24 August 2023 Observations

 

The Moon was up, at about 65% of full and 30-40 degrees above the horizon. Had just a few minutes to look at it before it went behind trees to the SW, so I didn't bother to align the scope, just slewed to the Moon and followed it with the hand controller.

 

Saturn was well up in the sky. so I did a 2 star alignment and then spent over 30 minutes observing. Started with the 25mm eyepiece and then was able to use the 13mm. I had several moments of steady seeing and was able to  make out separation of rings from planet, and a faint color band on the planet.See the sketch from 14 August.

 

The sky was fairly light from the moon. Light glow from town seemed brighter than usual, I think humidity in the air was magnifying it. I had checked out the Starry Sky kit from the library. Just after midnight I tried the SQM (Sky Quality Meter) and got readings of 20.81, 20.96, and 20.82. This is equivalent to a Bortle 4 sky. The Sqm measures "magnitude per square arcsecond". See then ote below for a description of what that means. I need to actually wait for the new moon to try it again, since the moon was still having some effect. I was up about 04:00 because of the dogs barking and the sky was much darker with most of the Milky Way visible.

 

After using the SQM, I decided to try for the Swan Nebula, M17. I used TLAO to locate it. Despite being low in the southern sky, it was clearly visible. I spent quite a bit of time observing and making the sketches.

 

 

Note on SQM units, from the Unihedron web site.

What are "Magnitudes per Square Arc Second"?

Magnitudes are a measurement of an objects brightness, for example a star that is 6th magnitude is brighter than a star that is 11th magnitude.

The term arcsecond comes from an arc being divided up into seconds. There are 360 degrees in an circle, and each degree is divided into 60 minutes, and each minute is divided into 60 seconds. A square arc second has an angular area of one second by one second.

The term magnitudes per square arc second means that the brightness in magnitudes is spread out over an square arcsecond of the sky. If the SQM provides a reading of 20.00, that would be like saying that a light of a 20th magnitude star brightness was spread over one square arcsecond of the sky.

Quite often astronomers will refer to a sky being a "6th magnitude sky", in that case you can see 6th magnitude stars and nothing dimmer like 11th magnitude stars. The term "6th magnitude skies" is very subjective to a persons ability to see in the night, for example I might say "6th magnitude skies" but a young child with better night vision might say "7th magnitude skies". You can use this nifty calculator created by SQM user K. Fisher to do that conversion, or this chart.

The "magnitudes per square arcsecond" numbers are commonly used in astronomy to measure sky brightness, below is a link to such a comparison. See the third table in section 10 for a good chart showing how these numbers in magnitudes per square arcsecond relate to natural situations:

  www.stjarnhimlen.se/comp/radfaq.html

Each magnitude lower (numerically) means just over 2.5 times as much more light is coming from a given patch of sky. A change of 5 mags/sq arcsec means the sky is 100x brighter.

Also, a reading of greater than 22.0 is unlikely to be recorded and the darkest we've personally experienced is 21.80.