[vsnet-chat 1217] From the "A Guide to Dwarf Novae Observing"

[Taichi Kato <tkato>]

From the "A Guide to Dwarf Novae Observing"

   Extracted and translated from the article, written by me in 1988.

                       Observation of UG stars

   Many observers may think UG stars (dwarf novae) are too faint for their
instruments, but there are a number of objects observable by small telescopes.
The important point is that the rising stage of dwarf novae usually takes
less than a day: this feature makes the data during the rise, even by small
telescopes, can be crucially important.  For those with small apertures
(~10 cm), I recommend to observe, after some experience with bright UG stars,
objects with very infrequent outbursts (table omitted).  Many suspected
dwarf novae are little known about their nature, because only one outburst
was recorded.  Some stars are completely neglected, despite its maximum
brightness, due to the rarity of outbursts.  Disclaimer: please don't be
disappointed if you can't catch any outbursts among them...

   I encourage observers with larger telescopes to observe faint UG stars.
Most of systems reaching mag 12-13 have been well monitored, and are most
likely being observed by other people.  However, systems whose maxima
below 13-th mag are little observed: outbursts only occasionally caught.
Down to 14-15 mag (at maximum) objects, one may be even suggested to change
the target to more "ordinally" ones ("It's only the waste of your observing
time...").  But, why don't we amateurs be more free from such restrictions?
Won't it be worth challenging to visually integrate the light at the eyepiece
to see 20-th mag stars, or to visually resolve 0.01 mag to detect superhumps?

   Rush to your telescope, and observe faint, and fainter, targets in order
not to be written in professional journals as: "objects below 13.5 mag are
omitted, because they will not become suitable targets of continous amateur
monitoring".  Observe all UG stars as many as you can; some systems may
undergo unpredictable bahavior, the clarification of which is entrusted to
your decision and endeavor.

                     How to observe faint stars

   Many observers don't seem to try to go fainter than the calculated limiting
magnitude (e.g. mag 11.8 for a 10-cm telescope).  Generally speaking, one
can clearly detect stars 1.5 mag fainter than this usually accepted limit.
It's not a sort of "tips", but is a natural physiological ability of human
eyes.  According to some physiology textbook, human eyes have an ability
of detecting light comparable to a 20-th mag star.  An observer visually
detected the Comet Halley at 20-th mag in 1984.  These data suggest one can
see 6 mag fainter than the usually accepted limiting magnitude.
There is a method what may be regarded as "mental subtraction of the sky
background": compare the blank field and the invisibly looking variable.
If the field of the variable is more frequently perceived, record the
count.  A more convenient way, used by some observers, is to compare the
variable (invisibly looking) and a brighter comparison (for example a 15-th
mag star), and estimate the ratio of the frequency of positive perception.
Experience with this method will enable you to standarize the sensation of
"1 mag difference" or "2 mag difference" to this faintness level.
Experienced observers with so-called "CCD-eyes" are called to be able to
estimate 17-th or 18-th mag stars without comarison stars...  Lesson your
perception in order not to miss any photons.

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