Cooling Down

[alan potts]

I know this is a scope/eyepiece question but the main part is to do with the eyepiece

Well we know that a telescope needs to cool down or warm up as is sometimes the case, before it works as intended. I can't get this out of my head ( sad or what )why do eyepieces not require to treated the same way? For that matter do Binos need to cool as well or is the magnification not enough to matter.

The other night I saw a gap in the clouds coming, weather moves slow here so I knew I had an hour or so. I put out the APM 115mm, very quick and got on to it at once. I saw something rather odd, Jupiter, one side was fringed in red and the other blue. 45mins later it was as sharp as a knife and no colour, Anyone have a reason for it?

Alan.

[estwing]

good point Alan..i await the answer too,big bins have got to need some time?

[mrphildog]

My understanding is that Newtonian scopes need most cooling because they are essentially large empty tubes with an opening at one end. The heat from the mirror slowly leaves the scope via the opening causing turbulence within the scope. It is this turbulence that affects the view. Other systems are less affected as they are closed systems without this big opening so they don't have heat escaping through the light path in such a big way.

No doubt someone with more knowledge will come along and explain more.

[michael.h.f.wilkinson]

Just to throw a spanner in the works. The cooling is not strictly necessary, assuming the external temperature gradients in the optical path are dissipated quickly by, e.g., wind. A closed-tube scope would work without problems so as there are no temperature differences which create convection currents. If a tube is sufficiently insulated, no internal temperature differences build up, and no convection takes place. In practice, temperature differences will build up, and generate convection currents in short fat scopes in particular. The reason they do not build up in thin scopes or EPs is that there is simply no space for a stable convection cell to build up. The space is too restricted. Baffles in a tube add to that. This is why (small) refractors and bins are not affected.

[michael.h.f.wilkinson]

My understanding is that Newtonian scopes need most cooling because they are essentially large empty tubes with an opening at one end. The heat from the mirror slowly leaves the scope via the opening causing turbulence within the scope. It is this turbulence that affects the view. Other systems are less affected as they are closed systems without this big opening so they don't have heat escaping through the light path in such a big way.

No doubt someone with more knowledge will come along and explain more.

SCTs are closed, but need cooling, for the reasons given above

[Charon]

Very interesting topic.

[John]

I've seen that effect with planets too Alan but this was when they were relatively low in the sky and was caused by atmospheric diffraction (I think thats the right term).

Both my ED refractors (4" and 4.7") need some cool down time before being at their best for high power viewing. I'd say 30 mins but there are further improvements over the next 30 mins too. This is in winter. On warm summer evenings the temperature gap is very slight so cool down time much reduced.

Where you are, in the summer it might be a case of warm up time !

[Space Dragon]

I've often thought the same thing. Started noticing things were distinctly sharper after 40-60 mins, so I now leave my Binoviewers and EPs in the utility room, which is quite cool, for an hour of two as well.

Haven't have that many chances to test out the theory recently and I also suspect it could just me my eyes getting properly tuned in over the course of an hour.

[alan potts]

John,

Yes I have seen that to when they are low but Jupiter is almost overhead, so I would have thougth that was not the case. The other night when this happened it was 2 degrees, very warm for us. In the house it's about 22-23C. I mean there are better scopes than mine maybe but it can't be the scope, I never see fulse colour in it.

Your right about Summer, in the house about 25C outside about 35c at 10 o'clock

Michael, Interesting I'm going to try to understand it now. I have a clear patch coming which should be good for an hours or so.

Crazy weather, it's plus 12 at the moment, last year same day minus 24.3.

Alan.