Re: [ATM] An intriguing toy

[Jarvis Krumbein <jkoptic@juno.com>]

Philco made a large screen projection television set which used similar
Schmidt optics.  This was black and white and I'm sure predated your
projector by many years.  Molded mirror blanks (8") and molded Schmidt
correctors were available on the surplus market very cheaply and I had
one of the mirror blanks that I used as an ashtray before I quit smoking.
 I think the bigest problem you will find in using the optical components
is that the are very fast and designed for fairly close focal distances. 
The abberations will probably be excessive when used at infinite focus
for astronomical purposes.

Jarvis Krumbein


On Sun, 13 Mar 2005 23:19:08 +0800 Bevan Harris <ngc2070@bigpond.com>
writes:
> Hi all,
> 
> I've just picked up a curious item on eBay that seems to hold a 
> certain 
> promise as a TM project.  When I say "just" I actually mean that I 
> picked it up on an opportunistic bid back in early January but have 
> only 
> just gotten it home today.  The item is an early model "three gun" 
> video 
> projector weighing about 80kg (176lb) - it was listed as pick up 
> only 
> and was situated across the other side of the continent.
> 
> Getting my booty home proved somewhat problematic and it often 
> seemed 
> that my $52 "bargain" was going to come back rapidly and bite me 
> rather 
> savagely with exhorbitant private shipping charges and the projector 
> 
> mysteriously going missing for a couple of days when it arrived on 
> my 
> side of the continent.  In the end though, I was able to get it 
> delivered to me for a shade under $100 and with the help of some 
> understanding colleagues.
> 
> What attracted me to this projector was that it was a rarity as such 
> 
> items go - the optical sets utilize a Schmidt configuration rather 
> than 
> the more familiar refractor style.  It was listed as non-working, 
> which 
> suited me perfectly, but the seller advised me after the close of 
> the 
> sale that he had been able to identify the fault and that it was now 
> 
> functional.  It's not often you think "d*mn, what a nuisance" when 
> delivered that sort of news.
> 
> Anyway, I got the projector home today and have verified its working 
> 
> status, but I was much more keen to lift the cover and inspect the 
> optical sets, so here goes a basic description.  Each of the optical 
> 
> sets is comprised of a 7.5" tapered primary and a 6.5" corrector 
> (clear 
> aperture ~6.25").  These are spaced apart by four threaded rods with 
> the 
> distance from the rearmost surface of the primary to the front of 
> the 
> corrector being 9.25".  The primary mirror has a barrel fan attached 
> to 
> its rear that blows air directly onto the CRT through its central 
> hole. 
>   The central obstruction by the prime focur assembly is huge (about 
> 
> 4"!).  This assembly is comprised of  a 2.5" diameter CRT that pokes 
> 
> about 4" into the unit and an 8" long tapering nose cone (which 
> hides a 
> large valve) protruding from the front.  The prime focus assembly is 
> 
> supported by an externally mounted four-vane spider and appears to 
> be 
> operated by  a toothed rubber belt runs around the spider mounting 
> points, though I can't see a motor that is used to operate this 
> function.
> 
> All-in-all it looks like an intriguing proposition.  It appears as 
> though the optical assemblies could be readily converted for 
> astronomical use, even if it be just for photographic purposes, but 
> I'm 
> wondering if anyone else has come across this type of system or has 
> any 
> advice to offer.
> 
> The projector was made by Projection Systems in New Jersey and 
> apparently converted for PAL use by Speywood Comm. in England.  The 
> 
> original eBay listing can be viewed at 
> http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=5741329319. 
> I've 
> also a handful of photos showing the optical assemblies that I can 
> make 
> available on my website if necessary.
> 
> Cheers,
> Bevan
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> ATM mailing list http://www.atmlist.net/
> 
> 
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