An Exercise in Frustration and Wishful Thinking or: Designing Telescopes

So I fall quite firmly into the stereotype of “cheap Asian”, in that I shop around, haggle, and then play manufacturers/resalers against each other for the best deals possible.

Sometimes though, that doesn’t get me exactly what I want for the price I’m willing to pay for it. One field where you literally cannot haggle at all is astrographic telescopes of any sort, and depending on what optical system you want, prices can be as high as $1500 per inch of aperture. It was prices like these which made me think “hey, I know of a few machine shops and design utilities, I could get a telescope custom built!”

Yeah nah.

Serious props and kudos to those brave few that grind their own mirrors, because I’m just trying to optimise critical structural values and have found myself having to compromise in areas where I really, really would not like to. I shudder to think what it would be like when you’re working with sub- micron tolerances.

 

TRIGGER WARNING: Technical shit starts here, folks.

Focal length and aperture were easy, since I was planning on buying an f/4.5, 406mm diameter parabolic primary mirror from GSO, a Chinese manufacturer, for a focal length of 1827mm. The choice of telescope tube structure was easy too, as with optics of this class, only a carbon-fibre Serrurier truss design would be light and strong enough to hold the optics in collimation. Focuser was a no-brainer as well, since the corrector optics set was built with a 3 inch focuser in mind. Hello Starlight Instruments Feathertouch! (A cool $700 investment)

So at this point, I think to myself – there must be some easy-to-use, free programs that will help me optimise secondary mirror size, mirror spacing and image illumination diameter, and lo and behold, there were. Easy as!

I plug what dimensions I know of into the program, and all I see are error messages.

I play around with secondary mirror sizes and tube diameters to arrive at what looks like an acceptable compromise, and I balk at the diameter of the circle that was fully illuminated at the imaging plane. A measly 18 millimetres. Eighteen. This isn’t even enough to cover the imaging chip on a commercial DSLR adequately, let alone a 35mm full-frame CCD camera. Back to the drawing board.

A quick search on the web gave me two mirror manufacturers that stood out –  Zambuto Optical Company, who in their entire operational history have not had a single bad mirror ship out to a customer and hold themselves to higher optical standards than the vast majority of mirror manufacturers, and Hubble Optics, who while having less stringent optical standards still offer quality mirrors for very, very reasonable prices.

Being a cheap Asian bastard (this can be taken literally as well as figuratively), I put cost savings over a small, but measurable gain in optical quality and revised my design for the Hubble Optics 16 inch hyperbolic f/4 mirror with included corrector lens.

More success was had this time (after three design further revisions), with a 42mm diameter full illumination circle. It will almost fully illuminate medium format CCD cameras, and the 75% illumination circle is almost large enough to fully illuminate a large format sensor. Compromise reached!

The dimensions are thankfully less ridiculous than my initial estimates, with a truss diameter of around 500mm, and a total length of around 1600mm.

 

The prospective materials cost?

I’m looking at around $4000 AUD just for the optics and focuser, not including the custom truss and specially designed 27-point floating mirror cell. I envision final costs to be upwards of $5000. The sad thing is – This is still around an eighth to a tenth of the cost of any equivalent astrograph. Still, I think I’ll wait to see some more cheap mass-produced Chinese astrographs in the 16 inch class before pulling the trigger on this scope.

In the eventuality that I do get this beast, I’ll be in possession of research-quality optics better than half of Perth Observatory’s telescopes. It’s pretty damn tempting, I gotta say.

 

Stay angry my friends.

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