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.  

Flat levels are measured with a sub-frame in the center of the imager during both the automatic exposure setting and when measuring the actual downloaded flat to minimize calculation time.  This is a new feature, starting with version 4.15.1.

When planning flats with a rotator, the Utility Window shows the Position Angles for your target(s) as a helpful reminder.

Flip-Flat support is integrated into CCDAutoPilot.  See Flip-Flat Notes.

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.    Since the description will ultimately be part of the data file names, it must not include the Windows "forbidden characters", '*/:<>?|'.  Such characters will be stripped from the target name if present. 
  • Rotation: This is the position angle at which the flat will be taken and consists of two parts, the angle and side of meridian.  If you don't have a rotator, these settings will be ignored.  You can enter the angle and then select from PA East (for flats corresponding to exposures east of the meridian or, PA West for flats corresponding to exposures west of the meridian.  If you want flats specified at a particular rotator angle, Select Rotator.  If you want no rotation, select None.I

    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
  • Series Order: When checked, flats will be acquired in the order they are entered on this page.  In this example, dusk flats for the blue filter will be acquired first.  The next series will not be initiated until the blue flats are acquired.  If series order is unchecked, each dusk flat planned will be tried to see it the ADU target can be met, regardless of the series order.  If you know your relative filter transparencies, you can take advantage of series order to increase your sky flat acquisition efficiency.  In the example shown, the blue flats are taken first at dusk, since that is the least transparent filter.  Then green, red and finally clear as the sky darkens.  At dawn, the flats are taken in reverse order starting with the clear flats and ending with the blue flats as the sky brightens.
  • Dusk Order: When checked, whatever order is entered for the flats will be inverted at Dawn.  Note that when this is checked, "Both" is automatically set for all series.  To use this option effectively, set your dusk flat order from least transparent filter to most transparent filter.  For example, Ha, B, G, R, L.  When your dawn flats are taken, the order will be reversed to L, R, G, B, Ha in this example.  This maximizes twilight use for the highest number of flats.
  • with Rotation: When checked, the dawn flats will be rotated from the dusk flats.  For example, if you select PA East for the dusk flats, then the dawn flats will be PA West and vice versa.  If you select Rotator for the dusk flats, the rotator will be moved by 180 degrees for dawn.  This, together with the Dusk Order option, will allow an appropriate set of flats to be acquired when using the stairstep method of data acquisition.
  • 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.
  • Readout Modes: Selects the readout mode to be used for flat frames.  See Readout Modes.  This selection is available only with Maxim version 5.

    Flip-Flat



    When the Flip-Flat option is enabled on the Settings page, a single setting is used for the Target ADU.  See Flip-Flat Notes for more details.


    Utility Window



    The utility window shows a listing of your target position angles as an aid in planning your flats.  Also shown is the estimated sky flat start time.