Skywatcher spider vane thickness?

[acey]

Does anyone know the thickness of the spider vanes on the Flextube 300P? Can't find it quoted anywhere and I don't have a micrometer.

Andrew

[Ben Ritchie]

Any idea if they the same as a solid-tube 300P? I have one of those and a micrometer...

[acey]

I would guess yes - certainly worth knowing if you don't mind measuring.

[John]

I thought it was 1mm ?

[Ben Ritchie]

Dammit, the micrometer's digital and the batteries are flat, so if someone else can measure it's going to be quicker ... sorry about that.

[johninderby]

SkyWatcher has the following on their website:

"All Sky-Watcher Dobsonian models feature 0.5mm Ultra-Thin Secondary Mirror Supports to reduce diffraction spikes & light loss"

John

[John]

This does beg the question regarding the optimum design of secondary mirror vanes, my understanding being that curved vanes reduce diffraction effects more effectively than straight ones. Would a single, slightly thicker, but curved design (as David Lukehurst uses) be better than the standard 4 thin vane design ?.

[johninderby]

Here's a quote from Barry at Orion Optics. Gives some insight into the issue.

" There are problems with 'circular' spiders, in manufacturing terms. They are particularly difficult to make and keep the curves symmetrical. When a straight spider leg is even a mm off, it is easily detected by the unaided eye and corrected. When all the legs are straight, provided they are all the same length, the central hole in the spider boss will be on the optical axis of the telescope.

The problem with circular legs is it is very difficult to detect a circular curve which is out by a mm, the eye just doen't like curved lines and

detection of irregularities is very difficult. When assembled to the central spicer boss, either one or both of these 'rings' can be asymetrical and as a result, can, and will, throw the central hole off axis, making collimation impossible.

A few years ago we made curved spiders for SPX models but, they all had to be hand finished to make sure they were symmetrical and the hole was on axis. It took longer to do this than the actual manufacturing process. To be frank, they are beggars to make properly.

They can be made relatively easily and cheaply but, to give them as much strength as a straight legged spider, the legs have to be around 50% thicker, this then makes the hand finishing (re-bending) a real problem.

We decided to drop the curved spider after realising the total manufacturing time, including the hand finishing time, was costing more than double it's straight legged counterpart and, if we were to continue fitting them, we would have no alternative but to increase the price of the telescopes using them. We decide against that.

However, if any customer would like a curved spider we can make and fit one but, it will be expensive and, we would need your tube to re-drill it to accept the totally different fixing method."

John

[acey]

"All Sky-Watcher Dobsonian models feature 0.5mm Ultra-Thin Secondary Mirror Supports to reduce diffraction spikes & light loss"

Cheers for this. Somebody on "another forum" (OK, it was CN) suggested that anything thicker than 0.015" (0.38mm) was "too thick" for a 12" (or smaller) scope.

About curved-vane, my understanding is that it creates a different diffraction effect (eliminating spikes) but gives rise to the same amount of contrast-reducing noise, the crucial thing being the area of obstruction rather than its shape.

There also seems to be some question about the thermal effects of vanes, and whether this contributes to visual noise.

Andrew