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My Latest Project...A Refractor of Course

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#1 Jeff B

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Posted 17 August 2013 - 10:34 AM

Specifically an 8" F8.8. The lens has ben sitting around for a while so I tinkered up this tube assembly.

My goal was to keep the weight low so I used a .050" wall, 9" O.D. tube with a simple, light weight lens counter-cell and focuser end cap. Based upon a laser marking a mask over the lens, there is no measurable flexure even with the short saddle.

My weight goal was to keep the bare tube assembly (no rings, saddle, finders/counterpoise weights, dew shield or diagonals) down to 35 pounds. It came in at 32 pounds.

As shown in the picture but with a bino-viewer and two fat lenses (Denk 21's), it flops in at a mere 45 pounds. Not too shabby. Note, only three, eleven pound weights.

Optically the lens is very good. It's figure is smooth with a mild amount of undercorrection and a mild central zone which is exactly what I saw in double-pass using a 100 LPI Ronchi. When everything was set up using the laser and Cheshire, I could see a mild amount of simple astigmatism at moderate power so I tweaked the lens collimation less than 1/4 degree to eliminate it.

Once the lens settles down thermally, it's quite sharp at the eyepiece with surprisingly little CA really. It's excellent on double stars and completely awesome on deep sky especially open and globular clusters. I'm going to make 7", 6" and 5" stops for it as well.

Using it with my Chromacors has been fustrating as there seems to be a very narrow window of spacings where they may actually improve the image in both CA and sharpness. This will take a lot of experimentation.

Jeff

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#2 Jeff B

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Posted 17 August 2013 - 10:38 AM

The "money shot".

Note that I use ventilated baffles where any tube currents flow through the gap between the bottom of the baffle and the tube wall instead of spilling over the center hole and into the light path.

Jeff

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#3 PrestonE

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Posted 17 August 2013 - 10:54 AM

Very nice Jeff :bow:

Best Regards,

Preston

#4 stevew

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Posted 17 August 2013 - 01:49 PM

Excellent job Jeff!
How does the G11 handle it?
I'd love to have an 8 inch refractor some day.

Steve

#5 Sean Cunneen

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Posted 17 August 2013 - 02:23 PM

Very cool, love your baffles!

#6 mikey cee

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Posted 17 August 2013 - 04:51 PM

Jeff no need to be secretive who made the lens?? :p Mike

#7 plyscope

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Posted 17 August 2013 - 05:53 PM

Well done Jeff, thanks for posting.

#8 youngamateur42

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Posted 17 August 2013 - 06:37 PM

Very cool project, that's one big lens!

#9 rwiederrich

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Posted 17 August 2013 - 07:13 PM

Well there you go Jeff......Nice. Did you make the cell adapter and back plate as well as the baffles?

Rob

#10 Jeff Morgan

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Posted 17 August 2013 - 08:49 PM

Looks like a fun scope! Clever baffle design, no matter which way the scope is oriented the vent side is "up".

That scope dwarfs your G11!

#11 Jeff B

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Posted 18 August 2013 - 10:35 AM

Actually, I got my weights confused with other projects.

The bare tube assembly weighs 28 pounds. Fully decked out with dew shield, main rings with dovetail, 60MM F6 finder/ring/counterpoise weight assembly, Chromacor II, diagonal, and Denk II's with Denk 21's weighs in at 41 pounds. So, fully assembled and ready to observe, that's one pound less than the bare 7" F8 APM triplet and 3 pounds less than the bare C14. The 41 pound dead weight is no problem for the G11 and it actually handles the moment arm really very well, certainly for visual use (which is what the scope is intended for)as focusing at high power is easy with a slow, low amplitude wobble that dies out in about a second or so.

The lens counter-cell and back plate were literally back-of-the-envelope drawings, taken to my local machine shop (who loves working on this stuff instead of tractor and auto parts)and made from 3/4" AL plate.

The lens is an IDK Fraunhafer. Further testing has shown that the "center zone" is really subtle and only an inch or so in diameter. The lens is most definitely better than diffraction limited with a very mild under-correction which goes away when stopped down to 7" or below.

Speaking of which, I made a series of aperture stops (7", 6" and 5")so that I can match the aperture to the prevailing seeing conditions and thermal state of the lens but also to further check out how my Chromacors behave with it (which is the subject of another thread in the refractor section).

At full aperture there is plenty of purple, similar to my D&G 11" F12 but not all that bad. Stars at high power show a rather yellow airy disk surrounded by reddish/blue purple which is completely normal for a fast achromat. Stopping it down to 6" cuts the subjective CA in half and I find I quickly ignore it. Stopping it down to 5" really knocks the CA back to the point where, while there, I have to remember to look for it to see it (however resolution and image brightness suffers).

I am very impressed with this lens and how well this build has worked out. A large aperture achromat that performs well,is easily one-man portable and G11 friendly.

Jeff

#12 Jim Curry

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Posted 19 August 2013 - 06:49 AM

Hi Jeff:

What are you using for baffle spacers?

Jim

#13 Jeff B

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Posted 19 August 2013 - 09:00 AM

Hi Jeff:

What are you using for baffle spacers?

Jim


I believe 5/16" wood dowel rods, hand selected for straightness.

Baffles are 1/8" model grade plywood. Baffle sizes are 7.5", 7.0", 6" and 4".


Jeff

#14 Jeff Morgan

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Posted 19 August 2013 - 11:08 AM

At full aperture there is plenty of purple, similar to my D&G 11" F12 but not all that bad.


I am very impressed with this lens and how well this build has worked out. A large aperture achromat that performs well,is easily one-man portable and G11 friendly.


Nice to see you managed to get a grab & go refractor. :grin:






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