Good Flat, Bad Flat

Another little lesson in astrophotography, one more in a very lengthy list… Do not use too short an exposure with the flat source.

Flat frames are used to calibrate out any uneven field illumination or dust in the field. This ever more important with the new camera, the larger, full frame sensor shows some vignetting at the corners. I use an electroluminescent source to do my flat frames, actually an old laptop back-light that has been re-housed in an acrylic frame. It provides an even illumination across the aperture of the telescope or lens I am using to acquire the flat field calibration frames.

Bad Flat
A bad flat field taken at 1/4000 sec with the EL back-light flicker interacting with the camera shutter
What I have discovered is that the EL backlight flickers. This is too fast to see with the eye, but if the camera exposure is fast enough it will create issues. This shows up as horizontal structure across the field as the flicker interacts with the camera shutter.

I discovered the effect as I took flats the first time with the Canon 6D. I had the ISO set to 6400 which resulted in a 1/4000 sec exposure. Fortunately I looked at the last flat and saw the problem before I dismounted the camera from the telescope, it was pretty obvious. Once the optical setup is disturbed it is not possible to re-shoot the calibration frames.

Good Flat
A proper flat field calibration frame, Canon 6D and AT6RC telescope.
It took a few seconds of thought to realize what the problem was. I never realized an EL light flickers, or rather I had never thought about it. Given that EL lights are driven with a high voltage switching power supply, flicker should be expected. The switching frequency of the supply should be above the human threshold of hearing to avoid an annoying whine, at least 20kHz. It can not be too much higher than that or it would not have shown up in a 1/4000 second exposure.

Slowing the camera down removes the effect. As I could not change the aperture I simply reduced the ISO to minimum, this slowed the shutter speed to 1/15 of a second for the nice mid-scale exposure needed for flats.

The corrected flats reveal the usual things that a flat is takes to correct. The dark corners reveal the expected uneven field illumination. the dust doughnut reveals at least one notable speck of dust on the cover glass of the sensor. There is a dark band at the bottom of the frame which I believe to be shadowing from the edge of the mirror. Looking at the flat I realize I will need to be conscientious about taking flats with the Canon 6D.

A Programmable Current Source

The usual engineering minutia takes up much of my week… Paperwork, documentation, purchase orders, meetings, etc. There are the occasional chances to just have a little electronic fun. Thus I greeted a request from a couple of our support astronomers with some enthusiasm. They needed a variable intensity light source for calibration of the OSIRIS spectrograph. This source sits inside an integrating sphere on the AO bench along with four spectral sources, small tubes of neon, argon, xenon and krypton.

One Amp Current Controller
The remote current controller PCB and case prior to final assembly
There was already a white light source in the calibration sphere, but control was limited to simply on or off. The spectrograph uses various filters that each let through differing amounts of light from the calibration source. This created problems during calibration, some sort of variable intensity source was needed. It also needed to be quite precise, with a well regulated output that would not vary during the hours a calibration script could run.

I set aside a bit of time each day to design and build the source. The last couple hours of the afternoon are best, at this point I have had quite enough paperwork. A chance to practice my trade sketching out the circuit, or simply sitting at the bench and soldering… Perfect.

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