When planning an observing session
using the Sierra Stars Observatory Network telescopes you need to consider many variables to get the optimum
results for your project. Are you doing quantitative photometry or astrometry or
are you striving to get the highest quality images for esthetic reasons (high quality
artistic work)? Your intended purpose determines how you schedule your exposure
times to achieve your intended goal. If you are taking images for scientific use
(photometry and astrometry), then you are most likely concerned with the quantitative
value of your data (how accurate and precise it is). If this is the case, then the
signal to noise ratio (SNR) is likely to be most important to you. If you are taking
images for the highest quality esthetic value, then you are likely to be striving
for high-contrast images that show interesting detail of the object (probably using
an LRGB or RGB color combination using various filters).
At SSON we take high-quality calibration
frames each night when possible to give you the best image data we can. Therefore, the ultimate
quality of your image data is primarily determined by the transparency and seeing
of our observatory sites during the time your images are scheduled and taken.
If your project is to do photometry
of object(s), your goal is to obtain a minimum (or optimum) SNR with specific filters
appropriate for the goal you have in mind. If your project is to do astrometry,
then you are likely trying to achieve a least the minimum SNR of the intended object
that will enable you to measure an accurate position (right ascension and declination)
and time (if the object is an asteroid, comet, or spacecraft).
The following discussion is meant
to give you general guidelines on how to proceed with your project. There are many
books and sources on the Web that you can refer to for a more detailed explanation
on how to be most effective and submit your data to scientific organizations and
institutions. These guidelines are only to point you in the right direction.
For photometry projects the goal
is typically to achieve a SNR that will give you a desired precision for measuring
the magnitudes of objects using specific filters. If your target object is relatively
bright then you can obtain high SNR data and precise magnitude measurements in relatively
short exposures. There are two major methods for taking photometric measurements:
all-sky photometry and differential photometry. All-sky photometry is much more
complicated, involves many computations, and requires pristine conditions. Differential
photometry is much easier to learn (and more forgiving) to use for doing photometry work.
Basically using the differential
photometry technique you compare the magnitude of the object being measured to other
non-variable stars within the same image. Measurements in the same field of view
in images cancel out the air mass and atmospheric disturbances that must be figured
into the calculations of all-sky photometry measurements.
Ultimately the accuracy of your
photometric measurements depends on the SNR of the object you are measuring and
being able to use stars in the image with an appropriate magnitude and color index
for comparison in your measurements. The absolute error in your measurements is
a direct function of the SNR. The higher the SNR you achieve the more accurate your
photometric measurements become. If you want to achieve an accuracy of 0.02 magnitude
or better for your photometric measurements, then you must achieve a SNR of 100
or greater. Also the more stars you use for comparison in the same field, the more
precise your measurements will be. You can use photometric measurements of fainter
objects in images with a SNR as low as 10 or 20 (for example, in the case where
the object may be so faint it is near the limit of practical exposure times of the
telescope). However, the inherent accuracy of your measurements will necessarily
measurements are often based on standards so that measurements can be compared equally.
During the past several decades optical filters were devised to only allow light
in a narrow part of the spectrum to pass to a photometer (used less and less today)
or CCD chip. The filters are referred to as band-pass filters because they only
pass light in a restricted part of the spectrum and cut off light outside of that
“band”. The standard series of photometric filters used for scientific research
with CCD cameras by research observatories today is the Johnson-Cousins UBVRI standard,
where the filters pass light in the Ultraviolet, Blue, Visual (green), Red, and
Infrared parts of the spectrum respectively. SSO uses a BVRI subset of the UBVRI
filters and a Clear (no filter) position to allow all ambient light to collect on
the CCD chip.
There are many opportunities for
doing photometry projects: variable stars, cataclysmic variables, asteroid light
curves, and so on. If you are interested in learning more about doing photometry
using SSON check out the Internet references below.
Astrometry is the technique of measuring
the position of objects in the sky. Most astrometry projects using the SSO network
are measuring the positions of asteroids and comets. To get an accurate astrometric
measurement of a moving object such as an asteroid or comet you must determine its
exact right ascension (RA) and declination (DEC) coordinates and the precise time.
RA and DEC coordinates in images are computed using the World Coordinate System
(WCS) method. The WCS method matches the pattern of stars in your image with the
pattern of stars in either the Hubble Guide Star Catalog (GSC) and/or the USNO Catalog
and, based on the image scale of your image, enables you to measure the exact RA
and DEC coordinates of any point in the image. You can use one of the many available
image processing software programs to perform these measurements.
Exact time measurements of when
an image was taken are critical for determining the ephemeris (orbital calculation)
of asteroids and comets. The SSON software records the exact time of the end of the
exposure in the FITs header of each image. The SSON control computer keeps time accurately
to a fraction of a second using an NTP server.
SNR is much less critical
than photometry work when doing astrometry work. Primarily you are measuring the
centroid of an object in your images. Even objects with a relatively low SNR (<
10) can be measured accurately.
Some of our customers do astrometry
work discovering new asteroids and follow up measurements of recently discovered
asteroids and comets. If you decide to do an astrometry project and want to submit
your data to the Minor Planet Center (MPC) with the MPC Observatory Codes you can find in the Observatories section of our web site.
If you are interested in imaging
objects for their beauty and unique characteristics, the SSON network offers huge
opportunities for you to explore. SNR is for quantitative analysis is not generally
a concern for this type of work. What you are most likely striving to achieve is
a high-contrast final image that shows interesting detail and, if you are doing
color composite images, a suitable saturation of the color bands filters used.
There is no fixed “formula” for
exposure times for taking images for esthetic projects. If the subject is faint
and extended then in general the longer the total exposure times the more contrast
and detail you will achieve. However, if there are bright stars in your composition,
then combining (stacking) shorter exposures that do not overexpose (bloom) the bright
stars might give a more appealing final result than combining longer exposure images.
Often it’s simply a matter of experimentation to see what works best.
When you first start out taking
images you are likely going to be fascinated with imaging large, brighter objects
(such as the well known Messier objects). These are good objects to try first if
you have little or no experience imaging astronomical objects and produce rewarding
results that you can compare with many examples you’ll find on the Web. But why
stop there? Beyond the dozens of such objects that are commonly imaged there are
literally thousands of interesting objects that are rarely, if ever, imaged well.
The SSON telescopes offer a relatively large image scale and wide field of view that
will enable you to image objects (and multiple objects in a field) that are too
small and/or faint to image well with smaller and shorter focal length telescopes.
For example, there are many smaller galaxies, galaxy clusters, planetary nebula,
and so on that would be interesting and attractive subjects to image. The opportunities
for doing this type of work are wide open!
SSON provides you with excellent raw imaging data for you to work with. To produce excellent high-quality
final compositions, you’ll have to use image processing software that enables you
to stack images, combine images for color composites, and other techniques that
will bring out the interesting details in the image data you take. Several companies
sell excellent astronomical image processing software you can use and most of them
offer trial versions for you to try out before you buy. There is also software available
for free that you can download from the Internet. See the references below for some
of the astronomical image processing software that is available.
Below are some links to places on
the Internet to get started. These are only a few places to start. Doing web searches
will enable you to find much more information.
A convenient SNR calculator for planning photometry and astrometry projects.
Signal-to-noise calculator for CCD photometry
AAVSO CCD Observing Guide
An Introduction to Astronomical Photometry Using CCDs
Guide to Minor Body Astrometry
How to do Astrometry with SIP