Hutech Astronomical Products
Equipment for the Serious Amateur Astronomer


Planet Town Astro Cameras

Mitsuboshi Astro Cameras

Astro Camera Frequently Asked Questions (FAQ)

What does the vacuum do for the camera?

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 outstanding astrophoto.

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 be achieved.

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 compromise position.

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 Mitsuboshi Knife-edge 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 position.

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 focuser work?

The Ronchi focuser consists of closely spaced alternating bars of clear area and opaque area. Use of a very fine metal-on-glass Ronchi 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 focuser work?

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 conditions).

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." Please note 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.

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