Re: Telephotography


Message posted by lone wolf on June 13, 2004 at 11:50:05 PST:

I've been using an old Pentax 500mmF4.5 lens that I modified to accept a telescope visual back. I don't write about it since it would be hard for anyone to duplicate unless you know a good machinist. [I've got one who takes Area 51 ink jet prints in trade for odd jobs. Such a deal!] I got a good deal on a 78mm refractor telescope that I will be using in the future.

For the most part, you can classify telescopes as either reflectors or refractors. Reflectors have a folded optical path, so you get more focal length for the physical size. The mirrors in the reflector telescope are very light. This combination of size and weight makes the reflector a good candidate for use on nasty hikes up steep slippery hillsides. Because the light doesn't go through glass, there are no chromatic abberations in a reflector telescope. There are disadvantages to the reflector. One problem is the secondary mirror is often pretty small, meaning the image won't fill a 35mm film plane. This means the image will be dark at the edges. To get around this, you add a barlow at the end of the optical path, or use a teleconverter on the camera. The problem here is certain combinations of telescope and barlow cause reflections off the secondary mirror, so you get a white spot floating in the image. I don't know how to predict what combos work and what don't. I managed to get a T-threaded barlow designed for photographic use that doesn't cause this problem on either the C-5 or ETX-90, but B&L stopped making it. The focus of the reflector is very temperature dependent. In astronomy, you let it cool and everthing settles down. In early morning Tikaboo photography, the rapidly changing temperature makes the focus drift.

The other type of telescope is a refractor. These use glass lenses and thus are heavy. Glass will have varying degrees of chromatic abberations, depending on the type of glass. If you are doing B&W photography, you can get around the abberations by filtering the light. Green works quite well. APO type refractors have very little color abberations, but cost more. You'll notice the better refractors use fluorite glass, as well as the better photographic lenses. Refractors do not have the folded optical path, so you get half the focal length for the same physical size. Given a reasonable physical size of the telescope and the requirement of getting at least 2000mm equivalent focal length for Tikaboo, you will need a 4x barlow with a refractor, or do eyepiece projection. The 2000mm focal length is to get around film grain and sharpness limitations. This assumes you are scanning the film at 4000 ppi. If you do prints, they had better be 8x10 to get reasonable sharpness. The film scanner (maybe $400 for an older generation 4000 ppi) will pay for itself eventually given the cost of 8x10 prints. FOr a given aperture, refractors cost more than reflectors. Fortunately a 3inch refractor is large enough for daylight photography.

Spotting scopes are generally a refractor telescope combined with an erecting prism. I guess I should have mentioned the use of a diagonal back in the telescope section. If you just put an eyepiece into a visual back of the scope, the image (ok, pulled from memory) is upside down and reversed. If you reflect it off a mirror, the image is right side up, but still reversed. Generally, who cares if it is reversed. I just reverse the image in the computer. However, if you were trying to read a tail number or license plate on scene, you would need an erecting prism. The prism can have ghosting problems and in general is optically inferrior to a mirror diagonal. If you were to shoot a photo, you would take the prism out of the path.

Astronomy isn't really concerned with field flatness, but rather sharpness. Stars need to be dots. However, in daylight viewing, field flatness is of some concern, unless you like buildings that look pincushioned. I think refractors have flatter fields. Reflectors have more bending near the edge of the field of view. The eyepiece used in the telescope also effects field flatness. For eyepiece projection, you should use an orthoscopic eyepiece. For viewing, there are all sorts of theories, but generally a simpler design has less field of view, but also less bending. A possl eyepiece is a good compromise.

Lastly, if you chose a refractor telescope, make sure it has a 2 inch visual back, or better yet a design that accepts "wide" t-rings that mount to the focuser. This will allow the telescope to be used in "prime" mode without the edges of the image becoming dark.

My ideal observation set up would be a good satellite, preferable at both directly above the base and at a 45 degree angle. Second would be a Predator drone. In all seriousness, the satellite has an easier task than you do shooting from Tikaboo. There is about 3 miles of breathable atmosphere between the satellite and the base. You have 26 miles of atmosphere between the base and tikaboo. Atmosphere reduces contrast, so it is best to photograph with slide film. I use Fuji Provia 100F.

You didn't ask about a camera, but most modern cameras have trouble setting the exposure with a telescope attached. I use a Nikon F3 with winder. The winder powers the camera. Most older cameras use mercury batteries that can no longer be purchased due to environmental concerns. However, camera winders generally power the camera. The F3 has a removable prism which can be replaced with a magnifying viewfinder. [You should always use some sort of magnifier on the viewfinder if the subject, i.e. base buidlings, are not very large in the field of view.] The DW4 viewfinder has a 6x magnifier that shows the full frame. A remote release is also needed so that your hand won't shake the camera. Obviously a tripod is mandatory for maximum sharpness. For a panoramic, you need a geared head for the tripod. I make sure the tripod is level. Otherwise the panoramic will have slanted lines in it as the photos are pasted together. How to paste the photos together without really nasty seams showing would be a post longer than this one. The short answer is you make sure the camera is in manual exposure. This is required on the F3 when the mirror lock up is used, and you shouldn't use a camera without mirror lock up. If the exposure remains constant across the panoramic, then all that is required for good edge to edge matching is that the scanner settings do not change from frame to frame.


In Reply to: Telephotography posted by jason rhoades on June 12, 2004 at 17:27:04 PST:

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