Re: [APML] OT: AP Oil Spaced Lenses

["John Gleason" <dvj@earthlink.net>]

Roland did not say what it was made of.  It is used in the telecom industry
to link bundles of fiber optics at terminations.  Heavier then oil, Roland
substituted it because of serious problems Tony Hallas was having with the
7" f/7 prototype.  The triplet set is adjusting to the ambient air
temperature at a different rate then the aluminum cell.  I guess this
caused some separation of the oil away from the lens.  Tony showed a photo
at one of the astrophotography conferences that looked awful.  I was
shocked to see something that looked like a giant snowflake (interference
pattern of light through different thickness of oil) across the 7" lens.  

I still see the same effect around the edges of my objective in cold
temperatures where I have used the instrument(50F to 26F).  When I check at
home in warmer ambient temperatures, the edge staining is gone.   

One of the real problems with the f/7 is thermal shock to the cell when
pulling out from a warm vehicle to set up. I keep the OTA in its box when
travelling and it is often warm to the touch when I take it out.  Roland
recommends keeping the dew shield retracted around the black anodized cell
to minimize the heat radiating from it during a cool down.  I use a
Kendrick dew heater at a low setting around the cell all night.  I was also
seeing some gross astigmatism with this system in cooler temps as well. 
Turns out that trapped air in the tube creates a temperature differential
and depending on the angle of the tube, astigmatism is apparent to a
greater or lesser degree.  Now I use a small fan to pull out "dead" air
with the butt end of the tube pointed straight up. 

Interesting to note, I know two people with late model 7" f/9's who do not
have any of these problems. ;-?  Could these be just a few of the reasons
Roland stopped making the 7's?    There are some 7 and 8 inch fluorites out
there from APM, I wonder how they work? 

jg


> [Original Message]
> From: Jim Janusz <jjanusz@ix.netcom.com>
> To: <astro-photo@seds.org>
> Date: 6/21/00 5:43:32 PM
> Subject: Re: [APML] OT: AP Oil Spaced Lenses
>
> Hey John,
> 
> You just gave me some info. I was unaware of.  I thought the 7-7 was oil
> spaced.  Now I understand what the slight stains are at the very outer
edge
> of the cell.
> 
> Gel, well I'll be damned!  Did Roland say what the gel is made of?  Is it
> petroleum based?
> 
> Jim
> ----- Original Message -----
> From: John Gleason <dvj@earthlink.net>
> To: <astro-photo@seds.org>; <astro-photo@seds.org>
> Sent: Wednesday, June 21, 2000 4:27 PM
> Subject: RE: [APML] OT: AP Oil Spaced Lenses
> 
> 
> > Chuck,
> >
> > My 7" f/7 is gel spaced not oil spaced. The gel used is from the fiber
> > optics industry.  I can see changes in the gel when there is thermal
shock
> > to the cell during cool down.  The cell is sealed, but there does not
> > appear to be a manifold to compensate for contraction or expansion of
the
> > lens elements.  Roland told me so.  The 6" f/7.1 that I also own (7
years
> > old) is oil spaced as per Roland.  This appears in early 90's AP
> > literature.
> >
> > jg
> >
> >
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--- John Gleason
--- dvj@earthlink.net
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