Curved or straight vane spiders in planetary Newtonians?

On Thursday, February 5, 2015 at 11:20:54 AM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 07:40:33 -0800 (PST), wsnell01 wrote:

On Thursday, February 5, 2015 at 10:21:56 AM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 04:10:46 -0800 (PST), wsnell01 wrote:

AEBE, larger aperture scopes always outperform smaller aperture scopes. End of subject.

You can't make that statement without considering magnification.

Yes, I can make that statement.

There is no visual difference at the eyepiece between a 14" telescope
operated at 50X and a 30" telescope operated at 50X. Of course, that
assumes unobstructed instruments.

A 30-inch at 50x wouldn't BE a 30-inch, assuming one's pupil is at 7mm or less. One could just as well mask down a 30-inch mirror and call it a 14-inch.

Try to present coherent, valid arguments from now on, if that's possible for you, peterson.

On Thu, 5 Feb 2015 08:37:06 -0800 (PST), wrote:

A 30-inch at 50x wouldn't BE a 30-inch, assuming one's pupil is at 7mm or less. One could just as well mask down a 30-inch mirror and call it a 14-inch.

Exactly my point.

On Thursday, February 5, 2015 at 11:40:49 AM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 08:37:06 -0800 (PST), wsnell01 wrote:

A 30-inch at 50x wouldn't BE a 30-inch, assuming one's pupil is at 7mm or less. One could just as well mask down a 30-inch mirror and call it a 14-inch.

Exactly my point.

No, that WASN'T your point.

A 14-inch working at 110x is still a 14-inch, but an (unmasked) 30-inch working at 110x is a 30-inch. Same mag, more light gathering power.

On Thu, 5 Feb 2015 09:08:26 -0800 (PST), wrote:

On Thursday, February 5, 2015 at 11:40:49 AM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 08:37:06 -0800 (PST), wsnell01 wrote:

A 30-inch at 50x wouldn't BE a 30-inch, assuming one's pupil is at 7mm or less. One could just as well mask down a 30-inch mirror and call it a 14-inch.

Exactly my point.

No, that WASN'T your point.

A 14-inch working at 110x is still a 14-inch, but an (unmasked) 30-inch working at 110x is a 30-inch. Same mag, more light gathering power.

Jeez.

On Thursday, 5 February 2015 18:08:27 UTC+1, wrote:
On Thursday, February 5, 2015 at 11:40:49 AM UTC-5, Chris L Peterson wrote:

Exactly my point.

No, that WASN'T your point.

Shall I start another telescope making thread so you can argue on that?

The REAL point is that I have an excellent 10" F:8 mirror and hoped for some expert advice on certain details of construction.

On Thu, 5 Feb 2015 09:55:54 -0800 (PST), "Chris.B"
wrote:

Shall I start another telescope making thread so you can argue on that?

Unnecessary. The guy is simply too much of a prick to deal with any
further.

The REAL point is that I have an excellent 10" F:8 mirror and hoped for some expert advice on certain details of construction.

It sounds like you're pretty sold on a conventional 4-vane secondary.
Any conclusions for the tube structure?

On Thursday, February 5, 2015 at 12:55:56 PM UTC-5, Chris.B wrote:
On Thursday, 5 February 2015 18:08:27 UTC+1, wsne... wrote:
On Thursday, February 5, 2015 at 11:40:49 AM UTC-5, Chris L Peterson wrote:

Exactly my point.

No, that WASN'T your point.

Shall I start another telescope making thread so you can argue on that?

The REAL point is that I have an excellent 10" F:8 mirror and hoped for some expert advice on certain details of construction.

And I gave my expert opinions in the second message of this thread:

Four vanes....small(ish) secondary... and if fine detail is what you -really- want, larger aperture. A good 16-inch Dob will beat the snot out of a 10-inch, visually.

On Thursday, February 5, 2015 at 12:44:38 PM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 09:08:26 -0800 (PST), wsnell01 wrote:

On Thursday, February 5, 2015 at 11:40:49 AM UTC-5, Chris L Peterson wrote:
On Thu, 5 Feb 2015 08:37:06 -0800 (PST), wsnell01 wrote:

A 30-inch at 50x wouldn't BE a 30-inch, assuming one's pupil is at 7mm or less. One could just as well mask down a 30-inch mirror and call it a 14-inch.

Exactly my point.

No, that WASN'T your point.

A 14-inch working at 110x is still a 14-inch, but an (unmasked) 30-inch working at 110x is a 30-inch. Same mag, more light gathering power.

Jeez.

If the mag is the same, WHY would I choose the 14-inch over the 30-inch, assuming that other factors were unimportant?

On Thu, 5 Feb 2015 11:31:22 -0800 (PST), wrote:

If the mag is the same, WHY would I choose the 14-inch over the 30-inch, assuming that other factors were unimportant?

Because you probably don't have one of each. A better question to ask
is, given a particular viewing desire, what aperture should you choose
in considering a purchase?

In this case, the aperture is fixed. It's 10". So how best to get the
best performance in terms of designing a telescope around a good
quality mirror? Tube and secondary have been reasonably covered. That
leaves a few other potential issues- mirror cooling, focuser, mount.
Chris B. didn't bring those up- possibly because they aren't issues
for him at this point.

A little additional information concerning diffraction 'spikes': In March 2007 I made a series of observations of a gibbous Venus using a 30cm (12 inch) Newtonian with a 4-vane spider. In addition to experimenting with various magnifications and filters, I took advantage of the large amount of available light to study the scope's *planetary* spider diffraction.

For a planet, I would call the diffraction pattern 'bars' instead of spikes.. For Venus the observed diffraction 'bars' had a constant thickness of about 1/4 Venus's diameter (with inhumanly super sensitive eyes, I suspect the bars would equal the planet's diameter, growing fainter near the planet's limb). I continued observing as Venus sunk lower in a continually darkening sky - enhancing the apparent brightness of the diffraction pattern. Eventually colors were noted in the diffraction 'bars' going from yellow to orange, red, blue, and green at increasing distances from the planet. The same color pattern repeated itself several times, growing fainter as distance from the planet increased.

A few things of possible relevance:

If the brilliance of Venus (especially under a 'dark' sky) is needed for a 'proper' study of diffraction of planetary images, then that (in addition to other well-documented observational evidence) would tend to indicate that under typical, visual, planetary observation the effects of spider diffraction on the contrast of planetary features is small enough that a little increase in aperture ought to be enough to overcome the greater contrast offered by an unobstructed aperture.

Also, the nature of the spider (4-vanes, 3-vanes, or curved) will not allow for substantially greater contrast in certain 'preferred' directions since in all cases, the light from all parts of the *extended* image undergo diffraction. A planet is not a star and does not exhibit "narrow" diffraction spikes with diffraction-free regions within the observed planetary disk. Yes, less light may be diffracted over some parts of the image than others, but *some* light gets diffracted everywhere - something that even occurs with unobstructed apertures.

Saturn's high-contrast Cassini Division as well as its low-contrast C-ring (observed against the background sky - not just against the planet's disk) are more easily visible with a 'large' aperture (spidered) Newtonian than they are with a 'small' aperture refractor. I've observed this with my own telescopes.

From a strict diffraction point of view, all other things being equal (thickness and rigidity) a straight, 3-vane spider would likely be about as good a spider as possible for a (visual) planetary Newtonian; but all other things are not equal. I would still favor a 4-vane spider, though the whole issue is overshadowed by optical quality, collimation, seeing, etc.

Sketcher,
To sketch is to see.

On Wednesday, February 4, 2015 at 1:39:29 PM UTC-7, Quadibloc wrote:
On Sunday, February 1, 2015 at 3:47:00 AM UTC-7, Chris.B wrote:

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

The "others" are right, curved spiders don't make diffraction disappear, they
just distribute it differently.

However, while diffraction still takes place, getting rid of the visible spikes
may still be preferable to having more detail visible in some directions only;
you can still get a refractor-like image, you just need to pay a larger premium
in aperture for the same detail.

So curved spiders can still be a better choice without having to credit them
with magically making diffraction disappear.

John Savard