Astrophotography - Photographing the Milky Way

I remember the first time I tried to do Astrophotography and I was disappointed by the results.  I kept trying and making the same and other mistakes, but I kept trying.  Standing out in the cold at 2 a.m., I wondered why I was I doing this.  I would usually shoot too fast or not have my ISO high enough, I did not know where the Milky Way was.  Then one night, everything just clicked.  I could make out that most amazing sight, the Milky Way.  Just seeing it with your naked eye is not quite enough, but when you see it in its bright and beautiful glory on the back of your camera's LCD you truly see it and it's finally put into perspective.

Astrophotography does not require all that much equipment.  

1.  Digital SLR - Any DSLR will work, but a camera with full frame sensor is a better choice.  It can handle higher ISO with better noise ratio and more light can reach the sensor with a full frame sensor.  My Nikon D810 works well for this type of photographing and they even make a D810A that is designed for astrophotography.

2.  Wide Angle Lens - The Milky Way is big and the easiest way to capture it is to use a wide angle lens that allows you to frame a large portion of the sky.  A lens with a focal length of 24mm or shorter works well, the shorter the focal length, the wider the field of view.  For better results utilize a lens with a low f/number, f/2.8 is a good choice.  The lower the number, the larger the aperture of the lens and the more light it can collect for exposing the stars in the night sky.  I use a a Rokinon 14mm f/2.8 lens when I photograph the night sky.

3.  Tripod - A tripod is a good investment in any photography but it is essential in astrophotography.  There are many different tripods to choose from, but there are somethings you might want to think about.  A tripod should be stiff and stable, ensure that you are not close to exceeding the load capacity of the tripod.  This will make the tripod less stable and could tip over with the wind.  Carbon fiber is stiffer and lighter than aluminum, I strongly suggest a tripod that ways less than 5 pounds.

4.  Headlamp - You will need you hands free at night to handle the camera in the dark.  Make sure the headlamp has the red "night vision" option.  This helps retain your night vision for seeing in the dark better and won't interfere with other stargazers in the area.

5.  Optional Items - A cable release or intervalometer.  A cable release will let you trigger the shutter with out touching the camera and is also required for exposures longer than 30 seconds.  You can set the camera to timer and use this option to trigger the shutter, but you can not trigger for longer then 30 seconds.  A intervalometer is a cable release that lets you program in different settings such as exposure time, interval between exposures, or number of exposures.  You can buy a cheap intervalometer for around $20 but most Nikons have a built-in intervalometer.  You will also want to invest in extra batteries as running the exposures and cold temperature will drain battery life quicker.

Finding the Milky Way

One of the first things you need to do when photographing the Milky Way is to find it.  With the advent of technology this is not too hard to do with the many apps that are out there for your smartphone.  Some of the best apps out there for finding the Milky Way include PhotoPills, SkyGuide, Stellarium.  The Milky Way is visible all year; however the galactic center is visible certain times of the year, February to November.  From March to May, the center is visible towards the Southeast and is more diagonal and panoramic.  May to July, the center is visible from the Southeast to the Southwest and is more diagonal.  July to November, the center is visible to the Southwest and is more vertical.

You will want to shoot on clear dark nights in order to get better results.  A night that is 30% or clearer during the new moon phase are the best nights to shoot the Milky Way.  The NOAA website has a chart that will show the sky cover for many areas through the United States.  Clear Dark Sky (www.cleardarksky.com/csk) is another site that can be used for planning purposes.  The best times to shoot is usually the time 1-2 hours after the setting of the Sun or Moon or 2 hours before the rise of the Moon.  For example, wait 2 hours after the sun sets.  The Moon is not as bright as the Sun; the time will be shorter, usually 1 hour.

Focusing Your Lens at  Night

Focusing is by far the most important skill you must learn for astrophotography.  No matter how well you perform any other skills, without a well-focused lens, your photos will not turn out the way you visualized them.

Method 1:  Preset focus point during the day.  It is far easier to ficus during the day than at night.  

Method 2:  Live view focusing at night.  Use live view to zoom on the brightest star, when you turn the focus ring notice the star getting larger and then smaller.  Turn the focus ring until the star is at its smallest, the star should be in focus.

Method 3: Focusing at night with artificial light.  Shine a light source on your subject of focus, manually focus your lens on the lighted subject.

Whatever method you use always use test shots to verify that stars are in focus, if is not in focus try and refocus until the stars are sharp.

Selecting Exposure Time for Astrophotography

The 500 Rule is used to calculate the maximum time a photo can be exposed without exhibiting star trail behind each star in the photo.  The 500 Rule calculated exposure time is only a function of lens focal length.  ISO and Aperture do not affect the 500 Rule calculated exposure time.

If you are shooting with a full frame camera, take the number 500 and divide it by the focal length you will be shooting.  For example, I have a 14mm lens so I take 500 and divide it by 14 and get 35.7 rounded up to 36 seconds.  If you have a crop sensor you will have to multiply the focal length and divide 500 by that factor.  You can get the crop factor by looking at your owner's manual, most will be 1.5 or 1.6 depending on brand.  For this example I have a camera with a 1.5 crop factor.  So 14 times 1.5 equals 21.  500 divided by 21 equals 23.8 rounded up to 24 seconds.  Of course this is a guide so you will have to shoot some test shoots to determine what time works best, fine tune the exposure time by 1-2 seconds either faster or slower.

Selecting ISO

Never increase ISO prior to fine tuning your exposure time based off the 500 rule.  I usually start off at ISO 800, this will usually be dark.  I increase the ISO by one stop until I find the Milky Way is clearly visible in the photos.

Shooting RAW

A RAW file contains much more data than what can be seen on your camera's LCD.  The image seen is a JPEG rendering of a RAW file, it can't show all of the data contained within your file.  You can only view all of this data on your computer.

Other Camera Settings

I shoot evaluative metering as this meters for all scene and not just the center.  Because I shoot RAW I have more leeway for white balance as I can adjust this in post processing, but if you shoot JPEG try using a cooler setting.  If you are not trying to shoot multiple exposure for time-lapse or exposure stacking, turn on your camera's Long Exposure Noise Reduction.  This will take two exposures for same length of time, one with the curtain opened and one with the curtain closed and compare the two and reduce the hot pixels for the exposed image.

Test you Camera Settings & Compare the Images

Take test shoots and try the different settings on your camera.  This lets you see what camera can do for you.  It is all about making multiple shots and experimenting.

Hopefully this will help you get started with Astrophotography, the secret is just keeping trying and not getting frustrated.