Ab Light Flat Calibration Panel

The Ab Light Flat Calibration Panel from Wa-chur-ed Observatory produces high quality flats for both broadband (RGB) and narrowband astronomical imaging. It uses LEDs arranged around an acrylic light pipe of a special type that provides more even (flat) illumination over the area.  Residual error is minimized by the use of a calibrated reflector behind the light pipe.

Ab Light Flat Calibration PanelThe standard Ab Light Flat Calibration Panel has an illuminated area of 7 inches diameter, which is large enough for almost all refractor telescopes. It is designed to take 12VDC power, which is supplied through a 5.5mm x 2.1mm barrel connector. If dimming is required, any 12VDC pulse-width modulation (PWM) device can be used, but a high frequency dimmer such as the Adim Controller from Wa-chur-ed Observatory is recommended to provide optimal flats even with relatively high shutter speeds.

The panel can be placed either directly on the front of your telescope’s dew shield, or some distance away from the telescope, such as mounting on an observatory wall (hangers are included). The optional custom fitted version adds a 1/4″ thick front cover that is lined with black felt. The aperture is cut to fit snugly on the front of your telescope so that flats can be taken without lighting up the surrounding area.

What makes Ab Light Flat Calibration Panel superior? According to Greg Marshall, owner of Wa-chur-ed Observatory, “Competitive flat light sources typically use electroluminescent (EL) panels.  Some use LEDs like Ab Light, but with inferior technology. EL panels are inherently pretty flat (even illumination) when properly implemented, and are light weight, but have issues with the color spectrum. Using LEDs as the light source allows tremendous freedom in choosing the color spectrum, since they are available in a wide range of color temperatures and color rendering indices (CRI). The LEDs used in Ab Light are chosen to provide excellent illumination for all types of commonly used astronomical filters, as well as DSLRs.”

Adim PWM Controller

The Adim PWM Controller from Wa-chur-ed Observatory is a USB or manual PWM (Pulse-Width Modulation) controller for 12VDC devices that are compatible with PWM. It is primarily intended to be a “dimmer” for flat calibration lights, such as the Ab Light. It can also be used to control the brightness of other 12VDC lights (LED or incandescent), or to control the speed of a 12VDC fan, etc. (Note that the photo is of a prototype unit built with a metal knob. Production models use a plastic knob.)

Ab Light Flat Calibration PanelAdim can provide up to 1.5 amps of current at 12V (the Ab Light draws only about 0.25 amps).  The PWM operates at 2kHz (2,000 cycles per second), so there is no visible flicker, and relatively fast shutter speeds can be used when capturing flat frames. Greater than 1/20 second (50 milliseconds) is recommended, as this is 100 times the duration of a single cycle, thus reducing any “shutter shading” to no more than 1% of the total exposure per frame.

The Controller’s manual knob has detents, so it works in discrete steps. These steps are in a logarithmic scale. In the default mode each clockwise step doubles the brightness and each counter-clockwise step halves the brightness. You can think of this in terms of photographic F-stops. You can also easily select steps of 1/2 or 1/4 stop. When using the provided Windows app to control the brightness you can select the output in 1/4 stop increments.

The Adim Controller application connects to the device via USB and helps to simplify the process of capturing flat calibration frames. There is an On/Off toggle button, and a slider to adjust the brightness. The current output level is displayed in F-Stops, with “full on” being 0, and the dimmest possible setting is -10 stops (1/1024 of full brightness).

A drop-down menu of presets can be set up to remember a wide variety of configurations (different telescopes, cameras, filters, etc.).  An Exposure field is also displayed.  It doesn’t actually control anything, but the value you enter here when defining a preset is remembered and displayed again when you select the preset, thus reminding you of the exposure value you entered. The PWM output level for each preset is also remembered, of course, and the output is set to that level immediately when you select a preset from the menu.

You can learn more about the Ab Light Flat Calibration Panel and Adim PWM Controller here.

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