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9/07/2012

A Strut Dob?



This is a 13.1 inch F4.5 and is from the DobStuff Website and is located at this link. This is similar to what my new scope should look like except it will have a dew shield and a couple of other items on it including a triangle string system to ensure no collimation shift.



Long time no post I guess. I have been swamped with the return to work and my own return to my advanced educational pursuits.  As I have mentioned, a couple weeks ago I took possession of a 14 inch Zambuto mirror that after two sessions, looks to be outstanding (of course).  It is now boxed back up and with some components from my XX14i, is being shipped to Dennis Steele at DobStuff to make a new Strut style dobsonian reflector. I knew that at some point I would be looking for a new structure to hold this mirror, and I considered heavily about building one, but lacking some of the major equipment I decided to have the old XX14i rebuilt into something better. Now don't take me wrong, my XX14i has been a wonderful scope and has served me faithfully for over 2 1/2 years.  It was sad to pull the "guts" out of the scope yesterday as I packaged everything and sent it off to Dennis.

The question will arise to why and why Dobstuff? There is another company back east that I had really considered but the wait time would be next May or latter and I am unwilling to wait that long.  Another reason is Albert Highe's new book that is out at Wilhelm Bell that discusses light weight scopes and that, hits to the reason I went they way I did.  I am not getting younger though I am still young I feel in the hobby in terms of my age.  More importantly my bad low back at L5/S1 dreads lifting the XX14i's bottom tube into my Pathfinder as I load and unload and load and unload on every dark sky trip.  The base though broken into 2 parts at home, isn't that friendly either to lifting.  The new scope will be much lighter and easier to transport. I had reached the point where lifting the bottom tube of the XX14i just made me dread thinking about observing. When your scope is getting you to question whether to go because of weight, its time to change some things in my opinion.

A strut? A STRUT? I've heard that from several people. I had considered Dennis' truss-hybrid design but after talking with Dennis on the phone, I felt more comfortable with the strut design.  The scope will be the 14 inch F4.62 mirror that I have at 1.4 inches thickness.  With that F ratio the notion of instability in the tubes is lessen because of the shorter length of the tube and I am adding a string design to make the scope more stable so that no variation occurs from horizontal to zenith in terms of movement of the scope so that collimation will hold. Dennis has a good explanation at his site located at this link. Jon Issacs over at CloudyNights owns a 16 inch strut from Dennis and he found the solution to his collimation shift using strings also.  The result is no collimation shift. For fun, here is a link to a 4 strut string telescope with some interesting features that is featured on Dennis' links.

Another concern is what about that fine Zambuto mirror? I observe in dusty, desert conditions and its easy for a small pebble to get thrown up.  Well, a dew shield for the primary put into place as seen in this link will protect the primary from dew, dust and small pebbles.  The advantage is the rapid cool down on the wonderful mirror and thus allowing better views in a quicker time which will maximize viewing times.  There are always trade offs in a scope design and I guess the main thing is to decide what trade offs are important enough not to compromise on, what one is willing to compromise on and how to minimize the impact overall on those items.

I have a dew heater for the secondary that will be installed and ready to go so that will ensure my secondary, especially in the fall and in the winter doesn't dew up.  I also have heard concerns about a shroud to help keep the dust out and to minimize stray light and I am sure I can make a shroud or purchase one from Shrouds by Heather (link) but this is one I believe I want to make myself. Keeping it out of the light path will be very important but I think that can be done either with black, rubber bungee cords, as shared over at CloudyNights: "On my 16" Lightbridge, I use 6 bungees stretched between the upper and lower tube assemblies to keep my shroud out of the light path. I use the black rubber type. I installed 6 hooks in UTA and 3 in the lower. I removed the S hooks from both ends of 3 straps and one end of other 3 straps. I slip the bungee ends over the hooks on the UTA, the three straps without S hooks on lower end are attached to hooks while the three with the S hooks are hooked to lower tube assembly truss securing knobs. I used to have a metal hoop to hold shroud out of light path but I find the bungee method works better."
As mentioned, I may use  two embroidery hoops position over the struts to keep the shroud out of the light path. We'll see on this after the scope is finished and I have it.  One thing is for sure, a scope is always a project.

So, what I am giving up by doing this? First, some money of course.  Next, in terms of the telescope I am losing the intelliscope feature.  Not a huge loss for me as I have only used it twice in the last year and those were at public star parties at our local library where there really are not any guide stars to find things at.  The rest of the time I star hop.  If I want to spend some more money then I could get the Sky Commander and the stalk with it.  The pro to this is that it can increase productivity since it takes you to an object instead of star hopping to it.  In the summer, when I may only have 3 to 4 hours to observe in good darkness, that can be a critical factor. However, I love to star hop, am extremely proficient at it and hunting down an object is part of the fun for me.  I don't do a lot of outreach so I think this may be something I opt out of.  One last pro though is that the scope becomes easier to sell if I ever choose that option if Sky Commander is on it.

Finderscope. Right now I think I am ready to not put both the Telrad and the 9x50 finderscope on the scope. I may change my mind on this as I have always enjoyed having a finder scope. Okay, the dovetail goes as I want the option of being able to put the finderscope on or not.  It will carry my Telrad and I have to state, dental floss made removing the base extremely easy. I am also have a Moonlight dual speed focuser installed on this new dob.  Color, I was thinking red, but I just like the notion of a black focuser on my scope.  It will fit with its new name, "Angel of the Night."  Hmmm . . . perhaps I birch should be stained black? No, it will be natural color as I like that the best.

Will this be a perfect scope? No, and I'll admit that upfront because I don't believe there is a perfect scope. I think we continue to play, and modify a scope for a long time seeking to improve its performance.  Will it be a scope that will last me for a LONG time? Yes. I like the 14 inch to 15 inch range, its the maximum size I want to use and going this route will make it lighter to transport.  Dennis offers a great product at a wonderful cost with premium results from what I have been told by a couple of people who own his product.  I'll be posting his images he sends to me of the build and of course, I'll post up when I get updates from Dennis as he builds it and as I receive the finished product from him. I intend to video tape the assembly so others can see how this works and give a review of the finished product after I have had it out under the dark skies of Utah's West Desert.  In the end I'm excited about this and will look forward to new scope as we move forward.  Comments are always welcomed.

My scope will be a little shorter than the measurements I give here since these are for a 14 inch F5 Hubble Mirror but the weights should be pretty close so I'll share them here.  They can be viewed at this link over at DobStuff. My eyepiece height should come in at 14x4.62=64.6 inches +/- 2 inches, or about what it is right now on the XX14i.


14" F5 with Hubble Optics mirror:
Weight of optical tube assembly w/mirror 44#
Weight of rocker box 25.5#
Weight of mirror box w/mirror 30#
Weight of mirror 19#
Height of eyepiece at zenith 71"
Rocker box size 21.75" x 20.75" x 13"
Length of struts 65"
Construction material Furniture grade Baltic birch 12-ply plywood

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