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Planning a folded open 8.5" f/12.5 refractor

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#1 Jim Chung

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Posted 19 January 2013 - 08:42 PM

Hey all,

I recently acquired an old D&G 8.5" f/12.5 achromatic doublet and wanted to make a open truss pole folded refractor so I can easily mount this on a midsized GEM. This is just a preliminary sketch but it looks like I will need a 6" optical flat in the first mirror, but if I undersize it to 5" diameter will I get the same effect as masking the lens objective down to 7"? This would make it an f/15 instrument with significantly better colour correction. Or do I still have to mask the lens down to 7" as well?

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#2 plyscope

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Posted 19 January 2013 - 11:19 PM

Jim I would not recommend giving up aperture just to increase the focal ratio. If you can, go with the larger folding mirror and maximise the aperture available. Use aperture stops at the objective to experiment with colour reduction later. Many targets will benefit from the larger aperture despite minor colour error differences.

#3 mconnelley

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Posted 20 January 2013 - 01:17 AM

Hello:

You may want to use a flat larger than 6" to maintain a reasonably large fully illuminated field of view. An open folded refractor is a neat idea. I don't see how you'll be able to mount this onto a GEM, unless you're going to have a plate that runs the length of the instrument (about 3 feet) which would then attach to the GEM. It would be interesting to experiment with an open vs. closed tube to see which gives you better seeing. One way to do this is to make a light weight shell (thin wood, sheet metal, cardboard) that you could easily put on and remove. Don't forget to add baffles, especially something to avoid a straight shot from the objective lens to the focuser.

Cheers
Mike

#4 plyscope

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Posted 20 January 2013 - 06:59 AM

This is a link to a 9" f9 folded refractor, might be of interest.

web page

Andy

#5 Mirzam

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Posted 20 January 2013 - 07:08 AM

Rather than design your scope for GEM mounting, why not build a dobsonian-style mount that uses just one fold mirror? Such a folded OTA would use a single 5" flat at the bottom, coupled with a small diagonal mirror near the eyepiece at the top. This would be more stable and allow comfortable seated viewing. Heat from the observer would need to be screened from the light path in either design.

I also think that a solid tube would allow better baffling and should be considered.

JimC

#6 ccaissie

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Posted 20 January 2013 - 07:47 AM

why not build a dobsonian-style mount that uses just one fold mirror?
I also think that a solid tube would allow better baffling and should be considered.
JimC


Our 6" f/15 folded triplet is such an arrangement. A 6" flat is used, but only 3" is utilized. Looks like an 8" f/6 dob.
Not very elegant looking, but planetary views are unforgettable.

#7 dan_h

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Posted 20 January 2013 - 09:34 AM

<< This would make it an f/15 instrument with significantly better colour correction.>>

Not really. There will be some improvement in the secondary color but I don't think I would call it significant. Both the 8.5" and 7" configurations are going to show visible color on bright targets like Jupiter and the moon. Niether are going to show much color on targets like M13 or M31. If you were to stop the lens down to 4" it then approaches apo performance with nearly all visible light in the airy disk but that is a huge scarifice of aperature.

You need to use good flats for folding and preferably, they need to be larger than the minimum size for full illumination. Any errors in the flats will likely be at the edges so the use of an oversize flat will keep the edges out of the light path. I also think the lower flat will need to have some sort of shield to prevent dew.

Good luck with your project. You are making a great scope. I hope it all turns out well and I get a chance to look through it real soon.

dan

#8 Lightning

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Posted 21 January 2013 - 12:15 AM

Keep the aperture and use filters to select whichever colours are useful for the subject you're observing.

#9 Jim Chung

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Posted 18 May 2013 - 03:53 PM

Some updates now that Spring/Summer is fully here and I can do some woodworking outside!

I've got the plywood bulkheads cut with baffles that will hopefully preserve the open nature of the OTA design. I used aluminum tubing from a place that sells it for making shortwave radio antennae masts and the struts expand from 2 feet to 3 feet with self locking buttons. Waiting for my 3 & 6" optical flats to be coated. Has a very solid oak base straddling one of the struts where I intend to mount a very long Losmandy style dovetail plate as it is quite nose heavy, although the 2" thick 6 inch optical flat at the rear will help.

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#10 Mirzam

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Posted 18 May 2013 - 05:24 PM

Very cool looking! Maybe you can get a bare aluminum focuser to go on the rear?

JimC

#11 catboat

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Posted 19 May 2013 - 03:58 PM

Great project! I'll be following with interest.

Can I ask where you got the telescoping tubes with button locks (thinking about dob struts)?

#12 Jim Chung

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Posted 20 May 2013 - 09:46 AM

Thanks guys!

JimC, unfortunately as you will see I installed a black JMI EV motorized focuser so I have the option of doing some planetary imaging.

Catboat, the tube connector buttons can be bought from McMaster Carr and the tubing from Maple Leaf Communications.

We're celebrating Queen Victoria's birthday today (Monday) so I used the bonus free time to install the 3" optical flat holder, Losmandy dovetail plate and focuser and tried it on my AP400GTO mount. Hmmm, much more intimadating than I expected!

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#13 Jim Chung

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Posted 24 May 2013 - 08:28 PM

It's Friday again, time to work on the scope!

I'm concerned that the dovetail attachment is not robust enough to support the weight of the scope once the lens cell is attached, literally tearing away from the structure so I got some 1/4" angle aluminum and attached the middle bulkheads directly to the Losmandy dovetail bar.

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#14 s800

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Posted 26 May 2013 - 11:19 PM

Nice aesthetics!

#15 Jim Chung

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Posted 27 May 2013 - 09:25 PM

Parts keep trickling in. Got some 1" and 7/8" ID shaft collars to connect the aluminum tubes to the bulkheads. I could have sunk a screw through the edge of the bulkheads into the aluminum tubing but that would have defeated the telescoping ability. As it turns out I had to sand the inside of each collar before it would finally slip onto the tubing. Very laborious. I will probably crazy glue the collars to the end bulkheads to keep them from moving outwards.

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#16 Jim Chung

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Posted 29 May 2013 - 08:03 PM

I couldn't find any 6" PVC drain pipe caps at the usual Home Depots or Lowe's but surely a city the size of Toronto must have specialized plumbing stores, just don't want to spend half a day crossing the city to find it. I lucked out when one had a branch outlet in my neighbourhood. Reduced the depth of the cap and mounted it. I intend to use a little silicone on the bottom and at three points on the sides of my flats to secure them to the adjustable cells. My flats just got coated by Normand Fullum and are in transit, should get them tomorrow and have first light this weekend.

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#17 dan_h

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Posted 30 May 2013 - 07:45 AM

Looking good Jim. I hope the weather holds for you.

dan

#18 Jim Chung

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Posted 31 May 2013 - 05:30 AM

Thanks Dan!

All the pieces are finally together, and it weighs 50lbs! Not sure where I could have saved more weight. Use only 1/2" plywood, ditch the shaft collars? Save 5 lbs? Anyway collimation is tonight's activity.

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#19 Mirzam

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Posted 31 May 2013 - 06:52 AM

Although your design is quite intriguing, if the entire scope were inside a tube then the strength would come from the tube itself, allowing the use of much lighter baffling. I think you would end up with less total mass.

JimC

#20 PhilHerring

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Posted 31 May 2013 - 04:23 PM

How much of the mass is in the objective and cell?

I recently built a 127mm f/9.4 folded refractor, using a thin plywood skin around a light wood frame. Most of the mass was in the objective and the focuser, so in the end I didn't save any weight, but the shorter scope was far easier to live with.

#21 Jim Chung

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Posted 31 May 2013 - 08:15 PM

I think the lens and cell is about 10 lbs. The original scope from the previous owner weighed 85 lbs and was 9ft in length so significant savings nonetheless.

I collimated the scope this evening, centered my Hotech Newtonian laser collimator into each mirror one at a time, first adjusting the focuser to be centered on the 3" optical flat. The lens cell has push pull hex screws so I adjusted the cells until the reflection from the posterior element reflected right back to the laser collimator. Now waiting for the rain to stop!

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#22 Jim Chung

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Posted 01 June 2013 - 11:16 AM

Cloudy .... cloudy .... cloudy.

Still I mounted the beast (and it is, needed my wife to give me a hand) and aimed it at some distant apartment buildings. Focus appears sharp! Will have to wait until nighttime but the open nature of the scope even in daytime did not exhibit any off axis light contamination.

Jim

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#23 Jim Chung

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Posted 01 June 2013 - 11:17 AM

er ... the front of it.

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#24 mikey cee

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Posted 01 June 2013 - 03:53 PM

Oooohhh neato but weird lookin' too! :waytogo: Can't wait for your visual testing. ;) Mike

#25 dan_h

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Posted 02 June 2013 - 07:36 AM

Oooohhh neato but weird lookin' too! :waytogo: Can't wait for your visual testing. ;) Mike


You're gonna have to wait! JIm has brought the new scope curse upon us in very big way! As soon as he mounted the mirrors the forecast went from partly cloudy to non-stop rain, rain and more rain. Based on the size of that scope the curse will probably last until November!

dan






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