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The Plate Camera Control Panel contains the controls used for the plate camera.
Description
The plate camera is located
immediately underneath the viewing screen (the magnification
ratio between main screen and plate camera is 0.885). On the
bottom of the projection chamber a guide is mounted along which
the tray containing the negative in its film holder will be slid
forward, out of the cassette, and backward after the exposure.
The complete exposure sequence contains the following steps:
The microscope will automatically restore the main screen to the position it had before the exposure and also restore the external shutter control to its previous setting.
Combining plate camera and CCD
The shutter control on the Tecnai microscope differs from that of
the CM series (because of occasional problems there in case
plate-camera use was combined with a slow-scan CCD camera; unless
one was careful the CCD camera would keep the shutter closed
during plate-camera exposure, resulting in blank negatives). On
the Tecnai microscopes the shutter control is not serial. The
shutter is controlled either by the plate camera or by the CCD,
with the switch under software control. Before a plate-camera
exposure is taken, the microscope checks the status of the
shutter control (internal = plate camera; external = CCD) and
automatically switches the external control off. The status of
the shutter control of the CCD is thus not important for
plate-camera use.
Warning: The previous note does not apply when the CCD shutter is using the beam blanker (alternate shutter). If that is the case, the user must ensure that the CCD does not close the shutter (i.e. blanks the beam) when the viewing screen is raised.
Note: Because of the construction of the camera, the negative will lie exposed to any light coming in through the windows of the projection chamber. Before taking any exposure, make sure:
The Plate camera control panel has a flap-out with Settings, Options and Stock.
The exposure time is measured continuously when the main viewing screen is down. The time is based on a conversion of the screen current coming from the main screen or the focusing screen, via the emulsion setting, to an exposure time. When the focusing (small) screen is in, the measurement is automatically adjusted for the smaller area of the focusing screen. If the image intensity is reasonably uniform across the whole viewing screen, the exposure time measured on the large screen, with the small, focusing screen out, can be used to determine the exposure time for the plate camera. If the image intensity is not uniform (strong differences, for example because part of the image is covered by grid bars), the exposure time from the small screen should be taken.
Note 1: When the small-screen exposure time must be used, do not move the small screen back by hand (the exposure measurement will immediately jump back to the main screen). Lift the main viewing screen (the small screen will be moved out automatically) or start the exposure itself by pressing the Exposure button on the left-hand Control pad. Alternatively, note the exposure time as measured with the small screen and set that as the manual exposure time.
Note 2: As a rough guide for diffraction patterns, 1/3 of the exposure time measured (and then set as manual time) is often a good value. Exposures of diffraction patterns should not be taken for very short exposure times. In general diffraction patterns (especially Selected-Area diffraction patterns) have very intense spots and because the microscope shutter is not instantaneously off, short exposure times will lead to spots displaying a (curved) tail. In general diffraction pattern exposure times should be 5 seconds or more.
From the exposure time it is possible to get an estimate of the beam current by making sure the beam is not (partly) off the viewing screen, then using the following formula:
Beam current (in nanoAmps) = 2.15 x emulsion setting / exposure time
If Auto is checked, the plate camera will use the exposure time as measured. If Hold is on, the microscope will not adjust the exposure time method to the mode defaults (automatic in image, manual in diffraction) but keep it as set.
The plate-camera exposure time method used by the microscope depends on the mode (automatic in image, manual in diffraction). If the user selects a method that deviates from the default, the microscope will use the method set. If Hold is not checked, this new settings applies only to the next exposure. Afterwards the microscope will reset to the default method. To keep the method as selected by the user for more than one exposure, check the Hold option. If Hold is on, the method will stay as set until the user changes the selection or switches Hold off.
If Manual is checked, the plate camera will use the exposure time set with the spin-edit control. If Hold is on, the microscope will not adjust the exposure time method to the mode defaults (automatic in image, manual in diffraction) but keep it as set.
The stock value displays how many exposures are still available in the plate-camera magazine.
The exposure number is a six-character code that is printed on
the negative. The first character can be a letter (A..Z) or a
number (0..9), the remaining five are a number that goes from
00000 to 99999. When the number exceeds 99999, the number
automatically switches to the next character up for the first one
(9 goes to A, Z goes back to 0).
The exposure number is updated immediately after the exposure has
been taken. The number displayed is therefore the number that
will be printed on the next negative.
The plate labeling system of the plate camera prints a number of settings on each negative. The magnification and plate number are always printed. Other values such as mm marker, date and user code are optional, set under Plate camera options. These values take up the two top lines of the plate labeling system. Underneath is room for four lines of 24 characters each, available for user comments. These four lines are defined in the edit control underneath the film text label. The size of the edit control is set automatically so that no more than 24 characters fit on a line (hence also the non-proportional Courier font used). New words that do not fit at the end of a line will automatically be wrapped to the next line.
Note: The film text is set only when the Enter button to the right of the edit control is pressed. Once the film text has been updated, the Enter button becomes disabled until the text in the edit control is changed. The status of the Enter button thus indicates the status of the film text. Only when the button is disabled, is the film text up-to-date.
Pressing the arrow button displays the flap-out containing the Options, Stock and Settings control panels.
Page last modified on 11/13/00
