I want to blow up the moon!

[michaelbsc]

Few amateur telescopes have a large enough focuser aperture to cover MF or LF films. Typical focuser diameters are 1.25 or 2 inches. Others are designed to cover larger format film, and if you have to ask the price...

So, let's talk more about this. (And maybe the CIA, FBI, or some other three letter bureau will chime in.)

If the focuser aperture is too small on a hobby store telescope, then the trick is to just build one. A sandwich honeycomb primary mirror on eBay is a lot cheaper than a lens, and my brother's been into the telescope hobby for 40 years now, so I have a resource to draw on. He doesn't know much about photography, but I've got that end covered.

If I'm making my own, then I can make the camera/film-back attachment point any way/size/shape I want it. (And if I don't want to keep it after I'm done then one of my brother's telescope buddies will probably buy it so my losses are managable. I just need to fabricate something that's astronomy friendly afterwards.)

So, assuming I'm building a lens, rather than a telescope, would my plan be to fab a telescope tube much like an amateur scope, expect where the eyepiece would normally go I would manufacture some kind of GG and Graflock assembly? Perhaps finding a scrap Mini-Speed is a reasonable plan after all. Then there's a shutter already in place.

Since this would be a mirror, not be a refractory type lens, will this have the trouble of vignetting in the field of view. I realize that the problem of mechanical vignetting will exist, but what about light fall off as with a lens?

MB

 

[Lee L]

There are a number of design considerations when making a Newtonian telescope as you're describing. One of the most important things will be for you to correctly size the secondary mirror, which has the property of choking the cone of light from the mirror in much the same way as a focuser tube. I'm not an expert in this, but you might find that a google search for software will get you where you need to go. Here's one MS Windows program that's been around a while, now freeware:
http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm
Once the secondary mirror reaches a significant percentage of the size of the primary mirror, you run into real problems with diffraction and loss of contrast.

I would think that it would be simpler to remount a refractor objective in a home made tube with a 6x9 or 4x5 camera back mounted to the tube. Amateur telescope tubes are typically constructed of aluminum tubing, cardboard concrete pouring forms, or wood.

Before going to the time and expense of building a special instrument for one shot, I'd recommend getting a T-adapter for whatever brand of 35mm SLR you have and trying that out on a good telescope with some fine grain film. You could shoot the moon and the building separately to judge relative size and print quality. You might also want to shoot the moon and building each at focus for the other object to check that you'll have sufficient depth of field. If you're going with a 2 meter focal length, shallow DOF could be a deal breaker. Does your brother have access to an 8" Schmidt-Cass? Those typically hit around the 2 meter focal length and could do the job if you find 35mm can work. How large do you plan to print?

Lee