I want to build a telescope

[Rakehellion]

I'm looking at buying one of these mirrors, putting it insied a trash can with one of these lenses. I know with such a short focal length, it'll probably look like I'm using Instagram filters, but it doesn't look like I have too many options. I'm sure it'll be just fine for looking at the Sun or Jupiter.

Have any of you done this before?

 

[MagnusTheBrewer]

Not since hs. It will help to coat the inside of the tube /trash can with a non reflective lining.

 

[Rakehellion]

The hardest part is finding the mirror. Most "parabolic" mirrors you see online are for solar ovens and have nonexistent focal lengths.

 

[cyclohexane]

Dude, you can buy a telescope with better optics at Walmart for like 40 bucks

 

[Hayabusa Rider]

That's a concave mirror, not a telescope optical element. If you want to try that's grand, but don't expect to see anything other than distorted images.

 

[MagnusTheBrewer]

The hardest part is finding the mirror. Most "parabolic" mirrors you see online are for solar ovens and have nonexistent focal lengths.

You can make your own, that's what we did in hs. You'll also need a small mirror at the focal point to reflect to your lens unless you tilt the main mirror but, then you lose some light gathering and may need a longer tube.

 

[MagnusTheBrewer]

That's a concave mirror, not a telescope optical element. If you want to try that's grand, but don't expect to see anything other than distorted images.

It's parabolic with a focal point and no one is claiming it's optical quality. You might be surprised by the resolving power of homemade optics.

 

[BoomerD]

Instead of using a trash can...how about some 16" white PVC pipe?

http://www.usplastic.com/catalog/item.aspx?itemid=65221&catid=727

 

[MagnusTheBrewer]

Instead of using a trash can...how about some 16" white PVC pipe?

http://www.usplastic.com/catalog/item.aspx?itemid=65221&catid=727

Good find but, that's 16" od. I think he'll need some 18" pipe if he goes that route.

 

[BoomerD]

Good find but, that's 16" od. I think he'll need some 18" pipe if he goes that route.

good catch. I forgot about OD/ID.

http://www.usplastic.com/catalog/item.aspx?itemid=65752&catid=727

 

[Hayabusa Rider]

It's parabolic with a focal point and no one is claiming it's optical quality. You might be surprised by the resolving power of homemade optics.

I've ground a few mirrors from pyrex blanks over the years and the last efforts were very good indeed.

If the mirror is parabolic that's nice, but we're looking at an f/1.5 setup more or less and while something better than what Galileo used could be had the overall quality of any telescope using that as a primary will be poor. Regardless it could be an interesting experiment. My point is to not expect much. I cringe when I think of what a 16" scope would cost at that focal length if properly made.

Still I'd like to see how he'd do it. Putting a mirror in a trash can is not a telescope. What is he doing for a secondary? He can't stick his head in front of the thing so he will need one.

I wonder what his expectations are.

 

[IronWing]

You don't need a solid tube to mount the mirror and lens. A wood or metal frame with a cloth covering to block light will work.

 

[Hayabusa Rider]

Oh I just noticed. What does he think he's going to do with a Barlow lens? He's not going to use it to look through, that's for certain. And look at the Sun? Egads.

 

[yhelothar]

You would need a secondary mirror.
Telescopes are dirt cheap on craigslist. I've seen 8" dobs regularly go for ~$200-300 on there, and that's often with a set of eyepieces.

 

[GagHalfrunt]

Anyone can build a telescope. Link up a dozen or so fresnel lenses and build a death ray instead.

 

[MagnusTheBrewer]

I've ground a few mirrors from pyrex blanks over the years and the last efforts were very good indeed.

If the mirror is parabolic that's nice, but we're looking at an f/1.5 setup more or less and while something better than what Galileo used could be had the overall quality of any telescope using that as a primary will be poor. Regardless it could be an interesting experiment. My point is to not expect much. I cringe when I think of what a 16" scope would cost at that focal length if properly made.

Still I'd like to see how he'd do it. Putting a mirror in a trash can is not a telescope. What is he doing for a secondary? He can't stick his head in front of the thing so he will need one.

I wonder what his expectations are.

It's still a fun project. The amazing part of a dancing bear isn't how well it dances but, that it dances at all. We built a 6' parabolic mirror in hs using lightweight concrete for the base and small triangular mirrors on the surface. It worked quite well.

 

[sdifox]

But it will be slimy...

 

[MagnusTheBrewer]

You would need a secondary mirror.
Telescopes are dirt cheap on craigslist. I've seen 8" dobs regularly go for ~$200-300 on there, and that's often with a set of eyepieces.

Anyone can build a telescope. Link up a dozen or so fresnel lenses and build a death ray instead.

You guys are missing the point. The idea is to be as accurate as possible with the tools at hand not, compare the result to an optic company's.

 

[Hayabusa Rider]

It's still a fun project. The amazing part of a dancing bear isn't how well it dances but, that it dances at all. We built a 6' parabolic mirror in hs using lightweight concrete for the base and small triangular mirrors on the surface. It worked quite well.

That sounds like a fun project. I still wonder if he understands any of the necessary concepts. You probably know that a "2x" Barlow lens isn't used for viewing, but rather an optical device which doubles the magnification of a given eyepiece and would be useful with a short focal length, but it's no more a substitute for an eyepiece than a flounder is for a piston when rebuilding an engine.

Might be fun to walk him through this process if this isn't a troll thread.

 

[MagnusTheBrewer]

That sounds like a fun project. I still wonder if he understands any of the necessary concepts. You probably know that a "2x" Barlow lens isn't used for viewing, but rather an optical device which doubles the magnification of a given eyepiece and would be useful with a short focal length, but it's no more a substitute for an eyepiece than a flounder is for a piston when rebuilding an engine.

Might be fun to walk him through this process if this isn't a troll thread.

Shhh! You're ruining part of the exercise.
I admire anyone willing to actually do the work to understand how something works rather than just accepting what they were taught. It gives perspective on previous challenges and dispells the commonly held belief that we're generally more intelligent than folks hundreds of years ago.

 

[Sho'Nuff]

I'm looking at buying one of these mirrors, putting it insied a trash can with one of these lenses. I know with such a short focal length, it'll probably look like I'm using Instagram filters, but it doesn't look like I have too many options. I'm sure it'll be just fine for looking at the Sun or Jupiter.

Have any of you done this before?

http://www.ebay.com/sch/Telescope-Mirrors-Prisms-/74925/i.html

 

[Rakehellion]

You would need a secondary mirror.
Telescopes are dirt cheap on craigslist. I've seen 8" dobs regularly go for ~$200-300 on there, and that's often with a set of eyepieces.

Here I can build a 15" telescope for half that much. And I doubt those are using anything like a parabolic reflector either.

 

[Rakehellion]

http://www.ebay.com/sch/Telescope-Mirrors-Prisms-/74925/i.html

Those are half the size for 10x the money. I'm just looking for a mirror with a long focal length, not trying to donate this thing to the Smithsonian.

 

[Rakehellion]

Instead of using a trash can...how about some 16" white PVC pipe?

http://www.usplastic.com/catalog/item.aspx?itemid=65221&catid=727

The trash can is a plastic tube, so it wouldn't be any different.

 

[rockyct]

Those are half the size for 10x the money. I'm just looking for a mirror with a long focal length, not trying to donate this thing to the Smithsonian.

Look, if you want a telescope pick one of these options: 1) have the experience of building your own personal telescope, or 2) save some money and buy a normal one. The mirror listed isn't going to show any detail. You'll see Jupiter but it will be a big fuzzy dot. I'm not sure why you even mention the barlow, you'll need a secondary mirror like everyone else has said and a set of eyepieces before that. You'll also have to think of a mount for your trashcan.

Oh, and you really can't use it as a solar telescope (assuming it had the detail as well) unless you projected the image from the eyepiece at a sheet of white painted metal. Even then, your eye wouldn't be able to pick out detail in bright spot and it probably wouldn't be great on your eye to look at. Telescopes usually have a solar filter before the light hits the mirror as that way is much, much safer. A projection setup really only is useful during solar eclipses so you can see the bite taken out of the sun.