Astrophotography III

In the last couple of blogs, we discussed astrophotography using a fixed tripod. We looked at several of the different targets that can be shot this way. But after shooting from a fixed tripod for a while, it becomes fairly obvious that there is a need for a way to track the motion of the sky. In this installment, we’ll discuss why a tracking mount is needed and the huge benefits of these for long exposure astrophotography.
When you look at the night sky, what you see is only a fraction of what is out there. It still surprises me what we could see if our eyes were more sensitive. The night sky would look drastically different. The problem is that most of the interesting targets are fairly faint. So as a photographer, how do you deal with faint subjects? Well, the first thing that might come to mind is using a flash. Perfect, except that most of the deep sky targets are more than 1000 light years away, so the light that you just fired from that flash will reach its target sometime in the next millennium with absolutely zero effect. Since that didn’t work, you’d increase the ISO. Now the camera has some level of internal amplification to help see faint objects. But what if that’s still not enough to form an image? Wouldn’t you do a long exposure next?

Here’s the rub. Doing long exposures of the night sky on a fixed tripod won’t work for recording deep sky objects, since the sky is always moving (relative to the Earth). The most you’ll get is a nice star trail image, like these.

These images have their own fascination. But if it’s the deep, dark, faint stuff you’re after, you’ll need another solution.

In order to take long exposures of the night sky, you’re going to need a device that holds a camera and moves at the same rate as the sky. With a device like this, it would allow you to take a long exposure photo with no star trails. It would be analogous to shooting a low light image of a stationary subject on a fixed tripod. So what’s the catch? The tracking motion needs to rotate around the North Celestial Pole (NCP), just as the sky does and at precisely the same rate as the apparent sky motion. When all this is done right, the photographer can aim the camera at a target in the night sky and have the subject remain stationary in the camera’s field of view.
With the general concept under our belt, we need a piece of equipment that will perform this task. There are many ways to establish this polar tracking motion. But the best and most reliable solution is the German Equatorial Mount (GEM). Most photographers may find a GEM a bit cumbersome, so there are portable tracking mounts on the market. But with these there are tradeoffs like stability, accuracy an easy of polar alignment. If you’re serious about trying some deep sky astrophotography, get a good GEM instead.

 

It may not seem obvious at first, but the equatorial mount is the foundation of your deep sky imaging platform. It’s the most important piece of equipment that you’ll use when imaging the night sky. Even with the best camera, lens or telescope, you’ll never get any useful images if the mount isn’t up to the task. So keep this in mind if you’re planning a serious endeavor into deep sky imaging. A mount should be very stable, easy to polar align and have accurate tracking. Sounds simple, right?

There are a multitude of mounts made for astronomy. However, doing visual astronomy (viewing the sky through a telescope) is vastly different than trying to form long exposure images. The stability requirements are much higher for imaging. I’ve used several different mounts, but my first serious mount for DSLR astrophotography was a Losmandy GM-8. It’s made entirely of materials machined from bar-stock, well built and easy to maintain. Many lower cost mounts are made from castings are not nearly as precise or robust. You’ll pay more for a good mount, but remember that it’s the basis of your imaging platform. I later upgraded to a Losmandy G11 which had more capacity and accuracy and finally ended up with a premium Astro-Physics Mach1GTO.

Although the vast majority of GEM’s are designed with the intention of holding a telescope, it’s fairly easy to mount a camera. This usually requires dovetail plate onto which the camera is mounted and then this assembly in attached to the GEM. The equatorial mount’s motion can be a little awkward at first, but you’ll understand its workings in no time. The key is to understand that one axis moves parallel to the Earth’s motion and the other moves perpendicular to this.

Some equatorial mounts also have computer systems that, once properly aligned, will move the mount to whatever target is programmed into its computer. These are called GOTO mounts. This makes it very easy and quick to locate specific targets in the night sky. This becomes increasingly important since the vast majority of your targets will not be visible with your naked eye. A GOTO mount will reduce the amount of time needed to locate and frame the object.

So the next task to think about is what you want to shoot. This is an important decision as there is a difference in equipment needed. Galaxies and asterisms (patterns or groupings of stars) can be shot with a stock DSLR and have better results with longer telephoto lenses. The first example is an asterism called Kimble’s Cascade.

Below is an image of Pleiades when Venus was near by.  This kind of image can be done with a stock, unmodified DSLR, since this is a reflection nebula and contains no hydrogen alpha emissions.

Nebulae are generally larger, fainter and are recorded more easily with a slightly shorter focal length (~200-500mm focal length). However, if shooting nebulae is where your interests lie, there’s a little more work to be done. This time the work is with the camera itself.

Nebulae generally look like clouds in the galactic expanse. But these are actually gigantic clouds of ionized gas. So what does this mean? Think of an electric neon sign. The sign glows because there are gasses in the tubes that are electrically excited. The atom’s electrons are moved to more energetic orbits by the electricity. But these electrons don’t like these energetic orbits and the fall back to the more stable orbit. When this happens, light is emitted. The color of the light is characteristic of the gas being excited (and some other things as well). This is important…why? Well, it turns out that one of the most abundant gasses in deep space is hydrogen. When excited by nearby stars, the hydrogen glows deep red (and some other colors, but not as intense). This red emission is by far the most important and most abundant color of nebulae. Great! Our cameras can see red, but here’s the catch. Stock DSLR’s and most other cameras have a filter (or set of filters) that is internal and usually lie just above the imaging sensor.  Here’s an example:

These filters are what block the ultraviolet and infrared light and pass the visible light that is properly colored for your photos. The deep red light from the hydrogen nebulae (called hydrogen alpha) generally gets blocked by these UV/IR cut filters.

So that’s great… I have a camera and equatorial mount aligned to the NCP. But now I won’t be able to see what I want to photograph. Well, some of the red hydrogen emissions are able to get through the camera’s filters, but not much. What’s needed is to have your camera modified so that your camera is able to efficiently record the hydrogen emission.

There are astrophotography specific modifications that can be had. But I find that the most flexible modification is a full spectrum modification.  I find this true also for IR photography. Having the camera modified in this manner allows you to install different filters that make the camera look at colors of specific gasses (for astrophotography), and easily changed for different wavelengths for IR photography.



With a full spectrum camera, a filter of some sort is generally needed, for either IR or astrophotography. The easiest way to get started photographing nebula (with a full spectrum modified camera) is to add an IR blocking filter. If you’re familiar with IR photography, you know that color and IR light focus at different positions on the film plane. This effect is seen in astro photos as “star bloat”. Since the stars also emit IR, they will sometimes appear out of focus. This generally depends on the telescope or lens that you’re using. Some telescopes are color corrected to fairly long wavelengths. Also, if you’re using a telescope with mirrors instead of lenses, star bloat from IR light is not really a factor. But generally it’s best to block the IR light when taking astro photos with a full spectrum modified camera.  Below is an example of the Heart & Soul Nebula.  This is one of my earliest DSLR astro photos:

This is an example of a DSLR shot of the Horsehead Nebula.  The hydrogen alpha emissions from these nebulae are quite dim and would be invisible without a long exposure.  This was shot with an EF70-200 f/2.8L lens at 200mm.  This should give some indication of the size of these objects in the night sky. Many of these nebulae are several times larger than the diameter of a full Moon but are too faint to see with your naked eye.

OK, now you have a mount and a camera that’s capable of seeing what you want to shoot. How should you proceed? You might want to pick a target first. A fantastic first target to shoot is the Orion Nebula. This one, and the one below was shot with a 480mm focal length apochromatic triplet refractor (telescope) and a full spectrum modified DSLR.

This nebula is fairly bright and can be shot with a stock or modified camera.  In fact, I’ve even shot Orion in IR with a special filter arrangement on my telescope.  These are both images of the Orion Nebula. The image below was shot with a 740nm filter installed.

But with even longer exposures the addition of deep hydrogen alpha emission can make the same target look vastly different.  This was shot with advanced imaging equipment, including a full frame monochrome CCD in place of a DSLR. For those interested, here is a sneak peak of some images that can be shot with more advanced equipment with very long exposures totaling many hours of exposure time.

The biggest problem with Orion is the large dynamic range. This usually requires some post processing work and maybe multiple exposure lengths to capture the entire dynamic range.   After a few outings you’re probably going to see that the images that you’re shooting are plagued with noise, grain and other artifacts. This is where some special shooting and processing techniques are helpful. I’ll be covering some of these details in the next segment in this astrophotography series. In the mean time, get out and shoot. See you next time.