Critical Focus

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To all appearances, focusing an astronomical CCD camera or digital camera ought to be simple, but often it is not. Observers report spending ten to fifteen minutes trying to obtain accurate focus, and then complain that they are not sure that they have succeeded.

The first step in getting sharp images is to be sure that the telescope optics are accurately aligned and produce good images. SCTs are prone to getting bounced out of alignment on the bumpy back roads that lead to good observing sites. Newtonians are prone to slip out of alignment at the slightest provocation. Make sure the optics are good, and check the alignment of your telescope with col-limation tools or a laser before every imaging session. Only well-made refractors seem to hold their alignment year after year.

In astronomical CCD cameras, the focus problem seems to be that the blocky star images on a computer screen do not look at all like the familiar pinpoints in an eyepiece, so the observer cannot decide which blocky blob is the "best" blocky blob. With digital cameras, the image in the reflex viewfinder is too small to judge whether stars are in sharp focus, so focusing must be done by taking an image, assessing its sharpness, adjusting the focus knob, and taking another image.

This process sounds easy, but downloading and displaying the last focus image takes some length of time. Since telescope shakes and atmospheric turbulence change star images somewhat randomly, a slow readout can make judging and attaining critical focus frustrating even for a skilled observer.

Another focusing bugaboo occurs with telescopes that change focus when they are pointed at different parts of the sky. SCTs and Newtonians are prone to shift focus slightly when pointed in a new direction. To use a Newtonian telescope for imaging, it helps to stabilize the primary and secondary mirrors by applying aquarium cement or bathtub caulking at three points around the edge of each mirror. With SCTs, special mirror lock-down kits allow you to steady the optics.

5.3.1 The Focuser

Focusers that stick, bind, or suddenly jump from one focus position to another simply will not do for digital imaging. Rack-and-pinion focusers are terrible in this regard; torque applied to the focus knob seldom results in predictable linear motion. Another offender is the SCT with a sloppy mirror-focus system; not only are focus changes unpredictable, but star images move every time the observer touches the focus knob. An add-on accessory that helps you deal with these problems is a focus counter that displays the exact focus position—but a solid, motorized focuser is a blessing that every serious imager should consider.

5.3.2 Focus Techniques

Even with steady air, a sturdy mounting, a fast-reading CCD camera, a rock-solid focuser, and stable telescope optics of high quality, the greatest difficulty in focusing is knowing how to recognize a properly focused image. Here are some methods in wide use among skilled observers.

The "Smallest Star Image" Technique. Some observers are adept at judging when the turbulent star-image blob is smallest. To focus using this method, select a star that is bright enough to produce an easily observable disk when it is out of focus, but not so bright that it saturates the sensor when it is near focus. Move slowly through focus in about ten steps, then back again until you find positions on either side of focus that have the same size disk. Find two new positions that have smaller out-of-focus disks, and finally find two positions where the star disk is just barely enlarged. Best focus is midway between the final two just-barely -out-of-focus positions.

The "Peak Pixel Value" Technique. If you have an astronomical CCD camera that reports the peak pixel value in an image or in a focus box, find a moderately bright star and observe the peak value as you change the focus position. The peak will fluctuate randomly; however, if you mentally average three to five images, the numbers will rise and then begin to fall again. Pass through focus several times until the peak pixel values are highest.

Chapter 5: Imaging Techniques Two-Hole Mask

Three-Hole Mask

Basic Diffraction Mask Hex-Diffraction Mask Hardware-Cloth Mask

Basic Diffraction Mask Hex-Diffraction Mask Hardware-Cloth Mask

Figure 5.3 Perfect focus often proves to be elusive. A variety of focus aids make it easier to determine the moment when the star image is smallest. With two-hole and three-hole masks, the star is deformed when it is slightly out of focus; with diffraction masks, you judge when an extended pattern is sharpest.

Figure 5.3 Perfect focus often proves to be elusive. A variety of focus aids make it easier to determine the moment when the star image is smallest. With two-hole and three-hole masks, the star is deformed when it is slightly out of focus; with diffraction masks, you judge when an extended pattern is sharpest.

The "Number of Faint Stars" Technique. The image of a sharply focused star concentrates its light in the smallest possible area. When stars are even slightly out of focus, their light is spread over a larger area, so fewer faint background stars will appear in focus-mode images. Set the exposure time to several seconds, focus approximately, then select a field with no bright stars in it. Slowly change the focus. As you move away from best focus, faint stars will drop out of the image; and as you approach best focus, more faint stars will appear. Proceed in very small steps—this technique is quite sensitive. Since the faint star images are literally "buried in the noise," their presence is subtle but easy enough to recognize. Even when you cannot see any change in the size of the star images, the number of faint stars increases sharply at best focus.

The "Longest Blooming Trail" Technique. The blooming trails that are so annoying in antiblooming CCD cameras can be harnessed as a focusing aid. Focus approximately, then center on a star that is bright enough to cause a readily visible blooming trail on the computer screen. When the star image is in the best focus, the concentrated starlight causes pixels to bloom; the more concentrated the light, the better the focus. An additional cue is that the better the focus, the thinner the blooming trail. Focus back and forth until the blooming trail is as long and thin as you can get it.

The "Diffraction-Spike" Technique. When the seeing is poor, star image blobs look the same over a considerable focus range. At such times, the contrast and visibility of diffraction spikes protruding from a star image may be easier to judge. You can generate handy diffraction patterns by placing one or many straight obstructions, such as wooden dowels over the front of the telescope tube. The thinner and closer the dowels, the wider (and weaker) the diffraction pattern. Two V^-inch dowels separated by two inches produce a useful spike, and an array of six dowels spaced an inch apart generates a big, bright spike. A dramatic alternative is to place a piece of hardware cloth (wire mesh with V^-inch spacing) over the front of the tube. When the extensive diffraction pattern looks bright and sharp, the camera is in focus.

"Holes-in-a-Mask" Technique. When a star is in sharp focus, rays across the entire aperture converge in the star image. If you place a two-hole mask (a piece of cardboard with holes cut in it) over the telescope tube, light from the two openings will converge at best focus. In poor seeing, it is relatively easy to tell when the two images merge into one. The holes in the mask should be about one-fourth the aperture and should be placed at opposite sides of the aperture. As you approach focus, the two images merge into an elongated blob, become round at best focus, become elongated past best focus, and finally split into distinct beams. After you focus, remove the mask.

A variation of this method is the three-hole mask. Cut three holes in a sheet of cardboard; they should be about one-fourth the aperture in diameter and spaced 120° apart. With this mask in place, you will see triangular images except when the telescope is near perfect focus. When you get round star images, remove the mask.

5.3.3 Automated Focusing

Focusing can be automated providing you have a good-quality motorized focuser, a CCD with a reasonably fast image readout, and appropriate autofocus software. When it's time to focus the image, simply identify a star and turn focusing over to the software.

The software usually runs the focuser well inside focus, and then systematically steps outward through focus, monitoring and measuring the size of the defo-cused image at each position. Plotted on a graph, the size is a "V"-shaped curve, with the smallest star image and best focus at the base of the "V." The software solves for the best position and then moves the focuser to that point.

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