My Community

Astrophotography forum => Reflectors Telescopes Forum => Topic started by: postbypopect on December 27, 2017, 10:28:22 PM

Title: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: postbypopect on December 27, 2017, 10:28:22 PM
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 (http://www.astromart.com/classifieds/details.asp?classified_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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: prehconbubun on December 28, 2017, 02:58:40 AM
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: hiswacoka on December 28, 2017, 04:21:50 AM
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: suctoleshe on December 29, 2017, 12:16:49 AM
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Dan Perez on December 29, 2017, 08:54:48 PM
I believe curved spiders are standard on teeter's sts line of scopes.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: asexdalo on December 30, 2017, 10:31:54 AM
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Nathan Mayienda on January 12, 2018, 04:01:32 AM
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Eric Hayes on January 12, 2018, 08:27:46 AM
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Gary Eldridge on January 13, 2018, 03:00:18 AM
Quote
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: trapoutampub on January 13, 2018, 12:00:13 PM
Quote
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Dan Perez on January 13, 2018, 12:59:04 PM
Quote
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: rackramasca on January 21, 2018, 05:29:45 AM
Quote

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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: ithoclirans on January 21, 2018, 06:53:15 AM
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Stephen Gupta on January 23, 2018, 01:40:22 PM
Quote
<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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: poithegepur on January 23, 2018, 03:42:00 PM
Quote
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
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: lolusthoumin on January 25, 2018, 03:41:17 PM
Quote
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Joe Wellard on January 25, 2018, 06:53:23 PM
Quote
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: frosperloacatch on January 31, 2018, 12:19:06 PM
Quote
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 (http://home.digitalexp.com/~suiterhr/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.
(https://s10.postimg.org/ufu108z39/17_attachment_00.gif) (https://postimg.org/image/ufu108z39/)
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: miswalltile on January 31, 2018, 01:08:54 PM
Quote
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: acbrawexel on February 02, 2018, 01:52:23 PM
Quote
Quote

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?
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Lamichael Evans on February 02, 2018, 02:42:15 PM
Quote
Quote

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 (http://www.cloudynights.com/page/articles/cat/articles/how-to/curved-spider-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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Jason Simpson on February 02, 2018, 04:31:46 PM
Quote
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 (http://www.astromart.com/classifieds/details.asp?classified_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 :-)
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: blacosticna on February 03, 2018, 12:55:16 AM
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.
Title: Re: Are curved spider vanes better? And if so, why are they not standard equipment?
Post by: Dave Fair on February 09, 2018, 11:17:06 AM
Quote
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.