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B9Robot
member


Reged: 10/08/13

16" Ultralight Dob
      #6125797 - 10/08/13 08:26 PM

Hey all,

I am thinking of building a 16" ultralight Dob. The one that captures my eye most is from Alan http://user.xmission.com/~alanne/DS4/DS4Main.html

Any pros/cons to this design?

Also there is a used Meade 16" F4.5 from a 1990 starfinder (plus secondary and spider) for $900 CAD - is this vintage of mirror for this price an OK deal?

thanks for the help!

Edited by B9Robot (10/08/13 10:12 PM)


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6126070 - 10/08/13 11:09 PM

Quote:

Any pros/cons to this design?





has to be f5 or faster.

I like how the opposing pairs are not parallel. makes for a stronger truss design.

I haven't tried a string spider so can't comment on it except for look how long that 2ndry stalk is...

he has 4 side blocks to keep the alt bearings on the rocker box. you could build a limit stop by putting a mating block on the alt bearings so it never takes a nose dive.

with his mirror box I like to hang the mirror from the top plate so you'd have front collimation bots. that means the mirror box would have to grow.



also with alt bearings like ) their nothing at the center of rotation to install DSC's. so you'd have to design them with a non typical strategy. and with no brace to form the D they may have a tendency to kangaroo when pointing close to the horizon.


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B9Robot
member


Reged: 10/08/13

Re: 16" Ultralight Dob new [Re: Pinbout]
      #6126129 - 10/08/13 11:42 PM

Danny, great info - thanks! I like the idea of using "D"s for the bearings, the limit stops and having the collimation bolts on the front (very convienient!)

One question - why F4.5 for faster? Simply to keep the overall length down?


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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6126176 - 10/09/13 12:19 AM

New Meade 16" f4.5 mirror is about $1300 US Dollars. Plus you run the risk it's a lemon, unless it's been tested. So could need refigure. That runs you at least $425 US Dollars, more from best opticians. Plus if Meade mirror not annealed properly (like mine), more problems with the refigure process. Meade mirror about 1.65 inches thick, takes a while to acclimate to air temp.
Maybe smarter to buy a mirror from an optician with a good reputation. That way you know your getting a good mirror + will be worth more if you sell someday.
I know it's not 16" but there's a great looking 14.25" mirror for sale in the CN Classifieds/ATM and Such section.
Quartz, thin, so would cool fast + lightweight. $1600 - very reasonable. If too small for you, maybe he'd make you a 16"? Just an idea.
Andy


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6126427 - 10/09/13 07:03 AM

Quote:

Danny, great info - thanks! I like the idea of using "D"s for the bearings, the limit stops and having the collimation bolts on the front (very convienient!)

One question - why F4.5 for faster? Simply to keep the overall length down?




Balance. If you go slower than f5 you need to add weights. I made a seperate set of poles for my travel scope in the above pic for a 8"f6 and I had to add 12lbs on the back of the cell. And that's for a 8" scope.


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Mirzam
Post Laureate
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 16" Ultralight Dob new [Re: Pinbout]
      #6126472 - 10/09/13 07:50 AM Attachment (19 downloads)

You really do need the focal ratio to be around f/4.5-4.6 as Danny says (I think the Meade is f/4.5?).

There are a couple things I would do differently than the scope you are using as a model. First, the top ring needs to be very stiff--thin plywood is not going to work, unless you forgo using anything but the lightest eyepieces. I used an aluminum ring, or you can use several layers of plywood bonded together.

There is a significant advantage in using the self-supporting spider design, made popular by the Obsession UC series of scopes. This allows the trusses to be several inches shorter and helps make a stiffer structure.

If you wish to use encoders you can add a cross-member on each altitude bearing. Mine are made of aluminum as shown in the 14" travel scope picture below.

Also, pay close attention in your design to moving the mirror and cell as far rearward as possible. Literally every millimeter will aid in balance.

Finally, the light baffle used in the model scope is way too big and heavy. It can be at least half this size, especially if you use a focuser baffle, which I highly recommend.

JimC

Edited by Mirzam (10/09/13 07:53 AM)


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B9Robot
member


Reged: 10/08/13

Re: 16" Ultralight Dob new [Re: Mirzam]
      #6126791 - 10/09/13 11:21 AM

Hi Jim - thanks for the great info - I will add it to the list.

I sure like that Obsession UC - very nice...

Do you have any more pictures of your build or a build log?

thanks
Don


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MitchAlsup
Pooh-Bah


Reged: 08/31/09

Re: 16" Ultralight Dob new [Re: B9Robot]
      #6126843 - 10/09/13 11:53 AM

Quote:

Any pros/cons to this design?




I built a DOB of similar architecture, and after debugging it, I see several areas where the DOB you provided a link too is deficient:

The altitude bearings are attached at 2 points. I was forced to add a third point at the back of the rocker box to fully stabilize bearings that were significantly beefier than the ones illustrated. Whth his ultra-low rocker box this might become problematic.

The upper ring lacks radial diameter, the ring on my 20 is 3 inches thick and is on the flimsy side. this can be fixed with either more thickness or with more diameter, or both. It could also be fixed with carbon fiber, but I digress.

The poles are on the thin side. I suggest nothing smaller than 1+1/4 inch tubes of 0.063 thickness.


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6127169 - 10/09/13 02:39 PM

I do have a few more pictures that I've posted here and there. I'll try to get them organized into a description of the build.

JimC


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

Loc: Blue Island Illinois
Re: 16" Ultralight Dob new [Re: Mirzam]
      #6127502 - 10/09/13 05:35 PM

I built an ultra-light style scope and while it looked great, using it for more than 20 minutes or so made me rebuild it in a more traditional style. The issue I had was flexing or bouncing when pointed lower *BLEEP* well as general flexing when working at high-power. It depends on what kind of observer you are though. If mostly low-power views are your thing, you might never be bothered. Consequently, I was able to get a smaller footprint when disassembled making it in a more traditional Newt configuration, YMMV.

Edited by Sean Cunneen (10/09/13 08:05 PM)


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JohnH
Carpal Tunnel
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Reged: 10/04/05

Loc: Squamish BC Moved!!!!!
Re: 16" Ultralight Dob new [Re: Sean Cunneen]
      #6127565 - 10/09/13 06:06 PM

I had considered a lightweight 16" and came up with some methods to do this. Most of them revolved around keeping the mirror mass as low as practicable.

To use a cellular blank like this one: http://www.cloudynights.com/photopost/showphoto.php?photo=14912&password=...

I had thought using this for a mere Newtonian would be a waste so I held out for some inspiration, and low and behold found a second mirror like this one.


After listening toa lecture and a meeting with Mel Bartels, I realized thin mirrors could work too, so I thought of cutting in half this mirror: http://www.cloudynights.com/photopost/showphoto.php?photo=21593&password=...

and then I could make a 1 1/2" mirror that weighs only 30 lbs.


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MitchAlsup
Pooh-Bah


Reged: 08/31/09

Re: 16" Ultralight Dob new [Re: Sean Cunneen]
      #6127748 - 10/09/13 07:52 PM

Quote:

I built an ultra-light style scope and while it looked great, using it for more than 20 minutes or so made me rebuild it in a more traditional style. The issue I had has flexing or bouncing when pointed lower *BLEEP* well as general flexing when working at high-power.




I solved these problems by finding the weak (points of low stiffness) and beefing them up. That is why i used the phrase "after I debugged" in my original post.

My DOB reminds me of using binoculars it has such low vibration suceptability. You can quite literally bang on the upper ring and the vibarion is all gone in under 1/3rd second, even when pointing at the horizon.

At the horizon I found two major low stiffness points, the point where the compression poles attach to the mirror box and changes these from single shear into double shear with about 5X stiffness gain. I also added the back brack to the altitude bearings to get rid of a horizontal rocking couple when the scope is supported entirely on the front of those bearings.


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

Loc: Blue Island Illinois
Re: 16" Ultralight Dob new [Re: MitchAlsup]
      #6127777 - 10/09/13 08:04 PM

Just to mention, I have NO idea what I wrote to incur auto-bleep in my above post. I know I was thinking "lower targets". I guess I should keep my comments to 35deg or higher.

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m. allan noah
professor emeritus


Reged: 08/14/09

Loc: Virginia, USA
Re: 16" Ultralight Dob new [Re: Sean Cunneen]
      #6127796 - 10/09/13 08:14 PM

Quote:

Just to mention, I have NO idea what I wrote to incur auto-bleep in my above post. I know I was thinking "lower targets". I guess I should keep my comments to 35deg or higher.




You wrote 'as' with an extra 's'.

allan


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B9Robot
member


Reged: 10/08/13

Re: 16" Ultralight Dob new [Re: m. allan noah]
      #6127868 - 10/09/13 08:56 PM

All great info gang - thanks very much. Wow - there are sure a lot of great designs out there. The Hubble, Obsession UC, Sumarian.

I do have the advantage of a excellent CAD tools (solidworks), an industrial CNC router, lathe and mill in the shop. So the manufacturing part of the scope is pretty straightforward. I just need to take all the good advice and bake it into the design.

thanks!
Don

Edited by B9Robot (10/09/13 08:59 PM)


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6128060 - 10/09/13 10:56 PM

Here's some design philosophy.


I use to design a scope from the mirror up.

Now I'm beginning to design from the experience then back into the scope.


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mrcotton73
member
*****

Reged: 10/31/08

Loc: Portland, OR
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6128293 - 10/10/13 01:50 AM

I would strongly recommend that you get a copy of Albert Highe's new book "Portable Newtonian Telescopes". I purchased this book and it is really an excellent reference if you want to build a lightweight telescope. The details he goes into for each design aspect, the inter-relation of certain design decisions to others, how they affect weight, stability, etc.. are simply awesome. And I particularly liked his chapter on specifying design goals -- that really makes you evaluate what features of the telescope are important and what decisions you may make based on that.

Also check out the telescope building pages of Reiner Vogel -- they are a great reference.
Reiner Vogel's site

By the way, I am currently building a 16" f/4 string telescope - with 8 strings. My mirror is 1" thick quartz mirror from Mark Cowan, weighs 15lbs; For the secondary cage I am using an aluminium bicycle tire rim (Sun Ringle - RhynoLite 507x29.5mm - weighs only 1.2 lbs and has an inside diameter of 19.5".

Good luck with your project!

Sameer.


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 16" Ultralight Dob new [Re: mrcotton73]
      #6128519 - 10/10/13 08:28 AM

Sameer--I hope you will give us more details of your build as it progresses, especially the UTA.

JimC


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Mirzam
Post Laureate
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 16" Ultralight Dob new [Re: Mirzam]
      #6128524 - 10/10/13 08:34 AM

Another thing worth mentioning is that counterweights need not be hunks metal. You could, for example, use a small eyepiece box with eyepieces inside as a counterweight. This way an airline travel scope does not need to bring along any wasted weight. I also am a proponent of elastic bands to provide some counterbalancing force.

Of course when you take the 31 Nagler out of the box the scope may still move!

JimC


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B9Robot
member


Reged: 10/08/13

Re: 16" Ultralight Dob new [Re: Mirzam]
      #6128742 - 10/10/13 10:31 AM

Sameer - thanks for the link - it is excellent! Also the books are on the way.

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