M31 from southern latitudes
Posted 06 November 2013 - 04:18 PM
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?
Posted 06 November 2013 - 10:58 PM
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.
Posted 07 November 2013 - 03:35 PM
Here are a couple of resources that might be helpful.
Posted 10 November 2013 - 12:31 AM
Posted 10 November 2013 - 01:48 PM
Posted 14 November 2013 - 08:41 AM
Posted 17 November 2013 - 12:49 AM
Posted 17 November 2013 - 02:32 AM
Posted 17 November 2013 - 12:13 PM
Posted 18 November 2013 - 03:01 PM
Posted 19 November 2013 - 12:55 PM
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.
Posted 19 November 2013 - 02:19 PM
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.
Posted 23 November 2013 - 06:21 PM
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.
Posted 24 November 2013 - 05:43 AM
Posted 24 November 2013 - 04:54 PM
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.
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.