- All-sky photometry is what's needed to obtain
calibrated magnitudes and colors of one or more objects.
Note that the success of all-sky photometry depends critically
on sky conditions remaining clear and stable while you're observing.
-
When doing all-sky photometry at Wallace the majority of your
observing effort will be spent collecting enough standard-star
data to monitor the stability of the photometric conditions
during your time at the telescope.
This means that the number of target object fields you can
robustly calibrate to
standard magnitudes in one night is quite limited - perhaps
as few as 1-3 fields for a novice observer,
and maybe up to 3 times as many for an experienced observer.
If your project involves a larger number of program objects,
such as a color-magnitude diagram of a star cluster,
you'll need to devise an observing strategy that combines
all-sky photometry of a few stars with (more robust and
more time-efficient) differential photometry
to ``bootstrap'' the rest.
-
You'll need to choose standard stars that are bright enough that you can point to and
image them absolutely as rapidly as possible.
On the other hand,
if you choose stars that are too bright
and the resulting exposure times are too short,
systematic errors arising from the finite time it takes to
open and close the camera shutter will dominate
and affect your results.
(For example,
if the
shutter operation time
leads to a true exposure time that's 0.03 s less than that requested,
then the systematic error introduced
on a 20 s exposure will be only 0.15%,
but for a 2 s exposure it'll be 1.5%,
and for an 0.2 s exposure it'll be 15%!)
It's preferable to choose standards
of color similar to that of your target object,
if possible, to reduce unwanted systematic color effects in your results.
-
It'll be important to design your observing plan so that
you image your standards over as large a range of airmasses
(altitudes) as possible, while at the same time minimizing the
elapsed time it takes to do so.
And if you're doing colors, this all has to happen
for each color filter you're using.
-
Objects down to V mag. approx. +12.5 have been successfully
observed using the C14's unguided for up to 3 min. exposures.
With effort, it should be possible to reach about to mag. +14
by manually guiding longer exposures, but of course if individual
exposures are longer then the total number of images you can shoot
in any given time is reduced.
Also, consider that guiding a C14 with the off-axis guider
is in general more difficult than guiding
using a separate guidescope (such as those on the astrograph or the 16-in).
-
The observing program needed to record an asteroid (or other moving object)
is similar
to that needed for an unmoving object such as a star, except for the additional
complication that the asteroid moves substantially each night.
As a result it takes more preparation to acquire the target with the telescope.
-
Another thing worth checking for when planning observations of
a moving object is whether
it'll have moved too close to
a background star at the time you want to observe,
a situation called an ``appulse''.
Appulses are bad because
when a star's image overlaps
and interferes with that of your target object
you won't get a good brightness measurement.
Workaround: if possible,
wait for your object to move away from the star
and then observe.
(Tinkering with a more
sophisticated image measurement scheme is
possible but beyond the scope of our class!)
-
``Keep your telescope out of the trees'' -
Try to observe your target objects at altitudes of at least 30 degrees
if possible,
but definitely don't bother observing lower than 20 degrees
altitude.
-
Keep objects of interest away from the edges of the frame,
because if they're too close to the edge you won't be able to get
brightness measurements.
-
When shooting ``discrete'' data points
do so in triplets,
so that you can have some confidence that your measurements are
repeatable and have a way to estimate the experimental uncertainty.
-
Sometimes observations of a relatively faint object can be contaminated
by light from a nearby much brighter object,
both by scattered light and
by CCD ``blooming''
(charge-transfer streaks along columns resulting from overexposed pixels).
To work around this problem you'll need to try one or more
of the following:
-
excluding the bright object from the field of view
while still including your target faint object and reference stars,
-
rotating the instrument so that blooming on the image is directed away from
objects of interest,
- shooting a larger number of shorter exposures of the same field,
and then summing the images later.
(works if your target objects aren't moving too rapidly)
