[vsnet-chat 9] Mira Comparison stars

[hz642@cleveland.Freenet.Edu (Robert J. Modic)]

     I'd like to clarify the remarks I made yesterday about Alpha
Cet and other comparison stars for Mira.  First, I assumed that
most observers are making estimates of Mira naked eye now that it
is mag ~2.6.  Secondly, I assumed  that these estimates where made
with the stars in focus.  In this case, the Purkinje effect will
cause Alpha Cet to appear ~0.2 mag or more brighter than its V mag.
Defocusing the stars and using averted vision will cause the opposite
to occur, as Brian Skiff mentioned.  I observed Mira, Alpha Cet, and
Beta Ari last night using these two methods.  With direct vision and
in focus, Alpha Cet was ~ 0.3 mag brighter than Beta Ari.  Using
a little averted vision, Alpha was 0.1 to 0.2 mag brighter.  Viewed
out of focus (I wear glasses) and with plenty of a.v., Alpha was
~0.15 mag fainter than Beta Ari.

     Finally, my suggestion of Beta Cet as a good comparison star
for Mira was based on its V mag and color.  Obviously, when Mira
is in the SW from mid-northern latitudes, Beta Cet is too low to be 
a useful comp star.  But when Mira is in the SE or when viewed from the
southern hemisphere, Beta Cet is a good choice.  Alpha Ari is a good
alternative for us northerners.

Regards,

Robert J. Modic
AAVSO

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