Update ...

I have passed along my design for a six-sided tube to my wood-working friend from church.  It will be 5" on a side, made from 9 mm (3/8") panels with longitudinal supports on the corners for strength.  It will have a minimum of 8" internal diameter which will provide for the 1" minimum clearance around my mirror.

Preliminary design indicates a tube length of 67" (see the "Newt on the Web" data sheets for details); so I will ask for 69-72" long and trim to fit after verifying the placement of the mirrors.  Easier to make it shorter than the other way around ...

My friend will first construct a much shorter (couple of inches long) 'mock-up' as a proof of concept of the design.  He thought it would be ready by Sunday; no real rush as until the materials I ordered from the States are in hand, the tube, primary cell and secondary holder are pretty much the only parts that can be built for now ... everything else (including the final length of the tube) depend upon the final placement of the hardware, including the focuser and eyepieces.

Just for giggles, I've been playing around with "SketchUp" a 3D drawing tool that helps put things into perspective.  Below you can see a 69" tube with a 6" mirror ...

Status Update ... sort of

I ordered some eyepieces and a Telrad finder today; estimated delivery by mid-February.  So, with the Moonlight focuser from the other day, everything that I think I need to purchase is now moving toward Sri Lanka.  :)

As I've been pondering my options for a tube, I've pretty much convinced myself that I'd prefer the look and feel of a hand crafted wooden tube.  I traced out a full sized sketch of a six-sided tube with 8" inside face-to-face and I like its proportions.  Will speak with my woodworking friend tomorrow and see what he thinks.  A nice dark stained wooden tube would really be sharp.

"Paint it Black" ... I bought some very flat black paint today to paint the inside of the tube and all the mounting hardware for the mirrors.  It is important to minimize or eliminate any stray light which reduces the contrast of the image.  Basically, the only light you want reaching your eye, is light that has been reflected (and focused) by the primary mirror.

I purchased a set of four eyepieces of 20/15/9/6 mm focal lengths.  All have good eye-relief and present a 66 degree apparent field of view.  They are often recommended as being very good for their price; i.e., "bang for the buck".  :)

The 20 mm will yield a magnification of ~80X, and will show a patch of sky ~0.8 degrees in diameter (the full moon is about 0.5 degrees in diameter, so it would be half again as large).  For a 6" telescope, this works out to about the maximum magnification for resolving details ... as we increase the magnification, the image gets larger, however no new details will be noticed.

The 15 mm provides a magnification of ~110X, with a field of view of ~0.6 degrees of sky (slightly larger than the full moon).  I don't anticipate using this eyepiece that much, wishing it were a 12 mm instead ... but that is the way it was packaged.

The 9 mm will provide a magnification of ~180X, which is about the maximum magnification one can use on a 6" scope under normal conditions.  I expect this eyepiece will be used the most.

Finally, the 6 mm will provide a magnification of ~270X, which is about the maximum magnification one can use under very steady 'seeing' conditions with a 6" scope.  Although the nights are rare when one can use that much magnification; it is nice to have the means to take advantage of excellent seeing when one gets the opportunity.  :)

The Telrad finder is a 1X finder; i.e., what you see through the finder is the same size as it normally appears ... just like looking through a window ... which is essentially what you are doing.  The main difference is the Telrad projects a set of circles on that window that can be used to help point the telescope at the target ... or at least get it close enough that you can see the target with a low power eyepiece (i.e., the 20 mm in my case).  These finders are simple to use and most everyone who has ever used one recommends it.

Working up the Design ...

... Crunching the Numbers.

A Newtonian telescope is a simple design consisting of:
1) a paraboloidal primary mirror, which focuses the incoming light to form an image;
2) a flat secondary mirror, which reflects the converging light beam to the side, where it can be observed without the observer's head blocking the incoming light; and
3) an eyepiece used to examine the image formed by the primary mirror.
The eyepiece is normally placed in a focuser, which allows small changes in the separation between the primary mirror and the eyepiece, to properly align the eyepiece with the image formed by the primary mirror.

The mirror Kevin F. sent me is a 6" f/10.6.  This simply means that the mirror is 6 inches in diameter (which for a Newtonian design will also be the aperture) and that the focus is 10.6 diameters away from the mirror, or 63.6 inches.

The secondary 'flat' mirror is an elliptical shape and 1" wide along its smallest dimension (minor axis).  Since it will be tilted at 45 degrees to reflect the light cone outside the tube, the elliptical shape will present a circular shape to both the incoming light beam and the eyepiece (held in the focuser) ... this minimizes the amount of light blocked by the secondary mirror.

We now have enough information to properly design the Optical Tube Assembly (OTA) and be confident that it will work after construction.  There is a very nice web-based program that will help us design our Newtonian telescope called 'Newt for the Web' and you can find it here:
http://stellafane.org/tm/newt-web/newt-web.html

Under the Specifications tab I entered the following data:

The information for the "Optics" section comes from my mirrors.  The information for the "Tube" section is based on my 'ideal' tube; it may be adjusted later if necessary to account for small changes in diameter or thickness.  The information for the "Focuser" section is based upon the data given for the Moonlight focuser from the company web site.

Moving to the "Ray Trace" tab, we can see what our design will look like:

I turned the 'baffles' option off, as there is no real advantage to baffling the tube of a Newtonian (unlike a refractor); and honestly, including tube baffles in a Newtonian may prove detrimental to the performance as the baffles will force the warm air rising from the mirror as the night time temperatures drop into the incoming light path ... remember those 'thermals' we are trying to avoid?

The "Performance" tab has some interesting information:

The main things here are that the 1" secondary mirror will produce a 17% central obstruction to the incoming light path.  We desire this to be small, the goal being <20%; so we are good here.  Also note that the fully illuminated field (100%) is 0.467"; close enough to the 0.5" that is often referenced as adequate for visual use.

And finally, the Dimensions" tab:

This tab is where we find the critical measurements for construction of the OTA.  I've already determined the diameter of the tube (which was input in the "Specifications" page); and now I can see that I'll need a tube at least 67" long.  Again, we will ignore the baffles data as tube baffles will not be used.  :)

Working up the Design ...

... the Optical Tube Assembly (OTA).

The mirror and a matching secondary arrived via FedEx today ... a day earlier than expected!  I anxiously unwrapped the package and was somewhat shocked to see the mirror was packed in an equally fine custom made wooden mirror box!  My eldest daughter, Thilini, thought it would make an excellent jewelry box (and she is correct); however, I think I will keep it for storing the mirror ... :p

That is the ceiling reflected off of the mirror in the first photo (awesome isn't it?).  The back is engraved with the 'Maker's Mark'  (Mark Harry).  :)

With mirror and secondary in hand, it is time to start finalizing the rest of the design.  There will be three main components: the OTA; the Mount; and the Equilateral (EQ) Platform.  The EQ Platform is designed to fit the Mount; and the Mount must be designed to fit the OTA ... so, the OTA is first.

The main part of the OTA is a 'tube' (surprisingly enough).  The tube is what holds all the other pieces (primary mirror, secondary mirror and focuser) in proper alignment with each other.  For a Newtonian telescope, it is recommended that the inside diameter of the tube be at least 2" larger than the diameter of the mirror, allowing for 1" of clearance between the edge of the mirror and the inside of the tube.  This is mainly to minimize the 'thermal effects' caused by warm air rising off of the primary and traveling up along the top of the tube wall as the evening air cools down.. So, the inside of my tube will be a minimum of 8 inches.

I would love to find a nice, strong cardboard tube to use as an OTA.  A common source are forms used for concrete construction; however, I am not certain those are available here in Sri Lanka.  My daughter has a friend, whose father  works with a firm that builds large construction projects (i.e., shopping centers and high-rise apartment buildings); so if such things are available here, perhaps he will know where I can find one.  If that does not work out, there are large cardboard tubes that rolls of linoleum come on.  They are 9" in diameter and come in 8' lengths ... they might be suitable ... if they are strong enough.  Another option (and perhaps preferable to the linoleum tube) is to have a wooden tube made.  It would certainly look nicer and could be made to exactly the dimensions that I want.  A six-sided tube would work well I think.  A member of my church is very handy with wood and could do this mirror justice I am certain.

I've also ordered a Moonlight focuser for this project.  They are highly recommended by those who have used them and very reasonably priced.  They claim they can deliver to Sri Lanka ... we shall see.

Two more items I need to purchase for this project are a finder and eyepieces.  I've got my eye on a Telrad finder and a set of nice eyepieces (20/15/9/6 mm) that I think will work well with my telescope.  I will need the focuser and eyepieces in hand to finalize the length of the OTA; however, they will not be needed for initial construction of the tube or the primary mirror cell.

Next: ... Crunching the Numbers.

A Telescope in the Works ...

An ATM buddy of mine (Kevin F.), whom I met and helped during a project of his, kindly offered to send me a 6" f/10.6 mirror of very high quality that was made by another mutual ATM friend (Mark H.).  I was hesitant to accept such a gracious offer, knowing the pedigree of the mirror; however, Kevin insisted that he was unlikely to use it (he likes BIG mirrors ... currently working a couple of 36" mirrors) and that he REALLY wanted me to have it.  So, I finally accepted his offer and the next day, he had delivered the mirror and a matching secondary (also figured by Mark H.) to the shipping company, where it commenced a journey that would take it half-way around the world from Pennsylvania, USA to Sri Lanka.  Shipping estimates the mirror's arrival on Friday, 17 January ... Kevin does not mess around when it comes to ATM projects ... :)

I had accepted the mirror, thinking I'd eventually use it in a CHief design (an unobstructed reflecting telescope, that uses two commercially available lenses, that are tilted to correct for image aberrations resulting from tilting the primary mirror).  I was hoping Ed J. (the designer of the CHief telescope design) would work up an optimized solution for that particular mirror using ZEMAX (a sophisticated optical design program).  Ed worked up a good design for me; however, I desired a slightly different arrangement to improve the performance and am currently awaiting those changes to be made.  More about CHiefs can be found in the Yahoo Groups "Spiderless": http://groups.yahoo.com/neo/groups/spiderless/info

Meanwhile, as there will be some delay in getting the CHief design finalized as well as requiring the purchase and delivery of the two corrector lenses and a larger 'flat' folding mirror; I have decided to use the mirror in a (much easier to construct) Newtonian configuration, until such time as the CHief can be built.

Living in Sri Lanka at a latitude of 7 degrees North of the equator presents some interesting mounting challenges.  I was originally thinking of constructing an inexpensive equilateral 'pipe' mount; however, the low latitude results in the polar axis being nearly horizontal ... which leads to problems with the counter-weight and scope clearing the pedestal supporting the equilateral head.  A lot of searching and head scratching, trying to find a way around this difficulty, finally convinced me that an Alt-Az mount, such as those used on the very common Dobsonians would be the most friendly so near the equator.  A clever device, known as an equilateral platform can be placed under the Alt-Az mount, which allows for tracking the stars for short periods of time ... perfectly acceptable for a telescope intended for visual use.

Next: Working up a Design ...