Homemade Refractor

[TimmySand]

I want to make a very cheap refractor, but I am having a hard time finding an objective lens. Since I cant find any cheap lenses with both an aperture around 70-80mm AND a focal length of around 1500mm, I was wondering if this would work:

Using a convex lens with a big aperture & a short focal length as the first lens, then directly behind it put a weaker concave lens that would lengthen the focal length of the scope to somewhere around 1500mm.

Does this logic make sense? Is there any problems I would run into with it? And if this would work, what equations would I need to use to know the focal length of the 2 lenses I need?

[Kaptain K]

Unless you get terribly lucky, you're going to have horrible chromatic abberation!

[Kyle Edwards]

I want to make a very cheap refractor, but I am having a hard time finding an objective lens. Since I cant find any cheap lenses with both an aperture around 70-80mm AND a focal length of around 1500mm, I was wondering if this would work:

Using a convex lens with a big aperture & a short focal length as the first lens, then directly behind it put a weaker concave lens that would lengthen the focal length of the scope to somewhere around 1500mm.


It would probably work on paper, but it sounds to me like your image is going go downhill in a big way stacking two cheap lenses like that.

In my opinion you might do better using something like this, http://www.surplusshed.com/pages/item/l3002.html , and then using a good barlow to get the long focal length. But I'm no expert on telescope making.

Good luck with your project.

[Maksutov]

Unless you get terribly lucky, you're going to have horrible chromatic abberation!Exactly. If both the convex (planoconvex I take it) and the "super-Barlow" lenses have the same index of refraction, you'll see lots of pretty rainbows where there aren't any.

Plus short focal length planoconvex lenses usually aren't all that accurate, and the negative meniscus lens will just pass these errors on to your eyepiece while adding its own.

You'd be a lot better off using the objective lens Kyle Edwards suggested.

Be sure to put a lot of care into making sure everything is on-axis. Once assembled most refractors are a bit of a challenge to collimate. The alternative is to add adjustment options, which will also require great care re design and manufacturing.

But, give it a shot. Having good optics to start with means you won't be creating a "crippled" scope right from the beginning.

Meanwhile, welcome to the BAUT, TimmySand!

Read the FAQs (http://www.bautforum.com/faq.php?faq=vb_faq), especially the rules (http://www.bautforum.com/about-baut/32864-rules-posting-board.html#post564845), and have fun.

[RickJ]

I want to make a very cheap refractor, but I am having a hard time finding an objective lens. Since I cant find any cheap lenses with both an aperture around 70-80mm AND a focal length of around 1500mm, I was wondering if this would work:

Using a convex lens with a big aperture & a short focal length as the first lens, then directly behind it put a weaker concave lens that would lengthen the focal length of the scope to somewhere around 1500mm.

Does this logic make sense? Is there any problems I would run into with it? And if this would work, what equations would I need to use to know the focal length of the 2 lenses I need?

That is sort of the principal of an achromat objective. But only if the convex lens is made of crown glass, the concave lens is made of flint glass AND they were made to work together. Then you need to know if they are to be cemented, air spaced or oil spaced. If air or oil spaced what is the required spacing for best color and spherical aberition correction? If either has any wedge you will need to shim that out. Most inexpensive lenses do have this problem.

Mounting such a long tube so it is steady would require a very good mount. The lever arm of a 2000mm tube (over 6 feet) would be very large. About the only way to deal with such a long refractor is to fold it using one or two flat mirrors. That would make for a much steadier telescope unless your machining skills are very high. Note that even though the flint lens is negative a 2000mm focal length refractor made this way would be about 2000mm long, it doesn't work like a barlow at all! While a single lens objective with a barlow of correct design could be made to work I doubt you could do it with less than 3 lenses using aspheric surfaces, expensive glass and a good lens design program. Aspheric lenses don't come cheap, if they did you'd see refractors made this way and I never have.

Far easier would be to make a Dobsonian reflector of small size.

Or find one of the typical dime store scopes people find so hard to use and give away at yard sales. Unscrew the objective and throw away the rest. Then you are all set at virtually no cost. Except for the not so minor problem if building a tube assembly with usable focuser and mounting it steadily enough you can focus the image. Those sometimes have fairly usable objectives and sometimes even the tube assembly is functional if you lap the gears in the focuser and lube it heavily. It's everything else that's junk. Though they are usually of about half the focal length you are after and thus have some residual color problems. Often with a good eyepiece the color is a minor issue. Those with plastic lenses or non achromats are worthless however.

I'll add my advice given here constantly to seek out a local astronomy club. They often have loaner scopes for beginners, ours loans out two 4" Synta (Chinese) refractors to beginenrs. They also have resources unavailable elsewhere you likely will find useful if you go ahead with the building project.
http://www.skyandtelescope.com/community/organizations

For basics of achromat design see:
http://alice.as.arizona.edu/~rogerc/
For free design software see:
http://www.atmsite.org/contrib/Fejes/DataEntry1-Refractors/RefractorData.html
I've not used it so have no idea how useful it is.

Rick

[TimmySand]

Yea, I think I will probably go with the lens Kyle suggested and buy a barlow lens. Thanks everyone for your help.

[JustAFriend]

I want to make a very cheap refractor, but I am having a hard time finding an objective lens. Since I cant find any cheap lenses with both an aperture around 70-80mm AND a focal length of around 1500mm...


Here's an 84mm x 1200mm for $65...

http://www.surplusshed.com/pages/item/l3548.html

They have others, browse around...