DSLR Imaging Tutorial

Imaging with a DSLR and Simple Tripod

Most of the imaging procedures for DSLR photometry will be very familiar to those who have dabbled in astroimaging. The goal of photometry, however, is to accurately record, not appearance, but the relative brightness of each star compared to other stars in the field. This accuracy is achieved first, by calibrating each image to eliminate both electronic and optical anomalies of the camera and second, by stacking multiple exposures. Stacking increases the signal (star photons) to noise (background photons) ratio and averages out the effects of atmospheric variability. Also, be sure to record all your images in "RAW" format.

The electronic anomalies of the camera are corrected by taking a series (~10) of "darks". These are images made with the lens cap on. Darks record internally generated electronic noise such as that produced by "hot" pixels. I should note that some DSLR imagers have suggested that darks are not necessary with exposures in the 5 second range.

Optical aberrations are handled using another series (~10) of images called "flats". Flats are images of a blank surface such as an evenly illuminated white foam-core board. Since the target is blank, flats highlight optical aberrations like dirt on the lens or distortions such as vignetting. Computer software will process the darks and flats to eliminate these imperfections and produce calibrated star field images.

There are a number of different methods used for taking flats. Light boxes and sky flats are two popular techniques. I use a simpler approach. I take flats by imaging a white foam core board in a dimly lit room. The dim lighting allows me to move the camera around while taking a longer (~5 sec) exposure. This motion eliminates the possibility of recording surface anomalies on the board. It is essential that the board be evenly illuminated and that the optical path (lens, focus, f-stop, etc.) be exactly the same as you used while imaging stars.

With the darks and flats in hand, it’s time to image the star field. In addition to the tripod, you’ll need a cable release. A right angle viewer can also be very handy, especially when aiming near the zenith. The focal length you choose should provide a field-of-view wide enough to include not only the target star, but also an array of comparison stars. These comparison stars will be used to compute a Transformation Coefficient and determine the final "V" magnitude of the variable star.

Locating target stars can be a challenge because they will typically be too dim to see in the viewfinder. I try to identify a bright star near my target star and, knowing my field-of-view, use a star chart to determine where I should place the bright star in the viewfinder.

Once the desired stars have been located, it’s actually best to defocus the camera slightly to spread the star images over a larger number of pixels. I do this by setting the focus to "manual" and turning the focus adjustment as far as possible toward "infinity". The infinity stop is actually slightly out of focus.

Taking between ten and thirty images at maximum aperture and 400-1600 ISO should work well. Exposure times will vary depending on the magnitude of the target star. Try a 5-second exposure as a starting point.

With your star images, darks and flats recorded, your ready to download the images and begin processing.