Comet Holmes(17P)

Catching Light

Photography by

Dan Lessmann

Oklahoma City Clear Sky Clock

Cheddar Ranch Observatory Clear Sky Clock

Comet Holmes(17P) - 2007

On October 24, 2007 this comet dramatically brightened from estimated magnitudes of 17 to 2.5 making the comet very easily naked-eye visible even in light polluted skies. This page is to be a series of shoots documenting the changing appearance of this comet. Click the date links below to view each entry.

Entry 1 - October 27, 2007

The comet is very high in Perseus on this night about 3 degrees north of Delta Persei. It is very easily naked eye visible even under a full moon and light pollution from Oklahoma City. The coma is easily visible using 7x35 binoculars and is spectacular through a 10" Newtonian telescope.

Two series of images were shot on this night. Both series were shot from my driveway between midnight and 1:45am.

First Series:

The first series was shot using a Canon 20D DSLR with a Canon EF 200mm 2.8 L telephoto lens and a 1.4X tele-extender. This resulted in a focal length of 280mm and f/stop of f/4. 203, 8 second raw images were taken at ISO 200 and then calibrated, average combined and digitally developed using Images Plus. Additional enhancements were done using Photoshop CS2.

8 second exposures were done to avoid overexposing the very bright core of this comet in any one frame. I shot 203 frames to average a smooth background and coma and also to be able to attempt detection of any faint tail evident through other combining methods. No tail was evident on this night though the coma is slightly asymmetrical.

Image 1 - The full frame image at 280mm.

Click images for larger version.

Even at this relatively short focal length and exposure time the comet's coma and brightness is impressive.

Image 2 - A crop of the full frame.

This version is a crop of the full frame and is also enhanced just a bit more to better show the extent of the coma. Note the bright inner core surrounded by a bright inner coma which in turn is surrounded by a much dimmer and more extended coma.

Image 3 - Enhancements to better show the outer coma extent.

The background sky glow has been neutralized and the image has been stretched much further to better show the outer coma and incidentally many more background stars. This of course burns out the core detail of the comet. Visually using a 10" Newtonian the outer extended coma was not visible on this full moonlit night.

Second Series:

The second series of images on this night was an afocal series using a 10" Orion XT Newtonian with a 26mm eyepiece. This yields a magnification of 46X and circular field of view of about 1 degree. The camera crops this field further. The 20D was mounted to the telescope using an Orion afocal extension tube and standard t-ring adapter. 115, 2.5 second images were taken at ISO 800 for this series. The higher ISO and shorter exposures were necessary to properly expose the comet's core without introducing motion blur at the telescope's magnification. A group of 10 images was shot between repositions of the Dobsonian mounted scope. The images were calibrated, planetary aligned on the core and digitally developed using Images Plus. Additional enhancements were done using Photoshop CS2.

Image 1 - Full frame

Note the relative motion of the comet over the 10 minute duration of the series as evidenced by the star trailing present.

This image offers the opportunity for some better analysis of the core of the comet.

Image 2 - Indications of multiple cores or concentrated gas clouds.

This image has been processed to better show the core detail of the comet. Note the obvious multiple brighter areas. I at first felt the upper bright region might be a motion blurred background star but the angle is incorrect as shown by the full frame view above. Either the comet's core has broken up or the larger lower bright lobe is an expanding eruption from the main core.

Image 3 - Possibly another and more distant fragment of the core?

This image shows yet another possible fragment of the comet well outside of the brighter coma region. Again the angle is wrong for this to be a motion blurred background star. I also considered the possibility of an in camera aberration but this is consistent relative to the comet center from frame to frame so it's not in the camera. Since the bright region is consistent from frame to frame over the series, this eliminates the possibility of a satellite pass or other more local object.

Entry 2 - October 28, 2007

The shoot this evening was 24 hours after Entry 1 an afocal series using the 10" Newtonian with identical exposure parameters as in Entry 1 for comparative purposes; ISO 800, 165 frames @ 2.5 seconds. Unfortunately my focus was a bit soft on this series.

Image 1 - Full frame

Image 2 - 24 hour coma size comparison.

The comet's coma expanded considerably over this 24 hour period. I estimate an overall increase in diameter of about 40%. The relative brightness of the core is abut the same. This analysis was done by equalizing the backgrounds of both images from the two nights to create a similar exposure on the core. However some error is probably introduced by the different position and slightly dimmer moon on this night. This would have made the overall sky glow less bright.

Image 3 - Core detail

The comet's core retains a definite lobed structure and appears to be little changed over the 24 hour period.

Image 4 - The mystery fragment

The possible fragment evident in Entry 1 is still visible but is no longer in such close proximity to the comet's main body. It has also taken on the appearance of possible multiple out gassing bodies. I at first felt this was an aberration introduced in the alignment of the multiple frames on the comet's core but if so this should also be evident in the background stars. It is not. The "fragment" has also moved relative to the background stars so this is definitely not a background deep sky object.

Entry 3 - November 10, 2007

Over that time Holmes has grown and dimmed and now requires longer and tracked exposures to be able to image it well. This shoot was done using an Orion 80ED at 600mm with a modified Hutech 350D and IDAS LPS filter on a Losmandy GM8. 43, 90 second images were taken for a total exposure time of 64.5 minutes.

Image 1 - Full Frame

There are certainly indications of a dim tail here but I was not able to pull out additional detail from the light pollution from home and could not capture more integrations due to a late fog.

Image 2 - The Core

Now that the coma has dissipated and spread it's much easier to see the comet's core as a distinct and small object amid the coma as seen in the inset.

Entry 4 - December 4, 2007

Comet Holmes has continued to expand and dim.

Image 1 - Full Frame

This shoot was done using a Canon 200mm f/2.8 L II telephoto lens with a Hutech 350D mounted on a Losmandy GM8. 100, 45 second exposures were made at f/2.8 for a total exposure of 75 minutes.

The inset in this image shows a scaled representation of the comet on November 10th for size comparison. The comet's coma has more than doubled in size visually while at the same time moving away from us from our vantage point. The density and brightness of the coma has decreased dramatically. Holmes is no longer visible through binoculars and is difficult to see from urban skies even with a 10" Newtonian. Only the brighter core region is evident. Though the comet appears to be of similar brightness in these images and the overall exposure times are similar, note the density and brightness of the background stars in the main image as compared to the inset. This main image was shot at f/2.8 versus the inset image at f/7.5. This makes Holmes less than 1/4th as bright overall as it was near the beginning of November.

Image 2 - The Core

As the coma continues to dissipate the comet's core becomes more and more visible as a distinct point within the coma. The tail is still evident sweeping off to the left and away from us as Holmes journeys outbound from the sun.