Re: [APML] 201XT- too hot outside?

[Glenn Shaw <gshaw@alaska.net>]

Regarding star scintillation:

In summer when the ground heats up by the sun, the air next to the
ground is heated and rises. The result is convection, which is
responsible for bringing afternoon cumulous clouds and afternoon
showers. The convection is more vigorous and deeper throughout the
atmosphere in summer. 

Scintillation of a star is caused by variations in the refractive index
of the medium through which the rays are passing. Since refractive index
is proportional to gas density, and since density varies in convecting
airmasses, scintillation is sometimes (not always, but sometimes) more
vigorous in summer than in winter.

You can't take these arguments too far, however, because in highly
stable atmospheres, such as those over central alaska in winter, you can
get extremely serious scintillation caused by gravity waves that
propagate and build up in "waveguides" in the lower regions of the
atmosphere.

So now that I have everybody confused, just suffice it to say that "yes,
its common to have poor seeing in the summer months".

glenn shaw
Alan Voetsch wrote:
> 
> Hello all,
> 
> I had a heckuva time getting my 201 to guide tonight. When cycling through
> the brightness & location displays, it would read: '25 at 55' for a couple
> of cycles. Then I got this reading:'05 at 33' for a couple of cycles.
> 
> The first time this happened there were other stars nearby, so I understand
> what may have been going on. The second time, no other stars were nearby.
> 
> It's been quite warm lately. It was probably still at least 70 degrees out
> after midnight.
> 
> My question: can the warm weather cause the 201 to SPAZ out? Just wondering,
> I haven't had this problem before. Proper dark frames were taken where
> needed.
> 
> Thanks in advance for any input.
> 
> Alan Voetsch
> 
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