Many observers record a lot of negative observations, monitoring CVs or
LPVs that are very faint
at minimum. After determining the variable is
definitely not visible, the next step is to determine an upper limit by
recording
the faintest comp star visible. One of the things that makes
doing these observations interesting and a challenge is to
see just how
faint you can actually see on a given night. This can become a test of your
optics, the atmosphere, and
your observing skills.
I never thought I would see a 15th magnitude star in Alex, my 10" SCT.
But, having learned some
tricks after 25,000 variable star observations,
this is not uncommon now.
Here are some tips that may help you push the limiting magnitude of the
aperture you use.
1- Make every effort to get fully dark adapted, and keep it as long as you
can. It usually takes about
twenty minutes to get adapted. Make every
effort not to have to start over again by getting blasted with unwanted
bright
light. This includes not looking at bright stars in the eyepiece. LL
Lyr is pretty close to Vega, so I use Vega to star
hop from. However, I
don't look at Vega in the EP. I line it up in the finder and then watch the
glow in the EP from
a safe distance as I slew the telescope south, until
Vega is out of the field of view.
2- The moon is the enemy. Avoid it, when you can.
3- Early morning is the best time. The sky is generally steadier and darker
after midnight. From about
1AM to twilight human activity and night
lighting are at a minimum.
4- Eliminate all distractions. It takes concentration to get the most out
of your scope.
5- Eliminate stray light. You don't have to observe all night with a
blanket over your head, but for
the few you really want to go after, try
it. A domed observatory is a great help. Even when the moon is up you can
point
the slot away from it to avoid stray light hitting the EP or your
eyes.
6- Use as much magnification as the sky conditions will allow. For me, the
12mm Nagler at 208x is just
about perfect all the time. I have a fairly
generous FOV, and the sky background is fairly dark on good nights. I have
a
10mm Radian (250x) and a 7mm Nagler (357x) that I use on rare occasions.
The 17mm Nagler (147x) I use for LPVs because
it has a wider FOV, and it is
the most comfortable, non-eye-straining EP I have ever owned. I don't use
it to see faint
though. I find I need more magnification to eke out the
faintest stars visible.
7- Relax! Take deep breaths or stretch or whatever makes you chill out. If
you feel yourself getting
tight or tense, take a break. Straining is
counter-productive.
8- Get comfortable. I can see fainter when I am comfortably seated than I
can when I am hunched uncomfortably
over the EP. Try to stay as warm and
dry as you can in the winter. Shivering is counter-productive too!
9- Master the art of averted vision. This really means find your 'sweet
spot'. When I want to see the
faintest star I can, I usually look away and
to the upper right of the star I am trying for. In most of the fields I am
familiar
with I have a 'go to star', the one I usually try for as my
fainter than observation. I also have another fainter one that
I'll try for
on good nights. More importantly, I have a 'go to spot' that I look at
while I try to catch the faint bugger
with averted vision. After you get to
know the fields, you know where to look, and what to look for.
10- You have to believe you can do it. I never looked for 15th mag stars
before because I assumed I wouldn't
see them. WRONG! As long as you are
there, studying the field, you may as well take a look. I just about fell
off my
stool when I saw 156 in the HT Cas field one night under excellent
conditions.
11- Mixed results with this one. Take a few deep breaths and hold the last
one. I don't know if it is
a function of additional oxygen or relaxation or
some body chemistry, but it seems to work sometimes.
12- Be patient. You may have to wait a minute or so for the sky to steady
just enough for you to glimpse
the faintest stars.
13- Observe fields at the highest point above the horizon you can. This
takes some planning, and it changes
the order you observe stars as the
seasons change. Just be aware of it, if you're going to push the limit.
14- Know your local conditions. I can always see fainter in the NE sky over
Lake Huron, because there
is less light pollution. So I may actually view
Cep, Cas, Per, And, and Aur before they approach the zenith, because the
sky
will be darker as they are rising.
15- To be sure you are actually seeing as faint as you think you are
seeing, you have to have a good
sequence, based on accurate photometry.
This eliminates many of the older AAVSO charts whose faint ends can be
pretty
far off the mark. Use a Henden, Stanton, Misselt or Zissell
sequence, or don't get too excited.
One final note. Don't let the pursuit of seeing to the absolute limit of
your scope spoil the
fun or wear you out. It really doesn't make that much
difference if a CV that is not in outburst is <14.5 or <15.0,
so don't go
blind trying. If your goal is to make as many quality observations as you
can in a night, don't waste extra
minutes waiting to glimpse the faintest
star you have ever seen in a field. Move on to the next one. There is
always
the next time.
