Tutorial:
Flat calibration and how to shot flat images for color cameras

Content:

1. Flat image for color camera
2. Pixel gain differences
3. Hardware setup
4. Exposure
5. Master flat 3D analyze
6. Alternative light source, RGB LED bulb
7. Test of TS flat panel
8. To be continued

7: Test of TS flat panel

I have used my flat calibration box that I built earlier for three years now: Project Flat calibrator. But lately I found that there are some weak circles on the calibrated images which I didn't had before. It came when I changed telescope to my my TS130 refractor. Partly it's related to internal reflexes in the adapters that I made, but even after I corrected that there some small traces of these rings. Could it be that my flat panel has some problem?

My astro friend Rolf had one flat panel that I could borrow for some test and maybe verify that my own DIY flat panel is okay or not.

TS flat panel:

This is how it looks from back side, if I'm correct the logo stands for TS. When I Google on it I couldn't find any information about it. It has an On/Off button which is combined with a brightness control. It's only 14 mm thick and I think only an EL panel can give that small thickness. It's powered from 12 Volt.

If you find electroluminescent panels interesting you have a link here to a DIY tutorial: How build flats panel imaging

An EL panel must be the perfect solution for a monochrome camera, but for a color camera I have my concern, there is no way to control the color balance electronically. But by adding colored filters (paper) on the surface the color balance of it can be adjusted in the case of a color camera. This was the reason that I built my own flat box where I can control the intensity for each channel but what use of it if the light isn't flat enough. That's what I'm going to test now.

On the front side there are four handles that grabs around the telescope's dew shield. This flat panel is optimized for a 6.5 inch (160 mm) telescope, the clamps is adjustable from 185 mm to 215 mm. My telescope is a 130 mm refractor and the dew shield has an outside diameter of 180 mm.

I made some test images and found that the light output is very bright and almost oversaturated the sensor. 1 second exposure (and 2 sec delay), 400iso at f/7. I want at least 2 seconds exposure to reduce the problem that panels like these flicker, in this case at high frequency so not very important.

I tested to add a sheet of white paper on top of the panel to reduce the brightness. The brightness control is set in its lowest brightness.

Looks to work and I cut a bigger size white paper that cover the panel.

And now it's ready to be mounted on the telescope for a test.

Squeeze the panel against the front of the dew shield.

Cover the flat panel and telescope with a dark towel to reduce the risk that light leaking into the system. I also have the room dark when I taking the sub flats.

I use KStars to control the camera, a Nikon D800. I could now increase the exposure time to 2 seconds. There is a built in histogram function in KStars, but it's useless to judge if the exposure is optimal. But I could at least see the green channel is much higher in level.

I have another software I use when I want to analyze in more details, Fitswork. A line is drawn cross the image and the intensity is plotted, to the left. The upper level is the green channel and it's between 8700 to 9700 ADU, very low vignetting to be a full frame sensor. The middle is the blue channel and at bottom the red channel. The histogram to the right shows were each color channel has its peak. Lower vignetting give sharper spikes. The stair on left side of each spike comes from that I didn't crop the image, a lot of area with low signal is then included.

After Debayer it's easier to see which color each peak belongs to. The red = 3500 ADU, Green = 9300 ADU, Blue = 7100 ADU. It had been better if all three channels have had its center at 7000 ADU. The optimal center is normally at 40% of the saturation level, in this case at 16000 ADU (14-bit sensor). It can be corrected with a pink colored filter. But it's okay as it is. The problems that can arise is that when a sensor isn't linear it compress higher levels, and too low signal it could be a noise problem.

I have now made new MasterFlats and shall calibrate my old data and see if the rings disappear. But there were no difference, same ghost rings over the image.
The good thing: I now know that there is no wrong with my own DIY flat calibrator.
The bad thing: there are some internal reflections in the telescope that I don't have control over.