Why a GEM for astrophotography?

*The GEM*

During a past on air hangout I spoke a bit about photographing Deep Space Objects and the equipment that I use to take those photos.  One of the things that was mentioned is that I use a GEM (German Equatorial Mount) on a very heavy tripod.  The GEM or Equatorial Mount is a really fancy way of saying it’s a tripod head that aligns the camera lens (either regular lens or telescope) with the earth’s axis.  I didn’t really explain in great detail why this is a must when you are trying to do very long exposures or “Stack” several shorter exposures. 

Below is a photograph I took from the back deck of my house.  I had my camera mounted on a “Regular” camera tripod.  The region of the night sky may be recognizable by many people, it is the Orion Constellation area.  This photograph was achieved by taking several 13 second exposures (@24mm) and “Stacking” them using a free program called Deep Sky Stacker.  The total exposure on the image is just under 6 mins.

Since the earth moves, I had to move the camera to keep the Orion Constellation in the center of the viewfinder.  You will notice in the photograph that the in center of the photograph the stars are nice and round, as you move out from the center the stars start to take on an “Egg” shape, and at the very edges of the photograph, you can actually see what appears to be “Star Trails” and the image also gets a fuzzy look to it.  The reason for this is quite simply stated because a regular tripod mount does not rotate on the same axis as earth.  Each time I took another photo the Earth had moved and since my tripod head doesn’t move relative to the Earths axis the stars moved in the frame.

If you don’t mind this look, you can actually take shots with the equipment you probably already own.  If you’re looking to have an image like this where ALL the stars are nice and round you will need to purchase a GEM or something like Astrotrac, both of which move your camera with the axis of the Earth.

Deep space photography is not as hard as you think it is!

Deep space photography is not as hard as you think it is!



Click for larger image


Yes, to get some spectacular images of galaxies and nebula you do need a good telescope (or lens) and GEM (german equatorial mount) so that you can take longer exposures without elongating the stars and get a bit closer to the object you wish to photograph.

There are however several deep space objects that you can photograph without using specialized equipment.

In 1771 Charles Messier was kind enough to amass a list of 45 objects (the list now contains 110) in space that can be seen quite easily with the aid of binoculars or a small telescope.

Many of these objects can even be photographed using high ISO and the maximum length of exposure your chosen lens and camera will allow (without getting star trails). More on that here

The photograph on this page is a good example of what I'm talking about.

The photograph was originally intended to be a test of my new camera and its high ISO performance, but when I looked at the photograph, I had unknowingly captured the 11th deep space object on Charles Messier's list, known as the Wild Duck Cluster.

I know it's not as "Impressive" as some of my telescope shots, but it is pretty neat to be able to say to your friends "oh yeah, that's Messier _ _" and give them a description of it.

Next time the skies are dark and clear, get out there and give it a try!

The specs on this shot are as follows
Camera - Nikon D7000
ISO - 3200
Exposure - 15 seconds - 18mm - f4.5

Minimal Processing in LR



The Power of Stacking Images

This was taken on an entry level DSLR with a 10 mp CCD sensor. Which is far from a great camera by today's standards.

The lens used was a very old kit lens from my old Pentax ME-F a 35mm 1.7

I was basically testing the lens performance at imaging stars, the night was not very good for astrophotography either, there was poor visibility and periodic clouds.

In the near future I will be doing another "stacking" of images with a bit longer (80mm) focal length using just my dslr, a regular lens, and a tripod.  I will make another post with the resulting image to show the power of stacking even more.

Settings used
Exposure - 15 seconds
Iso - 800
Lens - f 2.8
2 second delay timer to minimize shake

I took 5 images and stacked them using a free program called Deep Sky Stacker
and did some further processing in Photoshop.

I've also labelled the more known stars and some areas that contain some of the better known deep space objects. You can even see (faintly) the Orion nebula.

This image is several images stacked around the Orion Nebula, Running Man Nebula, and The Horsehead and Flame Nebluas.  The more images that you stack the more information will be in the stacked image.

Why Subtract Dark Frames?

I’ve seen many people doing night time / astrophotography I thought it would be beneficial if I did a really quick post on Dark Frame Subtraction.

Below are two images, one shows a section of a photograph before dark frame subtraction and the other shows after. (note the red dot in the before image)

Without Dark Frame Subtraction
Red Dot is Generated From Camera

With Dark Frame Subtraction
Note Red Dot is Gone!

First I’ll explain what a Dark Frame is for those who do not know. A dark frame is simply a photograph taken in complete darkness that is designed to capture the “Bad Pixels” on your camera sensor. When you take long exposures your camera tends to get “Hot Pixels” which show up in your image (sometimes as red dots). The intent of Dark Frame Subtraction is to reduce the amount of noise that is produced by your camera. 

Many of today’s cameras have “High ISO” or “Long Exposure” noise reduction. Essentially this does the exact same thing that I’ve done in post processing for this example. To explain this a little more, when you use these settings your camera will take the “Light Frame” (image that you are photographing), and then take the dark frame (image with the mirror down), it then subtracts the dark frame from the light frame, leaving you with the final image. That is why (you may have noticed) that it takes about twice as long for your camera to show the preview image when using these settings. Processing your images this way is just fine if you are taking a single exposure, but what if you are doing star trails, or even Deep Space Photography where several images are combined to give you the finished product. Star trails will have gaps and appear as a series of “Dashes” if in camera noise reduction is used. That is why it is important to learn how to do this in post processing.

Making your dark frame is easy, but there are a few things that you have to remember. Number 1 – Take your dark frame with the EXACT same settings as you did your picture (Noise changes with different camera settings). Number 2 – Take your dark frame at the same ambient temperature as you did your light frame(s) (outside temp). Temperature also affects noise.

How did I do this
Simple 
- Open your two images in photoshop (or elements)
- Now that the two images are open, go to the dark frame and select the whole dark frame by using the “Marquee Tool”
- Copy the image
- Paste the dark frame on top of the Light Frame
- Using the layer mixing options select “difference” (here you can play around with the “Opacity and Fill” levels to suit your taste.
- Once you have achieved your desired results flatten the image and voila, you’re done.

Important note
There are many programs out there today that you can use to do this automatically (using your dark frame) when combining deep space photographs or combining images for star trails. It is usually much easier to just use these programs, but it is always nice (and important) if you understand why you are doing something, or how it is actually being done.

Clear Skies!
Darryl W. Van Gaal