Been re reading this thread a bit from time to time. This last time through  I noticed that several of you were going to purchase the book sporting rifles and scope sights. by Truman Henson. How is the book and how helpful do you feel it is? My interest is in building a scope on  the line of the Unertl Small Game Scope, would this book help?
The internet says that my local University has a copy in its library and I am going to try to see if I can get a look at it and maybe also hit one of the local book sellers to see if they may have a copy available.
Sam

Mike,
20X is a pretty ambitious project. It will require at least an 1 1/2" objective (40mm) to get a reasonable exit pupil. (Objective diameter / magnification = exit pupil)

Working on a 6-7X, 3/4" x18" with Edmund lenses. Appears about medium quality. Getting some chromatic aberration (color fringes around the edge), which was expected with plain lenses. Has greatly increased my respect for optics designers.
Chuck

If my math is correct, with a 40mm objective at 20x, your exit pupil would be around 2mm, which would give you a surface brightness of around 10% (I think the brightness chart I have is for uncoated lenses).

If this is correct, you might get a pretty "dark" scope, compared to a lower power version. Some consider 2mm to be an optimum exit pupil size, but with older eyes, I personally like a "brighter" scope.

Just my two cents. 

I picked out 3 lens, one is a Plano 14mm X 279 FL objective, a Anchromat lens at 7.3 mm dia X 607.2 FL Erector lens, and a 14 mm dia X 42 mm Occular. This if I did the math correctly should be at a 21 power. 
Now this is using the cheap experimental lens but all grade 1. If I can get these in the better quality lens I will, but I will purchase the cheap lens to try it out first. The lady said I could always return them if needed.
Anyway, the math I used was the Oective FL + the Erector FL divided by the Occular FL. Anyway that was the way Strobels book described it. If this is not the correct way will someone let me know.
Thanks
Mike

Mike,

I didn't run your numbers but from my readings, I think your process is correct. If your erector lens doesn't magnify, add the additional focal length (out from both sides of the erector) to the objective focal length, and then divide that total by the ocular lens focal length.

There is a lens known as a Barlow that bends the light rays from the objective farther down the focal axis toward the ocular end of the tube extending the focal length of the objective lens, which increases magnification, and increases the scope length.

On the flip side, the collector lens appears to do the opposite of a Barlow, in that the collector bends the rays from the objective so that they converge sooner on the focal axis, which effectively shortens the focal length of the objective, decreasing magnification and allowing for a shorter scope tube length.

Frank,

I'm guessing that all of your lenses are double convex, with each lens having an equal focal length on each side of the lens (which means both sides are ground with the same curvature). I'm not saying that all the lenses have the same focal length. Let me know if I'm off track on your setup.

Anyway, the focal length of a lens is where all the light rays passing through the vertical axis of your lens, no matter what the distance above or below the horizontal center of the lens, will converge and cross the horizontal axis through your scope tube. The focal length is the distance from a lens to its focal point. You can put your cross-hairs at any focal point, though it's easier to repair the wires if you put them in front of the ocular lens extension.

The focal length of your 1st lens plus the focal length of your 2nd lens gives you the spacing between those two lenses, and so on.

Adding additional lenses will increase magnification, but each lens supposedly reduces light transmission to the next lens by about 4%. I haven't looked at the various coatings to see how that changes things.

BP, I blew the math and now I can see that. I did not add the FL from both sides of the anchromat lens that I picked to use as an erector lens. I only used one side and I knew something looked off, but just could not put my finger on it. Thanks BP and now back to the math board. I know also that they used the anchromat lens in several positions on these scopes but I am not sure if they were OK as an erector lens. And your question about does the lens magnify or not? I thought all of these lens did magnify, I have the formula complemts of Edmonds to figure the magnification amount out but have not used it yet. 
Mike

Hey BP, here is what Anchor says about there lens type double convex.

(Double-Convex (DCX) Lenses are lenses which have two outward curved surfaces. Double-Convex Lenses have a positive focal length (FL) and will act as a magnifying or condensing lens.)
So this looks as though you can use this lens to magnify or reduce or condense, I think condense is a fancy word for reducing the magnification power, but I may be wrong.  I just found a copy of the scope building article that Rudi from the ASSRA archives mailed me out a couple of years ago. In that article it was stated that the erecto lens power should not be to high and the occular power the same thing, other wise it is a trade off in magnification and other things. Malcolm did it with more lens some how and maybe we can figure it out. I have read Malcoms deal on lens making and he said that lens shape and curvature was more importand than lens finish. And I aked an optical tech that question and they agreed. In the set of lens I have from my 18 inch long original Malcolm scope the brass lens holders and light traps are all screwed together in a very short arangement. The only lens that is not screwed into this mess of lenns is the Objective lens as it was setting way out in front.
Mike