Overview

Star trail images are usually generated by taking the maximum pixel value of multiple frames and combining them. The idea being that stars will usually be the brightest objects in the frame and you will be able to follow their path through the night.

indi-allsky also uses some additional measurements to exclude frames with too much brightness, sometimes caused by clouds or other conditions. Images when the sun is above -15 degrees and when the moon is directly overhead are usually excluded.

The key to producing good star trails is excluding images which have too much brightness.

Settings

Setting	Default	Explanation
Star Trails Max Sun Altitude	-15	Images are excluded when the sun is above this altitude. This prevents the bright sky glow from overtaking the trails during dusk and dawn.
Star Trails Exclude Moon Mode	True	Exclude images when moon mode is active
Custom Max Moon Altitude	-91	Custom moon mode altitude (91 is disabled)
Custom Max Moon Phase	101	Custom moon mode phase (101 is disabled)
Star Trails Use Existing Data	True	Re-use the ADU and Stars data from when the images were created. Disabling this will cause the ADU and Stars to be recalculated. ADU is fast, but star detection can take 1-3 seconds per image.
Star Trails Min Stars	0	The minimum number of stars need to include the image
Star Trails Max ADU	65	Images with an average ADU above this value are excluded. This value should be less than the the night time target ADU for the exposure scaling algorithm
Star Trails Mask Threshold ADU	190	When the average ADU is below the Max ADU, but the percentage of pixels is above this ADU, the image is excluded
Star Trails Pixel Threshold	1.0	Percentage of pixels used for the Mask Threshold
Star Trails Timelapse	True	Generate a star trails timelapse video
Star Trails Timelapse Minimum Frames	250	A video will only be generated when this number of frames are available for the timelapse

Mask Threshold ADU and Pixel Threshold

These settings are used in combination. When the percentage of Pixel Threshold pixels exceed the Mask Threshold ADU, the frame will be excluded from the star trails.

ADU or Stars

Stars are probably the best metric of detecting if an image should be included, but ADU is a much faster calculation. Setting the Minimum Stars to 0 will use the ADU value exclusively. Setting the ADU to a high value like 255 will effectively cause the star values to be used exclusively.

Reusing Existing ADU and Star data

When individual images are captured, an ADU and Star count is initially performed. These values are calculated using whatever Detection Mask or Region of Interest is set at time of creation. If you have a new mask, you may recalculate these values for the star trails by unchecking this setting.