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Astronomical History in Clare

The People Who Discover Comets: JUSTIN TILBROOK

The People Who Discover Comets: Justin Tilbrook
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Above— Justin Tilbrook and his telescope
Below — The Tilbrook Observatory

A Tale of Discovery

Justin Tilbrook, one of our more experienced variable star observers, was not searching for comets. He just happened to be looking in the right direction at the right time, and was familiar with the starfield.


Justin was observing the dwarf nova TV Corvi on the evening of July 22nd (local time), when he noticed a nearby 10th magnitude blob about 60-70 arcseconds in diameter.

Not having seen it before, he took note of its position (1997 Jul 22 10:20 UT 12h18m23s -18d59' (2000)) using the VSS RASNZ chart for TV Crv.

A second look at 11:54 UT showed the blob had moved (12h18m25s -18d55'), so Justin began phoning other observers for verification.

As luck would have it, everyone locally was clouded out; but we were able to confirm that no known galaxies, nebulae or comets were in the vicinity.

An e-mail was then sent to the Central Bureau for Astronomical Telegrams.

IAU Circular 6705 announced the discovery on July 23. A preliminary orbit announced in IAUC 6707 showed the comet was close to perihelion at about 1.4AU from the Sun and a similar distance from the Earth.

At the time of discovery the orbital plane was almost at 90d to our line of sight.

If the comet had been 3 months earlier, it would have been visible in 7x50 binoculars from the Earth.

Extrapolating the orbit backwards a few months helps explain why this comet wasn't found earlier.

Although it has been over 60 degrees away from the Sun since March, it has spent most of the time cruising through Carina, Vela, and Centaurus - constellations with an abundance of stars, clusters, and nebulae.


Comet Tilbrook finally escaped the Milky Way in late June, but because it happened to be winter in the Southern Hemisphere the weather had been poor.

In fact, the night of discovery was the first semi-clear night Justin had for two weeks.

Justin Tilbrook lives on a dark hillside in the Clare Valley district of South Australia.

The Clare Valley is a bit of a magnet for visiting amateur astronomers; although it isn't clear if the attraction is the dark skies, the many excellent wineries, or both!

A Report taken from the Comet Observation page on the Internet

On 22 July 1997, Justin Tilbrook (of Clare, South Australia) discovered his first comet — C/1997 O1.

The majority of comets are found during deliberate searches by dedicated comet hunters.

Comet Tilbrook is one of the rare and accidental discoveries of comets, by variable star observers, in a variable star's vicinity.

Justin is shown in the picture (left) with the 8-inch f.6 Newtonian telescope he used to discover the comet.

The observatory structure was retrieved from a rubbish dump in exchange for a carton of beer.


On January 12, 1999, Justin Tilbrook discovered his second comet — this time he was looking for comets.


The Road to Discovering Comets — by Justin Tilbrook


"My interest in astronomy started when my father, Ian Tilbrook, received a 4½-inch reflecting telescope from my mother, Marie."

"It was in the early 1970s and we would sit outside on warm summer nights, watching satellites whiz overhead and marvel at celestial wonders through the new telescope.

Many nights were spent observing planets and other objects.

"There wasn't a lot of information about the night sky and astronomy in the early 70s.

Any information available was heavily biased towards the Northern Hemisphere, not much use for our southern skies.

This left me with a sense of wonder at all the mysterious objects in the night sky. This sense of wonder has stayed with me!"


"I dabbled with astronomy, on and off, while I grew up and did all the things one does:

-- Getting married to my very patient wife, Kathryn,

-- and having our children Rebecca and Richard.


It was the early 1990s when I came across an article about an Astronomical association that had started at Bowman Park near Crystal Brook.

I went along to a meeting and was hooked! It was great to be among like-minded amateur astronomers, sharing ideas and experiences."

The Serious Amateur Stage

"After joining the Bowman Park Astronomical Association my interest and knowledge increased ten-fold.

It wasn't long before I was building my own telescopes and trying my hand at all different areas of astronomy."


"We would run observing nights for school groups and the public – it was good fun!

We also had members of the Astronomical Society of South Australia visit us.

This is when I met Fraser Farrell, who suggested I should get into variable star observing.

Variable star observing is making visual estimates of how bright a star is – many stars vary in brightness due to varying factors."


"I had many new friends, and after one year was elected President of Bowman Park Astronomical Association, and remained President for 4 years.

Unfortunately the Club was closed when some key members moved on.

Being a small club, we needed all hands to run it".


"I continued with astronomy and my variable star observation.

In 1996, with the family needing more room and a need for a better observing site, we moved to Penwortham, 12kms south of Clare.

It proved to be a great move in many ways, especially for my astronomy, and my first comet discovery in July 1997.


The following charts and discovery stories will give you details of the events involved in the discovery of Comet C/1997 O1 and Comet C/1999 A1.

How Comet Tilbrook 1 was Found :
In just one word — Accidentally!

Justin Tilbrook, one of our more experienced variable star observers, was not searching for comets.

He just happened to be looking in the right direction at the right time, and was familiar with the starfield.


Above:  1997 - Discovery Area, Constellation Corvus

You can download a photograph of C/1997 O1 taken by Justin, using a 35mm camera with a 200mm zoom lens and 400ASA Fujicolor print film; on the evening of 1997 July 26 (10:45 UT).

The camera was hand-guided for an exposure lasting 10 minutes, to capture this faint image of Comet Tilbrook. As can be seen from the included star chart, no known comets, galaxies, nebulae or other fuzzy-looking objects were nearby.

The Road to Discovering Comets
The Serious Amateur Stage
A Tale of Discovery
How Comet Tilbrook 1 was Found
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Above: C/1997 O1 was Justin's first comet discovery

Below: 1997 - Orbit diagram of Comet Tilbrook

C/1997 O1 was Justin's first comet discovery.

The majority of comets are found during deliberate searches by dedicated comet hunters - an activity Justin now pursues successfully.

The first Comet Tilbrook (C/1997 O1) joins

  • Comet Williams (C/1998 P1),

  • Comet Kaho-Kozik-Lis (1936III) and

  • Comet Jones (1946VI)
    as one of the rare and accidental discoveries of comets - by variable star observers - in a variable star's vicinity.

Bitten by the Comet Bug
Tilbrook Strikes Again!


This time, Justin Tilbrook was actually comet hunting when he found a 10th magnitude arcminute-sized blob heading southwards;

on the evening of 1999 January 12 (Jan 12.486UT).

He found it at 23h38m -28.1d (2000), in a barren piece of sky on the Aquarius-Sculptor border.

Justin - armed with ephemerides of all visible comets - was sure it wasn't a known comet; and he could not recall seeing any uncharted galaxies (or similar fuzzies) during previous searches of this area.

Two Australian astrometrists (Gordon Garradd and Frank Zoltowski) were then contacted to provide confirmation and precise positions.

Then the observations were e-mailed to the Central Bureau for Astronomical Telegrams for checking.

At this stage, it was still possible that this object was a very faint known comet (not in Justin's ephemerides) that had brightened dramatically - or that it had just been discovered and reported by someone else.

Fortunately, Justin's anxious wait was ended on January 13 by IAU Circular 7084, announcing the discovery of C/1999 A1 (Comet Tilbrook).

The revised parabolic orbit from MPC 33651 is:

  • T = 1999 Jan 29.5750 (the time of perihelion)

  • q = 0.731240 AU (the perihelion distance, 109.4 million km)

  • peri = 232.5353 degrees

  • node = 259.0986 degrees

  • i = 89.5607 degrees

Below is a composite of 3 unfiltered CCD images taken at Stockport Observatory, shortly before midnight (local time) on January 14.

The observers were Jim Costello, Trish Ellin and Rob Purvinskis.


Although each exposure was only 15 seconds, they span about 5 minutes of time in total

- so the comet image is slightly trailed.

No tail was seen.

Comet Tilbrook 14-01-99.gif
Bitten by the Comet Bug — by Justin Tilbrook


"After having the good fortune of discovering Comet C/1997 O1 while variable star observing, Bill Bradfield suggested that I should give comet hunting a go! I didn't need much convincing, as the comet bug had well and truly bitten me."

"Initially, I used my 8-inch f.6 dob that was being used for variable star work. This soon became a pain in the neck, literally." 


"Apart from the increasing chiropractic bills, it was increasingly obvious that I was not covering enough sky at each comet hunting session.

Some of the other problems were

  • not being sure of the exact sweep limit,

  • the observatory dome opening being too narrow (30 deg width of sky) and

  • countless times forgetting which way I had been sweeping after checking out some fuzzy on the chart.

This all added up to lost time for the area of sky covered, about 30 x 30 deg in 1½ to 2 hours."


"In early 1998, I decided it was time to construct a purpose-built telescope.

Bill Bradfield came to the rescue with some diagrams and information on suitable designs.

A better mentor for comet hunting there is not.

I eventually decided on a cross-breed, a fixed height reflector that rotates at the eyepiece through the altitude bearings.

The rest of the mechanical side looks like a cross between a springfield mount and a dobsonian (see photograph at left).

I think it looks like the starboard engine of the star ship Enterprise with a weight lifting set thrown on.


Left: Specialised Hunting Telescope.

"When building this telescope, it was important to incorporate as many time saving devices as possible.


  1. Directional sweeping stopper. It has a 2-fold purpose – to stop the telescope at a given azimuth in both directions. The other is a switch connected to a red light. When sweeping clockwise the light is off, and on for sweeping anticlockwise. This stops over-sweeping, and now I don't have to sweep the same area of sky twice, after checking charts and forgetting sweep direction.

  2. Sky vector, digital setting circles. This has saved an enormous amount of time when checking out positions of those annoying objects which are not comets.

  3. Heated eyepiece and finder scope They are home made, using resistors stitched into a velcro band, and run from a 12 volt battery. They certainly prove their worth on those cold frosty nights.

  4. Black shroud to keep out intrusive light. Made from water-resistant card, with a layer of water-proof vinyl glued and stitched to the card.

  5. Padded sweeping handles. This is important, to stop frost-bitten fingers. Even with gloves on, fingers can seize up. Temperatures in the Clare Valley can hit minus 4 degrees Celsius on many winter nights.

  6. Rollers rather than teflon in the azimuth base This allows much smoother rotation.

  7. Roll off observatory with lift up door. This acts as a veranda to keep off the dew during those winter months. The observatory has a locking bar in the middle when placed over the scope, and is removed so that the observatory can be placed in any desired direction."


"I must mention the many good ideas suggested to me by Ian Bedford.

Many of you will have heard about or seen pictures of Ian's superb 30-inch f.5 telescope.

Currently my telescope is running 8-inch f.6 optics, but is designed to take 10-inch f.4.5. These will be installed in the near future.

The Discovery of Comet C/1999 A1 Tilbrook

"The story of this discovery actually starts 2½ years ago, with the decision to move out from the town of Clare, in the mid-north of South Australia, to Penwortham, a small village 12kms south of Clare.

The reason, increasing light pollution from ever brighter street lights, and personal security lights.

It was becoming so bad that I was dropping nearly one-half to one magnitude when trying to do variable star observations.

The decision obviously a good one – no street lights and an elevation of 470 metres at the observatory means that even in the middle of winter I am usually above any fog."


"On January 12th, after a relaxing Christmas break and the first day back at work, I decided it was time to get out and resume my comet hunting.

Starting rather early in the twilight sky I thought there was no harm in trying a sweep starting low on the horizon.

You never know your luck, you might find a bright comet.

After 20 minutes sweeping, I came across a fuzzy blob with a faint star overlaying it.

I knew from previous searches there are at least a few faint galaxies with stars in the same field."


This chart represents the night sky in winter.
It shows the constellations where the two comets were found.
Scl = Sculptor    Crv = Corvus


"A quick check of Sky Atlas 2000 showed no galaxies in this region of Sculptor. Next, I checked the Herald Bobroff atlas, knowing this showed far more deep sky objects.

Still no candidate By this time, I started getting the feeling I had read about so many times in the book, The Astronomical Guide to Discovery.


"It was time to record all the usual details of this fuzzy, and dig out all relevant information on known comets MPC, Perihelion, 1999 astronomy, and still no reference to anything in this part of the sky.

The next thing to do was to ring Dr Tony Beresford, of the Astronomical Society of South Australia.

Dr Beresford is our technical information officer.

Tony soon established there was no comet within 3 deg of that position, so back to the scope to check for movement, and a second position."


"To my surprise it had moved, motion being easy to detect because it was right over a star.

The suspect had moved approximately 200 arc seconds in 1½ hours.

I then called Tony with the details, and he kindly arranged for confirmation by Gordon Garrard, and Frank Zoltowski.

It was a nervous 48 hours waiting for that confirmation, but it certainly was a worthwhile wait, as this was something that I had set out to achieve! And it had certainly paid off."


"I would like to thank five people in particular.

  1. Firstly, Bill Bradfield, who is such an inspiration to aspiring comet hunters.

  2. Dr Tony Beresford, who with his expertise has helped enormously.

  3. Fraser Farrell, who is our variable star group leader and good friend, for writing up the ASSA home page, and supplying me with accurate charts,

  4. and to Gordon Garrard and

  5. Frank Zoltowski for dedication in their astrometric work; and to anybody else who helped with the swift confirmation of C/1999 A1."

Three Recent Images by Justin Tilbrook​ -

Below:  Another beautiful image of Comet C/2020 F8 Swan by Australian amateur Justin Tilbrook this morning.

Con Stoitsis on Twitter- 'Another beautiful image of Comet C-2020 F8 Swan by Australian am
The Discovery of Comet C/1999 A1
Tilbrook Strikes Again!
Three Recent Images by Justin Tilbrook​
All Sky Chart
Comet C-2019 U6 Lemmon and NGC 2440 on the 7th. Image Justin Tilbrook.jpg

Below:  The tail of Comet Lovejoy on January 6, 2012.

The “pointer” stars, Alpha and Beta Centauri, along with the Southern Cross and the Coalsack nebula, are on the left, and

the Large Magellanic Cloud (partially hidden behind a tree) is on the right.

Courtesy Justin Tilbrook in South Australia.

The tail of Comet Lovejoy on January 6, 2012.jpg
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