10" Ultra Portable Newtonian

Not just portable—this scope had to be airline baggage checkable. I used to hand carry my 4” Vixen refractor in the cabin on my trips to the Texas Star Party, with the mount and other stuff checked in as hold baggage (two 32kg cases!) After 9/11 carry-on item were strictly controlled, and in 2007 the weight limit for checked baggage was reduced to just 23kg.

After 9/11 I hand carried a pair of Miyauchi 100mm APO binoculars (x20 & x37) to the USA for star parties. These are great binos, pin sharp over the whole field and the eye relief is enormous. I could see faint fuzzies (amateur term to describe galaxies & nebulas!) down to about mag 12 and even managed to see Pluto over three nights, (mag 13.6). After a few years of taking them to TSP, I felt I wanted more magnification and aperture.

So the challenge was to build a truly portable scope. I quickly confirmed the DOB (Dobsonian) design as being the lightest solution, and a few calculations suggested that a 10” was the biggest that would fit in a single 23kg case. Using two cases was an option, but what about clothes and other stuff? Plus in Europe most carriers only allow one checked case per person.

I started with a 10” F4.9 Orion Skyquest optical tube assembly (OTA). I knew their quality was good and a quick star test confirmed I had an excellent set of optics. I first disassembled the OTA and threw away the steel tube. This left me with a top ring complete with spider and secondary, a 2” focuser, and a primary mirror and mounting cell. The focuser was modified to fix directly to the top ring and the secondary mount shortened to re-
align the secondary with the focuser axis.

After weighing these items, I determined where the centre of gravity was going to be (allowing for my favourite 17mm N
agler of course!). My original thought was to use tripod legs as truss tube supports. They are really quite flexible (even the expensive carbon fibre types) and so I would have to use 6 in the classic “V” configuration. Six legs was going to be expensive, bulky and quite heavy, plus the centre of gravity was around 300mm from the bottom of the tube, so the bottom “box” of the Dobsonian tube assembly was going to be big. Things were not looking promising!

IMG 4446

The rest was easier! The elevation bearings were made from 4” sewer pipe blanking caps. The base box was made from 12mm ply, held together with M8 bolts. To give a more stable base, the lower azimuth assembly was made as a low tripod. The azimuth bearingsare three pads of ptfe running on an old LP (Pink Floyd: Dark Side of the Moon!)

Avoiding the need for a large bottom box in the tube assembly could be done with a double Serrier truss (just like my18” scope), I just needed to find a neat way of fixing the truss ends to the scope parts so that I could pack the scope away easily. After a few weeks thinking on the problem I came up with some small ball & socket assemblies that if the retaining spring was removed, gave a nice snap on and off action.

Incorporated in the design is an Orion Intelliscope Computerised Object Locator. The trick here is to buy the pro version that is meant to fit their equatorial mounts and then you get two encoders. (The standard version meant for DOBs assumes you have an Orion DOB with the azimuth encoder already built in.) The encoders are magnetic rings with a little pcb holding a pair of Hall Effect sensors. When mounting the encoders you have to be very careful on two things 1. make sure the magnetic rings are very accurately concentric with the axis of rotation, and 2. position the sensors very close to 0.7mm from the magnetic rings.

The encoders as supplied are best reversed, ie the RA fits better on the elevation axis and the DEC fits better on the azimuth axis. This is because the Dec pcb has a hole in it (suiting the DOB azimuth bearing bolt), and the RA encoder is used as a junction box and needs to be nearest the handset. I made a little patch lead in which I switched the wiring thereby fooling the controller as to which was which.

Having seen baggage handlers at work (would you be careful if you had to lift 20+kg cases all day!), the case and packaging was critical. I have a number of Peli cases, which are almost indestructible and being plastic they deform a little to absorb shocks. The trouble is they are heavy! A decent size is about 12kg when empty. The most weight effective is their rifle case series. I selected the 1700 model which at 7.5kg would just take the long truss tubes and the rest of the scope.

The case protected the scope well so far on eight return trips to Texas and two to Florida, and I have got used to
being asked at check-in whether I had a weapon in it! US Security will want to open cases like this, so make sure you use the TSA approved padlocks so that they can open it.

Update: In early 2011 I decided to upgrade the focuser with a two speed crayford type. This was heavier and completely changed the balance and so I took the opportunity to refurbish the telescope and make a new bottom box. I removed more than a kilogram of weight from the telescope and box ensuring it would all continue to be under the 23kg airline check-in weight limit. With the new focuser and some other general improvements the scope is even easier to use.

AstroKeith 2014