Eyepiece temperature

[Captain Scarlet]

It’s common knowledge that until a scope, be it reflector refractor or cat, is properly cooled (or warmed!) to ambient temperature, its figure will be off and the view through it will be poorer than it could be.

So why isn’t the same accepted as being true for eyepieces? I can understand that small eyepieces will adjust temperature quickly, but surely monsters with many elements of thick glass, which description includes a lot of the top-end eyepieces, will suffer from cool-down effects and aberrations just like an OTA?

And yet the received advice is to keep them in your pockets to stave off the dew.

Cheers, Magnus

[Littleguy80]

I guess the most likely explanation is it’s a very small amount of air so seeing those effects is much more difficult. The effect is probably negligible compared to everything else that could affect the views. 

[dweller25]

Good point Magnus and I always cool my eyepieces, diagonal and especially my binoviewers with the scope.

Edited April 18 by dweller25

[John]

Very interesting topic, as someone who owns and uses complex eyepieces with large glass in them.

I've always tended to keep them just above ambient temperature to avoid misting / dewing when I come to use them (which, when it has occurred, out in the field, has been very annoying). 

I'm not aware of any performance issues arising from my approach but I suppose I'd have to run a comparison between a cooled eyepiece and one just above ambient to find out. Given that the eyepieces would need to be the same design and focal length to get a meaningful comparison, this might prove tricky to arrange 🙄

Perhaps it's a trade off situation - a small impact for a short time on view quality while the eyepiece cools VS the risk / inconvenience of fogging / misting as the warm eye gets close to the eye lens of a cool eyepiece.

I guess the jury is out on this one ?

Edited April 19 by John

[Steve Ward]

With reflectors the cooling to ambient is to reduce air currents inside the tube disrupting the view , it's not the figure of the mirror that is "off".

The relatively warm primary mirror when cooling down creates these currents by forming a thin layer of warm air that rises up the tube causing the shimmering seen at the EP.

Cats and Maks suffer the same causing small plumes of warm air around the internal baffle tube.

If on the other hand you cool your EPs to ambient you will find them misting up almost immediately when you bring your warm eye up to them so they are better of in a warmer enviroment such as your pocket.

It's the same thing as a refractor gathering dew on the objective when warm moist air hits the glass , hence the use of dew shields and dew heaters on the front.

Edited April 19 by Steve Ward

[JTEC]

Bill Paolini, in his comparative test of diagonals had this to say about thermal settling:

‘ Diagonals absolutely require thermal acclimation time for best planetary performance!  Going from a 70ºF inside temperature to a 30ºF outside temperature took a toll on the diagonals, particularly the 2" non-Prisms as they showed astigmatism even directly on-axis.  The prisms did not show astigmatism, but did show a less sharp view until thermal equilibrium was achieved.  Generally 15-30 minutes was required for optimal high magnification performance.’

Diagonals not eyepieces, but it seems reasonable to think that a chunky, multi-element eyepiece is going to need at least similar consideration.  I don’t think it’s just about the behaviour of air in the tube and above the mirror surface, though of course those are always an issue.

It makes sense, I think, that an eyepiece, especially one that is physically big and contains numerous lumps of glass with different thicknesses, figures and refractive indices, all supposedly ‘finely tuned’ to work together, stacked usually in a metal housing, is going to be subject to differential heating/cooling through its body - and it won’t perform as designed and intended until everything has settled down.  
 

FWIW, I expect to have to allow for this, especially when using big or complex eyepieces or the binoviewer with its GPCs, whenever I’m trying to get the best out of my frac.
 

 

Edited April 19 by JTEC

[Captain Scarlet]

8 hours ago, Steve Ward said:

With reflectors the cooling to ambient is to reduce air currents inside the tube disrupting the view , it's not the figure of the mirror that is "off".

It’s my understanding that the figure of a Newtonian mirror can significantly change shape whilst cooling, with the edges cooling faster than, and thus at a lower temperature than, the core, inducing turned-edge-type aberration. I’ve often seen it in my own 12”, starting off a session with seriously different in-and out-focus patterns, gradually becoming hardly-different later on.

This describes it: http://www.loptics.com/articles/fourlessons/fourlessons.html

[Mr Spock]

I keep mine in a tray with the eye dust cap on to prevent misting. I don't have enough pockets for all of them anyway, especially the larger ones 

[Carbon Brush]

Thinking about a newt.
A mirror at uniform temperature, whether 0C or 40C, has the same shape, and therefore consistent performance.
A decent mirror has very low thermal expansion, which means cold edges and hot middle should have a small effect on shape.
How much this matters depends on the temperature difference.  It is all a matter of degrees. OK I'll get my coat.

An eyepiece (even a jam jar size multi element) is small and low mass compared to a 10" or 12" mirror.
Temperature difference from centre to edge is going to be small. So performance hit should be small.
Cooling is going to be fairly quick. So by the time you have thought much about whether view it good, temperature has become stable.

In 2011 I trialled a solution to the 'foggy eyepiece' and 'not enough pockets' problem.
A fibreglass box (strong enough to stand on) containing large resistors and a fan on the aluminium plate.
Power from a 12V supply.
Just keep eyepieces in there when not in the scope
I posted about it on SGL and there was zero interest!

[Ags]

8 minutes ago, Carbon Brush said:

Just keep eyepieces in there when not in the scope
I posted about it on SGL and there was zero interest!

I have thought about putting a hand warmer in my eyepiece case!

I think the small size of eyepiece lenses is also relevant. Even the largest ones will be quite rigid compared a big floppy newt mirror for example.

[Don Pensack]

And yet, people heat their secondary mirror or corrector plates or eyepieces to prevent dewing.

A very slight change in the optical image is preferable to the problem of opaque optics.

[Captain Scarlet]

Interesting, thanks for all the responses. I lean towards Bill Paolini’s thoughts as presented by @JTEC, but local opinion seems to be evenly divided.

As for adding heaters to secondaries, I’ve avoided that for the same reasons, and decided that dew-opacity signals “end of session”. Luckily I’ve found that there’ve been overall surprisingly few heavy-dew nights, and this is Ireland!

Magnus

[Louis D]

I've found my 90mm Triplet takes about 30 minutes or longer to acclimate, or I get what looks like pinched optics.  That might just be due to a poorly engineered objective cell with a coefficient of expansion greatly mismatching that of the glass.  What I'm seeing is definitely not tube currents during acclimation.

I forget where I read it, but eyepiece acclimation is much less of an issue due to the smaller total thermal mass, and the fact that it is at the back end of the optical train where variance in the optical figure has less effect than at the objective.

As for dew heaters, the trick is to use as little as possible.  You want to warm the optical device just above the dew point.  That generally won't noticeably affect the figure of the optical element.

[John]

My Vixen ED102SS F/6.5 doublet needs 30 minutes or so to cool down. Prior to that, high power images are not quite as sharp as you might expect and it's really not worth pushing the magnification over 130x or so. Once cool the scope shows excellent performance and that is maintained to 220x-250x, seeing conditions allowing.

My Tak FC100-DL F/9 doublet seems to perform excellently, even at very high magnifications, almost straight out of the house. I've put the scope straight out and split 1.5 arc second doubles at 257x, for example.

The Tak performance at very high magnifications is slightly better than the Vixens. It's pretty comfortable even at 300x on suitable targets and the diffraction pattern around brighter stars is more tightly controlled - a very thin 1st diffraction ring, thinner than the Vixen's, even when the Vixen is fully cooled.

The cool down difference between these scopes I put down to a) thicker lens elements and b) the fatter Vixen tube (112mm vs 95mm). 

The performance edge at high powers I put down the Tak having a higher precision objective plus, possibly, the use of a fluorite element rather than the Ohara FPL-53 that the Vixen uses.

Both these scopes are extremely enjoyable to use though and present very nice images of the astro targets that I point them at. Their characteristics vary, which is why I'm happy to own both of them 🙂

Nothing to do with eyepiece cool down though, apologies for that 🙄

 

[Dave scutt]

I used to put a few strap around my ep to stop them dewing up, never thought about keeping them at ambient temperature 

[bosun21]

I keep my eyepieces outside in a case with the lid closed, taking them out one at a time as I use them. On nights of really heavy dew and for extended periods at the eyepiece I have sometimes been forced to use a dew strap on low power fitted around the eyepiece. I also refrain from using dew heaters on the scope as much as I can. Even on Maks and SCT's I prefer to extend the dew shield before employing a heater. Regarding a heater on the secondary of a reflector, never! We all learn about observing directly over house rooftops due to warm air currents rising which will degrade the view. Yet using a secondary dew heater is like placing a small roof inside the telescope tube. That's my reasoning anyway.

[Mr Spock]

Absolutely. There's no logic in allowing the scope to cool then heating it up again! If you use a dew heater, turn it on to get rid of dew then turn it off again. Or use a hair dryer.

[Stu]

The idea is to keep the eyepiece, objective or front plate JUST above the dew point, not to have it toasting away 😉. I agree with using them sparingly and on lowest setting to keep the dew away, but there are nights when the option is dew straps or pack away, so dew straps it is.