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Secondary spider designs?

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#26 careysub

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Posted 23 September 2013 - 02:19 AM

If viewing will be mostly deep sky, then go with 4.
If you plan on using the scope for planetary use, then go curved 3 or 4 vanes.


Why? Is it really better to spread the diffraction across the image rather than have it concentrated in four spikes? I don't think so.
...


If we are talking about visual use then the sensitivity of the human eye to small contrast reductions across an entire image is crucial.

It seems commonly accepted, for example, that illumination drops of 40% at the edge of field are acceptable since human vision is not very sensitive to reductions of this magnitude. (In fact it is often held that trying 100% illumination across the field is bad design, forcing compromises in other areas in return for an insignificant visual effect.)

Remember that the area of the vanes is small compared to that of the secondary, which is already causing contrast loss across the image.

Can the human eye actually detect the difference between, say, a 20% CO with no spider and a 20% CO + 2% spider vane? Is there evidence that it can?

Asserting that it can seems to conflict with statements in Suiter, and also seems unlikely simply due to the human eye usually being insensitive so small incremental changes of this kind. It is commonly held here that CO obstruction equal to or less than 25% causes minimal degradation. This seem inconsistent with the idea that a 20% CO + 2% spider would cause objectionable effects.

One poster here did an experiment a few years ago with different vane designs (mocked up with masks) and found even very thick curved vanes to be undetectable.

Suiter's analyses show very small contrast reductions from spider vanes.

It would seem that the only reason we would be aware of it at all visually is if, by chance, the entire effect where concentrated in a few small areas of the image and in such a way as to activate the eye in a way it is particularly sensitive to.

And that is what straight vanes do. The diffraction energy shows up in a few tin areas of the image, usually in a high contrast way (black background), and in a straight line for which the eye is a very sensitive detector.

Three and four vane spiders concentrate diffraction and thus render it quite visible, and a potential interfering artifact. It does not follow that spreading it over an image in a way that they eye cannot detect it is as bad or worse.

#27 Jon Isaacs

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Posted 23 September 2013 - 06:42 AM

Three and four vane spiders concentrate diffraction and thus render it quite visible, and a potential interfering artifact. It does not follow that spreading it over an image in a way that they eye cannot detect it is as bad or worse.



The eye cannot detect because it is spread out much the way you cannot see the California Nebula in a long focal length scope, it fills the field of view so there is no gradient in the contrast. But the fact that it cannot be identified does not mean it does not affect the contrast. Viewing a planet like Jupiter, the diffraction from the vanes is apparent, spread out or concentrated, it does affect the contrast..

In any event, the job of the spider is to support the secondary in a robust manner. Curved spiders are not as robust as a straight vane spider. In my mind, a spider that shifts as the scope moves around the sky is the concern and that has the potential to cause damage to the image quality.

Jon

#28 dpwoos

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Posted 23 September 2013 - 08:06 AM

Three and four vane spiders concentrate diffraction and thus render it quite visible, and a potential interfering artifact. It does not follow that spreading it over an image in a way that they eye cannot detect it is as bad or worse.


Interesting, and contrary to what I had always assumed. I have to say that I have never noticed any image degradation attributable to spider diffraction, so what you write is consistent with my experience. For sure there are mechanical challenges, but they are easily overcome. My 6" never had any issues, and my 10" vibration problem was easily corrected by adding a second curved vane. Both scopes are top-notch performers, and exhibit zero collimation issues.

#29 Starman1

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Posted 23 September 2013 - 01:00 PM

What percentage of surface is actually covered by thin straight vanes?

Take my 12.5" as an example.
The secondary holder is 2.7" across, per my calipers (2.6" secondary contained therein). That means each of the 0.01" vanes covers an area
(6.25-1.35) x 0.01" in area = 0.049sq.in.
Since there are 4 vanes, that's 0.049X4=0.196sq.in.
The area of a 12.5" mirror is 122.718sq.in.
That means the vanes cover 0.196/122.718=0.16% of the mirror.
Since the secondary covers 4.666% of the primary mirror, the increase is 0.16/4.666=0.03% increase.
Were it not for the linear nature of the diffraction, a 0.03% increase in diffraction would be invisible and might be below the increase in diffraction caused by dust on the mirror if the mirror hasn't been cleaned in a while.

Since each point in an extended image is also damaged by this increase in diffraction, and since bright stars (but not faint stars) display the diffraction spikes, is the degradation of the planetary images affected by this increase in diffraction?

Well, certainly mathematically the effect is insignificant. And, so far as I can tell, the effect is also visually insignificant. Very faint moons are still visible near the planet and planets and lunar details are incredibly sharp in good seeing.

One oddity: the field of view of my telescope is larger than the field of view of my eyepieces. Hence, a very bright star can be outside the eyepiece's field of view, yet still in the field of view of the telescope. I have seen, on nights of extreme clarity and darkness of skies, diffraction spikes extending out from some really bright stars (Sirius comes to mind) as much as a degree and a half away from the star (well more than an extra field of the eyepiece), complete with the wave-like variations in brightness along the lengths of the spike (many dips in brightness, actually) and the chromatic issues present in spikes. As soon as the star passes out of the field of view of the scope, the spikes disappear instantly.

Seeing this, I can totally understand why someone would prefer curved spider vanes. Alas, most such vanes have revealed collimation issues to me in the scopes in which I've looked for collimation changes with altitude. There is probably a size-related issue with curved vanes and weight. Where the cutoff should be, I am not certain, though it would seem, from their lightness of weight, that secondaries under 2" shouldn't suffer much, if anything, in the way of miscollimation with altitude due to flexure in the vanes. As I've posted before, the ")(" arrangement of curved vanes seemed plenty stiff up to 2.1" of secondary, though I don't know about heavier. They would seem to be more immune than some to the flexure that occurs as the scope points low.

#30 Jeff Morgan

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Posted 23 September 2013 - 03:08 PM

In any event, the job of the spider is to support the secondary in a robust manner. Curved spiders are not as robust as a straight vane spider.


I would tend to agree. In search of a "silver bullet" don't lose sight of the basics. Curved spiders are best kept to smaller Newtonians and low mass secondary mirrors.

#31 Kevdog

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Posted 23 September 2013 - 04:07 PM

Looking for input on spider designs. I'm building a 12" truss dob and am going to order a new spider for it. What are the pros and cons of the different designs. 3 or 4 vein, straight versus curved ?

Help me decide what is optimum.

Thanks
Tag


My spiders have 8, I thought they all did!

Posted Image

Of course this one is rather big (4" x 2.5" or so) and you don't want it anywhere near your scope!

#32 careysub

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Posted 28 September 2013 - 10:53 AM

What percentage of surface is actually covered by thin straight vanes?

Take my 12.5" as an example.
The secondary holder is 2.7" across, per my calipers (2.6" secondary contained therein). That means each of the 0.01" vanes covers an area
(6.25-1.35) x 0.01" in area = 0.049sq.in.
Since there are 4 vanes, that's 0.049X4=0.196sq.in.
The area of a 12.5" mirror is 122.718sq.in.
That means the vanes cover 0.196/122.718=0.16% of the mirror.
Since the secondary covers 4.666% of the primary mirror, the increase is 0.16/4.666=0.03% increase.


That should be:
0.16/4.666=0.034 which is a 3.4% increase.

Another way of looking at it: you are going from a 21.6% obstruction to a 22% obstruction. Put this way some one would be hard pressed to make a (supportable) case that this is significant.

#33 careysub

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Posted 28 September 2013 - 10:59 AM

Let us leave aside the separate question of mechanical stability (it is an unrelated issue and as Jeff Morgan indicates, it not a problem for sufficiently small scopes) and focus on the claim that curved spiders are harming contrast in a way that matters to the human eye.

There is no question the additional diffraction affects the contrast at some level, that is simple physics. The question is whether it is a significant effect.

Using Don's example, do you believe that a going from 21.7% obstruction (secondary only) to a 22% obstruction is a meaningful loss of contrast? Contrast loss is the same.

If the tiny additional diffraction from spider vanes is a matter of concern then the size of the secondary should be a matter of much greater concern. It seems generally agreed that below 25% CO it is really not an issue. Why does it become an issue when the cause are the vanes and not the secondary itself?

Logically one should object even more strongly to edge supporting secondary holders since they slightly increase the diameter of the CO, again using Don's example his holder increases the diameter from 2.6" to 2.7", this is a 7.8% area increase, more than twice the area of his vanes.

#34 BillP

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Posted 28 September 2013 - 12:46 PM

There is no question the additional diffraction affects the contrast at some level, that is simple physics. The question is whether it is a significant effect.


Yes. That is indeed the question. However, it is likely that there are no controlled scientific studies to suggest that the small contrast loss IS generally noticeable by the human eye. Conversely, there it is likely that there are no controlled scientific studies to suggest that the small contrast loss IS NOT generally noticeable by the human eye. So any conjecture either way is, and will remain, anecdotal. That being the case, some folks like to err on the side of being conservative, or they simply want to seek perfection making their instrument the best it can possibly be engineered. Nothing wrong at all with the pursuit of perfection. In fact it is admirable! I think this is the case we have here. I doubt the OP is looking for definitive scientific proof that the eye can or cannot see a contrast difference of a few percent. Instead what is important is probably to simply build to the best level technically achievable with his equipment. When one does this, it often brings all the more enjoyment to using the equipment and to the observing session. Just some thoughts on possibilities of the OPs motivations and what is important here.

#35 Starman1

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Posted 28 September 2013 - 01:21 PM

What percentage of surface is actually covered by thin straight vanes?

Take my 12.5" as an example.
The secondary holder is 2.7" across, per my calipers (2.6" secondary contained therein). That means each of the 0.01" vanes covers an area
(6.25-1.35) x 0.01" in area = 0.049sq.in.
Since there are 4 vanes, that's 0.049X4=0.196sq.in.
The area of a 12.5" mirror is 122.718sq.in.
That means the vanes cover 0.196/122.718=0.16% of the mirror.
Since the secondary covers 4.666% of the primary mirror, the increase is 0.16/4.666=0.03% increase.


That should be:
0.16/4.666=0.034 which is a 3.4% increase.

Another way of looking at it: you are going from a 21.6% obstruction to a 22% obstruction. Put this way some one would be hard pressed to make a (supportable) case that this is significant.

Doh! You're right.

#36 Starman1

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Posted 28 September 2013 - 01:37 PM

Let us leave aside the separate question of mechanical stability (it is an unrelated issue and as Jeff Morgan indicates, it not a problem for sufficiently small scopes) and focus on the claim that curved spiders are harming contrast in a way that matters to the human eye.

There is no question the additional diffraction affects the contrast at some level, that is simple physics. The question is whether it is a significant effect.

Using Don's example, do you believe that a going from 21.7% obstruction (secondary only) to a 22% obstruction is a meaningful loss of contrast? Contrast loss is the same.

If the tiny additional diffraction from spider vanes is a matter of concern then the size of the secondary should be a matter of much greater concern. It seems generally agreed that below 25% CO it is really not an issue. Why does it become an issue when the cause are the vanes and not the secondary itself?

Logically one should object even more strongly to edge supporting secondary holders since they slightly increase the diameter of the CO, again using Don's example his holder increases the diameter from 2.6" to 2.7", this is a 7.8% area increase, more than twice the area of his vanes.

That assumes you can make a sufficiently rigid curved spider with vanes only 0.01" thick. In my experience, curved vanes are often up to 0.1" thick and even more. So the coverage percentage is larger.
With equal thickness of vanes, though, your point is quite valid, and curved vanes would be just fine.

As a side note, it's even worse for my secondary mirror holder, as the Astrosystems secondary holder has a lip about 0.1" thick that surrounds the secondary, blocking some of its reflecting surface. I measure approximately 2.4" of exposed surface on the 2.6" mirror, which sits in a holder 2.7" across. So the 2.4" mirror actually covers 2.7" of the primary.
If I glued the mirror to a stalk, the obstruction would drop from 21.6% to 20.8%, and increase my edge-of-field illumination. Well, my current low power eyepiece has its edge illuminated to 75%, so improving that really isn't necessary.
Is it worth it? Only as an intellectual exercise.

I would like to experiment with the )(-shaped spider like on the 12.5" Obsession. It's been rigid enough on the scopes I've seen with that design (some 8" and 10" scopes), and it seems fairly rigid when tightening collimation screws (perhaps a little better than the 4 straight vanes I'm used to). If anyone out there has a 12.5" Obsession, I'd like to know the thickness of the vanes.

#37 AlBoning

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Posted 28 September 2013 - 06:03 PM

Amateur Telescope Optics has equations for calculating the effect on the Strehl Ratio by the vanes and by the central obstruction.

A year or so ago, I was contemplating replacing the spider of an AD8 and also down-sizing the secondary mirror. Lacking both experience and the opportunity to see what effect such changes would have I resorted to doing the calculations using the equations I mentioned above to assist me in coming to a decision. In each case the gain to be had in the Strehl Ratio was 0.02 (or rather the degradation in the Strehl Ratio by the vanes and the secondary obstruction would be each reduced by 0.02). An overall gain of 0.04 may well be regarded as not significant, but OTOH it can be regarded as cost effective.

Consider that in my case, assuming the manufacturer's claim that the primary has a wavefront error of 1/16th wave RMS is correct, the Strehl Ratio of the primary is 0.85 (another equation found at Amateur Telescope Optics). To get a 0.10 improvement in the primary mirror by replacing the primary with premium optics would run from 750 to over a grand depending on where it came from. An alternative would be to have the mirror refigured. Assuming the mirror wouldn't need to be reground then refiguring and coating would run 300+. Now compare this to replacing the spider and downsizing the secondary for a gain of 0.04 costing 300. Which in my mind is reasonable since the MoonLite I put on the scope ran 265. To bring the point home take a look at the cost per 0.01 improvement in the Strehl Ratio. For purchasing a premium mirror it is 75-100, for replacing the spider and downsizing the secondary it is 75.

As it turns out I've reached, what is for me, the point of diminishing returns. That scope is heavily modified already, the only things left to do that would improve it are thin the vanes, and replace the mirrors, but I think I'm going to be quite happy enough leaving it as it is now without spending another dime on it.

#38 Jeff Morgan

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Posted 28 September 2013 - 09:28 PM

Maybe Ed Turco has the right solution ... the optical window.

Well, for the 8" and under crowd anyway.

#39 izar187

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Posted 29 September 2013 - 03:39 AM

Except that even at 8" and under, you need to deal with dew and thermal bottling up of the ota.
For both of which there certainly are countermeasures.
But complexity has increased by two factors not present in an open tube, spidered ota.

#40 careysub

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Posted 29 September 2013 - 01:45 PM

Amateur Telescope Optics has equations for calculating the effect on the Strehl Ratio by the vanes and by the central obstruction.

A year or so ago, I was contemplating replacing the spider of an AD8 and also down-sizing the secondary mirror. Lacking both experience and the opportunity to see what effect such changes would have I resorted to doing the calculations using the equations I mentioned above to assist me in coming to a decision. In each case the gain to be had in the Strehl Ratio was 0.02 (or rather the degradation in the Strehl Ratio by the vanes and the secondary obstruction would be each reduced by 0.02). An overall gain of 0.04 may well be regarded as not significant, but OTOH it can be regarded as cost effective.
...


I had prepared a post using this equation, but you beat me to it. For those interested it is Eq. 64 (after suitable algebraic reformulation) on this page:
http://www.telescope...obstruction.htm

Al does a great job of showing the trade-off considerations on this.

#41 careysub

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Posted 29 September 2013 - 02:02 PM

...
I would like to experiment with the )(-shaped spider like on the 12.5" Obsession. It's been rigid enough on the scopes I've seen with that design (some 8" and 10" scopes), and it seems fairly rigid when tightening collimation screws (perhaps a little better than the 4 straight vanes I'm used to). If anyone out there has a 12.5" Obsession, I'd like to know the thickness of the vanes.


I wonder if spiders that look like this OO (two complete rings) would work using spring steel strips. They would be under tension, and (I think) resistant to vane twisting*. Would the tension make the hoops stiffer?

*A limiting factor in using wide, thin vanes is that unless the vanes are perfectly parallel and perfectly aligned with the optical axis the profile is likely to be much larger than the vane thickness. A 1" wide, 0.01" thick vane will have its profile doubled by a tilt to the optical axis of just 0.57 degrees.

#42 Sarkikos

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Posted 29 September 2013 - 07:06 PM

The Edmund Astroscan and Bushnell Voyager, both 4.5" reflector ball scopes, have an optical window instead of spider vanes.

Mike






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