The Importance of Flats

1 comment by AstronoMolly

The Importance of Flats

Darks, flats, bias frames, flat darks…if you are just getting started in astrophotography, all of the “extra” images people say you have to take can be overwhelming!  You may already know that dark frames help reduce noise, but what about flat frames?  Are they necessary?

Flats are a type of calibration image that are different from darks, biases, or flat darks.  Rather than being useful for noise reduction, flats address a different kind of image problem: artificial shadows.  Flat frames are used for two main sources of shadows in images: dust motes and vignetting.  Small specs of dust or dirt on your filters, camera window, and (more rarely) on your camera sensor will show up in your light frames as little round dark splotches.  While not too difficult to clone stamp out if they are over some background space, they can really detract from your image if they end up over top of a galaxy or nebula!  No matter how diligently you clean your optics, the little dust spots always seem to re-appear. 

Example flat frame, ZWO ASI294MC Pro with a Takahashi FSQ-106N telescope, using the Farpoint electroluminescent flat panel

Vignetting, on the other hand, is a mostly unavoidable problem: a square sensor on a round aperture leads to darkened corners.  Vignetting can also be caused by using filters that are smaller than your sensor’s diagonal length, as well as from using a camera with a sensor larger than the image circle of your telescope and any other attached optics (such as focal reducers or coma correctors). 

Example of vignetting in a flat frame, ZWO ASI1600MM Pro on a Celestron 8” Schmidt-Cassegrain with a 0.63x focal reducer

How Do I Take Flats?

The idea behind flat frames is to evenly illuminate the camera sensor through your telescope so that you can capture the dust motes and vignetting in the same place they will appear in your light frames.  There are several different approaches on how to illuminate your camera.

White t-shirt.  For this method, take a white t-shirt and stretch it over your telescope objective during daylight, and aim your telescope at a blank patch of sky.  It is a good idea to point opposite the Sun so that one side of the image does not appear brighter than the other.  You can use your dew heater strap, a stretchy headband, or other means of securing the t-shirt so that it lies flat over your objective, without wrinkles.

Image during twilight.  For this method, after sunset but before astronomical darkness, point your telescope at an open area of sky, and turn off the sidereal tracking on your mount.  Twilight flats allow you to take longer exposures, which can help with some camera quirks that happen in some CMOS cameras when they switch from “video mode” (short exposures) to long-exposure mode.  You may get stars in your flat frames, but if the tracking is disabled, they will be removed when you stack them if you select a clipping algorithm (like you would to remove satellite trails in light frames).

Illuminated flat panel.  There are a variety of ways to use an illuminated flat panel.  Some people with permanent observatories have a white-painted square on the wall or ceiling.  You can also purchase an electroluminescent panel, which are often battery-operated and sit over top of the telescope objective to provide an evenly-lit surface that is portable.  For instance, I got to test the Farpoint Electroluminescent Flat Field Panel on my refractor.  People who use tracing tablets for this often find that their camera catches the screen refresh rate flicker, which can cause problems with your flats.  The battery-operated Farpoint panel, however, is lit at a frequency of 2300 Hz, and with its ability to become very dim to allow for longer exposures, the flicker does not show up, and the flat frame is evenly lit.

The next step is to choose your exposure time.  There is no set standard since it depends on your camera, filters, telescope, and how brightly lit your flat field is.  The criteria to use when selecting your exposure time is the histogram.  You want the peak to be at about the halfway point of the histogram (in the midtones). 

Example histogram.  In this case, I used my color CMOS camera with a light pollution filter, which is why the red peak is much lower than the blue peak.  The average of the peaks is approximately in the middle.

You want a minimum of about 20 flat frames to reduce noise.  Finally, when you process your images, stack the flat frames to create a master flat.  DeepSkyStacker and PixInsight’s WeightedBatchPreProcessor do this step for you.  Don’t worry if the colors aren’t white-balanced – dust spots and vignetting appear in the same place at all wavelengths.

There are a few important considerations regarding how often you take a new set of flats.  The first is that you must take a new set of flats any time you take the camera on and off or rotate it to a new angle.  Even if you mark the rotation angle for the camera, it is difficult to get it exactly right, and the flat frame dust spots will likely be slightly off from the light frame dust spots.  In addition, taking the camera on and off the telescope is a prime time for new dust spots to appear.  If you set up and tear down your telescope every night, you will need a fresh set of flats each night.  (This is where electroluminescent panels can be of assistance – no daylight needed!)   Also, if you use an open tube-type telescope, such as a Newtonian, new dust spots are bound to appear.  If you have a permanent setup, then taking a new set of flats may become a rare occasion.

Second, you will need a set of flats for each different filter.  Filters are a hotspot for dust motes.  You will likely need different exposure times for different filters, which is just fine – again, the key is to have the histogram peak in the center. 

Third, you will need a set of flats for each camera-telescope-other optics combination.  If you move your camera from one scope to another or add a new focal reducer or a new filter, then the new setup will need its own set of flats.

If you stack an image that was taken over multiple sessions that need different flat frames, labeling your master flat files with the date range is helpful.  Simply calibrate the lights with their matching flats, and then you can stack all of your images at once.

What Difference do Flats Make?

The proof is in the pudding – let’s see the results.

Flat frame, Nikon D5300 on a Vixen 140mm neo-achromat

Without flats

Rosette Nebula, 23x300s, Nikon D5300 in Bortle 5, stacked and background subtracted

With flats

Rosette Nebula, 23x300s, Nikon D5300 in Bortle 5, stacked and background subtracted

While this camera-telescope combination had little vignetting to speak of, there were quite a few dust motes that the flat frames successfully reduced.  It is worth your while!

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