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Are curved spider vanes better? And if so, why are they not standard equipment?

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#51 SteveG

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Posted 17 July 2018 - 03:04 PM

I suppose there is always the potential of kidding youself that your 10" Newt with a curved spider is a 10" APO refractor.

Placing a Paracorr in it along with some quality eyepieces brings you there, provided your optics are good.



#52 SteveG

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Posted 17 July 2018 - 03:12 PM

Here is one I've pondered: This would only work for dobs probably but what about a single vane? It would protrude from either the bottom or top (picture the dob pointed to the horizon) so there would be no lateral load. It would be a stiff material that might be thicker to a degree but would only need one leg.
The old Coulters used such a system but ran the vane across the entire diameter.
This is two vanes, of course.
16 ga by two inches maybe?

Steve O.

Single vane only works on smaller scopes, and produces a huge diffraction spike that goes right through the center fov, on both sides of the object. It is the worst option IMO.



#53 Starman1

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Posted 17 July 2018 - 04:23 PM

Here is one I've pondered: This would only work for dobs probably but what about a single vane? It would protrude from either the bottom or top (picture the dob pointed to the horizon) so there would be no lateral load. It would be a stiff material that might be thicker to a degree but would only need one leg.
The old Coulters used such a system but ran the vane across the entire diameter.
This is two vanes, of course.
16 ga by two inches maybe?

Steve O.

A single vane or the vane going all the way across will yield the same diffraction spike, extending both directions from the star.


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#54 Starman1

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Posted 17 July 2018 - 04:28 PM

I use curved spiders in my 16 inch telescope and I'm very glad that I gave it a shot. So far, I didn't encounter any visible problems/issues. I use 3 polished stainless steel vanes with sufficient stiffness to keep the secondary mirror in its place regardless the altitude of the observe object. And to be honest I really like those bright stars without spikes!

 

attachicon.gif 20180311_175749 klein.jpg

You left the vanes polished to cool to the ambient temperature faster.

And, the polish will not matter to a star in the center, where diffraction is solely due to the coverage of the primary by the area covered by the vanes.

But for anything off axis, the light will be reflected by the vanes, and reflected at a low angle, resulting in reflected light very close to the object or star.

That low angle of reflection is why flat black paint in the tube usually is inferior to flocking, where contrast is concerned.

And it explains why nearly ever telescope tries to reduce the reflectivity of the sides of the spider vanes.

For everything except the axial image, polished spider vanes will compromise contrast.



#55 Starman1

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Posted 17 July 2018 - 04:41 PM

Spider vane diffraction:

https://www.cloudyni...-what-to-avoid/

http://www.fpi-proto.../img/spikes.gif

https://www.google.c...VHgWe4f7jmhKkM:

The area of mirror coverage determines the total amount of diffraction caused by the spider.

The shape of the vanes determines where the diffraction is relative to the star.

The idea of spreading out the diffraction by using curved vanes is an old one, and it works, usually, in small scopes.

But when the secondary nears 3" in diameter, its weight usually results in collimation changes with altitude for any spider vane system 

that is not under high tension, and curved vanes usually have little to no tension on them.  Stiffness is gained with thicker vanes, but that leads to more overall diffraction.

So it's a compromise.  How much do spikes on bright stars bother you?  Does it bother you more than having collimation change as the scope moves up and down?

How you answer that will determine what you choose to do in a 12" scope or larger.



#56 spaceoddity

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Posted 17 July 2018 - 05:06 PM

My 16" dob has a curved spider, seems to work just fine. Diffraction spikes don't bother me but I bought it used and it came that way. So far I haven't seen any reason to consider changing it.



#57 Jon Isaacs

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Posted 17 July 2018 - 05:09 PM

Here is one I've pondered: This would only work for dobs probably but what about a single vane? It would protrude from either the bottom or top (picture the dob pointed to the horizon) so there would be no lateral load. It would be a stiff material that might be thicker to a degree but would only need one leg.
The old Coulters used such a system but ran the vane across the entire diameter.
This is two vanes, of course.
16 ga by two inches maybe?

Steve O.

Back in the day, single stalk supports for the secondary were pretty standard on 6 and 8 inch Dobs.  Just a thick wire, to adjust the secondary, you tried to bend the wire..  I had a Pirate Instruments 8 inch F/6, I tried bending the stalk, it shot off into the distance.  Damaged the secondary.  The next step was a new secondary with a 4 vane spider.

 

The Celestron Sonotube 6 and 8 inch Dobs had single stalk secondary supports but they were cast and had adjustment screws for tilt.

 

Jon



#58 Starman1

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Posted 17 July 2018 - 05:25 PM

My first 6" f/5 scope had a single-vane supported the secondary, which had 3 screws to adjust it.

The vane slid up and down the tube with the "sled" focuser.

The single vane did fine with the 2" secondary because it was 1/4" thick aluminum.

Definitely not optimum, but it worked.  Collimation shifted as the focuser went up and down the tube because the secondary was centered.

I built a 6" f/5 after that that had a regular 2" focuser and a 3-vane spider.

Definitely better.


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#59 lphilpot

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Posted 17 July 2018 - 06:45 PM

Sounds like a late-80s C6...? I have a Comet Catcher with the same secondary / sled focuser. Good enough for that f/3.6 SN, but like you said not optimal.

 

On my new 12" f/5 I've not (yet) noticed any huge collimation shift through the altitude swing, but the few short looks from my humid and mosquito-infested patio were hardly exhaustive. Besides, I was still working out finder scope (mount) alignment and such, not really critically observing. I'll have to take a closer look once I get to a darker site. Maybe next month.

 

But I'm gonna do my best to enjoy what I have. If it turns out that there's too much shift, I'll deal with it, but despite my perfectionist tendencies I'm trying to stay cool about it.   :)


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#60 earlyriser

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Posted 17 July 2018 - 06:55 PM

If the spikes are a bother, how about using a glass window to support the secondary? Sure, it's another optical element, but if it's flat and multicoated, it shouldn't be a problem. Plus, it would keep dust and pollen off the mirror.



#61 lphilpot

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Posted 17 July 2018 - 07:21 PM

Flat optical windows in any moderate aperture aren't cheap... But they can be a nice solution. I had a friend with a killer 8" f/10 planetary Newt that had an optical window. Unfortunately one time during scope maintenance it rolled down the bench... right on to a concrete floor (with predictable results).



#62 Starman1

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Posted 17 July 2018 - 07:26 PM

Sounds like a late-80s C6...? I have a Comet Catcher with the same secondary / sled focuser. Good enough for that f/3.6 SN, but like you said not optimal.

 

On my new 12" f/5 I've not (yet) noticed any huge collimation shift through the altitude swing, but the few short looks from my humid and mosquito-infested patio were hardly exhaustive. Besides, I was still working out finder scope (mount) alignment and such, not really critically observing. I'll have to take a closer look once I get to a darker site. Maybe next month.

 

But I'm gonna do my best to enjoy what I have. If it turns out that there's too much shift, I'll deal with it, but despite my perfectionist tendencies I'm trying to stay cool about it.   smile.gif

Some designs are quite stiff in that aperture, for example: )o( as used in the Obsession 12.5", for example.

And, there is always the possibility of counterweighting the secondary mirror with equal weight on the top side of the spider.

That would prevent the gravitational sag that often occurs when the scope points low and all the weight is on one side of the spider.

So there is usually a solution even if you do run into collimation shift.

Don



#63 doug mc

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Posted 18 July 2018 - 01:27 AM

Correct me if i am wrong. A 3 vane has the potential of 6 spikes, but each spike has half of the energy of the usual 4 vane spider. Total diffraction would only be slightly less, but the spikes would be less obvious.


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#64 earlyriser

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Posted 18 July 2018 - 04:21 AM

Correct me if i am wrong. A 3 vane has the potential of 6 spikes, but each spike has half of the energy of the usual 4 vane spider. Total diffraction would only be slightly less, but the spikes would be less obvious.

I think this is correct.

 

This got me thinking, what about two vanes meeting at a 90 degree angle at the secondary? Each vane would prevent the other from flexing radially relative to the axis of the tube, so that arrangement should be pretty stiff and would have half the diffraction of the regular 4-vane spider.



#65 MrJones

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Posted 18 July 2018 - 08:30 AM

I use curved spiders in my 16 inch telescope and I'm very glad that I gave it a shot. So far, I didn't encounter any visible problems/issues. I use 3 polished stainless steel vanes with sufficient stiffness to keep the secondary mirror in its place regardless the altitude of the observe object. And to be honest I really like those bright stars without spikes!

 

attachicon.gif 20180311_175749 klein.jpg

Marvelous! I like how people tell you it won't work on a 16", you show them it does and they still say it won't work on a 16". And the ones that obsess over small eyepiece aberrations trying to rationalize away huge diffraction spikes are just bizarre. I love the lack of the spikes on my 12" and am also trying to work out a curved vane solution for my RC8. How are the bare vanes and apparently bare secondary working out? I've been thinking of removing the paint from mine. I remember a story about a professional observatory doing this but can't remember the details. Maybe it was in S&T?



#66 pjmulka

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Posted 18 July 2018 - 08:41 AM

20170614_151401 klein.jpg


Wooooo nice knobs!

#67 pjmulka

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Posted 18 July 2018 - 09:00 AM

I built the curved vein spider from the plans on Stellafane's website and the thing is solid as a rock. Granted it's only holding a little 1.3" secondary for a 6" mirror but I have no complaints. With one curved vein I can't imagine there is any more diffraction than 4 veins. and doesn't bending the ruler into an arch put it under tension?
The one complaint I had with the original design is that it only has one mounting hole in the center of each mounting tab. I was never able to tighten the bolt enough against the sonotube to stop it from rotating slightly. I stumbled upon a CN Post where I saw a curved vein with two mounting holes on each tab one on each end that solved the problem perfectly.
what about the curved vein spiders that are bent into a repeating sine wave the terms them into a tension spring when you tighten them?

#68 Jon Isaacs

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Posted 18 July 2018 - 09:29 AM

Sounds like a late-80s C6...? I have a Comet Catcher with the same secondary / sled focuser. Good enough for that f/3.6 SN, but like you said not optimal.

 

On my new 12" f/5 I've not (yet) noticed any huge collimation shift through the altitude swing, but the few short looks from my humid and mosquito-infested patio were hardly exhaustive. Besides, I was still working out finder scope (mount) alignment and such, not really critically observing. I'll have to take a closer look once I get to a darker site. Maybe next month.

 

But I'm gonna do my best to enjoy what I have. If it turns out that there's too much shift, I'll deal with it, but despite my perfectionist tendencies I'm trying to stay cool about it.   smile.gif

 

 My 16 inch Dobstuff had too much collimation shift and it was prone to vibration even though the struts were 2 inch diameter.  I added tensioning rods that triangulated the struts, made a real difference.  I don't have any good photos but you can see the rods in this photo and the turnbuckle in the foreground that I use to adjust the tension.  

 

Dobstuff Finder Mount 2.jpg
 
I replaced the center bolt on the three permanent short struts with an eyebolt..  
 
I also did this:  
 
4914398-Dobstuff Strut Clamp Detail.jpg
 
One has to be careful but it did stiffen things up.
 
Jon

 


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#69 Pierre Lemay

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Posted 18 July 2018 - 12:49 PM

You left the vanes polished to cool to the ambient temperature faster.

And, the polish will not matter to a star in the center, where diffraction is solely due to the coverage of the primary by the area covered by the vanes.

But for anything off axis, the light will be reflected by the vanes, and reflected at a low angle, resulting in reflected light very close to the object or star.

That low angle of reflection is why flat black paint in the tube usually is inferior to flocking, where contrast is concerned.

And it explains why nearly ever telescope tries to reduce the reflectivity of the sides of the spider vanes.

For everything except the axial image, polished spider vanes will compromise contrast.

Don, in his 1949 paper (http://www.astrosurf...thermique_e.htm) André Couder discusses the impact of using polished vanes and their impact on the reduction in contrast due to the reflection of the sky and stars off the polished vanes when compared to "flat black" paint painted ones. I put "flat black" in brackets because the question Couder raises, with regards to the paints of the time, is the flat black effect on grazing light when compared to fully polished metal. He indicates that, at the time, paints were not very absorbant of light when the light was coming in at a grazing angle, which would be the case for a spider vane. He says there would be a decrease in contrast with polished vanes, but also with flat black ones, albeit less, due to the reflexion of light at grazing angles on flat black paint. Maybe today's flat black paints are more absorbant at grazing angles? But probably not.

 

The advantages of polished vanes on the apparent thickness of the vanes when compared to equally thick flat black ones in rapidly cooling night environments has been demonstrated by Couder. I would like to see a quantative study of this phenomenon both on the impact of polished vanes on the apparent thickness of the vanes, due to the swelling of the turbulent air around flat black vanes compared to polished ones and, the effect of contrast increase of polished vanes compared to painted ones.

 

I hope to test this in the coming years but it will have to wait until I get my current ATM projects under control.


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#70 SteveG

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Posted 18 July 2018 - 12:55 PM

If the spikes are a bother, how about using a glass window to support the secondary? Sure, it's another optical element, but if it's flat and multicoated, it shouldn't be a problem. Plus, it would keep dust and pollen off the mirror.

I wanted to do this with a 6" f8 project. I couldn't find one for sale commercially, and read that I would have to make it myself.



#71 MrJones

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Posted 18 July 2018 - 02:02 PM

Don, in his 1949 paper (http://www.astrosurf...thermique_e.htm) André Couder discusses the impact of using polished vanes and their impact on the reduction in contrast due to the reflection of the sky and stars off the polished vanes when compared to "flat black" paint painted ones. I put "flat black" in brackets because the question Couder raises, with regards to the paints of the time, is the flat black effect on grazing light when compared to fully polished metal. He indicates that, at the time, paints were not very absorbant of light when the light was coming in at a grazing angle, which would be the case for a spider vane. He says there would be a decrease in contrast with polished vanes, but also with flat black ones, albeit less, due to the reflexion of light at grazing angles on flat black paint. Maybe today's flat black paints are more absorbant at grazing angles? But probably not.

 

The advantages of polished vanes on the apparent thickness of the vanes when compared to equally thick flat black ones in rapidly cooling night environments has been demonstrated by Couder. I would like to see a quantative study of this phenomenon both on the impact of polished vanes on the apparent thickness of the vanes, due to the swelling of the turbulent air around flat black vanes compared to polished ones and, the effect of contrast increase of polished vanes compared to painted ones.

 

I hope to test this in the coming years but it will have to wait until I get my current ATM projects under control.

The other possibly bigger issue than paint as an insulator is the much lower emissivity of bare metal vs. painted and even lower emissivity when highly polished. In the article I remember reading, the professional astronomers had trouble with dew forming on the painted vanes of their scope and removing it greatly reduced or even solved the problem for them.



#72 Starman1

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Posted 18 July 2018 - 02:13 PM

Spider vanes are so thin, though, temperature differentials disappear quickly without active cooling.

It would seem that cooling of the spider vanes is really only an issue to observers who want to take their reflectors outside from a warm house

and start observing the planets at high power immediately.

As for dew on the spider vanes, if the problem is encountered, the solution is simple--put them deeper in the tube, i.e. use a cylindrical light shield above them.



#73 Jeff Morgan

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Posted 18 July 2018 - 02:51 PM

Don, in his 1949 paper (http://www.astrosurf...thermique_e.htm) André Couder discusses the impact of using polished vanes and their impact on the reduction in contrast due to the reflection of the sky and stars off the polished vanes when compared to "flat black" paint painted ones. I put "flat black" in brackets because the question Couder raises, with regards to the paints of the time, is the flat black effect on grazing light when compared to fully polished metal. He indicates that, at the time, paints were not very absorbant of light when the light was coming in at a grazing angle, which would be the case for a spider vane. He says there would be a decrease in contrast with polished vanes, but also with flat black ones, albeit less, due to the reflexion of light at grazing angles on flat black paint. Maybe today's flat black paints are more absorbant at grazing angles? But probably not.

 

The advantages of polished vanes on the apparent thickness of the vanes when compared to equally thick flat black ones in rapidly cooling night environments has been demonstrated by Couder. I would like to see a quantative study of this phenomenon both on the impact of polished vanes on the apparent thickness of the vanes, due to the swelling of the turbulent air around flat black vanes compared to polished ones and, the effect of contrast increase of polished vanes compared to painted ones.

 

I hope to test this in the coming years but it will have to wait until I get my current ATM projects under control.

 

I was intrigued by this paper too. Anyone that has ever woken up to a frosted car knows that under the right conditions certain objects/materials can indeed cool well below ambient air temperature.

 

On an 8" project scope I left opposing stainless steel vanes untreated and painted the pair oriented 90 degrees to them. My reasoning was that on bright stars I should see the "vertical" spike more prominently than the "horizontal" spike (those spatial references being arbitrary for the sake of illustration).

 

In the short time I used that scope nothing prominent jumped out at me (qualitatively) on 1st magnitude stars.

 

  • It could be that the stainless steel vanes were bare metal and not polished. With aluminum, one does not get favorable emissivity without an actual polish. Stainless steel may behave the same way.

 

  • It could be that the effect is small and I was not looking rigorously for a difference (and I was not).

 

  • It could be that the cooling takes more time to manifest itself and fell outside of the window of session length.

 

Intellectually I am curious about the "Couder Effect". However, following developments on the ATM forum with offset wire spiders I concluded that could be a far better spider approach. I've incorporated one into my latest project.

 

EDIT - It could also be that Couder was wrong.


Edited by Jeff Morgan, 18 July 2018 - 02:53 PM.


#74 lphilpot

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Posted 18 July 2018 - 08:35 PM

 

 My 16 inch Dobstuff had too much collimation shift and it was prone to vibration even though the struts were 2 inch diameter.  I added tensioning rods that triangulated the struts, made a real difference.  I don't have any good photos but you can see the rods in this photo and the turnbuckle in the foreground that I use to adjust the tension.  

 

I replaced the center bolt on the three permanent short struts with an eyebolt..  
 
I also did this:  
 
One has to be careful but it did stiffen things up.
 
Jon

Mine came with "strings" as Dennis puts it, (i.e., cables with a turnbuckle):

 

side_view.jpg

 

end_ring.jpg

 

Since they're a pair (only) and pull (only) as opposed to your push-pull arrangement, they're not quite as versatile, but they're also lighter weight. That's important since without the counterweight it's functionally balanced and I want to be very careful about how much weight to add on the end of a substantial moment-arm. It would suck to have to add (much more) weight to the back of the "mirror cylinder", but I also have about an inch that I could move the alt bearings if necessary. I also thought about increasing the clamping power by cutting a few (larger) slots, but I've not gone there yet.

 

I've not spent any critical observing time with it yet, but I'm kinda encouraged by a mistake I made. I apparently tightened the "strings" too much and when they were later relaxed, it was clear the primary (when collimated under string influence) was actually pointing "up" toward the focuser / finder area instead of dead down the center of the 'un-strung' OTA. So it had flexed the tubes back a bit. That's encouraging since it showed me there's room for adjustment. I'm thinking I'll assemble it but leave the strings loose, then collimate it with the OTA as vertical as possible. Then, I'll drop it horizontal and see how much it creeps, then just now much string tension is required to bring it (roughly) back. That should give me a better feel for how tight to tension them.

 

Anyway, it's been HOT, HOT, HOT and humid, humid, humid and cloudy, cloudy, cloudy... well, you get the point.  smile.gif  So in my spare time I've been 'completing' the scope. Not that Dennis didn't do his part - I just elected to add the finders, DewBuster, etc., etc. Plus, I'm attempting to tweak a fundamentally dry-climate design into surviving our sub-marine viewing conditions down here.  :)  All of which I knew going into the purchase, so it's all good. I need to make light shroud (I was turned down by Heather Teeter lol.gif ), get a star party cover, add a battery tray to the front of the rocker box, yada yada yada. Quite a few other little tweaks. If it weren't so bloomin' hot I'd enjoy working on it!


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#75 MrJones

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Posted 18 July 2018 - 09:09 PM

I was intrigued by this paper too. Anyone that has ever woken up to a frosted car knows that under the right conditions certain objects/materials can indeed cool well below ambient air temperature.

 

On an 8" project scope I left opposing stainless steel vanes untreated and painted the pair oriented 90 degrees to them. My reasoning was that on bright stars I should see the "vertical" spike more prominently than the "horizontal" spike (those spatial references being arbitrary for the sake of illustration).

 

In the short time I used that scope nothing prominent jumped out at me (qualitatively) on 1st magnitude stars.

 

  • It could be that the stainless steel vanes were bare metal and not polished. With aluminum, one does not get favorable emissivity without an actual polish. Stainless steel may behave the same way.

 

  • It could be that the effect is small and I was not looking rigorously for a difference (and I was not).

 

  • It could be that the cooling takes more time to manifest itself and fell outside of the window of session length.

 

Intellectually I am curious about the "Couder Effect". However, following developments on the ATM forum with offset wire spiders I concluded that could be a far better spider approach. I've incorporated one into my latest project.

 

EDIT - It could also be that Couder was wrong.

Not a fan. I'm all for the crankpots giving it a shot but it's been 70 years. Also not a fan of the ignorant forum rats immediately criticizing some guy who's nice enough to post a good looking original creation here. I have to think there is something to reducing emissivity of the secondary structure though. Reducing dew and frost seems like an immediate benefit and well if I ever find that article I will post it.

 

And looks good lphilpot but also looks like a Destiny spider. The downfall of many curved vane systems seems to be the implementation and not the design and the Destinys well. I put these guys on mine to help lock down the push screws and recommend it!

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