To get the most from deep-sky image data, it is necessary to calibrate your subframes before stacking and processing them into a final image.
There are three types of calibration that can be applied to your images: bias frames to remove signal noise caused by the action of downloading your data, dark frames to remove thermal noise produced when your sensor warms up and flat frames to remove the effects of vignetting and dust motes.
Whereas bias and dark frames are simple to capture at any time, flat frames require careful planning and execution, so are often forgotten in the excitement of capturing new images.
A flat frame is a special image taken in such a manner that only a facsimile of the plain light cone passing through the telescope to the camera sensor is captured.
A typical way of doing this is to cover the front of the telescope with a white cloth and take a series of images with the telescope pointing at the daytime sky.
However, it is vital that the orientation of the camera and focus position are maintained between the capture of the celestial images and the flats, and therein lies the problem – you must not disturb the imaging train until the flats have been captured!
There is a fairly simple solution, though: you can use an electroluminescent panel to provide an evenly distributed light source to produce the illumination and capture your flats during an imaging session.
Electroluminescent panels comprise three components – the panel itself (a thin laminated plastic sheet), a 12V inverter that produces an alternating current in the 120V range and a 12V power source.
The panels are available in a range of sizes from A6 to A1; choose one that is slightly larger than the aperture of your telescope to ensure complete coverage.
Securing your panel
Because electroluminescent panels tend to be a little flimsy it should be affixed to a backing board.
The ideal material for this is the 4mm-thick corrugated plastic sheet often used for signboards as this material is very light, smoothly finished, moisture-proof and surprisingly strong.
By applying masking tape to the rectangular shape of the electroluminescent panel (allowing 40mm over the panel length to comfortably support their notoriously fragile electrical connections) you can cut a very neat backing board to size.
The panel can be attached to this backing board using double-sided adhesive tape.
As the electrical connections to these panels are so delicate, it is also worth making a loop in the cable near to where it enters the panel and securing this to the backing board with electrician’s tape to act as an emergency strain relief.
The inverter module can be fixed to the rear of the backing board with double-sided adhesive tape to act as a ‘handle’ and ensure that no physical tension is applied to the interconnecting cable.
To capture worthwhile flat frames it is important to get the exposure right.
Typically, this means achieving between 30 and 50 per cent of the full well capacity (saturation level) of the sensor elements.
With a DSLR, this is easily achieved by setting it to aperture priority (Av) mode and letting it automatically choose the exposure time.
Astronomical CCD users can use their capture software to determine the ADU (Analogue Digital Unit) value, which is a measure of the individual pixel values in a CCD image and should be in the range of 20,000 to 30,000 units.
You will need to capture 20 or so subframes, which then need to be stacked to produce a master flat frame using a median combine method.
These should then be divided into each image frame to apply the calibration.
Many stacking programs, including the freeware DeepSkyStacker, will do this calibration for you.
You will be astonished at the improvement in your images.
Steve Richards is a keen astro imager and astronomy equipment expert
This How to originally appeared in the August 2015 issue of BBC Sky at Night Magazine.