SUNSET: 
13Well, I've posted the article about 8P/Tuttle, so now I can discuss comets with a clean conscience.
Would you rather see web charts as color JPEGs or black-on-white PDFs?
Tony Flanders
Incidentally — while I'm on the subject — I decided to post the S&T charts in black-on-white format, as PDFs. The way I figure it, color charts are easier to read on the screen, and more attractive in the magazine. But when we post really detailed charts like these, most people are going to print them, so that they can carry them into the field. And on most home printers, charts with dark backgrounds are ugly, smeary, tend to jam the printer, and use lots of expensive ink. So I'm inclined to use traditional bright-on-dark for simple, at-a-glance charts, but black-on-white for detailed charts. What do you think?
But that's not what I set out to write about. Last weekend, I observed Comet Tuttle for the first time under reasonably dark skies. And naturally, I took a look at Comet Holmes too. What's odd is that I was more excited about faint, featureless Tuttle than dazzling Holmes. I'm beginning to take Holmes for granted.
Yes, Holmes is overwhelmingly big and bright, and shows amazing detail too. But it changes so little from one night to the next, either in position or appearance. It's almost as though the sky has acquired a new deep-sky object, a permanent fixture. As far as I can tell, it has dimmed not at all in the last 30 days. At this rate, it's going to remain a naked-eye spectacle for the better part of next year!
Comments
George Huftalen
December 12, 2007 at 12:09 pm
I don't know which comet Holmes you are posting about Tony! The once bright amazing Holmes has deteriorated to a dispersed cloud that is only barely visible naked eye to experienced observers and a difficult object in even an 8" SCT as the field of view is so small that the entire FOV IS Holmes and so transparent as to be flying through a cloud bank. No nucleus seems visible from SE New England. During a recent outreach--a week ago at U Mass Dartmouth Observatory--I had some difficulty showing an inexperienced observer even the "Holmes cloud" through my 10X50s. It was a good object lesson in "less is more" regarding magnification, however and allowed me to demonstrate the meaning of FOV to the group. Holmes changes day-to-day and one wonders if it will flare up again. However, (although one should never say never,LOL)I doubt that Holmes will be too much of an object through much of 2007, but Holmes continues to surprise...
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Haldun I. Menali
December 12, 2007 at 3:29 pm
Hi Tony,
First, I would love to have BW charts in PDF format. I agree with you that they are easier to use in the field, similar to S&T's Sky 2000 atlas I have. I'll look forward to them!
Second, you must have a better eyesight than I do (!), since I started not seeing Holmes with unaided eye for some time now (from Boston's suburbs). As it continues to expand and its brightness decreases a little every week or so, its surface brightness is decreasing more rapidly and thus I lost its sight! But it cannot escape from my scopes/binocs or camera :o)!
Lastly, as a quasi-veteran comet observer (Holmes is my 20th since 1985) I didn't have time to go to a dark site (such as ATMoB's clubhouse) to observe Tuttle yet. Maybe I'll join you there one of these nights!
Thanks for your interesting articles!
Clear skies,
Haldun.
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FER
December 12, 2007 at 3:58 pm
George Huftalen wrote:
"I don't know which comet Holmes you are posting about Tony! The once bright amazing Holmes has deteriorated to a dispersed cloud that is only barely visible naked eye"
Sounds like you have a light pollution problem. The amazing thing is that the comet's brightness is almost EXACTLY the same as it was a few days after the initial outburst. It's expanding at a steady rate so the brightness per square arcminute is decreasing. That means that if you look with the unaided eye, you can still see it very easily unless the background brightness of the sky at your observing site is brighter than the comet's brightness per unit area. So it's all a question of Moon interference and light pollution. From a dark site, the comet is merely larger but just as bright as it was a month ago. The time will come, perhaps in two or three months, when even normal skyglow at a dark site will defeat Comet Holmes.
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Marc Whitsett
December 13, 2007 at 6:31 am
Two nights ago (12-11-07) Holmes was not a visible eye target from Cincinnati, but was reasonably easy to see with 10 X 42 binocs.
I prefer the B and W charts for download, although aesthetically I like viewing the dark backgrounds on the web. Don't know if you could provide a "download link" for those that prefer to download in B & W (i know, more work..
::(()...
Thanks Tony. Love your column...a breath of fresh air.
Cheers.
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Terry Durbin
December 13, 2007 at 7:05 am
Black on white charts, yes, yes , yes. I've been socked in by the ice storm for more than a week and haven't seen a star, let alone Holmes, but at my last observing Holmes was still an easy naked eye object from SE Iowa.
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Tony Flanders
December 13, 2007 at 8:15 am
The disparate comments about Comet Holmes remind me of 1996, when Hale-Bopp and Hyakutake were in the sky simultaneously. People in cities, were amazed by Hale-Bopp shining night after night through the worst light pollution, but they were completely unimpressed by Hyakutake. But for people at dark locations, who could see Hyakutake's tail stretching all the way across the sky, Hyakutake was far more impressive than Hale-Bopp.
The comments about Comet Holmes are a sobering reminder that the great majority of amateur astronomers work under light pollution that's anywhere from strong to overwhelming. Trust me, seen in a reasonably dark sky, the comet is overwhelmingly bright. At the ATMOB observing field on the edge of Boston's suburbs, the comet is an obvious naked-eye sight to me, but not to some less experience people.
As for viewing the comet, an SCT, with its long focal ratio, is just about the worst possible instrument. The comet shows beautifully in the 2.5-degree field of my 7-inch Dob. But beginners definitely see it most easily through binoculars.
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Bob Groh
December 14, 2007 at 12:10 pm
Question - should star maps be black on white background. Short answer: YES. Long answer: YES. I do print them on my laser printer and take them out for observation. Much easier to see.
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m clark
December 14, 2007 at 6:22 pm
Since this time last month Holmes coma has expanded from about 1/2 degree to 1 degree or so. Assuming its integrated magnitude has not increased during this period (there is no evidence that it has), its angular size expansion alone has caused its surface brightness to decrease 4.4mag to a 5.5mag equivalent visual object (as a result of its contrast decreasing by about 60% during this time and based on a calculated equivalence with background sky brightness).
So, in this time period Holmes has gone from easy naked eye visibility to invisibility under 5.5 NELM skies. Even if it's integrated magnitude holds steady at 2.5, it will disappear from dark skies (6.5 NELM) as well once it reaches a diameter of ~140 minutes, as at this point its surface brightness will be less than the background sky brightness.
Holmes progressive disappearance from increasingly darker skies over time is directly related to its continued expansion combined with an essentially static integrated magnitude. If its rate of increase and integrated mag hold, its effective disappearance from view will occur in about 2 more weeks for 6.0 NELM skies and about the end of January under 6.5 NELM dark skies.
Holmes is a far cry from what anyone could reasonably call a "bright object" in any but the darkest skies.
(Above calculated using formulas published by R.N. Clark.)
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Björn Gimle
December 15, 2007 at 8:41 am
I agree that detailed charts are better in B&W/PDF.
Though it is very easy even with a freeware program like
IrfanView (excellent!) to convert to 'Negative' and 'Decrease Color Depth', doing that on a JPG decreases clarity/resolution.
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Claude McEldery
December 15, 2007 at 10:42 pm
I like M. Clark's explanation of the Comet's brightness posted Dec. 14th. From my location in Richmond, Michigan I can still see it naked eye, but it is very difficult. I agree with M. Clark's assessment that the comet will no longer be naked eye in a couple more weeks. I routinely can see 4th magnitude stars naked eye from my location. Not very good but better than some suburban locations.
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Tony Flanders
December 18, 2007 at 7:06 am
I went last night (2007-12-17) to my favorite inner-suburban observing site, in Arlington, MA. Thanks to fresh snow with a highly reflective rain glaze, the sky was magnitude 17.7 per square arcsecond as measured by my Sky Quality Meter -- just about 1/2 magnitude brighter than normal. I didn't know exactly where the comet was, but found it pretty quickly scanning around with my 15×70 binoculars. Having done that, I could also see it naked-eye, but that was not easy.
Here's my analysis of the comet's visibility. Current estimates put its magnitude close to 3.5 and its size close to 1 square degree. That works out to mag 21.2 per square arcsecond. As a rule of thumb, an object is pretty easy to see if it's two magnitudes fainter than the sky background, meaning that the comet should be easy to see under 19.2 mpss skies. So why could I see it under skies 4× brighter than that? Partly because I'm pretty good at this kind of thing. And partly because the comet's center has considerably higher surface brightness -- no doubt that's what I was seeing. I'm pretty sure I didn't see the full one-degree extent even through binoculars.
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Dave Roper
December 20, 2007 at 3:23 pm
Tony ... I appreciate your updates and help in following Comet holmes progress across the sky. Magnificent apparition! And, an enthusiastic YES on your suggestion to offer black-on-white PDF starmaps in the future. May I suggest offering both? I certainly prefer this format (B&W) for use under the dark skies. Thanks for the suggestion!
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Alison
January 3, 2008 at 8:28 pm
I hate PDF which for some reason often hangs up my computer, but assuming it does work, then yes, I would rather have black-on-white charts to print!
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