20 replies to this topic

[GOLGO13]

Comparing my refractors to my newts I've come to the realization that I don't really like deffraction spikes. Has anyone used curved spider vanes and seen a vast improvement? Would they be easy to install and setup?

[Calypte]

This is like a religious argument. If you don't like spikes, nothing I say will change your opinion. But when I discovered astronomy in the 1950s, my books had images by the great reflecting telescopes of the day, and they had diffraction spikes. I always thought of them as a mark of excellence. If spikes were OK with Palomar, then they were fine with me.

[Locoman]

Food for thought! Link

[*skyguy*]

I have a 6" f/8 reflector (Criterion Dynascope RV-6 with the standard straight spider) and a 6" f/10 homemade relector with a small secondary mirror (1") and a curved vane spider. Visually ... using direct comparisons ... the views through the 6" f/10 are far superior to the 6" f/8. The sky background is much darker (better contrast) and bright stars have no diffraction spikes. Planetary and deep-sky views are definitely better in the 6" f/10.

What I'd like to see are some direct comparison images taken through the same scope ... one using a straight spider then one taken with straight spider swapped out for a curve vane spider. It would be interesting to see how much ... if any ... the spread-out diffraction pattern of the curved vane spider would affect the overall quality of the image.

[GOLGO13]

Well...sometimes I also like seeing them in pictures...but I don't really like them at the eyepiece.

Maying cutting down the light a bit would help on planets?

[Starman1]

Note that the total amount of diffraction from spider vanes is directly related to the percentage of primary mirror covered. Curved vanes typically cover more primary, so actually increase the total amount of diffraction.
Note that since the extra diffraction is distributed over the entire field, it might not have much effect on the center. So the background around the planet will be darker.

An analysis of small points in the image (small details) shows that the visibility of small points will not improve with a curved spider.

The ability of a curved spider to hold collimation is also compromised because the curved spider is typically not held under the same tension as straight vanes. There is one curved arrangement that seems to be fairly stable, though and it is the curved arrangement I'd recommend. It also adds little to no extra primary coverage and is easy to make at home.
The shape is 2 curves, like )( with the secondary holder directly in the middle.
It seems to work well with secondaries up to 2.6". Above that, I recommend 4 straight vanes to be collimationally stable.

[GOLGO13]

Thanks guys...saves me some cash and effort

[Moravianus]

Don, anyone making the spider you describe ? The Destiny spider looks to me too brute than needed.

[GeneT]

Many people like them. I have used four vane spider vanes for more than 57 years. They work fine--and I am used to them.

[Chucky]

<< If spikes were OK with Palomar, then they were fine with me. >>

If curved spiders were OK with Clyde, then they were fine with me.

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[John Huntley]

I have curved secondary support vanes, in the pattern that Don describes, in my 12" Orion Optics F/5.3 dobsonian.

I don't see diffraction spikes and this is the first 12" scope I've owned that shows me Sirius B and the rille in the lunar Alpine Valley.

Whether the secondary vanes contribute to the above achievements to a large or small extent I'm not sure but I'm not complaining !

The scope seems to hold collimation very well too.

[Starman1]

Don, anyone making the spider you describe ? The Destiny spider looks to me too brute than needed.

I don't think anyone sells them commercially, but maybe Obsession does.
Here is a picture from the Obsession website:
http://www.obsession.../12.5/index.php
If it doesn't open correctly on your computer, the pic is down the page on the right.
For any spider to be stable, in terms of collimation, it needs to be symmetrical on either side of a central, vertical, line, like "+" or "X" or
"Y". This curved spider satisfies that requirement and can be made very stiff if fairly inflexible material is used. Thin stainless steel (like in a foot long ruler) or aluminum slat would work. Aluminum would have to be formed, while stainless could simply be in tension.
The ends would be bent at a 90 degree angle for attachment to the tube.
The Obsession type could easily replace a straight spider and even use the same attachment holes.

[planet earth]

An analysis of small points in the image (small details) shows that the visibility of small points will not improve with a curved spider.

I've tried a single curved spider in my 8 f7.6, but went back to the straight 4 vane after a year.
Didn't notice even a tiny hint of more fine detail on Jupiter with the curved vane.
Sam

[GeneT]

If spikes were OK with Palomar, then they were fine with me.

If curved spiders were OK with Clyde, then they were fine with me.

[GaryS]

Note that the total amount of diffraction from spider vanes is directly related to the percentage of primary mirror covered. Curved vanes typically cover more primary, so actually increase the total amount of diffraction.

Don's absolutely correct with the first half of this statement. However, the 2nd half isn't necessarily so. If you make a curved spider like the one detailed on my web page (a single arc), you end up with about the same diffraction as a 3-vane spider made with the same vane material.

The ability of a curved spider to hold collimation is also compromised because the curved spider is typically not held under the same tension as straight vanes.

The best straight-vane spiders actually are built so that they don't have to be under tension. You can make good or bad spiders of both types, one doesn't have to be inferior to the other. The curved vane spiders I (and many other ATMs) make and the ones Bryan Greer sells hold collimation just fine. As with anything in ATM'ing, you have to do a good job to avoid problems.

Gary

[Jarad]

I have had 2 scopes with curved vanes (10" with double-C style that Don likes, 14.5" Portaball with 3 60-degree arcs), and one with straight vanes (18" Starmaster).

This is essentially an aesthetic issue. If you don't like spikes, then go ahead and get a curved vane spider. It does eliminate the spikes. You will see an even glow around bright objects like Jupiter or Venus, and nothing around everything else. As others have noted, the total diffraction will be similar or slightly higher, but the difference is going to be small. You will see about the same amount of detail either way - the total difference due to spider diffraction is going to be small compared to the other, bigger issues that are more limiting (i.e. optical quality, thermal equilibrium, atmospheric seeing, etc.).

If the spikes don't bother you, then stick with a straight vane.

If you really want to minimize total diffraction, then switch to a wire spider. But before you spend money on that, make sure you have already gotten premium optics, good thermal control, and good baffling because those are usually much bigger issues than spider diffraction.

Jarad

[Starman1]

Well, it depends on what you mean by "hold collimation". I interpret that to mean "no visual change of collimation in an autocollimator over at least 50 degrees of altitude movement".

By that standard, the ONLY spider that passes with a secondary as big as 2.6-3.1" is a straight vane spider with the vanes under VERY high tension.

The reason is simple--the off-center torque of the secondary weight imparts a twist to the thin spider vanes as the scope lowers in altitude, and results in visible changes in collimation.

Now, one can reasonably argue that the amount of collimation change that occurs is within tolerances so long as the scope has a reasonable f/ratio of, say, f/5 and longer. But in this era of F/2.75-f/4 scopes, the tolerances are a lot tighter, and changes in Focuser Axial Error with altitude change are less 'tolerable'.

A little engineering to address this problem requires concentration on truss pole diameter and thickness, the rigidity of the truss pole attachments, sag in the UTA itself, twist in the spider, and movement in the focuser board and focuser drawtube. In rebuilding my previous scope, I learned a lot about how to make a truss scope more rigid, and was successful at eliminating visible autocollimator collimation changes from 20 to 90 degrees.
I was less successful below 20 degrees, but I so rarely observe there I didn't care.

Since I did that redesign, I have thoroughly checked hundreds of other scopes for the same issues. I found that loose spiders were the #1 cause of collimation changes with altitude, and also the most easily solved.
Like Rob Teeter, I found that collimation stability requires a symmetry of spider on either side of the center line. Many scopes with odd-angle secondary spider attachments could not be sufficiently beefed up or tensioned to avoid collimation changes.

The goal, I suppose, is not to eliminate ALL collimation changes as the scope moves, but to allow only enough movement that the scope stays in good collimation over the usable range. It is an issue addressed by only a few manufacturers, alas, and therein lies one of the best reasons for experimentation of methods to eliminate collimation changes with altitude.

[Zamboni]

Well, it depends on what you mean by "hold collimation". I interpret that to mean "no visual change of collimation in an autocollimator over at least 50 degrees of altitude movement".

By that standard, the ONLY spider that passes with a secondary as big as 2.6-3.1" is a straight vane spider with the vanes under VERY high tension.

The reason is simple--the off-center torque of the secondary weight imparts a twist to the thin spider vanes as the scope lowers in altitude, and results in visible changes in collimation.

Now, one can reasonably argue that the amount of collimation change that occurs is within tolerances so long as the scope has a reasonable f/ratio of, say, f/5 and longer. But in this era of F/2.75-f/4 scopes, the tolerances are a lot tighter, and changes in Focuser Axial Error with altitude change are less 'tolerable'.

A little engineering to address this problem requires concentration on truss pole diameter and thickness, the rigidity of the truss pole attachments, sag in the UTA itself, twist in the spider, and movement in the focuser board and focuser drawtube. In rebuilding my previous scope, I learned a lot about how to make a truss scope more rigid, and was successful at eliminating visible autocollimator collimation changes from 20 to 90 degrees.
I was less successful below 20 degrees, but I so rarely observe there I didn't care.

Since I did that redesign, I have thoroughly checked hundreds of other scopes for the same issues. I found that loose spiders were the #1 cause of collimation changes with altitude, and also the most easily solved.
Like Rob Teeter, I found that collimation stability requires a symmetry of spider on either side of the center line. Many scopes with odd-angle secondary spider attachments could not be sufficiently beefed up or tensioned to avoid collimation changes.

The goal, I suppose, is not to eliminate ALL collimation changes as the scope moves, but to allow only enough movement that the scope stays in good collimation over the usable range. It is an issue addressed by only a few manufacturers, alas, and therein lies one of the best reasons for experimentation of methods to eliminate collimation changes with altitude.

These are fair points, but almost entirely irrelevant when discussing the specific scopes the OP is asking about. For solid tube 10" and 6" reflectors slower than f/4, the flexure issues you talk about are not going to be even measurable.

As was previously noted, Protostar single-arc curved spiders cause diffraction roughly equivalent to a 3 vane spider of the same material, and in smaller scopes they stay very stiff so alignment fluctuations are negligible.

I you're replacing a 4-vane spider with a single-arc curved spider, you wind up with LESS overall diffraction in addition to the elimination of diffraction spikes.

[Calypte]

These are fair points, but almost entirely irrelevant when discussing the specific scopes the OP is asking about. For solid tube 10" and 6" reflectors slower than f/4, the flexure issues you talk about are not going to be even measurable.

I have a Cave 12.5-inch f/5 reflector whose original spider flexed and twisted as the scope was moved around the sky. Collimation was impossible. I replaced the spider with a Novak spider (I think Novak is long OB), which solved the problem. I don't consider spider flexure to be a trivial issue.

[Tom and Beth]

Did you read the link Locoman posted above?

[Starman1]

These are fair points, but almost entirely irrelevant when discussing the specific scopes the OP is asking about. For solid tube 10" and 6" reflectors slower than f/4, the flexure issues you talk about are not going to be even measurable.

If you're replacing a 4-vane spider with a single-arc curved spider, you wind up with LESS overall diffraction in addition to the elimination of diffraction spikes.

For the 6", maybe.
For the 10", though, it will have a large-enough secondary to where torque caused by the secondary weight will cause collimation changes with altitude.
The collimation changes ARE measurable and ARE visible, even, in an autocollimator. I can reliably see collimation errors as small as a couple thousandths of an inch--.05mm. (The maximum deviation from center for a laser beam in a 10" f/4 (FAE tolerances) is 0.32mm; at f/5, 0.63mm) . To hold that level of collimation over 50-70 degrees of altitude swing will require a very stiff secondary spider. It would be a lot easier in a 6".
The spider vanes will have to be substantially thicker, in the single-arc spider than they would in a 4-vane straight spider, so I wouldn't be too sure the overall primary coverage would be less. A little back-of-the-envelope calculation says the single arc would be a better design for the f/7-f/10 scopes of yesteryear than today's f/4-f/5. At f/10, the FAE tolerance is quite loose, at 5mm!