Message posted by gary on March 07, 2001 at 14:50:08 EST:
I have some web pages that might be useful to look at before the discussion. One is on eyepiece projection The other page is not ready for prime time, but rather is a work in progress. I did this page to figure out the requirements before I get the next telescope. I have a small reflector as show in the first page, but am inclined to get a somewhat larger aperture refractor. One of the things I've learned about telephotography is 99% of the market is astronomy, leaving 1% for terrestrial telephotograpy [ paparazzi (sp) photographing supermodels topless on the beach and those photographing restricted military sites]. OK, I'm making up the 99% to 1% ratio, but it must be skewed. Most astronomy is done at night, while the terrestrial telephotography is done during the day. You might ask what is the relevance. Well, the astrophotographers don't bother to meter their exposure because the light is so dim they can't meter. This is not true for those taking daytime photos. This is why I made the light meter adapter. The other difference is astronomers value seeing the stars as fine points, but don't get excited about things like how uniform the intensity of the image is across the film plane, nor spatial distortions (pincushion, etc). Those taking terrestrial photographs with the intent to make a panoramic value images that are clean from edge to edge, uniformly lit, sharp, and no spatial distortions. Reflectors tend to have spherical abberations near the edges (known as coma). There are two tricks I haven't mentioned on either page. One is using an electric motor focuser. This is more important for reflectors where the mirror is moved to focus the image. Twisting the knob jiggles the whole telescope. If you have an electric focuser, you don't touch the scope, so there is less jiggle. It is tough to focus when the image is jumping around. Some reflectors and most refractors use a rack focuser in the back of the scope. These tend to be smoother in operation. The other trick is to use a magnifier on the viewfinder or better yet have a camera where you can remove the prism and view the focusing screen. My camera can't do the latter, but Canon does sell a magnifier for around $40 that snaps on the viewfinder. This aids in critical focusing. I've seen a set up in a book where the photographer puts a small microscope on the focusing screen to zoom in 7x to 10x on the image to make sure the focus is sharp. The refractor versus reflector is a never ending war. [I'm lumping telescopes that use corrector lenses, like Mak-Cassegrain's, in with pure reflectors.] In short, reflectors are more compact because they have a folded optical path. They are also lighter. You can get more aperture (light gathering ability) with reflectors since it is very difficult to make large lenses. [The main lens used in an ASM stepper for integrated circuit processing takes 18 months just to cool the glass, which must be done very slowly. Yes, it is a Zeiss lens.] In daylight, you don't need a great deal of aperture, so reflectors don't have an advantage. Reflectors have a central obstruction (the secondary mirror) that reduces the sharpness of the image and contrast. The camera used for telephotography doesn't have to be fancy. Howver, it needs to have two features. One is the ability to use a remote shutter release so you don't shake the set-up. The other is "mirror lock up". Mirror lock up first moves the the mirror, waits a bit, then takes the photo. This lets the set up recover from the shake due to the mirror. Incidentally, I found a mechanical camera release climbing Tikaboo. One problem taking photographs on hill tops is the wind shaking the telescope. I'd like to hear some ideas about how to get around this problem. Incidentally, the atmosphere is the weak link in the chain. Take your telescope and look at the moon. It looks very sharp, yet it is over 200k miles away. Wind is just part of the atmospheric degradation.
In Reply to: Thursday's Chat Program posted by Andre' M. Dall'au on March 07, 2001 at 12:13:10 EST:
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