Asteroid Lightcurves

 

 

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SAM III

HHO SAM III Geomagnetic monitor is now offline until a new fulltime monitoring PC can be installed.

22 June 2011

   

 

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Sky conditions improve at HHO

After months of seemingly never ending cloudy skies, things look up and the observatory is taking data once more

12 April 2012

   

 

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Spectra L-200

The L200 spectrograph has been calibrated and seen first light on Sirius and Spica.  See here for details

04 May 2011

   

 

 

Click here for Lightcurve Results

 

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.