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Latest News... Archive!

Fact-bites, stories, and points of interest relating to SOHO comets and SOHO images.

January 2006 through October 2007

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October 11, 2007 -- Predicting revolutions...

Once again, SOHO comets have made headlines with the recent announcement of SOHO's first officially periodic comet, P/2007 R5! This discovery was due to the brain power of the highly succesful SOHO comet hunter Sebastian Hoenig, who first proposed that SOHO comets C/1999 R1 and C/2003 R5 were actually the same object, and Terry Lovejoy (Australia), Kazimieras Cernis (Lithuania) and Bo Zhou (China), who found the comet in our data at each of its respective apparitions in 1999, 2003 and 2007. Huge congratulations are due to all involved!

    This image (click for larger version) shows P/2007 R5 at each of its three observed perihelion passages in 1999, 2003 and 2007. You may note that the size, shape, brightness and location of the comet in each image is remarkably similar -- as expected, of course!

So this leads us to the question of just how many of SOHO's comet discoveries are actually periodic objects? Well, we know that the Kreutz comets are technically periodic but as we have never knowingly seen the same Kreutz comet twice (their orbital period is at least several centuries), we can't call them periodic. A more positive link is made amongst the Marsden and Kracht group comets. Here, we are quite confident that they are all related to 96P Machholz, and we even have some possible linkages between individual objects seen approximate five years apart in SOHO data. Admittedly, some of these linkages are made at a stretch but some also look quite compelling. (Maik Meyer has a good summary on his website of Kracht and Marsden comets and some of the proposed linkages.)

Recently, inspired by Sebastian's work, Rainer Kracht -- the reigning champion SOHO comet hunter! -- has been looking for new potential linkages within the Kracht group (guess who that group is named after...), and has made some very tentative predictions about possible returns of some of the Kracht objects next year (2008).

Rainer has been kind enough to share this work with us, and so with his permission, here is a list of his predictions for Kracht group comets next year:
Designation  	         Possible return date:
C/2002 N2  (SOHO-501)    2008 Feb 12/13
C/2002 Q8  (SOHO-504)    2008 May 12/13
C/2002 Q10 (SOHO-505)    2008 May 16
C/2002 S4  (SOHO-519)    2008 Jun 28
C/2002 S5  (SOHO-520)    2008 Jun 30
C/2002 S7  (SOHO-522)    2008 Jul 03/04
C/2002 S11 (SOHO-526)    2008 Jul 22/23
Rainer used the free SOLEX program to calculate the orbits, making sure to incorporate gravitational influences of planets and, of course, the Sun, on the comets. Full documentation of Rainer's work can be found on his website. It should be noted clearly that these are simply rough predictions based on the limited observations we have of these objects and some basic assumptions about their orbits and behavior. As Rainer notes, "Some of them could be already too faint at their return in 2008 to be found again in the SOHO LASCO images. Some of them could fragment further with unknown consequences on their perihelion times.". We will have to wait until 2008 to find out for sure, but based on our current knowledge, it seems certainly seems likely that we should see a return of at least some Kracht group comets next year.

January 23, 2007 -- Fantastic comet McNaught!

You do not need to be an astronomer of any kind to appreciate the amazing spectacle that was (and still is, in the Southern Hemisphere) comet C/2006 P1 McNaught. I had mentioned in a previous post that at perihelion the comet would disappear from view for a few days for everone except the Sun-watching spacecraft SOHO and STEREO. It turns out I was quite mistaken as comet McNaught brightened to the point that it was visible in broad daylight! However, the best views at the time were still reserved for space-based observers...

The LASCO C3 images from January 12-15 were dominated by the massively saturated images of comet McNaught. We tried to turn the exposure as low as we could, and that made some difference, but the comet was just so bright! As reported in several news sources, we did take color filter observations of the comet. These images were still suffering from saturation, but to a lesser extent.

    These images (click for larger version) are raw FITS file images of the comet in LASCO C3. The image on the left was taken with a "Blue" filter, the one of the right with a "Deep Red" filter. The images were taken two minutes apart. Exposure times are 10.1 and 4.1 seconds, respectively.

While the LASCO images were very impressive, for the first time in it's 11-year history, SOHO was arguably outperformed by the new kid on the block -- STEREO/SECCHI! As mentioned in my last entry, all of us here in the SECCHI operations room were thrown into a stunned silence when we opened the door on the SECCHI HI-1B instrument for the first time. The images of the comet were truly outstanding! Also as mentioned, the reason for our surprise at seeing the comet was because we hadn't realised that the roll angle of the "B" spacecraft was (for operational reason) such that the comet was visible. Just a few days later the "B" spacecraft rolled back to where it should be and the comet was gone... from that instrument at least.

So the comet images from SECCHI HI-1B instrument were a pleasant surprise, but the images from the HI-1A instrument proved to be the ultimate prize. Below I have selected three images taken from HI-1A during the period of January 11th to 19th. The image show the comet tail in spectacular detail, especially once the bright comet head left the field of view and stopped saturating the images. These images are very likely the most detailed images ever taken of a comet while it is that close (0.17AU) to the Sun, and also (in my opinion) one of, if not the most, beautiful uninterrupted sequence of images of a comet ever made.


Finally -- and this is really the best bit of all -- here come the movies! I have three movies to share with you all. They are the full sequence of images taken by the SECCHI HI-1A instrument between January 11 and 18, 2007. Each HI-1 image is a sum of twenty-four, 24-second exposures taken two minutes apart. One of these "summed" images is taken every two hours. You might find it better to right-click on the below links and save these files to your computer. (I tend to get better results this way than watching the movies in the browser itself.) Also, then you have them saved to your computer so you can watch them later without having to download them again.
Note that also visible in these movies is Venus (bright object left of center at the botttom) and Mercury (appears from te right later in the sequence). There is an internal reflectio noticeable on the left hand side of the images for a short period (the instruments were not designed with mag -5.5 objects in mind!). Our processing of these images is not yet perfected -- it takes time to fully understand the response of the CCD and account for that in our processing. Hence the bottom of the images (particularly as Mercury slides in), look a little darker than the top.

Please note that all of the SECCHI and SOHO data is free for public use, but if you use them in presentations, published work, websites, etc, please give the appropriate image credits ("STEREO/SECCHI" or "SOHO/LASCO" will usually suffice). You can also always email me for information, too. Enjoy!!

January 11, 2007 -- Wow...

  This image (click for larger version) from the SECCHI/HI-1B instrument on the NASA STEREO-B (Behind) spacecraft was taken on January 11, 2007 just after the door covering the instrument was opened for the first time after the STEREO launch on October 26, 2006. The image is dominated by a spectacular view of comet C/2006 P1 (McNaught) The full field of view of the HI instrument is centered at about 14 degrees from sun center and is 20 degrees wide. (Note that the image opposite is a close-up view of the comet.) The comet tail is approximately 7 degrees in length and shows multiple rays. The coma is saturating the image even at the shortest exposure time of 1 sec. The images are full resolution 2048 x 2048, which corresponds to 35.1 arc sec/pixel. The SECCHI/HI instrument was built by a consortium consisting of NRL, the University of Birmingham (UK), Rutherford Appleton Laboratory (UK) and Centre Spatiale de Liege (Belgium).

This was the sight that greeted us when we opened the HI-1B doors for the first time today. Wow!! Just as a note to you all, the reason we can see this object in HI-1B is because the 'B' spacecraft is currently rolled (for operational reasons) at a roughly 140-degree angle. Ordinarily we would not have been able to spot the comet.

  This is a full-field (but not full-size -- I rescaled it) image from the SECCHI HI-1 B camera showing comet C/2006 P1 (McNaught). Note the saturation in the HI images shows itself as vertical streaks instead of the horizontal streaks we see in LASCO. This is due to the way the HI CCDs are read by the instrument.

As many of you are aware, the recent discussions of this comet have centered around the question "how bright?". Well right now we have not had chance to analyze the images and determine a magnitude (we're still somewhat in shock), but as a benchmark, it appears to be brighter than Venus is in our HI-1 A images.
  This is another close-up image of the comet. I have rescaled the image to show more fine structure in the tail, but it has dramatically increased the apparent saturation in the comet head. Note also that this was a longer exposure image (twenty-four seconds) so the saturation is pretty bad anyway. The glare in the lower-right corner is from the Sun's F-corona.

January 10, 2007 -- It's almost time... (and we still don't know how bright it will be!)

All eyes are beginning to turn towards the SOHO/LASCO C3 images in the next couple of days as the much-publicized comet C/2006 P1 (McNaught) prepares to put on a show in the images of history's most prolific comet discoverer.

In my last update I mentioned that nobody really knew how bright this comet would be. Estimates were ranging from about mag +5 to mag -6. Since then, the comet has appeared in the twilight skies for Northern Hemisphere observers and brightness estimates have been refined... a little. The popular consensus now seems to be somewhere between mag -1 and mag -3, but due to its position in the sky, these are only very approximate estimates. There are many images of the comet now available online, all of them very beautiful.

At around 0300UT on January 12th, the comet should start to appear in the LASCO images. It will track from the top-left to the bottom-left of the images, crossing the Heliospheric current sheet as it goes by the Sun. While most of you reading this are familiar with how and where to obtain the latest SOHO images, for the benefit of any newcomers amongst you here are the locations where you can download the realtime LASCO data:
Please note that while we do have "real time" data from SOHO/LASCO, we do not have continuous, uninterrupted 24hr contact with the spacecraft. This means that there will be periods where the images are not updating. These periods typically last only a couple of hours, but sometimes our ageing data processing machines do crash and a technician has to manually restart the process. At weekends this can take time, so please be patient with us! You can always check the spacecraft contact schedule to see when the planned data gaps will occur.

So now all that is left to do is sit back, pull up your internet browser, and prepare to be dazzled by what could be one of the brightest comets in decades!

December 26, 2006 -- We know when, we just don't know how bright...

For those of you who follow the goings-on in the world of comet astronomy, you may have noticed that one particular object has been the source of much discussion recently. That object is the comet C/2006 P1 (McNaught). It was discovered on August 7th, 2006 by the hugely successful comet discoverer Rob McNaught (Siding Spring Survey), and is the 31st comet to bear his name. (Admittedly, the SOHO satellite scoffs at such a small tally, but among us humans it's a formidable achievement.) At time of discovery, the comet was a very faint mag17 object, but the published orbit -- giving a perihelion distance of just 0.17AU and a perihelion date of January 12, 2007 -- indicated that the object has the potential to be a very bright object indeed. The question was, and still remains, just how bright will it be?

Ordinarily, (pre-perihelion) estimates of a comet's maximum brightness are accurate to within a magnitude or two, often better. In the case of this comet, however, I have seen estimates that range from magnitudes of +6 to -5! So why the huge uncertainty? Well, the single biggest problem is that for several weeks the comet has been at a small elongation (apparent proximity to the Sun as viewed from Earth) and so it has been extremely difficult for observers to get good measurements of it. (Important aside: Never, ever, point a telescope at or near the Sun without the correct solar filters!!)

So nobody really knows just what this comet will look like at its closest point to the Sun... and that is where SOHO comes in! As you are probably aware, the LASCO instrument on-board SOHO has the ability to watch comets as they get extremely close to the Sun. Fortunately for us, C/2006 P1 is going to pass right through the LASCO C3 field of view in just over two weeks' time!

  The image opposite (click for a larger version) shows the expected track of the comet through LASCO C3. Unfortunately this object will not come close enough to the Sun for it to also be in the C2 instrument field of view, but hopefully we will get a spectacular view of the object, maybe even rivaling the comet NEAT or comet Bradfield passages through LASCO C3!

The comet will appear in C3 at around 1000UT (0500 EDT) on January 12th (a few hours before perihelion) in the upper-left of the images and travel almost vertically down, exiting C3 in the lower-left at roughly 0300UT on January 16th.

It is worth making one final note that this comet should also be visible in the "Heliospheric Imager 1 (HI-1)" instrument on STEREO-A spacecraft. However, while these images will be publicly available, we do not currently have processing routines in place to produce a regular stream of "pretty pictures" from any of the STEREO instruments. I will try and post some HI-1/A images of this comet as soon as we have them.

November 22, 2006 -- No news for two months? What gives?!

So, it has been an awful long time since the news page was updated and lots has happened in that time! I have been wrapped up in numerous projects and distractions, but should soon have a little more free time to keep you updated. We have a few potentially exciting comet-related events coming up in the new year but for now here's an overview of what has been going on these past few weeks...

First off, SOHO found its 1,200th comet. Congratulations to Bo Zhou for that one!. We also had a very bright Kreutz discovery, courtesy of John Sachs' expert hunting (See picture, below. Click for larger image), and several other smaller comets. In fact we've had 31 comets since the end of September.

Comet SOHO-1213, found by J.Sachs

But probably the most exciting piece of news we've had is the successful launch of the STEREO mission!! Why should comet hunters care about this? Well, if all goes to plan, the STEREO spacecraft should be able to observe and discover comets along-side SOHO! STEREO consists of two near-identical satellites that will view the Sun-Earth system from two different angles, allowing us our first stereoscopic view of the Sun, it's atmosphere and corona, and CMEs. The primary imaging suite on board STEREO is a set of instruments known as "SECCHI" (Sun Earth Connection Coronal and Heliospheric Investigation). Each SECCHI 'package' contains four coronagraphs and one EUV imager. The coronagraphs (named COR1, COR2, HI1 and HI2) behave like LASCO does, blocking direct sunlight with an 'occulter', allowing us to see the fainter solar atmosphere. The EUV imager (named EUVI) takes extreme ultra-violet images of the Sun like those taken by SOHO's EIT instrument. The coronagraphs on LASCO currently take images that cover the range from ~3 to ~32 solar radii. The four SECCHI coronagraphs will cover from 1.1 to 215 solar radii! They cover the entire area from the Sun to the Earth (allowing us to track CMEs all the way to Earth).

I'm am going to spare you more details for now, but it is my intention to post something a little more substantial about SECCHI some time in the near future.
The official set of SOHO comet confirmations for the past couple of months is as follows:
Soho#  Date/Time of Post  Discoverer   Tel   Group  Images of..
1193  Sep26,06 17:40:42  R.Matson     C3,C2  Kreutz  Sep26-27,06
1194  Oct02,06 11:14:54  H.Su         C3,C2  Kreutz  Sep30,06
1195  Oct02,06 13:31:51  R.Matson     C3,C2  Kreutz  Oct02,06
1196  Email report       H.Su         C3,C2  Kreutz  Oct03,06
1197  Email report       G.Sun,T.Chen C3,C2  Kreutz  Oct03,06
1198  Oct06,06 03:59:29  W.Xu           C2   Kreutz  Oct06,06
1207  Oct24,06 10:13:49  H.Su         C3,C2  Kreutz  Oct24,06
1208  Oct24.06 14:03:06  T.Chen         C2   Kreutz  Oct24,06
1209  Oct25,06 10:48:26  H.Su           C2   Kreutz  Oct25,06
1210  Oct29,06 09:27:08  H.Su         C3,C2  Kreutz  Oct28,06
1211  Oct29,06 10:08:34  L.Cane       C3,C2  Kreutz  Oct28,06
1212  Oct29,06 10:14:50 Kracht/Su/Chen  C2   Kreutz  Oct30,06
1213  Nov01,06 20:01:32  J.Sachs      C3,C2  Kreutz  Nov01-03,06
1214  Nov02,06 10:09:56  H.Su           C2   Kreutz  Apr06,03
1215  Nov02,06 21:28:40  H.Su           C2   Kreutz  Nov03,06
1216  Nov03,06 20:26:45  H.Su         C3,C2  Kreutz  Nov04,06
1217  Nov05,06 11:09:32  H.Su           C3   Kreutz  Jan22,03
1218  Nov07,06 08:47:59  T.Hoffman      C2   Kreutz  Nov07,06
1219  Nov07,06 11:43:31  H.Su           C2   Kreutz  Nov07,06
1220  Nov08,06 09:42:39  H.Su           C2   Kreutz  Nov08,06
1221  Nov11,06 16:27:39  R.Kracht       C2   Kreutz  Nov11,06
1222  Nov13,06 08:57:30  B.Zhou       C3,C2  Kreutz  Nov13-14,06
1223  Nov13,06 19:30:00  H.Su           C2   Kreutz  Nov13,06
1224  Nov19,06 21:02:21  T.Hoffman      C2   Kreutz  Nov19,06

September 21, 2006 -- Quiet skies, stormy skies...

The past few weeks have been very unusual for SOHO comets. August was a shockingly quiet month for comets. In fact, we went from August 4th to September 2nd without a single comet discovery! That's one of the longest "dry spells" in recent years. The spell was eventually broken by a Meyer-group comet, found in C2 images by Hua Su. That comet was our first Meyer-group comet in nearly four months -- another unusually long comet-free period. And when Tony Hoffman spotted a Kreutz-group comet on September 11, so ended a 38-day period with no Kreutz-group comets -- yet another very uncharacteristically long "dry" time. But as we all know, when it has been dry for such a long time you have to expect a storm sooner or later, and two days ago we got ours...

It all began with a Kreutz group comet spotted by expert SOHO/LASCO (and SOHO/SWAN) comet hunter, Rob Matson, late at night on September 17th. Shortly after discovery, SOHO entered into a reasonably long data gap that lasted into most of the next day. During these data gaps, even though we do not have "real time" contact with the spacecraft, data from the instruments is recorded on board ready for downlinking at the next time of contact. I often give advice to comet hunters at times like this to watch the spacecraft contact times and await the influx of new data. Clearly, Rob was following this advice when the data began to flow because, in the space of 42 minutes and 14 seconds, he reported another four Kreutz comets in the LASCO C3 and C2 fields of view! By the time a few other comet hunters had rejoined the hunt, Rob had already cleared up. This was the first time that five Kreutz comets were visible in C3 in less than twenty-four hours, and certainly the first time a comet hunter has found five comets in such a short period. Congratulations, Rob!

Pictured opposite are three of the five aforementioned Kreutz comets (click for a larger image). The other two (SOHO-1189 and SOHO-1191) were a little way ahead of these, so I chose these three as a nicer image. You can see a short tail on SOHO-1190, and (even though it was the brightest of the five comets) a much shorter one on SOHO-1192. SOHO-1190 had no noticeable tail at all.

The official set of SOHO comet confirmations for the past couple of weeks looks as follows:
Soho#  Date/Time of Post  Discoverer   Tel   Group  Images of..
1186  Sep02,06 12:54:52  H.Su          C2   Meyer   Sep02,06
1187  Sep11,06 15:04:09  T.Hoffman     C3   Kreutz  Sep11,06
1188  Sep18,06 03:11:18  R.Matson    C3,C2  Kreutz  Sep18,06
1189  Sep18,06 17:01:56  R.Matson    C3,C2  Kreutz  Sep18-19,06
1190  Sep18,06 17:04:52  R.Matson    C3,C2  Kreutz  Sep18-19,06
1191  Sep18,06 17:12:05  R.Matson    C3,C2  Kreutz  Sep18-19,06
1192  Sep18,06 17:44:10  R.Matson    C3,C2  Kreutz  Sep18-19,06
So, as you can see, SOHO is now up to 1,192 comet discoveries!

August 30, 2006 -- Argh!! What was wrong with my comet?!

Ever since SOHO comet hunting became popular in the early part of this century, thousands of reports have been made to the sungrazer website... but only a thousand-or-so of those have been confirmed as comets. Unfortunately, due to time constraints, it has never been possible for the sungrazer webmaster to personally address every comet hunter who has had a claim rejected and explain exactly why their claim was rejected.

So I feel that it is time, at last, to address a large proportion of rejected comet hunters and explain exactly what was wrong with their report. You see, many, many newcomers to SOHO comet hunting make the same mistake with their reports, and their reports share a common fault: they don't obey the laws of physics!

Imagine the scene: a comet is flying through space at whatever ridiculously high speed it happens to be going at. We may not know much about this comet, but there are two things we can say with absolute certainty:
  • 1) Unless something hits or pushes it, the comet is not going to make any change in direction other than the gradual arc it follows as part of its orbit.
  • 2) Unless it hits something, or passes through some yet-to-be-discovered dense medium, the comet is not going to change its speed, other than, perhaps, a gradual acceleration/deceleration as it swings past a solar system body.
Obvious, right? Of course! Barring gradual changes of speed at (for example) perihelion, and the obvious arc of an elliptical/parabolic orbit, a comet will always move in a constant direction at a constant speed. No exceptions.

"But how does this apply to my rejected claim?", I hear you ask. Well, like a large proportion of comet reports before yours (and undoubtedly like many after yours), according to the positions you gave, your comet just decided to slow down and change direction! "Ohhh....", you reply, as inspiration suddenly strikes.

So how can you avoid becoming one of the rejected masses? Well, you can't really because SOHO comet hunting will always involve false reports. That's just the way it is and we accept that. It's very difficult to tell faint noise from faint comets and even the best comet hunters make false reports. But what you can do is limit your reports to those that at least are physically possible. The way you do that is as follows.

Before submitting your report, just take twenty extra seconds to look at your positions. I'll use an example that I just made up:
Time --  X -- Y
16:18   186  528
16:42   189  539
17:18   193  532
17:42   215  536
You do not need a calculator or a physics degree to spot the two problems here. We will start by looking at the x-positions of this report. Between 16:18 and 16:42 (24 minutes), the comet moved 3 pixels in the x-direction. OK, nothing wrong with that. Then, between 16:42 and 17:18 (36 mins) the comet moved another four pixels. Great, still sounds fine. But then between 17:18 and 17:42 (another 24 min gap) the comet moved 22 pixels! That can not be correct! For the comet to do that would require an enormous acceleration (and we've already discussed why that can not happen).

So on this basis alone we should either reject the 17:42 position, or reject the entire report. But let's not rush into things. Instead we'll look at the y-positions. OK, between 16:18 and 16:42, the comet moves 11 pixels in the y-direction. That sounds good. But wait! Then, between 16:42 and 17:18 it moves 7 pixels in the opposite direction! I think that at this point we can safely assume that this is not a real comet, and so we certainly do not post this report!

I can not imagine just how many hundreds of comet reports I have seen on the sungrazer site where one, or both, of those rules have been broken. My hope is that my simple demonstration above will help new comet hunters understand that "constant speed and direction" are the key to SOHO comets, and that in turn will help them find their first comet.

August 09, 2006 -- One one-thousand, two one-thousand...

Almost exactly one year after SOHO's 1000th comet discovery, another amazing milestone has been reached -- SOHO's 1000th Kreutz-group comet discovery! The comet was the sixth, and last in line, of an unseasonal "comet storm" that produced six LASCO-C3 Kreutz-group comets in just three days! (In May or June we expect this, but in August...)

The discoverer of the comet -- SOHO-1185 -- was Arkadiusz Kubczak from Poland. It was only his third SOHO comet find, following only three days after his first SOHO comet! Well done, Arkadiusz!

To make it official, all of the recent comet discoveries were measured and sent to the MPC (Minor Planet Center) for reductions and orbit determination. As always, the tireless Dr Brian Marsden -- who has single-handedly been responsible for the reductions and orbit calculations of the great majority of SOHO's 1,185 comets -- released the orbits on two MPECs (Minor Planet Electronic Circulars). The published orbits showed all comets to be obvious members of the Kreutz-group, typified by their extremely small perihelion distance (~0.005 to ~0.008 AU) and high inclination (around 144 degrees). The comet in question, SOHO-1185, was given an official designation of "C/2006 P7 (SOHO)". (Note that comets found in SOHO images get named after the spacecraft itself, not the person who found the comet in the images.)

I am sure you are all now very eager to see this fantastic and beautiful comet, so.... behold SOHO-1185!

image showing a tiny comet

Ta-da! ... ... ... OK, click on the image and you should be able to see it a little clearer. (Yes, it's that tiny little white blob in the center of the yellow box.) So it may not be the most spectacular object ever discovered in space, but it does signify another fantastic achievement: the discovery of one thousand members of a comet group that, prior to 1995, was only known to contain only about thirty members!

August 01, 2006 -- Incoming...

Good news...
As mentioned in the last news item, the SWAN instrument on-board SOHO recently discovered it eighth comet. The good news is that this comet will definitely be crossing both LASCO C3 and C2! Unfortunately, the comet is remaining relatively faint (about mag 9 as of late-July, 2006), and will not reach perihelion (closest approach to the Sun) until a few weeks after it has left the LASCO field of view, so we are quite confident that this will not be a sensational comet transit. However, even at its current brightness it should show up in the images.
So, where and when will it appear? Well, the below images show the approximate path of the comet through our images. (Note that subsequent alterations to the current orbit, could change these times by as much as a few hours and the positions by as much as a few pixels.)

path of comet through lasco C3   path of comet through lasco C2
Track of C/2006 M4 (SWAN) through LASCO C3
(Click for larger image)
  Track of C/2006 M4 (SWAN) through LASCO C2
(Click for larger image)
Bad news...
Unfortunately, the SWAN comet is perfectly timed to coincide with the SOHO keyhole. For those of you who are not familiar with these, it means that for the majority of the time that the comet is in the LASCO field of view, we will be suffering from poor quality data (lots of missing blocks from the images), large data gaps (several hours or more of unrecoverable data), and closed instrument doors (no images at all!). The impact of this upcoming keyhole is yet to be know, but right now it is not looking good for observing the comet in all of C2 and the last two weeks in C3... More information on this will be posted as I receive it.

July 21, 2006 -- Another SOHO/SWAN comet discovery!

A slight underachiever by SOHO standards, the SOHO/SWAN instrument recently discovered its eight comet -- C/2006 M4 (SWAN). The comet was found and reported independently by Rob Matson and Michael Mattiazzo. As with SWAN's previous discovery, Rob Matson has been kind enough to share his discovery story, plus some images from Rob McNaught and Sebastian Hoenig.

"I first spotted the comet around 10:30 am exactly a week ago today when the 7/5 SWAN image was first posted. I should have spotted the comet a few days earlier, but as I later explained to Brian Marsden and Tim Spahr at MPC, I missed it because it was hugging one of the superimposed gridlines. By the time of the 7/5 image, though, the comet had enough separation from the grid line to draw attention to itself.

I e-mailed Eric Christensen at Catalina Sky Survey (with whom I'd been working on a different SWAN candidate) that our old phantom had disappeared, but had been replaced by something much more promising, and I would get back to him once I confirmed it wasn't a known comet.

By 11 am I had checked known bright comets and found no matches, but wrote Eric back that due to the sky location, the southern hemisphere was the only hope for confirmation. I measured pixel positions for six nights from 6/20 to 7/5, converted to ecliptic coordinates and then to equatorial. I checked residuals on a parabolic orbital solution, tweaked a few positions until I was satisfied, and at 1:22 pm sent a discovery report and rough ephemeris to Brian and Tim Spahr at MPC. Ten minutes later I sent the same positions to Eric.

Since the southern hemisphere was the only hope for confirmation, the next step was to contact Rob McNaught and Gordon Garradd at Siding Spring Observatory to see what they could do. Like most observatories, SSO was down for full moon and wasn't planning their next run until Thursday. But just in case, I sent Rob and Gordon predicted comet positions for Monday and Tuesday, and told them if no additional SWAN images appeared by Wednesday, I'd extend the ephemeris a few more days.

That evening I was able to check my e-mail and confirm that Tim had gotten my message; also, Rob replied that he would try for the comet on Thursday from the Uppsala telescope. There were no new SWAN images Tuesday so I went ahead and extended the ephemeris through Friday for Rob, running some excursions to give him an idea of positional uncertainty. A few hours later, Rob replied that Michael Matiazzo had independently found the same comet and reported it to him. He also indicated that he was thinking about heading up to SSO a day early if the weather cooperated.

Well, late Tuesday night (Wednesday afternoon for Australia) Rob informed me that Terry Lovejoy had located the comet in one of his June 30th survey images from Thornlands, QLD, and forwarded me Terry's crop of an image stack showing a noticeably green cometary object. The weather had cleared sufficiently for Rob to make the decision to head up to Siding Spring, so I checked Terry's position against my rough orbit to see if it needed some real-time tweaking before Rob made the recovery attempt. I didn't have all of my astronomy tools on my laptop, but working from my hotel room I decided that my predicted position needed to shift a bit to the east and to the north. I forwarded the correction to Rob and at 1 am cross my fingers and called it a night.

Wednesday morning I got the blow-by-blow in a series of messages from Rob. The comet wasn't at the nominal position (no surprise), but it was off by quite a bit more than I would have expected -- nearly 3 degrees. Indeed -- it is a testament to Rob's skill and perseverance that he was able to spiral out and locate it before the comet set at his location."

The image on the left is a stack of four July 12th images taken with SSO's 0.5-meter Uppsala Schmidt by Rob McNaught at Siding Spring, New South Wales..
The image on the right is a stack of 26 15-second exposures taken by Sebastian Hoenig, J.-C. Pelle and N.Teamo at the Hibiscus Obs in Tahiti.
Robert H.McNaught,RSAA/ANU, 2006 S.Hoenig,J.C.Pelle,N.Teamo, 2006

It's looking likely that C/2006 M4 (SWAN) will cross the LASCO field of view in August. The question, however, is whether it will be bright enough to see... More on this next week!

June 7, 2006 -- Time to draw breath, at last!

So after a fairly slow start to the year for SOHO comets, things really picked up in May! The following comets have been confirmed since the last news item:
Soho#  Date/Time of Post  Discoverer(s) Tel  Group   Images of..
1138  May15,06 12:34:24   G.Sun         C2   Kreutz  May15,06
1139  May16,06 23:19:05   R.Matson    C3,C2  Kreutz  May16-17,06
1140  May18,06 04:16:36   L.Cane        C2   Kreutz  May18,06
1141  May18,06 17:40:54   B.Zhou        C2   Kreutz  May18-19,06
1142  May19,06 19:31:36   R.Matson      C2   Kreutz  May19,06
1143  May20,06 10:13:46   W.Xu          C2   Kreutz  May20,06
1144  May20,06 15:57:44   H.Su          C2   Kreutz  May12,98
1145  May22,06 13:52:08   H.Su        C3,C2  Kreutz  May22-23,06
1146  May22,06 14:24:31   H.Su          C2   Kreutz  May22,06
1147  May22,06 01:13:16   H.Su          C3   Non-grp May22,06
1148  May24,06 10:05:52   G.Sun         C2   Kreutz  May24,06
1149  May24,06 18:58:25   R.Matson      C2   Kreutz  May24,06
1150  May25,06 06:40:03   T.Chen        C2   Kreutz  May25,06
1151  May25,06 18:33:34   R.Matson      C2   Kreutz  May25-26,06
1152  May25,06 19:39:29   R.Matson      C2   Kreutz  May25-26,06
1153  May26,06 06:07:50   G.Sun         C2   Kreutz  May26,06
1154  May29,06 11:19:05   H.Su          C2   Kreutz  May29,06
1155  May29,06 12:31:31   T.Chen        C2   Kreutz  May29,06
1156  May29,06 12:55:28   V.Bezugly   C3,C2  Kreutz  May29-31,06
1157  May26,06 14:54:06   H.Su          C2   Non-grp Dec12,99
1158  Jun02,06 23:54:48   G.Sun         C2   Kreutz  Jun02,06
1159  Jun05,06 09:06:09   W.Xu          C2   Kreutz  Jun04,06
1160  Jun03,06 09:55:22   H.Su          C2   Kreutz  Nov14,99
This makes May 2006 the busiest month for SOHO comets in SOHO's history, with thirty comets (plus one archive comet) in 31 days! Amazing! All but two of these comets were members of the Kreutz comet group, with the remaining two belonging to no known group.
Congratulations are due to Luciano Cane for finding his first SOHO comet, Gouyou Sun for finding his first four SOHO comets, and Rob Matson and John Sachs for both making their 50th SOHO comet discoveries.
As I write, LASCO is preparing to open its doors after SOHO performed its quarterly roll maneuver. While we are currently experiencing quite large data gaps, these should decrease as we move out of the keyhole. It will be interesting to see if June -- which is traditionally as busy as May for comet discoveries -- can continue this record-breaking run...

May 12, 2006 -- And for my next trick...

Behold the psychic powers of Mr Sungrazer! Just days after mentioning a potential "comet storm" period, the comets started "storming"! It is really quite incredible that between April 11 and May 04, there were just 10 comets discoveries. Then, between May 8 and May 11, there were another 8 discoveries! Eight in three days... well done everyone! Here are those eight:
Soho#  Date/Time of Post  Discoverer(s) Tel  Group   Images of..
1130  May08,06 09:36:01   K.Battams    C3,C2 Kreutz  May07-09,06
1131  May08,06 11:14:51   H.Su           C2  Meyer   May07-08,06
1132  May08,06 17:34:58 R.Matson,J.Sachs C2  Kreutz  May08,2006
1133  May10,06 00:26:51   T.Hoffman      C2  Kreutz  May10,2006
1134  May10,06 00:27:29   T.Hoffman      C2  Kreutz  May10,2006
1135  May10,06 10:06:43   H.Su,T.Chen    C2  Kreutz  May10,2006
1136  May11,06 01:33:34   T.Hoffman      C2  Kreutz  May11,2006
1137  May11,06 14:31:43   J.Sachs        C2  Kreutz  May11,2006
Among the highlights were a nice bright Kreutz found by the elusive K. Battams, and a nice Meyer-group comet found by Hua Su. But perhaps the most interesting was a "double comet" found by Tony Hoffman. It... they... were quite faint, and only visible in C2, but were clearly two distinct objects. I have created an image (below) that gives a zoomed view of the objects. The image on the left is taken straight from the gif image of the comets. The image on the right is a contrast enhanced image, showing more clearly the two objects. (The blob in the lower part of the image is simply a piece of noise from a cosmic ray hit).

Double comet
Comets SOHO-1133 and SOHO-1134, found by Tony Hoffman

May 05, 2006 -- Long time no news...

The following comets have confirmed by Mr Sungrazer (after a long period without confirmations...)
Soho#  Date/Time of Post  Discoverer   Tel   Group   Images of..
XC54  Apr08,06 06:38:35   J.Ruan       C2   Kreutz   April08,06
1120  Apr11,06 02:44:10   T.Hoffman    C2   Kreutz   April11,06
1121  Apr14,06 09:52:17   J.Sachs      C2   Kreutz   April14,06
1122  Apr17,06 09:22:23   H.Su         C2   Kreutz   April17,06
1123  Apr19,06 12:11:02   K.Battams    C2   Kreutz   April19,06
1124  Apr21,06 08:43:34   A.Ambrus    C3,C2 Kreutz   April21-22,06
1125  Apr24,06 11:55:09   H.Su         C2   Kreutz   April24,06
1126  Apr28,06 10:24:47  H.Su,J.Sachs C3,C2 Kreutz   April28,06
1127  May03,06 02:20:05   H.Su         C2   Kreutz   May03,06
1128  May04,06 08:19:01   T.Hoffman    C2   Kreutz   May04,06
1129  May04,06 08:24:18   H.Su         C2   Kreutz   May04,06
Congratulations are due to Adam Ambrus for finding his first SOHO comet! It was a nice one, too, visible for quite some time through C3 before appearing in C2, sporting a short, thin tail.

As we move into the month of May, this is traditionally the "comet storm" season. However, this year has been slower than usual for SOHO comet discoveries, so it will be interesting to see what comes through...

Jan 31,2006 -- Pretty pictures for everybody...

A question I am frequently asked by comet hunters is how we make...
raw FITS files image
into this:
raw FITS files image
(yes, they are the exact same image! Click on them for a larger version.)

I'll start by explaining what each image is. The image on the left is a raw (LASCO C3) FITS file image (NOTE: I have converted it to gif so you can view it in a web browser). The image data is sent down from SOHO as part of a data "packet" and is saved directly into this FITS format with no processing -- this is a completely "raw" image. As you can see, it is not much use for comet hunting! In this example, it is possible to make out the very bright comet Machholz, a small number of bright stars, and the planet Venus -- but that's about it!
The image on the right is a processed image saved as a gif file. This is what most comet hunters use. Notice is has the familiar blue color, hundreds of stars, comet Machholz (including tail), Venus, and the fine structure of a beautiful CME (Coronal Mass Ejection)! We have also added the date/time and our logo.

How we do it...

So how did we get from the "ugly" grayscale image to the beautiful blue/white one? Well, there are several steps involved. One of the first we do is to read in a "background" image. The background image needs to be subtracted from the FITS file data to remove the effects of sunlight being scatted by dust (also known as the "F corona"). We also subtract a camera bias value from the image and rescale it to give a smoother appearance. We then apply a mask to the image to cover up the non-useful parts -- namely the occulter disk, pylon and corners of the image. Then we rotate the image so that solar north is "up", add the white circle to the occulter disk (it represents the size of the Sun), add the date and time in the lower-left corner, and our logo in the lower-right. Finally, we apply the blue/white "color table" (we can make the images any color we want, but several years ago blue for C3 and red for C2 were chosen to be the standard colors) and save the image in whatever format we choose (in this case, gif).

How you can do it...

Another question I am often asked by comet hunters is if it possible to make their own images using the raw FITS files? The answer is yes... sort of. We use a programming language (IDL ) which is expensive and usually unavailable to non-scientist. It is not possible to possible to prefectly recreate our images outside of the IDL environment. However, with some inexpensive (or free) software, it is possible to create very good comet-huntng images from the raw FITS files.
So first, you need to know where to get the data. Most FITS files can be downloaded from the LASCO database query form. Next, you need background images (also FITS files), which can be downloaded from here. It is important to get the correct background image. For C3 images, you are looking for FITS files that begin "3m_clcl_" or, in the rolled/ sub-directory (for when SOHO is rolled) files that begin "3mr_clcl_". For C2, the file names begin "2m_orcl_" and "2mr_orcl_" respectively. Next, you need to select the background image that has the date closest to that of the image(s) you are working with. This information is in the filename in the form: yymmdd. So, for example, the file 3mr_clcl_060121.fts is the background image for C3, rolled images, dated within ~14 days of 2006/01/24.
Once you have the background image, subtract it from the LASCO image and you should instantly see a big improvement in image quality. You can then play with the scaling yourself (it varies depending on the software you use) until you are happy with the way the images look. Note that it is recommended you leave the images as grayscale -- if you try and add color to them, you will probably lose detail.

LASCO image
To prove a point I have created the image, opposite. (click for larger version)
All I did to create this image was, using the FITS files, subtracted the background from the image data and rescaled. The files I used were 32082087.fts (C3 image from 2002/01/08) and the background file 3m_clcl_020114.fts. It was quick and easy and shows that you don't need elaborate calculations and calibrations. Just a simple subtraction and rescale, and you're set!

Older news stories: Oct-Dec 2005, 2008/2009