Histograms and Curves

All imaging processing programs have the ability to display a histogram of the image. This graph describes the distribution of brightness of the pixels in the image. The left-hand side is the black point, and the right edge is the white point. Any pixel at or near the black point will appear black, and any pixel at or near the white point will appear white. Your goal in processing is to create a beautiful image with a balanced tone and a wide-dynamic range, which usually has a bell-shaped...

Focusing

Accurate focus is essential, and benefits from a good focuser. Most SCTs are difficult to focus by using the main focus control that moves the primary mirror. For these scopes, consider purchasing a dual speed Crayford focuser for the visual back, and locking the main focus knob if possible. Manual focus is easier to fine tune with a dual speed focuser, or an electric dual speed focuser. If you are purchasing a new refractor, look for a dual speed focuser that is also sturdy enough to support a...

Autoguiders

Nothing has popularized astrophotography more in the past decade than autoguiding. Before autoguiding, long exposure astrophotography required the astronomer to keep his or her eye glued to the eyepiece of a guide scope, making small manual adjustments to keep a guide star centered on a reticle, sometimes for an hour or more. Now, a small CCD chip keeps track of the guide star, and through a computer the autoguider gives instructions to the mount to keep the guide star centered. An autoguider...

Investing Wisely in Software

Digital imaging requires software for both controlling the camera and processing the images. Many CCD manufacturers include basic software for operating their cameras, which may be sufficient for acquiring your data. When you are ready to consider a more sophisticated and versatile program, check manufacturer Web sites for free trials to make sure that you will be satisfied with the investment. As of this writing, you can download trials of image processing programs like CCDstack or integrated...

Luminance Layering

The human eye sees details in the luminance (brightness) component of an image and not in the color component. Therefore, the color information of an image can be obtained with a lower resolution than the luminance data, without degrading the perceived quality of an image. Because color data can be acquired with lower resolution, the color exposures in an LRGB sequence can be binned 2 x 2, accumulating in 15 min the same signal-to-noise as a 1-h color exposure unbinned. A monochrome camera with...

Beginner Scopes for Imaging

Among the best 100 targets in section one, about one-third were taken with small 3-4-in. refractors with focal lengths of 300-600 mm one-third were taken with a mid-size 5.5-in. refractor with a focal length of 1,000 mm and another one-third with large 8 or 12-in. Schmidt-Cassegrain telescopes at a focal length of about 2,000 mm. Different focal lengths of telescopes are useful for different angular dimensions of various celestial targets. No single focal length is effective for every object....

Choosing a Camera

Three categories of camera are available for modern deep sky astrophotography monochrome CCD, single-shot color CCD, and conventional digital cameras. Conventional digital cameras have a big advantage most of us already own one. But for the best deep sky imaging, I suggest a dedicated CCD. Both color and monochrome CCD cameras come in three categories and price points. The least expensive have no active cooling, although some of these have methods of passively dissipating heat. Because...