How to find Deepsky Objects Rapidly
How To Find Deep-Sky Objects Rapidly
copyright © 2001, Jay Reynolds Freeman
Sometimes I look at 200 deep-sky objects a night, and people ask how I log so many so fast. In part, I rarely look at one object for long: I generally view things near the chart limit of Millennium Star Atlas, and with only the fourteen-inch aperture of Harvey, my Celestron 14, there is little detail in most. With more aperture or brighter targets, I would be more leisurely.
Yet time spent hunting objects is time not spent observing them. So perhaps advice on how to find things rapidly will be useful even to folks who take longer looks.
I locate objects by star-hopping. If you use setting circles -- analog or digital -- or have a computer-controlled telescope, this article won't help. I do not object to such devices, they just aren't part of my hobby at present.
Five areas contribute to finding things quickly. These are (1) having the right equipment, (2) having "deep" charts, (3) knowing how to use equipment and charts properly for finding things, (4) planning and observing systematically, and (5) creature comforts.
THE RIGHT EQUIPMENT:
For me, the right equipment means a telescope which is
- Solidly mounted,
- With the eyepiece in a convenient position,
- With a sidereal drive,
- Which is easy to slew by hand,
- With slow motion axes oriented to the compass directions (north, south, east, and west, on the sky),
- With a good wide-field eyepiece for finding things,
- With one or more straight-through magnifying finders,
- With finder aperture sufficient to see all the stars on my charts.
The virtues of solid mount and convenient eyepiece position are obvious. By "solid", I mean both that the mount does not jiggle, and that it does not weathervane in wind. Add a sidereal drive to such a mount and your telescope will remain pointed accurately not only while the wind blows, but also while you check your charts for the next step in the star-hop, or change eyepieces.
The easier hand slewing is, the more it can substitute for slow motions, and hand slewing is generally faster. My Losmandy G-11 came with lubrication that had hardened like glue in the bearings. I cleaned and re-lubed it, and gained an immense improvement in hand slewing, so much that I rarely use its electric slow motions when looking through the finder. I can often set on an object entirely by hand.
Having axes oriented to the compass points -- or at least having a control-paddle mode for motion in those directions -- means that when you know from your charts that you need to move the field northwest, you will be able to tell what direction that is at the eyepiece.
The wider the field in the main telescope, the more stars plotted on your charts will be visible at one time, and therefore, the easier it will be to know exactly where to look for an object that is not obvious at first sight. I use a 40 mm Erfle with my Celestron 14, which gives about a 40 arc-minute field, enough so there are usually several stars visible in it which are plotted on Millennium Star Atlas. Often more magnification makes it easier to see a galaxy, but I usually don't put in a higher magnification eyepiece until I am confident I know exactly where to look, and figuring that out generally takes a wide field.
I use magnifying finders instead of unit-magnification ones because I need to see more than just naked-eye stars to point the telescope accurately. I use straight-through ones because I can keep both eyes open and use the finder cross-hair as a reflex sight, fused by the brain with the view through the other eye. Users of this trick will have little use for unit-magnification finders.
With my larger telescopes, there seems to be no place to put one finder so that it will always be convenient, so I use two, oriented about like the warp engine pods on the star ship Enterprise. They also serve to frighten off Klingons.
Have enough finder aperture to show all the stars on your deep-sky charts, but not too much more, or you will be confused by additional stars. In dark sky, with averted vision, a 10x40 or 8x50 finder will reach the eleventh-magnitude stellar limit of Millennium.
Several of these criteria seem to rule out undriven Dobson-mounted Newtonians. That is a bit too strong. I have used three Dobsons extensively for deep-sky work, logging between 1500 and 2000 observations with them altogether, but though Dobsons certainly give satisfactory views, I cannot find things as quickly with them as with my C-14. The main reasons why are confusion as to which direction is which in the eyepiece, inconvenient eyepiece position in larger Dobsons, and inability to walk away from the eyepiece without having the object walk away as well. I do not say I cannot find things at all with Dobsons, or recommend that you sell yours, yet I myself cannot find things as fast with them as with driven, equatorially mounted telescopes with handy eyepiece positions.
To star-hop is to use the bright and obvious -- stars -- as stepping stones to the faint and subtle. You need enough charted stars to find many in every finder field, and to put a few in every low magnification field at the main eyepiece. For my big telescopes, the only amateur atlas with enough stars is Millennium Star Atlas, which goes 1.5 magnitude deeper than Uranometria 2000.0 or the old AAVSO Variable Star Atlas. The popular Sky Atlas 2000.0 has fewer stars still. Having charted stars visible means not just knowing that a faint galaxy is somewhere in the field, but knowing exactly where it sits with respect to specific stars. If you can only see the galaxy via averted vision, and only when you know exactly where to look, that is a big difference.
If you use a telescope smaller than my C-14, your low magnification eyepiece will likely have a wider field than forty arc minutes, so that you will be able to make do with a cheaper atlas, with fewer stars. Yet I cannot stress too much: If your atlas does not put several plotted stars in the field of your low-magnification eyepiece, you are cutting your throat before you start, for finding faint deep-sky objects fast.
I do use other atlases -- I have an old Norton's Star Atlas that I use for orientation, when I am not sure where in the sky is the field I am looking at in my big atlas. You will probably need an atlas for that purpose, as well.
USING EQUIPMENT AND CHARTS:
There are two star-hopping tasks which I accomplish regularly. The first is finding an object "from scratch", starting with the telescope pointed somewhere else entirely, with my atlases open to pages other than the ones I need. The second is a "differential" star hop, in which I start not far from the new object. Several sub-procedures and required bits of knowledge are common to both tasks, so let me describe the pieces just once. Finding "from scratch", involves the following steps, more or less in the order given. A "differential" star hop omits several, depending on whether I need to use the finder as well as the main eyepiece, which in turn depends on how far away the next object is.
Locate the Target Object on Your Deep Charts:
You should be familiar enough with your deep charts to know where in the stack of pages to look for any given constellation, or any given right ascension and declination. At least, you should know which way to turn the pages if you open to the wrong one.
Locate a Handy Start Point on Your Orientation Charts:
The starting point for a "from scratch" star-hop is likely a star or group of stars, sufficiently bright to be obvious to the naked eye, as close to the target object as possible. You will use your orientation charts to locate one. The knowledge you need is the same notion of what's on which page that you need with your deep charts. Many lists of targets identify the constellation where objects are located, so it also helps to have an idea where all the constellations are, or at least, where they are located with respect to the bright and obvious ones. Thus you should know that Sextans and Crater are generally south of Leo.
Lining up for Finder Star-Hopping:
Swing the telescope so it points generally toward your chosen start point. With both eyes open, look through one of its straight-through finders, and move the telescope so the image of the finder cross-hair, as seen with the eye using the finder, moves toward the start point, as seen with the other eye. When the start point is close enough to where the finder is pointing, the start point will appear in the finder field. If the finder is inverting (some straight-throughs are not), it will appear on the side opposite where you see it with the non-finder eye. In any case, line the start point up with the crosshair. But don't look through the main eyepiece just yet, we'll see why in a moment.
Star-Hopping in General:
The essence of star-hopping is the selection of patterns of stars on your charts that are easy to recognize, and that are small enough to fit inside the field of view with plenty of slop left over, so you can still recognize them even if they are not perfectly centered. That is true whether you are using the finder or the main telescope for star-hopping.
Unfortunately, I cannot easily tell how to pick the right patterns. There is a knack to it. Getting a pattern small enough to fit the field is the easy part, what's hard is picking something that stands out in that particular area of sky. If most plotted stars in that area are isolated single stars, then a small group of two or three may be a good pattern, but if there are lots of such groups, you might want to pick something else. If there are lots of isolated sixth-magnitude stars, you probably don't want to use any one of them by itself as a pattern. And so on.
You also need either a good visual memory or a set of concise descriptive names, to remember what the patterns look like for the short interval as you step from chart to telescope. I use several geometric names, like "straight line", "equilateral triangle", "right triangle", "isosceles triangle", "square", "parallelogram", "circle", and "trapezoid". I also have some fancier descriptive names that work for me. I have tried to set up the following text with a constant-width font, so that I can show what some of them are in the next few lines. Don't feel obliged to use my patterns, though -- pick ones that work for you.
* * *
* * -- dart or arrow * -- butterfly or bow tie
* * *
* * * -- dipper
* * * -- fish (a fish is any kind of oval with a few
* * * extra stars to one side)
* * * -- space ship (think, "Klingon battle cruiser)", or
* * broad arrow (pointed up). Hey, at least my
big telescopes have engine pods mounted like on
the star ship Enterprise.
There is a complication if the optical system reverses the image. Refractors with star diagonals and Cassegrain configuration telescopes with star diagonals reverse the image, as do finders with simple 90-degree star diagonals. Newtonians do not reverse the image, and neither do straight-through finders, or refractors and Cassegrain systems used without star diagonals, or finders with fully erecting diagonals. If the image is reversed, you must mentally reverse what appears on your charts, when you step to the eyepiece. There is a knack to doing that as well, and I have no way to describe how to learn it. All I can say is that it can be learned. It might help to look the wrong way when you are crossing the street, or to practice tying your shoes with knots that are mirror reversed from what you usually use. I don't say for sure that it will help, just that it might, and do be careful when you are crossing the street.
You also have to know directions. Maintain the habit of keeping track of which way north, south, east and west are in the finder and main telescope eyepiece at all times, even if one is reversed and the other isn't. When you rotate the star diagonal to a new position, make sure you still know which way is which. It helps to have slow motions that move the telescope in these specific directions, to remind you. Note that many people are confused by directions on star maps anyway: On a map of the Earth, you are looking down from the top, so if you orient the map with north up, east is on the right and west is on the left. On a star map, you are looking up from the bottom, so if you orient the map with north up, east is on the left and west on the right. That's one more thing to think about in addition to whether or not the optics reverse the field.
It may also help to think of directions using the face of a clock, with 12:00 being north. It is pretty easy to convert back and forth for reversed fields. Just imagine a clock dial and you can easily see, for example, that what is opposite to 9:30 is 2:30, and so on.
Your finder and telescope field diameters are natural yardsticks for star-hopping. If you don't know how big they are, you will get lost. Some recommend circles drawn on clear plastic, one field in diameter at your charts' scale. I don't use them, because my charts all show how big a degree is. If you know your field diameter in degrees, as well as how big it looks on the chart, you have two ways to navigate. You can either "go half a degree", or "go three-quarters of a field".
Thus a set of "directions" for a star-hop, that I remember from my charts as I walk to the eyepiece of the main telescope, might be, "Go a field and a half east to the arrow pointing east/west. Then turn left 45 degrees (that would have been "turn right" on the charts, if my optics were reversing) and go two arrow-lengths to an equilateral triangle that is about half a field wide. The galaxy is about 40 percent of the way down the west side of the triangle, a little within the triangle itself." I can remember all that until I get to the eyepiece, most of the time, and I can also usually remember that the field is reversed in my C-14, and that the way the star diagonal is rotated right now has north -- say -- downward and to the left of the center of the field.
With "clock" directions, and with reversing optics, that same star hop would be "Go a field and a half toward 3:00, then two arrow-lengths toward 1:30..."
A finder star-hop might be "Go a field and a half north, to a wide pair of bright stars, then another field and a half to a much closer, prominent finder double." I would then set the cross-hair on the finder double. But I would *not* look through the main telescope immediately, to be sure I had the finder double in sight. Why bother? If it's not there, then my finder has slipped in its bracket, or the drive has stopped, or something else is wrong, and that doesn't happen very often. Before I look through the eyepiece, I go to the charts again, to memorize the directions for the main-eyepiece star-hop from that finder double to the actual object. There's no point going to the eyepiece unless I know what I have to do when I get there.
PLANNING AND OBSERVING SYSTEMATICALLY:
What you do in the field is the tip of the iceberg of an observing program. Preparation is important, so you will do what you set out to do, and so you will do it without wasting time.
I generally have a master target list for my current program, or at least a set of charts with a plan to observe what's on them. From them I prepare a one-night list of objects not yet looked at, in areas of sky that are well placed as the night wears on, with consecutive targets short star-hops apart. I usually order objects by increasing right ascension, in long east-west strips whose north-south extent matches the width of my charts. Thus most star-hops are differential, over short distances, and take much less time than star hops from scratch. In some parts of the sky, objects are so close together I may not need the finder for hours, till the next long hop to a target, as when starting a new strip of sky.
About all that remains to be said in this section is to do what you plan. Prepare an observing list for each night, take it along with you when you go out, and use it.
It's hard to do anything well when you are uncomfortable, except maybe whine, and there are a lot of creature comforts to improve your efficiency. There are enough for several articles, but let me say at minimum that you should make sure you are warm, well-rested, and well-fed when you observe, and not get sore muscles or cramps from staying in one position too long. When I observe, I carry lots of warm clothes and plenty of instant edibles, as well as good instant coffee and several thermoses of hot water. I have catalytic hand warmers for when it is really cold.
I wish I could build a chart stand, so I could use my charts from the telescope, but I do not know of a good design. The requirements are stringent. Such a stand would have to hold a volume of Millennium, plus a notebook for observing lists, plus my orientation atlas, all at eye level, close enough for me to see in dim light without getting out of my observing chair. It would have to protect the paper products from dew, without providing any surface on which the dew could condense and hide the paper. It would have to be sturdy enough not to blow over in the breeze, and light enough to move easily when I switch from one part of the sky to another. Think of a high-tech music stand, with a hood over it, open at one side, ballasted and with legs far enough apart so it won't tip.
I don't have one of those, but I do have a wide, flat, plastic box that holds my Millennium open, with a piece of Lexan that nearly spans it, to keep the pages from getting soggy or blowing over. In dry weather, I sometimes set it up on a table near the eyepiece; otherwise, I leave it inside my van, where the Lexan is out of the dew.
See, finding stuff quickly isn't that hard. Just don't talk about those Klingon battle cruisers loud enough for anyone else to hear you. And keep your phasers ready to fire.