by David J. Watkins
Beginning Astrophotography 101
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The learning curve:
Wide angle Milkyway, star trails, and Moon shots are relatively easy to capture and can be obtained with just a
tripod, camera, and lens. Photographing planets, galaxies, nebula, and star clusters is where Astrophotography
becomes a challenge. It involves much more than pointing up at the sky and shooting away.
Most deep space objects (DSO's) require exposures of at least two hours! It is not unusual for me to
collect 15 hours worth of exposures for faint nebula! Digital sensors are
much more sensitive than film and will not handle single exposures of an hour. So stacking multiple photos
is required to get the proper length of exposure. Deep Space Objects move too quickly to shoot on
a tripod. If you shoot with a focal length greater than 600mm, your stars will blur after a 1 second
exposure on a tripod. It would take 3600 one second images to equal an hour of exposure. Even
at the higest ISO you will not collect enough photons for a stackable image with a one second
exposure. So you will need a GEM tracking mount. A GEM mount has an RA axis that rotates parallel
to the earth's rotation, so you can track the stars across the sky. Typically, you will want to be able to
image anywhere from 30 seconds to 10 minutes. If you image with narrowband filters expect to expose at least
20 minutes even up to 45 minutes per exposure! Even GEM mounts are not perfect, and they will
begin to blur the stars after two minutes, depending on how perfect your polar alignment is.
Even if you polar alignment is perfect, GEM mounts have mechanical gears, and those gears will have some
error that will repeat each cycle of the gears. That error is known as PE, or Periodic Error.
Many astrophotographers use autoguiders for more accurate tracking to compensate for PE, and seeing or atmospheric
conditions. An autoguider
consists of another
small telescope with an image sensor that is aligned with a guide star.
As the guide star drifts, the autoguider will send the correction signals to the GEM tracking mount.
That process is usually controlled by software so a laptop is required for many autoguiding
systems. With autoguiding you can expose past the exposure limit of your DSLR sensor.
You don't have to be an astronomer to acquire good images but it does require a lot of patience. This is a hobby that requires a lot of experimentation and learning. Don't expect to buy a bunch of equipment, put it all together and have everything work flawlessly the first few times out. There are a lot of pieces involved; optics, electronics, mechanics, computers, and software. They do not always play well together! The equipment required all depends on what you want to photograph and how deep you want to get into it. You might be able to get away with a point-and-shoot camera for wide angle images, but to image most deep space objects (DSO's) you will need a super-telephoto lens or a telescope and at least a DSLR camera. The most important thing to master first is your camera. It is important that you have a good working knowledge of you camera like how to determine and manually set the ISO, aperture, shutter speed, and how to focus on a star. If you don't have a good working knowlege of how to operate your camera in manual mode, you will struggle with astrophotography.
As I mentioned in the introduction, start off slow and simple, don't jump full into astrophotography all at once! If you do, you will end up buying expensive things that you may not use more than once when you find out you needed something a little better. A good way to start is with equipment you already have. I had a 600mm lens that I used for wildlife photography, I tried that for awhile then realized that I needed an equatorial mount for photos longer than 1 second. I researched and talked to other astrophotographers and determined that a GEM mount was the way to go. I knew that I would eventually want a telescope to shoot with in addition to my super telephoto lens, so that meant looking at a mount that could handle the extra weight of a telescope, and that meant figuring out what telescope I would want in the future! Do your research and plan ahead. Talk to other astrophotographers, join a few of the on-line astrophotography forums, read equipment reviews. The more informed you are, the better idea you have on what you will need. And don't rely on one source, a local astrophotographer highly recommended to not use my lens but to get a refractor scope and not an SCT. After reading reviews and more research, I ended up getting an 8 inch SCT. The Celestron EdgeHD 8 is specifically designed for photography and with a full frame sensor. I am happy I chose what I did. Though I found when I started imaging with a CCD, that for larger objects that less telescope is often better! I bought a 120mm refractor scope for viewing while my 8 inch scope was imaging. But I ended up using the refractor for imaging some of the larger DSO's. For nebula like the Heart and Soul, I ended up adapting the CCD camera to my Canon 70-200mm lens! So the largest scope is not always what you want!
What equipment do you need?
So what equipment is needed to get started? Before you can ask this question, you need to
decide what types of images that you want to shoot. And if you're like me, you're
thinking "I want to be able to shoot everything!". The problem with that is no
one single lens or telescope will allow you to capture every type of image!
Surely there has to be one general purpose telescope or lens that can do it all, right? Unfortunately, the answer is no.
The moon is very bright and very large. A small telescope or 400mm lens can capture detail on the moon with a very fast exposure. Planets are also very bright and can be seen with the naked eye, but they require super high magnification to be able to see any atmospheric detail. Jupiter can be seen along with it's four moons using a small telescope or long focal length lens (600mm), but it will be very tiny and lack atmospheric detail at that size. At 600mm you will barely be able to see the ring around Saturn, and that is "IF" the Earth's atmosphere is stable enough, at the time you are shooting, to get a sharp image.
A wide angle lens is great for capturing images of the Milkyway band, but if you try to shoot the Moon, even at 200mm you will not capture much detail, it will appear as a very tiny disc. Large emission nebula like the North American Nebula, California Nebula, Pelican Nebula, etc can be captured with a 200 or 300mm lens, but they are so faint that they require several hours of exposure! Since the Earth rotates, those nebula and stars move across the sky. So a simple tripod will not cut it for exposures over 30 seconds, and that is with a wide angle lens. An exposure over one second on a 600mm lens will cause the stars to trail or blur. A motorized equatorial mount will allow exposures greater than 30 seconds. DSO's (Deep Space Objects) like galaxies and planetary nebula are very tiny and very faint. They require long exposures and higher magnification.
Telescopes, which is best?
Eventually you will probably want to get a telescope if you do not have one already. There are alot of resources on the web to research telescopes, so I will not go into great detail here. The things to keep in mind while reading this is what focal length range you are looking for, how much light or apeture you need, and how big a piece of equipment do you want to transport around to get your images. There are 3 basic kinds of telescopes:
A refractor telescope is very similar to a camera lens, it contains several glass elements. This makes refractors
more expensive and heavier. The advantages are that they do not require collmation alignment.
They are sealed so there are no internal parts to clean. They are not as temperature sensitive as
the other telescopes, so they do not require a cool down period to stabilize. The disadvantage is that they
suffer from chromatic aberration, that is color fringing Different wavelenghts of light focus in slightly
different planes when focused through glass elements. Color fringing appears as solid red, purple, blue, or
yellow along the edges of objects. There are refractor telescopes that minimize this effect, APO
or Apochromatic refractors, but they are more expensive. Refractor telescopes typically have smaller
apertures, 6 inches or less. But they do not have obstructions in the light path for secondary mirrors
like reflector or catadioptric scopes so, so more light reaches the eyepiece. You can get refractors with larger
apertures than 6 inches but they can cost you as much as a new automobile! Refractors typically range from
300mm to 1500mm focal length. They are also heavier than
other scopes because of all the glass. They are best for imaging
the moon, large galaxies, and large emission nebula. The smaller aperture is limiting for distant
galaxies and planetary nebula, as it limits the maximum useful magnification.
Many astrophotographers perfer imaging with refractor telescopes over reflectors or catadioptric scopes. They are easier to use with autoguider scopes. One issue to be aware of with shorter focal length refractors (400mm and less) and DSLR's is "Backfocus". You may find that when you attach your DSLR that you cannot achieve focus because the sensor is too far away from the focal plane! Adding a focal reducer may help move the focal plane so that you can achieve focus.
Reflector scopes tend to be the least expensive telescopes. You can get a 10 inch aperture telescope for under $600.00. Larger aperture means collecting more light and this will bring out detail in planetary nebula, distant galaxies, and star clusters. Some light is lost by the obstruction of the secondary mirror. The disadvantages of reflectors are that the they require frequent collmation alignment of the mirror. Also the tube is open so dust and particles will accumulate inside and on the mirror. They are lighter and easier to transport, but are also more delicate because of the exposed mirror. Reflectors cover the entire range of focal lengths, but the longer focal lengths will be awkward to mount. Reflectors typically have a very short backfocus, so you may find that you cannot achieve focus when you attach your DSLR. Adding a focal reducer may help achieve focus with the DSLR. Do your research, don't get stuck with equipment that you cannot use right from the start!
The Schmidt Cassegrain telescope or SCT uses a primary mirror and a secondary mirror at the objective end
to reflect the image to the
eyepiece at the rear of the scope. This allows the scope to be shorter than a
newtonian reflector scope of the same focal length. An SCT has a corrector plate lens on the objective end
of the telescope. So like a refractor, it is sealed so this keeps out dust and particles.
An SCT still has to be collimated, but less frequently than a reflector telescope. All telescopes produce
coma or elongated distorted stars at the edges of the field of view. There are models of SCTs that
have corrector plates to nearly eliminate coma. The Celestron EdgeHD series SCTs are pretty much coma free
making them ideal for imaging. The EdgeHD series scopes are collmated at the secondary mirror.
If you never take the secondary mirror out, you will rarely have to collimate. You can also add a
focal reducer lens to an SCT to reduce the focal length for a wider field of view. This makes the SCT a
more versatile telescope. SCTs are more expensive than reflectors, but less expensive than refractor
scopes. The thing to be careful about with SCTs, is that when you buy the scope and the mount, many
SCTs come with a fork style alt azimuth mount. Alt azimuth mounts are NOT what you want for
astrophotography. See the paragraph below on mounts.
SCTs are excellent for imaging due to their versatility. For autoguiding with an SCT, an off-axis guider is recommended over a separate guide scope. The primary mirror on an SCT will shift as the scope tracks across the sky causing the image to shift slightly, though many SCTs have mirror lockdown knobs that can be tightend after focusing. An off-axis guider uses the same optical path as your eyepiece or camera so it will allow the guider to compensate for the mirror shift while guiding. SCTs at full focal length are more of a challenge to image with, so many astrophotographers use focal reducers when imaging on an SCT. SCT's have long backfocus, so you don't run into backfocus issues like on reflectors or short refractors when adding your camera. There is usually plenty of room for an off-axis guider, a filterwheel, and the camera!
The only type of mount you should consider for astrophotography is a German Equatorial Mount or GEM. These types of mounts rotate about two axis; Right Ascension (RA) and declination. The RA axis gets aligned parallel to the Earth's axis of rotation. If you get a perfect polar alignment, all of the astro objects in the night sky will rotate perfectly around the RA axis. Many GEMs have a port for an autoguider. Since even the best mounts are not perfect, an autoguider will send correction signals to the mount to compensate for any error in the mount gears. It will also correct for a less than perfect polar alignment. I go into mounts in much more detail in the Aquiring Astro Images section.
Learning the night sky:
What? I thought you said I don't need to be an astronomer to do this? You don't, but
if you want to image galaxies, nebula, and star clusters, and you don't want to learn the night sky,
you have to get a successful three star alignment on your
GOTO GEM mount. And the night you start your three star alignment and your handset displays slewing to
Zubenelgenubi, you will want to have some idea where that is at, because most likely it will not appear
directly in the middle of your eyepiece when your mount is finished slewing! One option
is to only choose the stars that you know. But ocassionally it will only give you a choice
of stars that you will not know. I installed the program
on my Windows 8.1 tablet. Stellarium is a program that will plot the night sky for your location.
You can even get it to control your telescope with an RS232 connection and the proper driver for
your mount. The downside to Stellarium is that on a tablet, the controls and buttons are
microscopic and difficult to use with the end of your finger or even a stylus! I ended up
buying a small keyboard the size of a television remote. The keyboard also has a touch
pad. This works very well for what I need. If a star shows up in the handset that
I am not familiar with, I can go to Stellarium and search for where it is in the sky. The
other option is to have a sky chart handy. You should also learn how to read a sky chart.
That being said, another option with a laptop, and a special USB cable, you can use EQMOD, Stellarium Scope, and Stellarium to slew your scope to your target. With EQMOD you can choose your own alignment stars! You can choose as few as one star, or as many as you like! I have been using EQMOD for almost a year now and have no intention of ever going back to using the GoTo Handcontroller!
Now that you have a basic idea of telescopes and mounts, we will discuss the types of astrophotography
objects or targets to shoot.
There are six types of Astrophotography targets:
The next six pages give a brief description of what equipment is required to capture the six different types of astrophotography targets. There is a quick reference review at the end of each of the six pages on what equipment is recommended and what equipment you might be able to get away with as a minimalist approach. The equipment is discussed in more detail in the Aquiring Astro Images pages.