10" f/8 Arachnophobia [Fear of oversized spiders!]

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The high frequency vibration in the curved secondary support suggested that it might be subtly vibrating all the time. Which would obviously spoil the view through the telescope. 

Some commentators also suggest that the curved secondary is a poor fix for reducing diffraction effects from the vanes. Instead of being concentrated in the familiar spikes the diffraction would be smeared all over the image.  

The need for a stiff, but curved spider vane requires it be rather thicker than desirable.

I decided to make a 4-vane spider instead but quickly discovered that the construction materials were difficult to find. 

Fortunately my search online for inspiration produced an advert for a secondhand spider which suited my needs perfectly for a relatively small sum. 

It was found on Astrobuysell the popular, UK, amateur astronomical equipment, small ads website. I paid for the spider and a few days later it arrived. [Thank you, John!] 

 U.K. Astronomy Buy & Sell 

It is decades since I last handled a commercial spider/secondary support and. I imagined they would still  be made of solid aluminium alloy bar. It was not the case. The spider I had bought was mostly made from a tough, engineering plastic. Only the adjustable stalk was of solid aluminium. The difference in weight from the use of this material was readily apparent. Had the entire thing been made of solid alloy it would probably have weighed twice as much.

Not that spider weight is a serious issue with most people. Provided they don't need to carry their telescope/OTA very far. When light weight is a priority everything which reduces weight is worthwhile. The elderly amateur may no longer have the physical strength to carry anything far!

The secondary mounting block and collimation plate were a smidgen over 50mm. Which suited me perfectly as I can turn these down in the lathe to  to suit my 47mm secondary. This reduction in diameter will have no effect on the mechanical needs of the spider and its adjustment. 

I plan to replace the solid 19mm alloy stalk with a tube to save some weight. 

I shall probably make another shroud to hold the secondary mirror in place. The present one is a bit short of a complete wrap. Leaving a larger gap between the meeting edges than desired. I am not a fan of the present silicone glue attachment. Unless perfectly cleaned off  the silicone can [apparently] off-gas during re-aluminizing.  

These images show my purchased spider completely assembled and with it increasingly dismantled for clarity. 

The offset vanes have the potential for reduced diffraction effects in comparison with perfectly radial vanes. Though this may be at the expense of more smearing. At least they won't allow vibration! I may make some longer collimation nuts to provide easier [i.e. no-tools] adjustment of secondary alignment. If they will not clear the vanes then I may fit wing nuts instead of plain nuts. Though another possibility is to use a socket wrench I don't fancy using one in the dark. Not with the primary necessarily exposed below where I am working on optical collimation.  

Click on any mage for an enlargement.

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