Dew on triplet refractors

[Richard136]

 

Hi.

I am getting a second hand triplet refractor. It's air spaced. 

I am concerned about the potential effects of dew between the elements of the objective.

Concern is along the lines of:

 - dew builds up on all cold surfaces, including presumably the inner surfaces in the overall objective.

- dirt, water marks etc condense on the inner faces.

- it's not practical to clean these inner faces.

- potentially leaves an uncleanable lense

Is this concern grounded or off base?

I don't have a dew heater yet. I'll get one of course. The question is do I need one for the care of the objective in terms of dew, or just to extend observing sessions.

Thanks for the advice. R

[johnturley]

A dew heater will help, I keep one on all the time on a low setting when not in use, to try to prevent this happening with my Esprit 150 (situated in an observatory shed), only problem is that its probably costing me a fortune on my electricity bill.

In addition I have a desiccant dehumidifier in the observatory shed, plus also use a desiccant cap.

John

[michael.h.f.wilkinson]

I haven't had any real issues with my APM 80mm F/6 triplet. I do not have a dew heater (should get one), but dew always first builds up on the outer surface, so imaging is curtailed when that happens (which is rare, given the deep lens hood). When I bring the kit inside, the front element does generally fog up, but I only store it in its case once all signs of dew have disappeared. I have had this scope for about 8 years without any issues.

[John]

I've not had any dewing on my 130mm TMB/LZOS triplet while observing. As with Michael, the objective has misted up when the scope is bought cold into the house after observing but that clears quickly and without residue when as the scope warms back up. I've not had any misting between lens elements or on the inside surface of the objective even when the glass is cold and the house warm.

The tube on my scope is made of Kruppax which seems to be very good at stopping the objective dewing even when the outside of the tube is dripping wet.

I guess if triplets routinely dewed / misted on more than the externally facing optical surface, they would be deemed pretty impractical scopes in UK conditions ?

 

 

 

[Richard136]

Thanks, all.

So this sounds like I'm concerned over nothing. Good to know.

What then is the general practice with dew heaters?:

- turn on the lowest useful setting to prevent dew whilst scope is outside. How warm do they get and doesn't this heating creates thermal currents in the tube?

- leave the heater on when bringing the scope inside after observing(?)

I'm assuming the heater does not prevent dewing-up when bringing the scope back indoors, i.e. it doesn't heat it up that much.

I'm clear on the points about not packing the scope away whilst wet; that makes total sense.

[smudgeball]

Hi

I've got a WO GT102 which is air spaced.

I've had some of the issues you mention, but running a heater band close to the elements cures them.

In my case I think it's a bit of inner condensation.

Ultimately I'd love to send away the front element section to have it professionally cleaned, but I don't think this service exists (well not in the UK)

Otherwise mine would be away to be cleaned.

But I don't have any real issues with my imaging.

 

Neil

 

[Richard136]

Thanks, Neil.

Sounds like a different experience to the others if you are getting inner condensation. Love to see a pic of the objective after the dewing has gone, if possible please.

[joe aguiar]

just wanted to add in case you didn't know dew is a problem for most scopes if not all.

there is a thread just like within 2 weeks where a reflector (I think was a 6"f/5) mirror dews up, I have also had my 2ndary mirror dew up on reflector too, and of cours the scts /maks always have this issue.

so dew is a problem that probally all scopes will have some more then others and of course depending where you live ie: by lake, by ocean front, in a country with high humidty

joejaguar

[Space Hopper]

A precautionary warning with refractors......and more so if it has a Fluorite elememt

Be careful not to induce 'thermal shock' ie taking a refractor outside from a warm 20ºC house into sub zero temperatures, and then vice versa afterwards.

My routine is :

1. Take scope in its closed case outside

2. Leave 30 mins

3. Open case an inch or two and leave for another 30mins

4. Mount scope and begin observing (Leave case outside in cold. Not closed, just ajar to stop interior getting dewey)

5. After session put scope back in cold case and close it up.

6. Return case indoors to a cool room and leave for 30 mins

7. Open case an inch or so and leave for another 30 mins

8. Open case some more, let scope gradually reacclimatise and then check the objective for dew. If it is still dewey, i'll remove the covers and let it evaporate off.

9. Always let the scope dry out completly. Don't leave a wet cold dewey scope in a locked case.

 

Never rush the above process. Maybe i'm being a little over cautious here, but its better to be safe than sorry.

Its only an issue really if its winter and you have a 20º to 30º temperature differential.

 

Enjoy your new scope ! Let us know how you get on with it ?

Clear skies  😀

 

 

 

 

[dweller25]

27 minutes ago, Space Hopper said:

A precautionary warning with refractors......and more so if it has a Fluorite elememt

Be careful not to induce 'thermal shock' ie taking a refractor outside from a warm 20ºC house into sub zero temperatures, and then vice versa afterwards.

My routine is :

1. Take scope in its closed case outside

2. Leave 30 mins

3. Open case an inch or two and leave for another 30mins

4. Mount scope and begin observing (Leave case outside in cold. Not closed, just ajar to stop interior getting dewey)

5. After session put scope back in cold case and close it up.

6. Return case indoors to a cool room and leave for 30 mins

7. Open case an inch or so and leave for another 30 mins

8. Open case some more, let scope gradually reacclimatise and then check the objective for dew. If it is still dewey, i'll remove the covers and let it evaporate off.

9. Always let the scope dry out completly. Don't leave a wet cold dewey scope in a locked case.

 

Never rush the above process. Maybe i'm being a little over cautious here, but its better to be safe than sorry.

Its only an issue really if its winter and you have a 20º to 30º temperature differential.

 

Enjoy your new scope ! Let us know how you get on with it ?

Clear skies  😀

 

 

 

 

I think you are being overly cautious when taking your scope out.

I have a 5” 22 year old Fluorite telescope and have been taking it from the centrally heated house straight outdoors often in freezing mid winter temperatures without issue.

I think the fluorite thermal shock issue is fake news - although rumour has it that Roland Christen once dropped cold water on a very hot fluorite element and it shattered - as glass would - but even that may be a myth 😂

But each to his own 😉

As you say making sure the OTA dries out well after a session is very good practice.👍

Edited December 3, 2019 by dweller25

[dweller25]

18 hours ago, Richard136 said:

 

Hi.

I am getting a second hand triplet refractor. It's air spaced. 

I am concerned about the potential effects of dew between the elements of the objective.

Concern is along the lines of:

 - dew builds up on all cold surfaces, including presumably the inner surfaces in the overall objective.

- dirt, water marks etc condense on the inner faces.

- it's not practical to clean these inner faces.

- potentially leaves an uncleanable lense

Is this concern grounded or off base?

I don't have a dew heater yet. I'll get one of course. The question is do I need one for the care of the objective in terms of dew, or just to extend observing sessions.

Thanks for the advice. R

You could buy a thin black foam camping mat and some elastic webbing to fashion a long dewshield for very little money - it will keep dew off the objective for much longer than just the standard dewshield.

Edited December 3, 2019 by dweller25

[Rob]

I've had dew form between the optics, and on the front lens with no ongoing effects. I don't own a dew heater, however like John I also bring the scope into the house after observing.

Then once back in the warm house. Leave all caps off, wait an hour (or just leave over night if late) before re capping and storing.

Rob

[Space Hopper]

10 hours ago, dweller25 said:

I think you are being overly cautious when taking your scope out.

I have a 5” 22 year old Fluorite telescope and have been taking it from the centrally heated house straight outdoors often in freezing mid winter temperatures without issue.

I think the fluorite thermal shock issue is fake news - although rumour has it that Roland Christen once dropped cold water on a very hot fluorite element and it shattered - as glass would - but even that may be a myth 😂

 

Possibly yes ; I'm over cautious with mine being a new scope. 

But this product here comes with a warning......in Red ink.

https://www.baader-planetarium.com/en/accessories/optical-accessories/barlow-lenses-coma-and-glaspathcorrectors/fluorite-flatfield-converter-(ffc)--3x-8x.html

Apologies if i'm going off topic a bit.

 

I think the real danger with Fluorite (if you're not careful) is if the front exposed element is Fluorite. I think some Tak refractors are configured like that. Fraunhoher designs ? Some Borgs too....

But it is certainly fragile and will degrade in moisture.

 

[John]

The FC-100 Taks have Fluorite as the rear element and use the Steinheil design.

 

[dweller25]

Naturally occurring Fluorite is fragile and absorbs moisture.

I believe synthetic Fluorite used in high end photographic lenses and telescopes is fully multicoated and does not absorb moisture.

However it always pays to be careful with our kit 👍🙂🙂

 

Edited December 4, 2019 by dweller25

[michael.h.f.wilkinson]

I tend to store my scopes in a only slightly heated garage (just enough to keep the frost out), and that means temperature differences with the outside world are not that severe. Having said that, I do not believe going from say 20 Celsius to -20 Celsius or lower would cause thermal shock, as the heat capacity of air is so low, and free convection is pretty inefficient when the objective is well recessed within a dew shield.