Windows / Tabs > BadPixelMap / Dark Frame

The bad pixel map tab allows the user to create a bad pixel map which can be used to repair hot or cold pixel on acquired images. It also allows the creation of a dark frame image which can be used for dark frame subtraction. In general it is not recommended to use both dark frame subtraction as well as a bad pixel map at the same time. The tab has the following layout:

Creating a Dark Frame Image

Generally (probably due to changes in temperature which affects dark frame subtraction) I have found that using a bad pixel map works better for my setup than using dark frame subtraction. So using a bad pixel map is what I would recommend but if you find that dark frame subtraction works better for you, you can create one quickly by using the following steps:

•	Set the number of images and exposure time to be used

•	Cover the camera and place in a dark location to ensure that no external light can reach the sensor.

•	Click 'Create'

•	Once the images have been taken and the stacked dark frame has been created click the '(Save Dark Reference Image)' button

•	Click and select a file name and location and save the file

The saved image can now be set as the image to use for dark frame subtraction (see below for details).

Creating a 'Bad Pixel Map'

Creating a bad pixel map to save is fairly straightforward. The steps are as follows:

•	First set the number of images and exposure time to use, to capture the images to be used for the bad pixel map. This might take some experimenting but generally the default settings work reasonably well.

•	Cover the camera and place in a dark location to ensure that no external light can reach the sensor.

•	Click 'Create' - the images will now be taken and once acquired, the dark reference frame will be created

•	(Alternatively, instead of capturing new images, you can also load an old reference fits file by clicking the '(Load Dark Reference Image) button.

•	Now set the Cold and Hot Pixel thresholds (see below for details) and click the 'Evaluate Bad Pixel' button

•	Once the evaluation is complete a message below the button will show the number of detected cold and hot pixel

•	Keep adjusting the cold and hot thresholds until you are happy with the result. You can use the image preview on the right (see below for details) to help in deciding on good thresholds.

•	Click the 'Save Bad Pixel Map' button to save the file to a location of your choice

You can now use the saved file as the bad pixel map file to be used in the settings (see below for details).

Cold / Hot Pixel Thresholds

The thresholds basically determine how much dimmer/brighter (compared to the surrounding pixel) a pixel must be to be considered as a cold or hot pixel. The thresholds are measured in 'standard deviations' (a measure of how much pixels in general differ from the image mean value) but that's not really of great importance. The important bit is that if you lower the threshold, then more pixel will be included in the bad pixel map and vice versa.

Depending on the camera used, good settings can vary quite a bit so it's best to take a 'trial and error' approach to find suitable thresholds.To do this click the 'Evaluate Bad Pixel' button which will give you an indication of how many cold/hot pixels have been found. This should give you a good idea wether the thresholds need adjusting or not. If there are no or very few hot pixel detected for example, lower the hot pixel threshold or conversely if a large number has been detected raise the hot pixel threshold to reduce. More importantly though you can use the preview image on the right to display the detected hot and cold pixel on top of the created 'stacked dark frame' image. By toggling 'Show Hot/Cold Pixel' and 'Show Image' you can alternately see where the bad pixels have been detected and what the image looks underneath. Most likely you will need to adjust the 'Stretch' setting to see how bad the hot pixel are.

Preview Controls

The preview section allows the user to display an image showing:

•	The detected hot and cold pixels shown in red and cyan

•	A stacked 'average' image created from acquired images

The following controls are available to manipulate the preview:

•	'Show / Update Preview' button: Updates the preview image with any changes made to the controls

•	Clip: The black point where the preview image should be clipped

•	Stretch: The stretch 'midpoint'. A value of 0.5 shows the original image. Any value below 0.5 will stretch the image

•	Show Hot/Cold Pixel: If ticked detected hot/cold pixel will be shown in red/cyan on the preview image

•	Show Image: If ticked the stacked image will be shown

•	Zoom: The preview image can be zoomed by holding the shift key and using the mouse wheel or touchpad scroll

•	Pan: The preview image can be panned by holding the control key and dragging the image

•	Clicking the 'Four pointed Arrow' symbol located at the bottom right of the preview image gives access to a view finder and additional center and zoom controls

Settings

Once the dark frame or bad pixel map file has been saved it can then be set in the application settings to apply it to acquired images: