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wsDK - weak secondary Dall Kirkham telescope

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#76 Ajohn

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Posted 03 February 2013 - 09:53 AM

The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.
Mike


Telescope services do one Mike which they say will work down to F3 but it's rather expensive and long so would only be of use on an astrography with the camera on axis.

http://www.teleskop-...S-Optics-2-5...

There is also a Bader to F3.5 as well as the other one they do
http://www.teleskop-...info/p486_Ba...

There is a syntra / skywatcher one to F4 around on there as well. Long again which means huge central obstruction I suspect.

John
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#77 Ajohn

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Posted 03 February 2013 - 12:30 PM

I came across this corrected DK by Wynne. He refers to it as a wide field cassegrain, spherical 2ndry plus corrector with a parabolic main mirror. UV to IR using ubk7 plus a rather flat field. Nice even spot size too.

It's as per his prescription other than I have left it in mms rather than cms and have set the 2ndry dia for 0 field angle which oddly according to oslo doesn't cause any light loss across the field.

John
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#78 coinboy1

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Posted 05 June 2013 - 09:22 PM

Any updates with this thread? I have been looking at this design with interest. My calculations with cassegrain calculators also show low coma for such a design. I have a 25" F/4.7 mirror that would make a good candidate for this DK system. Not using a ladder would be a HUGE plus.

#79 Mike I. Jones

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Posted 06 June 2013 - 08:14 AM

Here's one possible design for your 25" f/4.7 primary. 0.38-0.9┬Ám spectral band, 1.4" circular FOV. The 2-element corrector is made from affordable glasses (no high-$$$ fluorocrown glasses).

Mike

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#80 Mike I. Jones

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Posted 06 June 2013 - 08:15 AM

Here's what it looks like, complete with baffles.

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#81 Mike I. Jones

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Posted 06 June 2013 - 08:17 AM

and here are the spots. The circle is the first Airy dark ring, so the images are extremely sharp.

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#82 Mike I. Jones

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Posted 06 June 2013 - 08:35 AM

Here's the OSLO-EDU file for this Cass, including the baffle tube apertures.
Mike

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#83 coinboy1

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Posted 07 June 2013 - 08:38 AM

Wow! Thanks mike!! What's the system f/ratio? I have OSLO but I'm not too great with it but I will defintley load the file into it. Thanks for looking into it :)

#84 coinboy1

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Posted 07 June 2013 - 03:28 PM

Nevermind I see that it is an F/14 system. I just loaded it into OSLO EDU and looking at the design now. Would it be possible to scale this down to an optimized F/8 with a corrector lens. I would be interested in that even if I have to make the mirror a different F/ratio and corrector lenses. I have not any experience making lenses but willing to try!

#85 Mike I. Jones

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Posted 07 June 2013 - 03:45 PM

I think an f/8 is do-able, but the secondary obscuration with your f/4.7 primary would be enormous. If you can speed the primary up to f/4 or even f/3.6 or so, the obscuration would be reasonable. The secondary would still be spherical and the primary an ellipsoid. I'm away for now but will look at it when I get back.
Mike

#86 siriusandthepup

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Posted 23 July 2013 - 11:12 PM

Hi Gang,

Here is a little update on the 25" wsDK project. Ross Workman aka "The Planetman" and I have completed some woodwork for the upper ring (actually three 1/2" baltic birch rings now glued together).

The mirror box is also complete, my photos are too big and I must reload software to shrink them before I can post the mirror box construction pics. That will have to wait a while, but I do have some small pics to post so you can at least see it and get an idea of where this project is going.

First, the mirror box - 1/2" baltic birch, 60" tall. I was concerned for stiffness using 1/2", but also didn't want too much weight. So we built the box "model airplane style" with inner reinforcing baffles - also 1/2" BB. It turned out nice, approximate weight 80 lbs for just the box without hardware. It went together without a single glitch and it all fit perfectly and the box is totally square in all directions. How often does that happen?

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#87 siriusandthepup

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Posted 23 July 2013 - 11:14 PM

And the inside pic.

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#88 siriusandthepup

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Posted 23 July 2013 - 11:22 PM

This one shows how the spider/tertiary mirror will be placed. Some mounting blocks are being fabricated to attach/tension the spider vane tips.

This pic will have to wait also - too big to post. :foreheadslap:

#89 siriusandthepup

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Posted 23 July 2013 - 11:32 PM

So.... on to the upper ring construction. Most of these pics are smaller. First we cut out 34" x 34" squares from Baltic Birch (BB). Outer ring diameter 33.5". A sandwich construction with outer rings being 2" wide and the inner ring 3" wide. That brings the inner ring diameter to 27.5". A spiral solid Carbide 1/4" router bit was used and punched through the 1/2 BB nicely in about 3 rounds (one stop in the middle for vacuuming out the sawdust) without any bogging or burning - a new favorite bit for this job.

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#90 siriusandthepup

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Posted 23 July 2013 - 11:35 PM

The inner cut - Remember sports fans - always cut the outer diameter first else you loose your center for your router circle cutter.

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#91 siriusandthepup

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Posted 23 July 2013 - 11:40 PM

Doing this with a helper is highly recommended for a number of reasons. The trusty helper (Me) here is in charge of vacuuming and router cord management during the actual routing process.

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#92 siriusandthepup

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Posted 23 July 2013 - 11:48 PM

Notching the outer rings with a 1" Fostner bit to allow clearance for the vane tip screws. They will probably be changed to Allen heads in the final form. No fingers were harmed in the process. The pic does not show the ample clearance between digits and the bit.

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#93 siriusandthepup

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Posted 23 July 2013 - 11:49 PM

Hula Hoop anyone?

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#94 siriusandthepup

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Posted 23 July 2013 - 11:55 PM

Applying glue to one of the outside rings. After spreading the glue evenly with a brush, the outer ring was inverted and placed on top of the inside ring. After carefully measuring the position with respect to the center ring the outside ring was secured in place with a few pins from the pin nailer. Rinse and repeat for the other side and you now have a sandwich ready for clamps.

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#95 siriusandthepup

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Posted 23 July 2013 - 11:57 PM

Glued and clamped to dry overnight.

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#96 siriusandthepup

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Posted 24 July 2013 - 12:04 AM

Checking the spider fit. Looks good for now. 27" vane edge to vane edge - NOT counting the the little tip thingies at the ends. 27.5" inner ring diameter. So that leaves 1/4" each side, vane to inner ring clearance - perfect.

This upper ring will be supporting the 8" spherical wsDK Cass secondary. The ring alone with no hardware or secondary included weighted 8 lbs on the bathroom scales.

After the glue dried the outer edges of the three glued together rings were given a good belt sander sanding with 120 grit (80 grit would have been good too) until the outer edge was nice and smooth all the way around.

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#97 siriusandthepup

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Posted 24 July 2013 - 12:33 AM

And lastly for tonight a close up (minus the aluminum mounting blocks for the spider tips) showing the approximate fitting and positioning.

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#98 MKV

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Posted 24 July 2013 - 05:30 AM

This upper ring will be supporting the 8" spherical wsDK Cass secondary. The ring alone with no hardware or secondary included weighted 8 lbs on the bathroom scales.

That spider seems awful thin to me to support an 8-inch mirror at 90 degrees without flexure or vibration.

#99 siriusandthepup

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Posted 24 July 2013 - 06:51 AM

I understand that concern. The spider is an Astrosystems Super Duty II Spiders - Style 4 Astrosystems spider page. Vane thickness is .048" and we will see how well it works. The secondary will only weigh 3.5 lbs. when Mr. Royce gets that conical blank finished. We will just have to cross our fingers. If necessary I can always talk to Randy at Astrosystems about making me a custom heavy duty spider. The spider vanes feel fairly stiff on their own, so I'm optimistic at this point. :)

#100 coinboy1

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Posted 09 August 2014 - 11:19 PM

Updates?








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