Challenge #1 – C42, Globular Cluster in Delphinus
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C42 (NGC 7006) | Globular Cluster | 21h 01m 29s | +16° 11′ 14″ | Delphinus | 10.6 | 3.6 arcmins |
Overview
William Herschel discovered this cluster in August 1784, recording it as “Very bright, small [and] round”.
Consider that this cluster is about 135,000 light-years away from us, possible even as far as the galaxy’s outer halo, and you realize just what a beast of an object this is, given how bright it is in our night sky.
However, don’t let this fool you. This a compact cluster, just a few arcminutes wide and reminiscent of a comet in the eyepiece of a larger scope. In a smaller scope, it’s a significant challenge to see this cluster as anything more than a faint, slightly fuzzy star.
Learn how to find this remote cluster with your telescope.
Finder Charts
This first sky chart, from SkySafari 6, which shows the sky looking south at 10:00 p.m. in mid-September. The green circle marked NGC 7006 is our cluster and you should note its location with respect to the Great Square of Pegasus and Cygnus, that are both marked by orange ovals.
Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.
With these more obvious parts of the sky located, we need to find the small constellation of Delphinus. The second star chart below, still with stars to magnitude five, shows where it is with respect to the edge of the Great Square. Note also the bright star Altair in Aquila.
In this third chart, stars are shown to magnitude seven, comfortably visible in binoculars and magnifying finderscopes. Use a line drawn from Sualocin to Gamma 2 Del. and onwards to an inverted ‘T’ of five fainter stars that are all of similar brightness.
C42 is on the line forming the bottom half of the T\s stalk, between HR 8037 and HD 200393. These two stars both shine slightly brighter than magnitude seven.
Now you have this cluster in your eyepiece, what should you expect to see?
Individual Telescope Views
The following views will help you find C42 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Stars are shown to magnitude 10.0 and the bright stars closest to N and E inside the 1.5° field of view circle are the two stars at the base of the T’s stalk.
Each image can be clicked on for a full-screen version.
Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
Despite its not especially faint magnitude, this object can’t be picked up in binoculars and is little more than a faint star in the eyepiece of a smaller telescope. With a dark sky and patience, the ‘star’ can be seen as a circular body with volume at higher magnification, and perhaps the hint of a brighter core.
With its brightest stars just a few shades brighter than magnitude 16, this is one of the hardest globular clusters to resolve. Reports of owners of telescopes with apertures of 12″ or more have reported being unable to resolve stars in this cluster.
It seems that Sir Patrick Moore included the object in his list for its spectacular distance and for dwelling in the Milky Way’s halo, rather than the joy it brings the backyard telescope observer.
PDFs for Printing
Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [C42-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.