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The Webfooted Astronomer - March 2001

 

Minutes: Astrophotography

By Leslie Irizarry

THE February SAS meeting began with Ken Applegate who has calendars. See him if you'd like one. Judy Schroeder asked members to let her know if your address has changed. The club gets charged for returned mail.

Project ASTRO

George Best introduced the first speaker, Linda Schagler, who talked about Project ASTRO. Linda works in the astronomy department for Woody Sullivan and the center for astrobiology. Project ASTRO began in 1993 through the Astronomical Society of the Pacific, which had a national science foundation grant that pairs astronomers and engineers with grades 3-12 teachers in the schools. The astronomer works with the teacher throughout the school year. The astronomer decides with the teacher which activity will be done at what particular time. Most of them are elementary school teachers, and most of the elementary school teachers are 5th grade teachers. The exercises can be adapted for all grade levels.

To volunteer, fill out an application form and submit it to her. After the first of May, the astronomer is assigned a teacher according to geographical location and astronomer grade preference. The astronomer and teacher meet at a mandatory summer workshop July 13-14 to work together to figure out a realistic schedule. The first school visit is for observation.

Astrophotography

The second presentation by Keith Allred, who runs the photography group, covered the topic of basic astrophotography and astro-imaging. Astrophotography is one of the more difficult things that he's done. This presentation is about things that he has learned through experience.

Film, which is involved in astrophotography, and CCD, which is involved in astro-imaging, share everything in common except how the photons are saved. The basic equipment required is a mount that compensates for Earth's rotation, an optical system, and a camera imager that saves the information that the photons provide. Mounts must rotate. This can be accomplished through manual rotation, such as a barn-door mount with a polar alignment scope and a motor for right ascension adjustment, or through mechanical rotation. These include German equatorial mount, fork mounts, Dobsonian mounts, and exotic mounts. Common problems with mounts include rigidity. In fork mounts, the wedge part tends to be weak. The fork can vibrate, which is death to a good astro-image. Other problems include gear and motor problems. Periodic error can cause variability in rotation. Some of the best gears around are the Byers Gears. If it varies by one-half a degree, Byers shuts his factory down. Put your money in a mount.

There are various optical systems: the advantage of refractors are that they don't have a central obstruction and thus no collimation is required. Other optical systems include: camera lens, reflectors, such as Newtonians, Ritchey, and Catadioptric—Schmidt-Cassegrains (SCTs). Common problems include collimation. The stars are blurry and one can't get them into focus. Another problem is Mirror Flop in SCTs. The machining of the housing and tube that it rides on is not as precise as it should be. It is filled with grease. With heat and cold, the grease comes out. So when you're in one part of the sky, you're in focus. In another part of the sky, you're not in focus. So what people are doing to address this problem is that the primary mirror is locked down, and the focusing mechanism is outside the telescope. The draw tube is placed outside the primary mirror.

Coma is a problem due to the mirror being out of alignment, and the focus is in the wrong spot. It puts a halo around the image, and the stars are oblong. Vignetting (the darker area outside the image) is due to the image area being smaller than the film frame. Chromatic Aberration is common in refractors. As the light comes in, different colors have different points of focus. Many of the older refractors have this problem. Some of the newer refractors, such as Takahashi do not.

Cameras should be lightweight, and should not be battery-powered. If the shutter is open between 10 and 30 minutes, you can burn through all the juice. It is better to have a mechanical camera with a mirror lock and replaceable screen.

The CCD is used as a photon gatherer. This is used in all the major observatories. Basic CCD technology involves a silicon chip; the surface has thousands of pixels. When a photon hits, it generates a certain amount of current. That current is fed through the circuitry to a converter, then to a computer. A certain level of brightness on the computer screen corresponds to the number of photons that hit the chip. It is useful due to quantum efficiency. It registers photons very efficiently.

CCD imagers vary from do-it-yourself to top-of-the-line. There is a trade-off in price and features. Variables include chip size, pixel size (if under sampled, get square stars), chip sensitivity (many are not as sensitive in blue as in red), cooling (a chip must be cool enough so you're not getting noise--50C below 0), guiding, and whether separate computer is required.

A Cookbook Camera is the least expensive. It requires some basic knowledge of electronics and some mechanical skill. You can buy pre-machined aluminum housings and ready made circuit boards (costs $350). If you buy housings and circuit boards, you're out $525. Manufactured cameras can be obtained by Santa Barbara Instruments Group (SBIG), Apogee, Finger Lakes J, and Meade. High end cameras have back-illuminated chips, providing higher sensitivity, particularly in blue and green. It dramatically improves quantum efficiency, and has much larger chips: Apogee 6540. In terms of fields of view, the Cookbook has a small field of view.

Common problems include pointing, tracking/guiding and focusing,. It is hard to find the object. This depends on the field of view and the brightness of the object. Mechanical pointing is done via a Telrad or finder scope. Because the area of the CCD is generally very small, it is difficult to center the image on the CCD. A flip mirror is a common solution to this. Rotate the mirror, so you can see it through the eyepiece. (If the object is very faint, however, this will not work.) Another means of pointing is electronic pointing. The Meade LX20 Series are pretty good. Astrophysics GoTo series can take you to within arc seconds of where you want to be.

Tracking/Guiding is important to keeping the object on chip or film. Factors include focal length—the shorter the focal length, the easier it is to guide. Mechanical guiding involves picking a guide star and putting it in the cross-hairs, but you must stay with the telescope! Electronic guiding can be done by off-axis guiding in which light comes in through a pick-off prism, and it runs it up to the guiding CCD. Other methods include guide-scope guiding and built-in guiding. Another method is through in-line guiding such as Apogee, which is a brand new product and provides an elegant solution.

Polar Alignment is important. Alignment scopes include Off-axis scopes and bore scopes. A bore scope is better, providing much more accurate alignment. Keith uses drift alignment. If the scope if not polar aligned, you will get field rotation, which can cause stars to appear shaped like kidney beans. A focusing screen such as the Orion Intense Screen helps a lot. It makes starlight appear brighter in the viewfinder. Another device is the Verimagnifier, which hooks on to the back of the camera and can be rotated 30 degrees. Due to temperature changes, an optical tube can get out of depth of focus.

Focusing imaging methods include star reduction; light insensitivity measuring; Hartman Mask openings; and the diffraction spike method.

Black and white film has a finer grain. For color, recommendations include Kodak Ektachrome EDH P1600; Kodak PPF, PJM; Fuji 800. Gas hyper sensitization makes the film more sensitive (hypering). The gas used in this process drives away the moisture of the film. Dry ice is an older method and is not used much. For developing, use a quality lab. Do it quickly and don't allow them to cut the negatives. CCD Accessories include narrow bypass filters.

Keith offered his favorite solutions:

  • Build an observatory
  • Get a big telescope. Make it rigid.
  • Get the best instrumentation you can get.

Keith presented a spectacular slide show that included such celestial delights as the Horsehead nebula, Moon, NGC 1055, Whirlpool Galaxy, Sculptor Galaxy, (the second largest galaxy you can see), Crescent nebula, Stephan's Quintet, Saturn Nebula, Helix Nebula, M 13, Eagle Nebula, Pinwheel Galaxy, M33, Dumbbell M27, Lagoon Nebula, and more.

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