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Observing Wirtanen

Posted: Sun Dec 09, 2018 2:04 pm
by Apollo XX
Here's a very good article about observing the current bright comet in the sky - 46p/Wirtanen. I say very good because it's written realistically, without the usual overhype about brightness and visibility that articles about comets usually possess. This one says it like it is and offers all the information one needs to see, track and understand the comet without generating unrealistic expectations.

https://earthsky.org/space/46p-wirtanen ... -394841273

Re: Observing Wirtanen

Posted: Tue Dec 11, 2018 9:46 am
by Paul D
As you know I have been hunting comets for many years now and I have learned that you can never go by predictions of comet brightness Most predictions don't even come close to what they say. The thing with hunting comets is patience. If you don't find it one night try it on another night. A big but is that you go to a website or a program and look and see the comet is mag 9. You say to yourself, "Wow I look at mag 9 objects in my yard all the time. This is an easy find." Well there is nothing further from the truth. I really don't know how these comets are rated as far as magnitude but it is unrealistic. If your looking for a mag 9.0 comet its like looking for a mag 13.0 comet. Mag 6.0 comet will probably be in the 9.0 range

I say this not to discouarge people but to have expectations of what you should be looking for. Hunting comets is fun and each one can be very different if you closely examine them. If by chance you dont catch one comet always be prepared for the next one. You may get a surprise like comet Holmes that brightened to naked eye status for months and was one of the great comets that I have observed.

Re: Observing Wirtanen

Posted: Tue Dec 11, 2018 4:01 pm
by Pete
Believe that comet magnitudes are figured same as galaxies. Magnitude is the brightness you'd get if all of the photons in the coma were stuck back in a star-like pinpoint.

Re: Observing Wirtanen

Posted: Fri Dec 14, 2018 11:12 pm
by NGC7000
Hey Mike, Thanks for posting the article. I got a lot out of it.


Tom

Re: Observing Wirtanen

Posted: Sat Dec 15, 2018 10:29 am
by Apollo XX
Pete wrote:
Tue Dec 11, 2018 4:01 pm
Believe that comet magnitudes are figured same as galaxies. Magnitude is the brightness you'd get if all of the photons in the coma were stuck back in a star-like pinpoint.
It's interesting in that there is very little information out there on exactly how the visual magnitudes of diffuse objects are figured, but your explanation is correct. In this day and age of instant access to information, I was expecting it to be easier to find an answer than it was. The best one I found put it like this - Let's take the galaxy M33 as an example. Listed at a visual magnitude of 5.72, M33 is notoriously difficult to see in the eyepiece under even moderately light-polluted skies, whereas a magnitude 5.72 star is no problem. To gain a better understanding of how the number 5.72 was determined for the much harder to see galaxy, find a mag 5.72 star and defocus it to the same size as the galaxy, that is spread out to about 1* in diameter. Reading that brought immediate clarity to my mind about how the visual magnitude of diffuse objects is stated. Anyone who has hunted objects in a daytime sky understands how critical it is to have your optics at least near to tight focus, because if they're not the light of the object will be spread out and you won't see it - plain and simple.

That's all well and good, but it doesn't really help when it comes to comet-hype, unrealistic expectations, and letdown at the eyepiece. And there's no easy answer. The problem is light pollution and the infinite degrees of variation of it. Light pollution has a dramatic effect on diffuse objects, and light pollution levels vary so much that there is no good way to state the magnitude of a diffuse object in a way that everyone who wants to observe it can know what to expect.

We often hear of the term 'surface brightness' and it's typically a good indicator of how challenging an object will be to observe visually. We use it all the time when looking at galaxies, and again, M33 presents a good example. Possessing a surface brightness rating of magnitude 14.2, an experienced observer can surmise upon seeing that figure that the galaxy will be challenging to observe under suburban skies. But here's the kicker - under dark skies that very same galaxy can be seen naked eye. So in one way the galaxy presents itself as visible without optical aid, but in other ways it's nearly invisible with even sizeable optics? It's true. In my 4.5" reflector under the skies of Springfield, VT, M33 shows spiral structure, whereas with that same scope from my driveway in Bridgewater, MA, I'd be hard pressed to see anything at all. The difference is that dramatic.

We can't very well go around saying 'well if you live here it will be mag6, but here it will be mag10, and there it will be mag12 - it's just too complicated to figure for every variable. With galaxies it's typically not a problem. They're fixed objects that don't vary and quite frankly don't garner much attention from the public at large. Visual magnitudes and surface brightness are fine for those of us that pursue seeing them.

Comets though, are a problem. On one hand it's now clear how the brightness has been determined and on the other we can see how difficult it is to factor in the sky variables when publicizing them. It's different for everyone. 46p/Wirtanen may have achieved naked-eye visibility from the darkest parts of the world, but Pete wrote this after viewing the comet through his 14" telescope from Barrington, RI:
Comet brightness and visibility is almost always exaggerated. While Wirtanen may brighten significantly Spaceweather.com is as bad as Popular Science when it comes to promises. We could see no tail and the coma was dim.


So I guess we're just stuck with it. The real problem isn't the system - apparently the magnitudes are being factored consistently - it's the light pollution problem. Because of that, those of us that like to view and share these objects need to understand our local conditions, reconcile them in our own heads, and be prepared to explain the situation when a guest observer turns from the eyepiece and says, 'wow, that's dimmer than I thought it would be'. Difficult words to hear, for sure, but then perhaps that can be turned around when we respond with 'yes it is, now maybe you could go home and turn off some lights, eh?

Re: Observing Wirtanen

Posted: Wed Dec 19, 2018 7:01 am
by bluemax
Mike, thanks for this fine contribution to the club. The de-focussing of the star to emulate M33 was enlightening. This light pollution thing is a real bug-a-boo for us in the northeast. There really isn't much we can do about it. I think Pete made some valiant efforts in this regard with communities in the past (the new Providence bridge?) but our hobby is so small that I guess the public/local government are not interested in it. We have to suck it up or move like Jack or Tom Rinaldi, who moved to Arizona! This whole issue is why I shifted a while back to imaging. But, I am always humbled by the club's observers and dob-pushers (I mean that in a good way :wink: ) and how they still seek the stars in spite of the obstacles.

Re: Observing Wirtanen

Posted: Wed Dec 19, 2018 10:19 am
by Pete
Sixteen or seventeen years ago Steve the AstroGeek had me by for a night of observing, and we did it remotely through a Adirondack Video camera (an early version of the Mallin Cam). Blew me away with how clearly we could see the star link in M51.

After 10 years of effort, light pollution here in town has been significantly reduced. We've gone from cobra head sodium luminaries to full cutoff sodium luminaries to full cutoff LED streetlamps. And those around my property have been specially shielded so that the yard is DARK.

We are still in a larger light dome, and as my aging eyes grew dimmer viewing on a monitor has been the solution. Have reverted to visual for the fall class and the last 2 months due to computer replacement problems, and after so many years of observing faint fuzzies just don't cut it anymore. Cheers for electronic enhanced viewing.

Pete

p.s. Made several brief excursions out last night but was unsuccessful in catching Wirtanen in the 15X70s. Maybe being 30° from the moon had something to do with it.

Re: Observing Wirtanen

Posted: Fri Dec 28, 2018 3:19 pm
by Apollo XX
So here we are nearing the end of 2018 and I've yet to see a good map produced of 46P's path past the 3rd of January. This comet is hanging in there brightness-wise and it's not unreasonable to believe it will remain a good target throughout the next couple of weeks before the moon reappears on the scene and snuffs it out. I did find an ephemeris up through the January full moon (when of course we can all shift gears and become eclipse lunatics) and the comet is moving further away from the ecliptic, so there's that. I've drawn an arc of the comet's path on a star chart for rough reference and attached the ephemeris for more precise locating on a day to day basis. Happy Hunting!

46P PathStroked.png
46P PathStroked.png (439.66 KiB) Viewed 2351 times
Ephemeris for 46P Through Jan22'19.pdf
(180.71 KiB) Downloaded 81 times

Re: Observing Wirtanen

Posted: Fri Dec 28, 2018 3:36 pm
by NGC7000
Beautiful job, Mike. I've been popping out the last few nights, but have been met with either cloudy, hazy or overcast skies.

So it is even more satisfying to see the projected path come alive on your very colorful and well illustrated chart.

Thank you! :D

Tom

Re: Observing Wirtanen

Posted: Sun Dec 30, 2018 12:39 pm
by Apollo XX
Got out last night to see Wirtanen again. It's just crossed into Lynx and is holding it's brightness well. It's also better in binos than in a scope, which seems to have been true all along with this one. It's a good sized fuzzy patch in 10x50 binos once your eyes are adapted (which is critical, BTW - it was a lot tougher to see it before I adapted), but in the scope (last night I used my SV80ED @19x & 30x with the latter being better) the fuzziness is reduced. Tonight looks like another good night to catch it, at least before 19:00 when my clear sky predictors show the transparency going away.