Effective Deep Sky Observing
By Paul Markov, February 2000.
If you are city dweller like me, a night of observing under a dark country sky has become a rare event. Work and family commitments have made these kind of observing sessions so infrequent that when they do happen it is imperative to make the most of them. I cannot afford to be unprepared or disorganized because I know the next opportunity may not come for several months. In this installment I will discuss how to prepare yourself for a productive night of deep-sky observing, and how to be as effective as possible once you're under a dark sky.
Tools of the Trade
The basic prerequisite to a successful night of observing is to have all the required equipment with you at your observing site. To help yourself to be prepared on a moment's notice, try keeping all your equipment in one location in your home. Do not keep the tripod in the garage, the telescope in the basement, your accessory case in the bedroom, and your flashlight in kitchen cupboard! If you do, it is a sure way of forgetting something behind. Keep all that you need in a common area of your home so that after you're done loading your car you can go back and do a quick visual inspection of that area - if you forgot something, you'll notice immediately.
Aside from the basic components like the tripod, mount, telescope and eyepieces, here are some items that can be easily forgotten that will "kill" your observing session: dew cap and heating element for the main scope and finder scope, star atlas, observing list, red flashlight with fresh batteries, and appropriate clothing and footwear. If you forget just one of these items, your observing session may be a very short one. Assuming you have all these necessities under control, you must pay particular attention to one of them - the observing list. This is not a static item that you can just pick up and go. It always changes depending on time of the year, sky conditions, and your recently logged observations.
The Observing List
The observing list is simply a tabulation of deep sky objects you plan to observe on a particular night. Its purpose is to list the objects in the order you plan to observe them, so as to save you time in the field. The list can be just a few objects long, or several dozen objects if you have the whole night ahead of you, and should include the object's basic data such as catalogue number, type, constellation, magnitude, size, coordinates, and star chart number.
Generally, the observing list will be a subset of a master list. For example, your master list could be "The Messier Catalogue" or "The Finest NGC Objects" from the Observer's Handbook, and once you complete those, you may tackle the Herschell 400 or the New General Catalogue. No matter which list you are aiming to complete, I strongly recommend you prepare an observing list before each observing session. The last thing you want to do under a clear rural sky is fumble through pages and pages of potentially observable objects trying to decipher which ones to hunt down.
If you have the time, you can take this preparation exercise a step further, and actually locate each object you want to find on your sky charts, and familiarize yourself with its location. You can even put pencil marks next to each so that you will be able to spot them quickly on the charts once you are at the telescope.
I have made the mistake a few times myself of not having observing lists prepared in advance. The result was very unproductive and frustrating observing sessions. For example, rather than observing my typical dozen "previously unseen" objects during an observing session, I only observed two or three because I was spending too much time creating an observing list in the field. On another occasion, I was very happy with having observed twelve "previously unseen" galaxies, until the next day when I checked my computer database and found that I had already seen ten of these twelve!
In order to create useful observing lists, you should do a preliminary "cleansing" of your master list, especially if it is a computerized database. From your master list, remove objects that are below the horizon from your location and remove the ones that are much too faint to be seen with your telescope. But do not delete them permanently just in case you travel south or you have the opportunity to observe with a larger telescope. This initial preparation will ensure you do not waste any time sifting through objects that would be impossible to view in the first place. Next, make sure to keep your master list up-to-date by marking observed objects as "seen" as this will ensure next time you prepare an observing list you will not re-list previously seen objects.
Think of your observing list for a particular night as your "game" plan, and as such it should be well thought through with an emphasis on accuracy, efficiency and effectiveness. Perhaps this is starting to sound too much like a corporate business plan, but as I noted above, when dark sky observing sessions become a rare "commodity", you must make the most of them!
Your first step in creating an observing list will be to determine what constellations will be in the sky during the length of your observing session - a planisphere or planetarium software can make this task simple. You will want to create an observing list for each constellation starting with the westernmost. Next, select the limiting magnitude for your particular site, and select objects brighter than that limit. If you expect a particular night to be less transparent than usual, then select object types that are less vulnerable to poorer sky conditions, such as open clusters and globular clusters. Try to list your objects based on proximity to one another, as opposed to listing them by the Right Ascension. If you list them by Right Ascension, as per the New General Catalogue, you may find yourself needlessly star-hopping all over a constellation (see Scope, Oct. 1999, pg. 14 for a more details). For a higher "hit" rate and more pleasing views, try to observe objects that are at least 20 degrees above the horizon. This is because an object 18 to 20 degrees above the horizon will suffer a loss of half-a-magnitude due to atmospheric extinction (see 2000 Observer's Handbook, pg. 51), which gradually decreases to a zero-magnitude loss at +52 degrees or more above the horizon.
At the Eyepiece
You are finally set up under a dark country sky, you have all necessary equipment ready to go, and you have an observing list - time to get busy!
Remember that your observing list is your "game" plan, so follow it and try not to stray from it. As you observe each object remember to mark it as "seen" on your list, and ideally try to write a few descriptive words about your observation before moving to the next one. Cannot seem to locate or see one of the objects on your list? If this is the case, make sure not to spend too much time searching for one single object. Perhaps it is a particularly difficult object and during the time you spent looking for this elusive "fuzzy", you could have spotted another couple of objects. There could be many reasons why you may not find certain objects - perhaps these are too faint or the sky is not transparent enough on a particular night, or maybe your data is incorrect (wrong magnitude, wrong apparent size, wrong coordinates or mis-plotted in the sky charts), or possibly you just lost your bearings and were looking in the wrong place. The important thing to remember is to move on if you just cannot seem to locate an object which should otherwise be readily visible. That raises another important observing practice - always record failed observations, so the next day you can double-check your object data to ensure there were no mistakes, and then you can create a "try again" observing list.
As you begin to "bag" new objects, try to pace yourself. Do not rush through each object to the point where you forget what you saw just a few minutes earlier, but also do not celebrate each new observation with a coffee break! As an aside, make sure you are properly dressed so that you will not spend more time in the car warming up rather than being at the eyepiece. Also worth mentioning is to keep your observing list and observing notes safely fastened to a clip-board or inside a binder - an unexpected gust of wind could blow away your papers into the darkness, never to be found again!
Copyright (C) 2000 by Paul Markov
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