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Equipment Discussions >> ATM, Optics and DIY Forum

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Sean Cunneen
Let Me Think
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: gatorengineer]
      #5614642 - 01/09/13 10:46 PM

The secondary cage will be 2ft long and I am seriously considering an extendable dew shield as well. Where I observe there is a lot of stray light!

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Jeff Morgan
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Reged: 09/28/03

Loc: Prescott, AZ
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5615184 - 01/10/13 10:22 AM

Quote:

The secondary cage will be 2ft long and I am seriously considering an extendable dew shield as well. Where I observe there is a lot of stray light!




Will you be using a filter slide?


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gatorengineer
Carpal Tunnel
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Reged: 02/28/05

Loc: Hellertown, PA
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Jeff Morgan]
      #5615216 - 01/10/13 10:38 AM

The Deep upper ring is great. The other thing to factor into the design is that you are going to need a Ton of tail weight, especially if you put a grenade in the focuser. You will want something that you easily adjust, so a sliding weight on the top of the mirror box may be in order. in addition to several lead counterweights in the mirror box Even at F5 on large dobs its tough to find the balance between a 31 Nagler and 6 orthoscopic. F7 will even be more challenging, unless you depart from a traditional rocker and move the bearing substantially cageward.....

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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Jeff Morgan]
      #5615400 - 01/10/13 12:10 PM

Quote:

Will you be using a filter slide?




I like to build adjustability into my dobs. Either the mirror has been adjustable 4-6" or the top of the OTA has been adjustable to accomodate filter slides, polarizer filters, binoviewers (perhaps not all at once, one does have to keep humble)

Edited by Sean Cunneen (01/10/13 12:16 PM)


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: gatorengineer]
      #5615409 - 01/10/13 12:14 PM

The bearings are 3-1/2ft up the tube because for planetary/highpower viewing the moments have to be kept as short as possible to minimize vibrations and maximize stiffness.

for this scope, imagine a 10ft long teeter-totter with 45lbs 3ft away from the balance point and 7ft of poles and a cage on the other. My concern is coming up with enough weight on the top end so I can avoid having counterweights suspended above the mirror.

I am considering going with conduit for truss poles as the added weight in this case is a benefit.


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Jeff Morgan
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Reged: 09/28/03

Loc: Prescott, AZ
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5615788 - 01/10/13 03:58 PM

Balance is a tough nut to crack for the longer focus scope. Extending the mirror box upwards can be done, but the with diminishing returns.

I've been immensely pleased with the double truss layout. Building the center box for the attachment of altitude trunnions and trusses frees you to make the mirror box exceptionally light. Which just makes the whole concept work even better. In hindsight, I should have ordered an aluminum mirror cell for mine to eliminate even more of the dead weight.

To keep the rocker box stiff I use thicker upright sections. Usually I use hollow core construction for weight, although in my current project I used the cut-outs you saw in the photo. The hollow core approach is light, but lacks hand holds. The cut-out approach is slightly heavier, but is easy to grab and handle. Both approaches take several hours extra work in the shop.

There is another balance idea I'll past you. For my next project I was considering an RFT - something like a 8" f/4.5 with a focuser coincident with the center of balance. The reason is to make eyepiece balance irrelevant, and put the focuser at a fixed and comfortable height of around 40 inches. The issue here of course is balancing a scope that effectively rotates around the focuser! To make it work without using forward weight extensions I was thinking of something like a pulley. That is, suspend a weight on a chord that wraps around the altitude trunnion on the side of the scope opposite the focuser. The issue of course is that the arm of the weight is limited to the radius of the altitude trunnion. I haven't fleshed it out yet, but from that position it would not at any time be suspended above the primary mirror.


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Jeff Morgan]
      #5620867 - 01/13/13 02:01 PM Attachment (41 downloads)

Spent most of my weekend in my basement practicing my stripping. It's been a while and I am a little rusty but after an hour session last night, I can state confidently that I am stripping like a pro. If you are unfamiliar, stripping can be pretty dangerous. I wear gloves and I always keep my hat on.

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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5620888 - 01/13/13 02:12 PM Attachment (40 downloads)

I received a Windows phone last week and there are all sorts of doodads I can play with picture-wise, so please be patient with me while I figure things out...

Here are the parts for the dob base stripped and partially dressed before finishing. Looking dapper!

Edited by Sean Cunneen (01/13/13 02:18 PM)


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droid
rocketman
*****

Reged: 08/29/04

Loc: Conneaut, Ohio
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5636664 - 01/22/13 11:14 AM

This project rocks, awaiting further updates.

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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: droid]
      #5637114 - 01/22/13 03:06 PM

I've been busy! Currently I'm gathering parts. My Antares 1/16th PV Secondary just came in, I picked up a bunch of plywood and hard-board and I'll be cutting lots of circles in the next week.

I also had a bunch of projects to get out of the way the past couple of weeks. I rebuilt a Criterion RV-6 Mount and an old Edmund Equatorial. I am waiting for a brass Crawmach focuser which has been shipped to complete my Chester Brandon/Max Bray 3" f15.3 Objective (which you may see in this forum soon!)

I can hardly wait to get back to this thing myself!


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782393 - 04/06/13 06:45 PM Attachment (26 downloads)

Goings have been slow, but I am making progress now, I hit some snags and made some twists and turns in my initial design, working out some of those took quite some time...

I picked up three sheets of 18"x4' planked pine sheets. These are narrow pine boards glued together to make larger sheets, they are finished sanded and look nice, though they are on the heavy side. I cut 12" off of three, then glued the scrap up to make a 4th 3' section. I do love being frugal. Some sweating with a plane and some sanding, and the franken-piece will work fine.

Edited by Sean Cunneen (04/06/13 06:47 PM)


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782401 - 04/06/13 06:49 PM Attachment (17 downloads)

Next up is an 8' x24" sheet of birch ply... This will be cut into 24" squares and the scrap used for support pieces for the cell. A couple are used for the rings of the UTA, and 3 are cut for baffles for the LTA. Below is my circle jig for the OTA... The clamping knob makes it look so... so... classy.

Edited by Sean Cunneen (04/06/13 06:51 PM)


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782415 - 04/06/13 06:52 PM Attachment (19 downloads)

For the baffles, I then ran the router around the inner circle with an angle bit to rough cut what will be knife edges after some finish sanding...

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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782417 - 04/06/13 06:56 PM Attachment (20 downloads)

Next I had to make a jig to true and flatten the circle that will become the altitude bearings. I picked up a planked pine 24" round that had been dropped. It split in half so the floor manager let me have it for $2. I'll take that any day. Some glue, some clamping, wood putty and sanding and it looked like new.

For the jig I made a radius extension for my router table...


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782418 - 04/06/13 06:58 PM Attachment (15 downloads)

The circle mounts onto the threaded shaft then I spun the wheel against the cutting bit of the router. That gave me a true circle with an accurate center hole!

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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782421 - 04/06/13 07:00 PM Attachment (22 downloads)

A shot of my rigged radius control knob assembly...

Crawford Machining, beware, I'm coming for ya...


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782664 - 04/06/13 09:38 PM Attachment (26 downloads)

Now that I have the LTA sides cut, baffles cut, side trim cut, UTA rings cut, base cut, base sides cut, it's time for the cell. The mirror is a stout 40#, 1.75" thick Pyrex, so it won't be overly sensitive to mounting supports, but it will take a while to cool. After some Plop research and some Kriege and Berry(sp?) research, I've decided on a 6pt cell with 3 each 2 points on a teeter/totter. I am constructing the cell out of the rounds from the routing of the UTA with a stiffening structure in between that will hold the mirror supports. This design took a while to work itself out dimensions-wise. There was more than a few times I searched for a ready-made 16" cell. My children are cute however, and I am not allowed to trade them for Telescope parts.

Below shows the upper cell round getting slots routed for the mirror supports. If you look close you can see all the geometry worked out and you might get a hint of what's to come!


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Sean Cunneen
Let Me Think
*****

Reged: 08/01/07

Loc: Blue Island Illinois
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782667 - 04/06/13 09:38 PM Attachment (22 downloads)

and one more...

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jgraham
Postmaster
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: My 16" f7.2 Newt project code named "Grace" new [Re: Sean Cunneen]
      #5782708 - 04/06/13 09:55 PM

What a neat project! My 16.5" is an f/6.5. I ground it long to give me a good chance at producing a good figure and allow me to use a small, affordable, diagonal. I've since bought a 16" f/4.5 to serve as a more compact companion, but I love my biggo 16.5".

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Jeff Morgan
Postmaster
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Reged: 09/28/03

Loc: Prescott, AZ
Re: My 16" f7.2 Newt project code named "Grace" [Re: Sean Cunneen]
      #5783485 - 04/07/13 10:28 AM

Quote:

Goings have been slow, but I am making progress now, I hit some snags and made some twists and turns in my initial design, working out some of those took quite some time...

I picked up three sheets of 18"x4' planked pine sheets. These are narrow pine boards glued together to make larger sheets, they are finished sanded and look nice, though they are on the heavy side. I cut 12" off of three, then glued the scrap up to make a 4th 3' section. I do love being frugal. Some sweating with a plane and some sanding, and the franken-piece will work fine.




Thanks for the progress report Sean, it's fun to watch your progress.

I'm starting a rebuild on my 16" f/7 today. The explanation is lengthy and I don't want to hijack your thread, perhaps I'll start a new thread.

The design allows me to build up some of the larger wood sheets like you're doing from the numerous scraps I have laying around. I'm done with furniture-quality clear/poly finishes, going with paint so the seams will never show.

Have you made a decision on truss connectors? I just got a set of quick release clamps from Aurora Precision, a clever design with excellent build quality. Check out their video on Youtube. Pretty reasonable on cost too if you go with the simple lower clamp system vs. the more traditional design.


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