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My 16" f7.2 Newt project code named "Grace"

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#26 Sean Cunneen

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Posted 04 January 2013 - 10:47 AM

Attached is a PDF of a simple sketch, 1:9 showing the general proportions. Kinda diggin the long look!

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#27 Sean Cunneen

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Posted 07 January 2013 - 10:29 PM

It's heeeerrrrreeeeeee!

Whew!

Rather than have this thing ride in a truck and sit by my door I opted to pick it up at the shippers store. I had to wait the weekend and picked it up this afternoon. It's a Pegasus mirror and came with the specs on the mirror. The numbers are very good, I am all tingly...

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#28 bremms

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Posted 07 January 2013 - 10:57 PM

+1 racing terms for up one lap.

#29 Sean Cunneen

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Posted 08 January 2013 - 12:21 AM

Here are some of the parts congregating:

The sides of the rocker box: I'll strip them and clean them up over the weekend...

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#30 Sean Cunneen

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Posted 08 January 2013 - 12:22 AM

The poor donor:

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#31 Sean Cunneen

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Posted 08 January 2013 - 12:24 AM

Finally a beautiful piece of 18"x18"x2" pine which will become the bottom of the rocker box. This piece was give to me by my good friend Tom, I figure this is a good enough use!

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#32 Darren Drake

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Posted 08 January 2013 - 12:35 PM

Yea both you and Napersky (Mark) got a major package yesterday. (He just gotta 6 inch AP superplanetary on a Losmandy mount.)That explains the week of clouds we have coming. Thanks a lot. lol.

#33 magic612

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Posted 08 January 2013 - 01:10 PM

Yea both you and Napersky (Mark) got a major package yesterday. That explains the week of clouds we have coming. Thanks a lot. lol.


As a Chicago-area observer myself, I will second that - especially since the clouds are coming right when I have time in the evening this week! ;)

Seriously, congrats on the mirror, Sean. Looks like you've got some good pieces of this already thought out and in-process.

#34 JohnH

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Posted 08 January 2013 - 02:03 PM

Jeff, simply beautiful! The split truss is the way to go for resistance to misalignment. It should be an awesome all-purpose telescope. Do you have a 3 or a 4 wave spider. A 3-vane is better diffraction-wise. How big is your central obstruction?

Mladen


I would say four vanes is somewhat better diffraction-wise.

The four vane type consisting of two pairs of two perpendicular vanes with make a four-point pattern, whereas a three vane one will generate a six-point pattern.

#35 Howie Glatter

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Posted 08 January 2013 - 03:22 PM

> I would say four vanes is somewhat better diffraction-wise.

If all vanes are the same size, there will be less light in the six spikes produced by the three vane spider than in the four spikes of a four-vane spider.

#36 niteskystargazer

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Posted 08 January 2013 - 06:45 PM

Sean,

I knew, that some day, you would have a use for that piece of wood, that's why we hung on to it. We knew you would come-long, and need it.

CS,KLU,

:thanx:,

Tom

#37 gatorengineer

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Posted 08 January 2013 - 08:58 PM

You might want to rethink the secondary a bit, 2.6 is a bit large, 2.14 would be plenty.....

http://www.loptics.c.../diagonals.html

I have run 3.1 in the several 16"s I have had over the years. Your building a great planetary instrument.... keep the secondary down, and consider a protostar curved vane spider.

Just thoughts looks like a great project....

#38 Jeff Morgan

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Posted 08 January 2013 - 11:25 PM

> I would say four vanes is somewhat better diffraction-wise.

If all vanes are the same size, there will be less light in the six spikes produced by the three vane spider than in the four spikes of a four-vane spider.


I do slightly prefer the 3 vane pattern. The complication in my case was the size/weight of the secondary was a bit more than I have previously used. So, I opted for the stability of the four.

Once the scope goes operational and I have completed a few finishing touches, I would like to go back and see if I could build a wire spider. If I like the result, I can retrofit it to the existing cage during some full moon period.

#39 Sean Cunneen

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Posted 09 January 2013 - 08:44 AM

I haven't decided on a secondary or secondary arrangement yet. I wanted to put off buying expensive stuff until the mirror arrived safe and sound.

I've built a couple of curved vane spiders for smaller scopes and I was never able to get them stiff enough to my liking. I've done a fair amount of reading about wire spiders and I've collected parts for one. I love the look but for an 18" expanse, I'm not sure if the wire can be thin enough to do it's job. For a big scope, I don't think you can do better than a thin 4 vane spider in terms of elegance.

#40 gatorengineer

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Posted 09 January 2013 - 08:14 PM

The other thing you will want to factor in is that you will want to have a very large piece of flock board opposite the focuser. This makes it very difficult for wire spiders, as you have more points to work around.

#41 Sean Cunneen

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Posted 09 January 2013 - 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!

#42 Jeff Morgan

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Posted 10 January 2013 - 10:22 AM

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?

#43 gatorengineer

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Posted 10 January 2013 - 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.....

#44 Sean Cunneen

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Posted 10 January 2013 - 12:10 PM

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)

#45 Sean Cunneen

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Posted 10 January 2013 - 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.

#46 Jeff Morgan

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Posted 10 January 2013 - 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.

#47 Sean Cunneen

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Posted 13 January 2013 - 02:01 PM

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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#48 Sean Cunneen

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Posted 13 January 2013 - 02:12 PM

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!

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#49 droid

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Posted 22 January 2013 - 11:14 AM

This project rocks, awaiting further updates.

#50 Sean Cunneen

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Posted 22 January 2013 - 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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