27 March 2012

The president of AAA has been divesting himself of his astronomy book collection and Peter picked up one of the books, Star-Hopping: Your Visa to Viewing the Universe by Robert Garfinkle (ISBN 0-521-41590-X) for me.  I have started reading the introductory chapters and found mention af appendices so I immediately skipped to the back of the book to check them out.  I have reformatted the data from two of them dealing with stars here since they are objects that I routinely observe.

When we observe double stars we often refer to them as being wide or close or even very close.  For me these have always been relative terms and would therefore change with the magnification I was using.  What appeared a close double at 25X could become a wide double when viewed at 75X (now that I have telescopes and can change my magnification).  But now I see that these terms have actual numbers behind them and that there are Open and Standard categories of doubles that I have never used or, in the case of Standard, even heard of.  I have color coded it with information about the resolving powers (Dawes Limit) of my scopes to make the table relevant for me.  Now I have to become proficient at estimating distances in the eyepiece.  I have heretofore use descriptive terms like the size of a pea, the width of a paper match, or whatever else seems appropriate at the time.

Double Star Separation
Open	>30.0"
Wide	10.1" - 30.0"
Standard	5.1" - 10.0"
Close	2.1" - 5.0"
Very Close	0.5" - 2.0"
Naked-Eye
Both scopes
Dobsonian only
Not Possible
6" Dobsonian 0.76"	70mm Mini Mak 1.65"

When Peter and I observe open clusters we often describe their appearance to us to each other while at the eyepiece.  We speak of levels of star brightness - I can often discriminate two levels of brightness and a "crunchy" under level behind them.  We also speak of the clusters being "rich" or "poor," but again these have been relative terms for me.  In the Double Cluster, NGC ___ appears to me to be a ring of bright stars with a basically empty center which I term sparse while NGC ___ appears as a filled in circle which I have called rich.  But somewhere I read a description of this feature that described both clusters as being rich.  Now I have numbers that will allow me to more accurately describe what I am seeing.  This ability will obviously be tempered by my eyesight which isn't great ar seeing faint things.

                    

Trumpler Classification of Open Clusters
Concentration	Magnitude	Richness
I Detached from surrounding central concentration      II Detached with little central concentration      III Detached with no central concentration      IV Not well detached	1 Small range from brightest to faintest member      2 Moderate brightness range      3 Large range	p Poor: <50 stars      m Medium: 50-100 stars      r Rich: >100 stars
Example: NGC 1891 (Cr 73)	III2p	Detached, no central concentration Moderate brightness range; fewer than 50 stars