Spiders and secondary holders

[pmrid (Peter)]

I'm about to embark on a long-delayed project to refurbish a fast 10"newt I have had sitting here for some time. I've reached the secondary mirror section now and am wondering about the relative stability/strength of the various alternatives out there. This unit uses a secondary mirror with a 3 inch minr axis so it's a relatively heavy unit. When you take the weight of the secondary holder into account, that's half a Kg or better that has to be supported/held by the spider assembly.

I've had some serious disappointments with different spiders I've bought for this scope. The worst faults I've encountered so far have included (in no particular order):

The mirror-holder shaft made of plastic that flexes when the scope moves - impossible to hold collimation or that strips its own threads when you tighten the hex nut;

The captive brass threads in the spider centrepiece that slip around in their housing preventing any proper tension on a collimating screw;

Captive brass threads that actually have a different thread from the collimating screw (yep, I've had one of those).

So I've become more than a bit cynical about these products. So I'd be glad to have some thoughts from others about how to identify a decently made, reasonably solid and reliable secondary/holder combination that will actually hold collimation.

Peter

[mental4astro (Alexander)]

Hi Peter,

I read your thread asking about spiders and holders. What I'm not clear about is do you want to purchase a unit or make your own.

Purchasing an off the shelf unit is just like any instrument - you get what you pay for. You've experienced this with the list of disappointments you've noted. But more expensive units can come with their own baggage too. Sure they are more stable and predictable, but usually they are heavy, long in profile, and not often suited to the scope we are trying to fit them into.

I ran into this problem for the scopes I build. The mass production spiders and holders are just not up to the task, and the better quality ones are just totally too large for the low profile I require. The only option I had was to make my own. And what I came up with is everything the better quality expensive units are, but lighter in weight and very low profile, and allows for an integrated dew heating system to be added to the secondary.

Such a holder and spider is not the easiest to make; it is time consuming (especially the designing), a lot of care is needed, and some screws and inserts need to be modified. While I had some components specially made for me, for a one-off this is not necessary. You need to design the spider and holder assembly for the scope. This is why the instruments I make are not inexpensive - I design and build the whole thing (other than the focuser) because this is the only way I can produce the scope I want. Everything is custom/hand made. Use someone else's components, and the entire instrument is a compromise to other people's components. Make it yourself, and you get what you want.

This is not intended as a sales pitch. It is meant as a pep-talk to show you YOU are in control of your scope, not someone else. Select the correct materials, a clever, well thought out design, and you are the master. Clever design can also mean cannibalizing bits and pieces from other spiders/holder if these give you what you want.

The better quality, more expensive units will work the way you want a spider & holder to work, don't get me wrong. They are marvels of engineering. But they are expensive as the spider-holder unit is actually a complex little machine.

So, make it yourself or buy a compromise? Both options are as valid as the other, and it is best to consider both.

Alex.

[pmrid (Peter)]

G'day Alex,
thanks for the motivational injection.
Peter

[Rod]

Hi Peter

I find wire spiders are very strong, light and have little diffraction. I like the ones where the wires crossover like Mel Bartels uses on his scopes:

http://www.bbastrodesigns.com/ZipDob/ZipDob.html

Scroll down the page and you will see some pictures and a description.

For the hub I have been reading this page:

http://www.reinervogel.net/index_e.h.../3_MBox_e.html

I like Reiner's idea of removing the central bolt completely.

Hope that gives you some ideas.

Rod

[ZeroID (Brent)]

Just done my first wire spider and rather impressed with it's stability. It's a simplified version of Rolfs (Skyviking). My secondary is a lot lighter than your 1/2 kg prediction and I'm only using .23mm top 'E' guitar strings but nothing to stop you moving up a string or two in diameter to gain some strength with the heavier holder. Although I must add here that having all the adjustments away from the holder simplifies and lightens that considerably as well.