Message posted by JB737 on December 12, 2007 at 23:05:30 PST:
I agree 100% that the C5 is a great choice. On a lower budget the Celestron C90 (90mm diameter instead of 5in = 127mm) is also excellent, as are the Meade equivalents ETX125 and ETX90. A really good test target is the moon. Just like the base, it is lit by sunlight, so the exposure time will be about the same as any other daylight photo, hence holding the camera steady will be equally important. The moon's angular size is equivalent to an object 800 yards long at the distance of the base from Tikaboo. The largest craters are about 70 miles diamter or 1/30th the diameter of the moon, so equivalent to an object about 25 yards in diameter at the base. A C5 on a good night can almost resolve 1 mile craters on the moon, or somewhat over 1 foot resolution on the base. You're doing well if your C5 photo actually resolves two 2-foot objects separated by 2 feet, in average air conditions and with average ability at getting it in focus. Expect a 400mm camera lens to resolve maybe in the 5-10 foot range at best unless it is of extreme quality and held rock steady (literally sandbagged onto a boulder). Pixel size or film grain size come into play as limiting factors as you get to shorter, higher quality lenses, but don't matter much if your lens is extremely long. When in doubt, shoot with and without tele-extenders or Barlow lenses, to get different magnifications and exposure times. The base isn't moving, so longer exposures are OK if the camera is held down well enough. Counter-intuitively, in some cases a long exposure can reduce camera vibration from "mirror slap" which is common at moderate shutter speeds (such as 1/8 second to 1/250 second with a 400mm lens, or maybe even quicker exposures on a longer lens). So crank down the ISO setting (equivalent to film speed) for a few longer exposures while you have all your extenders stacked on to make the exposure as long as possible. Similarly, when you have the extenders off, try different ISO settings to get different shutter speeds, including the fastest ones possible, in case camera movement is your limiting factor. A good used manual focus 400mm lens with even a cheap 2x extender can get you into the pretty-good-base-photo game at relatively low cost if your DSLR body is of the 1.5x or 1.6x type relative to 35mm film. JB737
In Reply to: Re: what's the minimum size camera optics for good photos of the base? posted by Joerg (Webmaster) on December 12, 2007 at 17:10:29 PST:
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