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Flat Fields Made Easy: Aurora Flatfield Panel Review

I always recommend using flat field frames to calibrate deep sky images. My most recent imaging setup with a large full-frame camera and an extreme 0.66x focal reducer made it even more important to get perfect flat field calibration frames. Reaching perfection isn’t easy but the Gerd Neumann Aurora Flatfield Panel claims it will get you there.

Are Flats Calibration Files Essential?

If you’re doing deep sky astrophotography, then the answer is a big YES! No telescope, no matter how well designed and built, will be able to illuminate the focal plane perfectly (sensor in our case) in a completely even manner. We typically get more brightness in the center of the field compared to the corners. The drop in illumination, known as vignetting, becomes more pronounced the larger the sensor, and with the use of reducers and correctors. Furthermore, our filters and sensors are bound to attract dust, which shows up in the image as small circular shadows (dust donuts or dust bunnies).

A single exposure before and after flat field calibration. Note the evenness of illumination and the removed dust donut on the lower right after calibration in the “After” image. Credit: Rouzbeh Bidshahri

A flat field calibration frame can completely fix both these issues, a reference image of a perfectly illuminated (“flat”) light source, provided it is done properly. This flat field image provides a reference that shows deviations from perfect illumination across the field. We can then use this calibration image to correct our images for those deviations. The main requirement is a perfectly “flat” illuminated light source at the right brightness.

There are a few options here, with the most common being LED backlit panels. These work in many cases, but as LEDs are point sources so the illumination is not always perfectly even. Another popular method is using natural, indirect sunlight by pointing at the sky during twilight, sometimes with a white cloth covering the telescope. While some have success with this method, I find it impractical to have to time the flats for a specific time to get the exposures just right.

Gerd Neumann Aurora Flatfield Panels

The Gerd Neumann Aurora Flatfield Panels differ from conventional LED panels as they use a thin electroluminescent (EL) foil that is completely illuminated. They provide even illumination and eliminate the need for diffusing screens and the resulting bulk.

EL foil requires high voltage to operate but the current is extremely low, so the power requirements are low as well. Gerd Neumann sells power inverter for either AC (110v-220v) or 12VDC power sources. The 100mm version can run a small battery powered inverter.

I opted for the 12V invertor as it makes field use much easier. The inverter is relatively small and only weighs 170grams (0.37lbs), so it can be mounted permanently it on a telescope, if desired. The only controls are an on/off switch and dimmer knob located on the panel but no USB control.

The Gerd Neumann Aurora panels are well built, with the rubber outer edge protector making it feel more robust than some other panels.

I like is that the back of the panel is opaque. This is great for use at star parties where you don’t want light from the panel to disturb others or get you kicked out!

The 420mm Aurora Flatfield Panel and the 12v power inverter. Credit: Rouzbeh Bidshahri

Brightness Control

The brightness of the panel is appropriate for most uses. The supplied 12V inverter has a dimmer knob that allows about a 40% reduction in brightness, if needed.

If you require less light for longer exposures, you can add one or more sheets of neutral density (ND) filter. There are several darkness options here. I ordered some but I found the standard setup worked for me, so I haven’t used the ND filters.

The disassembled panel with one layer of ND darkening filter sheet. Credit: Rouzbeh Bidshahri

Brightness requirements also vary depending on focal ratio, sensor gain settings, and filters. Most imaging sensors are less sensitive in red light, and the panel’s spectrum seems to be less bright towards the red end of the spectrum as well. 

Below are values for flat field exposure times on my 14-inch telescope at f/4.7 and the QHY600M with gain 0, LRGB exposure times, and the panel at full brightness (dimmer = max, no ND sheets). Exposures were to 40% of maximum histogram values.

Exposure Time
Luminance 0.7s
Red 4.3s
Green 2.1s
Blue 1.1s

I expected narrowband exposures to be longer, but they were a bit longer than I had hoped, even at maximum panel brightness. Test results with my 3nm and 6nm narrowband filters at full brightness on the same setup were as follows:

3nm Bandwidth 6nm Bandwidth
  Exposure Time Exposure Time
Ha 50.7s 26.1s
Oiii 8.1s 3.9s
Sii 82.5s 45.4s

Sizes and Physical Dimensions

The Gerd Neumann Aurora panels come in several standard sizes, from 100mm (4-inch) to 420mm (16-inch). Even larger sizes are available on request. My 420mm panel’s illuminated area checked out as 420mm (16.5 inches) in diameter.

The 420mm’s circular shape is a perfect fit for my 14-inch telescope. Credit: Rouzbeh Bidshahri

One of my favorite features about the panel is its extremely slim profile, only 13mm ( 0.5 inches), and its very light weight, just 1.2kg (2.6lbs). This makes it even more practical for field use.

I’ve noted some online users concerned that the panel’s wire connector is too fragile. Taking a close look, however, it’s clear that what you see is a rubber sleeve, the actual connection to the foil located safely inside the panel.

The wire and protective rubber sleeve. The sleeve is sandwiched and locked between the rear panel cover and front diffuser. Credit: Rouzbeh Bidshahri

Flatness Test

To test the evenness of the panel’s illumination, I averaged a series of flats in one position and then rotated 90 degrees. Below is a side-by-side comparison of the averaged flats.

Visually identical Analysis plots: Left is an average of frames with the panel at 0 degrees. Right is the average of frames after the panel was rotated 90 degrees. Credit: Rouzbeh Bidshahri

A more stringent test would be to divide these two sets of rotated flats using PixelMath. A perfect result would be perfectly white, i.e, 1/1=1. Below we see the Aurora panel deviating from a perfect 1.000 on the left side. Note that the scale on the right is compressed, though, and the lowest value is still 0.997. Thus, the maximum difference detected between flats with the panel rotated 90 degrees was 0.3%. That is near perfect.

Rotated flats divided with PixelMath and displayed in a contour plot. The darkest parts indicated a difference 0.3%. Credit: Rouzbeh Bidshahri.

Final Thoughts

After having tested many flat field light panels, the Gerd Neumann Aurora is my favorite.

I didn’t test it with a color camera, but with my mono camera I would have preferred it to be slightly brighter in the red spectrum so it wouldn’t require such long Sulfur (SII) exposures. A USB dimming controller rather than a knob would also be a nice touch but it’s not essential, and not practical due to the nature of the EL light source. 

What I like is that the illumination is very flat, and it is the lightest panel I’ve ever seen in this size. Along with the 12v power and opaque rear side, it’s very practical for field use.

I would definitely recommend these panels for those looking for a great flat field panel that is also very portable.

 

Website: www.gerdneumann.net

MSRP: $56 (100mm) to $1740 (750mm) (not including VAT)

 

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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