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Comet C/2006 P1 (McNaught)

The appearance of comet Mcnaught in the SECCHI HI-1 images in early 2007 remains one of the most memorable highlights of the STEREO/SECCHI mission thus far. Leading up to the event, we were not certain if we would be able to see the comet in our field of view, and there was great uncertainty regarding the brightness of the comet. But when the time came, so did McNaught... in spectacular fashion!

We placed the images and movies online as quickly as possible at the time, to share the fantastic sight with everyone, but it now seems appropriate that this event get its own page on the Sungrazer site. However, rather than rewrite the occasion, it is perhaps better to leave the words as they were written then, as they do such a great job of capturing all the excitement of the weeks and days leading up to its arrival, and our subsequent shock and delight at the amazing images we captured.

The following text was taken from the "Latest News" archive page from December 2006 through January 2007. The SECCHI/STEREO and LASCO/IMAGES images are under an "open data policy" meaning that they are considered free and unrestricted for use. However we do ask that image credits be given in publications, websites, etc. We recommend you contact us with specific questions.

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.

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 aging 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!

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 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 everyone 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 realized 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 bottom) and Mercury (appears from the right later in the sequence). There is an internal reflection 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!!