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 ...