Hints & Tips for Digital Astrophotography
Written by Stefan Parmark (gamma@coronaborealis.org) in cooperation with Societas Coronae Borealis (contact@coronaborealis.org)
All rights reserved
I will herein describe the experiments I have performed within digital astrophotography, and what I have learned. Hopefully this will help you, using your digital camera to capture beautiful images of the stars and nebulae.
Equipment
The equipment I use is:
Camera: Canon Powershot G2, 4 Mpixels, 3x zoom
Telescope: Skywatcher 150/75EQ3-2 focal length 750 mm Newton, diameter 150 mm
Focal multiplier: 2x Barlow lens (optional)
Camera adapter: Scopetronix MaxView 40
Software: Mira, Photoshop, TifSplit
Observation locations: Oxie and Västra Ingelstad, both just outside Malmö in southern Sweden.
1. My First Experiments With a Camera On a Tripod
My very first astro photo session with a digital camera was with the camera mounted on a normal tripod.
I always set the camera to manual mode, RAW image format and maximum aperture.
1.1 Maximum exposure time
I took pictures with the camera's maximum exposure time, 15 seconds. When pointing the camera to the south sky,
I noticed that stars became lines, because of the Earth's rotation. By experimenting a little, I found that for 3x zoom, 5 seconds seemed to be an upper limit. In the north sky, the rotation is less noticable, so here 15 seconds was OK.
A rule of thumb I found, was that (focal length/mm) x (exposure time/s) x cos(RA) shall not exceed 750. This allows the stars to move about 1.5 pixels.
1.2 Focusing
I focus the camera on a distant light subject, and then select manual focus to lock the focus.
1.3 Thermal noise
When taking pictures, there is a problem with thermal noise. The CCD is getting warmer while you use it. This will cause electrons to excite, produce false pixels on your image.
Before taking any picture, I will let the camera cool down to the same temperature as the night air. Usually 20-30 minutes will be long enough to wait.
My experiments have shown that if I turn the LCD display off, and wait for about 1 minute between the exposures (for 15 s exposures), there will be no thermal noise.
It seems like the CCD chip is active all the time that you have the LCD display on, and that it actually shuts it off when you do not take photos or use the LCD display. When is is shut off, it will cool down, to be ready for the next shot.
Unfortunately, turning the LCD display off also disables the MF mode. But, to my surprise, I found that this does not matter! The camera tries to autofocus, but since the image is so dark, it cannot find anything to focus on. So it just stays where it was! I don't think this was intended, but it is very useful. Of course, the camera will be able to focus on the moon and other big objects, but not on tiny stars!
This is a 5 second exposure of Orion.
Click for larger image
2. Using the Telescope for Guiding the Camera
Attaching the camera piggy-back on the telescope, I have no problem with long exposure times any more. Well, as long as I have polar aligned the telescope.
This is a 15 second exposure of Orion.
Click for larger image.
This is a sum of 15 exposures, 15 seconds each, of Orion.
Click for larger image.
3. Photographing With the Camera Through the Telescope
With a MaxView 40 adapter, I connect the G2 camera to the telescope.
Magnification is 60x, or 120x with a 2x Barlow lens.
3.1 Focusing through the telescope
I set the camera to manual mode, RAW format, zoom 3x, max aperture 2.5 and manual focus set to infinity. That way if I accidently touch any controls, which I occasionally do, it is easy to restore the settings again.
The image shakes quite a lot when I touch the camera, so I prefer to focus with the telescope instead, and then lock it. I also feel that I have better control that way.
I get the magnified LCD image by putting a rubber band across the camera, and right over the MF button. I put a small wooden plug between the rubber band and the MF button, to hold it pressed. I then press the up button, which activates the magnification, and press it until the distance is set to infinity.
Now, without releasing the MF button, I move the telescope to a bright star, and focus with the telescope until the star is as small and sharp as possible.
Using the autofocus through the telescope does not seem to work, so skip that. Use manual focus instead.
This is an average of 7 exposures, 15 seconds each, of the Orion Nebula.
Click for larger image
4. Photographing With a Web Camera Through the Telescope
In the future I will experiment with connecting a web camera to the telescope.
5. Creating a flatfield
A flatfield is a picture taken on an evenly lit object. A clear sky is very close to this. A very foggy day is also good. I take about 20-30 images and average them. The more the better. It is important that the zoom and aperture are the same as you are going to use, so set them to 3x and 2.5. Time does not matter, but you will notice that you have be out early in the morning or late in the evening, so that the picture will not be overexposed. The shortest time seems to be 1/500 s.
I move the camera randomly around over the sky as I take the flatfield pictures, and discard those where birds or clouds appear. It is important that there are as little structures as possible.
Now I convert the pictures to 16-bit TIF, and average them. I do this in Mira. Mira can't handle 16-bit colour images, so I have to split the picture into its' R, G and B components first. A wrote a small C program, TIFSplit, to do this.
You will find that the blue component is clearly dominant, and red is weakest. Make sure that blue was not overexposed, otherwise the picture is useless. Values should be as high as possible, without overexposing, to get as much precision as possible.
The flatfield that you create is specific for your camera, and can't be used for any other G2. It tells how responsive each pixel in your CCD chip is.
Now that you have the flatfield, you are ready to stack some real images. In an astronomy image program, you flatfield the images first, before you do anything else. After that you can register them (move them, so that the stars are in the same pixels). Then you can add them or average them.
All rights reserved by Societas Coronae Borealis (contact@coronaborealis.org)
This is a 5 second exposure of Orion.
This is a 15 second exposure of Orion.
This is a sum of 15 exposures, 15 seconds each, of Orion.
This is an average of 7 exposures, 15 seconds each, of the Orion Nebula.