Demystifying Flat-Frame Calibration

In my last blog I wrote in regards to the variations and significance of darkish and bias calibration frames. Proper picture calibration makes your job a lot simpler when you begin stacking, stretching, and tweaking your photos for an aesthetic presentation (fairly footage). Calibration can be important step to quantitative scientific work. However, darkish or bias body calibration is just half of our information discount course of, and we have to discuss our third and sometimes uncared for calibration body, flat-field photos.

Vignetting before and after
Left: Vignetting is widespread in all optical programs and darkens the outer areas of a picture. A flat calibration body (heart) is used to even out this artifact within the remaining picture (proper).
Richard S. Wright, Jr.

Recording correct flat-frame calibration frames could be difficult, so many informal imagers simply skip them solely. The notion is that flats solely appropriate vignetting, the way that gentle falls off as you progress from the middle of the body. Many image-processing packages have vignette-correction instruments with easy-to-use sliders that make quick work of the darkening you see across the edges of most optical programs. But flats really do excess of this!

While a darkish or bias body corrects your picture for artifacts launched by the digicam electronics, a flat body corrects your picture for illumination artifacts primarily launched by your optics. This contains not solely vignetting but in addition different shadows and obstructions within the gentle path in addition to yet one more noise supply, photo response non-uniformity (PRNU), which can’t be eliminated by stacking alone.

How to Take Flat Fields

Unlike darks or biases, that are taken at midnight, flats are totally illuminated photos that should have uniform lighting throughout the whole body. Flats would not have to be in excellent focus, so just a little change in focus place on account of temperature the following day is not going to trigger issues.

You can document a flat by capturing the blue sky, utilizing a flat panel (a white laptop computer or pill display will work with small apertures), or by masking the top of the scope with a white blanket or material to chop down on the sunshine depth. If you shoot flats throughout the day, chances are you’ll have to wrap the focuser and filter wheel with aluminum foil to get rid of gentle leaks which may contaminate your flats with sizzling spots.

handkerchief flats
Any white material you could find might help you get daylight flats utilizing the skies pure illumination. Two layers of material or extra will cut back the prospect of recording the sample weave in your flats.
Richard S. Wright, Jr.

High-signal flats are finest, with two provisos. First, you have to keep away from saturating the pixels of your detector; a strong, white discipline the place each pixel is similar most worth is not any good.

Second, your flats should be saved throughout the sign vary the place your sensor is linear. There’s some math that I’m going to spare you, however flats received’t work effectively if the sensor shouldn’t be recording information linearly. Nonlinear flats had been one of many causes photos from early CMOS cameras had been so tough to calibrate — they go non-linear in a short time. A very good rule of thumb is that the center of your flat-field picture needs to be about half the utmost ADU worth that your digicam can present. The histogram studying for a correctly uncovered flat discipline appears to be like like a giant hump proper across the center.

Using Flat panels
A devoted flat panel is right for taking flats every time handy. In a pinch, a pill with a white display also can work.
Richard S. Wright, Jr.

Just like darks and bias calibration frames, you need to take as many flats as you possibly can tolerate and stack them collectively to make a grasp flat discipline. This reduces the digicam’s learn noise and shot noise.

Another, often-overlooked requirement in your flats is that they should be darkish subtracted in an effort to work 100% correctly. “Dark flats” is a time period I take advantage of and have seen different use. If your flats are 3-second exposures, take some 3-second darkish frames on the similar temperature. Make a grasp darkish, subtract that from the flats, and stack them. Doing all this could lead to wonderful flat-frame calibration, with scientifically correct sign values throughout your body when your favourite astro-image software program does the calibration of your gentle frames.

Example flats
These mud donuts will seem in your whole photos, and every filter has it is personal distinctive signature. Taking flats is the best way to take away these artifacts.
Richard S. Wright, Jr.

Why Bother with Flats?

As famous earlier, many imagers skip flat fields altogether and proper vignetting manually with software program. This is ok aesthetically, however for scientific work, corresponding to photometry, you possibly can’t simply eyeball it.

In addition, small grains of mud on the duvet slip over your digicam’s sensor will depart small, darker-colored spots. These shadows develop bigger and fainter relying on how far they’re from the sensor. Dust on a filter wheel, for instance, will create massive round options, typically referred to as mud donuts. Dust on the first mirror or entrance goal of your telescope, however, is so removed from the sensor that its total impact can’t be seen. So you possibly can cease furiously cleansing your telescope or digicam lens — the spots you see are from the digicam cowl glass, your colour filters, flatteners, coma correctors, and different parts up close to the digicam. Some readers have written to me about these mud donuts, and blamed them on the picture sensor, the telescope, or each. Regardless the place they arrive from, although, making use of a great flat will make them disappear like magic!

Histogram for a well exposed flat field image.
If you’re utilizing a histogram to gauge your flat discipline exposures, a hump close to the center, and never too far to the fitting is a secure wager.
Richard S. Wright, Jr.

The remaining motive to take good high quality flats is the PRNU noise I discussed earlier. This is much like the mottling you get from dark-pattern noise, however this noise supply is particularly brought on by small variations in sensitivity to gentle from one pixel to a different. Flats actually “flatten out” this mottling. Because the energy of this undesirable sample goes up with elevated sign, it turns into particularly dangerous below light-polluted skies. If you are rigorously guiding on a star, you aren’t simply aligning the stars, you’re aligning the PRNU, and that sample will not go away with stacking. Dithering will assist significantly, however it’s nonetheless higher to take away this sample noise from the very starting.

Example of PRNU
Noise is not at all times random fluctuations. It’s additionally undesirable sign, corresponding to this PRNU noise brought on by variations in pixel sensitivity.
Richard S. Wright, Jr.

Remember too that your optics will not be the one supply of gradients or illumination artifacts in your picture. Light air pollution, for instance, can introduce a gradient that flats is not going to appropriate for. Nevertheless, for the scientifically minded, picture calibration or information discount is an important first step in any workflow. For the astrophotographer making an attempt to squeeze probably the most out of their photos as doable, flats, together with correct darks or bias frames will take away a variety of pointless baggage early on and aid you make a clear well-processed picture.

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