OF THE ROGER N. CLARK

error of only one quarter the field diameter,
or 10.5 arc-minutes for this case.
A 7-power finder can do this if it is accurately
aligned with the main telescope and has
thin cross hairs. (An 8-inch focal length finder
objective and a 7 X eyepiece would require
cross hairs 0.01 inch or less in thickness
- a reasonable value.) But now consider that
after an object has been acquired, the magnification
may be increased to 150x or 200x. If
the object is lost from the field of the main
telescope, where the true field is now only 20
or 15 arc-minutes, it would be very difficult to
reacquire the object. A low-power eyepiece
would have to be reinserted in the main telescope
and you'd have to start all over again.
Because this book advocates observing at
magnifications well above 100x on some
deep-sky objects, a relatively high-power finder
is also a good idea. Higher power, however,
means a reduced field of view, and that
can hinder comparisons with star charts. The
field of view must not be less than about 5° or
this becomes a real problem.
The solution to this dilemma is one of two:
1) a finder with exchangeable eyepieces, so
that wide fields can be used for finding an
object and higher magnification for precise
aiming, or 2) two finders, one low power, one
higher. Option 2 has the advantage that the
higher-power finder could also be of greater
aperture, and then very faint objects become
easier to find. Disadvantages are the added
expense and weight.
The ease of locating objects with two finders,
a small one of low power and a larger
one of higher power, cannot be overstressed.
For many years I had a single finder on the
8-inch Cassegrain used for most of the drawings
in this book. It is far better than most,
because it has a 60 mm objective of 300 mm
focal length (from an old telephoto lens) and
a war-surplus Erfle eyepiece of 38 mm focal
length and a 65° ·apparent field of view. The
combination gives a breathtaking 8° field on
the sky and 7.9x. The cross-hairs are graduated
in 1° increments for easy star-hopping.
However, after much of the research for this
book was completed, and I was using magnifications
consistently higher than in my early
years of observing, I added a 3.1-inch, f/7.9
refractor. With a 20 mm Erfle, this telescope
gives a 2° field of view at 31 X.
The ease of getting the general region of
VISUAL ASTRONOMY OF THE DEEP SKY
the object in the low-power finder, then
ing to the 3-inch and almost always
the object and centering it, and then
to the 8-inch, makes the added cost
worth it. Except for a very few objects
8-inch is used at magnifications of 117 X '
higher. The FMR ratios are then only
from the 7.9 X finder to the 31 X finder
only 3.8 from the 31 X finder to the 117X
be used at a magnification greater than 31
for the FMR ratio to be greater than 10
higher-power eyepieces can also be usd
the 3-inch refractor at such times. In
tion, the 3-inch is a richest-field
giving some marvelous views of the sky
possible with the 8-inch Cassegrain.
With these thoughts in mind, Table
was drawn up to recommend finder
figurations.
the main telescope. The 8-inch would have
MISCELLANEOUS TOPICS
Caring for optics
If the optics are to perform to their limit,
must be clean and free from scratches.
true of eyepieces, finders and the
telescope. All optics get dirty with use,
they must be cleaned with great care.
The worst problem is invisible:
abrasive dust that will scratch s
during cleaning. Before anything else,
gently brush off the surfaces with a
hair brush (which like other cleaning items
available at camera shops). Some
sell cans of pressurized gas for blowing
off surfaces such as lenses and mirrors,
before using one of these be sure it says it
not leave a residue and is safe for optics.
To clean small lenses such as eyepieces,
next step is to use lens-cleaning
(obtainable at most camera stores) or
cotton swabs and a mild soap, alcohol, or
cleaning liquid (also obtainable from
camera stores). Use only genuine
surgical cotton, not synthetic "cotton"
The glass should only be patted and
rubbed because any rubbing action will
to scratch dust across the surface.
Mirrors can be cleaned the same way,
the aluminum coating is extremely easy
scratch, so the utmost care is required. If
mirror is removed from its cell, it can
--
Main telescop e
ap erture
--
2- to 3-inch
3- to 7-inch
8- to 11-inch
12- to 16-inch
1 6- to 24-inch
=
THE EYE AND THE TELESCOPE
Table 3.5. Recommended two-finder combinations
The lower-power
finder
power
7x
1) 8 X
or 2)
12 X
8 x
8 x
8 x
. mersed in lukewarm water and mild,
Im . F'
un
scented pure soap for cleanmg. Irst run
h .
r
u . h
I kewarm tap water over t e mirror lor
several minutes to dislodge dirt on t e
surface. Then soak it in a plastic tub or sink
full of lukewarm soapy water. (Place a towel
in a sink to cushion the mirror if it is
dropped.) Now swab the mirror very gently
underwater with sterile cotton. Turn the wad
of cotton in a backward-rolling motion so
that as soon as part of it rubs the surface, it is
carried away and won't touch it again. When
the wad has been completely turned, throw it
out and use a new one. Rinse by running tap
water over the mirror again for several
minutes. Finish with a rinse of distilled or
de ionized water (this does not leave stains),
stand the mirror on its side on a towel, and let
it dry. If your tap water contains many
impurities, you should use distilled or
deionized water for the whole process.
Eyepieces should be stored in a safe case.
Some companies sell eyepiece cases, or
camera cases can be adapted, or you can
and many amateurs simply place the
build your own. Most cases are foam lined,
eyepieces in slots or holes in the foam. Such
practice can damage the eyepiece if any lens
surface touches the foam. The extreme case of
damage would occur if the case was exposed
to sunlight; the foam can melt onto the lens!
The eyepieces should have some form of lens
over, like a plastic cap, before being placed
In a foam box. Or keep them in plastic
sandwich bags.
The high power
finder
aperture power aperture
35 mm none
50 mm and 15X 50 mm
50 mm none
50 mm and 25 X
60 mm and 40 x
60 mm and 50 x
60 mm
80 mm
100 mm
Dew
Dew is extremely hazardous to optics.
Repeated dewings can turn mirror coatings
brown because when dew settles, dust settles
with it. The presence of water can also start a
chemical etching process by impurities.
A telescope brought into a warm house
from the cold will immediately dew up. The
optics should be sealed before entering the
house so that little water will condense on
them, and after reaching room temperature,
the seals should be loosened so any trapped
moisture can escape.
Dew can also form on a telescope while it is
being used outdoors, fogging the view. This
can be prevented to some degree by having a
long tube in front of the objective, about
twice as long as the tube's diameter.
Reflectors only dew in the severest
conditions, because the mirror is at the end of
a very long tu be. Finder telescopes should
have a long shield in front of the objective to
prevent dew as well as to keep out stray light.
Eyepieces are more difficult to keep from
dewing up, especially since they are near the
observer's breath. Eyepieces should not be
left uncovered, but put in a protective box
when not in use.
In really wet conditions, the box could be
heated with two to three watts to drive off
dew. The objective can also be kept dew free
by placing one to three watts of heating
elements around it. Such heaters can be
made with small resistors or resistive wire
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