Taking Great CCD Images
Submitted: Wednesday, 10th October 2007 by Philip Holmes
I’ve found over many years of capturing CCD images there is a big learning curve which makes capturing these beautiful images part of the fun that goes into a long night at the telescope and computer , a bit of sleep some processing and then waiting till night falls again so I can do it all again - providing the weather OK.
There is nothing like sitting watching the computer screen downloading that next image. I got into CCD imaging as a bad neck injury hindered my ability to scan the sky looking through the eyepiece at these lovely objects . The ability to take CCD images has really helped me with my neck injury. I feel the goal of taking these images for me is that after a nights imaging I have a beautiful image I can share with my friends and to share with people that do not have the equipment to do so. It’s funny showing someone a photo of the night sky and then saying “Is that really up there?“.
I couldn’t believe that these photo where taken with a black and white CCD camera . How do we get a colour photo out of a B/W CCD camera. The answer is we take a number of exposure through different filters which may include RED, GREEN, BLUE or Luminance (L), R, G, B . Now we are seeing narrowed band photos which let through specified wavelengths of light . These may include H/alpha, Sulfur II, Oxygen III to name a few.
So what goes into capturing these photos with a CCD camera?
is just a part of how to capture these photos.
A Typical Night at the Telescope
Well where do we start? I can’t stress enough about polar alignment . Its one of those things when your starting out where you say “that’s close enough that will do” but when you have just taken 30mins or longer of exposures you soon find out how good your alignment is. So why do we need to polar align? The answer to that question is field rotation , mis-alignment of images, good tracking, and the ability to put the subject on the chip.
How do we polar align? I use the drift alignment method there is plenty of information on the Internet on how to do this. If you are lucky enough to have an observatory this only needs to be done once.
The Subject I am going to Image
How long should the exposure be? The key here is to pick one subject and spend the whole night on it or several nights if need be. The more exposure time through each filter, the less noise in the final image.
Another part of collecting these images is collecting reduction images which includes darks, flats, and bias frames. As much as I hate taking dark frames they should become your best friend and you should take lots of them. Why? Because of noise. There are many sources of noise. Cooling the CCD helps combat noise.
What is a dark frame? A dark frame is an exposure of the chip in total darkness which is the same time and temperature as your light frames. To do this we may put the lens cap back on the telescope or if the CCD has a shutter we then expose the CCD to darkness. The dark frame reads the CCD noise the same noise that is in your light frame. We then subtract the dark frame from the light frame giving us a much cleaner image.
Flat frames are used to map the optical path of your equipment being used, these can be used to clean up optical problems such as vignetting, internal reflections and dust donuts. There are many ways of taking flats this may include dome flats, sky flats or a light box.
Once I have found a subject and my telescope is polar aligned I start the same way as I would for visual work. I align the telescope. Find my subject and let the CCD cool down. Once the CCD has cooled down I may take a couple of dark frames. Then I spend some time focusing both the guide camera and the imaging camera. Once I know I have good focus I then calibrate the guide camera, pick a guide star in the field that I am imaging and start the guide camera guiding.
Then I select the first of the filtered exposures and fire away. I repeat this process until I have enough data for my photo (this includes taking more of those dark frames). There are many programs that let you set up a whole nights worth of imaging, and once set up properly you press start and walk away. I like to take each exposure then recheck the focus and so on. After all this is done and its about 2.30am in the morning I turn everything off and go in for a cuppa thinking about what I have just been imaging.
A couple of hours sleep does wonders before you jump in and process all that data you have collected. If all went well you should have a beautiful image you can be proud of. Processing this darta is a whole different chapter to CCD imaging.
I find online galleries and magazine galleries are great for finding exposure times. Go to the photo gallery and look at the photos, see what the subject is, how long they exposed the CCD for and give it a go.
The best way to learn is by your mistakes, which I have made a lot of - it’s part of the process of learning the tricks to imaging. I find IceInSpace a great website for lots of information on CCD imaging along with lots of other great stuff and its Australian owned. Also I have read Ron Wodaski book "The New CCD Astronomer" - this is a great book and a must have if you would like to get into CCD imaging.
At the end of the night you have some great images to share with other people and there is nothing like showing someone your photos and watching their face light up.