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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Homemade folded refractor
      #339783 - 02/10/05 05:36 PM Attachment (340 downloads)

Just finished this little winter project.

I bought an Orion 120mm f/8.3 last year and was so impressed with the optics that I decided to buy its big brother the 150mm f/8 Synta OTA. The problem was that it was too long to use on my Universal Astronomics UniStar mount (alti azimuth) and too awkward to use at zenith, so I decided to try a new approach by folding the light path and bring it out the side. This was accomplished using a Newtionian secondary mirror (3.1 in) and mirror holder.

This addresses both objectives, reducing the tube length to 36 inches and by the use of a 2 inch diagonal, keeps the eyepiece comfortably oriented throughout the altitude range from horizontal to vertical. The diagonal is rotated in the focuser as the altitude angle is changed, keeping the eyepiece in the same relative position within the 13 inch arc of rotation.

This allows for easy sweeping of the telescope, which is intended for rich field viewing of the Milky Way starfields and also comet searching.

The "tube" is constructed of 1/2 and 3/4 inch birch cabinet plywood, and the interior is completely lined with 3M flocking paper to maximize contrast.

The focuser is a custom MoonLite refractor focuser similiar to the model used to retrofit the 120 and 150mm Syntas. I had it made completely matte black and a flat base to allow mounting to the tube.The inside of the drawtube is lined with flocking paper to completely eliminate unwanted reflections.

The objective is the standard Synta; I hacksawed off the collar that screws onto the tube, retained the lower part of the cell that allows collimation.

The entire tube assembly complete with AP diagonal and 40mm Konig ep and Telrad finder weighs about 28 pounds.

Mr. Bill

Attachment

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Anonymous
Unregistered




Re: Homemade folded refractor new [Re: Mr. Bill]
      #339801 - 02/10/05 06:00 PM

Nice work. How do the star tests look (ie. before vs. after the folding?)

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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: ]
      #339820 - 02/10/05 06:10 PM Attachment (180 downloads)

Ariel Antares Optics secondary mirror (1/15 wave) and Astro Physics diagonal have negligible effect on image.

Objective is slightly undercorrected; color is what one would expect from this size and speed achromat. As Synta optics go, this is on the good side of the curve.

Remember, this is NOT intended for planetary or star splitting, but rich field viewing at 30x to 100x.

Attachment

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Tom L



Reged: 01/07/04
Posts: 31037
Loc: Sunny Oregon
Re: Homemade folded refractor new [Re: Mr. Bill]
      #339874 - 02/10/05 07:08 PM

Nice design and great solution on the long focal length.

--------------------
Tom
Tele Vue 102mm f/8.6 on an EzTouch
Vixen 80mm f/5 A80SSWT on a grab-n-go mount


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Anonymous
Unregistered




Re: Homemade folded refractor new [Re: Tom L]
      #340180 - 02/11/05 12:28 AM

Very nice design, I like it.

Ever thought about building another one and placing them side by side? You'll have a reverse binocular telescope! Mounting would be very tricky though, as would alignment.


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Tom L



Reged: 01/07/04
Posts: 31037
Loc: Sunny Oregon
Re: Homemade folded refractor new [Re: ]
      #340191 - 02/11/05 12:51 AM

It could be done, though. Good idea Ken.

--------------------
Tom
Tele Vue 102mm f/8.6 on an EzTouch
Vixen 80mm f/5 A80SSWT on a grab-n-go mount


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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Homemade folded refractor new [Re: Tom L]
      #340599 - 02/11/05 12:34 PM

Nice job Mr. Bill! I've always wanted to do something like that with a long-focus lens. Call it "refractor in a suitcase".

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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Anonymous
Unregistered




Re: Homemade folded refractor new [Re: Mr. Bill]
      #340656 - 02/11/05 01:51 PM

I was imagining multiple reflections in that box so I was wondering if the slight shift off-axis was important. But with your internal picture I can see that isn't the case.

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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: ]
      #340700 - 02/11/05 03:06 PM

Posted another picture to show mount and dovetail plate attachment detail....

Mr. Bill

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: Mr. Bill]
      #340702 - 02/11/05 03:09 PM Attachment (182 downloads)

Opps....try to post picture again. Still getting the hang of this

Attachment

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: Mr. Bill]
      #410651 - 04/18/05 07:25 PM Attachment (166 downloads)

New incarnation....

Moved focuser over to dovetail side to put weight closer to tripod CL. This increases stability to tipping and lowers dampening time.

Attachment

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Homemade folded refractor new [Re: ]
      #411943 - 04/19/05 09:26 PM

Quote:

Nice work. How do the star tests look (ie. before vs. after the folding?)




I've been interested in doing this for some time and am considering it for my next project. Using Newtonian diagonals just makes a lot of sense. One conern I had was the conventioanl wisdom of keeping the angles small. I want a right angle solution like Mr. Bill. It occured to me that flats should be like most other optics - the errors are predominantly near the edges. I had a conversation with Bryan Greer at ProtoStar about this, and he confirmed it. By using oversized diagonals, the wavefront error is ridiculously small, allowing for large angles and great performance. Looking at the interfeogram on my ProtoStar mirror, I can see it is numerically about 1/17th wave (red light). Qualitatively, I can see almost all of that error occurs at the edge. The center is better by perhaps an order of magnitude. Probably below the ability of the intferogram to measure it.

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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erik
telescope surgeon


Reged: 01/30/04
Posts: 24789
Loc: Hawaii
Re: Homemade folded refractor new [Re: Jeff Morgan]
      #412003 - 04/19/05 10:17 PM

very cool! i bet you get some curious looks at star parties, huh? looks great!

--------------------
-Erik Wilcox
Starstuff
Homebuilt 16" Truss Dob in Ohia Forest Observatory
(12' X 12' rolloff under Mag 7 skies)
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos
Homebuilt 80mm f/5 refractor
Mirador 60mm f/12 1960's refractor


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Anonymous
Unregistered




Re: Homemade folded refractor new [Re: erik]
      #412078 - 04/19/05 11:20 PM

For terrestrial use, this configuration has a correct image if you stand alongside the scope!

-- William


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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Homemade folded refractor new [Re: ]
      #412643 - 04/20/05 01:30 PM

As long as you keep the number of mirrors even, that should be the case.

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: erik]
      #412984 - 04/20/05 06:49 PM

Quote:

very cool! i bet you get some curious looks at star parties, huh? looks great!




Thanks! Don't know, haven't been to public starparty since I built it. Had some very positive comments from knowledgeable observers, though.

Thinking of attending the Oregon SP this year and bringing.

Mr. Bill

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Homemade folded refractor new [Re: Jeff Morgan]
      #413099 - 04/20/05 08:32 PM

Quote:

Quote:

Nice work. How do the star tests look (ie. before vs. after the folding?)




I've been interested in doing this for some time and am considering it for my next project. Using Newtonian diagonals just makes a lot of sense. One conern I had was the conventioanl wisdom of keeping the angles small. I want a right angle solution like Mr. Bill. It occured to me that flats should be like most other optics - the errors are predominantly near the edges. I had a conversation with Bryan Greer at ProtoStar about this, and he confirmed it. By using oversized diagonals, the wavefront error is ridiculously small, allowing for large angles and great performance. Looking at the interfeogram on my ProtoStar mirror, I can see it is numerically about 1/17th wave (red light). Qualitatively, I can see almost all of that error occurs at the edge. The center is better by perhaps an order of magnitude. Probably below the ability of the intferogram to measure it.




That's why I used a 3.1 inch (oversize) 1/15 wave secondary for my relay mirror. You hit it exactly!

--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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astroN00B
sage


Reged: 10/21/06
Posts: 393
Loc: Bakersfield, CA
Re: Homemade folded refractor new [Re: Mr. Bill]
      #1465596 - 03/06/07 02:21 PM

Sorry to drag this thread up from the grave, but I have a couple questions. I'm interested in building something like Mr. Bill's scope and would like to know how to calculate the positions of the secondary mirror and focuser. I thought the NEWT software might work. Any ideas... other than eyeballing it?

--------------------
SV 80/9D 10:1 focuser
WO 66mm SD APO
100mm f/5 ATM achro refractor
WO TR-188 tripod
Modified DS-2000 goto mount
Orion AZ-3
Vixen Polaris
Meade 5000 1.25" EPs, Telextender
GTO 7mm-22mm zoom


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refractory
Pooh-Bah


Reged: 02/05/05
Posts: 1337
Re: Homemade folded refractor new [Re: astroN00B]
      #1466249 - 03/06/07 06:40 PM

Something like this- modified for my bino project- might just do the trick. How much does the OTA weigh including focuser?

Jess Tauber

--------------------
"Keep an open mind, but not so open that your brain falls out." - Richard Feynman


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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Homemade folded refractor new [Re: astroN00B]
      #1466347 - 03/06/07 07:17 PM

No problem dragging it up, always an interesting topic. Since this thread went cold I have acquired two lenses for use in this type of project, a 4" f/14 achro and 80mm f/7 APO. The achro will be the easier project, so I have started that one first and should be making sawdust this weekend.

My first step was to purchase PVC pipe and mount the cell and a focuser to verify the focal length. I also noted the focuser position for the eyepiece in my collection that has the most inward focuser travel (a 31 Nagler).

With focal length and the most inside focuser position confirmed I used a drafting program to lay my project out. I could have used paper and pencil too.

First, you want to draw a cone. The base of the cone is equal in size to the aperture of your lens. The height of the cone is equal to the focal length of the lens. When you place the focuser on the drawing, you want to be sure that the vertex (tip) of this cone is in the center of your focuser drawtube travel.

Second, decide how large of an 100% illuminated circle you need at focus. As a visual obersever, I decided that 1/2" was adequate (more than adequate actually). I then drew a 1/2" line centered on the tip of the vertex, and perpendicular to the light path. I then drew lines connecting each end the of the 1/2" line to the base of the cone (representing the objective lens) on the same side. This gives you a somewhat bigger cone with a "snipped" tip. Anything you place in the scope (mirrors and baffles) should be large enough to accomodate the larger cone.

Third, check what size mirror you have available . Lets say for example you choose a 2" minor axis newtonian diagonal. You want to place that mirror in such a way that the light cone doesn't touch the outer edges of the mirror. For my project, I decided that I would not use the outer 0.15" edge. So I placed the mirror at the point where the cone was only 1.7" wide.

Takes much longer to explain than do. Fire up a drawing program and give it a try.

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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