The Dolidze-Dzimselejsvili Open Clusters
By Paul Markov, Oct. 1, 2001

Beyond the New General Catalog there are dozens of other catalogues of deep sky objects – the Dolidze Dzimselejsvili (DoDz) open clusters catalogue is one of them. Little information is available on this “tongue twister” of a catalogue; all I was able to find about the DoDz open clusters on the Internet were observing reports for a couple of objects.  There was no information anywhere about the two astronomers who discovered the clusters and the actual catalogue was nowhere to be found on-line.  Despite the lack of information, it is easy to gather enough details about these clusters from the Saguaro Astronomy Club deep sky database (www.saguaroastro.org), the Digitized Sky Survey (http://stdatu.stsci.edu/dss) and software like Earth Centered Universe or Starry Nights, to create an observing list with all relevant information. The DoDz catalogue is comprised of 11 rather unimpressive open clusters of moderate magnitude but because these can observed with a small telescope even from light polluted skies, I think observing all DoDz objects would make for an interesting short-term observing project.

Although I have been observing deep sky objects for nearly twenty years, I did not know about the DoDz open clusters till the summer of 2000.  The reason being that the deep sky database I use to prepare my observing lists (Saguaro Astronomy Club database) lists all objects with unknown magnitudes, such as the DoDz open clusters, as magnitude “99.9” and all these years I have been doing database searches on objects of magnitude “13.0” or brighter, inadvertently excluding all objects with unknown magnitudes! 

 As mentioned above, the integrated magnitudes for these clusters are not available, however by using software like Earth Centered Universe, I was able to find out the magnitude ranges for the stars in each cluster, as shown in the table.

Object

Const

R.A.

Dec.

Size

# of stars

Mag. range

DoDz  1

ARI

02 47.4

+17 12

12.0'

12

8.5 - 11

DoDz  2

ORI

05 23.9

+11 28

12.0'

12

9 – 10.5

DoDz  3

TAU

05 33.7

+26 29

15.0'

10

9 – 11.5

DoDz  4

TAU

05 35.9

+25 57

28.0'

15

6.5 – 9.5

DoDz  5

HER

16 27.4

+38 04

27.0'

7

9 - 11

DoDz  6

HER

16 45.3

+38 17

17.0'

5

9 - 12

DoDz  7

HER

17 10.6

+15 32

20.0'

6

~ 10

DoDz  8

HER

17 26.2

+24 11

14.0'

6

8.5 – 9.5

DoDz  9

HER

18 08.8

+31 32

34.0'

15

8.5 – 11

DoDz 10

CYG

20 05.7

+40 32

20.0'

12

8.5 - 11

DoDz 11

CYG

20 51.0

+35 57

13.0'

12

9.5 - 12

- Data from the Saguaro Astronomy Club database and Earth Centered Universe

          

The apparent angular size ranges from a small 0.2 degrees to a more reasonable 0.6 degrees (for comparison, the impressive open cluster M35 in Gemini is 0.5 degrees in size).  The reason I say these clusters are unimpressive is because they have very few loosely-arranged stars and if one is not careful to identify the field of view accurately, the entire open cluster can easily be dismissed as background stars.  You may observe a few more stars than listed, but chances are those “extra” stars are not cluster members.  If you plan to star hop to these clusters, then the Uranometria 2000.0 sky atlas will greatly improve your chances of positively identifying these clusters.

The sketches I made are not very precise, but should give you an idea of what to expect.  I used a 10-inch Schmidt-Cassegrain telescope at low power (50x and 62x), yielding an actual field of view of just under one degree.  I observed five of the eleven clusters with relative ease from my backyard in Scarborough and because the remaining objects are similar in magnitude I expect them to also be visible from the city.  Here are my observations for the five clusters:

DoDz5 – Scattered and poor. The cluster occupies only the upper right portion of the drawing above. I counted 7 stars, 6 of which are arranged in the shape of a “boat anchor”.

DoDz6 – Very small and poor. The cluster made up of just the 5 stars in the very centre of the drawing.

DoDz7 – Very easy to locate, but very easy to dismiss as just background stars. Just 2 degrees north-west of Alpha Herculis. Quite faint, only a few stars visible, about a quarter-degree in size. The poorest of the Hercules clusters.

DoDz8 – Very easy to locate, just 2 degrees south-east of Delta Herculis. This is the brightest of the Hercules clusters, but it is also scattered and poor with only 6 or 7 stars spread over half-a-degree. I could see a hint of this cluster in my 9 x 60 finder scope.  There are three sets of double stars in this cluster, although I did not notice this in my observations, perhaps because they are close doubles.

DoDz9 – The richest of the Hercules clusters encompassing about half-a-degree. I counted about 25 stars, with the brightest ones forming a rough circular pattern. The catalogue says only 15 stars are part of the cluster.

The notes below are not observations I made at the eyepiece, rather they are my comments based on “seeing” the objects on charts generated by Earth Centered Universe and on actual photos available on the Digitized Sky Survey.

DoDz1 – small, few stars, the three brightest stars, each of which is a double star, form a triangle.

DoDz2 – small, few stars, unclear as to whether the 7th magnitude double star in the same field of view is part of the cluster. 

DoDz3 – small, few stars, very easy to dismiss as background stars. About half-a-degree north-west of DoDz4. 

DoDz4 – large, loose and scattered, brightest of the DoDz clusters, contains 7 sets of close double stars. With a wide-field eyepiece one should be able to see both DoDz3 and DoDz4 in the same field of view.

DoDz10 – larger, loose and scattered. The two brightest stars in the cluster are close doubles.

DoDz11 – small, faint, stars are arranged in a linear fashion. The two brightest stars in the cluster are close doubles.

Even in late October or early November the clusters in Hercules and Cygnus are still well above the western horizon, so if you observe these now, you will be able to complete the entire DoDz catalogue by early winter.

This 1 degree x 1 degree negative shows DoDz 3 (upper right) and DoDz 4 (bottom centre to bottom left). The Digitized Sky Survey copyright 1994, Association of Universities for Research in Astronomy, Inc. http://stdatu.stsci.edu/dss/index.html

 

Copyright (C) 2001 by Paul Markov


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