Tuesday, March 10, 2015

Astro-Tech 8" Imaging Newtonian, Part VII

I've blogged extensively about the modifications and improvements I've made to my Astro-Tech AT8IN Imaging Newtonian.

Part 1 - spacing the Keller reducer, and first light

Part 2 - oblong stars, and thinking about reinforcing the tube under the focuser

Part 3 - reinforcing the tube under the focuser, and relocating the collimation locking bolts

Part 4 - replacement (stiffer) collimation springs

Part 5 - an even larger and thicker reinforcing plate under the focuser (at some point I also replaced the stock GSO linear bearing focuser with a Feathertouch)

Part 6 - gave up on the Keller reducer, switched to a Paracorr, and extended the tube to reduce focuser racking out

Now for what is hopefully the last modification to the AT8IN.  After all the improvements made above, the newtonian was usable - it held collimation, and the stars were round across an 8300 sensor. Unfortunately, one side effect of all the modifications (as well as the tube falling a couple feet) were a lot of dings, dents, holes, and rivets on the tube.  In other words, the AT8IN became very ugly.

A new carbon fiber tube from Klaus Helmerichs would cost about $500, which I was not really willing to spend.  Instead, I commissioned a local machine shop to roll an aluminum tube out of 2mm thick sheet.  The tube ID is 232mm and the OD is 234mm.  The tube is 840mm long and has the correct spacing for the lengthened original tube.

The tube weight is about 3kg - about the same as the original steel tube, which had 0.8mm wall thickness.  So there is no weight savings.  The new tube is (I calculate) about three times stiffer than the original steel tube.

The only challenge at this time is that the tube is unpainted (I asked the machine shop not to paint, powder-coat, or anodize it, because I had to drill holes in it for the hardware and focuser).  I could remove all the hardware and have the tube powder-coated white; that's the best and most aesthetically pleasing solution.  However the tube cost $200 and a powder coat will cost another $100 plus, bringing the total cost very close to that of a carbon fiber tube from Klaus!

Teleskop-Service sells replacement carbon tubes for the GSO newtonians (actually made by Klaus Helmerichs), the weight is about 1kg - 1.9kg.  So there really is not that much weight savings in going to a carbon tube (right now the telescope weighs 12kg, it would be reduced to 10kg with a carbon tube).

So I am probably just going to wrap it with 3M vinyl wrap with a carbon fiber weave pattern.  So the tube will look like carbon fiber but will be heavy and metal.  A 1-meter length of the vinyl wrap is only $20 so the price is definitely right.

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