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Fun with doubles and Questions on Light Pollution

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#1 BigMatt

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Posted 03 December 2012 - 10:30 PM

Hi guys I'm new to this, but I'm quickly becoming fascinated with double stars. I'm having a lot of fun working on a few I've found here in the forums, or just pointing my Nexstar 8se at a random spot and finding the closest doubles it can find with the identify feature. (By the way, I find identify to be a really cool feature for learning the sky even with a goto, just point and find out some nearby objects to learn more about and see.) Soon I think I'll settle down to a somewhat more methodical approach, but I haven't even had my scope a week yet. I was tantalizingly close to seeing A and B in Zeta Cancri last night, but every time it started to look notched, it would blur up again, I think looking over my house probably hurt the seeing.

Two quick, probably answered, but not that I could find questions. First, I'd guess light pollution doesn't hurt double splitting a bunch as long as the components aren't particularly faint, is that accurate?

Second, when in focus, I never see any rings around stars, just the central visual disk of the airy pattern, but most of the pictures of doubles appear to have it. I had a couple of ideas, namely that either my light pollution is washing that ring out (I'm solidly in the white area of the D/FW metroplex on the maps) or that the rings show up more clearly in pictures with their increased light gathering. I'm hoping it's one of those things and not something I'm doing wrong.

Matt

PS This is fantastically helpful for understanding what everyone's talking about, http://www.cloudynig.../resolution.pdf thanks Ed.

#2 Cotts

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Posted 03 December 2012 - 10:41 PM

Matt, bad light pollution is unrelated to resolution. You are correct that the light pollution could drown out faint diffraction rings, leaving the central spurious disc of thr diffraction pattern.

Dave

#3 Asbytec

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Posted 04 December 2012 - 12:01 AM

Dave, sure, light pollution could raise the sky brightness to that of the rings thereby drowning them out. Makes sense.

Seems everyone agrees light pollution has no affect on resolution. And by that, I think they mean contrast between two resolved features. But, resolution requires contrast, as you might agree. I am not entirely convinced light pollution does not dampen contrast, thus dampen resolution.

Light pollution is a type of noise imposed over a signal, say from Jupiter. This noise can be strong enough to dampen some of Jupiter's most delicate contrast features. Now, I suspect this to be the case both in theory and, I tend to believe, in practice. The effect may be minimal, but I do seem to observe slightly degraded views of Jupiter (and Saturn) when the full moon is nearby.

One might imagine enough sky glow would do the same. And maybe more so on extended objects with bright, low contrast detail than the higher contrast between sufficiently separated point sources. But enough sky brightness, say like the daytime sky, could even dampen contrast between point sources. That's why we cannot see them, normally.

Anyway, that's just an opinion with no hard evidence to back it, and it's certainly debatable.

#4 Cotts

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Posted 04 December 2012 - 07:29 AM

Norme, the OP was asking in the context of double stars. Light pollution will affect the ability to see faint companions and the diffraction rings but not the resolution of pairs bright enough to be seen through the muck.

I totally agree about planets, though. At the Okie-Tex SP I observed Jupiter under the worst light pollution possible - daylight - and found the disc to be wan, washed out and very bland. Bad nighttime LP must have the same but lesser affect. And, while I'm on about it, just check out the difference in the moon's features naked eye between night and daylight views. The contrast is noticeably affected......

Dave

#5 Asbytec

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Posted 04 December 2012 - 08:29 AM

Thank you, Dave, your input is greatly appreciated. I apologize for getting off topic. When it's an interesting subject, there's usually more to talk about. And before we realize it, well... :)

#6 blb

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Posted 04 December 2012 - 10:18 AM

I too live in a white zone and the light pollution will limit the faintest star you can see. For example, at home with my 4-inch refractor I can only see down to about 11th magnitude, but from a dark sky site I can see a little beyond 14th magnitude with the same scope. The diffraction ring pattern is another thing though. I have only seen them at very high magnification and never at low to medium powers.

#7 Ed Wiley

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Posted 05 December 2012 - 02:02 AM

Good that you are on the doubles! Perfect for the urban environment. I observe from my backyard observatory and spend most of my time imaging doubles. The major limitation if have found is seeing, I rarely have a 4/5 night,

For an observing program, you might try the Astronomy League double star list. Another good one for the urban environment is the Hass book, by constellations, Yes, there are some faint ones, but also plenty of bright pairs to enjoy.

Relative to you questions, I think Dave answered them. And yes, Jupiter was washed out, I saw it myself through Dave's scope. On the other hand, it was my first look at Jupiter if Daylight. Thanks Dave! :jump:

#8 BigMatt

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Posted 05 December 2012 - 03:09 AM

Yeah I've printed out their list, I'll probably start going through it soon. My only problem with the list is I've really been enjoying close doubles, and their list isn't really geared towards that. Outside of maybe 15" or 30" depending on brightness and they lose the perception as one system for me.

There's a certain patience required (something I often lack) and it's very rewarding to have a pair that when first viewed you see one star or perhaps a hint of two, but then once you've seen it for a while, even after looking away you can't see just one star anymore.

#9 Ed Wiley

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Posted 05 December 2012 - 08:32 PM

Yep, you are correct. Patience is key to splitting close doubles, you have to wait for those small periods of good seeing that you get even when seeing is only fair. I agree about the AL list, lots of wide pairs, but many of them are rather pretty. Check out the Hass book, it has some close ones. And then there is always the WDS catalog and "roll your own."

Clear skies,
Ed

#10 TonyDralle

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Posted 09 December 2012 - 09:35 AM

Hi, Matt,

I observe from suburban Pittsburgh, also with a Nexstar 8SE. I quickly began to focus (so to speak!) on double stars after attending a star party under really dark skies and realizing my backyard is not a very good site for deep sky observing.

Unfortunately, the built-in list of double stars in the Nexstar database is rather meager -- only about 50 or 60. So I make extensive use of the Eagle Creek Observatory double star database, nicely arranged by constellation, and available in the "LINKS to double star resources" thread at the top of this forum. Here it is

Eagle Creek Observatory Double Stars

One nice feature of this database is the inclusion of the SAO number for nearly every double star. These make finding the stars with our Nexstar hand controllers very easy.

- Tony






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