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

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In summary: If you want to capture more of the light from the mirror, you'll need to move the focuser closer to the mirror.If you want to capture less of the light from the mirror, you'll need to move the focuser away from the mirror.
  • #1
vincentm
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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.
 
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  • #2
vincentm said:
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!
 
  • #3
Ai52487963 said:
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?
 
  • #4
vincentm said:
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.
 
  • #5
f3.3 is very fast and $50 is very inexpensive for a such a mirror. Is it fully figured (parabolized) or spherical?
 
  • #6
Chronos said:
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.
 
  • #7
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!
 
  • #8
HallsofIvy said:
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
 
  • #9
Sounds like you have a spherical f3.3 mirror - which has the optical properties of a goldfish bowl.
 
  • #10
Chronos said:
Sounds like you have a spherical f3.3 mirror - which has the optical properties of a goldfish bowl.

It's concave.
 
  • #11
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.
 
  • #12
vincentm said:
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.
 
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  • #13
And the length of the tube should be nominally equal to the FL of the mirror as well.
 
  • #14
turbo-1 said:
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 said:
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. :cool:
 
  • #15
Chronos said:
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 [tex]3.3 \times 6[/tex] for the distance from the secondary mirror and the primary mirror?
 
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  • #16
http://www.dalekeller.net/ATM/Newtonians/newtsoft/newtsoft.htm
 
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  • #17
vincentm said:
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 [tex]3.3 \times 6[/tex] 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.
 
  • #18
Chronos said:
. 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.
 
  • #19
Piano wire would be my choice for mounting the secondary mirror. You will need two strands in each plane to keep it from twisting.
 

1. What materials do I need to build a telescope with a 6" mirror and a focal ratio of 3.3?

To build a telescope with a 6" mirror and a focal ratio of 3.3, you will need the following materials:

  • A 6" mirror
  • A telescope tube or enclosure
  • A mirror cell or mounting system for the mirror
  • An eyepiece
  • Focusing mechanism (such as a rack and pinion or Crayford focuser)
  • Additional accessories (such as a finder scope or star diagonal) may be desired, but are not necessary for the basic functioning of the telescope.

2. How long does it take to build a telescope with a 6" mirror and a focal ratio of 3.3?

The time it takes to build a telescope with a 6" mirror and a focal ratio of 3.3 will vary depending on your level of experience and the complexity of your design. It could take anywhere from a few hours to several days.

3. Can I use any type of mirror for this telescope?

Ideally, you should use a high-quality, parabolic mirror for the best image quality. However, you can use a spherical mirror if necessary, although the image quality may not be as good.

4. Is it possible to build a telescope with a 6" mirror and a focal ratio of 3.3 for under $50?

While it is possible to find some inexpensive materials and tools to build a telescope, it is unlikely that you will be able to build a high-quality telescope with a 6" mirror and a focal ratio of 3.3 for under $50. You may need to spend more to ensure the best image quality and durability of your telescope.

5. Will this telescope be suitable for astrophotography?

This telescope can be used for basic astrophotography, but it may not be ideal for more advanced or detailed images. The focal ratio of 3.3 may not be fast enough for capturing faint objects, and the quality of the mirror may also impact the image quality. It is recommended to do some research and potentially make modifications to the telescope for better astrophotography results.

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