Contents - Index

Flat Frames

Get any group of professional astronomers together and sooner or later the subject will turn to flat fielding. Flat frames are essential if the faintest parts of a target are to be detected. The basic concept is to point the system to a uniformly illuminated light source and take an exposure. The resultant exposure will capture all of the imperfections in the system - vignetting, dust on the optical surfaces, off-axis optics and other disturbances. Various illuminated screens are normally used for a target. Or, the twilight sky itself can be used! There is a specific point in the twilight sky called the null point that has the most uniform lighting. When dawn or dusk flats are selected, the telescope is automatically slewed to this point at the appropriate time.
While some may avoid flat frames due to the assumed difficulty in acquiring good ones, CCDAutoPilot makes the acquisition of high quality flats very easy.  By pointing the telescope to the proper point in the twilight sky, a very uniform light source for flat fields is obtained.  The difficulty of dealing with the changing brightness of the twilight sky is resolved by the efficient auto exposure routine that dynamically adjusts the exposure to the changing conditions to meet the desired exposure target, expressed in ADU (Analog-Digital Units) or "counts" as it is commonly called.  Even with an artificial light source, such as a dome screen or light box, flats can be taken automatically.

Exposures
There are 8 series of exposures possible for your flat series and these can be obtained at dusk, at dawn or both.



For each series, the checkbox at the left of each series enables or disables that series.  When disabled, the series information is grayed out.  The remainder of the series information is:
  • Dawn/Dusk/Both: The specified series will be taken at the selected point in the day.
  • Number: This is the number of exposures that will be taken in this series before moving on to the next series.
  • Filter: This is the filter that will be used for this series.  Filter naming and selection will be according to the names and capabilities of your camera control program.
  • Binning: This is the degree of binning that will be used for this series.  Binning will be according to that reported by your camera control program and your specific camera.
  • Target ADU: This is the desired count level for your flats.  The level should be chosen to be in the linear range of your camera.  The linear region is 30-45% of the full count level.  For a 16-bit camera, typical levels are 20,000 to 30,000 ADU
  • Description: Here you can enter a specific description for the exposure.  If no description is entered, the file name for the exposure will be automatically generated as follows: <Filter>_Flat<Rotation>_<Target>_<sequence number>.fit.  <Filter> is the name of the filter for the series.  <Rotation> is the position angle for the flat.  <Target> is the target name, as defined in the target pulldown.  The sequence number is a 5-digit number that identifies one exposure from another.  If you enter a description, the file name will be <Description>_<Target>_<sequence number>.fit, where <Description> is the description entered.
  • PA: This is the position angle at which the flat will be taken.  If you have any field asymmetry due to camera rotation, best practices encourage taking a flat at all rotations.  Assume you have a target that you tracked across the meridian at a position angle of 120°.  If you wanted a flat to match target exposures from both sides of the meridian, you would enter 120e for one series and 120w for the other.  The two series of flats would be obtained with one rotated 180° from the other.  This assumes a rotator of course.  If you wanted flats at a specific rotator setting, just enter the number without 'e' or 'w' and the flat will be taken at that specific rotator setting.  Leaving this field blank results in no rotation and the flats will be taken wherever the rotator happens to be.

    Below each item in Series 1 is the button, which replicates the series 1 settings into all of the other series for easy editing.

    The need for separate flats for each filter and rotation depends on each situation.  Experimentation is needed to determine what kind of sensitivity your system (and you!) have to flat optimization.  CCDAutoPilot gives you all the tools and capabilities you need to get the flat frames of your choice automatically and with minimal work on your part.

    Options
  • Flat Frames Now Hitting this button immediately begins acquisition of the flat frames specified in the active series.  This is most useful when an artificial flat illumination source, such as a screen or light box is available.  Exposure will be automatically determined and flat acquisition will proceed unattended.
  • Tracking When Tracking Off is selected, tracking will be turned off when the telescope slews to the null point. If your telescope does not support turning tracking off, or you choose to take flats with tracking on, select Tracking On. When Tracking On is selected, as in dithering during image acquisition, the telescope will be moved appropriately between flat exposures. This becomes significant when the master flat is created. With tracking off, any stars that appear in the image are trailed and lower intensity. With tracking on, stars appear as point sources and are displaced from one flat exposure to the other. In that way, a median combine should effectively eliminate either the point source stars or the trailed star images.
  • Exposure Limits This sets the minimum and maximum exposure over which you will allow the auto-exposure function to operate.  Too short and the camera shutter movement itself will distort your flats.  Too long, and you may not get all the flats you need at dawn or dusk.  Recommended starting values are 2 seconds minimum and 30 seconds maximum for cameras with a mechanical shutter.  Shutterless cameras can go as short as .01 sec. for the minimum setting and .02 sec. for the maximum setting.
  • Null Point This is the point in the sky calculated for most uniform illumination, based on studies by professional astronomers.  It is recommended that the Automatic setting be used but if you wish to experiment, you can select Manual and enter your own azimuth and altitude settings.
  • Sun Elevation Here you can enter the sun elevation at which the taking of sky flats will begin. These settings should normally not need to be changed but it may be appropriate for some conditions. For example, if you want to take dusk flats through a narrow band filter which needs more light, you may set the dusk elevation at 1 or 2. Defaults are Dusk: 0.5, Dawn: -8.0.