*** Abell 2151 ***

A2151.jpg (50321 bytes)

Figure 1 - Some of the brighter galaxies near the  center of A2151. Image from the STScI Digitized Sky Survey. Width is 15' and North is up.

Observing Challenge - 100+ Galaxies in the Hercules Galaxy Cluster, Abell 2151

Albert Highe

For those who want to jump straight to the observing list, click here.

Download the finder chart I've created for the central 1º X 1º field. 



My observing project of the Perseus Galaxy Cluster, Abell 426, was very rewarding. I found I enjoyed searching for and reading the academic sources, sorting through the data to identify cluster vs. non-cluster galaxies, preparing my own observing list, and hunting down galaxies that are seldom, or have never been, seen by human eye. Consequently, I decided to perform similar studies of other rich Abell galaxy clusters.

Uranometria 2000.0 shows a close up plot of A2151 on chart A9. A dashed circle with a diameter approximately 1º indicates the location and size. As with other galaxy clusters, I tried to answer three questions. 

  1. Do all the galaxies within the circle belong to the cluster? 
  2. Are there observable cluster members that are not plotted in Uranometria? 
  3. Do cluster members lie outside the circle?

Sourcing the Data


Analyzing and Verifying the Data


To insure the coordinates in Table I are correct, I downloaded a DSS image for each galaxy and made sure it appeared at that location.


The Observing List

The observing list is contained in Table I – Galaxies within 1º of the center of Abell 2151.

The headings of the columns in Table I are described in Table II below.

Table II - Description of headings in Table I


Name Generally the most common catalog name.
Alternate name 1

Identification in an alternate catalog.

Alternate name 2 Identification in an alternate catalog.
RA Right Ascension for Equinox 2000 in hours, minutes, and seconds.
DEC Declination for Equinox 2000 in degrees, arc minutes, and seconds.
Magnitude For most of the data, this is the photographic apparent magnitude.
R (arcmin) Distance of galaxy from center of cluster in arcmin.
Radial velocity Reported heliocentric radial velocity in km/sec.
Cluster member? Remarks indicating likelihood of being a cluster member.
Date observed Date of most recent observation with my 17.5" f/4.5 reflector.
Magnification The magnification used for the reported observation
% averted vision The percentage of time I could hold the object with averted vision.

You’ll notice several designations in Table I, including objects from NGC, IC, UGC, MCG, CGCG, and PGC catalogs. However, some less common designations are also used. Full names for these other designations are shown in Table III.

Table III - Some less common object designations.

KUG Kiso Ultraviolet Galaxy Catalogue
MAPS-NGP Minnesota Automated Plate Scanner - North Galactic Pole
AGC Arecibo General Catalog
2MASX 2 Micron All Sky Survey Extended Objects
ABELL Abell Clusters of Galaxies
ARK Arakelian Emission Line Objects

Observing Methodology

Preparation is very important. The most useful charts are those I prepared from STScI Digitized Sky Survey images of the central 1º X 1º field and selected outlying areas. I labeled the target galaxies in Photoshop. For use in the field, I print out charts of regions approximately 20' X 30'. Feel free to download the finder chart I've created for the central 1º X 1º field. 

Although a large number of galaxies are clustered together in the central 1º, using a low power, wide field eyepiece will show very few at one time. Most of them are very small and faint. Higher magnification improves contrast and shows more detail. For observing faint, low contrast galaxies, I find using an exit pupil of approximately 1 - 1.5mm provides the optimum view. For my 17.5" scope, I generally use a 7.5mm Takahashi LE or 7mm Nagler T6 eyepiece, providing magnifications of 267X and 286X, respectively. For viewing the smallest and faintest galaxies, and for splitting the very close galaxy pairs, the 5mm Takahashi LE, providing 400X, works best. Detecting the faintest members also requires very dark skies with good to excellent transparency.

Most of the galaxies on the observing list are faint. Most of the recorded descriptions are brief. In order to provide some indication of the relative difficulty of seeing an object, I record the magnification used for the observation and the percentage of time I was able to hold the object with averted vision. Any object that I could hold less than 50% of the time is very challenging.

Selected Observing Notes

Currently I've observed 69 objects on the list. There are over 100 targets left, so I expect to significantly increase this number.

Creating a database for "The Sky"

Contacting me

If you have questions, or want to report any errors, please contact me at: ahighe @ ix.netcom.com.

I also would appreciate hearing about your observations, especially if you use a scope of a different size. Thanks.

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Copyright © 2003 by Albert Highe, unless otherwise noted. All rights reserved.

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Page created by: ahighe @ ix.netcom.com
Changes last made on: November 26, 2003