Monday, 19 December 2011

Confirming the recovery of Comet P/2006 T1 (Levy)

Comet P/2006 T1 (Levy), due back to the Sun's neighbourhood in mid-January 2012, its first return to the Sun following its discovery in 2006 was eventually recovered by Richard Kowalski from the Mt. Lemon Survey early on 17 Dec 2011 during a search of the object's entire uncertainty region.

It was over 2° from the predicted position at the time and magnitude +19, about 9 magnitudes fainter than predicted, based on its brightness during the 2006 apparition, though it is thought that the comet underwent a brightness outburst just before being discovered.

The recovery was confirmed by further positional measurements from observatories Farra d'Isonzo (595) and Great Shefford (J95), both observatories noting the object as diffuse and with an elongated coma or tail pointing to the North-East (upper left in the image below).

Images taken from Great Shefford Observatory of the comet at its 2006 discovery apparition can be found here.

Friday, 9 December 2011

2011 November notes

Poor weather in the first half of November (that stopped the very close fly-by of 2005 YU55 from being observed from Great Shefford) was replaced by a decent last two weeks and plenty of newly discovered NEOs were found to be in need of observation, although a fair number of these were rather faint.

However, there were a few close passes observed and probably the best of those were by 2011 WP4 and 2011 WQ4. My last sighting of 2011 WP4 was on the morning of Nov 23 at mag +17.6 with it moving almost due south at 48"/min. At that time it was just over 3 Lunar Distances (LD) from Earth. Two other observatories observed it in the few hours following but no one reported it at its closest just over 1 day later when it had approached to within 1.5 LD and was moving 5 times faster but had reached high southerly declinations of -60 to -70 degrees. 2011 WQ4, although a similar absolute magnitude to 2011 WP4 (H=+27.4 vs H=+27.2 for WP4, equating to likely diameters in the range of 8-13 meters) was always a fainter target and only approached to 2 LD. It was also last seen on the morning of Nov 23, heading out from its close approach a day and a half earlier but by then was only mag +19 and moving at 55"/min made it a much harder target than 2011 WP4 had been.

At 10pm on the night of Nov 17th I received an email from Michel Ory (discoverer of Comet P/2008 Q2) requesting help to try and recover a fast moving 19th mag. object that Claudine Rinner had discovered two nights earlier from station J43 with a 0.5-m f/3 reflector (she would go on to discover Comet P/2011 W2 less than 2 weeks later with the same instrument). With only three positions spanning just 41 minutes from Nov 15th, the uncertainty in the position predicted by FindOrb for Nov 17th was expected to be fairly large and so a number of fields would need to be taken to stand any chance of recovery. Although Michel had sent the positions into the Minor Planet Center, the object had not yet appeared on the NEO Confirmation page (NEOCP). I started taking images for it within the hour, but already it was getting low in the southwest, the altitude of the field was only 27 degrees and getting lower but even more of a problem was thickening cloud coming in from the west. I managed to search for nearly 45 minutes before being completely clouded out but could not positively identify anything, especially difficult with the poor quality, cloud affected images.

The next night was clear and so I started searching again for the object, earlier in the evening and in better conditions than the night before. By now it was listed on the NEOCP, together with an uncertainty map showing its likely location was expected to be anywhere within a strip of sky over 2.5 degrees long. After 1h 45mins I eventually found the telltale expected movement in blinked images about 3/4 degree ahead of the NEOCP nominal predicted position. To help other observers, at 10:30pm I posted a comment on the NEOCP Blog ( stating where I'd found it and about 15 minutes later, a set of positions from the Catalina Sky Survey Mt. Lemmon telescope (station code G96) were posted from images taken some 18 hours earlier.

In the meantime I had used FindOrb to improve the orbital elements, combining the discovery positions with those I had just measured and worked out an ephemeris for the previous night. With the accurate ephemeris and much improved values for the rate of motion, I was able to re-stack the images from the night before and managed to identify weak images of the object. These were quickly measured and both nights of measures sent in to the Minor Planet Center and by 10:52pm the Minor Planet Center announced the new discovery as Amor object 2011 VP12 in MPEC 2011-W18, containing the discovery positions from J43 together with those from G96 and my two nights. Congratulations to Claudine on this discovery as well as for P/2011 W2!

On Nov 26, NASA launched the Mars Science Laboratory (MSL) on top of an Atlas V rocket and, using an ephemeris from the JPL Horizons web page available from I picked it up on the morning of Nov 28 at mag 16.8-17.0, moving at a fairly gentle 5"/min. The next day a 16th mag object was added to the NEOCP with very similar RA and Dec to where I had observed MSL the night before and moving in the same direction with similar speed. It was quickly determined however that this object was NOT MSL, but rather the Centaur second stage of the Atlas V rocket, about 3/4 degree away from the MSL spacecraft. The Centaur stage is used twice, once to achieve a low Earth orbit after launch, then again to take the MSL out of Earth orbit and accelerate it towards Mars and although not part of the experiment is accompanying MSL on its journey to Mars. I imaged both the MSL and the Centaur 2nd stage on the morning of Nov 30th and by then MSL had more than doubled its distance from Earth since my first observation, 2.8 LDs compared to 1.3 LDs two days earlier and was also a magnitude fainter than before at +18.1. The Centaur stage was much brighter than MSL, maybe by a magnitude or more and was also varying rapidly in brightness, very obviously changing between consecutive images, in marked contrast to the steady light from the more stable MSL spacecraft.

Wednesday, 5 October 2011

2011 September notes

A big improvement over recent months, September ended with a remarkable and record breaking heatwave in the UK and more than half the nights in the month being at least partially usable at Great Shefford, though fog and mist stopped work several hours before dawn on a number of occasions.

After a quiet summer, both PANSTARRS and the Catalina Sky Survey (CSS) telescopes were in full survey mode, looking for new Near Earth Objects and astrometry was contributed for 43 of the objects posted on the NEO Confirmation Page.

PANSTARRS picked up 2011 SP25 on September 20th, relatively faint at mag +20 and slow moving, I observed it 2 nights later and measured it at (red) mag +19.7. It is not a NEO and at the time of writing (Oct. 5th) there are now two weeks of astrometry available and the elements show it to be in a highly eccentric retrograde Halley-type orbit with period 89 years and perihelion at 2.27 AU due this coming November. It looked completely stellar in my images and will probably stay that way, but may be worth keeping an eye on in the next couple of months.

A couple of moderately close NEO passes were well observed, 2011 LJ19, discovered back in June by CSS came closest mid-month, at about 17 Lunar Distances (LD) and maintained 16th mag or brighter for about 2 weeks. A CSS discovery from September, 2011 SR5, passed about 22 LD towards the end of the month and peaked at mag +16 for about 5 days, reaching a speed against the sky slightly faster than 2011 LJ19 at 35"/min.

The closest observed approach though was 2011 SE58, a very faint mag +20.8 discovery made early on September 23 by the Spacewatch team using the 1.8-m reflector on Kitt Peak, when it was moving slowly at 0.8"/min. Spacewatch observed it again the next night by which time it had tripled in speed and was about 0.7 mags brighter, closing in fast at a range of about 14 LD.

I observed it 11 hours later on September 24 at 21:00 UT and found it more than a magnitude brighter at +18.7 and moving at 4"/min. Its distance had approximately halved to 8 LD in that time and a very close approach to 0.6 LD was predicted for September 27.1 UT. The next night was cloudy in Great Shefford but the night of the close approach started out clear. By 23:00 UT 2011 SE58 had risen to an altitude of 14 degrees, just high enough in the east to clear my house roof but it was moving so fast due east and still accelerating that it almost kept pace with the Earth's diurnal motion and stayed in approximately the same place just above the rooftop, edging in azimuth towards the south for the next hour, until mist came down and stopped any further observation. Although registering between mags +14-15 it was moving so fast and at a low altitude that it was unfortunately never recorded strongly enough for any accurate photometry to be attempted.

Here 2011 SE58 is seen passing from west to east through the 18'x18' field of view in less than 2 minutes

When first picked up that night it was moving at 480"/min at 0.80 LD and when last recorded 58 minutes later had accelerated to 600"/min, had approached to 0.72LD and moved a total of 9 degrees against the sky background. The Minor Planet Center Daily Orbit Update published the next morning included the measures I had made that night and also included some pre-discovery PANSTAARS astrometry from 2 days before the Spacewatch discovery observations, when it was 22nd magnitude and had only moved 9" in the hour it was under observation, quite a change in a week!

Friday, 9 September 2011

2011 August notes: An amateur discovered tumbling NEO

August continued the poor run of weather from July and most of the time it was a case of making the best of what clear sky there was.

One opportunity came early in the morning on August 9th when a new discovery was posted on the NEO Confirmation Page, eventually designated as 2011 PE2, this one picked up by the amateur run La Sagra Sky Survey in Spain. The discovery images were from 24 hours earlier which was quite unusual, often La Sagra post their discoveries in near real-time. Jaime Nomen from the survey mentioned later that their reduction software had crashed during the previous evening and those images that had not been processed at the time were put into the queue for the next night, so it was only detected a day later.

Because of the delay the positional uncertainty was growing, with the Minor Planet Center's uncertainty map indicating the likely area it might be found in was 1.75 degrees long. I eventually picked it up 20' from the nominal prediction and then followed it for 20 minutes to get enough images to measure astrometry to send to the MPC. The ephemeris indicated it should be about mag +18.3 but during that 20 minutes it could be seen to vary in brightness from barely visible to very obvious within about 10 minutes. Because of this, I decided to continue to follow it until dawn to try and obtain a lightcurve, ending up with nearly three hours of measurements and showing the total variation to be very large at about 2 magnitudes (a factor of 6). 2011 PE2 was about at its brightest when discovered and on subsequent nights had already faded too much for any more useful photometry to be obtained.

Canopus was used to reduce the brightness measurements obtained that first night and to plot a lightcurve. Although the rises and falls from max. to min. in approx 10 minutes that were noticed at the time the images were taken were visible, the variations were not regular, indicating that it may not be simply rotating in one axis, but probably tumbling. In the diagram below, "beating" can be seen in the maxima and minima as two competing periods cause constructive and destructive interference to the overall curve, indeed at about 0.57 on the x-axis a minimum is almost completely absent.

Solving for a single period gives a value of 43.8 minutes for the main variation (so 11 minutes between each maxima and minima). However, the second period has not yet been satisfactorily determined and so the final value of the main period is still undetermined and could be somewhat different to that given above.

Raw (unfolded) lightcurve of 2011 PE2 from 348 data points obtained 2011 Aug. 09 00:14-03:08 UT

Sunday, 28 August 2011

Bright Supernova 2011fe in galaxy M101

Discovered automatically by the The Palomar Transient Factory on August 24 at magnitude 17, this was quickly determined to be a very young type 1a supernova, expected to brighten by possibly 6 magnitudes within the next week or two.

The animation below consists of two frames, the first taken on 2003 Jan 28 using a 0.3-m telescope, before the supernova appeared and the second on 2011 Aug 27 using a 0.4-m telescope, with the supernova at magnitude +13.0. This image was obtained during a 4 minute gap with the object passing between trees, when M101 was almost at its lowest point over the north horizon, poor weather preventing any attempt in better circumstances.

Friday, 19 August 2011

The Sky At Night comes to Great Shefford

The BBC's Sky at Night team visited Great Shefford Observatory on July 6th and filmed a short interview following the very close approach of NEO 2011 MD at the end of June (see the June and July notes).

The interview was included in the program "Dawn at Vesta" and was first transmitted in the early hours of Monday 8th August on BBC1.

Watch the interview with Paul Abel here:

Tuesday, 9 August 2011

2011 July notes: 2011 MD and recovering 2000 CP101

Early in July the remarkable NEO 2011 MD was still under observation following its exceedingly close pass to just 7600 miles above the Earth's surface at the end of June. It was visible at high northerly declinations before and after the near-miss only because of the severe perturbations caused by the Earth's gravity, putting a 130° kink in its trajectory (see the JPL news story here)

The final images I obtained were taken during the early morning of July 3 with 2011 MD moving relatively slowly at 5"/min in Cassiopeia in a very rich Milky Way star field, near the border with Cepheus and Lacerta. Even though more than 5 days after the close approach, 2011 MD was still only 2.5 Lunar Distances from Earth and with the object at nearly 20th magnitude this combination caused a rather unusual problem when trying to measure positions. Plenty of images needed to be taken to register a 20th mag target and so I took 200 x 20-second exposures between 00:37 and 02:07 UT, before twilight started to interfere. However, because of the rich star field, wherever the asteroid passed too close to stars many of the images would have to be discarded. Normally, this process is time consuming but straightforward, involving identifying the stars on a line from the first to the last ephemeris position and estimating which of the corresponding images should be left out before stacking the remaining images together to enhance the feeble light from the faint asteroid.
When I did this and started stacking various combinations of images with Astrometrica I kept on finding the faint image of the NEO was contaminated with star trails, which should not have been happening if I had properly identified the images to discard.

The cause of the problem was that the object was so close to the Earth, but moving so slowly against the sky that the Earth's diurnal rotation was causing the apparent track of the NEO to be significantly curved. The image shows the difference between the more usual straight track and the actual curved track.
Track of 2011 MD on 2011 July 3.07 UT, 9'x9' field of view
Armed with the new track and this time leaving out the correct images I managed to get three sets of images stacked and measured and sent off to the Minor Planet Center. The position measured from the final stack, shown here was the last reported sighting of 2011 MD received by the MPC.

Later in the month I searched for NEO 2000 CP101, discovered by LINEAR on 2000 Feb 10 and observed for the next 30 days but not seen in the 11 years since. By July 24 it was expected to be 18th magnitude but the 3-sigma uncertainty area was a daunting +/- 21 degrees long, stretching east and west from the nominal prediction, far too big to sensibly search the whole area using my 0.3 degree field of view.

In an effort to try and reduce the scale of the search I retrieved all the original astrometry via the Minor Planet Center's MPCOBS facility at and then used FindOrb, brutally filtering out any observations that had residuals bigger than 0.5" and then producing an ephemeris from the resultant orbit. The MPC prediction by contrast was much more lenient and included most of the original observations. My ephemeris placed the object a couple of degrees to the east of the MPC prediction and hoping for the best I started searching from that point, working outwards both to the east and west.
I started at dusk and after 4.5 hours, covering 13 fields of view, I eventually picked up the telltale motion of 2000 CP101 just before dawn. It turned out to be 3.7 degrees east of the MPC prediction. After obtaining confirming images the following night, the recovery was announced in Minor Planet Circular MPEC 2011-O29.

The 13 fields taken during the search for 2000 CP101, Delphinus is at top left and Altair at lower right.
Graphic generated using SkyMap Pro 9

Tuesday, 28 June 2011

Confirming the discovery of close approach asteroid 2011 MD

The Minor Planet Center (MPC) added two new LINEAR discoveries to the NEO Confirmation page (NEOCP) just after 01am UT on 23 June while I was taking images of another object discovered a day and a half earlier by the SPACEWATCH team (which would eventually be designated 2011 MF). With only 45 minutes left before twilight would get so bright that imaging would have to be abandoned, I decided to stop what I was doing and try for one of the LINEAR discoveries.

The two new objects were posted on the NEOCP with the temporary designations assigned to them by the LINEAR team, BZ52584 and BZ52587.

BZ52584 was in reasonably dark sky, about 4 degrees north of M13, the Great Globular star Cluster in Hercules while BZ52587 was much further east, about 3 degrees west of M31, the Andromeda galaxy and already in the glow of the approaching dawn. As BZ52584 was moving twice as fast as BZ52587 and better placed I decided it would be more useful and more likely to succeed to try and confirm it in the short time left before dawn. (BZ52587 would turn out to be a comet and be designated C/2011 M1 LINEAR a couple of days later).

However, the MPC prediction for BZ52584 was a bit odd - LINEAR had only observed it for 60 minutes some 18 hours earlier and normally the MPC would provide both a predicted position for the new object as well as an uncertainty map showing the likely area of sky the new object might be found in. However, this time only the predicted position was given, no uncertainty map. I took a set of images centered on the predicted position between 01:20 - 01:40 UT but when examined, there was no trace of the new object. With only about 20 minutes of usable sky left I started to hunt for BZ52584. With time only to take one or two more fields of view I chose to start with the field of view immediately to the east of the MPC predicted position.

As there was so little time left, the images were examined as soon as they were downloaded from the camera and after 7 minutes collecting 13 images it looked like there was a probable candidate, moving with the right motion, over 1/4 degree from the original prediction and just 17 pixels from the bottom of the images!

Two fields taken during the hunt for BZ52584. White denotes the first, centered on predicted position, yellow indicates the field where the object was found, very close to the bottom of the frame
One or two of the images had been spoiled by clouds that had started to thicken up and in a desperate attempt to positively confirm the new discovery before being clouded out I repositioned the telescope to centre the suspected object in the field of view and get some more images. The clouds continued to thicken and only 5 of 23 images taken after repositioning were at all usable, but fortunately they did show the new object in the expected place.

All of the good images were then measured and positions sent off to the MPC at 02:05 UT.

Animation of 2 sets of 5 x 20 second stacked exposures, showing motion of 2011 MD
01:41-01:45 UT 23 June 2011

The three positions I had just measured and the four provided by LINEAR were then put into FindOrb to work out an orbit and to provide an early view of where the new object was going to be in the next few days. It was immediately apparent that it was headed for a very close approach to Earth in 4 or 5 days time and so, to alert other observers and the MPC, I posted on the MPC's NEOCP blog at 02:16 UT:

"J95: BZ52584 probable v. close approach on June 27.2 UT"

"FindOrb gives ~23,000Km on June 27.2 UT (leaving out 1 of 704 positions). Worth
some more follow-up."

With further positions measured from other observatories in the following hours, the new object was announced by the MPC as 2011 MD later on June 23 and the close approach turned out to be somewhat closer than that first prediction, at 18,700 km from the Earth's centre on June 27.7 UT, or just under 1 Earth diameter from the Earth's surface.

Sunday, 8 May 2011

2011 April notes: 2011 GP59 and NEOCP changes

April continued the good spell of weather that set in during March and ended up being the warmest April since records began for the British Isles. Plenty of Near-Earth asteroids were picked up by the surveys but probably the most interesting object was 2011 GP59 which was discovered by the amateur run La Sagra survey in southern Spain just before midnight on April 8th. It was described as mag +17 by the discoverers and was heavily observed from Europe in the next few hours, with 45 positions being reported by the time I sent my first position in, just 2 hours after discovery. It was immediately obvious that it was varying greatly in brightness in the space of just a few minutes and it could be seen to brighten up and then fade completely from view in real-time as sequential images were captured and displayed on the pc. Also evident early on that first night was that it would brighten over the coming days and make a close approach about a week later. It passed Earth at 1.4 Lunar Distances on the evening of April 15 but at a southerly declination and running into evening twilight so difficult to observe from the UK. Nick James posted a very good animation on YouTube showing the rapid brightness changes from the night of April 11th.

I last observed it in the early hours of April 12th when it was 16th mag and still moving relatively slowly at 9"/min., obtaining 376 images over a three hour period to try and determine a lightcurve. The initial reduction of the data showed a dramatic curve with a 2 mag amplitude and a period of just over 7 minutes. However, there was quite a noticeable scatter in the brighter segments of the curve, just where the errors would normally be expected to be at their smallest.

Initial lightcurve reduction, showing large scatter at the brightest part of the curve, where scatter would normally expected to be least

So Canopus was used to try and determine whether the object was tumbling and if a secondary period was contributing to the scatter of the main curve. Canopus has functionality to determine an initial lightcurve and can then subtract that modelled variation from the original data points. The adjusted data points can then be used to try and solve for a secondary period. If a secondary period is apparent, then this can in turn be subtracted from the original data points and a fresh determination of the primary period  made. A few of those iterations for 2011 GP59 resulted in the two lightcurves shown here, the main one with a 7.352 +/- 0.002 minute period and 1.8 mag amplitude, while the secondary period was found to be 10.25 +/- 0.02 minutes with an amplitude of approximately 0.4 magnitudes. There is a great deal of scatter throughout the secondary lightcurve and it is best viewed from several feet away(!) when the sinusoidal lightcurve becomes much more apparent. The overall noise is mainly due to the secondary period being superimposed on the large 1.8 magnitude variation of the primary period and therefore both the maximum and minimum of the secondary curve have data points contributed from the faintest parts (as well as the brightest parts) of the overall variation, so low signal/noise ratio measures, with large scatter are unfortunately present throughout the secondary curve.

Primary lightcurve with secondary subtracted

Secondary lightcurve with primary subtracted
Using measurements obtained by the Lowell Observatory from the previous night, tumbling asteroid expert Dr Petr Pravec reported similar results in the Minor Planet Mailing list here with periods of 7.3501 +/- 0.0004 minutes and 10.258 +/- 0.003 minutes.

Users of the NEO Confirmation web page will have noticed during late April that the number of objects on the page has exploded from what in previous months had been at most around 30 objects at any one time to much higher numbers, for a while in the first few days of May there were over 160 objects listed. The surveys are not necessarily picking up any more NEOs than they had done a few days earlier, rather the Minor Planet Center (MPC) has changed the threshold "NEO probability" that a newly discovered object has to score to get onto the page. Following a workshop in March that included representatives from all the NASA funded surveys, JPL and the MPC, the attendees overwhelmingly voted for more objects to appear on the page, so objects that only have a small chance of actually being a NEO (such as objects with Mars crossing orbits, Hungarias, Phocaeas etc.) are now appearing on the NEOCP along with more definite NEOs.

As an observer, the sheer number of objects on the page makes target choice very difficult and to help, the MPC are making some modifications. Initially they have started displaying the NEO probability as a percentage against each object, so the observer can choose objects with high scores, say 50%+ if they want to have a good chance of observing a new NEO. In beta test now and hopefully soon to be introduced fully is a means of filtering the page by magnitude, declination and NEO probability which should make planning an observing session much easier.

Sunday, 10 April 2011

2011 March notes: 4 close approach NEOs

Fortunately March broke the 4-month run of very poor observing conditions at Great Shefford, with 16 usable nights and additionally plenty of Near-Earth objects to observe. March and Oct/Nov are the months that the surveys are generally most successful at discovering very close-approaching minor planets and this month they again had a bumper crop.

2011 EY11 discovered on March 5th with the Mt. Bigelow Schmidt by the Catalina Sky Survey team made a very close pass to just 0.34 Lunar distances (LD) from Earth at 03:36 UT on March 7th. It was picked up from Great Shefford just before 8pm on March 6th when it was already 16th mag and moving at 170"/min. It had come inside the orbit of the Moon about an hour before, but when last recorded, at 01:55 UT on March 7th was 0.38 LD away and travelling at over 800"/min. Heading almost due South its declination decreased from +16° to -25° in the 6 hours it was under observation and was likely to be only about 6 meters in diameter. It showed large and very rapid variations in brightness of 1 mag or more, but a lightcuve has not yet been reduced.

Another discovery from March 5th with the Mt. Bigelow telescope was 2011 EC12, which was to make an approach to 3.3 LD during the early evening of March 8th. It was observed at mag +16.7 and moving at 100"/min on the night before closest approach but when observed at the point of closest approach on March 8th was up to 0.7 mags fainter due to the rapidly increasing phase angle and it had accelerated to 150"/min too, both factors making it a more challenging object that second night.

2011 EU20 was first picked up by the Mt. Lemmon 1.5-m telescope of the Catalina Sky Survey on March 8th, 3 days before making a pass to within 1.62 LD of Earth. Observed on the night of March 9th at mag +17.2 and again the next night at mag +16.1. When last detected at 03:36 UT on March 11, 7 hours before closest approach it was at 1.7 LD and moving at 160"/min. Again, relatively small, with an estimated diameter of about 11 meters.

2011 EW74 discovered on March 15 from Mt. Bigelow was a larger object with an estimated diameter of 65 meters and made an approach to 10 LD on March 21. Even though more distant than the other objects mentioned, because of its larger size it would still reach mag +16.1 for a few days either side of closest approach. Unfortunately this coincided with the full Moon and for the three days when 2011 EW74 was at its brightest it was always less than 40° from the Moon, reducing the signal/noise ratio of the images and making photometry more difficult. No obvious brightness variations were noted but a full reduction of the images obtained has yet to be completed.

Thursday, 10 February 2011

Visit to the Catalina Sky Survey

During the first week of February my job took me to Phoenix, AZ and before flying home, I took the opportunity to visit the Catalina Sky Survey (CSS) team in Tucson. I met up with Principal Investigator Ed Beshore and co-P.I. Steve Larson at the Lunar and Planetary Laboratory within the University of Arizona campus for lunch and we were joined soon after by Richard Kowalski and Rik Hill from the CSS and by Carl Hergenrother from the Minor Planet Center (MPC), also based in Tucson.

Later in the afternoon Ed drove me up the winding road to Mt. Lemmon, a 1.5 hour journey, initially crossing the Catalina foothills covered with magnificent Saguaro Cacti. Half way up the mountain, stunning views unfolded across Tucson, to Kitt Peak 55 miles to the southwest and to Mt. Hopkins with the MMT 50 miles south, not surprising the area is dubbed "Optical Valley". Some miles before reaching our destination we passed the snow line and saw the devastation caused by the forest fires that came close to destroying the CSS observatories back in the summer of 2003. Once we arrived at the summit of Mt. Lemmon (9200ft above sea level), the Large Binocular Telescope on the summit of Mt. Graham 50 miles to the northwest was plainly visible, with the lengthening shadow of Mt. Lemmon pointing almost directly at it.
The shadow of Mt. Lemmon pointing almost directly at the Large Binocular Telescope on Mt. Graham 50 miles away (top right in insert)

As the Sun was setting, Ed opened up the dome of the 1.5-m reflector (observatory code G96) and initialised the equipment before the start of the nights NEO surveying. The 4K x 4K CCD is normally operated at -100C but that temperature has recently been difficult to get down to, an air dryer unit used to extract water from the cooling system suspected of being full of water and needing replacing soon.
Ed Beshore opens up the lower shutter of the Mt. Lemmon 1.5-m telescope
The observer for the night, Alex Gibbs arrived and continued preparing the telescope, running through focussing sequences and also choosing fields in the early evening sky ready for the survey work to start. Two computer flat screen monitors arranged side-by-side displayed the sky divided up into the fixed 1.2 x 1.2 field centres that are used night after night for all the survey work. Settings allowed the areas of sky that CSS and the other surveys such as LINEAR and Spacewatch had covered in recent nights to be colour-coded so that fresh unsurveyed sky could be targeted that evening.

Field centres for the 1.5-m telescope, colour-coded showing where NEO surveying had been done in recent nights by other surveys

Alex clicked on sets of 12 adjacent fields with the mouse, each set of 12 fields would be automatically imaged one after another, going around the set a total of four times. All the images would then be fed into the processing pipeline for automatic moving object detection. Lines on the computer screen marking out 60 elongation from the Sun indicate the westernmost boundary and surveying then proceeds to the east as the night progresses, with the observer manually choosing the areas of sky to be targeted. Earlier in the week Pan-STARRS had been given a complete night dedicated to NEO detection, resulting in 27 Pan-STARRS objects being on the NEO Confirmation Page, most very faint at mag. 22-23 and out of the reach of amateurs. Alex selected some of the sets of survey fields to cover the uncertainty areas where some of the Pan-STARRS objects were located to try and help recover them. As astronomical twilight ended the exposures started and soon the first set of 12 x 4 images had been processed and was ready for examination.

Alex Gibbs selecting fields at the start of a night of NEO surveying by the 1.5-m Mt. Lemmon telescope

The CSS software detects objects that have consistent movement on all four frames of each field, down to a signal-to-noise threshold of about 1.2 (i.e. deep into the random noise in the images) and then matches those detections against the latest orbits in the MPC's MPCORB Minor Planet database. Each set of four images is then blinked, with known objects marked in green. All the remaining detections are then visually checked by the observer and those that look convincing are manually selected, the rest discarded. A score is then determined for each new object depending on its rate and direction of motion, together with where it is located in the sky to indicate its chance of being a NEO. Astrometry is measured for all the objects and sent off to the MPC, any objects that look like they are particularly interesting have further exposures scheduled for later in the night to help with the follow-up effort.

With the 1.5-m telescope in full operation, Ed and I left to drive the 7 miles down to Mt. Bigalow where the CSS 0.68-m Schmidt is located (observatory code 703), 1000ft below the Mt. Lemmon telescope. That night Andrea Boattini was the observer (he was the only member of the CSS team I had met before, at the MACE meeting in Mallorca in 2003) and as we arrived he was busy blinking a set of images with 100+ moving objects, expertly selecting or rejecting the automatic detections at a remarkably fast rate!

Andrea Boattini blinking newly detected moving objects at the controls of the CSS 0.68-m Schmidt
Ed and Andrea discussed recent adjustments to their procedures for the Schmidt, increasing exposure length to 60 seconds to try and detect fainter objects, at the expense of reducing the amount of sky they would be able to cover per night. We left Andrea blinking the latest set of images processed through the pipeline and drove back down the mountain with both telescopes continuing to work flat out.  What a night...

Thursday, 6 January 2011

2010 XM56

December and November were very disappointing months, the smallest amount of clear skies here for seven years, indeed my counts of nights used (169) and hours spent imaging (702) for the year were also the lowest since 2003. I recall writing this time last year how 2009 had produced the best observing figures since my observatory was commissioned in May 2002... I should have kept my mouth shut!

On one of the rare decent nights in the month, I followed Apollo 2010 XM56 for 5.9 hours on December 16th. It had been  discovered a week earlier by LINEAR and with an estimated diameter of only about 30 meters it was predicted to reach mag. +15 as it passed by Earth at slightly less than 2 Lunar distances. During the night its apparent speed accelerated from about 160"/min up to 250"/min and only stayed in the same field of view for 4 minutes at the start of the night and 3 minutes by the end. It was obviously varying substantially in brightness with a period of about 2 hours. I stacked all the usable images from each of the separate fields of view taken during the night and ended up with 96 photometric measures. The raw lightcurve shows a plot of relative  magnitude against fractional Julian Day (0.5 = December 17.0 UT) and shows the amplitude of the brightness variation increasing from an already substantial 2.5 magnitudes at the start to 4.4 magnitudes by the end of the night! By the end, the object was changing from being very well recorded on individual 2 second exposures at maximum, then fading within 1/2 an hour to being completely invisible on individual images at the deep minimum, though the multiple image stacks made at minimum still recorded it well. Some of the increase in amplitude will be due to the rapidly increasing phase angle (47° increasing to 65°) as shadows lengthened on the surface of the object although there may also be some variation due to tumbling. The period was determined to be 2.35 +/- 0.02 h.
Raw lightcurve of 2010 XM56 from 2010 Dec 16/17th