What go into taking CCD photos

[Phil/downunder]

Taking Great CCD Images
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 myself 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 L,R,G,B . Now we are seeing narrowed band photos which let through specified wavelengths of light . These may include H/alpha,Sulfur11,Oxygen111 to name a few.
So what goes into capturing these photos with a CCD camera. Precise polar alignment , a good mount , good optics long guided exposures and image reduction. Is just a part of how to capture these photos
A TIPICAL 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 , miss alignment of images, good tracking, and the ability to put the subject on the chip. How do we polar align ?I myself 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 hole night on it or several nights if need be. The more exposure time through each filter you give your subject 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 then, 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 firer away. I repeat this process unit I have enough dater for my photo, this includes taking more of those dark frames . There are many programs that let you set up a hole nights worth of imaging, once set up properly you press start and walk away. I myself 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 of 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 dater you have collected. If all went well you should have a beautiful image you can be proud of. Processing this darter is a whole different chapter to CCD imaging. I find mags like this one is 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 ticks to imaging. I find Iceinspace and this site a great web sight for lots of information on CCD imagining along with lots of other great stuff . 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.
Thanks for reading.
Phil

[Bob_riggs]

Hey Phil!

Thanks for taking the time to write such a comprehensive explanation of the process of CCD imaging. Nice overview! Me and my astro-bud David are looking forward to the challenge when our ob gets finished up, hopefully before the snows fly.

You filled in some of the blanks for me, and I really appreciate it. Hopefully, for others "in the same boat", it will ease the trepidation of imaging, as you have for me.

CDS!

Bob

[budguinn]

Phil,

Nice writeup......I don't think I've ever seen anyone put it down on paper.

you did forget the apart about letting the little Obs partner (in my case my dachshund) sit on my lap about an hour into the process. She has her own little bed, but always wants to sit on the lap.

[Phil/downunder]

Thanks for your replys guys.
Phil

[AstroImaging_KindaGuy]

If I may take the liberty of summing it all up in two words......extreme patience!

This is the one thing I find that most beginners do not really know about CCD imaging or emulsions. I have personally witnessed several folks giving the hobby up after the first couple sessions because their perception of what it is and what it takes has been tainted by terrestial photography.

When I started I didn't know a thing about 35mm cameras or film and CCD's were just beginning to come to market let alone be afforded with discretionary income. I never ever used a camera beyond an instamatic in the full light of day. And as it was since I was starting completely from scratch in both hardware and the understanding of light, I set out on a most ambitious first target using a used OM-1...and that was Saturn.

Needless to say I spent an entire 1996/97 summer shooting Saturn. Having used 100 rolls of B&W film I kept meticulous records during this time of what equipment I used and the various scope and camera configurations and settings and associated each emulsion result to its respective record.

Finally after near continuous studying of my records it all began to come together (at least for emulsions) and the light bulbs began to flicker as I noticed patterns between the exposure timings, aperture, the amount of light and the position of the object relative to the atmosphere and horizon. It all culminated in the B&W picture of Saturn I posted in the gallery under my moniker which for me was like graduating from high-school and now ready to enter college.

After spending about 3 years in emulsions I finally mustered enough confidence to buy my first CCD camera and then I pretty much had to start over again. Some things translated over from emulsions but many things were completely new. The most challenging aspect for me was and still is the processing of the RAW images into final frames.

I was completely surprised to find that many of the processing techniques as described by Ron Wodaski in his book "The New CCD Astronomy" I have already been doing using my meticulous record method of trial and error to learn. There were no books out about CCD imaging when I first started so my "trial and error" technique that I used for emulsions proved for me to be the best way to learn.

I eventually took a near 4-year hiatus starting in 2004. I spent 3 years getting away from it completely to reboot my brain and then I spent the last year (2007) reading all the fine CCD imagng material that has come to be recently published. I just started back into it in September of this year. I was again surpised to find that the time off did me some good as my processing techniques are beginning to show positive forward improvement.

At this stage I have pretty much mastered the the equipment configurations, timings, taking the RAWS and calibration frames but processing is still my most welcome challenge.

This however is where the payoff is for me because I don't think I would want to do any hobby that as time passed the learning would cease to happen any more. That's why this hobby is a keeper for me as I will enjoy its rich learning curve until I finally depart from this space marble!!