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M31 from southern latitudes

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#1 alexvh

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Posted 06 November 2013 - 04:18 PM

Hi guys
I was lucky enough to observe from very dark skies recently. From South Africa at this time of year M31 reaches a maximum height of about 40 degrees ( I think... Maybe someone can verify this) I was looking through a 12 inch dob, and as I said, there were very dark skies and good seeing. Despite the good conditions, M31 was a grey blob with no structure at all. Was this because it was low on the horizon? Or do I need more aperture?

#2 IVM

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Posted 06 November 2013 - 10:58 PM

The structure in M31 is comparatively subtle. Having a good photograph at the telescope to compare with the view helps. I begin to observe M31 when it reaches 40 deg. altitude. Although it does get better when it is straight overhead (as it gets here), the difference from 40 degrees is not very dramatic. You should be able to see the two main dust lanes at least without difficulty, but compare your view with a photograph to orient yourself. Beyond that there are at least a dozen specific astrophysically defined features in M31 that are within reach of an 8" under dark skies, but to find them a good photographic chart is needed. I use the Hodge Atlas of the Andromeda Galaxy. It is long out of print but you may find a digitized version somewhere on the NED (NASA Extragalactic Database) website.

#3 Tony Flanders

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Posted 07 November 2013 - 06:12 AM

From South Africa at this time of year M31 reaches a maximum height of about 40 degrees ( I think... Maybe someone can verify this).


Not even close!

Joburg is at latitude 26S, so the farthest north object you can ever see would be at declination 90-26 = 64. M31 is at declination 41, so it's 23 degree up at its highest. That's really very low for viewing galaxies.

I was looking through a 12 inch dob, and as I said, there were very dark skies and good seeing. Despite the good conditions, M31 was a grey blob with no structure at all. Was this because it was low on the horizon? Or do I need more aperture?


I'd say that it's not hopeless. But M31 is a very tough galaxy to observe even when it's directly overhead. It's tilted an extremely awkward angle to our line of sight.

I doubt that more aperture would help; 12 inches is ample to see M31's two main dust belts. Do you see how M31 ends much more abruptly on the M110 side of the core than on the M32 side? That abrupt edge is the inner edge of the inner dust lane. Scan out from the core toward M110 to see if you can pick up the outer edge. Then, once you've seen the dust lane, start to trace it forward and back.

#4 aatt

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Posted 07 November 2013 - 03:35 PM

I have been making a regular go at this one and the details are coming with more experience. They are surprisingly tough to see with this galaxy being so "close" to us. I can get more dust lane structure in M51 or M64 on a good night than this one. The minor dust swirls are starting to pop near the core for me this fall. In my experience 40 degrees has been adequate for studying it.

Here are a couple of resources that might be helpful.

http://www.cloudynig...hp?item_id=1559

http://ned.ipac.calt...las/frames.html

#5 alexvh

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Posted 10 November 2013 - 12:31 AM

Thanks guys! I'm feeling better now! If it's max height is 21 degrees, maybe that's why it's structure did not pop out! I will try m51 ...

#6 WeltevredenKaroo

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Posted 10 November 2013 - 01:48 PM

Where are you observing from, Alex? If within 100 km of Jo'burg, Durban, or CT you may have to drive a bit further. Sodium II glow from urban street & township lights cause secondary emission from the mesosphere 80-100 km altitude that is roughly 40% of the brightness of typical light domes one sees if 20 km or so distant from direct view. I observed M31 for roughly 15 mins two nights ago from a very dark site 13 km w. of Nieu Bethesda, which is 68 km from the nearest sky glow at Graaf-Reinet. Local limiting mag was about 7.3 to 7.5. At 20° elevation M32 and NGC 205 stood out sharply from a very busy star field. The NGC 206 star cloud glowed continuously though faintly using averted vision. It was only moderately less luminous than when observing it from 49° N last year in France under mag 6 skies. I checked in on M33 and easily saw distinct spiral structure and NGC 604 plus four other H2 and OB associations. These are not do-able from suburban Graaf-Reinet or Grahamstown with mag 5.5 skies & township lights.

#7 HellsKitchen

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Posted 14 November 2013 - 08:41 AM

M51 will be even lower as it is about 6° further north than M31. So it will be only about 15° above the horizon from your location.

#8 alexvh

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Posted 17 November 2013 - 12:49 AM

I was observing from spionkop dam. What scope were you using ??

#9 WeltevredenKaroo

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Posted 17 November 2013 - 02:32 AM

Is the Spionkop Dam the big one across from the Spionkop Lodge at Ladysmith? Looks nice flat veldt there. How is the nearby habitation light intrusion? What a luck that you have KZN to observe in, open and warm. My Karoo site has no human-originated lights for 50 km but drops to near-freezing even in November. A lot more stars than mozzies, which compensates somewhat. I use a 200 mm Mak and 4" achro on a T-arm alt-az mount.

#10 alexvh

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Posted 17 November 2013 - 12:13 PM

Yes that's the one. There was virtually no "glow" on the horizon... That's why I was so surprised that M31 was just a grey blob! I was using a 12"...

#11 WeltevredenKaroo

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Posted 18 November 2013 - 03:01 PM

Hi again, Alex . . . I'm at a loss to understand why M31 & companions seemed so blah. But, as suggestd by a couple of other posts here, at the eyepiece M31 appears less riven with lanes and patches than as seen the home-brew astrophotos posted here on CN. M31's sheer size and luminosity fills an eyepiece field in ways other galaxies do not. M31 is best in my 15x70mm binocs, certainly more majestic looking. You might give nearby M33 a try. It is fainter but also just fills a field at 60-75x. I've always felt it more textured than M31. The spiral structure really does come across well, what with it being nearly face-on. I find that if I stare at an object steadily for 5 to 10 mins a great deal more detail comes out.

#12 alexvh

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Posted 19 November 2013 - 12:55 PM

I wonder if in my stupidity I was looking at the companion.... hmmm

#13 Tony Flanders

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Posted 19 November 2013 - 01:00 PM

I wonder if in my stupidity I was looking at the companion.... hmmm


No, I'd say that's completely out of the question. M31 is so much brighter than M32 or M110 that you couldn't possibly miss it.

Again, M31 is super-bright -- but also very hard to see as anything other than a blob.

The obvious great spiral for a southern observer to start with is M83. That reveals its spiral structure far more readily than M31.

#14 WeltevredenKaroo

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Posted 19 November 2013 - 02:19 PM

There is also NGC 253 in Cetus. Size-wise it is more manageable, which gives a more satisfying 'galaxy' feeling. We see it as a cigar not a spiral due to its being so edge-on to us. It is great galaxy to hone one's skills distinguishing the subtle mottled textures that give galaxies their variety. The mottles are bright star forming regions. The dark patches are gas and dust that are often in the midst of collapsing into star-forming regions far in the future. NGC 253 glitters with field stars from our own galaxy that happen to lie in front of it. These provide a fireworks-in-slow-motion effect that enchants one for hours. Nearby NGC 55 is another wonder, the 'hole-in-the-wall' galaxy because the more elongated end has an easily visible dark patch. It is quite literally a hole, caused when another now-unseen galaxy sailed right through millions of years ago and took a lot of NGC 55's stars with it. (The Magellannic Clouds did this to the Milky Way a long time ago.) In your scopes both NGC 253 and 55 will fully fill the field at 50x to 75x, and even stretch outside it. The longer we look, the more we see. You can look most any object up on WikiSky by its common name, NGC 253 or M83 etc, and find a digestible and easily understood wealth of fact. I find that wealth just as important as the wealth in the eyepiece. Happy hunting!

#15 Tony Flanders

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Posted 20 November 2013 - 06:15 AM

There is also NGC 253 in Cetus. Size-wise it is more manageable, which gives a more satisfying 'galaxy' feeling. We see it as a cigar not a spiral due to its being so edge-on to us.


Right. The tilt and appearance of NGC 253 are actually quite similar to M31 -- though obviously on a much smaller scale.

#16 madbadgalaxyman

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Posted 23 November 2013 - 06:21 PM

M31 is a low surface brightness galaxy.

It is extremely extremely extended, so a telescopic view many only show part of it.

In some circumstances, the contrast of a very distended object like this one can be increased by adopting very low magnification.
(because to go any higher will reduce the surface brightness to undetectable levels

In addition, the use of a binocular telescope will increase the contrast of the object against the sky.

You may well see M31 better in large binocs (80-125 mm) than you do in your telescope.

The detail within this galaxy is extremely subtle and requires repeated observations or long-duration observation;

- the star formation rate of this galaxy is low, especially for a galaxy that is larger than the Milky Way, so there are few really luminous supergiant stars to make the spiral arms really stand out.
- M31 is poor in gas and dust, so there are no heavy dust lanes which help in defining the spiral arms.

#17 alexvh

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Posted 24 November 2013 - 05:43 AM

And it being at 18-20 degrees on the horizon makes it harder right?

#18 madbadgalaxyman

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Posted 24 November 2013 - 04:54 PM

Absolutely. At low elevations you are looking through so much more atmosphere than when you are observing near the zenith.

#19 Starman1

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Posted 27 November 2013 - 02:09 PM

Absolutely. At low elevations you are looking through so much more atmosphere than when you are observing near the zenith.

Right.
To wit: at 30 degrees altitude, you're looking through 2 atmospheres (the zenith being 1).
The Extinction Factor (the number of magnitudes of dimming from the thicker atmosphere) can vary a lot:
--on the high mountain sites without light pollution, it can be as little as 0.15 magnitude per atmosphere.
--on low altitude sites, even without light pollution, it can vary from 0.25 magnitude to up to 0.5 magnitude (really thick air, there).
Since the atmospheres at the horizon are from 10 to 12, you can see the thickness of the air increases dramatically below 30 degrees and Extinction goes up radically.

This is a good reason to avoid looking at deep sky objects below 30 degrees if you can. Obviously, we have to make an exception for those objects that just cruise over the southern (or northern) horizons. Even then, I put stops on my scope to disable my ability to view below 10 degrees (to avoid the temptation). For me, the horizon is dimmed by well over 5 magnitudes due to light pollution anyway, so "dirt diving" (what we call looking low) isn't really worth it.

#20 alexvh

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Posted 29 November 2013 - 08:01 AM

Thanks guys...






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