Return to Home Page
Re: (fwd) Airmass, extinction and seeing at moderate altitude (h) above the horizon Gianluca Masi asked: > Dear All, > I've been trying to find a description of the problems encoutered > observing lower than 45above the horizon (h<45ccd, 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. There IS an expression. Among the most widely used formulae, Hardie (1962) gave the following expression. (z = sec Z) X = z - 0.0018167(z-1) - 0.002875(z-1)^2 - 0.0008083(z-1)^3 I know that at least some observatories use or used this (or a similar) formula as their standard way of analysis. This and similar formulae would be expected to work up to z=4. However, photometry at such large airmass requires special caution. First, the formula is given for monochromatic light. Broad-band photometry is very sensitive to second or higher order extinction terms which depends on the color of objects. Unfiltered CCD photometry is most severely affected. Even differential photometry usually requires additional empirical second-order correction. Secondly, atmospheric refaction plays an important role. The shape of the PSF (and even astrometric quality) depends on the color of the object. The distortion of the image sometimes need to be properly handled in correctly locating the object (from our experience, we had a considerable difficulty in measureing CI Aql around its solar conjunction). I presume comet photometrists have much more experience at such large airmasses than variable star observers? Regards, Taichi Kato
Return to the Daisaku
vsnet-adm@kusastro.kyoto-u.ac.jp
