[vsnet-chat 1035] Re: Some remarks

["Bjorn H. Granslo" <b.h.granslo@astro.uio.no>]

Dear colleagues,

Timo Kinnunen writes in message [vsnet-chat 1030]:
>  With more experience I have become aware of problems with
> observing near horizon. Usually I don't observe from under 20
> deg above horizon but I think that SS Cyg and RX And, which are
> circumpolar here, are still worth observing while they are at
> north.

According to my experience it is possible to produce observations of
reasonable quality (class 2) of low altitude objects and/or in a
bright sky background (due to twilight, Moon, auroral light etc.)
provided they are made with care. It helps to not to use the lowest
powers (in order to reduce the sky background) and estimate the
brightnesses extrafocally (i.e. using the Bobrovnikoff method in
comet terminology).

I have on a number of occasions observed objects (variables and
comets) at true altitudes 3-10 degrees and have not found any major
deviations when comparing to observations made higher in the sky. In
such cases it is important to take extinction into consideration and
when possible only use comparison stars at nearly the altitude as the
object. Variables with considerable amplitudes and not strongly
defined variations (e.g. Mira stars and cataclysmic objects) should
in my opinion be covered over as much of their observing seasons as
possible and if possible during the whole year.

There is another advantage in adopting such a strategy. One has a
greater chance to discover comets as they tend be brightest when
close to the Sun. Such comets can only be seen at small Solar
elongations and hence low altitudes.

Stars like SS Cyg and RX And can be followed quite closely even
around the time of Solar conjuction provided there are enough
observers at suitable latitudes who are willing and able to overcome
the obstacles in observing at low altitudes and during late nights or
early mornings. Among the brighter dwarf novae the following objects
may be observed from Northern hemisphere through the Solar
conjunction (at least during outbursts): RX And, AR And, SS Aur,
UZ Boo, RU Peg, IP Peg, FO Per, UV Per, KT Per, TY Psc, SW UMa,
BZ UMa, CY UMa and ER UMa (ecliptical latitudes of these objects are
20-41 degrees N). Objects likewise accessible from the Southern
hemisphere include AT Ara, HL CMa, BV Cen, V436 Cen, V442 Cen,
EX Hya, CW Mon, CN Ori, V1159 Ori and EK TrA (ecl. lat. 22-45 deg.
S). The recurrent nova RS Oph is located 16.7 deg. N of ecliptic and
may thus be checked when near Solar conjunction at least to a
limiting magnitude that excludes any major outburst.

> The problems are: extinction (which I believe is very colour
> sensitive), twilight, and everchanging atmospheric conditions.

The atmosphere of Earth is subject to three sources of extinction:
1) Rayleigh scattering in air molcules (mainly exygen and nitrogen)
2) Aeorosol scattering on dust, water droplets, man-made pollutants
and other larger particles, and 3) molecular absorption. Rayleigh
scattering is strongly wavelength dependent (it varies with the
fourth power of the inverse wavelength) and is the chief component
from high altitude sites and other locations with limited amount of
dust and humidity in the air (e.g. at high Northern and Southern
latitudes). The aerosol component is very site and time dependent due
to rapid variations in the amount of the larger scattering particles.
It is much less colour dependent and is approximately proportional
with the inverse wavelength. This source is particularly important
from sites of lower latitudes that are subject to high humidity, dust
and particles from e.g. forest fires. The molecular absorption from
ozone etc. is usually small and may be neglected. The total
extinction is the sum of these components. [More information is
available from Hayes and Latham (Ap.J., 197, 593, 1975) and Green
(ICQ, 14, 55, 1992). The paper by Green is available from
http://cfa-www.harvard.edu/cfa/ps/icq/ICQExtinct.html].
 
What we see is the visible part of the spectrum weighted such that
the eye is most sensitive to the green-yellow part of the spectrum
and gradually less sensitive towards the blue and red parts. Thus
the effective wavelenghts of nearly all objects we observe are not
subject to as large variations as indicated by their spectra. As
pointed out by Taichi Kato in [vsnet-chat 1031] the colour dependent
part of the extinction can be neglected in cases which only cover a
limited part of the spectrum and where no great precission is needed
(as in visual observations). In my opinion, however, this matter
should be investigated further, especially when dealing with very low
altitudes (large airmasses), say below 5-10 degrees.

> So I think Taichi may have commented some apparent changes
> (in RX And) which may be non-astronomical.

I believe the short-lived dips detected by T. Kinnunen and me in
March and April are real and that the status remains unclear due to
lack of confirming observations. I have discussed this with my
Norwegian colleague O. Midtskogen who also observes RX And regularly.
After checking his observations he found that RX And was faint on
March 27-28 (he obtained mv=12.9 on Mar 27.84 UT, which confirms my
sighting at mv=13.0 on Mar 28.11) while it was at its standstill
brightness a couple of nights before and afterwards.

                               Yours sincerely,
                              Bjoern H. Granslo
                            (bgranslo@astro.uio.no)

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