NexStar 11 Internal Condensation

For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with other
telescopes.

Puttimg a dew shield ahead of the corrector plate does allow extra observing
time, which is extended by using a hot air blower onto the outside surface
of the corrector to periodically remove external condensation.

What I do not overcome is the intenal condensation on surface of both the
secondary mirror and the primary mirror.
Once this condensation occurs the only remedy is to await the next morning
sunshine to gradually evaporate the condensation, or to take the NexStar 11
into a warm dry room for some hours ( i.e. the rest of the night + ).

Does any one have practical experience of solving the internal condensation
on primary and secondary mirrors of the NexStar 11 ? I do not think my
climate ( near London ) is very extraordinary.

Does mounting a Kendrick heating element around the outside of the tube as
near the primary mirror as possible , plus another heating element around
the top of the scope as near the corrector plate as possible prove effective
against preventing condensation on telescope inside surfaces ? or is image
degradation from air currents then a problem ?

Could the solution in fact be to cut a hole in the NexStar 11 tube and mount
an extraction fan on it. Is it practicable to do that with the carbon fibre
tube ?

Anthony - dewed internally near London

On Wed, 17 Sep 2003 09:08:31 +0100, "Anthony Stokes"
wrote:

For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with other
telescopes.

Puttimg a dew shield ahead of the corrector plate does allow extra observing
time, which is extended by using a hot air blower onto the outside surface
of the corrector to periodically remove external condensation.

What I do not overcome is the intenal condensation on surface of both the
secondary mirror and the primary mirror.
Once this condensation occurs the only remedy is to await the next morning
sunshine to gradually evaporate the condensation, or to take the NexStar 11
into a warm dry room for some hours ( i.e. the rest of the night + ).

Does any one have practical experience of solving the internal condensation
on primary and secondary mirrors of the NexStar 11 ? I do not think my
climate ( near London ) is very extraordinary.

Does mounting a Kendrick heating element around the outside of the tube as
near the primary mirror as possible , plus another heating element around
the top of the scope as near the corrector plate as possible prove effective
against preventing condensation on telescope inside surfaces ? or is image
degradation from air currents then a problem ?

Could the solution in fact be to cut a hole in the NexStar 11 tube and mount
an extraction fan on it. Is it practicable to do that with the carbon fibre
tube ?

Anthony - dewed internally near London



This is an uncommon problem in my experience, there must be something
about the way your scope is stored that makes it prone to internal
condensation. With an eyepiece in place it's effectively a 'closed
tube' scope so there is no way excessive moisture should get inside to
cause a problem. However, if it's *stored* in damp conditions then
it's already got moisture in there when you take it outside and start
using it.

I suggest using some desiccant to absorb internal moisture when it's
not in use - I've found silica gel held in a suitable container works
well enough. A fan is not an answer to this particular problem, when
it's 99% humidity outside on a dewey night you don't want to be
blowing it all into your tube! But the desiccant is just a temporary
fix - you need to look carefully at your storage conditions and find
out why it's happening in the first place.

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

"ChrisH" wrote in message
...
This is an uncommon problem in my experience, there must be something
about the way your scope is stored that makes it prone to internal
condensation. With an eyepiece in place it's effectively a 'closed
tube' scope so there is no way excessive moisture should get inside to
cause a problem. However, if it's *stored* in damp conditions then
it's already got moisture in there when you take it outside and start
using it.
I sometimes get the same problem with my LX55SN-10 with condensation on the
inside of the corrector plate. The scope is normally stored in a cool dry
room with eyepiece tube sealed.

I suggest using some desiccant to absorb internal moisture when it's
not in use - I've found silica gel held in a suitable container works
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2 - 5 mm beaded
0.5 - 1.0 mm granular
1 - 3 mm granular
3 - 6 mm granular.

GeeJay Chemicals Ltd.
16 Gosforth Close, Sandy, Beds., SG19 1RB.
Tel : 01767 682774. Fax : 01767 692409.
e-mail :
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500 gram tub for £12 or a 1.5kg tub for £22. These prices
include return delivery - cheque or VISA
---------------------------------------------------------------
The silica gel can be heated and then re-used.

blowing it all into your tube! But the desiccant is just a temporary
fix - you need to look carefully at your storage conditions and find
out why it's happening in the first place.
Maybe the OTA has been opened up and then re-assembled in humid conditions.
If this is the case then the dessicant solution may be all that is required.

Sally

Anthony Stokes wrote:
For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with other
telescopes.

Puttimg a dew shield ahead of the corrector plate does allow extra observing
time, which is extended by using a hot air blower onto the outside surface
of the corrector to periodically remove external condensation.

What I do not overcome is the intenal condensation on surface of both the
secondary mirror and the primary mirror.
Once this condensation occurs the only remedy is to await the next morning
sunshine to gradually evaporate the condensation, or to take the NexStar 11
into a warm dry room for some hours ( i.e. the rest of the night + ).

Does any one have practical experience of solving the internal condensation
on primary and secondary mirrors of the NexStar 11 ? I do not think my
climate ( near London ) is very extraordinary.

Does mounting a Kendrick heating element around the outside of the tube as
near the primary mirror as possible , plus another heating element around
the top of the scope as near the corrector plate as possible prove effective
against preventing condensation on telescope inside surfaces ? or is image
degradation from air currents then a problem ?

Could the solution in fact be to cut a hole in the NexStar 11 tube and mount
an extraction fan on it. Is it practicable to do that with the carbon fibre
tube ?

Anthony - dewed internally near London



I had a C8 throughout the 80's and often had similar problems, but
mainly concerning the corrector plate. My scope was stored outside in a
shed, and despite keeping the back end sealed moisture always got in. In
those days, the corrector plate was mounted on cork pads and retained by
a ring-I dont think this could be described as watertight. I guess heavy
dew could seep inside quite easily.
Interestingly, when it dewed up internally, it started off as bands of
dew,always in the same places,suggesting that the corrector plate had
not been cleaned properly prior to assembly.
I eventually took the plate off and cleaned it as I would a mirror, and
this greatly reduced the problem. (Note that the plate was uncoated). At
the same time, I removed dust particles from the mirrors these being the
points at which condensation occurred on the mirrors.
After the cleaning, I only experienced internal dewing under exceptional
circustances. I made a point of always bringing the scope in to dry out
overnight, and of course used a dewcap.
I think that the suggestions about dessicants are good, but based on my
experience, you should look at the possibility of dirt on the optical
surfaces and consider the option of getting them cleaned if possible.
Have you spoken to anyone at Hinds or BCF about this?
Phil Bishop

Hi Anthony,

I too own an NS11 and have never had the problem you describe. The problem
could get really serious if the moisture is not eliminated completely and
quickly because you could end up with mold and/or mildew on your mirror and
corrector surfaces. Storage in a dry air conditioned place is the answer but
if not possible, I would recommend this product:

http://www.buytelescopes.com/product.asp?pid=5690

Take Care,

JAS


"Anthony Stokes" wrote in message
...
For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late
night
observing totally impossible. I have not had this problem with other
telescopes.

Puttimg a dew shield ahead of the corrector plate does allow extra
observing
time, which is extended by using a hot air blower onto the outside surface
of the corrector to periodically remove external condensation.

What I do not overcome is the intenal condensation on surface of both the
secondary mirror and the primary mirror.
Once this condensation occurs the only remedy is to await the next morning
sunshine to gradually evaporate the condensation, or to take the NexStar
11
into a warm dry room for some hours ( i.e. the rest of the night + ).

Does any one have practical experience of solving the internal
condensation
on primary and secondary mirrors of the NexStar 11 ? I do not think my
climate ( near London ) is very extraordinary.

Does mounting a Kendrick heating element around the outside of the tube as
near the primary mirror as possible , plus another heating element around
the top of the scope as near the corrector plate as possible prove
effective
against preventing condensation on telescope inside surfaces ? or is image
degradation from air currents then a problem ?

Could the solution in fact be to cut a hole in the NexStar 11 tube and
mount
an extraction fan on it. Is it practicable to do that with the carbon
fibre
tube ?

Anthony - dewed internally near London

Anthony Stokes wrote:

For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with other
telescopes.

etc.

I wonder whether the carbon fibre/fiber tube is better or worse (or no
change) than an aluminum/aluminium tube as far as internal condensation
is concerned? Any real-time experience out there? I'm more concerned
with experience with both choices for a Celestron 9¼ AS but any and all
experience is of interest.

On Wed, 17 Sep 2003 09:51:50 -0400, Dan Wenz
wrote:

Anthony Stokes wrote:

For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with other
telescopes.

etc.

I wonder whether the carbon fibre/fiber tube is better or worse (or no
change) than an aluminum/aluminium tube as far as internal condensation
is concerned? Any real-time experience out there? I'm more concerned
with experience with both choices for a Celestron 9¼ AS but any and all
experience is of interest.

The key requirement for internal condensation is the OTA being full
of damp air immediately prior to a drop in temperature. The water will
deposit on the coldest surfaces, so if the tube walls are slightly
warmer than the optics then it's the mirrors/corrector plate that
catches it. I don't know if a CF tube cools more slowly than an
aluminium one, if it does then it might make things worse. However, I
should add that I have *never* had internal condensation in my 10"
LX200 and it's been out in some awful dewey nights! I've seen water
literally dripping off the outside but never anything inside. My
scopes are kept at normal room temperature when not in use (well, a
bit cooler than room temperature in winter as the central heating is
turned off in that room - but it is quite dry).

ChrisH

UK Astro Ads: http://www.UKAstroAds.co.uk

ChrisH wrote:
I've found silica gel held in a suitable container works well enough.

One example of a "suitable container" is a muslin bag inside a
perforated 35mm film can. Just slip it into the visual back instead of
an eyepiece.

Best,
Stephen

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"Dan Wenz" wrote in message ...
Anthony Stokes wrote:
For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late night
observing totally impossible. I have not had this problem with
othertelescopes.

I wonder whether the carbon fibre/fiber tube is better or worse (or no
change) than an aluminum/aluminium tube as far as internal condensation
is concerned? Any real-time experience out there? I'm more concerned
with experience with both choices for a Celestron 9¼ AS but any and all
experience is of interest.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's now seventeen hours since bringing my NexStar 11 GPS scope indoors into
a warm and quite dry room. I removed the eyepiece to improve air
circulation.

Moisture takes only three or four hours to evaporate from the internal
surfaces of the optical elements: but there are still patches of moisture
visible on the inside wall of the carbon fibre tube. I do not think a
metal tube scope could be as bad as this !

Anthony ( near London in dry weather ).

Anthony,

I would recommend - once you get the moisture out - that you consider using
a skylight filter on the visual back. I use one for my [lowly] N5 and don't
know if they are available for your 2" visual back, but nothing gets inside
my OTA. Well, very little. But I would think that this would help keep the
moisture out of your N11.

Jim

"Anthony Stokes" wrote in message
...

"Dan Wenz" wrote in message ...
Anthony Stokes wrote:
For several months I have been using a NexStar 11 SCT but have found it
suffers from a terrible internal condensation problem that makes late
night
observing totally impossible. I have not had this problem with
othertelescopes.

I wonder whether the carbon fibre/fiber tube is better or worse (or no
change) than an aluminum/aluminium tube as far as internal condensation
is concerned? Any real-time experience out there? I'm more concerned
with experience with both choices for a Celestron 9¼ AS but any and all
experience is of interest.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's now seventeen hours since bringing my NexStar 11 GPS scope indoors
into
a warm and quite dry room. I removed the eyepiece to improve air
circulation.

Moisture takes only three or four hours to evaporate from the internal
surfaces of the optical elements: but there are still patches of moisture
visible on the inside wall of the carbon fibre tube. I do not think a
metal tube scope could be as bad as this !

Anthony ( near London in dry weather ).