What does the vacuum do for
The vacuum back is a modification made to
the camera's film holder. Its primary
purpose is to hold the film completely
flat during the exposure. Especially
with fast f-ratio optical systems, the
slight bowing of the film in commercial
film holders will cause parts of the
image to end up out of focus.
The secondary purpose of the vacuum
system is to draw air through a dessicant
so that only dry air is comes in contact
with the film during exposure. Moisture
in the air can cause curling and/or
desensitization of hypered film during an
exposure. The severity of this problem
depends on your environment -- a humid
tropical location requires more attention
to this than a dry mountain top. When a
dessicant reservoir will be used, a
larger vacuum pump is recommended.
Why do the astro cameras use 220
film and not 120 film?
Unlike 220 film, 120 film has a
paper backing behind the film. This
interferes with the vacuum plate's
ability to hold the film flat. However,
120 film can be "modified" in a darkroom
procedure to remove the paper backing
(see Hutech's procedure:
Converting 120 Film for Astro Camera Use).
Why is focusing such a big issue?
Precise focusing of an astronomical
camera is a "big deal" if you wish to
get the best performance possible out of
your astrophoto system. Not only does it
allow big enlargements to be made, but
also insures that the full sensitivity of
the film is used. Better focus means
stars are focused to a smaller area on
the film, allowing more chance for the
film to react to the light of dim
objects. Precise focusing can make the
difference between a good astrophoto and
The traditional focuser for normal
photography consists of using your eye to
judge an image projected onto a
ground-glass screen. This is inadequate
for astrophotography because of the low
light level, variability in quality
(and fatigue)of the photographer's eye,
and inadequate magnification to reach the
level of focus necessary to critically
focus a lens. The last point is
especially important with fast f-ratio
telescopes such as the Borg line.
Hence astro cameras generally use an
eye-independent precision method such as
the knife-edge or Ronchi screen
technique. In practice, this means a
precision focuser is supplied with each
camera which is calibrated to the film
plane for the camera. When focusing,
the camera body is removed and the
focuser is substituted. After focusing,
the camera is replaced for the shot.
Precision machining of the camera and
focuser components assure that this can
Can I use one focuser for several
cameras or film holders?
In general it is possible to do this with
the precision machined astro cameras.
Buying them at the same time as the
focuser or sending an older camera back
for matching at the time of purchase will
insure that this is the case.
For commercial camera systems such as the
Pentax 67, this is not generally possible
due to the manufacturing differences
between cameras. The best that can be
done is to set the focuser for a
Can I calibrate the focuser myself?
This can be done yourself but requires
the proper tools and care. It is best
done by using a micrometer to measure the
camera flange-to-film plane distance as
is done at Hutech (see
Focuser Calibration procedure).
Alternatively, the camera can first be
focused precisely (usually by placing
a user-supplied knife-edge at the film
position and focusing on a star), and
then finishing off by adjusting the
focuser to indicate focus at the same
The micrometer adjustment method,
however, will be more consistent in
assuring a good calibration as the second
method is subject to your ability to
initially achieve perfect camera focus,
which in turn depends on factors such as the
seeing at the time of calibration
and the quality of the knife-edge
focuser you use on the camera back.
How does the Planet Town Ronchi
The Ronchi focuser consists of closely
spaced alternating bars of clear area and
opaque area. Use of a very fine
screen (1250 lines per inch)
allows focusing by centering on a
point source, such as a bright
star, and then adjusting focus to minimize
the number of bars seen crossing the
image of the star. This works because
the Ronchi screen is set to be at the
exact focal plane of the camera, so when
the star's image is minimized (spanning the
minimal number of lines), the focus knob
is set to the best position.
If the optical system and seeing allow
for focusing a star smaller than the
width of a Ronchi screen bar, the opaque
area can be used as a traditional
knife-edge, or the exact focus position
can be interpolated.
How does the Mitsuboshi knife-edge
The Mitsuboshi knife-edge focuser works
on the principle that when a cleanly
defined edge at the camera's focal plane
cuts the focused image of a point source
(star), the image should wink out
instantaneously (assuming good seeing
The Mitsuboshi focuser makes focusing
easier by implementing the "knife-edge" as
a clear circular area in a metal film deposited
on glass. So no matter which direction
the target star is moved, it will
encounter an edge of the mask where it can be cut
off by the "knife-edge."
that this knife-edge circle is fixed at
the center of the field-of-view and so it
may not be suitable for you if you are
trying to focus off-center (e.g. trying
to compensate for a curved focus plane).
Can I use a Ronchi or Knife-edge focuser
with a field flattener or focal reducer?
Yes. These focusers can and should be
used with all optics in place which the
film will see through. If a dense filter
prevents seeing a star for focusing,
these may be temporarily replaced with a
clear filter of the same thickness.
Why does Hutech recommend buying
the camera and focuser as a set?
The camera and focuser must be set to
match each other precisely for reliable
focusing. When they are purchased
together, they can be precisely adjusted
before shipment (+/- 5 microns).
If you also know what type of film you
intend to use for the camera, it can also
be factored into the adjustment since
certain films (such as Tech Pan) are
thinner than other films such as color
slide film. If two different types of
film such Tech Pan and slide film will be
used, for perfect focus two separate
focusers are recommended especially for
fast f-ratio systems.
When purchasing a focuser for a camera
you already own, we recommend shipping
the camera to Hutech or sending the camera's
measurements to Hutech so that the focuser
may be calibrated precisely. Especially
commercial camera bodies such as the
popular Pentax 67 have too much
manufacturing variation for a focuser to
be set properly without having the measurements
for the particular camera body to be used
with the focuser.