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Piggy Backing my TEC140APO
of the 10-inch Calver

TEC140APO_2.jpg TEC140APO_04.jpg TEC140APO_06.jpg TEC140APO_07.jpg TEC140APO_04hiresRT8.jpg
TEC on CALVER update 14OCT2003 hi-res images
tube ring register seat upstand register seat in backplate TEC140APO piggy backed TEC140APO piggy back tailpiece with turret the long sky view


As soon as I used my TEC140APO mounted on my Vixen GP-DX GEM it was all
too evident that vibration induced when focusing, even with the FeatherTouch
rackmount, would prevent my using powers in excess of x50/inch.

The only way I was going to be able to get the most out of my TEC140APO
was too piggy back it off my 10-inch Calver. The question was, would the
Calver carry the additional load, and how should I make the support structure
in a way that minimized additional weight yet afford the necessary stiffness?

I first of all toyed with the idea of having support shoes cast that would
bolt onto the inner cradle rings, and mount an aluminium thin section box
frame off it that would carry the TEC's tube rings. I sketched my initial idea
but decided it would be too awkward to make, and too heavy.

My next attempt involved a 3"x1" backplate out of T6 dural flat bar and
a pair of 8"x3" T6 'U' channels, profiled by hand, with register seatings
to provide the offstand and support the TEC's cradle. This had a calculated
weight of 9lbs 8ozs which was acceptable.

I purchased the material from Thames Stockholders at Ponders End, Enfield,
and profiled the channel upstands by hand using a dreadnaught file. It was a
lengthy and exacting business, and very hard work, but I considered it better
to do it by hand than trust it to a machinist because it was too easy to get
the dimensions slightly out and that would mess up the collimation with the
Calver itself. I had already dismissed the notion of providing collimation adjustment
because it would be a source of flexure. I decided to reply on my ability to
work with hand tools to within ±0".001. I can measure with an engineer's
rule to ±0".005, and with a micrometer to less than ±0".001.
All went well, and I then marked out and filed the TEC tube ring register seatings.

The reason for the register seatings was to accurately locate the tube rings
and ensure there was no way they could move once bolted down.
The back plate was a 17-inch length of 3"x1" T6 flat barstock. I marked out
the channel offstand seatings and took it round to Ron Irving who machined
a pair of 4"x3"x1/16" deep register seats using a shaper and drilled the bolt
holes using a jig borer. I then deepened the register seats to smooth off the
shaper's machine marks, and to ensure they were flat and parallel within

Fitting the backplate between the inner (fixed) cradle rings entailed profiling
tapers on each end and a central radius in two planes to match the profile of the
cradle ring. I then located the plate between the cradle rings and marked
out the centres of the bolt holes and then centre popped them and ran a pilot
hole thru' taking care not to drill into the tube wall, and making sure swarf did not
get into the bearing faces of the rotating cradle. After which I opened the holes
up to 0".302 and tapped each hole 3/8"-16BSW.

I bedded the backplate in situ with epoxy metal putty, and removed the excess
with a sharp bladed srewdriver, and then bolted it down using csk.hd. Allen bolts
accurately cut to length so as not to touch the tube wall.

The channel offstands, which raise the TEC140APO 8-inches away from the Calver
so as to see past the tube furniture around the top end of the Calver's OTA
were then bolted in place, and the TEC cradle and dovetail plate assy. bolted
into their register seatings.

The Calver is far too massive to manhandle. The next job, before placing the
TEC140APO into its cradle was to disengage the worm drives and chock the
Calver pointing vertically upwards, and then fit the TEC140APO and the Unitron
UniHex turret, and then add the additional counterweights to the declination axis.
This is not as easy at it sounds. The TEC plus backplate assy weighes 30lbsf
and two additional counterweights had to be added, a 25lbsf threaded weight, and
a 42lbsf fixed weight on the dec shaft extension. I had to remove a 52lbsf c'weight
off the dec. axle, undo the shaft extension using a chain wrench; unscrew the outer
threaded dec. c'weight, and add the 25lbsf threaded c'weight so it was between the
inner and the outer, before replacing the shaft extension and adding the 42lbsf
inboard of the 52lbsf c'weight and then locknut them in situ.

The balance could then be checked by hand before re-engaging the worm drives
into the wormwheels. Now for the moment of truth, would the drives cope
with the additional inertial load. I had already performed some calculations
which indicated the Dec. drive would see virtually no increase in inertial load.
The RA drive however would see a 5.5% increase, how would the stepper motor

Well, for a start it stalled in Dec when climbing uphill, indicating the Dec c'balancing
still wasn't quite right. That is why I added a central threaded c'weight; I was able
to screw it further out along the Dec axle until the Dec motor stopped stalling.
Once that was done, it was obvious the Dec motor was not feeling any noticable
increased inertial load because the <SLEW> & <ACCL> settings were still OK.

The RA motor had been set to 2º.7/sec <SLEW>, and with the 5.5% increased
inertial load it was beginning to resonnate, which is symptomatic of a stepper
motor that is about to stall. Its not a good idea to push a stepper motor to the
limit because there is a possibility of missed steps to the virtual encoder. I reduced
the setting to 2º.5/sec, and the motor was fine at that, running quietly and smoothly.
I did not need to reduce the <ACCL> setting.


The TEC140APO was piggy backed off the Calver on Friday 10th October 2003.
The job of bolting the backplate between the fixed cradle rings, placing the
TEC140APO into its cradle, and resetting the dec. counterbalancing and stepper
drives took all day. I was eager to see how stable the new setup was. Would the
additional load push the Calver's mounting beyond it's stability limit, or would
it cope?

My first view through the newly mounted TEC140APO came on the Saturday morning
between 04h:00m & 05h:15mUT in Seeing II and excellent transparency.
I observed Saturn using a variety of eyepieces, ranging from a 1-inch Zeiss aspheric
Orthoskop; 16.3mm Kowa Konig; & 12.5mm; 9mm; & 6mm Zeiss Orthoskops.
My second view came on the Sunday morning (12OCT2003) between 01h:15m & 02h:45mUT in Seeing II-III
and slight cirrus haze. I observed Mare Crisium & Petavius, and Aldeberan, using the same eyepieces,
plus a TAK 4mm HiOrtho & a TMB 8mm Mono & a TAK 3.6mm HI LE Ortho.
The seeing wasn't good enough to support the 3.6mm Ortho. (x272), but the views
even in the 4mm (x245) were crisp and contrasty.

I ended the second session observing the Hyades with a giant exWWII 44mm Erfle.
This eyepiece has a calibrated focal length of 36.26mm (using an Dall focometer)
& an apparent fov of 71º.333; yielding a power of x27 and a real fov 2º.7.
Its field curvature pretty well matches that of the TEC140APO's O.G. The outfield is
affected by slight astigmatism, but no radial false colour.

Stability was not a problem. I tapped the TEC140APO's tube and what minimal vibration
was induced died down almost instantly. I tapped the Calver's tube and the image oscillated
for only a fraction of a second, less than a quarter I'd say.

The most important aspect from my point of view is observer comfort. I've positioned
the TEC140APO almost opposite the dec. saddle plate, and sufficiently down the Calver's tube,
so I can view the Moon and planets whilst seated, and looking down into the eyepiece.
Oh the comfort compared to being perched aloft atop my library step stool. Observer
comfort counts for a lot in visual observing. The TEC140APO may not have the raw
resolution of the 10-inch Calver, but I am able to see a lot more, and see it for longer,
without becoming fatigued.

The cost of the T6 barstock and channel section offcuts was £55, and Ron Irving
charged me £25 for the machining of the backplate. £80 is not a bad price
for a first rate job.

I've read review articles in Sky & Telescope, Astronomy, and Astronomy Now magazines
about the Software Bisque ParamountME GEM. Martin Mobberley in his Astronomy Now review article
(October 2003 issue) claims it to be the ultimate German Equatorial head, because it has a
load capacity of 125lbsf; a pointing accuracy of ±2arcmins and only 5arcsecs periodic error.
My Calver has a load bearing capacity in excess of 500lbsf; a pointing accuracy with its AWR
microstepper drive system of ±30arcsecs and less than 5arcsecs periodic error.

Martin Mobberley reckons that the $10,000 price tag (£10,000 by the time you've
paid shipping charges, customs duty & 15%VAT, and airport handling and clearance fees) is good value.
Well even including the cost of my TEC140APO, the all up cost of my restored Calver, including
the 12.5ft. aluminium, motorised, domed observatory, and the AWR drive system, cost just under £10,000

£10,000 for just an equatorial head does not look such good value by comparison.
I know which setup I'd sooner have.

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