|David Lloyd-Jones - Astronomy||
Weighing in at 313 grams the GStar-Ex3 colour CMOS camera is quite a bit heavier and larger than the LP-Guide colour camera (59 grams or 92 grams with focal reducer) probably due to the new camera's incorporated cooling fan. By comparison, the old GStar-Ex reached 367 grams when decked out with its external fans. Nevertheless, with one reservation (see *** below) the Vixen R130sf's simple focuser seems to cope well with the additional weight and retains focus in all orientations of the OTA. The camera's sensor is quite small (8.92mm on the diagonal) but its small pixel size (2.4 x 2.4 microns) generates impressive image resolutions of either 3040 x 2048 pixels or 1520 x 1024 pixels in binned mode. It can be used as a standard video camera with exposures ranging from 0.224 seconds up to 5 seconds. It can also be used as a "still' camera with exposures from fractions of a second up to many minutes duration.
This fan-cooled camera has an available autoguiding port but it is important to remember that it is primarily intended for electronically assisted on-screen viewing. That it can achieve reasonable astro-imaging results is a bonus.
A recent addition has been an adapter to allow a focal reducer to be added to the camera. A focal reducer widens the camera's field of view (rather like using a wider angle lens on a regular camera). Experiments with a few focal reducers of differing powers showed that the best matches delivered reduction powers of x0.86 and x0.70. Trying to use a greater reduction power resulted in unacceptable coma.
*** Without a focal reducer the GStar-Ex3 needed to be racked out almost to the full extent of the R130sf's focuser's travel. This simple focuser was probably never designed to accommodate such a relatively heavy weight when fully extended and this showed up in a slight side-to-side play of the draw tube affecting the OTA's collimation. The R130sf's supplied screw-on focuser extension tube was too long to be used to cut down on the focuser's travel but a shorter extension tube was found at the back of a drawer and it does the trick. The focuser now needs to be racked out only a short distance from its fully closed position eliminating most side-to-side play of the draw tube.
The supplied screw-on extension tube can be used to advantage when using a barlow (i.e. tele-extender) lens such as when imaging the Moon.
The camera is now used with the SkyWatcher PDS 6 inch telescope.
The accompanying image capture software is an updated version of ToupSky that came with the LP-Guide camera. In addition to handling the new video and still image modes it also provides a new feature for real-time stacking of captured images. Stacking can be either "additive" or "average". Additive stacking brightens the image by simply adding the pixel values from each consecutive image. Average stacking removes random camera noise to give a cleaner result. Other features added to the software are dark frame subtraction and flat field division to correct for hot pixels and any uneven illumination across the camera frame.
Initial impressions were that the camera itself is well built and works well. At first the myriad of camera setting combinations provided by ToupSky was bewildering for a new user and it took some time before the full potential of this camera could be realised. For most deep sky work the maximum gain setting combined with a gamma reading in the range of 0.38 to 0.42 generally works best. The short video mode exposure settings are fine for composing the layout of your subject on screen before switching to "trigger" (i.e. longer exposure) mode and setting the camera to run in "loop" mode. Experimenting with exposure settings lets you determine the best setting for your subject. Monitoring the light distribution curves of the red, green and blue channels in the dynamic histogram feature makes it easy to level them. And dragging the histogram's left bar up to the start of those curves invariably improves the displayed image's contrast. The colour saturation setting can often be increased to advantage. It is probably best to build a library of previously recorded dark and flat frames from which you can select those appropriate to the image being taken. As a last step you set the stacking process in motion. I have found that average stacking delivers the best results and that for diffuse nebulae stacking just four frames delivers a nice noise-free image.