Pinched Optics?

Hi,
I left my scope out last night (teen temps) for the first couple of hours all was great. When I took a look this morning (5:30 a.m.) I got the triangle out of focus image and the classic pinched optics in focus pattern. Is this wierd or does it make sense that for the first few hours out everything is ok but sustrained cold temperatures (11 hours of it) can ultimately cause pinched optics and that I have to loosen the retaining ring or is something else at play? This is the first time in two winters that this happened. Also is there a daytime test that could be done to dtermine pinched optics?
Thanx
Clear Skies
Itz

Itz,

Can you see if any of the spacers are displaced? Is this a triplet or a doublet? If the former, don't do anything yourself.

Dmitri

If it truly cold temperature induced, it will probably clear up as the lenses return to more normal conditions. I would wait and see how it looks next time out and if all is fine, I would avoid the real cold with it. I once had a cass primary develope a clam shell fracture at the centre perforation when left in deep cold for an extended period.

dan

It appears the temeperature at your location (Brooklyn)
dropped from 36F to 22F between 5PM and 5AM. So yes,
the effect could worsen during the night.

Whether you can usefully do anything depends on the design of
the lens cell. The problem is that the metal cell contracts
faster than the glass elements as it gets cold.
Often the cell will have spring tabs machined into it
which hold the lens elements together while allowing
some expension / contraction. Or rubber or cork pads
to do something similar. If the cell is a crude design
with no allowance for contraction,
then loosening the retaining ring might do something
useful. Another scenario is that the diameter of the
metal cell is becoming too small for the glasses, and
is squeezing on the circumference of the glasses --
in this case there is probably nothing you can do
yourself -- the cell would need to be carefully
machined to a slightly larger diameter.

I am not aware of any daytime test. It seems you
have already done the relevant test -- star testing
at cold temperatures. I would keep some record of
the temperature of the scope and whether you see the
effect in the future. I would not push it too
far, lest anything get broken -- when you see a strong
effect I would move the scope somewhere warmer.

Maybe an anti-dew heater strap could be used to get
you a few extra degrees of good images. Since it would
be directly warming the metal cell, it should have
a useful effect.

If you send me a private message with details of the lens,
I might be able to supply more info.

If the lens cell is too tight versus the diameter of the glass, then it can pinch when it gets to a certain low temperature. Aluminum shrinks at a rate more than twice that of glass. If the temperature gets too low, say below zero, the cell will put intense pressure on the glass and the optics could crack in half.

These simple cells really cannot be made with too much clearance because that could produce coma in warmer temperatures.

Rolando

Robert-

Itz has an air spaced doublet, similar to one I used recently.

Being that I am at 26° S I can't help with the effects of sub-zero conditions on optics/lens cell design.

RC mentioned a good point on this RE contraction rates.