Observing M36 with the VAO
Astronomical Laboratory ASTR:4850, Spring 2018
by Philip Kaaret with text from Robert Mutel
Reading
Equipment
Introduction
In this lab, you will take images of the open cluster M36 with
the Van Allen Observatory (VAO) on the roof of Van Allen
Hall. You will use these images in the later labs on
photometry and astrometry, so make sure you obtain good quality
and unsaturated images. You need two images of M36 taken in
the B-band and one in the V-band (G filter). One B-band
image should have the cluster centered in the view of view and the
other image should be offset by about 5 arcminutes in
declination. The V-band image should have the cluster
centered in the field of view. These should be raw images with no
processing. You will also need bias and dark frames. A
bias frame is a CCD image taken with the camera shutter closed and
an exposure time that is as short as possible. A dark frame
is taken with camera shutter closed and an exposure time equal to
the that used for the astronomical image. Bias and dark
frames are used to correct for imperfections in the camera as we
will learn about in the next few labs. Make sure that you
save all your images in FITS format.
The Van Allen Observatory
The VAO has a 0.43m (17 inch) primary mirror and an SBIG CCD
camera, similar to the Orion cameras that you have been using in
lab, but with many more pixels in order to cover a larger field of
view. The read noise and dark current are also lower. The
available filter set includes L, R, G, and B photometric color
filters, and a narrowband [5 nm width] at Halpha [656.3nm]
filter. There is a also a 2048 -channel fiber-fed
spectrometer with a spectral range 350 nm - 750 nm and a
resolution 1.2 nm that we will use later in the semester.
Details about the VAO along with photos of the equipment are given
here http://astro.physics.uiowa.edu/vao/equipment-2.html.
Have a look over that web page before continuing.
You need to be trained to use the VAO before using it
yourself. For the first part of the training, watch the
following two instructional videos. These videos will likely
make more sense after you have been up in the dome and introduced to
the equipment, so it would be good to watch them again after you do
your training.
Now go up to the roof, do your training with the "Telescope
masters" (Patrick, Chris, and Zach), and get your images of M36,
along with your bias and dark frames. Be sure to write down
all the steps in opening the dome, setting up the equipment,
observing with the CDK17 and the SBIG camera, shutting down the
equipment, and closing the dome since if you group chooses to do a
research project with the VAO, you will be need to do all of
theses steps on your own without their help.
A note on saturation: As we discussed in the first class, the CCD
chip in the camera contains an analog to digital converter
(ADC). The Kodak KAF-6303E CCD chip in the
SBIG STXL-6303E camera has a 16-bit ADC. This means
that the largest numerical value it can produce is 216-1
= 65535. The actual saturation level, where the number of
ADU is no longer proportional to the number of inputs photons, is
somewhat lower than this. If part of an image is too bright,
then the ADC records values around the saturation level and the
proportional response is lost. This makes it impossible to
determine the number of incident photons from the ADC counts, thus
making it very hard to accurately measure the flux from the
saturated star. To avoid saturation, look at the peak pixel
value using CCDOps. Keep the peak value less than about
30,000 to be on the safe side. You might also want to double
check your final set of images using ds9 before deciding that
you're done.
Observing M36
M36 is an open cluster. It is useful target for learning
about photometry and astrometry because it contains a good number
of well separated stars.
Point the telescope towards M36, select the B filter in the filter
wheel and start taking images. Find an image or map of M36 and
verify that the telescope pointing is correct. Look at the
peak pixel value in the images and adjust the exposure time until
the peak value is below 30,000. Note that if the peak value is
very low, you should increase the exposure time. Once you have
obtained a good image with non saturation, save it in FITS format
and record the camera settings including the CCD temperature in the
file information and your lab notebook. You might save a few images,
just in case.
Then move the telescope pointing by 5 arcminutes in declination,
either north or south, your choice. Take another image with
the same setting, save it in FITS format, and record all the
information.
Now move back so that you are centered on M36 and select the G
filter. Again, adjust the exposure time until you get a good peak
value. Save at least one image.
Now take bias and dark frames. Close the camera shutter and
take 5 exposures with the same integration time as your best B-band
and G-band images. Save all 5 frames in FITS format under
names including the word 'dark' and, of course, record
everything. Then set the exposure time to the minimum allowed
for the camera, take 5 frames, and save them in FITS format with
names including the word 'bias'.
Now you should be done with observing. However, before you
shut down or walk away from the telescope, review your images to
make sure that your target stars are not saturated and that you
recorded the filter, exposure time, and time at which each image was
taken. Since clear sky is not so common in Iowa, it is (much)
better to err on the side of too many images rather than discover
later that some of your stars are saturated or some of your images
are too dim. Also, make sure that you have your 5 bias and 5
dark frames at the exposure times for the B and G images (if those
exposure times are the same, one set of darks is ok) and that they
look ok. We will be using these images for the photometry and
astrometry labs.
Epilogue
If you are the last group observing for the night, shut down the
telescope and close the dome.