Astro Photography

Some Astrophotography is pretty easy and doesn't require special equipment.

Note: focusing digital cameras on stars is actually rather difficult, especially if your camera or phone only has a digital viewfinder.  Practice doing this before getting near a scope. :)

Cell Phones

If there is a bright object (Moon, Jupiter, Saturn) in the eyepiece, you can use your cell phone to take pictures.  The hardest part will be lining up the phone and getting it to focus.  Take a bunch and check the images.

Cell phones are also getting very good at low light photography.  Some even allow long exposures.  With wide angle shots of the sky, you can get maybe 30 seconds of exposure without the stars forming trails from the earth's rotation.  you'll need to prop it up against something or use a tripod.

Cameras (DSLRs, etc.) - Tripod

Cameras, whether they be SLRs, DSLRs, Point and Shoot, Mirrorless, etc., can take great shots of the sky.  You WILL need to use a tripod or be able to prop up the camera.

Those MilkyWay shots you see everywhere?  All you need is a dark sky and a wide-to-normal lens.  The amount of light pollution will limit how long you can expose a shot.  Bright skies wash out the stars.  In a dark sky, your exposure length is limited by your focal length.   Expose too long and the earth's rotation causes star trails. 

A rule of thumb for non-tracking shots is the 500 rule:

• Shutter Speed = 500 / focal length (for full-size (35mm) sensors)
• Shutter Speed = 500 / (1.5 * Focal Length) (for APS-C crop sensors)

For example, a 10mm lens on a Sony A6500 can shoot for about 30 seconds without trails (500/(1.5*10).

Set your ISO for the lowest you can go and get good detail without too much noise.

Cameras on Tracking mounts

At some point, the focal length of your lens or telescope (just about any telescope) requires you to mount your camera/scope on a tracking mount - one that tracks the movement of the sky (well, compensates for the rotation of the earth).

Once you start using tracking mounts,  then you're into the realm of true astrophotography.  You're no longer limited by star trails, but by vibration, alignment, light pollution and a thousand other things.

Many telescopic mounts can accept a camera adapter and many telescopes allow cameras to ride "piggyback" on them.  If you have a telephoto lens and a way to get your camera on a tracking mount, you're good to go.  Expect to take a bunch of shots and make a lot of bad images.  But some may be pretty good.  If you're at an outreach event, recognize that "popping a camera on" may take a lot of time and it may impact the visual observers.

Here's a shot on a tracking mount using a 200mm lens.  Notice the width of the field of view: from Orion's Belt to below the Sword.  The Orion Nebula is prominent and the Flame Nebula can be made out (left of the Belt).

Note the field is still quite wide.  These focal lengths make nice shots with something in the foreground (landscapes, etc.).

Cameras Attached to Telescopes

Once you want to start getting details of objects, you pretty much will want to mount your camera on a telescope.

"Prime Focus" Photography is when you mount the camera on the telescope instead of an eyepiece.  At this point, the focal length is so long, you absolutely need a tracking mount.  Focusing is not easy and autofocus will not work (your camera cannot tell the scope to focus).  This also takes a long time at an outreach event...

You need an adapter that connects the camera to the telescope focuser.  One side of the adapter fits your camera mount, the other side fits in the telescope as if it were an eyepiece.

The shot below is an unprocessed (straight from the camera) shot of the Orion Nebula (It is cropped).  The camera was mounted on the telescope instead of an eyepiece. 

Taking Astrophotography to the Next Level

All the shots above are single exposure straight out of the camera.  They are pretty legitimate Astro Photos.  Call them One Shot Wonders.

Stacking

Few "professional-looking" images are made up of single exposures.  There are a number of reasons.  A major one is that the amount of light pollution will limit how long you can expose before the sky glow starts to wash out your target.

"Stacking" is a way to take multiple photos and combine them to simulate a longer exposure.  For example, if your sky limits your individual exposures to 30 seconds (or your camera does), just take a whole bunch of 30-second exposures and Stack them (using Stacking software).   Fifty 30 second images can be stacked to give you the equivalent of a 15-minute exposure (more or less, bad shots (airplanes, camera bumps, etc.) can be excluded).  Noise can be cancelled out and the signal boosted with the software = much better images.

Stretching and other Processing

Stretching is a technique to adjust the brightness so that the subtle differences between low-light variations (most objects are very dim and the differences in the brightness in the details are VERY subtle).  Stretching essentially allows you to use the digital light variation storage to be used just for the brightness range you need.  You stretch a narrow brightness range across the full range of the photo format.

A LOT of other magic can occur to make your astrophotos look spectacular.  General software (like Photoshop and GIMP) as well as specialized astrophotography processing packages can be used. 

Here's a single, unprocessed shot next to a stack that got the expert treatment:

Compare this beautiful image of the Orion Nebula by Dr. Fogel to the "one shot wonder" above