OF THE ROGER N. CLARK

amateurs tend to. use taa bright a red light.
They have the false impressian that if it is
red, night visian will nat be impaired. In
practice, hawever, I find that very dim green
ar yellawish light warks fine. Strang calors
shauld be avaided because they distart the
eye's calar perceptian at the telescape, if an
abject is bright enaugh to. shaw calar at all.
A gaad light level can be abtained fram a
twa-battery flashlight by replacing the bulb
with ane meant far three, faur, ar five batteries.
This bath dims and reddens the light,
while extending battery life. The flashlight
shauld cast a smaath glaw with no bright ar
dark spats. This can be ensured by placing
ane ar two. pieces afpaper in front afthe bulb
to. act as diffusers. The brightness can be
fine-tuned by using two. pieces af palarizing
material between the bulb and paper to. act as
a variable filter. Buy a high-quality flashlight
- it is very annaying to. have to. shake ar bang
it to. make it wark.
A drawing can caver any amaunt af sky
and be any size an the paper yau wish. Same
authors recammend using just ane eyepiece,
dra wing a circle to. represen t its field af view,
and sketching what is seen in this field. This
is a bad idea far several reasans. The mast
impartant is the need to. use many magnificatians
to. explare an abject properly, ane afthe
main thrusts af this baak. Anather is that yau
may want to. draw a larger area than fits in
the field, ar just a small detail.
One ratianale far the circle methad is to.
give athers a feel far what they might see in
their eyepiece at the same magnificatian. But
even at the same pawer, ane eyepiece can
shaw a field twice as big as anather. The right
way to. indicate an abject's apparent size is to.
give the distance from which to. view the paper
sa the abject laaks as big as it did in the
telescape. Angular distance an the sky carrespands
to. linear distance an a drawing ar
phatagraph. The relatianship between them
is called the scale (ar frequently the "plate
scale", a carryaver fram phatagraphic
plts). Often it is given in arc-secands per
mllhmeter. If an angle at in the sky carrespands
to. the distance ls an the drawing ar
phatagraph, then the scale Ps is
Ps = at I ls. (equatian 5.1)
Once the scale is set, the apprapriate viewing
distance v to. the paper depends anly an the
VISUAL ASTRONOMY OF THE DEEP SKY
56
magnificatian m af the telescape:
v =
ls I (2m tan(a,!2)). ( equation
scale and viewing distances for vario ot
The drawings in Chapter 7 give b h
nifications. By halding yaur eye at th:e
tances from the page, yau see J' ust ho
an 0 ect appears at a given
If an eyepiece's true field in your
i knaw . n (see Chapter 3), you can find
hnear size of the eyepiece field of vie I
the d. rawing (or a photagraph) fr;' e
equatlOn
le = 2mv tan(at/2),
where at is now the true field of view of
telescope with that eyepiece.
Even thaugh many magnifications
be used to view an object, one will often
the bst ovrall impressian of the object.
magmficatlOn, along with a viewing
afabout 25cm (10 inches), will define a
scale far the drawing.
Any drawing takes considerable time
at the telescope, then indaors doing a '
ric correction and a finished version
sed below). A very simple subject
cauple of field stars, such as a faint,
less galaxy, may take only 10 minutes at
telescope. Most subjects take over 30
nutes, and complicated anes like the
nebula (M42), several hours. The more
plex a subject, the more you must go
and forth from eyepiece to paper, and
time you return to the telescope it takes
minutes or sa to regain full night vision
when using a very dim flashlight.
After the drawing is completed at the
scope, a finished version is made .
the finger smudging method. For this
drawing, a geometric correctian s
made using known positions of
"straighten out" positional errors.
drawing usually takes longer than the
made at the telescope. In general, the
time spent, the mare accurate the
likely to. be.
tic talent ar experience, but they do
clase attention, much time at the
much time redrawing with finger
and honesty in not recarding details
bered from photagraphs but not
seen. All this can require mare time
b .
Goad drawings do. not require special
w
--
MAKING DRAWINGS AND KEEPING RECORDS
h h But the
taking a lang-exposre f . p atograp .
result is often as satlS ym ar mare so.
raW
Th .
1
D ing even has certam advantages over
h raphy One is law cast. e slmp est,
b p ot ;ones tlescape is all you need - no
d . 1
clock drive, alignment eVlces, ar e ectramc
'der not to. mention camera, film, and a
toraphic darkrpm. Secondly, . the eye
h a far greater lautude, or dynamic range,
t: s n film. This means bright and faint details
be seen at the same time, which is sameca
( . h
thing film cannot do Wit out very special
techniques). For example, have yau ever seen
faint auter wisps and the bright Trapezium
a photagraph of M42 that shaws the large,
See Chapter 7 far many examples.
Drawing method 1:
the initial-blind method
For this methad yau start "blind" with no.
infarmatian abaut the subject that might bias
you; a phatagraph shauld nat be at the telescope.
First the field stars are drawn, then the
object itself is sketched in, using the stars to.
positian its parts carrectly. Finger smudging
is nat used, just gentle shading with the pencil.
The warking light should be sa dim that
you can't tell pencil shading from finger
smudges anyway; the smallest detail visible
in very dim light at a distance of 10 inches
(25.4 cm) is abaut a sixteenth af an inch (1.6
mm).
Because af this, a drawing made at the
telescape will laak terrible in narmal roam
lighting. Mare to. the paint, a subject cannat
b redered very faithfully under dim illummatIOn.
But detail will be remembered far a
shor while after leaving the telescape. This is
the tue to. redraw the rough sketch indaars.
While daing sa, a geametric carrection can
be performed. This is dane by finding a suitable
phatagraph af the abject and tracing
onḷy the field stars (and, if the abject has very
bnght portians with sharp baundaries these
as well.) But remember that brigh;nesses
my appear different an the phata than perceIVed
by th D
e eye. raw the star bnghtnesses
:s remembered and recarded, nat as they
ppear on the phatagraph.
Lastly th h
and th h e p tagraph should be put away
e azy abject finger-smudged in Here
one needs to b h
_
e
very anest and nat biased by
.
.
.
.
57
the phata just een. Only wen the drawing is
camplete may It be campared with the phata.
The final drawing should be checked for
accuracy during anather observing session,
preferably under similar skies at the same
time the next night. The object should first be
s . tudied in the telescope, then a bright enough
hght shauld be used to. confirm the details in
the drawing.
Drawing method 2: filling in details
In this ethad yau start with a pre-drawn,
geametncally carrect sketch and fill whatever
details yau can see in the telescape. Such a
sketch cauld be traced fram a photagraph,
but trouble may result because phatas
usually recard stars that are taa faint far
visual use and the proper magnitude cutaff
can't be tald. A better saurce is a drawing
already made by methad 1. Such a drawing
can be traced or capied, leaving aut the detail
at the limit of detectian.
Using this preliminary sketch, yau add any
newly abserved detail. Then a new final
drawing is made indaors using finger
smudging as in methad 1.
Drawing method 3:
the double-blind method
In this procedure no phatographs are used
befare, during, or after the drawing at the
telescope is made. This methad, the freest
from bias, requires an accurately driven
telescape and a grid reticle in the eyepiece.
The stars and subject are drawn on graph
paper, where the squares match the reticle
grid seen in the eyepiece. Everything can be
platted accurately this way; there is no. need
far a geametrical correction in roam light.
Hawever, nebulasity will still need to. be
redraw by . finger smudging. The grid may
be retamed m the final versian, ar remaved.
This i . s the mast difficult drawing methad,
and retlcle eyepieces are nat available in
today's telescape market. They have to be
custam made. A grid is nat needed in all
eyepieces, but in at least ane sa that some star
positians can be accurately plotted.
SUMMARY
Gaod astronamical recards provide a useful
baseline far seeing the same and possibly