Building a Telescope with 6" Mirror & Focal Ratio of 3.3 - $50

[vincentm]

Hello,

I just purchased a 6" mirror that has a focal ratio of 3.3, ready to go for $50. Is there any links one can provide on building a telescope for such a mirror? I decided to do away with my previous idea, and keep that one around.

 

[Ai52487963]

Hello,

I just purchased a 6" mirror that has a focal ratio of 3.3, ready to go for $50. Is there any links one can provide on building a telescope for such a mirror? I decided to do away with my previous idea, and keep that one around.

That sounds like a job for a homemade Dobsonian telescope! I built one for my senior project in high school for use with an 8" mirror, but I assume you can take a lot of the functionality of what I did and retool it for your mirror.

I can't find the original link that I got out of Sky and Telescope some years back, but it followed very similar guidelines to this. Of course, it assumes you have a mount for your secondary and all those fun things, but even if you don't have those, there will probably be no problem in finding a compatible one with your mirror.

Hope that helps!

 

[vincentm]

That sounds like a job for a homemade Dobsonian telescope! I built one for my senior project in high school for use with an 8" mirror, but I assume you can take a lot of the functionality of what I did and retool it for your mirror.

I can't find the original link that I got out of Sky and Telescope some years back, but it followed very similar guidelines to this. Of course, it assumes you have a mount for your secondary and all those fun things, but even if you don't have those, there will probably be no problem in finding a compatible one with your mirror.

Hope that helps!

Thank you very much for that link. What size will the secondary mirror need to be?

 

[Ai52487963]

Thank you very much for that link. What size will the secondary mirror need to be?

I'm not entirely sure. I know there's some correlation between the two, but I don't know the exact size.

 

[Chronos]

f3.3 is very fast and $50 is very inexpensive for a such a mirror. Is it fully figured (parabolized) or spherical?

 

[vincentm]

f3.3 is very fast and $50 is very inexpensive for a such a mirror. Is it fully figured (parabolized) or spherical?

From what i was told and what i can see it's ready to go, i'll post a picture when i get home. He has another one but doesn't know the focal ratio for it, same size and price though.

 

[HallsofIvy]

Saying it is "ready to go" does not answer the questions- one can use a spherical mirror in a telescope if one is willing to accept the spherical aberation or if the focal length is long enough to make it not important. Also I doubt that anyone could tell spherical from parabolic from a picture!

 

[vincentm]

Saying it is "ready to go" does not answer the questions- one can use a spherical mirror in a telescope if one is willing to accept the spherical aberation or if the focal length is long enough to make it not important. Also I doubt that anyone could tell spherical from parabolic from a picture!

Gotcha

 

[Chronos]

Sounds like you have a spherical f3.3 mirror - which has the optical properties of a goldfish bowl.

 

[vincentm]

Sounds like you have a spherical f3.3 mirror - which has the optical properties of a goldfish bowl.

It's concave.

 

[chemisttree]

I think Chronos was referring to the difference between a spherical mirror and a parabolic one... both are concave of course. It is possible that you have a parabolic one at this price if it is surplus or used.

 

[turbo]

Thank you very much for that link. What size will the secondary mirror need to be?

Since nobody has answered this, here's how you go about it. Draw a 6" vertical line, then from the midpoint of that line draw a line perpendicular to that one, and make it as long as the mirror to eyepiece distance, based on your focal ratio. Draw a line from each end of the 6" line (representing your mirror to the point of the perpendicular line. This 2-dimensional triangle represents the light cone from the mirror. Now from the vertex of the 3 lines at the focal point measure back toward the mirror. You want to measure the length of the focuser plus half the outside diameter of your optical tube. Draw a vertical line at this point that intersects the two slanted lines representing your light-cone. Measure this line. If you want to capture all the light coming from that primary mirror, the length of this line is the size of the minor axis of the secondary mirror you should buy.

The secondary will be tilted at 45 deg, so that looking down the tube the major axis of the secondary will subtend the same width of the light cone as the minor axis of the secondary.

 

[chemisttree]

And the length of the tube should be nominally equal to the FL of the mirror as well.

 

[vincentm]

Since nobody has answered this, here's how you go about it. Draw a 6" vertical line, then from the midpoint of that line draw a line perpendicular to that one, and make it as long as the mirror to eyepiece distance, based on your focal ratio. Draw a line from each end of the 6" line (representing your mirror to the point of the perpendicular line. This 2-dimensional triangle represents the light cone from the mirror. Now from the vertex of the 3 lines at the focal point measure back toward the mirror. You want to measure the length of the focuser plus half the outside diameter of your optical tube. Draw a vertical line at this point that intersects the two slanted lines representing your light-cone. Measure this line. If you want to capture all the light coming from that primary mirror, the length of this line is the size of the minor axis of the secondary mirror you should buy.

The secondary will be tilted at 45 deg, so that looking down the tube the major axis of the secondary will subtend the same width of the light cone as the minor axis of the secondary.

And the length of the tube should be nominally equal to the FL of the mirror as well.

Thank you guys very much, that's what i really needed to know. I'm thinking this mirror is going to be good. I bought it from the treasurer of the Tri-Cities Astronomy Club, I can tell he's an experienced builder given that he had a scope he built himself in his living upon picking up this mirror. He also informed me that he's taught Astronomy at the local Columbia Basin Community College. so I'm sure i didn't get swindled, I am just a newb when it comes to this. I'm also heading out to LIGO here in Richland to check out the "Sample The Sky" event on the 9th, It's going to be my first real stargazing event and i know I'm going to have fun. I'm going to bring a bunch of food and beverages for folks, as well as my $30 fee to join the TCAC.

 

[vincentm]

Sounds like you have a spherical f3.3 mirror - which has the optical properties of a goldfish bowl.

Sorry to bump this thread, but i have to ask is this a good mirror given such "properties"

Also i plan on making the tube from cardboard. What are the exact measurements needed for it?Is it 3.3 \times 6 for the distance from the secondary mirror and the primary mirror?

 

[montoyas7940]

http://www.dalekeller.net/ATM/Newtonians/newtsoft/newtsoft.htm

 

[Chronos]

Sorry to bump this thread, but i have to ask is this a good mirror given such "properties"

Also i plan on making the tube from cardboard. What are the exact measurements needed for it?


Is it 3.3 \times 6 for the distance from the secondary mirror and the primary mirror?

. An f3.3 6" mirror is very nice if parabolized. If not, it's very challanging if you're not an experienced mirror maker. The tube need only be about 20" long. PVC pipe might be an option.

 

[vincentm]

. An f3.3 6" mirror is very nice if parabolized. If not, it's very challanging if you're not an experienced mirror maker. The tube need only be about 20" long. PVC pipe might be an option.

Thanks Chronos,

So I've got most of the parts needed for prepping of the tube ( sandpaper, bondo, urethane, flat blak paint, etc.

So instead of making another thread I'll just go ahead and save the server some space and post my next question here. My main concern however is the secondary mirror and the spider. I want to decrease diffraction as much as possible, which material(s) should i proceed with? And if anyone has some tutorials on building one that would be great.

 

[Chronos]

Piano wire would be my choice for mounting the secondary mirror. You will need two strands in each plane to keep it from twisting.