Cool Down Eyepieces?

[Sunshine]

Here's a question I almost feel dense asking but, is it vital for eyepieces to reach temp also? i tend to set my scope out but, leave the eyepieces inside for the time being. Can a warm eyepiece cause visual distortions of any kind until it too cools down?   Interesting to know if some designs with larger voids between elements can trap warm air.

Edited March 7, 2021 by Sunshine

[vlaiv]

I think that it is best to do the opposite - keep the EPs warm.

At least I have that problem - as soon as I bring my eyeball close to cold EP - it starts to fog up. I guess it is due to moisture in my eye that condenses real fast on cold EP eye lens.

As for "tube currents" - I don't think it is important as wavefront is really "dense" in the eyepiece - unlike in telescope tube where it is spread over almost all of interior space of the tube.

EP is also much smaller by volume and there is not much room for gradient to form - that amount of air is going to equalize in temperature very fast.

[John]

I find they work better when above outside temperature - they don't mist up then when you put your eye to the eye lens !

 

 

[Louis D]

Not really.  The thermal inertial of the glass in eyepieces is very low compared to objective lenses and mirrors.  They are also very small pieces of glass in comparison, so temperature differentials within them even out in very little time.

As others have said, keeping them warm enough to prevent dewing is a good idea.  I wear eyeglasses at the eyepiece, so they act as a moisture barrier between my eye and the eye lens.  As a result, I rarely have dewing issues.

[jetstream]

A diagonal can have a cooldown, espc prisms.

[Sunshine]

Good to know, thanks! regarding dew forming when eye is brought to eyepiece, I have encountered that a few times.

[mikeDnight]

2 hours ago, Sunshine said:

Here's a question I almost feel dense asking but, is it vital for eyepieces to reach temp also? i tend to set my scope out but, leave the eyepieces inside for the time being. Can a warm eyepiece cause visual distortions of any kind until it too cools down?   Interesting to know if some designs with larger voids between elements can trap warm air.

When I first take my eyepice box outside, I generally leave it open to the air for a while so as to allow the warmth trapped inside to escape. After ten or fifteen minutes or so i will then close the lid to prevent the eyepieces from becoming excessively cold, and to prevent moisture forming on any metal or glass and becoming accidentally trapped within the box. A warm eyepiece will kill fine definition when you're looking at intricate or subtle planetary detail, so it's a fine balancing act on particularly cold nights, and if it does that with the planet's, it dose it with everything. At the other extreme, after an eyepiece gets so cold that it wants to fog over, I will hold it in my hand, with my hand in my pocket, to allow it to warm up; but not so much that it interferes with the fine detail. I can't say that I've ever really noticed much of a difference in heat retention between simple and more complex eyepiece designs, as it only ever takes just a few minutes for the heat in an eyepiece to escape. Prisms or mirror diagonals cool rapidly, but because their optical components are not exposed to the outside air, they don't fog over, and don't hinder the view in any way. Dew heater strips can be used to prevent eyepieces from dewing over, but that's just another gadget to contend with. 

[Carbon Brush]

I made this 10+ years back. Specifically to keep eyepieces warm, and remove dew from used eyepeices.

A plastic (thermally insulating) box. Contains a couple of resistors to provide a bit of heat and a small fan to stir the air. Runs from 12V.

The Hyperion eyepieces give you an idea of scale.

[JTEC]

From Tom Trusock’s 12/07 Part II of IV Report on Nagler Eyepieces on CN.  I suppose if anyone knows the answer to this, it would be Televue: 

 ‘As an interesting side note, I asked Tele Vue if the infamous "Eyepiece Cool Down" myth holds any water in the real world. Their answer? Nope. There's no cool down effect for eyepieces. The only "gotcha" that might exist here is that massive eyepieces could cause some visible tube currents is taken from a warm house out into the cold.   Specifically Tele Vue says:

"the eyepiece is in such a low power-space that it is impossible to see any
image change due to thermal changes of the lenses.  This myth seems to revolve around the 31mm Nagler which produces low power not suitable to observe changes in performance due to equilibration of the optics. "’
 

[mikeDnight]

It's an easy issue to resolve (pun intended). Simply leave a telescope outside to acclimatise for an hour or two on a winters night, then place any short focal length eyepiece straight out of a warm house into the focuser and aim your scope at a planet. It doesn't have to be a complex multi element affair, even a simple Mono or orthoscopic will do. The view will boil and agitate until the eyepiece reaches thermal equilibrium. 

[Louis D]

13 minutes ago, mikeDnight said:

It doesn't have to be a complex multi element affair, even a simple Mono or orthoscopic will do. The view will boil and agitate until the eyepiece reaches thermal equilibrium.

Reaching thermal equilibrium should take just a few minutes given the small element sizes.  The metal barrel may pose more of a cool down issue.  Perhaps at high power this boiling might be briefly observed.

At low powers, I've never noticed any boiling effect when doing this with large eyepieces I grab from cases left in the house for later comparison.  For instance, I was swapping my 40mm Meade 5000 SWA with my 40mm Pentax XW-R, and could see no initial boiling effects this winter despite grabbing the Pentax from off the dinette table by the back door since it doesn't have a permanent eyepiece case home yet.  The temperature drop was about 30 degrees F.  After 45 minutes of swapping on different targets, the difference in views between the two eyepiece remained unchanged despite both reaching thermal equilibrium by then.

I've repeated this observing pattern dozens of times in the past 20+ years when comparing eyepieces against other eyepieces left in the house in auxiliary cases, and I've never seen boiling attributable to low to mid power eyepieces not being thermally acclimated.  I'm incredibly picky about image sharpness, and I just haven't seen this effect.

I was surprised to see spiky stars in my 90mm triplet APO for 30 minutes or more after bringing it out into the cold.  Everyone always talks about how refractors don't require cool down time.  Well, I found out otherwise.

Edited March 8, 2021 by Louis D

[vlaiv]

2 minutes ago, Louis D said:

The metal barrel may pose more of a cool down issue.

Heat capacity of glass is higher than most metals and comparable to aluminum and magnesium and glass elements in eyepiece are often heavier than barrel - it's unlikely that metal will pose more problems than glass.

4 minutes ago, Louis D said:

I was surprised to see spiky stars in my 90mm APO for 30 minutes or more after bringing it out into the cold.  Everyone always talks about how refractors don't require cool down time.  Well, I found out otherwise.

Of course they do - triplets much more than doublets and aperture plays a part too.

I guess people often say that because they use relatively small aperture size refractors - those cool down quickly compared to say similarly priced MCT of 6-7" of aperture

 

[John]

15 minutes ago, mikeDnight said:

It's an easy issue to resolve (pun intended). Simply leave a telescope outside to acclimatise for an hour or two on a winters night, then place any short focal length eyepiece straight out of a warm house into the focuser and aim your scope at a planet. It doesn't have to be a complex multi element affair, even a simple Mono or orthoscopic will do. The view will boil and agitate until the eyepiece reaches thermal equilibrium. 

I just don't find that Mike. I keep my eyepieces in a centrally heated room all the time and each one only goes outside when it goes into the scope, then back into the centrally heated room again until needed again.

I've observed this way for the past 10 years I guess and I've had no problems whether using simple orthos or very complex hyper-wide eyepieces.

I have tried having the eyepieces out with the scope when at star parties and outreach events and have sometimes had problems with fogging when eyepieces have got too cold but never optical issues when they are warm.

Interesting how two observers can have very different experiences isn't it but there you go

Refractors do need cool down times of course but I have noticed that the Tak FC100 is mininal in this respect. My Vixen ED102 on the other hand seems to need a good 30-40 minutes for the optics to get "on song". Fatter tube and thicker glass perhaps ?

 

 

[Louis D]

32 minutes ago, vlaiv said:

Heat capacity of glass is higher than most metals and comparable to aluminum and magnesium and glass elements in eyepiece are often heavier than barrel - it's unlikely that metal will pose more problems than glass.

I was thinking in terms of a 4mm orthoscopic eyepiece that the quoted post might have been referring to.  In that case, the metal barrel thermal mass far outweighs the thermal mass of the tiny amount of glass.  Perhaps that poster has large transference of heat from the metal barrel into the glass leading to the image boiling they saw.

I was just theorizing to try to explain what they are seeing.  I don't use tiny, high power eyepieces due to their limited eye relief; so I have no direct experience to draw upon with regards to that person's experience.  My high power Pentax XL and XW eyepieces sit outside with the scope cooling in their case, so there's no equilibrium issue with them.  I could try leaving the 3.5mm XW inside sometime on a cold night to see if there is any effect on the image.  However, we're rapidly moving toward spring here, so the nights will be warming up dramatically over the next month, limiting any time to try such experiments this season.

[Don Pensack]

58 years of telescope observing under my belt and I've never seen an issue caused by heat in the eyepieces.

Mirrors, objective lenses, corrector plates, yes.  Eyepieces no.

Easy to see: defocus a star and look for movement of small air pockets in the out of focus star image.

Looks like a can of worms when I try this before the mirrors cool down.

If I try this with eyepieces straight from the box, which don't feel cold to my bare hand, after the mirror is fully cooled, 

no noticeable movement in the out of focus star image.

[jetstream]

2 hours ago, Louis D said:

I was thinking in terms of a 4mm orthoscopic eyepiece that the quoted post might have been referring to

I have a very good sample of a 4mm UO ortho that gets frequent use. I havnt seen a time where this eyepiece caused thermal issues. Just my experience.

[Mr Spock]

Warm, cold, doesn't matter. Sometimes it's better to keep them in pockets