I may need some math/logic help here, if someone could help?
- The end purpose, is to calculate the minimum secondary size.
Quote:
A good place to start in your quest for the ideal secondary mirror is by figuring the absolute minimum diagonal size that will catch all the converging light rays from the telescope’s primary mirror. To make this simple calculation, you need two numbers: the primary mirror’s focal ratio (f) and the distance from the secondary mirror to the focal plane (L). The focal ratio is simply the telescope’s focal length divided by the diameter of the primary mirror. For example, an 8-inch mirror with a 48-inch focal length would have an f/ratio of 6, or f/6 as it is usually expressed.
To determine L, you need to locate the focal plane. You can find it by removing the eyepiece and projecting an image of the Moon (or a daylight object at infinity) onto a small square of tracing paper taped over the end of the focuser. (A layer of frosted tape will also work for this.) Rack the focuser in and out for best sharpness and measure the distance from the paper to the center of the tube — this is L.
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Okay..
So, I'm working this out for a Type 1 (1992) TAL reflector. So, it has a 110mm primary, currently has a 27x38mm secondary, and the official ratio I see around is f/7.3 (but there are two TAL versions - one original - one a later model white one, with a shorter tube which I don't have). (The scope is discussed here:
https://www.cloudynights.com/topic/4...ian-from-1990/ - At the end of the article, Andie Howie describes some of the differences between this and the later version).
I raised the primary mirror in the tube 28mm, and the distance from the primary to the center of the secondary mirror is now 66cm (26"). This allows modern 1.25" eyepieces to be used.
"L"
The focal plane on a TAL is interesting. I need a moon or something, then need to project it onto tracing paper in the eyepiece holder without an eyepiece? I guess I can do that, currently, moonrise is a little late. For arguments sake,m let's imagine it's 13.5cm (5,3") - that's with the focuser about half way, and I can adjust/update the formula once I understand how it works.
How do I put all that together correctly, to find the minimum secondary size?
