Re: [APML]: APML: Wind and seeing

[Wil Milan <wmilan@airdigital.com>]

That's very interesting about you sometimes having good seeing with high winds
at Mt. Pinos, yet here in the desert I've had the opposite experience. After
thinking about it a bit, I have a theory to explain the difference:

- If memory serves, the night winds at Mt. Pinos are primarily driven by the
general coastal wind patterns. There's not a lot of temperature gradient as the
winds flow over the top of the ridge, so there are not a lot of density
variations which cause bad seeing.

- Here in the desert, on the other hand, there is no coastal flow and the winds
are primarily driven by local temperature differences. When there's significant
wind it usually indicates a lot of mixing of warm and cold air, and that does
generate density differences and therefore changes in refraction which result
in bad seeing.

If this explanation is correct then I would expect that when you have a Santa
Ana flow (high-pressure cold air in the high desert flowing down to sea and
being adiabatically heated) then I would expec the seeing *would* be poor
because then you do have large temperature gradients and moving air. But I
wouldn't think you'd ever have strong Santa Ana flows over Mt. Pinos because
the valleys north of Mt. Pinos are not very high. (That's theory only; am I
wrong?)

Do you observe at the top of the peak, or in the parking cul-de-sac below the
summit? Years ago when I lived out there I used to spend a lot of time in that
area, both at Mt. Pinos and in the valleys below (west of Gorman, Frazier Park,
Piru Creek region, etc.). My wife and I used to go to the open summit of Mt.
Pinos to watch for condors, but for astronomy I much preferred the parking lot,
not just for the convenience but precisely because it's much better protected
from wind. (Bloody cold up there this time of year -- and I'm sure you don't me
to tell you that. :-) )

Wil M.


Martin C Germano wrote:

>
> Wil,
>
> >> Bob Fera wrote:
> >>
> > > I, like many other astrophotographers, am of the opinion that you can't
> >>have too much mount -- my C-11 sits on a AP 1200. Given that, I'd
> >>definitely recommend the 900 over the 600. I previously owned a G-11 and
> >>felt that it was inadequate for shooting with a C-11 (fine for visual,
> >>though). The slightest amount of wind was enough to give it  and the
> >>autoguider fits.
> >
> > I don't shoot when gale-force winds are blowing :-), but under more
> normal conditions I've experienced no such problems with a C11 on a G-11
> mount.
> >
> > I *have* had the experience of high winds playing havoc with guiding even
> though the winds were *not* blowing on the mount. In that case the
> guide-star jitters were due to the wind howling over the hill behind me
> (the "hill" was actually the top of a mountain ridge; I was located just
> below the top). The extreme turbulence caused by the wind cresting over the
> peak
> > above caused jittery guide stars and erratic guiding, looking very much
> like mount vibration, but in fact the mount was in still air. (That was an
> AP mount with an smaller scope, but it wouldn't have mattered if it was
> Palomar 200" mount; the motion was due to the air moving, not the mount.)
> >
> > As a practical matter I've found that when strong winds are blowing the
> seeing will be so poor that it's not worth bothering with narrow-angle
> photography. Even when wind on the mount is not a factor (as above), the
> star images will balloon to the point that the photo will never be
> outstanding.
> >
> > Wil M.
>
> Bob Fera was just voicing a fact of life for Mt Pinos, where it can be
> windy AND have *good* seeing. As a long time (1976) photographer from the
> Mt Pinos (California) parking lot, I have quite often experienced good
> seeing and high (est 15-30mph) winds. At nearby Pine Mt (due south across
> the Lockwood Valley, known for its terrible seeing) some of the best seeing
> I have ever experienced occurred with the wind howling 50 feet over my back
> at the ridgetop.
>
> Conversely, at another mountaintop ridge I frequent (Figueroa Mt), the
> seeing is most often fair to poor when the wind blows. I guess the real
> reason we like to "over-mount" our tube assemblies is for stability when
> the wind strikes our unshielded telescopes, as is unfortunately a way of
> life in our Southern California mountains. *Bad* seeing will cause problems
> whether the wind is blowing or not. I guess we just have to take what we
> get out in the field, as my experience with wind verses seeing is a mixed
> bag in the Southern California area.
>
> Martin C Germano
> Thousand Oaks, Calif
> mgermano@gte.net



--
"When I consider your heavens, the work of your fingers,
the moon and the stars which You have set in place,
what is Man that you are mindful of him,
or the son of Man that you care for him?"  -- Psalm 8