Going Deep with a DSLR
Using a modified Canon EOS 20Da digital SLR, one astrophotographer aims for unusually faint nebulae.
Obtaining the Raw Image
After initial tests with the EOS 20Da, I replaced the camera’s internal infrared-blocking filter with a Hutech Type 1 filter to improve its sensitivity to hydrogen-alpha light. Also, just like my experience with film astrophotography, I found that the longer the total exposure time with the digital camera, the better the final image. I consider 3 hours to be my minimum duration per subject when imaging under dark skies. As with cooled CCD cameras, creating long cumulative exposures is best done with many shorter ones combined; due to the greater sensitivity of digital sensors, they reach the skyfog limit far quicker than film.
From my location, I’ve standardized my exposure length to 10 minutes per frame at ISO 800. Lower ISO values cause undesirable posterization (visible steps between brightness levels) in the faintest areas of my images, and I don’t see much advantage to ISO 1600, with its decreased dynamic range. I save all my images in RAW format, to preserve the entire 12 bits of data produced by the camera. I also turn off the automatic noise reduction and instead record dark exposures (images of the same duration as the light frames, recorded with the camera’s lens covered) at roughly the same ambient temperature so I can calibrate my images later. Of course, accurate guiding of the telescope is a must for any image longer than a few seconds, so I use an old SBIG ST-4 autoguider to ensure round stars every time.
Processing the Raw Image
I used Adobe Photoshop to process my first images with the 20Da but quickly discovered that it isn’t a very effective tool for performing critical steps such as the calibration of individual frames. Also, the plug-in that was included with the Canon software package for converting RAW images to other formats wasn’t as effective as I had hoped, especially in handling color balance. So I purchased the program ImagesPlus (reviewed in the July 2006 issue, page 82), which was specifically created to convert, calibrate, and process digital SLR camera files. (Cyanogen’s MaxDSLR performs many of the same functions.) I now use ImagesPlus for RAW file conversion, dark- and flat-field calibration, and combining the individual images.
I try to record at least as many dark frames per night as light frames and then combine them using ImagesPlus to create a master dark frame, which averages any temperature changes and cosmic-ray hits in the individual darks.
I also use a master bias frame, which is a zero-length exposure that records the inherent electronic signal in every exposure. Bias frames are used when dark frames need to be scaled. While I don’t take new flat-field images every night because my instruments are optically very stable, users of telescopes with moving mirrors that can shift over the course of the night should consider recording new flat-field images each imaging session. Finally, I’m very careful to clean dust off the filter in the camera each night. Shadows from any remaining dust motes I can remove in Photoshop later.
After a night of capturing light exposures and calibration frames, I convert the RAW files to 16-bit FITS format in ImagesPlus using no white balance. In the Calibration Setup menu, I select Auto in the Scale Factors section to compensate for any remaining differences in my darks due to temperature changes. (All digital SLR cameras lack temperature regulation, so differences from one image to the next are inevitable.) Once all the calibration is performed, I align the frames and combine them using Adaptive Addition, which is similar to performing a sum of the exposures with much of the random noise averaged out. With the exception of noise reduction, I do all other processing with Adobe Photoshop.
The images from my EOS 20Da are very red because of the wider red passband of the Hutech filter. To compensate for this, the image needs to be white balanced. White balancing simply means that if an image of a gray card is recorded with a camera in RGB format, the individual red, green, and blue channels are adjusted until the image appears gray. The standard technique in astrophotography to achieve white balance is to photograph a G2V (solar-type) star and adjust the green and blue channels until the star appears white. My G2V test on an overhead star achieved proper white balance when the green channel’s white slider in Photoshop’s Levels palette was set to 133 and the blue channel’s was set to 126. While other white-balance calibrations can be used, anything other than G2V calibration will change the color balance compared to daylight photos.
The next step is to perform a gamma adjustment. Using the Levels palette, I move the center slider until it is just to the right of the primary data within the histogram. This adjustment starts to bring out faint nebulae and galaxies without saturating the already-bright stars in the image.
After the gamma adjustment, I now adjust the sky background. It’s necessary to find an area of neutral sky in the image that’s free of any faint detail I wish to preserve in the final image. Images of nebulae that permeate the entire field can sometimes complicate this step. To monitor any changes I perform on the background, I first open the drop-down menu Window > Show Info (which displays the pixel values of the point under my cursor) and set it to display both RGB and Grayscale readings. I also select the Color Sampler tool and change the sample size from point sample to 5 × 5 average. I then click on the image in my chosen neutral area, open the Levels palette, and adjust the black (left) sliders of the green and blue channels until they equal the red levels.