The equipment
Although a number of components can be added to the rig, such as a filter drawer or filter wheel there are very limited degrees of freedom to develop a setup that is truly flexible. For example, there is such limited back-focus with probably any camera, that it is impossible to fit even a very thin camera rotator to facilitate the framing of astronomical targets. There is room to fit a very small number of spacer shims so by varying the number and thickness of these and tightening up the camera, it may be possible to approximate the desired framing, or not!
It should be possible to build in a camera rotation system into the scope’s design without reducing the back focus. This is of no consequence if the scope is used as a guide scope, but is of huge importance for an imaging device for which it is heavily marketed and hopefully Touptek will consider this.
Considering the components that we used for the first test, a number of important lessons can be learned.
The GS-100 is a fast scope that may cause problems with larger sensors due to the angle of incidence of some of the rays hitting the sensor. The faster the scope, the steeper these can be. This can increase crosstalk between pixels resulting in poorer colour fidelity and contrast.
Probably the biggest problem with using the ZWO ASI585MC camera concerns reflections back and forth between the sensor and the filter. The camera used does not have anti-reflection coatings on the sensor window glass which means that the reflected light is not inhibited and passes back and forth between the sensor and the filter wheel, passing through the quadruplet lenses on its journey. The shifting of the originally parallel light rays leads to the development of Newton’s rings interference patterns which are particularly noticeable towards the ends of the long axis of a rectangular sensor. There are limited options for changing the distance between the filter and the sensor, which would probably have little effect anyway. There is no tilt plate built into this camera so no way of using tilt to mitigate the problem. The filter itself has inadequate anti-reflection coatings which could have reduced the impact of the interference patterns. After consideration, the quadband filter may not have been a good choice. I am not sure whether ours is V1 or V2. If it is V1 then this filter was renouned for poor anti reflection coatings and producing bad halos around stars. We clearly had reflection problems and some stars did have bad halos which were unsatisfactorily cosmetically treated during processing. A high quality UV/IR filter may have been a better choice.
Unlinked channels stretched image showing the concentric interference bands
The images were improved by cropping out the central area where the interference was less evident. However, this partly defeats the object of using the ZWO ASI585MC camera.
Cropped and reoriented images
North America and Pelican nebulae
For a future test our Player One Mars-C II IMX662 Colour Astronomy Camera will be used. Learning from our previous experience, this camera seems to have a number of advantages over the ZWO ASI585MC. It has Diameter 21 * 1.1 mm High Quality AR Plus (Anti Reflection) Multi-Layer Coating. Instead of a single layer of anti-reflective material, this optical element has multiple microscopic layers cooked onto the surface. Multi-layer coatings are superior because they cancel out reflections across a much wider spectrum of light wavelengths (colours), rather than just one. It also has a built in front tilting plate that allows for adjustments to avoid interference patterns if required.
Further camera details:
SONY IMX662 1/2.8" CMOS sensor (color). 2.1 Mega Pixels. Maximum Resolution 1936×1100
2.9μm square pixels. Sensor Size 5.6mm×3.2mm
If we centre on the star HD198597 the field of view is such that much of the Cygnus Loop can be framed.
Stellarium simulation of the field of view framed by the GS-100 with the Player One Mars-C II camera. The framed area is within the red frame lines.
