Dark Frames and Bad-pixel Maps


Introduction

Cameras used for guiding are typically not temperature-regulated and may produce images that appear quite noisy.  As a result, guide exposures frequently show obvious defects in the form of  hot ("stuck") pixels or regions with spurious brightness levels.  If there are too many of these defects, you may have trouble identifying and selecting a good guide star - trying to calibrate on a hot pixel is a common problem for beginners.  Even after guiding has begun, a spurious hot pixel close to the guide star can disrupt the calculations needed for smooth guiding and may cause the software to "jump" between the real star and the hot pixel.  These sorts of problems can be mitigated by using either of two approaches in PHD2: dark frames and bad pixel maps.  All functions related to dark frames and bad-pixel maps are located under the top-level 'Darks' menu.

  

Dark Frames

PHD2 will build and use a library of dark frames that match the range of exposures you use for guiding.  Once the library is built, it will be saved automatically and will be available for use across multiple PHD2 sessions.  As a result, you can spend a modest amount of time to build a good dark library, then use that library for an extended period of time.  Once you have connected to your camera, you can build a dark library from the 'Dark Library...' item under the top-level 'Darks' menu.  That will start a dialog that looks like this:



You use the two controls at the top to specify the minimum and maximum exposure times that will be used to acquire dark frames.  The starting, ending, and intermediate values match the exposure times used in the main PHD2 window, so you can acquire dark frames that will match any exposure time you choose for guiding.  The third control specifies the number of dark frames that will be acquired and averaged for each exposure time.  The averaged image is referred to as a "master dark frame."  Historically, PHD has used 5 dark frames for this purpose, but you may want to increase that number to improve the quality of the master dark frame.  You can also add a note or comment if you wish - this will be embedded in the header of the master dark frames for later reference.  

The two radio buttons above the Notes field let you specify whether you want to modify/extend your current dark library or build a new library from scratch.  If you've gotten alert messages saying the dark library must be rebuilt, you should choose the 'Create entirely new dark library' option.  This insures that all of the master dark images match the format of the camera you're currently using.  Otherwise, you can simply refresh or expand the current dark library by taking new dark frames at the specified exposure times

One you've set your parameters, click on 'Start' to begin the process.  If your guide camera does not have a shutter - most do not - you'll be prompted by PHD2 to cover the guide scope.  To get the best results, be sure there is no light leakage into the guide camera - doing this in daylight is not likely to work well.  PHD2 will systematically work through the range of exposure times you've chosen, taking the specified number of frames for each exposure time.  Progress will be displayed on the status bar at the bottom of the window, so you can see where you are in the overall process.  Once you've started the process, the 'Cancel' button above will change to a 'Stop' button.  You can click on this if something goes wrong or you want to change the parameters before the entire sequence completes.  Stopping in this way will discard whatever data has already been collected, so you'll need to make your corrections and then restart the process.   Once all the frames have been collected, PHD2 will compute the master dark frames, store them in a dark library data file, then show a message box summarizing the results.  If your camera has no shutter, you'll also be prompted to uncover the guide scope so you can return to normal imaging.

Once your dark library has been built, you control its use by the 'Use Dark Library'  item under the 'Darks' menu.  The checkbox on the menu item will toggle on or off each time you click on it.  The setting of the item is retained across program executions, so if you choose to leave the menu item checked, PHD2 will automatically load the dark library and resume its use the next time you run the application.  The dark library itself is retained on disk until you build a new library, so you can freely change the setting on the 'Use Dark Library' menu item without loss of any data.  If you are using a dark library and there is no master dark frame that exactly matches your guide exposure time, PHD2 will use the nearest fit.  However, you are encouraged to obtain matching master dark frames for best results.  If you have a dark library that has missing exposure times, you can simply acquire the missing data and it will be added to the existing dark library - there is no need to start over.  By changing the setting of the 'Use Dark Library' menu item, you'll be able to see the effect of using the dark library and determine whether your guider images are sufficiently improved.

Remember that a dark library is associated with a particular camera.  PHD2 will check to be sure that the dark library matches the camera you are currently using.  If it does not, you will see an alert message telling you the dark library can't be used and must be rebuilt.  This can happen when you've changed cameras inside an existing equipment profile, something you shouldn't do unless you have upgraded your guide camera and have no plan to revert to use of the old camera.

Bad-pixel Maps (Defect Maps)

For some guide cameras, dark frames don't do an adequate job of removing the defective pixels that are visible in the guide frame.  In those situations, you can probably get better results by building and using a bad-pixel map.  This approach directly measures and compensates for specific areas of the sensor that produce false signal (hot/stuck pixels) or don't respond correctly to incoming light (cold pixels).  Such a "map" is created by taking a sequence of comparatively long dark exposures (e.g. 15 seconds), averaging them, then statistically analyzing the resultant frame to identify the locations of defective pixels.  These pixel locations are saved for future use.  During normal guiding, each of these pixel locations on the guide image is replaced by a statistical sample of the surrounding pixels, thus eliminating all or most of the effect of the "bad" pixel.  The final result is usually an image with a smoother background and fewer obvious defects.  For any defects that remain, PHD2 also provides a way for you to manually click on bad pixel locations and add them to the map.  This entire process of obtaining and analyzing dark frames is handled for you by PHD2, so it's easy to build a bad-pixel map.

Building a bad-pixel map is done by clicking on the 'Bad Pixel Map...' item under the top-level 'Darks' menu.  If you are doing this for the first time, you will be prompted to obtain a sequence of dark frames for analyzing your camera sensor and building the map:

Defect_Darks

This is a slightly different version of the dialog used for obtaining dark frames, described in the previous section.  Because the analysis is based on statistics, you should use relatively long exposure times (> 10 sec) and at least 10 frames.  Since the bad-pixel map can be re-used for fairly long time periods, you won't have to repeat this operation very often, and it's worth spending some time to get higher quality data.

Once the dark frames have been captured, PHD2 will compute the statistics and identify an initial set of defective or suspect pixel locations.  After a short delay, you'll then see a dialog that looks something like this:

Bad-pixel Map

The 'General Information' section shows a summary of the statistics computed by PHD2 during the identification of bad pixel locations.  Normally, you won't need to look at these, and you can hide this portion of the display by clearing the 'Show Master Dark Details' checkbox.  The "Results" group shows the counts for hot and cold pixels based on the current settings of the two "Aggressivness" sliders below them.  If you're doing this for the first time, the aggressiveness sliders will be set at their default values, 75 within the range of 0 to 100.  You'll need to experiment or make some judgment about whether the counts look reasonable based on what you see on your normal guide frames.  If you adjust the aggressiveness sliders left and right, you'll see the hot and cold pixel counts change.  The sliders control how "aggressive" PHD2 should be in identifying suspect pixels and flagging them as being defective - so higher aggressiveness settings will result in higher pixel counts.   Once the settings are where you want them, click on the 'Generate' button to compute and load the new defect map.  

At this point, you'll probably want to examine the results.  The main window of PHD2 is still active, so you can take a normal guide exposure to see how things look.  If you want to quickly see the result of using the defect map, just toggle the 'Use Bad-pixel Map' menu item under the 'Darks' menu.   Keep in mind that you don't need to achieve a perfectly smooth, black background in the guider image - you just need to have a sufficiently small number of remaining hot/cold pixels that neither you nor the PHD2 guiding algorithms will mistake a bad pixel for a star.  If you over-correct with very aggressive settings, you may create so many bad pixel areas that they interfere with detection of usable guide stars.  It's easy to make adjustments with the sliders  - just change the slider settings, click on 'Generate' again, and look at the results in the main PHD2 window.

You may find  this approach still leaves some hot pixels that you'd like to eliminate.   Since the default approach relies on statistics and needs to apply to a wide range of cameras, it isn't a "fire-and-forget" operation - you will often need to fine-tune it using the steps below.

Step-by-Step Guide to Refining a Bad-pixel Map

The following steps are recommended for refining a bad-pixel map to control pixel-level artifacts in your camera:
  1. Cover the guide scope and start looping 5-second exposures
  2. Open the Refine Bad-pixel Map window (Menu/Darks/Bad-pixel Map), then drag it to the side of your screen so you can see both the BPM and guiding windows
  3. Adjust the gamma slider in the main window until you can see the hot pixels - this may require a brighter image than you are accustomed to seeing
  4. Select the option "Show defect pixels."  With the box checked, red dots will appear for any hot pixels that are already known.  
  5. Slowly drag the hot-pixels aggressiveness slider left and right until most of the hot pixels are covered by a red dot, with a much smaller number (or even zero) hot pixels not covered.  Click the 'Generate' button
  6. Now pick up the remaining hot pixels by manually adding them to the bad-pixel map
   
Once your bad-pixel map has been built, you control its use by the 'Use Bad-pixel Map'  item under the 'Darks' menu.  This setting is retained across program executions, so leaving it checked will tell PHD2 to automatically load the defect map and use it for all guide exposures.  The settings for 'Use Dark Library' and 'Use Bad-pixel Map' are mutually exclusive - you can use one or neither, but not both at the same time.  As with the dark library, the bad-pixel map data file is stored permanently, so you can disable its use without losing any data.  Both of these data structures can be used for extended time periods, but it's worth remembering that camera sensors do change over time.  As a result, you may want to rebuild the dark library or bad-pixel maps at periodic intervals or when you start to see a degradation in the appearance of your normal guide images.  In these cases, it is also advisable to click on the checkbox for 'Rebuild Master Dark Frame', which will tell PHD2 to reacquire the underlying dark frames and recompute a baseline bad-pixel map.  You'll then need to refine the map as you did before until you're happy with the results.  There is no reason you should need to interact with either the dark libarary or bad-pixel map data files, but you can find them located in the 'AppData\Local' logical directory used by your operating system.

Like dark libraries, bad-pixel maps are associated with a particular camera.  PHD2 will check to be sure that the bad-pixel map matches the camera you are currently using.  If it does not, you will see an alert message telling you the bad-pixel map can't be used and must be rebuilt.  This can happen when you've changed cameras or binning factors inside an existing equipment profile, something you shouldn't do unless you have no need for the old settings.  

Reusing Dark Frames and Bad-pixel Maps

If you're using the same camera in multiple profiles, you may want to re-use the dark libraries or bad-pixel maps you built for that camera.  This can be accomplished by importing the camera-related data files into a profile that doesn't already have those files.  For example, suppose you built an original profile - call it Profile1 - that uses your Lodestar guide camera, and you built both a dark library and bad-pixel map for it. Some time later, you create a new profile, Profile2, that has different mount or focal length properties but still uses the original Lodestar camera.  In that case, you would connect your gear using Profile2, then use the 'Import From Profile...' menu item under the top-level 'Darks' menu.  You would select Profile1 as the source of the import function for the dark library, bad-pixel map, or both.  You will be shown only those profiles that have a camera with compatible sensor geometry (size and pixel size).  Clicking on 'Ok' will copy the dark/bad-pixel map files and will associated them with your new profile, Profile2.  Since they are copies, changing the data files in one profile will not affect other profiles.  Keeping them synchronized, if that is what you want to do, will require a subsequent 'import' operation.