Doublet foil spacers

[Herbig]

Hi, anyone any experince in making new foil spacers for a doublet.

I found an old 70's/80's 5" refractor in a junk shop, very mucky so set about cleaning the objective. Having removed the lenses it apears that one of the foil spacers was missing and the other 2 damaged.

Will any foil the right thickness do if I measure one of the ones remaining with a micrometer?

[Merlin66]

These doublets usually have about 0.004" spacers - check out your HD aluminium kitchen foil...I believe Scotch tape is also 0.004" thick ;-)

[johninderby]

There's a post worth reading on the subject of doublet spacers in the Carton Scope Club yahoo group

http://tech.groups.yahoo.com/group/cartonscopeclub/message/2501

John

[AndyH]

Interesting stuff.

I've been following the Carton spacer thread too.

I asked the question to the Tal designers, a while back, regarding the spacers required for the 100RS(4"F10). They said 0.3mm +/- 0.005mm which is equal to 0.012"(?).

I believe the 125R(5"F9) is even a wider gap.

Would it be safe to assume that in a std designed doublet, the faster the lens, thereby the bigger the lens curvature, the wider the gap ?

Cheers,

Andy

[ronin]

The gap is defined by the design of the lens so the answer is no. I have a 5" F/10 doublet lens somewhere that has a large seperation in the region of 10mm (big brass ring) between the 2 component lens.

All you can determine is the minimum, in that the 2 faces cannot come into contact with each other. But I doubt that that value will be the one the lens was designed to be.

There is a high chance that it might be close but not a certainty.

Unless you can measure the 2 curvatures and work out the minimum gap I can only think of one, somewhat mapcap, method of getting the minimum spacer thickness.

[cs1cjc]

Postage stamp paper is I understand the traditional spacer.

[Herbig]

Thanks for all the replies guys. I've measured the only one decent remaing spacer and it's just under 3 thou. I've been through the kitchen measuring all the foil but it's all too thin, by chance I remembered a roll of Gas closure tape I had when I instaled a new balanced flue on our oil fired boiler and it seems to be the correct thickness and added bonus is it's self adhesive so I might give it a go, but going to have alook at the thread on the forum mentioned above first.

[Herbig]

Many thanks Johninderby for the link to the Carton Scope Club Yahoo group.

The thread confirmed my thoughts that Gas closure adhesive foil tape will do the trick, thay called it HVAC duct sealing tape, same thing.

Well I've reassembled the lens cell and will fit it to the tube tomorrow and see how it collimates. No much chance of star testing for a while :-(