Lua/Scripts: Fixed Exposure Intervalometer

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fixedint.lua - An astrophotography oriented script for long exposures + fixed exposure intervalometer


This script shoots one or more shots at a constant exposure. It is primarily intended for astrophotography, either single long exposure shorts, sequences for "stacking", or timelapse. It is distantly related to Meteor_Intervalometer_with_Dark_Frame_Management, but is optimized for the authors particular workflow.


Download version 1.2: (note: The zip originally uploaded on Jan 3 2016 contained the wrong files. Re-uploaded on Jan 5)

Discussion / Bug reporting thread:

Change HistoryEdit

Version 1.2Edit

Fixed the interval being ignored and "sleep" not being logged

Version 1.1Edit

Initial release

General usage notesEdit

This script is requires CHDK 1.4 or later. Options are described in detail in the options section below.

Set focus to infinity in the Canon firmware or SD overrides before shooting.

Picture review and flash should be turned off in Canon settings.

A tripod is recommended. For small P&S, $5 minipod will give adequate results.

Use the "start delay" to avoid picking up camera shake when you start the script.

Unless you use a tracking mount, longer exposure don't allow you to go "deeper" once trailing becomes significant. Accumulation of background light actually makes it harder to pick out faint objects. When trailing becomes noticeable depends on sensor resolution, focal length and where in the sky you are aimed. For wide angles, trails generally become significant over 15 seconds.

In moderate light pollution, I find unguided exposures readily pick up stars down to magnitude 6 - 7 (roughly the limit of naked eye visibility under very good conditions.) Faster moving objects like meteors and satellites must be much brighter to show up.

For satellite passes, exposures of 1-4 minutes can give a nice continuous track, with moderate star trails.

Aperture (Av) is not controlled by the script, but should generally be the widest value. If you shoot in P or Auto mode, this should happen automatically at night.

With raw, shooting low ISO and Ev adjustment in post processing often gives similar results and more flexibility than shooting at high ISO, while avoiding significant "processing" time on the camera. The optimal settings can vary significantly depending on the camera sensor hardware.

Getting good results from raw requires using raw processing software, not just some image program that happens to be able to load raw. The examples linked below were processed using Raw Therapee (

The script is optimized for using raw with Canon dark frame disabled and set of "master" dark frames for standard combinations of exposure and focal length. This allows you to shoot long exposures without waiting for the Canon dark frame processing between each shot, while not sacrificing quality. The Tv and ISO Mode settings allow quickly selecting from a limited set of standard values. All the Tv options are added together so you can use the Tv+... options when something outside the standard range is required.

"Disable Canon Dark Frame" should be checked both when shooting the actual shots and when creating dark frames. You should also have CHDK manual badpixel removal (badpixel/badpixel.txt) disabled: The dark frame will take care of hot pixels, and you do not want them "fixed" in your dark frames.

Making master dark framesEdit

  • "Enable Lua Native Calls" must be set in the CHDK misc menu.
  • leave Tv, ISO and zoom settings the same as the actual shots.
  • check "Make dark frames",
  • Set "shots" to 5-10, run the script, and combine them on camera with "raw average".

The averaged frame is your master dark. Raw therapee can automatically select dark frames placed in the appropriate directory based on camera and exposure settings.

For optimal results, the dark frames should be taken at a similar temperature to the light frames. You can use the sensor temperature in the log to organize both light and dark frames by temperature.

Amp glowEdit

Some cameras (especially those from before about 2012) show very significant "amp glow" for exposures of a few tens of seconds or more. This shows up as a purplish glow in one or more corners of the image. Amp glow can be mitigated to some extent using dark frames. More recent cameras tend to perform better, with exposures of several minutes not showing significant amp glow.


Elph 130, 30s exposure. Pleiades + comet C/2014 Q2 (Lovejoy)

D10, 64s exposure. Pleiades + comet C/2014 Q2 (Lovejoy). Shows amp glow, and removal with dark frame.

Elph 130, 512s exposure. Polaris + aircraft + star trails

D10, Timelapse of 180 x 64s exposures. Orion rising + clouds

Elph 130, 128s exposure. Tiangong 1 space station.

Script optionsEdit


Number of shots to shoot. Set to zero to shoot until one of the shutdown conditions is reached.

Interval Sec/10Edit

Minimum shooting interval, in 1/10ths of a second. Set to 0 to shoot as fast as possible.

Use rawEdit

Control CHDK raw saving.

  • Default = Use current CHDK UI setting.
  • Yes = Save raw
  • No = Do not save raw

Disable Canon Dark FrameEdit

Force canon dark frame subtraction off. This is useful to increase the shooting rate of long exposures. Standalone dark frames can be created and subtracted later.


Select a shutter speed from list of values, from 256 seconds to 1/64th.

Tv + secEdit

Add a specified number of seconds to the value above.

Tv + sec/10000Edit

Add specified 1/10000ths of a second to the values above

Start delay (ms)Edit

The number of milliseconds to wait before shooting starts. Use to allow vibrations from pressing the shutter to settle.

ISO (by CHDK)Edit

ISO value to set, using CHDK overrides, in Canon display units. Ignored if 0.

ISO (by ISO mode)Edit

ISO value to set, using Canon ISO modes.

Use cont. mode if setEdit

Shoot by holding the shutter button down, if continuous shooting is enabled in the Canon menu. If not checked, or if continuous mode is not enabled in the Canon menu, shooting is done by holding down half shoot and clicking full. Continuous mode is faster on some cameras.


Display power saving mode. On = always on, Off use set_lcd_display off. Blt_Off use set_backlight every shot. Pressing set while the script is running will turn the display on for 30 sec.

Shutdown on finishEdit

Shutdown after "shots" images are taken.

Shutdown on low batteryEdit

Shutdown if the battery voltage falls below the CHDK OSD low battery value.

Shutdown on low spaceEdit

Shutdown if the SD card space falls below 50 MB.

Make dark framesEdit

Shoot with the shutter closed. Requires "Enable Lua Native Calls" be set in the CHDK menu.

Log modeEdit

If enabled, the log is written to A/fixedint.csv

  • None = do not record a log
  • Append = append to existing log
  • Replace = replace any existing log

Log file descriptionEdit

The log is a CSV file. The exposure settings are logged recorded in the desc column of the first row. Timing, temperature and memory information is logged per shot.

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