Please, no giggles. And yes, flat frames are a widely used astrophotography technique. But like many tricks that astrophotographers use, you can make use of flat frames yourself to do some clever things.  If you’re impatient you can skip ahead and discover how Flat Frames can be used below and decide if it’s worth reading the rest of this article.

What Is a Flat Frame?

A flat frame is a normally exposed image with the entire field of view of the image lit as uniformly as possible.  Like dark frames, flat frames are rather dull and uninteresting things to look at. They are visually white or gray and quite boring.

Flat Frame: Canon 5D II with 16-35mm f/2.8 L II lens at 20mm

How Do I Create a Flat Frame?

There are many ways to create a flat frame, let me quickly run through a few. First make sure your zoom, aperture and focus are as you will be using them because vignetting and center of field brightness change as you adjust the zoom, aperture or focus. I usually set my camera on aperture priority mode and let the camera meter for me.

But how should I take a flat frame image?

1. Is your sky cloudless (or uniformly gray)?  If so, point the focused camera at the sky (and only sky) and take a couple of normally exposed images.  Because it’s probably not as uniform as you think, try rotating the camera and pointing it differently to get a good average flat frame. If you are using a very wide-angle lens, it may be hard to get “only sky”.
2. Take a clean white t-shirt. Drape it over the lens or lens hood. Smooth it out. Shoot a few frames, rotate the shirt, shoot a few more. Obviously if you’re doing this at night, you’ll need a uniform light source – the good news is the color temperature doesn’t matter much.
3. Select a uniform white or gray display on your iPad, iPhone, Mac or laptop computer. Hold the tablet up against the lens – making sure the lens is completely covered by the display and take several exposures. Rotate the camera or light source to avoid hot spots.

What Can I Do with a Flat Frame?

Did you read this far hoping that flat frames could in fact be useful somehow? Well then, here is the good news. With flat frames you can:

1. Remove Dust – since dust tends to move around having taken flat frames very near to the same time you took your normal shots increases the effectiveness of the dust removal.
2. Remove smudges on the sensor
3. Reduce or eliminate vignetting.

To effectively use a flatframe, however, you must be able to use layers.

Using a flat frame you can get this result

Even though you started with this:

Notice how the bright center of the field has been normalized. You may not think of the center of a lens as being brighter, but you are probably quite familiar with the outer edges being darker, that is, vignetting. Because these images have been cropped the bright area is not centered as you would expect.

What about Dark Frames – Are They Related?

Flat frames and dark frames are not related at all and are used for very different things. Astrophotographers will normally take Dark, Flat, Offset and Light frames… all of which  serve different purposes.  We do recommend taking dark frames for night or low light photography.

How Do I USE a Flat Frame

You’re welcome to look up how to use Flat Frames in any of the references below, but we will be providing a “part two” article with the details.  We’ll also cover Flat Frames both in the next Astrophotography 101 and Photo Manipulation webinars.  If you want to be in the next webinar, please join our subscription list and we’ll let you know when we schedule it!

References:

• http://cs.astronomy.com/asy/astro_imaging/f/16/t/45752.aspx
• http://www.cloudynights.com/ubbthreads/showflat.php/Number/4875702
• http://www.willbell.com/ccd/photoshop_astronomy_samp.pdf
• http://geoastro.co.uk/lightbox.htm
• http://www.dl-digital.com/astrophoto/p2-astro/Astro-Workflow.htm