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Some Golden Rules of Astrophotography

By Gerry Aarts

Username - Gerry Aarts

Difficulty Level - Depends on your goal. Expect to work hard.

Rule1. Get an f6.3 or f3.3 focal reducer if you have an f10 scope. If you have an f15 like a Meade ETX you definately need the f3.3 focal reducer.

Imaging dim, deep sky objects at f10 or f15 is nearly impossible and certainly very difficult. The required exposure times are far too long for most beginners at f10. At f10 you would require a minimum of 3 to 10 minute exposures to get a decent galaxy or nebula image. You need perfect polar allignment for exposures that long. Most beginners (and many experienced astronomers) are not that good at achieving perfect polar allignment. I know I'm not but I am working on it..

At f3.3 I can image a dim galaxy with say 30x30 second exposures or 60x15 seconds exposures that are automaticaly combined or "stacked" by the DSI software. (a total of 15 minutes exposure time). The smaller the f ratio the "faster" the camera can download your target image. I use an f3.3 focal reducer for most of my imaging and it works well with the Meade DSI camera. If your scope is an f4 to f6 you can image deep sky objects without a reducer.

The f3.3 focal reducer also expands the field of view. At f10 my Celestron GPS11 and little Meade DSI chip only captures a very tiny area of the Moon or the Orion Nebula. At f3.3 the field of view is around one fifth of the moon. A huge increase. To determine your field of view with your set up dowload this free program called ccdcalc. http://www.newastro.com Then use the drop down box in the software to determine the field of view at f6.3 and f3.3.

Rule2. For planetary imaging you need to go the other way.

An f10 or f15 telescope is fine, if you insert a 2 or 3 times barlow you effectivly image at f20 or greater. The primary target planets are very bright and your exposure times will be less then a tenth of a second in most cases. A web cam is the best option, you take hundreds of images and stack the best of them in Registax. Again the Meade DSI is also very capable of planetary imaging and fantastic value for a beginner as an all round performer. See my Jupiter image I took last month on the wsaag website.

Rule3. Use a ccd camera designed for astronomy.

I will likely stir up some differing opinions on this subject. I do not believe standard DSLR cameras, (eg Cannon Pentax) are designed for astrophotography. Modifications are required, eg IR filter. You need third party software to drive the camera on a computer. All this leads to frustration for most beginners. Finding focus on that little LCD of the camera aint easy!

For beginners I strongly reccommend you spend $500 on the Meade DSI color imager. The DSI comes bundled with fantastic software that helps the beginner become an expert with around one year of practice. Yes, that's right, don't expect to be an expert in one month. It has software developed by NASA called "Drizzle". You cross-hair two stars and the DSI "locks" onto the image. In other words it's "Software Guiding" the image, not the telescope "hardware guiding". As long as the telescope is tracking (not guiding) accurately the DSI will do the rest. For those of you with a Meade telescope, the DSI will guide as well.

The DSI allows me to capture pleasing images even in Atazimuth Mode. The DSI software de-rotates the image on the fly. In Equatorial Mode (EQ mode) the need for derotation is not required. But if the telescope is not accurately polar alligned again the DSI software will "lock" onto the two selected stars and produce a pleasing image of a galaxy or nebula.

I would recommend the Color DSI not the one where you need to image in LRGB. (Individual Luminance, Red, Green, Blue images later merged in sotware.) Yes all the experts will tell you that is the way to go, but that is ok for experts. For beginners taking one colour image is hard enough, never mind 4 images.

There are other astonomy CCD cameras out there. Starlight Express, SBIG to name a few. They are much more expensive, starting from around AU$2000. For beginners this is really a little over the top. Learn the craft with affordable, easy to use equipment, that's my advice.

Rule4. Accurate Tracking

Accurate tracking is important regardless as to wether you are tracking in Altazimuth or Equatorial mode. One of the best tips I can give you here is to select a nearby guide star of the galaxy or nebula you want to image. eg the Tarantula Nebula. I would choose Acrux because it is close to the Tarantula nebula and centre it in the field of view of the image on my laptop. This is also a great star for finding focus because it splits into two stars when properly focused. Anyway, when it is dead centre of the image I re-align the telescope by pressing the align button. Then I input the instructions for the GOTO to slew to the Tarantula nebula. It should land dead centre of the CCD chip. You would repeat this process every time you want to image a new object. That dim galaxy should land dead centre of the ccd chip if you follow this advice. All you have to do is increase the exposure time until the galaxy comes into view.

Rule5. Focus Focus Focus.

I spend a lot of time on focusing. One big tip here, you can not achieve visual focus through an eyepiece if you have an f3.3 focal reducer. You can at f6.3 or greater. Anyway achieving visual focus does not do you any good because the camera's ccd chip focal area is located in a different position to your eye. With a DSLR camera you can try to find focus on the little LCD screen of the camera. It is far better however if you can aquire some software that will allow you to to view a live image on your laptop computer.

This is where specialised CCD astronomy cameras come into their own. The DSI updates an image at my required timing, say evey 1 to 2 seconds for bright stars for initial focus. Then when I point the telescope to a Galaxy, I might only have some dim stars visible and increase the exposure time to 15 or twenty seconds. The galaxy will now come into view as well. The DSI allows me to zoom onto a particular star near the galaxy to help achieve final focus.

Here is another excellent tip. No Donuts. If your dim stars appear to look like little donuts you are out of focus in one direction. If the stars appear a little blurry you are out of focus in the other direction. Find the happy medium between the two and you should be as well focused as your set up will allow.

Rule6 Join an astronomy club.

Get as much advice as you can. Go to Star parties and learn from others. That is how I did it.

Rule7. Post Processing

Acquire Photoshop. The attached images and all the images you see in the magazines and on the internet are processed. The images do not look like this to begin with. The detail needs to be extracted. The raw data we collect in those images has all that detail built in but not always visible initially. I spend more time on the post processing then on acquiring the image. Over-processing is not desirable. Photoshop is a powerful program with many features. You and I will never master all of them.

Well I hope this article helps some of you to achieve the thrill of capturing that first pleasing image. My first pleasing image was the Keyhole Nebula (Eta Carina) taken at the Arkaroola Star Party in the North Flinders Ranges SA held each January.

Regards
Gerry
President WSAAG