vkastronomy.com - Astrophotography Weblog
Equipment
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.
Actual imaging is then done using Canon EOS 350D digital SLR, which has been modified for greater sensitivity. Stock Canons have internal IR cut off filter which reduces astronomically important wavelenghts, such as H-alpha at 656.3nm. My camera has this filter removed which allows me to use filters designed for astrophotography. Having no filter in front of CMOS means I have to use IR block filter to reduce star bloating in my two element apochromatic imaging scope, Skywatcher ED 80 Pro. I also have Astronomik 13nm H-alpha, OIII and SII filters, which give deep contrast in emission nebulae. These 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. The filter drawer doesn't fit between the camera and the reducer. William Optics reducer has an exotic thread in the other end (2" x 32TPI), so I needed to have a special adapter made to it.
Skywatcher ED80 Pro has (as every doublet) significant field curvature with large format cameras such as DSLRs. I found William Optics 0.8x reducer a great help on this. Putting the reducer directly to the T-adapter for Canon won't give the best results, since the optimal distance to the sensor is 52.5mm and Canons installs a bit too close. I had nearly an optimal spacer ring with 7.5mm thickness which allows the reducer to remove the field curvature almost completely. This combination yields very good guality images across the whole frame.
Focusing is one of the most important phase in taking an astrophoto. I use software called Nebulosity 2 for focusing and capture. 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. TS Crayford is also 10mm shorter than the original which allowed me to insert filter drawer in the optical path.
Image aqcuisition is controlled by computer software Nebulosity 2. It controls the bulb exposures via a serial port shutter cable. Image is transferred to computer between exposures, that way I can do quality control during the exposure sequence. Other noteworhty part in my setup is TC-80N3 timer remote from Canon. It is modified to fit EOS 350D and controls my dark frame exposure sequence. Compared to computer control it has advantage when taking dark frames - I can haul my computer inside and leave camera and timer outside taking darks. Before disassebling everything and leaving the camera taking darks, 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, is powered using 18Ah jump start battery or when available, a 12V 5A power supply.
