Tuesday 10 April 2012

Naked eye test - M44

I did a little test on the visibility of Messier 44 in Cancer without optical aid. As it is impossible to make identical observations of the same object on two different nights, the results are directional at best. More data will be added later on but it would seem that in order to see the cluster with the naked eye, the limiting magnitude in the region should be close to 5.0. This is slightly less than my previous estimate of 5.2 (Deep Sky Naked Eye Limiting Magnitude 2005 / Saloranta). Next in line would obviously be the visibility of M35 (unfortunately "out of season") and M13.

Based on personal experience, in order to see M44 with the naked eye from suburban location:
  • Delta Cnc (mag 3.9) needs to be visible with direct vision.
  • Gamma Cnc (mag 4.7) should be visible at least 50% of the time with averted vision.
  • Theta (5.4) and Eta Cnc (mag 5.3) will probably be invisible.

M44
Castor, Pollux, Mars and M44 barely visible in the pinkish hue of light pollution

How were the observations made?

  1. The same observing spot was used on all occasions.
  2. M44 at an altitude of 40 - 50°.
  3. The weather was very similar on all occasions.
  4. Dark adaptation (night vision) was limited to 5 minutes.
  5. The SQM-L reading was taken directly from M44 and mean value was taken based on 5 separate measurements.
  6. At 23.00 the city shuts down half of the streetlights on the small walking path leading to the observing spot used.

7.3.2010, 21.11 - SQM-L 17.70 (NELM ~4.85)- M44 invisible.
13.3.2010, 22.06 - SQM-L 17.98 (NELM ~4.99) - M44 fairly easily visible with averted vision.
13.3.2010, 22.36 - SQM-L 17.94 (NELM ~4.97) - M44 perhaps slightly more difficult than on the previous try.
13.3.2010, 23.06 - SQM-L 18.01 (NELM ~5.00) - M44 fairly easily visible.
13.3.2010, 23.36 - SQM-L 18.02 (NELM ~5.00) - M44 fairly easily visible.
13.3.2010, 23.56 - SQM-L 18.02 (NELM ~5.00) - M44 fairly easily visible.
14.3.2010, 21.36 - SQM-L 17.84 (NELM ~4.92) - M44 very difficult.

Naked eye limiting magnitude = (SQM reading - 8) / 2