Sunday, November 27, 2005

Application of Info Index - "2"

Described here is an example of the Info Index. This past Friday night I had a list of galaxies I wanted to search for but it turned out that I only observed one: NGC7331. The illustration below is an image I marked up from the cd-rom set of Real Sky. On the galaxy I overlaid an oval that approximates the size and shape I saw. NGC7331 companion galaxies can be seen below it.

I first hopped off of eta Peg where the galaxy is just under 4.5° to the NNE. In a low power eyepiece of 32x, I was unable to see it. It was assigned a "0" with a sensation in the general area. At 63x and greater powers the galaxy was observed with direct observation, and I settled on 97x for most of the observation to assign a "2".

With a rating of "2" I was able to easily locate the galaxy relative to the starfield and determine its orientation and size. The orientation was north to south with a brightness profile of __^_. The brightest part of the galaxy observed was off center toward the southern side. Using the closer pair of two horizontal stars below, I was able to deduce that the galactic nucleus and core were about 2 arc minutes in length. Its width was a fraction of this, appearing long and slender. The surface brightness of the inner 2.0 arc minutes is 20.2 mag/sq" based on info in Rachford's table (described below).

The three bright galaxies below NGC 7331 were not seen.

I compiled a list of candidate galaxies with a good probability of detection. I am still refining this list to find a suitable cutoff. It has been NGC6946 but this is to be revised, and declination needs to be taken into account. The list is based on the work of Brian Rachford.

I found reference to this from Tony Flanders' Astronomy Site, after reading his abbreviated version of the information within the site in a Sky & Telescope article concerning urban/suburban astronomy and observation reports for Messier objects. A very good article.