Back in the late 1980s and early 1990s robotic telescopes were the rage. These automatically did the photometry. While most indeed did work and some are still in use there are some downsides to them. They are complex, expensive and far from foolproof. They also have alienated many amateur photometrists and potential photometrists, which is sad. One of the best reasons for doing photometry is to learn. With this comes a great satisfaction that is hard to find elsewhere. Using a robotic system pretty much isolates the user from the universe. Would you be happy just reading a book about something exciting or would you prefer to actually do it yourself. Remember, during a trip, getting there can sometimes be more interesting and rewarding than arriving.
The Backyard Observatory
For those serious enough to pursue astronomical photoelectric photometry, an observatory which allows the equipment to remain setup, particularly the telescope and mount, is essential.
Permanent Pier
Perhaps the most important part of the telescope equipment is a permanent pier. Even if the telescope and photometry equipment are removed between observing sessions, a solid pier and well aligned mount will make observing much easier and setup much quicker.
Getting the telescope to the proper height for comfortable use is very important. The best place to observe stars for photometry is when the stars are close to the zenith or at least at their high point in the sky. It's good to have the eyepiece close to eye level whith you standing near errect when the telescope is pointed at the zenith. Design and build your pier with that in mind. With Cassegrain telescopes the eyepiece is at its lowest point when the telescope is pointed at the zenith. Mount a Cassegrain telescope high enough so you can use the eyepiece comfortably in this position. Use a small platform or stand for viewing at other positions. Remember it is easier to use a platform for a too high an eyepiece than to continually try to get beneath a too low eyepiece. Newtonian telescopes will have a very different situation, but the same rules apply. The Newtonian mount should also provide easy observing close to the zenith. Because of the atmospheric extinction, observing far from the zenith is seldom done when doing photometry.
The pier should be solid enough to easily hold the telescope with photometer head and prevent vibrations when touching the telescope or photometer head.
Mount
As noted above, a permanent pier is desired if not a requirement. A solid and smoothly operated mount should also be used. If no enclosure is provided, some protection for the mount should be considered so once set up the mount does not need to be removed between observing sessions. Plan to spend time to get an accurate a polar alignment of the mount. It will save much frustration later and once set, will not need changing.
Any mount will do if it is sufficient to provide control for tracking and fine adjustment. Fork mounts are great and provide excellent access near the zenith. While altazimuth mounts will work for Cassegrain telescopes with fork mounts, they are far from ideal as they put the photometer head within the fork when viewing near the zenith. If one of the newer altazimuth Cassegrain telescopes is used, it will be better to use it in the polar mount position. German equatorial mounts work fine as do cradle type mounts. The key criteria is "easy use" when the telescope is pointing near the zenith.
As with astrophotography an accurate drive is essential. Slow motion controls in declination and right ascension will make centering a star much easier. A great deal of frustrating time can be spent if slow motion controls are not available. Even manual control for declination will make use easier.
For optimum tracking and smooth operation, precisely balancing the telescope with the photometer head and any other equipment mounted is important. A well balanced telescope will track better. If you use the telescope for other purposes, mark the position of counter weights and other balancing items for the photometry setup. This will make switching back and forth easier.
Telescope
Most any telescope can be used for photometry. The aperture size is not too important unless faint star work is desired. Apertures from 4" up have been used, with 8" being most popular. For photometry in the ultraviolet band, telescopes with large amounts of glass in the optical path should be avoided. The glass absorbs most of the ultraviolet light. This means refractors are not a good choice if U band data is desired. For the same reason, Maksutov telescopes are less than ideal. Some people have expressed concern about the Schmidt-Cassegrain telescopes with their corrector plates. These plates are very thin and do not seem to have significant effect on U band measurements. The best telescopes for all the bands are the straight Cassegrain and Newtonian telescopes.
Figure 1 is a photograph of the Hopkins Phoenix Observatory equipment which includes an 8" Celestron C-8 Schmidt-Cassegrain telescope and home-built photometry equipment. Note the heated dew shield made from an ice cream container.
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| Figure 1 Hopkins Phoenix Observatory Equipment |
Figure 2 is a photograph of the Blue Hills Observatory Equipment which includes a 12.5" Cassegrain telescope and Starlight 1 photometry equipment.
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| Figure 2 Blue Hills Observatory Equipment |
Finder Scope
A good finder scope is essential. At least a 50 mm finder, preferably larger if faint star work is desired. One of the frustrating parts of astronomical photometry is finding the correct stars. While the first time can take a while, even repeated observing of the same star can be speeded with a good finder. More than one photometrist has taken data on a wrong star.
Roll-Off/Slide-Off Roof
While an enclosure is not required, it will make doing photometry much easier. In addition to protecting you and your equipment from the elements and stray light, it will allow you to leave your system set up. With high walls, the roll-off or slide-off roof can offer some of the same advantages of the dome plus a much better view of the sky. This is not only nice for viewing the sky visually between readings, but allows you to see approaching clouds.
Atmospheric extinction increases dramatically the closer the star is to the horizon. Most photometric observations will be done above 30 degrees of the horizon and probably closer to 45 degrees and thus views to the horizon are not needed. Photometric observations of a star should not be done close to the horizon unless absolutely necessary. It is far better to wait until the star is higher in the sky.
A 12 or even 8 foot square observatory with seven foot walls will provide excellent protection from wind and stray light. Lower walls give more view close to the horizon at the expense of more light and exposure to the wind.
Figure 3 shows a photograph of the 12 foot square two story Hopkins Phoenix Observatory and slide-off roof. The observatory is approximately 1,200 above sea level and is located on the west side of Phoenix, Arizona. While not elegant, it has served well for over 20 years.
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| Figure 3 Hopkins Phoenix Observatory |
Dome
If you can afford a dome, they are certainly nice. They are expensive and more complex than a roll-off /slide-off roof observatory, however. They excel in providing protection from the wind, stray light and allow observing from the zenith to the horizon, They exemplify an observatory.
Figure 4 shows a picture of Stanley Gorodenski's Blue Hills Observatory located 5,200 feet above sea level just outside Dewey, Arizona (about 90 miles north of Phoenix, Arizona). Construction was started in 1983 and completed around 1990. The observatory is 16' square two story 2" X 6" stud construction. The pier is made of concrete block. The observatory and dome are Stan's own design and construction.
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| Figure 4 Blue Hills Observatory |