Author Topic: Are curved spider vanes better? And if so, why are they not standard equipment?  (Read 204 times)

lolusthoumin

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Curved Vane Spiders ... potentially compromise the collimation.

Jon


Forgive me Jon, for not having your knowledge in engineering, but I can only attest from experience that the spider in the Teeter holds collimation very well.

Joe Wellard

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This mimics my experience as well when viewing Planets through a friends 10" F5 Teeter.

Mike

Yes, that's the model I have. It's great on the Moon and globulars, as well.

frosperloacatch

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The idea behind a curved vane is to distribute the diffracted energy from a concentrated spike into a far less intense 'fan.'

Because a curve is longer in length than a straight line when the end point separation is the same, the total diffraction generated is greater for the former. This is a more significant factor than the curved vane's typically greater thickness.

Of course, 3 curved arcs may have a shorter total length than 4 straight vanes, which could then reduce total diffraction.

In the end, the matter comes down principally to aesthetics; does one find the readily seen spikes for straight vanes distracting? Their presence does not impact small-scale contrast appreciably differently from the more diffuse pattern resulting from warping the diffractor.

Well, this has all been discussed before of course - but since the curved vane is necessarily thicker the angular distance over which it distributes the diffracted light from the source is far less.  Seehttp://home.digitale.../TM/Spiders.htm

Which, at least for extended targets like planets, means you have more diffraction onto the disc itself, rather than out into the surrounding area as is the case with sufficiently thin vanes. 

Stare at the center dot of this image for a while and you'll see the surrounding ring vanish. Same thing happens with planetary smears if they are faint, the brain decides they aren't real and edits them out IME. Very thin straight vanes with negligible thermal mass (i.e. wires) do this.


miswalltile

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The question is in the title. Are curved spider vanes better and if so then why are they not standard equipment?


Your question starts with a couple of false premises - that excellence is the driving force in manufacture, and that curved vanes eliminate diffraction.

If this were the case, manufacturers would be using the offset spider design (rigidity without high tension) or the wire spider design (minimal thermal effects and diffraction).

As so correctly pointed out earlier, the curved spider just averages the spikes across the image. And then there are the rigidity issues. If the secondary mirror subtly de-collimates with scope movement (or is just difficult to adjust), any imagined gains in diffraction are wiped out.

But Excellence does not drive design and what we see is manufacturers using the "cross" spider design. It may be due to custom, inertia, or ease of manufacture. But is certainly not due to excellence.

acbrawexel

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Curved Vane Spiders ... potentially compromise the collimation.

Jon


Forgive me Jon, for not having your knowledge in engineering, but I can only attest from experience that the spider in the Teeter holds collimation very well.

Yes, they can work - in smaller apertures.

What is the largest scope you have ever seen a curved spider in? 12" maybe?

Lamichael Evans

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The question is in the title. Are curved spider vanes better and if so then why are they not standard equipment?


Your question starts with a couple of false premises - that excellence is the driving force in manufacture, and that curved vanes eliminate diffraction.

If this were the case, manufacturers would be using the offset spider design (rigidity without high tension) or the wire spider design (minimal thermal effects and diffraction).

As so correctly pointed out earlier, the curved spider just averages the spikes across the image. And then there are the rigidity issues. If the secondary mirror subtly de-collimates with scope movement (or is just difficult to adjust), any imagined gains in diffraction are wiped out.

But Excellence does not drive design and what we see is manufacturers using the "cross" spider design. It may be due to custom, inertia, or ease of manufacture. But is certainly not due to excellence.
The question came out of this article I found last night on Cloudy Nights.

http://www.cloudynig...ider-vanes-r500

It's a glowing report of how well properly designed, fabricated and installed curved vanes can work.  So much so that from a new persons perspective I wondered why they were not standard now. If everyone is not using them then there must be a reason for that as well.

In my manufacturing process we tend to give customers what they want (within reason) if they are willing to pay for it. Everything else is driven by cost. But if we manufactured dobs and most of our customers insisted on curved vanes then that is what we would be using whether it was an improvement or not. Occasionally a larger customer (like Walmart) will force us to do something that is just not smart (in my opinion). And we comply. I want to know the whys and why nots of curved vanes.

All of these response are helping me with the bigger picture. Thank you.

Jason Simpson

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The question is in the title. Are curved spider vanes better and if so then why are they not standard equipment? I do not own (or have ever owned) a reflector and have never looked through one with a curved spider. I am a beginner who would like to know why.

This astromart ad is what peeked by curiosity. http://www.astromart...ified_id=924330

There seems to be an entire industry devoted to supplying upgrades for telescopes. I have no way of knowing if an upgrade is a meaningful improvement or a waste of money. And if an improvement, then how much so? Negligible, modest, substantial, extreme? Then the issue of trying to quantify someones else "substantial" improvement. My eyes and value system might see little to nothing of value while they saw enormous. Does not mean people are always lying or being dishonest, just that we are different.

It gets really complicated.

Had curved vanes on my Portaball. Didn't notice much difference in the views except for the absence of spikes. Experienced no collimation problems. Decided newts are SUPPOSED to have spikes, and I feel a loss without them :-)

blacosticna

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Years ago, before I knew of CN, I considered getting curved secondary holders because I mistakenly thought that they eliminated diffraction. For DSO observers like me, that hardly ever look at anything bright, curved vanes are not worth the trouble or expense.

Dave Fair

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Years ago, before I knew of CN, I considered getting curved secondary holders because I mistakenly thought that they eliminated diffraction. For DSO observers like me, that hardly ever look at anything bright, curved vanes are not worth the trouble or expense.



The essence of the matter. Your habits and preferences rule this choice too.