The Asteroid lightcurve program at Hunters Hill Observatory has evolved considerably since
it first started in 2002. Below is my own brief history of time since 2002, along with a
general discussion of the asteroid lightcurve program. If you'd like more
details about CCD photometry and lightcurve determination and analysis, then I
recommend Brian Warners book, "A Practical Guide to Lightcurve Photometry and
Analysis", available from Springer.
Equipment
The observatory has run with a range of equipment over the years. Its
started out in 2001 with a Meade LX90 8" SCT and Starlight Xpress MX516 CCD.
In 2002 I upgraded to a Meade 0.25m (10") LX200GPS SCT and a Starlight Xpress
MX716. In 2003 I upgraded again, this time to an 0.35m (14") LX200GPS SCT
and then in 2004 was able to purchase an SBIG ST-8E. In 2005 I
purchased a water cooled SBIG ST-9E to overcome the hot summer nights we get in
Canberra. Both SBIG cameras had parallel interfaces and as such were a
pain to operate and ate into observing time with length image download times.
I later sold the ST-9E to someone who would make more use of it than I had been.
The camera was run at -10°,
even though lower temperatures could be reached during non summer periods, so as to
keep consistency in the images regardless of the time of year. In early 2002, I
erected an observatory to house the telescope and then purchase software that
would allow the autonomous operation of the observatory. The ST-*E is
connected to the 0.35m SCT via and f/3.3 FR resulting in an f/5 optical train
and a 1.31"/pixel resolution. The 0.35m telescope was mounted on a wedge
atop the supplied giant field tripod. Although the observatory shed is
relatively small, it does allow full access to the sky in all directions above
24 degrees altitude. Of course with the CCD fitted, there are limitations
on just how far south the telescope can be safely pointed. In this regard,
a -65 declination limit has been set though I rarely go further south than -60.
The telescope/camera combination is run by its own PC located in the
observatory building. It is networked into the house via a hardline dug into the
ground (Power, 2 LAN lines and 2 Phone lines). The system is operated via VNC
though any of the homes networked PC's and more recently via WiFi/3G through an
Apple iPad from anywhere in the house or anywhere I can get a 3G phone signal.
I undertook a successful test operating the observatory whilst located at Mt
Stromlo Observatory 30km away.
Exposures
Exposures throughout the program have been, on average, 60-120s, depending on
the brightness of the target asteroid. As fainter targets came into the program,
the exposures have been increased up to 240s (4 minutes). All exposures are
guided. The Meade LX200GPS suffers from a woeful mount generating 120
arcseconds of periodic error that can only be corrected to around 20 arcseconds
at certain points around the gear. Many people report that their system
can track well enough for short exposures out of the box - well I must have got
a dud - BUT -Meade representatives stated that since they do not quote a PE
standard in their specs, and PE is possible and therefore not their
responsibility.
Generally, the 0.35m scopes can reach good SNR values (> 50, 0.02m)
down to 15.5 with 180s. Library Dark and Flats are taken every 3 months and raw
images are taken and stored from each observing session (several hundred
gigabytes to date).
Software
Telescope and camera control is done with
ACP, a custom program with simple scripting written by Bob Denny (DC3 Dreams). It is capable of sending a telescope to several targets,
maintaining focus, changing filters, and all the other requirements of a
research level astrometry or photometry program.
Image processing and measurements are done with
MPO
Canopus written by Brian Warner (BDW Publishing) who has been my Mentor
throughout my journey. This is another commercially available program written at PDO that
was the first to include Dr Alan Harris' industry standard Fourier period analysis
algorithm in a program for general use by amateurs. Brian keeps Canopus (and the
associated suite of software) developing to meet the needs of amateurs and new
findings in the world of asteroid observations. His major breakthroughs
include Star-B-gone, LCInvert, Dual Period Lightcurve Analysis and a means
of photometric calibration to an internal standard that allows us to link
nightly data to within 0.05m (or 0.02mag with the APASS catalogue). The
latter has revolutionised the ability to analyse very long period asteroids
without the need for the amateur to revert to All Sky photometry and data
reduction to the standard system (a near impossible task for those of us without
photometric quality skies).
General Program Description
Asteroids targets come from 3 sources. BINAST, CALL and Programmed.
As a member of the Binary Asteroid Photometric Survey, my primary responsibility
is to tackle targets identified as binary prime candidates by Dr Petr Pravec of
the Astronomical Institute, Czech Republic. I have been an invited member
of this group for many years now and as a result of my observations have
uncovered or assisted in the uncovering of the binary nature of more than 2
dozen asteroids and NEO's. The CALL program is managed by Brian Warner.
Each quarter, Brian, in conjunction with Drs Alan Harris, Petr Pravec and
others, put together a lit of targets in need of observation to either determine
the rotational period, improve the quality of a previously derived period or add
to the lightcurve database to enable spin axis or shape determination.
Programmed targets are those where I get specific requests from professional
astronomers. These could be to support radar observations or to support
theoretical analysis such as YORP/BYORP targets.
Sometimes, there are periods when assigned targets are light on. In
these cases I do a search for my own using Astroplanner. I enter BINAST
survey parameters and come up with my own list of targets. Once the list of potential targets is made, it is usually reduced by
filtering out those asteroids with already well-known lightcurves. Sometimes,
however, even those with known periods are observed, either as a check of the
original results if they are somewhat dubious or to assist with shape modelling
project. The filtering is done by referring to the
list of
lightcurve parameters maintained by Alan Harris and Brian Warner.
Learn more: Astroplanner
Once asteroids are selected, a script is prepared that will automatically
take images of the asteroids all night, quitting when the asteroid reaches 24° in
the west or twilight begins. On most occasions the telescope will have at least
2 targets to ensure that it is kept busy all night. Sometimes I will be
tracking only long period targets. In this case I might follow up to 5
asteroids in one night. The number chosen will be determined by their
relative positions, the amount of overlap and the length of integration to
ensure that gaps in the observed data are kept to a minimum.
Usually, observations are observed using a Clear filter. Calibration from night
to night then depends on the feature in
MPO
Canopus to adjust the nightly zero points visually to get data from
different nights to align. Once the script is started, I periodically monitor its progress, making sure
that focus is holding - sometimes it changes rapidly and needs adjusting before
the auto-focus command is reached in the script (usually once an hour). Besides
that, I can and often do go on to other things such as reading and watching TV
or improving my math skills.
Learn more: Automation with ACP
Data Reduction
Data reduction is done with
MPO
Canopus. For each image, the following information is stored in a database:
UT Date/Time of mid-exposure
Air Mass
Instrumental magnitude of the target and comparison stars
Catalog derived magnitudes of the target and comparison stars (optional)
SNR of the target and comparison stars (to allow computing the estimated
error per observation)
Average magnitude of the comparison stars
Comparison - Asteroid magnitude
Differential photometry techniques are applied to the data reduction. Several
comparisons are used (two minimum, and up to five) to provide additional
stability to the average value of the comparisons and to assure that there will
be at least one comparison, preferably two, that is not variable.
In addition, the distance of the asteroid from earth and its predicted
magnitude are kept as part of a larger record associated with all the data for a
given night's run. These are used to determine the corrections required for
phase angle differences and light-time corrections. The mean value of all the
averages for the comparisons is also stored. This can be adjusted per session so
that all data is eventually referenced to a common, but arbitrary, zero-point,
i.e., the comparison value used for all data points, even over several nights is
the same.
Period determination is accomplished using a routine based on the FORTRAN
program FALC developed by Harris et al. This performs a Fourier Analysis on the
data, allowing different parameters such as number of harmonics, period, size of
period steps, etc. to be held constant while others are varied. This routine is
also included in the
Canopus
software. Finally, a plot of the raw data or phased (all data merged into a
single cycle from 0 to 100% of the derived period) is generated. This plot can
be saved as a Windows BMP for reproduction and manipulation at a later time.
If you would like more information about the details of the asteroid
lightcurve program, equipment, or software at the Palmer Divide Observatory,
please drop contact Brian Warner.
