|
The Sierra Stars Observatory is a completely automated robotic observatory system.
Talon, our Linux-based observatory control software, sends instructions and monitors
the status of the telescope, camera, filter wheel, and dome continually throughout an entire observing session - all without any
human intervention. The telescope, CCD camera, instruments, weather monitors, dome
and so on all work together seamlessly in a fully integrated system.
The SSO Nightly Observing Run Process
- Throughout the day until 4:00 PM (PST) we accept observing requests from SSO users. Schedules are very easy to set up using our on-line Observing Request Form.
You can request observations
of virtually any object you choose from fast moving
solar system objects such as near earth asteroids (NEOs) and comets to the most
distant and faint galaxies and galaxy clusters.
- After we receive all the observing requests for the day, Talon's very sophisticated
and highly optimized scheduling program, Telsched, sorts the requests and sets up
the schedule of images to be run for the current upcoming observing run. Telsched
arranges the schedule to take each image in an optimal position as close to the meridian as possible.
- Before opening the dome, Talon monitors the integrity of the observatory system
and weather and environmental factors to ensure that it is OK to start the observing session. At the appropriate time Talon opens the dome, sets the CCD camera to the
proper cooling temperature and performs various other tasks in preparation for taking
the first images of the observing session
-
The first images taken each evening are bias, thermal, and flat field calibration frames.
These images are used to remove artifacts (noise, dust shadows and so on)
in the integrated
imaging system. For a detailed description of SSO calibration
frames, see “SSO
Image Calibration Process Explained” below.
- Talon takes images throughout the
observing session each night using a master schedule file created by Telsched from
schedule requests submitted through the online SSO Observation Request Form. Immediately
after Talon takes each image it automatically applies the calibration corrections
to create fully calibrated images.
The raw image is also saved to the local SSO archive server before performing the
auto-calibration process.
All images are saved to the local file server to ensure immediate archiving to safeguard the
valuable data and then copied to the online server where users can upload their
images as soon as they are available.
- Immediately after Talon takes scheduled
images and saves them locally the files are compressed and transferred to a remote
server where SSO users can later download their image files.
- After all the image files are uploaded
for the evening’s scheduled observing run, SSO runs a process that checks which
schedules were run, which images were taken and uploaded, performs various database
and accounting functions, and sends emails to all users that have scheduled images
ready for them to download.
SSO Image Calibration
Process Explained
Sierra Stars Observatory offers excellent quality optics and instrumentation for
taking very high-quality image date. Even so obtaining the ultimate quality image
data requires equally high quality calibration frames to correct for noise and imperfections
introduced by electronics, heat, and artifacts in the optical path. Below is a description
of the image bias, thermal, and flat field calibration frames SSO uses to create
fully calibrated high-quality image data.
Bias Frames
Bias Frames are zero-duration exposures with the shutter closed that measures the
electronic noise in the camera only. We take 20 Bias Frames and average them together
to create a Master Bias Frame that provides a mean value for the readout noise.
Automatically calibrated images have the field BIASFR added to the FITs header of
each image with the value “Bias frame: Averaged=xxx” where xxx is the number of
Bias Frames averaged together to create the Master Bias Frame.
Thermal Frames
Thermal frames are taken for a finite exposure time with the shutter closed. They
are used to remove (subtract out) noise generated from heat in the camera system.
Many systems refer to this as a “Dark” frame. However, Talon subtracts the master
Bias Frame from the “Dark” frame to create the Thermal Frame. Talon assumes that
the accumulation of thermal noise is proportional (linear) to the exposure time
of the image and scales the values according to the ratio of the exposure time of
the correction frame to that of the image being taken. Removing the Bias Frame enables
a more accurate scaling of the Thermal Frame because the bias correction is not
scaled as well. We take 20 60-second Thermal Frames to create a Master Thermal Frame.
Automatically calibrated images have the field THERMFR added to the FITs header
of each image with the value “Thermal frame: Averaged=xxx Bias=yyy” where xxx is
the number of Thermal Frames averaged together and yyy is the name of the Master
Bias Frame subtracted to create the Master Thermal Frame.
Flat Fields
Flat Fields remove the irregularities
in the optical path of the camera system such as “dust doughnuts”. They are exposures
taken with the shutter open for each of the filters.
Flat Fields are inherently the most difficult calibration images to take well. There
are many different methods used by observatories around the world to take flat field
images including various methods for creating twilight flats, all-sky flats, and
dome flats.
After experimenting with various methods we developed a method for creating Flat
Fields that produces excellent and consistent results. We use a dome flat method
with a canvas (a framed canvas used by artists for painting) that is coated with
a full-spectrum paint used for high-end projection systems. The dome flat is mounted
on the observatory dome directly opposite of the dome opening. When taking our dome
flats the dome shutter is opened and the dome is rotated so that the shutter opening
is pointed
at the azimuth of where the sun has set. The dome flat is very evenly
illuminated by the ambient light from the twilight sky. When the sun has set about
2 degrees below the horizon the intensity of the ambient light on the dome screen
is about right to start taking the first Flat Field images.
Talon monitors the intensity of the pixel values and, when the pixel values are
approximately 35,000 (about midway to the saturation level [65,000]), 10 Flat Field
images are taken for each filter. The intensity and timing for when the exposures
are taken varies for each filter and the order for taking the Flat Field images
is B, V, R, I and finally C (no filter). The C filter is completed when the sun
is about 5 degrees below the horizon. The 10 individual Flat Fields for each image
are averaged to create Master Flat Fields for each filter.
Exposure times are also important to consider when taking Flat Fields. If the exposure
times are too short, there is a noticeable shutter “vignetting effect” caused by the center of the image being exposed proportionately longer than the edges. By
experimenting we found that 4-second (or longer) exposures alleviate this problem.
The resulting Flat Field images using our overall method produces excellent Flat
Fields with minimal imposed gradients. An enormous advantage of using our system
over twilight flats is that you have much more time to work with without worrying
about stars affecting the images and even more importantly we can take our Flat
Fields even when the skies are partly or entirely cloudy.
Automatically calibrated images have three fields added to the FITs files of each
new image:
FLATFR with the value “Flat frame: Averaged=xxx Bias=yyy Thermal=zzz” where xxx
is the number of Flat Field images taken and averaged together, yyy is the name
of the Master Bias Frame subtracted, and zzz is the name of the Master Thermal Frame
subtracted.
FLATMEAN which is the average pixel value of the entire image used during Flat Field
computations to improve performance.
FILTER which is a single letter designation for the filter used for the Master Flat
Field.
|