Introduction to Astrophotography - Part III

My astrophotography home in Chiefland, Florida, a 10 foot dome (center) that houses my equipment. Left is a roll off roof observatory belonging to a friend, right is a pod dome belonging to another friend.

This is Part III of a multi-part series of posts on astrophotography. In Part I, I discussed astro images that can be taken with basic photography gear. In Part II, I took it up a notch and delved into imaging solar system targets like the moon and planets. In Part III, I'm going to take you into the world of imaging through telescopes without breaking the bank.

 DEEP SPACE – THE FINAL FRONTIER

Two types of mounts - (L) Altitude-Azimuth mount; ® German Equatorial Mount.

Celestron Advanced VX mount - $799 new, less if you buy used.

For deep sky targets such as nebulae and galaxies a motorized mount is a must. There are two types, Altitude-Azimuth mounts (Alt-Az) and German Equatorial mounts (GEM). GEMs are the best choice for astrophotography. Once the mount is polar aligned, lengthy exposures can be taken without any target movement. Expect to pay $500 to $1500 for a quality GEM that can handle a DSLR with long lenses and/or many telescopes. To ensure smooth operation while imaging, when selecting a mount, make sure that you review the mount's specifications. The mount’s rated weight capacity must be twice the weight of the equipment you intend to use so purchase wisely.

 All of the mounts in the $500-$1500 price range are motorized and come with a "Go To" feature. "Go To" mounts allow you to select a target from a menu on the hand controller, push enter, and the mount will automatically slew the telescope to the selected target.

Screenshot of Stellarium software after a search for "Great Nebula in Orion"

Alternatively, the mounts can be hard wired to a computer and controlled the same way through planetarium or imaging software. You simply open the software, type in the name of a target (e.g., Great Nebula in Orion) and the software finds the target in the night sky. If it's visible on that particular evening, it appears on the computer screen.

Stellarium screen shot showing what you get when using the "ocular view" feature for a target. In this case, the target is the Great Nebula in Orion.

There are several planetarium programs on the market, some free and some that you must buy. My favorite among the free programs is Stellarium which has a lot of features that make it a program I keep open on my desktop even though I don't use it to control my telescope mounts. One of the features I really like is the ability to see what a target will look like in an image depending on the telescope being used. Stellarium has a built-in cache of images depicting most celestial targets and by accessing them through the "ocular view" feature you can get a preview of whatever target you have chosen. If you like what you see, you can then proceed to the imaging phase of your evening. If not, you simply move on to another target. This makes finding image targets a relatively simple process even if your astronomical knowledge is limited.

If Stellarium isn't your cup of tea, Cartes du Ciel is another free planetarium program you can try. You can also go to http://freeware.intrastar.net/planetarium.htm for a list of many other free astronomy programs available for download.

Learning To Walk

Sky-Watcher Pro ED80, a good, starter 80mm refractor suitable for astrophotography. 600mm focal length @ f7.5. Expect to pay approximately $600 new, less for a used one.

Deciding how to shoot deep sky targets will determine equipment selection that will in turn dictate the size, quality and detail of the targets being imaged. Wide field images can be captured with a DSLR, a modestly priced 80mm (3-inch) refractor telescope, and a Celestron Advanced VX GEM mount (30 pound load capacity). Not counting the camera, your investment would run approximately $1,500 to $2,500 by the time you add desirable accessories such as flattener, a dew heater, a T-mount, and an auto guider.

Wide field image of the nebulae in the constellation Orion. Orion’s Belt is the diagonal line formed by the three blue stars on the left, ending with the blue star in the center nebula.

Above is an example of what is possible with a similar setup, including the accessories. I shot a total of 160 images at various shutter speeds ranging from 10 sec to 45 sec at ISO 1600, and 10 images with 1-3 minute exposures to layer in a sky saturated with stars at the end of the Photoshop process. Before wrapping up, I shot ten “flats” at each shutter speed, twelve “darks” (half before starting the imaging process of Orion's nebulae and the other half at the end of the imaging session) at each shutter speed, and ten “bias” frames for the pre-Photoshop processing in software that “stacks” the images together.

Say What???? Flats, Darks, Bias Frames and Stacking???

“Flats” are images taken to minimize or eliminate dirt, dust, reflections, and other undesirable things in astro images. They can be created by covering the end of the lens with a white T-shirt and shining a flashlight on it or taking an image of an evenly lit surface such as a white computer monitor, any other flat screen light source, or the sky at twilight. They’re used by the "stacking" software to correct any difference in brightness in the main images. 

“Darks” are images taken by covering the end of the lens with the lens cap. Half of the darks are taken at the beginning and the other half at the end. Darks correct the dark signal flaws in image sensors. 

“Bias” frames are images taken with the fastest possible shutter speed the camera can shoot and the lens cap on. They contain only the noise generated by the camera’s electronics on the sensor and is subtracted from the data in the darks to identify the true sensor noise. These extra images are time consuming but they will allow you to create the best possible final image.

All of these images are used in the pre-Photoshop processing of images in software that is commonly known as "stacking" software. Darks, flats, and bias frames will be covered in the next post (Part IV) of this series; stacking will be the topic in Part V.