[vsnet-chat 5095] Airmass, extinction and seeing at moderate altitude (h) above the horizon

[Gianluca Masi <gianmasi@fr.flashnet.it>]

Dear All,
I've been trying to find a description of the problems encoutered
observing lower than 45° above the horizon (h<45°)ccd, particularly the
effect of the airmass in limiting the fantest observable stars with an
unfiltered CCD. As a general rule, I have reviewed (in an old message to
this list) the formula:

  X = sec(Z) = 1 / cos(Z)
  
for zenith distance (Z) less than 60 degrees. As I would like to extend
this formula to reach h=25°, is there a large deviation from it in this
range? Also, is the 0.25mag loss per airmass still valid?

Also, it is of interest to me to know if there is any formula/comment
describing the effect on the seeing of these quite low altitudes; in
other words, if at the zenith I have a 1" seeing, what value I can
expect looking at h=30°?

Any comments, personal experiences would be very appreciated.

Thank you very much,
Gianluca
-- 
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* Gianluca Masi                           "Two things fill the soul   *
* Via Madonna de Loco, 47               with awe and reverence [...]: *
* 03023 Ceccano (FR)                     the starry sky above me and  *
* ITALY                                    the moral law within me"   *
* e-mail: gianmasi@fr.flashnet.it                                     *
* web:  http://vsnet.bellatrixobservatory.org             I. Kant       *
* Minor Planet Center Obs. Code: 470 - Ceccano                        *
* Center for Backyard Astrophysics - Italy                            *
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* CCD-astrometry-photometry@egroup.com mailing-list moderator         *
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