EL panel for flat fields in astrophotograpy


Here some images of my EL (Electroluminescence) panel mounted in a photo frame to take flats with my telescope. Flats are typically used to address practical problems like vignetting and dust. Vignetting causes the light tends to drop at the corners from 90 to 75% depending on the telescope and optical train. Since my telescope is a wide field one, using the twilight sky can be tricky for making flat fields due to the unequal illumination of the sky. A EL panel is than a better choice.  Flats made with the EL-panel look good and symmetrical. Comparing two set of 100 flats, one with the EL-panel 90 degrees turned indicate it is within 0.6% uniform.

EL-panel on top of the 100 mm telescope. Note the telescope front diameter is about 140 mm:
EL-panel on top of the telescope


The EL panel was bought for about 15 Euro in Asia. Powered by USB connector at 5 volt.  The panel didn't look perfect and three black dust particles could be seen inside. :(  Since the telescope is not in focus it isn't a problem.
 


If powered it lights up with a bluish white color


A simple wooden photo frame was used the protect/house it:


Backside of the frame. Components are simple glued on the back plane using a glue gun:


For a CCD camera without any narrow band filter the  EL-panel is too bright so a solution was sought to reduce the brightness. At 5 volt supply is consumes about 0.5 amp. The invertor makes from the 5 volt a high voltage AC mayby 120 volt at  a high frequency typically 400 hz. In theorie the EL brightness can be controlled with both the voltage and AC frequency. As a test I connected a regulated power supply an while reducing the voltage the panel kept working down to 2.6 volt supply and consumed a reduced current 70 mA average. This brightness reduction was later implemented by putting a resistor of 100 ohms (emperical value) in series with the 5 volt supply. With the switch the resistor can be bypassed for maximum brightness. With the 100 Ohm resistor in series the brightness reduced with about a factor 40. Exposure times for 25% reading increased from 20 ms to 1 second. With an H-alpha 7nm filter in place at full brightness about 7 seconds exposure  are required for 25% reading.



The EL panel in service at maximum brightness:

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Last update 2017-5-10

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