Then along came a spider

Nov 12

Written by:
11/12/2012 7:44 PM  RssIcon

Reflectors always seem to pose a challenge to the visually obsessed among those in the amateur astronomy community.  Unless you are building a Herschelian or offset-axis reflector - in other words, one with no central obstruction - then there is always the problem of the best way to place that obstruction in the light path with as little diffraction occurring as possible.  Some don't mind the straight, four or six diffraction spikes caused by typical 4-vane or 3-vane spiders, and to be honest, they don't really bother me all that much.  In fact, that may be the direction I go.  But something that has intrigued me for quite a long time is the curved spider.  Oh, believe me, if I could figure out a way to afford an optical window for my reflector, I wouldn't hesitate.  So the curved spider is the next best thing.

What is a "curved spider" you ask?  Glad you did.

A curved spider is one where the vanes of the spider assembly are, well, curved (duh), but the reason for that is so that the diffraction caused by the incoming light doesn't cause straight lines.  Rather, when the curved spider has either 180 or 360 degrees of "curve" in the light path, the diffraction energy is curved as well - and therefore the straight spikes 'disappear.'  Well, not exactly 'disappear' - it's just that the diffraction occurring in a circle becomes... well, not at all obvious around point-like stars and round planets.  So the 'spikes' aren't gone per se; merely (as my friend Sean says) 'smeared' across the view.  I don't think of it as smeared as much as 'better camouflaged into the background.'

So here's a few pictures of the one I am making.  I bought some brass stock, and designed the shape in AutoCAD.  Here's one of the curved pieces on the page template I printed:

Curved vane for spider assembly

I am banking on collimating this thing all of once, because I plan to bend the soft brass into place and leave it there.  Collimating it that one time will be a real pain, but it will likely never change either.  And with a 6" f/10.9 scope, it won't need much in the way of tweaking, and I'll leave myself a little wiggle room on one side of the spider-attachment screws.  Shouldn't need much else, I wouldn't think.  Anyway, the brass is fairly soft, and it wasn't too hard to bend around a 2" PVC pipe, and from there it was a simple matter of gently bending it to shape.

Bending brass for the spider vane

Here's the 3 spider vanes set in place on the template.  I drilled a 1/4" hole into a piece of plywood.  This let me place a screw into the wood to hold the brass tube in place.  Next I placed the spider vanes in position, as seen.  Unfortunately, I didn't have a soldering iron with enough wattage to heat all the surfaces and melt the solder.  So I screwed some drywall screws into the wood to hold the vanes in place, took the whole thing outside in my garage, and heated up the central portion with a propane torch (like people do to sweat pipes in plumbing).  

3-vane curved spider on template

Worked like a charm, except I'm a little sloppy with solder, so I had to grind off the excess with a Dremel tool.  Here's the central area of the spider assembly, with the solder sanded down to a nice curved look.  Doesn't look like much at the moment, but it cleaned up nicely and the flat black paint job I added really covered up the ugliness due to the solder, flux and propane torch.

Central area of spider assembly





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The Nightlight

This blog includes what to see in the night and daytime skies, thoughts on telescopes, binoculars, and other astronomy observing accessories and equipment, plus my own occasional notes on objects I've seen and observed. Oh, and the random theater or other "my take on life" post. In other words, there is always something interesting. Check it out.