Curved or straight vane spiders in planetary Newtonians?

The arguments over the optimum, secondary mirror, spider vane form continue.

Some claim that curved spiders reduce diffraction effects to nil.
At least compared with 3 and 4 legged {straight vane] spiders.

While others claim the diffraction effects are merely smeared into the object's image. Thereby reducing contrast and fine detail.

Four vane spiders are claimed to offer the greatest stiffness in use.
Helping to maintain collimation at the expense of "crucifying" bright objects.

A third type involves thin tension wires but is much less popular.

Do the members have any opinions based on personal experience on the efficacy of the different types of secondary spider form?

On Sunday, February 1, 2015 at 5:47:00 AM UTC-5, Chris.B wrote:
The arguments over the optimum, secondary mirror, spider vane form continue.

Some claim that curved spiders reduce diffraction effects to nil.
At least compared with 3 and 4 legged {straight vane] spiders.

While others claim the diffraction effects are merely smeared into the object's image. Thereby reducing contrast and fine detail.

Four vane spiders are claimed to offer the greatest stiffness in use.
Helping to maintain collimation at the expense of "crucifying" bright objects.

A third type involves thin tension wires but is much less popular.

Do the members have any opinions based on personal experience on the efficacy of the different types of secondary spider form?

Four vane is to be preferred over three vane. Thin wires can be difficult to implement properly, and are rare. Curved vanes are also rare, but some who have tried them might swear by them, for whatever -subjective- reasons they might have.

An "oversized" secondary does far more damage anyway.

The best way to see finer detail is to use a larger telescope, AEBE.

On Sunday, 1 February 2015 15:50:20 UTC+1, wrote:

Four vane is to be preferred over three vane. Thin wires can be difficult to implement properly, and are rare. Curved vanes are also rare, but some who have tried them might swear by them, for whatever -subjective- reasons they might have.

An "oversized" secondary does far more damage anyway.

The best way to see finer detail is to use a larger telescope, AEBE.

Thanks, but larger apertures are much less forgiving of poor seeing due to the cellular nature of atmospheric disturbance. The optimum aperture in typical/average seeing is probably between 8" and 12". [20-30cm] Anything larger becomes extremely cumbersome [and expensive] on a suitably robust equatorial mount.

The secondary m.a. for an optimized planetary Newtonian will usually be chosen to provide adequate illumination over [say] a 1/2" 10mm field. This will ensure that the secondary obscures no more than the 20% of clear aperture before image quality is lost. A long focal length and low profile focuser will allow a smaller secondary to be used. Very small secondaries risk reducing the available aperture if taken too far.

On Sun, 1 Feb 2015 02:46:57 -0800 (PST), "Chris.B"
wrote:

Some claim that curved spiders reduce diffraction effects to nil.
At least compared with 3 and 4 legged {straight vane] spiders.

That claim is patently false. A curved spider simply changes the
nature of the diffraction. You can't fool Mother Nature.

While others claim the diffraction effects are merely smeared into the object's image. Thereby reducing contrast and fine detail.

The first part of this is certainly true. As is the second, although
this is equally true for straight spiders.

Technically, you evaluate this by looking at the PSF and MTF of the
system. And you look at the intent of the instrument. Visually, it
isn't going to be an issue except for planetary viewing. For DSOs, we
always observe at a much lower resolution than the optics are capable
of. For imaging, these are much more important concerns, because we
are typically resolution limited by either the optics or the seeing.

For a planetary Newtonian I would expect aperture, optical quality, collimation, and seeing to be more critical than the differences in the diffraction effects of the different spiders. That being said, I would choose the old, reliable, sturdy, straight, four-vane spider.

Sketcher,
To sketch is to see.

On Sunday, February 1, 2015 at 3:47:00 AM UTC-7, Chris.B wrote:
The arguments over the optimum, secondary mirror, spider vane form continue.

Some claim that curved spiders reduce diffraction effects to nil.
At least compared with 3 and 4 legged {straight vane] spiders.

While others claim the diffraction effects are merely smeared into the object's image. Thereby reducing contrast and fine detail.

Four vane spiders are claimed to offer the greatest stiffness in use.
Helping to maintain collimation at the expense of "crucifying" bright objects.

A third type involves thin tension wires but is much less popular.

Do the members have any opinions based on personal experience on the efficacy of the different types of secondary spider form?

On Sun, 1 Feb 2015 14:24:57 -0800 (PST), Sketcher
wrote:

For a planetary Newtonian I would expect aperture, optical quality, collimation, and seeing to be more critical than the differences in the diffraction effects of the different spiders. That being said, I would choose the old, reliable, sturdy, straight, four-vane spider.

It's worth noting that this is the design of choice of nearly all
professional telescopes with monolithic mirrors, both ground and space
based. Four-vane spiders produce the simplest diffraction patterns,
and professionals frequently make rigorous compensation for
diffraction effects. Telescopes that have mirrors made of hexagonal
segments typically use six-vane spiders, with the vanes aligned with
mirror segment edges. Again, this produces the simplest diffraction
pattern.

So while there are good practical reasons for a four-vane spider (as
you itemize), even where very large budgets and superior engineering
resources are available, we seldom see anything else. That should tell
us something.

On Sunday, 1 February 2015 23:37:07 UTC+1, Chris L Peterson wrote:
On Sun, 1 Feb 2015 14:24:57 -0800 (PST), Sketcher
wrote:

For a planetary Newtonian I would expect aperture, optical quality, collimation, and seeing to be more critical than the differences in the diffraction effects of the different spiders. That being said, I would choose the old, reliable, sturdy, straight, four-vane spider.

It's worth noting that this is the design of choice of nearly all
professional telescopes with monolithic mirrors, both ground and space
based. Four-vane spiders produce the simplest diffraction patterns,
and professionals frequently make rigorous compensation for
diffraction effects. Telescopes that have mirrors made of hexagonal
segments typically use six-vane spiders, with the vanes aligned with
mirror segment edges. Again, this produces the simplest diffraction
pattern.

So while there are good practical reasons for a four-vane spider (as
you itemize), even where very large budgets and superior engineering
resources are available, we seldom see anything else. That should tell
us something.

All very good points. Thanks. The main problem I discovered with single curve spiders was severe vibration of the secondary. This required beefing up the original vane to use a sturdy, stainless steel rule. As the curved vane thickness increases one presumes any supposed diffraction advantages over a nicely thin, 4-vane spider would be lost anyway.

The alternative of a three vane curved spider would seem to be a solution looking for a problem to solve. The three vanes would probably have several kinds of diffraction problems to deal with. As well as reduced rigidity, the ability to keep the vanes perfectly aligned with the optical axis might greatly increase their damaging obscuration. Particularly since curved vanes need to be made deeper simply to increase their natural stiffness when not under beneficial tension loads.

On 02/02/2015 11:00, Chris.B wrote:
On Sunday, 1 February 2015 23:37:07 UTC+1, Chris L Peterson wrote:
On Sun, 1 Feb 2015 14:24:57 -0800 (PST), Sketcher
wrote:

For a planetary Newtonian I would expect aperture, optical quality, collimation,
and seeing to be more critical than the differences in the
diffraction effects of
the different spiders. That being said, I would choose the old,
reliable, sturdy, straight, four-vane spider.

It's worth noting that this is the design of choice of nearly all
professional telescopes with monolithic mirrors, both ground and space
based. Four-vane spiders produce the simplest diffraction patterns,
and professionals frequently make rigorous compensation for
diffraction effects. Telescopes that have mirrors made of hexagonal
segments typically use six-vane spiders, with the vanes aligned with
mirror segment edges. Again, this produces the simplest diffraction
pattern.

Some mid sized scopes in the 0.5-1m class do use a 3 vane spider.

The intensity of the diffraction effects are roughly proportional to the
area obscured by the vanes and are distributed in a direction
perpendicular to its longest dimension. I slightly prefer the less
aggressive hexagonal symmetry of the 3 vane support perhaps due to
familiarity - at least on a good implementation. YMMV

A four vane spider has a Fourier transform which can be thought of as
very close to being from a circular aperture with two independent thin
rectangles missing across the diameter and the obviously missing centre.

So while there are good practical reasons for a four-vane spider (as
you itemize), even where very large budgets and superior engineering
resources are available, we seldom see anything else. That should tell
us something.

All very good points. Thanks. The main problem I discovered with single curve spiders was
severe vibration of the secondary. This required beefing up the
original vane to use a sturdy,
stainless steel rule. As the curved vane thickness increases one
presumes any supposed diffraction
advantages over a nicely thin, 4-vane spider would be lost anyway.

You can compute the effective diffraction pattern of an ideal aperture
by taking the absolute magnitude of the aperture Fourier transform with
1 = clear aperture and 0 = obscured. See further down this old thread:

http://stargazerslounge.com/topic/20...upports/page-2

It is instructive to experiment with wire frame based shadow masks in
front of a normal refractor to gain experience of how diffraction
effects from different central obstructions and supports can interact.

A hexagonal aperture mask can sometimes make awkward double stars easier
to see if you can control where the spikes sit.

The alternative of a three vane curved spider would seem to be a solution looking for a problem to solve.
The three vanes would probably have several kinds of diffraction
problems to deal with.
As well as reduced rigidity, the ability to keep the vanes perfectly
aligned with the optical axis
might greatly increase their damaging obscuration. Particularly since
curved vanes need to be made
deeper simply to increase their natural stiffness when not under
beneficial tension loads.

The curved vanes might avoid putting an obvious diffraction spike
pattern on to stars but they necessarily distribute the missing energy
somewhere. Basically it is smeared around rather than being linear.

--
Regards,
Martin Brown

On Mon, 2 Feb 2015 03:00:28 -0800 (PST), "Chris.B"
wrote:

The alternative of a three vane curved spider would seem to be a solution looking for a problem to solve. The three vanes would probably have several kinds of diffraction problems to deal with.

Such a spider produces six diffraction spikes instead of two, but
typically with somewhat less energy in each spike. That introduces
aesthetic issues that can be seen as good or bad, but scientifically
is usually somewhat more difficult to deal with. And as you note, it
can be a somewhat more difficult system to manage in terms of
centering and tensioning.

On Monday, 2 February 2015 16:21:32 UTC+1, Chris L Peterson wrote:

Such a spider produces six diffraction spikes instead of two, but
typically with somewhat less energy in each spike. That introduces
aesthetic issues that can be seen as good or bad, but scientifically
is usually somewhat more difficult to deal with. And as you note, it
can be a somewhat more difficult system to manage in terms of
centering and tensioning.

So, in summary: Tensioning of straight vanes has the advantage of raising the natural frequency of the secondary to a point where excitation due to external shocks is very unlikely. Tensioning also raises the stiffness of the vanes such that they can be made much thinner. It being extremely unlikely that one could reach the yield strength of steel vanes. The necessarily small cross section ensures that the choice of steel vanes has zero effect on the weight of the secondary support in comparison with supposedly lighter materials. Which would require greater thickness for the same tensile strength with resulting diffraction disadvantages. The weight of the hub and secondary mirror itself will dominate.

The radial stiffness of the supporting main tube, secondary cell or ring is important to avoid local flexure and loss of tension. Which might lead to alterations in collimation due to changing tube orientations with equatorial mounting.

The mechanical demands will be much reduced from the natural selection of a smaller secondary mirror in a planetary Newtonian. Though the laxer demands for collimation in such a long focus system should not lead to deliberate sloppiness in achieving perfect optical alignment. The use of high powers demands care in collimation to ensure the highest contrast and detail in the highly magnified image.