascension and declination caused by imperfections in your mount's mechanics. These can result in distorted star shapes. There are several ways of dealing with this: autoguiding, periodic error correction (PEC), and adaptive optics. The latter remains very expensive at this time, so let's concentrate on the first two. Autoguiding uses a separate telescope and camera to track the motion of a guide star. As mount imperfections and wind cause the guide star to move, software sends pointing corrections to the mount.  Used alone, autoguiding does a good job of eliminating drift.  When used with PEC, autoguiding reduces tracking errors even more.

Some imaging CCD cameras come with a built-in autoguiding sensor and use a guide star from the edge of the field of view. In some models this places the autoguider sensor behind the narrowband filter, which is a disadvantage. Many people prefer a separate autoguiding camera and telescope  for this reason, and for the reduced cost.

Thanks to modern software, your autoguiding telescope's focal length can be much shorter than that of your imaging telescope.  A favorite autoguiding telescope is the ubiquitous f/5 80mm achromat. These are generally easy to buy used. The autoguiding camera can be something designed specially for guiding, or an older CCD camera such as a Meade DSI.

If you decide to use an autoguiding telescope, you must mount it securely.  Flexure is what happens when the orientations of the optical axes of the guiding and imaging telescopes change relative to each other. Usually this happens when the mounting hardware bends or shifts. You don't need the two optical axes parallel, but you do need to keep them from changing their relative orientations.

The most popular software for autoguiding is called Push Here, Dummy (PHD). It's freeware and excellent. PHD is simple to use, but it has some parameters you can adjust to make it work better with your setup. Experimentation is encouraged.

Some autoguiders incorporate the guiding software within them. This convenience raises their price.

None of the above methods will do you much good if you don't balance your imaging system properly. it's important to balance the mount's load around both the polar and declination axes. In my experience, the nights I've had  the most trouble are those when I didn't do a good job of balancing. Usually I balance with the imaging telescope in the orientation it will have during imaging.

Next Time: Setting up

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