Author Topic: Are curved spider vanes better? And if so, why are they not standard equipment?  (Read 329 times)

postbypopect

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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.



prehconbubun

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I think it really depends upon whether you are bothered by diffraction spikes. In other respects (secondary mirror adjustment and flexure, contrast, cost, etc.) there are probably some tradeoffs depending upon the particular design.

hiswacoka

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The essential problem with curved vanes is that they cannot use tension to support the mirror; this means the vanes must be thicker to compensate, which increases total diffraction and reduces contrast.

So its a matter of choosing your problem.

suctoleshe

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A curved vane does not reduce the diffraction of the spider, it just spreads it out so it is not as apparent when viewed against a dark background.  Since it will need to greater in area since it cannot be tensioned, the overall effect on planetary contrast will be greater.

For smaller tube diameters with smaller secondaries, it makes a certain amount of sense if one is bothered by the diffraction spikes.

Myself, it believe the most important thing about a spider is that it holds the secondary in the most secure and rigid possible manner. A straight vane spider is better suited for that.

Bottom line: Curved Vane Spiders are not inherently better and potentially compromise the collimation.

Jon

Dan Perez

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I believe curved spiders are standard on teeter's sts line of scopes.

asexdalo

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IMO Curved Spider Vanes are far superior as the little more scatter they add (over straight Spider) is minor compared to the Imparted "Cross" on Planets with straight Spiders.
I only wish these were available for larger Newts (20"+).

Mike

Nathan Mayienda

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IMO Curved Spider Vanes are far superior as the little more scatter they add (over straight Spider) is minor compared to the Imparted "Cross" on Planets with straight Spiders.
I only wish these were available for larger Newts (20"+).

Mike


And just how does that make them "far" superior? The actual planetary contrast is reduced with the curved spider, the collimation is potentially compromised.. Technically, there is no cross on the planet itself, the cross is seen against the night sky.

The curved vane spider philosophy: "Take this pile of dirt here, add some more, take a broom, spread it out across the floor and voila, the floor is clean."

Jon

Eric Hayes

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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.

Gary Eldridge

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

Three straight vanes are also doable... It is not only the length but the area..

Jon

trapoutampub

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Quote

IMO Curved Spider Vanes are far superior as the little more scatter they add (over straight Spider) is minor compared to the Imparted "Cross" on Planets with straight Spiders.
I only wish these were available for larger Newts (20"+).

Mike


And just how does that make them "far" superior? The actual planetary contrast is reduced with the curved spider, the collimation is potentially compromised.. Technically, there is no cross on the planet itself, the cross is seen against the night sky.

The curved vane spider philosophy: "Take this pile of dirt here, add some more, take a broom, spread it out across the floor and voila, the floor is clean." 

Jon
Jon, Hence the "IMO" designation. Not seeing that dreaded "Cross" makes them far superior (IMO  ).

And although a micro layer of dirt spread out evenly on the floor is still dirty, I'd much prefer that to the congregated pile of dirt which is easily seen in the middle of the floor.

As per usual, YMMV.

Mike

Dan Perez

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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.

Never thought about this but this is an Important factor. 3- Curved vs. 4 straight. Easy decision for me!

Mike

rackramasca

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It's dirt either way and the bigger issue is making sure the secondary is supported in the most rigid possible manner. And that is what a straight vane spider does in a "far superior" manner..

"Dreaded Cross?" Not around here.. Newtonians are simple telescopes. The diffraction effects of the spider and secondary are small when compared to the diffraction effects of an SCT secondary. I prefer to keep the piles of dirt small and well organized..

My mom would have said, "I thought I told you to sweep the floor." My dad would have said:, "What's all the commotion about? That's an amazing view. You're an engineer Jon, you know better than make something simple more complicated just to spread the dirt around."

Jon

ithoclirans

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I believe curved spiders are standard on teeter's sts line of scopes.


The one I have has the curved spider. It performs very well and I do not miss diffraction spikes. If anything, it enhances contrast and you do not get distracted by spikes.

Stephen Gupta

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<p class="citation">QuoteJon, Hence the "IMO" designation. Not seeing that dreaded "Cross" makes them far superior (IMO  ).
And although a micro layer of dirt spread out evenly on the floor is still dirty, I'd much prefer that to the congregated pile of dirt which is easily seen in the middle of the floor.
As per usual, YMMV.
Mike


It's dirt either way and the bigger issue is making sure the secondary is supported in the most rigid possible manner. And that is what a straight vane spider does in a "far superior" manner..

"Dreaded Cross?" Not around here.. Newtonians are simple telescopes. The diffraction effects of the spider and secondary are small when compared to the diffraction effects of an SCT secondary. I prefer to keep the piles of dirt small and well organized..

My mom would have said, "I thought I told you to sweep the floor." My dad would have said:, "What's all the commotion about? That's an amazing view. You're an engineer Jon, you know better than make something simple more complicated just to spread the dirt around."

Jon[/quote]
Jon, and that reason (Secondary support) is the only reason the big boys (big Dob makers) don't push the Curved Vanes as they haven't figured out how to implement such a system.

As for the effects of the Spider in relation to an SCT. Well, again we disagree as I find my C14 has a better Planetary Image than a friends 16" Dob (and he agrees also).

Making something that is simple more complicated is usually the result of a reason. Obviously you don't see any reason so you're good with "simple"
For me there is a reason simple isn't good enough and hopefully one day the big dob makers can figure something out so we all aren't stuck to "simple".

Mike

poithegepur

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Quote

I believe curved spiders are standard on teeter's sts line of scopes.


The one I have has the curved spider. It performs very well and I do not miss diffraction spikes. If anything, it enhances contrast and you do not get distracted by spikes.
This mimics my experience as well when viewing Planets through a friends 10" F5 Teeter.

Mike