Contents - Index


Sub-Exposure Calculator

As discussed in Imaging Strategies, characterization of the imaging camera and site sky glow can aid in determining sub-exposure duration.  The Sub-exposure calculator consists of two parts, Measurement and Analysis.


First, the camera read noise and gain must be measured.  Many vendors supply values for these terms but it is good to measure your specific camera.  The read noise measurement requires no special setup but the gain measurement does.  For the gain measurement, you will be asked to somewhat uniformly illuminate your imaging camera to approximately 20,000 ADU by adjusting the exposure to reach that level.  If you are not prepared to do that, you can accept the camera manufacturer's value and proceed.

Before measuring the camera, select a binning and a filter.  I suggest your first measurement be at 1x1 and you use a clear filter.  Hit the Measure Camera button.  You will be prompted through the steps for the measurement.  When complete, there will be an entry for Read Noise and, if measured, Gain.  In theory, read noise should be unchanged when binning, if the binning is done in hardware and there are no other noise sources introduced by the binning process.  With some cameras, this is not the case.

The next step is to measure your sky flux.  Try an exposure of 180 to 300 sec., depending on your ambient light.  This measurement will take some time as a light exposure and an auto-dark exposure will be taken.  Once the exposures are complete, the Sky Flux will be displayed at e/sec.  You can also determine this manually by using a calibrated previous exposure through the filter of interest.  Simply measure the average ADU level of an area free of stars.  The sky flux is simply that average ADU times the gain as measured, divided by the exposure time in seconds.

Gain, read noise and sky flux are the key ingredients necessary to proceed to the Analyze box.


Select the contribution to total noise you wish to assign to the camera read noise.  This is a percentage of the noise that will be contributed by the sky flux.  The underlying concept is to expose long enough so that the noise from the sky flux overwhelms the read noise such that the read noise contributes the indicated percentage to the total noise.  A good starting point is 5%.  Entering a Read Noise Contribution value automatically calculates the minimum sub-exposure duration in sec.  For more details and analysis, see my paper on Sub-Exposure Times and Signal-to-Noise Considerations.

Next, choose your image sensor.  If necessary, consult your camera documentation or vendor for your sensor type.  If your sensor is not available, you can add it if you have the necessary data.  See here for the file structure.  Enter your planned sub-exposure time.  Select your Sensor temperature, Dark Noise contribution and the number of sub-exposures in your stack to get the number of mean combined darks you need.  (Because any master dark is subtracted from each sub-exposure, it is correlated.  This means any noise that is in the master dark eventually will appear if the stack is deep enough, just as faint details appear when the stack size is larger.)  Experiment with changing your Sensor Temperature and Dark Noise Contribution.  As you further cool your camera, you will see the number of darks needed slowly decreases.  Now warm up the camera.  The number of darks will increase slowly at first then faster.  You want to be in the area where it increases slowly if possible. For more details and analysis see my paper on Sub-Exposure Times and Dark Frames.

Compare Stare mode to Dithered mode.  "Stare" means undithered.  You can readily see the impact of dithering on the required number of darks.

It should be mentioned that the number of darks is based on a min./max clip combine. You would normally use this combine method to eliminate cosmic ray hits and similar random artifacts.

Caveat: This is a bit of a simplification of a very complex topic.  Narrow band imaging, extremely dark skies, etc. will mandate much colder camera operation since the sub-exposures will become longer.  Nevertheless, this is a good starting point for determining a starting point for your sub-exposures, camera operating temperature and number of darks.

Test Buttons

These buttons cause the indicated actions to be performed.  They may be used to test communications to the ultimate hardware through the various software layers before committing to an automated session.  It is recommended these buttons be used whenever hardware changes or a suspicion of things not going right arises.

IMPORTANT: CCDAutoPilot can only send the Tracking Off and Park command to your telescope control program. It is your responsibility to verify that your mount behaves properly when it receives these commands. Please take advantage of the test buttons to verify proper communications with your mount and dome. Hit these buttons one at a time to test whether your chosen telescope control program turns off tracking or parks the mount. If you don't get the expected results, consult the manufacturer of your telescope control program for support. CCDAutoPilot sends standard commands for tracking off and park in accordance with ASCOM and TheSky's defined interfaces.  Watch the status panel below the buttons for reports of activities.
  • Tracking On: This button should turn the mount tracking on.
  • Tracking Off: This button should turn the mount tracking off. 
  • Park: This button should send the mount to its park position.  With most telescope control programs and/or mounts, this park position must be pre-defined by the user.
  • Close Dome: This button should close the dome shutter.
  • Open Dome: This button should open the dome shutter.
  • Move Dome To: The current dome azimuth is reported to the right.  Change the value, hit Move Dome To and confirm the dome does indeed rotate to the entered position.  
  • Move Rotator To: The current rotator position is reported to the right.  Change the value, hit Move Rotator To and confirm the rotator does indeed move to the entered position.
  • Guider Alarm: This gives an example of the alarm that sounds when guiding fails and the AGRS cannot restore it.
  • Alarm Off: This turns off the alarm.

    Guide Calculator

    This calculator allows you to determine your optimum minimum and maximum move parameters for your system.  Proper setting of these parameters is important for optimal guiding.  When connected to your camera control program, the guide calculator will automatically determine whether DirectGuide (CCDSoft only) or camera relays is in use and calculate minimum and maximum recommended moves automatically.  These parameters can be loaded into your camera control program.  For more information, see the online Guide Calculator.
  • Max. Allowable P-P Error: Here we try to determine when we want your mount to actually make a correction. If your guider error is only .05" arc-seconds, there is no need to make an correction. If your min move setting is set to low, then you guider relays will move the mount every guider cycle. This can result in mount oscillations or chasing atmospheric turbulence. My best suggestion in setting this value is about 75% the image scale in which you are imaging. For example, if you are imaging at 1.2" asp, then set this value to 0.9" asp. This way a guider correction is only sent when the centroid of the guide star is 0.45" asp off in either the X or Y axis from the selected guide star position.
  • Max. Allowable Movement: This value is not required to achieve great autoguiding, but it does protect us from some extreme situations that could ruin a sub-exposure. For instance, if a cosmic ray hit is sensed on the autoguider CCD, this could cause your autoguiding software to think that the centroid of the star has moved many, many pixels from center. Therefore it is going to attempt to correct the mount for this error. If you have no max move setting, this will result in a very large correction and a ruined exposure. If you have a limit to the size of the correction, i.e. max move, then this effect can be mitigated by not allowing a large correction. Setting this value too low may result in under correction, so its best to set this value about 2X - 4X larger than the above 'Peak to Peak Maximum Allowable Error' setting.
  • Suggest: Hitting this button will cause recommended settings to be entered for the above two values and represents a good starting point.  If you are unsure of what to enter, hit the Suggest button.
  • Recommended Min. Move: Based on the above entries, this is the recommended minimum move amount for your guider. 
  • Recommended Max. Move: Based on the above entries, this is the recommended maximum move amount for your guider
  • Aggressiveness: With the recommended minimum and maximum move, this is the recommended aggressiveness setting for your guider
  • Apply To <guider>: Hitting this button will automatically enter the recommended values into your camera control program.  The button name will indicate your camera control program (CCDSoft or Maxim) and the corresponding guide method (Relays or DirectGuide for CCDSoft, Relays or MicroGuide for Maxim).