Perplexing Refractor

In my quest to build a 5 3/8" refractor, I have stumbled upon a most unusual problem. Building the tube and the other parts of the assembly were of real no issue's, but when I got the installation of the objective lense I have come across my isuue. I am using a {American Optics 18" lens cell} { not much info on this cell}. I did a homemade focal length test and came up with about 41" for focal length. Installed the cell in the tube and then installed the focuser and diagonal. the view is as it should be {backwards and reversed}. NOw come's the issue. As soon as I drop a eyepiece into the whole assembly it turns out all fuzzy and almost white. I tried to move mthe lense cell to varying lengths to see if my calculations were wrong, but to no avail. Is there something I am doing wrong? If there is any advice one can give, it would be most appreciative. Thanks.... :S

Hi

Two thoughts

1. The lens you have may not be a complete lens.

2. If you tested it in daylight, and haven't flocked or baffled the tube you may have problems from stray light.

I am puzzled by your comment "the view is as it should be {backwards and reversed}." How are you viewing through it before you put an eyepiece in?

Also, how did you measure the focal length? I'd suggest stopping the lens down to an inch or two and measuring the focal length by imaging the Sun or Moon on a white surface.

Clear skies, Alan

Just a few questions...

1. Where did the objective come from???

2. How did you measure the focal length?? I think Alan's method is a good one.

Jon

Here goes a wild guess: The focal length is 18". When
you did your light bulb test, you had the lightbulb
and paper roughly equidistant from the lens, and hence
you got roughly twice the focal length (36") + a bit
for internal structure of the lens ~5" = 41".

The fact that the image "looked reversed" when you
peared in the focusser means you were very far
OUTSIDE focus -- again consistent.

The good news is that you goofed in the "easy" direction.
You just need to measure again and get out the saw.
Some wise man once said "Measure twice and cut once"
-- good advice.

The lens is from am opaque projector. It is designed to project a picture of a brightly lit object inside the machine onto a wall 10 to 20 feet away. It is not designed to focus at infinity and performs poorly as a telescope. I doubt that it can handle anything over 15X at all and that would be a 25mm eyepiece.

Opaque projector lenses come in different focal lengths. I think that 15 inches is the most common. The 18 inch lens may be better than I've seen with the 15 inch versions.

Good Luck
Tom