About Astrophotography

Astrophotography is one of the most technically challenging form of photography. In addition to the fact the targets are extremely dim and needs hours of exposure, earth slowly turns and makes stars trail in the image. To get a sharp image from the sky this motion has to be compensated precisely using a tracking mount. This puts great demand on the accuracy of the machining of the mount gears. Proper mount is the ground of where astroimaging is build. Even the brightest deep sky objects need hours of exposure to reveal the dimmest parts of the target. These exposures are usually taken in shorter parts, but it is common to expose 10 minutes at a time. During the ten minutes camera collects also lots of unwanted signal like thermal noise and light pollution from cities. These are minimised using cooling and filtration or moving to a darker site.

Currently my imaging setup builds on a Vixen GP-D2 mount. I have an excellent PowerFlex MTS-3 controller from Boxdoerfer Electronics controlling the stepper motors. Powerflex controller has also a serial port connector for computer control. As Vixen (like any other reasonable priced mount) doesn't track well enough for astrophotography alone, I have to autoguide the mount using computer. Computer reads star positions from Imaging Source DMK21AU04.AS USB camera attached to a Orion ST80 guidescope. This information is used to calculate and command small corrective actions to the mount. I use PHDGuiding to handle the autoguiding process. Before I got the Vixen mount I imaged with Orion Skyview Pro mount with the MTS controller. I got some quite nice results with it, but it was extremely difficult to get it to track well, even when quided. The mount has to be aligned precicely towards the celestial pole. I use Kochab Clock method which gives accurate alignment for my needs.

The imaging camera I use is Atik Instruments' 4000LE cooled CCD camera. It has a monochrome sensor which is very sensitive and cooled several tens of degrees below ambient. The camera is specifically designed for astronomical imaging and is very good in narrowband imaging which was very limited using DSLR. Atik 4000LE is completely depended on computer compared to self sufficent DSLR. I did most of my imaging before 2009 with Canon EOS 350D digital SLR, which had been modified for greater sensitivity. Stock Canons have internal IR cut off filter which dims astronomically important wavelenghts, such as H-alpha at 656.3nm. My camera had this filter removed which allowed me to use filters designed for astrophotography. I have sold this camera and jumped into CCD world. I still have a stock Canon 1000D for scenery shots and of course for normal daytime use.

Some objects like Nebulae emits their light in very specific wavelenght. This gives a good opportunity to do some serious imaging under suburban light polluted skies. Narrowband filters are tuned to these specific wavelengths and blocks any other wavelengts from entering to the sensor, including light pollution. I use Astronomik 13nm H-alpha, OIII and SII filters, which give deep contrast in emission nebulae. These three filters are used to create color narrowband images. I use Gerd Neumann's filter drawer system to exchange filters without removing camera and thus affecting focus or camera orientation.

My current imaging scope is Skywatcher ED80 Pro. It is a 80mm f7.5 refractor, with apochromatic objective. The lens is a doublet ED lens and it has significant field curvature with large format cameras such as DSLRs and Atik 4000LE. William Optics 0.8x reducer flattener works fine with this scope. Putting the reducer directly to the CCD camera or Canon T-adapter won't give the best results, since the optimal distance to the sensor is 56.5mm from the thread base. Canon DSLR's has a body thickness of 44mm which isn't quite there with a T-adapter. I used 7.5mm extension to get the best result. However this combination didn't have enough space to fit filter drawer between the reducer and camera so I had to find a very exotic adapter ring to fit the filter drawer system in front of the reducer. Atik has shorther distance from threads to the CCD, 18mm. This gives me an opportunity to place a filter drawer between the reducer and camera and still there is some space left for future accessories.

Focusing is one of the most important phase in taking an astrophoto. I use software assisted focusing using Artemis Capture. The software reads star image from the camera and displays FWHM value in screen which has to be minimised. Critical focusing is very precice job and focuser plays very important role in it. Camera has to be positioned in greater than 0.1mm accuracy to make a sharp image. With f6 focal ratio, critical focus zone is only 0.079mm. I've replaced the original focuser with TS Crayford focuser. This focuser has 1:10 reduction and a locking screw. One revolution with the reduced knob moves the focuser only 1.4mm.

Image aqcuisition is controlled by computer software Artemis Capture. The software handles also cooling in the camera and allows me to do sequencing of the images. Image is transferred to computer between exposures, that way I can do quality control during the exposure sequence. With DSLR I used Nebulosity 2 for the job. It controled bulb exposures via a serial port shutter cable. Before disassebling everything I shoot flats using self made flat target. My flat target uses electroluminence foil to generate evenly illuminated flat area. Also one key factor in long exposure photography is dew control. I use Kendrick V7 controller with Dew-Not heater stripes. Everything, including the camera, computer, dew control and mount, is powered using 45Ah battery.