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ScopeScavenger 12" truss prototype 1.0

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#1 woodscavenger

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Posted 03 January 2014 - 01:03 AM

I wanted to share a writeup on the scope that I just built. It is my first but not last truss scope. It was built fast due to a deadline of a shoulder surgery and wanting to have a scope ready for the spring for a vacation to Arches NP. Enjoy the writeup. Unfortunately although it is cloudless the foggy humidity has kept me from good first light with the new design but since the optics were well vetted in their previous life as a Z12 I am not too worried about performance unless I really botched the alignment.

Enjoy the writeup.


ScopeScavenger 12" Truss Prototype 1.0

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#2 Mirzam

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Posted 03 January 2014 - 07:26 AM

I could not get your link to work? Scope looks nice and light weight though.

JimC

#3 Messierthanwhat

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Posted 03 January 2014 - 08:07 AM

Corrected link (the colon evidently wandered away) Really nice write-up on an intriguing, ambitious project. Thanks!

#4 woodscavenger

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Posted 03 January 2014 - 10:31 AM

I hate it when my colon wanders away!! :confused: I think I got the link corrected. thanks

#5 Pinbout

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Posted 03 January 2014 - 10:41 AM

nice job on the scope. :waytogo:

they sell rivet nuts for your next iteration. very convenient.
there's a specific word for press fitting a nut into a hole as you did in your poles, I do it in wood [sometimes], just don't remember it.

#6 StarStuff1

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Posted 03 January 2014 - 11:35 AM

Very interesting build. :cool:

#7 BigDob Al

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Posted 03 January 2014 - 12:40 PM

Your scope looks great, you used a lot of interesting ideas in this build.
I would be curious to learn how well is the mirror cell working and what mirror thickness is in the scope.
I always thought a whiffle tree primary mirror cell would be needed for mirrors like yours based on PLOP calculations.

#8 woodscavenger

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Posted 03 January 2014 - 05:46 PM

Your scope looks great, you used a lot of interesting ideas in this build.
I would be curious to learn how well is the mirror cell working and what mirror thickness is in the scope.
I always thought a whiffle tree primary mirror cell would be needed for mirrors like yours based on PLOP calculations.


I will get a thickness measurement posted. I wish I knew enough about mirror cell construction to know what "whiffle tree" meant. I have heard of PLOP but have not investigated it either.

I did contemplate other designs but opted for this.

I terms of mirror performance what clues would I get that my mirror support is inadequate?

#9 Pinbout

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Posted 03 January 2014 - 06:09 PM

It would be real easy to turn that into a float cell support. 6 points.

#10 woodscavenger

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Posted 04 January 2014 - 01:51 AM

I knew when building this the mirror cell would be the most debated issue. What real world difference does a multipoint floating cell make? Will i only notice it under certain conditions? Will it make a visual observation difference?

#11 Pinbout

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Posted 04 January 2014 - 08:48 AM

how thick is your primary.

a real inexpensive float cell is a sheet of bubble wrap. make a 10" circle with a 4" hole in the center. It doesn't take much to modify and is very inexpensive and doesn't hurt to try it. See if you can see a difference when star testing at high mag. :grin:

#12 mattflastro

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Posted 04 January 2014 - 02:27 PM

I knew when building this the mirror cell would be the most debated issue. What real world difference does a multipoint floating cell make? Will i only notice it under certain conditions? Will it make a visual observation difference?

If your mirror is thin (all modern production mirrors are) you'll see cell induced astigmatism with improper cell design. PLOP helps you design a support system that equalizes and distributes evenly the support forces on the mirror back. Imagine for a second that your plywood back is a little warped, not enough to notice with the naked eye but enough to have a high spot. It's true that the soft backing will make this impossible to feel with your hand but the support force will be higher in that area (as applied to the back of your mirror) . It doesn't take much force difference to warp a mirror that's 12" diameter and 1.5" thick.

#13 woodscavenger

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Posted 05 January 2014 - 12:40 AM

Matt you nailed the thickness. It is 1.5"

I just got inside from my first real test. I was out for about 15min on the moon earlier then just spent about an hour on Jupiter and then Orion. It was cold!!

Collimation was a snap. With the open truss it made it super easy to align both laser points on the secondary. The central locking knob was firm and holds it great. At the end of the session I rechecked and it had not moved.

Star testing was clean. I looked on google for examples of astigmatism and saw nothing similar. Donut---pin point star---donut as I moved across the focal point.

I need to work on the friction of the bearings. To tight. It is nice when changing EPs but making small adjustments were not easy. Too much stiction. Will look at formica for the bearing as they are currently just a plywood edge. May also look at just waking the bearings.

I tried out my new Astrotech 60degree EPs, (18,12, and 8mm) The 24mm is hard to find but I will get it. They are nearly parfocal, great optics up until the very edge, M42 was just stunning. Even though seeing was dropping quickly I was able to make out the star between trapezium A and B for the first time. The overall shape of the nebula was more apparent and was great to watch drift through the new EPs.

GREAT NIGHT!!

#14 woodscavenger

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Posted 14 January 2014 - 12:51 AM

Update: The AZ rotation was a little rough. The lazy susan had poor tolerance and was wobbly front to back and side to side. I placed some furniture slides but it didn't help enough. Went back to the original Z12 and realized it has a really slick bearing sandwich with 2 metal discs, and a plastic disc with small cylindrical needle bearings. Drilled my center holes, placed a T-nut and it was done.

Took the scope out tonight for the moon through some thin clouds. The rotation is smooth as butter. The wobble is gone! Now I need to smooth out the stiction on the ALT bearings and figure out my finder.

I just recieved a tactical green lazer light with pigtail pressure switch. I am deciding between that and the right angle finder from the Z12. Likely will use the laser.

I also used some O-rings to set my EPs very close to parfocal. Makes EP swaps so much simpler. I don't have to monkey as much with the focuser.






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