Defines when you want to be notified that there is need to refocus. When your telescope or lens is focused, click on the button Set Focus Mark in Tools Tab to set focus mark on the temperature that focus was achieved. For example if 4° is selected, when APT detects change of 2° (half of the defined value), the temperature reading in top of the APT main screen will start flashing to show that soon there will be need to refocus (in minimized mode there will be flashing “F” letter). When APT detects change of 4°, the temperature will be displayed with bright font and message will be printed in the Log panel than there is need to refocus (in minimized mode there will be non-flashing “F” letter). The value 0° disables this feature. Required is external Temperature / Humidity Sensor.
Tips & Tricks. Don’t forget to click on the “Set Focus Mark” button every time you focused to place the “Focus Mark” on the right temperature reading.
Defines when you want to be notified for closing the dew point. There is no need to heat your optics when the dew point is not even close. For example if 4° is selected, when APT detects that dew point is on 2° or less from the current temperature, the DewP reading in top of APT main screen will start flashing (in minimized mode there will be flashing “D” letter). When APT detects that temperature dropped under the dew point, the DewP reading will displayed with bright font and message will be printed in the Log panel (in minimized mode there will be non-flashing “D” letter). The value 0° disables this feature. Required is external Temperature / Humidity Sensor that is able to measure the humidity.
Tips & Tricks.To get more accurate readings, place the sensor close the objective, but not on the heating strips.
You have to experiment when you have to start the heating. It depends on your heating solution, telescope size and etc.
Instructs APT that there is no additional external sensor. APT will still use the readings from the ASCOM / INDIGO / INDIcompatible focuser if they are available.
Defines if you want to use the Sky Quality Tracking system in APT. The sky quality tracking can be made automatically by using the Unihedron SQM or compatible devices and manually by observer's estimation of the limiting magnitude or by using the SQM-L manual device. More information about supported devices see Sky Quality Meters
When the Sky Quality Tracking system is enabled you can use Graphs to monitor the value changes through the imaging session, to use the readings as part of the image file name - see Name Parts in Main Settings tab. Also the readings will be stored in the FITS files.
Defines MPSAS (magnitudes per square arc second) as default sky quality metrics to use in Status panel, for the value entered in Sky Quality Calculator when SQM-L or manual entry is selected and for storing in FITS files.
Defines NELM (naked eye limiting magnitude) as default sky quality metrics to use in Status panel, for the value entered in Sky Quality Calculator when SQM-L or manual entry is selected and for storing in FITS files.
Defines that you want to use your estimation by eye, SQM-L or compatible device to measure and track the sky quality. After that you have to use theSky Quality Calculator enter the value you have determined.
Defines that you want to use SQM-L, SQM-LU-DL or compatible device to measure and track the sky quality.
When this option is selected there is need to define how often to read the sky quality value and to which port is connected the device in the section that will become visible.
If you are using mySQM or other device that needs slower COM port speed, enable the option "Slow COM Port"
Defines that you want to use SQM-LE or compatible device to measure and track the sky quality.
When this option is selected there is need to define how often to read the sky quality value and the IP/Port of the device in the section that will become visible.
The APT Settings dialog box is accessible from 
Related videos: 
