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SQM-L Sky brightness question

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#1 Seldom

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Posted 20 February 2013 - 01:06 AM

Is it possible to view objects whose mag/arc second is darker than the sky? Last night was just past quarter moon, and my SQM-L showed a zenith MPAS of 16.13. I looked for the following objects M33 (MPAS 23.1), M101 (MPAS 23.7), and M31 (MPAS 22.8). All Messier MPAS values are per Ronald Stoyan's Messier Catalog.

I didn't expect to see M33 or M101, and couldn't. I could see the core of M31, but nothing outside of the core, no dust lanes or extended galaxy.

Does this mean that the core is actually brighter than 16.13 MPAS, or am I missing something?

#2 Tony Flanders

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Posted 20 February 2013 - 05:20 AM

Is it possible to view objects whose mag/arc second is darker than the sky?


Absolutely! The skyglow is added to the brightness of the object you're viewing. So if you're viewing an object that's inherently 1/4 as bright as the skyglow, it will appear 25% brighter than the background, which is ample to detect it.

It's certainly possible to detect objects that are two magnitudes fainter than the background, and three magnitudes is conceivable.

Last night was just past quarter moon, and my SQM-L showed a zenith MPAS of 16.13.


Ouch! That's incredibly bright -- far brighter than the Moon alone can cause. Brighter than the sky over midtown Manhattan on a good night. Where were you viewing from? Are you sure you didn't clip the Moon while taking the reading?

I looked for the following objects M33 (MPAS 23.1), M101 (MPAS 23.7), and M31 (MPAS 22.8). All Messier MPAS values are per Ronald Stoyan's Messier Catalog.


These are averages over the entire huge objects as defined by CCD photometry. Much of the areas included in these measurements are rarely if ever visible to the eye even under ideal conditions. The parts that you actually view are much, much brighter.

M31's core is intensely bright; it has a tiny nucleus that's indistinguishable from a star.

#3 Seldom

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Posted 20 February 2013 - 10:19 AM

I must have clipped the moon. J. Tomshine's map puts me in a blue area, semi-rural, 5 miles outside of (but with a straight line view of) a small town. I've only had the SQM-L for about a week, and the best reading I've taken was 21.94 MPAS at 10:00PM on 2/14, with the crescent moon near the horizon. At 8:00PM the same night the reading was 20.1.

#4 GlennLeDrew

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Posted 20 February 2013 - 12:59 PM

As a guide to the visibility of DSOs, you may wish to look over a chart I created based on information contained in the RASC's Observer's Handbook. It will be found in my Cloudy Nights Gallery, the link to which is just below, in my signature. The image description contains a fairly detailed user guide.

As Tony pointed out, surface brightness values for an entire object are the average, which include the fainter (usually) outer portions. Most objects have at least some variation in brightness, some markedly so. As a class, galaxies almost uniformly have notably brighter centers, with peak central surface brightness of 17-20 MPSAS being fairly common. This makes most galactic cores visible in bad light pollution, sufficient aperture being applied as required, of course.

The single most important aspect to bear in mind when observing the fainter DSOs especially, is that as one or both of surface brightness and contrast decrease, the larger on the retina the object must subtend in order to be detected. In the worst cases, this angle exceeds 5 degrees (!), as compared to a mere 'star-like point' (e.g., the nucleus of M31) for those bright enough to easily stimulate the color-sensitive cones in the fovea.






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