Using a Flat Field Panel for Astrophotography
Flat frames are calibration files are used in the pre-processing stages of an astrophotography image. To capture a successful flat frame, you need to evenly illuminate the field through your telescope or camera lens.
When used correctly, flat frames can correct the uneven field illumination of your light frames, which reduces vignetting and gradients significantly. It also removes dust and other artifacts that may be present in the optical train.
One of the most challenging aspects of capturing flat frames is producing consistent, accurate results in terms of brightness. A flat field panel can help you create a uniformly lit flat frame to correct uneven field illumination in your astrophotography images.
This issue can arise whether you are using a DSLR camera, CCD, or dedicated astronomy camera. Previously, I have used the DIY “white t-shirt method” to take flat frames. In order for this method to work, you must find the right balance of t-shirt layers between your telescope’s objective, and the daytime sky.
While this method is often useful to help correct an uneven field in your astrophotography images, it has a number of drawbacks including having to wait until daylight to complete the task, and inconsistent results.
Enter, the Artesky Flat Field Generator…
The Artesky Flat Field Generator
I’ve been fortunate enough to receive a demo version of a dedicated flat field panel for astrophotography, the Artesky Flat Field Generator. This device was designed to make taking flat frames easier, and deliver reproducible results that can be used for future imaging projects.
The model I have is the Flat Box USB 250mm unit, which connects to my PC for complete control over the settings. The Artesky Flat Field Generator was designed in Italy, and the team approached me about demoing this product at NEAF.
The Artesky Flat Field Generator creates a fully illuminated area for creating high-quality flat frame images for astrophotography. This unit uses a color-neutral LED light to produce a pure white image. I have found this flat field panel (often called a “flat box”) to work exceptionally well with my one-shot-color camera (ASI294MC Pro).
To control the brightness of the LED, you can either use the 1:10 dimmer switch (on the non-USB models), or the use the software-controlled method on the USB version like I have. Here is a look at the Artesky flat field software on my computer.
The Artesky flat field panel software allows you to control the exact brightness of the panel you need.
An important design aspect of a quality flat panel is that it does not flicker whatsoever. The Artesky Flat Field Generator is flicker-free (using the correct 12V power supply), which comes in handy when collecting short exposure flat frame images.
Currently, there are 2 sizes of Artesky flat field panels available, 250mm and 550mm. The 250mm model I have will fit all of my refractor telescopes with room to spare. There are also non-USB controlled versions available, that include an on-off switch and 1:10 dimmer for complete field illumination control.
All versions include a digital voltmeter for accurate and reproducible exposures. You have the option of monitoring the exact voltage used to collect the perfect flat for your desired exposure and use it next time.
Using the Flat Field Panel on Your Telescope
The panels offer evenly illuminated, uniformly lit light to cover the objective of your telescope. To use the flat panel, I simply point my telescope straight up towards the zenith and place the flat box on the end of the dew shield.
The panel unit itself is strong and was designed with an anti-scratch, dirt-proof external surface. I do not plan on leaving the panel exposed to the elements, especially the illumination side. The Artesky 250mm Flat Field Generator came with a padded canvas bag to store the flat panel in when not in use.
The flat panel placed on top of the dew shield of the telescope’s objective lens.
The Artesky website states that this flat panel requires a 12V 1A (minimum) power source to function properly. I used the included 12V DC cigarette lighter power cord to plug the flat panel into my 12V 10A AC DC adapter in the backyard.
Creating Perfect Flat Frames
I used the Artesky USB 250 flat box to collect flat frames through my Sky-Watcher Esprit 100 ED APO refractor, with a ZWO ASI294MC Pro camera attached. The resulting flat frames were evenly illuminated and calibrated out the uneven background sky I have become accustomed to here in my light-polluted backyard.
Because this model is USB controlled, I had to install the necessary Artesky software on my computer to communicate with the device. When you change the brightness of the flat box on your computer, the onboard digital voltmeter reflects the changes in real-time.
The digital voltmeter display on top of the Artesky Flat Field Generator.
The camera control software I use to run my dedicated astronomy camera (Astro Photography Tool) includes a feature called the “CCD Flats Aid“. This tool calculates the perfect exposure length to use to create a useful flat frame for your project.
For one-shot-color dedicated astronomy camera’s like the ASI294MC Pro, I have found it is best to use a flat frame with a target ADU of around 25000. This is in the mid-range of the spectrum, as indicated in the histogram shown below. This value creates the perfect flat frame to even illuminate the field during the calibration stages of pre-processing.
Creating a flat frame using the Artesky Flat Field Generator in Astro Photography Tool (CCD Flats Aid).
After collecting flat frames in APT using the Artesky flat panel, I then import the files into DeepSkyStacker to create a master flat for my project. I have never collected flat frames with such a balanced and evenly illuminated field. This is, no doubt, thanks to the precise brightness controls that the flat field generator provides.
I love the level of accuracy the Artesky Flat Field Generator provides when collecting calibration frames that are as tricky as flat frames. The software is extremely straightforward and easy to use, and the results out of the box were impressive. I can honestly see my self using this device from now on, ditching my old white t-shirt method for good.
I like having the option of controlling the flat panel from my computer remotely, as the software allows you to control the LED brightness without being anywhere near the telescope. This is great news for those with an automated observatory that prefer this level of control.
I wish that the USB model included an on-board 1:10 dimmer switch as well, in the off-chance that I want to take flat frames without being connected to my computer. This would be a nice feature when using the flat panel with a DSLR camera that I am controlling with a remote shutter release cable – not PC controlled software. However, I feel that most people looking for a flat panel option like the Artesky Flat Field Generator are looking for a CCD flat frame solution.
The Artesky Flat Field Generator was used to create flat frames for the image of IC 1318 above.