Create Master Dark
The Create Master Dark command combines raw dark frames to create a master dark frame having higher signal to noise ratio than any of the raw frames. A bias correction may also be applied to the individual darks before they are combined. The dark frame accrues thermal signal between the time the detector is flushed at the beginning of an exposure and the time the detector is read out at the end of the exposure. Sophisticated camera controllers record this using a special keyword, such as DARKTIME. A good approximation is to use the exposure time stored in the EXPTIME keyword. However, using EXPTIME will give the wrong amount of thermal integration time in situations such as having is a dwell time between flushing and the shutter opening or the shutter being closed and reopened during the exposure.
The Create Master Dark command opens from
the Image Window > Process >
Calibration menu and 
button
on the Main Toolbar. The "immediate"
form, shown first, opens when an image or image set is displayed in
the top-most window. The general form can be used to select source
images from folders, windows, or by matching templates.
|
Create Master Dark Properties |
|
|
Profile [|>] |
Manages the parameter collection used by this dialog. See Profile Control for details. |
|
Select |
Opens the Select Source Images dialog for selecting source images to be processed into the final calibrated result. The name of the first selected file or displayed image is listed in the field to the right. |
|
Name of the Master Dark to be created |
Enter a name for the master dark being created or click [...] to browse files to choose a name.. |
|
Create in subfolder: |
Enter the name of the subfolder where the Master Dark will be created. This is a subfolder of the folder containing the first raw Dark frame in the source image list. |
|
|
Check this box to overwrite an existing file having the same file name in the output subfolder. If this box is not checked and a file of the same name exists, an underscore and number will be appended to the output file. For example, if MasterDark.fts exists and this box is not checked, then the output file will be named MasterDark_2.fts |
|
Combining Strategy... |
Click this button to set properties for combining the dark frames. |
|
Selection Criteria... |
Click this button to set properties for selecting raw dark frames based on their range in dark exposure. This allows Mira to use only raw darks that are statistically valid by having a minimum exposure but not saturated beyond a maximum exposure. |
|
Bias Correction... |
Click this button to open the Bias Correction Properties dialog for applying a bias correction to the raw dark frames. |
|
|
Check this box to display the resulting Master Dark. |
|
|
Check this box to list verbose processing messages in a Text Editor window. If the window already exists from previous processing by this or another calibration command, then its contents are appended. |
|
Log processing to: |
The drop list provides options for listing messages: --None -- lists no processing messages This window lists processing messages in the "Calibration Message Log" text area at the bottom of the dialog. Editor lists processing messages in a Text Editor window named "Calibration Message Log". |
|
Notification: |
Select the type of notification Mira issue when processing is finished: Do nothing Quietly ends the processing without notification. Beep creates a notification sound, Message displays a notification message in a top level dialog that locks out Mira use until you dismiss the message. |
The [Selection Criteria...] button opens a dialog for specifying how raw dark frames are selected and processed. In this dialog you specify the image keyword holding the thermal integration time (or exposure time) and the range of time allowed for raw dark frames to be including in creating the master dark.
|
Dark Frame Selection Criteria Properties |
|
|
Keyword for Dark Time |
Enter the name of the keyword that holds the thermal integration time for the image. This might be DARKTIME or EXPTIME, as described in the introduction, above. |
|
Filter Keyword |
Specify the name of the filter keyword in the raw flat frames. When Filter Selection Criteria options 2 and 3 are selected, this property specifies the name of the filter keyword. Typically, this is the FITS format standard FILTER, although any keyword may be used. |
|
|
Check this box to select raw dark frames based on the value of the specified keyword. Enter the minimum and maximum acceptable time, measured in seconds.. |
|
Min Value |
Specify the minimum acceptable dark integration time. |
|
Max Value |
Specify the maximum acceptable dark integration time. |
Unlike the other Create commands such as Create Pixel Flat, the Create Master Dark provides no combining options. The individual darks are combined as the Sum without normalization. The EXPTIME (exposure time) and DARKTIME (thermal integration time) keywords are also summed to preserve the dark current (count per second).
If you wish to cancel the task before it completes, either click the [Cancel] button on the Progress window or press the Esc key.
|
Tip |
To abort the calibration before it is finished, click the [Cancel] button on the progress window or press the Esc key. Mira will prompt you for confirmation to quit. If you confirm, the task will terminate immediately. |
If the Messages option is selected for the List results in option, Mira logs processing messages in the Results box at the bottom of the dialog. Messages appear after the processing finishes. TheResults field has a right-click menu as shown below. Using this menu you can save messages to a text file and copy from the box to the Windows clipboard or a Mira Text Editor. Only selected (highlighted) text is copied. Commands for editing and copying the text are available in a pop-up menu opened by right-clicking inside the results box. The menu is shown below. You can also elect to save the messages to a Text Editor which offers similar functionality as well as printing capability.
To prevent accidental deletion of calibration messages, you cannot clear text simply by highlighting and deleting the text using the mouse. To clear messages, you must use the Clear command in the pop-up menu.
Calibrating Images, Create Master Bias, Create Master Flat, Create Illumination Flat, Create Pixel Flat, Select Source Images, Express Image Calibration, Combine Files, Combine Image Set, Bias Correction Properties
Overwrite existing
files