How To Find Deep-Sky Objects Rapidly
Jay Reynolds Freeman
Sometimes I look at 200 deep-sky objects a night. People ask how I
log so many so fast. Partly, I rarely look at one object for long: I
generally view things near the chart limit of _Millennium_Star_Atlas_,
and with only a fourteen-inch aperture, 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 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 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
A. Solidly mounted,
B. With the eyepiece in a convenient position,
C. With a sidereal drive,
D. Which is easy to slew by hand,
E. With slow motion axes oriented to the compass directions
(that is, north, south, east, and west, on the sky),
F. With a good wide-field eyepiece for finding things,
G. With one or more straight-through magnifying finders,
H. 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 the slewing, 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. These
also serve to frighten off any Klingons I meet.
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, but only a bit. 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 low eyepiece positions.
The essence of star-hopping is to use the common, bright, and
obvious as guideposts to the rare, faint and subtle. Most guideposts
are stars, so you should have lots of them on your charts, enough so
many are in every finder field, and so you can expect at least a few, on
average, in every low-magnification field at the main eyepiece. For my
telescopes, the only amateur atlas I know that meets the latter
criterion is _Millennium_Star_Atlas_. It goes a magnitude and a half
deeper than _Uranometria_2000.0_ or the _AAVSO_Variable_Star_Atlas_.
The more popular _Sky_Atlas_2000.0_ has fewer stars still. The
difference between having charted stars visible and not, is the
difference between knowing that an all but invisible galaxy is somewhere
in the field, and knowing it is a little north of a point 40 percent
from this star to that one. If you can only see the galaxy via averted
vision when you know exactly where it is, 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, in which
case 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.
Know How Big Your Finder Field Is:
The diameter of your finder field is a natural distance yardstick
for star-hopping. If you don't know how long your yardstick is, you
will get lost. Some recommend a circle drawn on clear plastic, one
finder field in diameter at the scale of your charts. I have never used
one -- it's one more thing to keep track of, I need a different one for
each set of charts, and I have never encountered a chart that didn't
have some easy way to tell how big a degree was. If you know your field
diameter in degrees, as well as by how big it looks on the chart, then
you have two options for navigating. You can either think "go half a
degree", or "go three-quarters of a field", whichever seems right.
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 star-hopping.
Unfortunately, I cannot easily tell how to pick the right patterns.
There is a knack. 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 groups, you might want to pick something else.
If there are lots of 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. If you view this text with a constant-width font, I will show what
some of them are in the next few lines, but don't feel obliged to use
mine -- 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.
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.
The natural distance scales for star-hopping will be the diameter
of the field of view itself, and the sizes of the patterns you have
picked as landmarks. You can also use the sizes of the patterns as
a check on whether you really know how big the field is.
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
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. That's redundant -- 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 list for my current program, or at least a
set of charts with a plan to observe what's on them. I use those to
prepare a nightly program of objects I have not yet looked at, sorted to
put consecutive objects close together in the sky, so I won't have to
star-hop too far. When I use _Millennium_ as a deep atlas, objects are
usually in increasing order of right ascension, organized in strips of
right ascension that match the six-degree declination width of the
charts. For a night's observing in an extensive program, I might plan
to work the strip at 24 degrees south declination between 10 and 12
hours right ascension, followed by the strip at 18 south in the same
right ascension range, and so on. I will have selected areas of the sky
that are well placed as the night wears on. By planning this way, I
ensure that most star-hopping will be "differential", over short
distances, which take much less time than the same number of "from
scratch" star hops. In some parts of the sky, objects are so close I
may not use the finder for hours -- I need it only for a particularly
long hop to the next target, or 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 whining, 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.
Some might wonder why I do not build a chart stand, so I could use
my charts from the telescope. If I knew of a good design, I would
probably do so, but 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 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.
Or if you do, have your phasers ready to fire.
Thanks to Jay Freeman for kindly letting me publish this article on this site.