Re: Problems with dew
- From: "Roger Hamlett" <rogerspamignored@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 05 Oct 2005 13:54:00 GMT
"Andy Grove" <andygrove73@xxxxxxxxx> wrote in message
news:1128515174.453992.191220@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I was attempting to image Mars for the first time a few days ago using
> an 8" LX90 with the Meade LPI camera. I had the telescope set up for 4
> or 5 hours before Mars was in view (I had to wait for it to rise above
> my neighbours house). When I started to take images all I could see on
> my laptop screen was a very bright disc with fuzzy edges. I tried using
> an orange filter and that reduced the brightness and gave me a nice
> orange disc with no detail at all on the surface. Frustrated, I packed
> up the scope and it was only when I got it indoors that I realised that
> the lens was covered in dew. Presumably this is why I could see no
> detail.
>
> For my next attempt I will put the scope out just an hour before I can
> view Mars and I will keep the dist cover on and keep the scope pointing
> down until I'm ready (tips I've read over the last couple of days) but
> my question for the group is .. should I really be buying a dew
> prevention system? I'm reticent to spend more money at the moment as
> I'm already way over budget on my new hobby but am I wasting my time
> trying to image planets in this weather without such a system?
>
> Cheers,
>
> Andy.
Assuming you have a suitable power source, consider just getting a small
12v hairdryer.
Basically, dew _will_ form on any surface that cools below the dew point
of the atmosphere. In conditions where the humidity is high, this will not
involve the parts cooling by very much. The dew shield, reduces the angle
of the sky being 'seen' by the front of the scope (the sky is a relatively
'cold' thing, and the scope experiences radiative cooling as it looks at
this, reducing the area looked at, slows this). It also tends to trap a
volume of air, slowing the rate at which dew can actually migrate to the
scope front. However it only slows the inevitable, and eventually the
scope will drop below the dew point, and problems will appear. The
solution is to slightly heat the surfaces, but obviously only by a small
amount to avoid too many air currents. A few seconds burst from a
hairdryer, between image sequences, is cheap, and easy, and keeps the
problem away. These dryers are a lot cheaper than a dew heater system, and
even with such systems tend to be needed to deal with areas unheated by
the system. The dew heater is great, for long exposure imaging, where the
steady small heat input is ideal, but are relatively expensive, and for
many applications the dryer is easier. You will see these in just about
every bag being bought to star parties...
Best Wishes
.
- References:
- Problems with dew
- From: Andy Grove
- Problems with dew
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