The Reference Deep Sky Object
by Paul Markov, June 1999.

Deep sky observing requires good sky transparency, among the other obvious things like a clear dark sky, and as much aperture as possible. Sky transparency is a term astronomers use to describe the clarity and cleanliness of the sky, and it is crucial to deep sky observing because it dictates the faintest magnitude visible on any particular night.

Summer time in Southern Ontario usually produces very hazy and murky skies, which means the sky transparency is much worse than at any other time of the year. Unless you pay close attention to transparency this summer, you could be wasting your time searching for deep sky objects that are hidden by poor atmospheric conditions, or that are dimmed so much that the views will be disappointing.

However, just because the sky may be less than perfect, it does not mean you should not observe. Rather, use a few reference deep sky objects to evaluate your current sky conditions and adjust your observing plans accordingly. This same methodology can be applied to skies suffering from light pollution, or a combination of poor transparency and light pollution. Also, if you suspect the sky might have improved or deteriorated during the course of your observing session, you can always repeat this check.

The criteria for selecting a set of reference deep sky objects is as follows; objects are easy to locate, they are in different parts of the sky, of varying magnitudes, and should preferably be galaxies.

Your reference deep sky objects should be easy to locate so that you spend as little time as possible finding them. Remember, the point of this exercise is to do a very quick check in order to evaluate your current conditions, so you don’t want to spend more than one minute finding each reference object. With that in mind, choose objects that are close to a bright star or that are close to other bright deep sky objects, and preferably within the same field of view. Two examples are NGC 6207, a small galaxy just north of M13, and NGC 404, another small galaxy in the same field of view as Beta Andromedae.

For an all-sky transparency evaluation, the reference objects should be in different parts of the sky, such as overhead, and in the eastern, southern, and western sky. This should give you a good indication of whether there is more or less haze in certain areas.

However, if you’re concentrating your viewing efforts to a confined area of the sky, the Virgo cluster for example, then save yourself some time and do a reference check of that area only.

You want to choose reference objects of gradually decreasing magnitude. For example, look at three galaxies with a magnitude of 10.5, 11.0, and 11.5. This will give you a good idea of "how faint you can go" during a particular observing session. When your faintest reference object becomes difficult to see, then you know you have reached the limit for that night.

I suggest you select galaxies as your reference objects. The reason is that there are plenty of galaxies throughout the sky, which means you will have a better chance of finding the magnitudes you require near a bright star. Do not use deep sky objects that are stellar in nature, such as open clusters or globular clusters, as these can withstand poor sky transparency and light pollution much more than diffuse objects like galaxies. You may be able to use planetary nebulae, however there are not many to choose from. Bright nebulae could be used, except that typically their magnitudes are not reported in catalogues.

Given the above criteria, I have compiled a list of 40 reference galaxies that should be relatively easy to find, due to their proximity to bright stars. The list below includes objects found in the Spring, Summer, and Fall sky, as this will ensure you will always have a few to choose from during the next several months. Unless otherwise stated, the objects are plotted in Sky Atlas 2000.

NGC2859, NGC2793, NGC2832 same field of view as alpha Lyncis.

NGC3162, one degree south-west of zeta Leonis.

NGC3226 and NGC3227, one degree east of gamma Leonis.

NGC 3877, same field of view as chi Ursae Majoris

M109, 1 degree east of gamma Ursae Majoris

NGC4485 and NGC4490, same field of view as beta Canum Venaticorum

NGC4668, NGC4666, NGC4653, NGC4632, NGC4592 all within 1.5 degrees north of Gamma Virginis.

NGC4961, same field of view as 41 Comae Bernices, which is 1 degree west of beta Comae Bernices.

NGC5746, NGC5740, same field of view as 109 Virginis.

NGC5885, 1 degree south-west of beta Librae.

NGC6018, NGC6021 same field of view as gamma Serpentis (not plotted in SA2000)

NGC6181, two degrees south of beta Herculis

NGC6196, 1 degree south-west of M13 in Hercules (not plotted in SA2000)

NGC6207, 1 degree north of M13 in Hercules

NGC6675, 1 degree north of alpha Lyrae (Vega) (not plotted in SA2000)

UGC11465, same field of view as 16 Cygni (not plotted in SA2000)

NGC6928, NGC6930 1 degree south of epsilon Delphini (not plotted in SA2000)

NGC7013, 2 degrees west of zeta Cygni (not plotted in SA2000)

NGC7606, 1 degree north-east of Psi Aquarii.

NGC16, 2 degrees south of delta Pegasi (same star as alpha Andromedae)

NGC404, same field of view as beta Andromedae

NGC681, same field of view as zeta Ceti.

NGC890, 1 degree south-east of gamma Trianguli

NGC955, NGC941, NGC936, 1 degree west of 75 Ceti.

NGC1055 and M77, 1 degree east of delta Ceti.

NGC1465, same field of view as zeta Persei

Copyright (C) 1999 by Paul Markov

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