new UTA for my 14"

[PhilW]

I have tried to take my mind off Melbourne's weather by rebuilding the UTA on my 14". This time I've gone for a curved spider, made from two laminations of 0.025" brass shim. The baffling is a sheet of 1mm birch ply. Total weight of the UTA & spider is 1kg (less focuser, secondary & finders).

Photos attached if anyone is interested in the approach (especially if you're looking to de-Kydex your scope).

Phil

[Rod]

Hi Phil,

Looks great and seems like it would be both light and strong. Can I ask why you abandoned the wire spider?

Rod.

[madtuna]

Looks good Phil.
I'm very interested in how you made your spider. Is it possible to get a close up view or a more detailed how you did it?

[PhilW]

Regarding the wire spider: it works very well, but I just felt like experimenting with a curved one on this scope. I have liked curved spiders on other scopes I've built.

I made this one by routing a curve of the right radius in a piece of plywood (each arm is a 60-degree arc, so they sum to 180 degrees). Then I covered one side of a strip of brass with epoxy, pressed it against another strip, and taped it to the plywood form for a day or so, in order to make each arm. The arms are then riveted to an alum tube, 1" diameter. I'll take some more photos in daylight & post them.

[Just added two pics]

[Satchmo]

Why build a properly curved 3 vane 60 degree spider ?

Well, once you get your first view of bright stars and planets in a large Newt with no diffraction spikes you can never go back to a straight spider :)

[Omaroo]

Looks fantastic Phil :) Nicely done - great workmanship by the looks.




Absolutely agreed Mark. I can't fathom why people find spikes attractive - either in real life or in photographs. They are unnatural, and are NOT meant to be there so why do some people look for ways to add them? LOL!

[PhilW]

It was all surprisingly easy to build, & is extremely rigid. I should add that I routered the upper & lower UTA rings from 12mm hardwood ply.

Only downside is that the curved birch ply baffle makes the UTA look like a Swedish lampshade from certain angles.

[Omaroo]

LOL! I like it, but as long as the rocker box matches. Otherwise - do as the Stones told us - "paint it black"!

[tnott]

Nice work. Yes the weather in the southern states is certainly more conducing to building rather than viewing at the moment.

I have read that even though curved vanes get rid of the spikes, they introduce slightly more error to the image. The difference is probably negligible though and not perceptible to the eye.

A friend of mine got rid of curved vanes in his scope because he reckoned they changed shape with change in temperature, throwing out his collimation (like a thermostat?). I don't know if this was due to the materials he used or whether it was only a perceived effect:shrug:. Be interesting to see how it goes in your scope.

[madtuna]

thanks Phil!
Looks good and food for thought

[Omaroo]

I've not really ever seen this happening on any curved vane spiders that I've actually had the pleasure of using. I could possibly see this happening if he'd used two dissimilar metals, which have wildly different coefficients of expansion. Phil here has used brass/brass - so they should be thermally "centred". Even if they were dissimilar, because they are in a spiral pattern, each on a third, I'd expect them to all contract or expand at the same rate each, still resulting in a centered secondary.

If one vane was bent the opposite way to the other two (i.e. the dissimilar metal strips were accidently reversed on one arm) then maybe... Anyone??

[Satchmo]

The surface area in the 3 vane curved spider is probably similar to standard 4 vane. There are some claims on the internet that they cause bloated star images on bright stars. This is not so. The diffracted light ( with a corecctly designed 3 vane with 60 degree arc) is spread evenly over the field of view rather than concentrated in visible spikes. In poorer seeing its probably that the light is not drawn out of the star image into the spikes.

In general diffraction spikes are an optically cosmetic problem. They have to be very thick before they effect the telescopes MTF or contrast function. Suiter did an excellent analysis in `Star Testing Astronomical Telescopes' that showed that your contrast transfer function is still good up to a thickness of 1/128 aperture. Inother words a 16" scope is safe optically up to 3mm thick vanes.

As modern spiders are somewhat thinner then spikes are a cosmetic issue I'd rather do without :). I've had curved spiders made for my 14" bino under construction.

[stringscope]

Very nice Phil. Well done :thumbsup:

Cheers,

[GrahamL]

Does look nice phil ..cant see any hint if IKEA in there myself but I've never been to the store :)