Lens cover on or off?

15 years 4 months ago #7949 by Bill_H
Lens cover on or off? was created by Bill_H
Just a tiny silly question. when I put my scope out to cool down prior to using it, should the lens cover be taken off or left on, will leaving it on during this stage encourage condensation, or doesn't it really matter?

Astronomers do it with the lights off.

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15 years 4 months ago #7950 by Seanie_Morris
Replied by Seanie_Morris on topic Re: Lens cover on or off?
From my own personal experience Bill, I leave the scope out for about 30 minutes to 1 hour before I go out, with the cover OFF. I also leave the tube horizontal. This way, the minimum amount of condensation forms and rests on the objective lens (same principle for a reflector). If you have it tilted upwards (more vertical), then condensation, and dew, will settle on it. If you tip it down, the the condensation and dew will form on the focuser! (well, on a refractor anyway.)

Seanie.

Midlands Astronomy Club.
Radio Presenter (Midlands 103), Space Enthusiast, Astronomy Outreach Co-ordinator.
Former IFAS Chairperson and Secretary.

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15 years 4 months ago #7951 by Bill_H
Replied by Bill_H on topic Re: Lens cover on or off?
Many thanks Seanie.
Bill.

Astronomers do it with the lights off.

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15 years 4 months ago #7964 by John OBrien
Replied by John OBrien on topic Re: Lens cover on or off?
This is what I do with a camera - put it inside a plastic bag and take it outside and leave it for 30 minutes to cool, that way codensation forms on the bag.

Given that a scope is a bit bigger, I would leave it to cool in a unheated room like a porch or shed first. If I can't do that then outside horizontal with the cap off the front and the eyepiece opening faceing down.

Still though, it's hard to avoid getting condensation after stuff has been outside for a while - any more options?

"We are the music makers ... and we are the dreamers of dreams." - W.W.

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15 years 4 months ago #7965 by albertw
Replied by albertw on topic Re: Lens cover on or off?

Still though, it's hard to avoid getting condensation after stuff has been outside for a while - any more options?


When I do bother to let it cool, I put a dew shield on (camping carry mat!) and leave it in a horizontal position for a while. I need the dew shield since the corrector plate can quickly fog up without it.

Cheers,
~Al

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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15 years 4 months ago #7972 by dpower
Replied by dpower on topic Re: Lens cover on or off?
I leave mine in the conservatory all of the time, it's the coldest room in the house and condensation isn't a problem, so the scope is generally nice and cold when I start.

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15 years 4 months ago #7975 by Seanie_Morris
Replied by Seanie_Morris on topic Re: Lens cover on or off?

Still though, it's hard to avoid getting condensation after stuff has been outside for a while - any more options?


If your scope allows, a small fan is never any harm. A low rpm fan just blowing air accross the objective lens or objective mirror, helps keep it a stable temperature. This limits the amount of expansion, keeps ita constant temperature, and prevents dew/condensation from settling on the surface.

Just that in winter, have a few spare batteries handy - they tend to die quicker, especially rechargeables, in extreme cold.

Seanie.

Midlands Astronomy Club.
Radio Presenter (Midlands 103), Space Enthusiast, Astronomy Outreach Co-ordinator.
Former IFAS Chairperson and Secretary.

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