Why are custom Observing Lists so Helpful?
Because general lists list everything for everywhere. It's TMI (too much information).
What you want is the best stuff you're likely to see on a given night from where you're observing.
Your Observing List
Okay, I'm going out astrologying, or is it astronomying? How do I know what to look for?
Is there a list of what I should look for?
The short answer is: not really.
The complete answer is: you or someone else should create an Observing List for what you might see on a given night.
There are resources that will get you a list from which you can pare down based on the date, your location, your sky and your equipment.
How do I make my Observing List?
Start with a General List and then customize for:
- From Where?
- With What?
The General Lists
Everything we've ever discovered in the Universe goes on at least one list.
Just about everything most amateur astronomers will ever look for are in two catalogues: Messier's List and the New General Catalogue.
- Messier's List is 109 bright objects, but they're not the 109 Best Objects. He compiled this list of things not to look at - they were NOT COMETS, which is what he was looking for.
- The New General Catalog (NGC) came from systemic full-sky surveys of the brightest objects in the heavens. It list several thousand targets, most of which require a good telescope and a dark sky.
Messier's List is "the Default" for new observers. While not perfect, it's a great start. (here's a great resource for info AND how to find them): But not for tonight, not where you're at. You need something more tailored.
Location! Location! Location! (And Time)
Lists List the Whole Sky = Too Much
Lists list the whole sky. You need ONLY the stuff for tonight at your place. For any given observing session you probably only care about 20% of the sky spatially (unless you're planning an all-nighter).
So the first steps in getting a custom observing list are finding a general list and then cutting out the 90% of the sky that you are not interested in. Exclude the objects the sun washes out and our stubbornly non-transparent Earth blocks.
What can you see from your Location When you're there?
Given a list of cool things in the sky, exclude the objects you can't see from your location..
Down With the South!
Your view to the South is your hard limit on the objects you will ever be able to see from your location. Ever.
If you were on the equator, you'd see almost all the sky in the North-South direction . If you were at the North Pole, you'd be able to see only the Northern Sky. As you move further South from the North Pole you'd get to see further into the Southern Celestial Hemsisphere.
As an example, if you live in San Diego, at lattitude 33, you get to see all but the lower 33 degrees of the southern sky (down to Declination -57 (declination = Space Lattitude). To see further south, you have to travel further south.
So, unless you travel, you can throw away, permanently, all the objects that are permanently too south for you to see.
(Note: Southern Hemisphere Observers have the same problem but with the opposite limits.
West to East
East-West limits are far more forgiving. You're limited by time.
Sunset limits how far West you can see. Your Eastern view is limited only by your obseving time and Dawn.
In a given moment, if you can see from the east horizon to the west horizon, call it half the sky east to west. This east-west half of the sky after sunset truncated by your southern viewing limit should be the start of your observing list.
You can add objects based on how long you might observe.
If you're willing to stay up all night, you can see most of the sky above your Southern limit. As the earth rotates, you can see stars rise in the east; you get to see more stars rising until the break of dawn. So, from east to west, you get to see almost the whole sky, maybe 90% of it. The north-south wedge of sky that the sun is in gets washed out by dusk and dawn.
Further restricting your viewing is atmosphere: when you're looking close to the horizon, you're looking through a lot of air, which degrades the viewing. You will not be able to see objects well that are close the horizon. Eastern "slop" can be waited out until the target rises higher. Southern slop is permanent.
What about Buildings? Trees? Mountains? Again, eastern obstructions can be waited out.
You may of heard of "Spring Targets" or "Winter Objects" or "Galaxy Season". Those designations are based on one assumption: that you are observing right after it gets fully dark. That's a really really good assumption for 99.99% of eyeball-time spent observing. Know that all the objects are always "up" over the course of an evening (except for the wedge of sky the sun blocks out right at sunset and sunrise).
Seasonal recommendations essentially break the sky up into four subsets centered on the slice of sky away from the east and west horizons at an hour or two after sunset for a day in each season. This is better than an "All Sky" list.
Keep in mind that the sky drifts west from day to day (by one degree). Rather than publish 365 daily recommended lists, publishers prefer four Seasonal Recommendations. Based on the date and time you might be between lists (there is plenty overlap, though).
Planets and the Moon
Because these positions vary by time and date, these aren't on any permanent list.
Your Sky Brightness
If a Celestial Target is up, can you see it?
Is the sun up? Then you can't see anything else (except the Moon).
Are you observing from a city center? You probably can't see it.
Are you in a dark rural area? Your odds are getting better...
Is the moon up? Yes? %(&@#^**&!! Deep Sky Observers hate the Moon. It's like a dimmer Sun, washing out all the dim stuff.
The short answer is the sky brightness at your observing site will limit what you can see, even with the largest telescopes.
There is zero point in adding invisible objects to your observing list.
The sky brightness at your site is a quantity that can be used to limit what objects get on your observing list. "Limiting Magnitude" is one term that some list generators use to filter out any objects too dim for your sky. (the Bortle Scale is a more general rating system that can help you determine what you might be able to see).
The Naked Eye Deep Sky Object List is very short.
Telescopes have two purposes: to make things bigger and to see dimmer things (and for many serious amateur observers: to impress our friends (inevitabley stressing our spouses)). As your skies get darker and your telescopes get bigger, your target list gets correspondingly bigger.
Limit your list by the equipment you expect to use. No need to distract with things you cannot resolve.