Tips on Star Hopping --Starseeker 15:30, 14 October 2007 (UTC)
Why Star Hop?
In a world flush with GoTo telescope mounts why learn to find space objects without them? I think there are two reasons why someone might engage in the dated art of Star Hopping to find an astronomical target.
- The telescope you need or choose to use isn't equipped with the latest computer based GoTo mount drive.
- You are a star explorer at heart and were meant to be on the bridge of the USS Enterprise, but unfortuately you were born a few years to early. To you, exploring space is at least as important as getting there fast.
At a recent star party I was reminded that while most of the techniques of star hopping are pretty simple, they are not all immediately clear to someone who has just taken their first real telescope out under a clear night sky and decided to try and find something other than the moon or a bright planet. If that describes you, maybe you will find something here helpful.
What is Star Hopping
"Take the second star to the right, and straight on til morning." Peter Pan
Star hopping is simply the technique of moving (or hopping) your telescope's field of view from one known star field to another know star field until you get close enough to your target object that you can just sweep the telescope a little ways in a known direction to find what you want. Sounds simple right? Well it is, but it may be easy or not depending on how well your prepare for the journey.
When I prepare to hunt for a new object with a non Goto telescope, I ask myself four questions:
- What is the nearest bright guide star that I can see with my unaided eye?
- What is the nearest bright guide star that I can see with my finder scope?
- What is the nearest star field that I can easily and positively identify with my telescope and eye piece?
- How do I sweep my telescope from that last star field to my target object?
If you think these questions out before you go out looking, your chances of sucess are better. I have found all the M objects many NGC objects and dozens of comets using pretty small non GoTo telescopes (8 inches or less) by star hopping.
Finding the Nearest Bright Naked Eye Guide Star
It should go without saying that if the object that you want to see is a nake eye object like Mizar, M31, or Albereo life is good you just point your telescope at it using the finder scope if necessary. But if you are looking for something that is not naked eye visible things are more sporting and star hopping probably will be useful.
In order to begin star hopping you need to start from a known location. Fortunately the stars don't move around much from night to night, decade to decade, and they move just a little bit from century to century. Yes, the Earth spins around and that causes everything to move across the sky, but relative to each other the stars appear as fixed objects in space. So unless there has been recent nova or supernova, any bright star that you can see with your own eye is a known fixed starting point. That being the case, it is easiest to start with a star that is as close as possible to the object you wish to find.
Which stars are visible with your eyes varies from night to night and from location to location. Even if the sky is clear, the amount of moonlight will drastically impact what you can see in the sky. This raises the question of how many guide stars are visible at each magnitude. David Haworth has created some nice tables of that information at his web site. http://www.stargazing.net/David/constel/howmanystars.html His charts tell me that on a good night in the country I might have 5000 guide stars but if I were observing in the city I might only have a few hundred.
Because I often don't know how many guide stars may be visible, I print a couple of star charts. I use The Sky, but there are several programs available that print out starcharts that would work as well. I print out a chart with a threshold stellar magnitude of 3.5 for a city location, one with the threshold of 4.5 for an suburban location, and one at 5.5 if I am going to a rural location. For each of those charts I ask the question, "Which visible guide star is the closest to my target object? "
Finding the Nearest Guide Star in a Finder
Once you have found the nearest naked eye guide star to your target, aim your telescope at it using the finder scope. Even if the finder scope has a small objective lens it is increasing the number of possible guide stars from a few thousands to 10,000 or 100,000 or more. This can be a great help normally or sometimes not if you happen to be looking down the spine of the Milky Way and the star clouds take your wits away. In addition to the increase in potential guide stars, two other factors are now relevant. First the image you are now seeing may be inverted, if you are using an straight through view finder, or the image may be upright but now a mirror image, left to right of the actual sky if your finder has a right angle prism. Second you are now dealing with a limited field of view. That is, instead of taking in a huge region of the sky of approximately 100 degrees or more as you do with your eyes, you are now looking at maybe a 3 to 5 degree circle of space. It is like looking through a soda straw, you only see a little bit of the sky at a time.
Lets consider these issues a little more. All stars are not created equal, well at least not to your eye. You will be attracted to the brightest stars in the field of view, so if there are no stars or if there are many that are just hovering on edge being detectable, they won't help much. Ideally you want to find starts that are at least a magnitude brighter than the limit magnitude for the finder. They will be more easier to detect and to arrange into meaniful patterns.
While a finder scope may create a limited field of view, you can at least calibrate that field of view to a known size. If the known that the size of the finder field of view is say 4 degrees and you can find a interesting arrangement of stars say 6 degrees north of your last guide star on your star charts you know to move the telescope one and a half finder diameters to find the expected star patterns. This means that you will want a set of charts that set the limit threshold magnitude to the expected magnitude of the stars visible in your finder. You probably don't want to plot these charts out for the whole sky, just for the region between your last naked eye guide chart and your target object.
Using this approach you can hop from star pattern to star pattern in your finder scope until you come to the last guide star that is visible in the finder. Just be sure to view the charts up side down if your finder inverts the image. If you have a right angle finder, you may need to use some imagination for matching the star patterns unless your software allows you to print out charts mirrored left to right.
As you reach the last finder guide star and if the target object should be in the field of view of the finder, you may find it helpful to estimate where the object should be in the finder scope, for example if the target object should be 2.5 degrees to the west, simply place the telescope at the estimated position using the finder's reticle. Then search around slightly with your widest field of view eyepiece in the telesope to see if you get lucky. Often this will work for medium bright objects.
Finding the Nearest Star Field
Now is the time to use the widest field eyepiece you have because if using a finder scope is like looking through a soda straw, using the telescope is like looking through the eye of a needle. On the plus side you may have 30 million guide stars to help you locate you position. Otherwise the technique is very much the same as star hopping through a finder scope.
Again, determining your field of view is very helpful. There are several calculators on the web that will help you estimate your telescope's field of view with a given eyepiece , if you don't know it from the information supplied with your telescope, or with your star atlas, or star charting software.
To help me keep track of my position on my star carts, I use a circle template, a piece of plastic with a set circles from 1/64th of an inch to 1.25 inches cut out, with my charts that I can place right on the star charts to show what the field of view should look like in the telescope. I invert my charts to match the view in my telescope as I move the telescope slowly and carefully, making sure I can positively identify each star field from the star charts as I move from field to field.
In addition to the issues of inverted images or mirrored images, another concern may cause some confusion. Most star charts are plotted with equatorial co-ordinates, that is right ascension and declination. If your telescope is on an alt alzmuth mount, it does not naturally want to move along the grid lines of the star chart. Some charting software allow you to plot the charts with alt alz grids, however be aware that these grids are a function of time and change greatly over the course of an evening of observing. The important issue is to get a good understand of how your telescope motion relates to movement on your star charts.
Identifying a star field through a telescope takes time and practice. Don't get discouraged if you get lost from time to time, it is part of the adventure. In some ways this is like a meditation, "OK I am now lost, go back to the last place where I wasn't lost and try again." The exercise of identifying your field of view precisely will pay dividends if you become interested in observing comets, variable stars, asteroids, or even if you want to search for Pluto. Pluto looks like just about every other 14.5 magnitude star, all 50 million of them.
That Final Sweep
“Traveling through hyperspace ain’t like dustin’ crops, boy!” Han Solo, Star Wars IV
Sometimes, the last easy star field is still a few degrees away from your intended target object. Even though you can still see stars, they don't form useful patterns for star hopping. By this point however you should be fairly comfortable with moving your telescope and have a sense of how small movements shift the field of view. At this point you must trust yourself and boldly go into the unknown. (Sorry about being a little melodramatic, but I really like this part. Especially when it works.;-)))
If your target is about two degrees away, move your scope what feels two degees in the correct direction and slowly search around this point for your object. If your target is 4 degrees from your last star field, clearly you must estimate a 4 degree motion. It is often helpful bracket your target between two star field or between a star field and a bright star. Your ability to aim your telescope will improve if you know it must be half way between two positions that you can easily find.
It takes practice to be good at navigating a telescope through the universe, but it is not as hard as many other worthy goals. And hopefully for you, the practice should be fun because in the process you will see beautiful things only a handful of people that have ever lived on this planet have also experienced.
Good Hunting. --Starseeker 12:51, 14 October 2007 (UTC)
Easy Star Hops
Here are a few easy targets if you are just learning to star hop:
- M 31 Andromedae Galaxy
- M 57 Ring Nebula - Planetary Neb between Beta and Gama Lyr
- M 13 Globular Cluster in Her
- M 11 Open Cluster in Scu
- M 28 Globular Cluster in Sag
- M 22 Globular Cluster in Sag
- M 8 Diff Neb in Sag "The Lagoon"
- M 4 Globualr Cluster in Sco
- M 35 Open Cluster in Gem
- M 41 Open Cluster in CMa
- M 15 Globular Cluster in Peg