Embracing the Dark Side -or- Recording features on the night side of Venus

Phil Miles Anthony Wesley 19th May 2017 During 2016 the topic of recording thermal emissions from the dark side of Venus came up in many of our conversations. One of us (Anthony) had attempted this in 2013 using an older generation camera and 1000nm longpass filter with some success, but we thought that much better results might be possible with the new cameras and better equipment available in 2017. Phil was keen to try this and so we started a project to do this over the 2016/17 conjunction. In particular we thought that using Phil’s 508mm aperture scope in combination with the more modern Point Grey camera should allow for a significant increase in signal to noise when looking for the very faint thermal signal at 1010nm. Christophe Pellier had proved the concept in 2004, and others have since repeated this detection, but in all cases there has been a significant problem with the glare (reflected sunlight) from the lit crescent. In amateur scopes the light scatter from this source can dominate in the image, making it very difficult to separate from the thermal signal. After the experience in 2013 Anthony purchased an additional filter – a Semrock bandpass filter covering 850nm-1020nm. When used in conjunction with the existing Thorlabs 1000nm longpass filter this gives an effective narrowband filter centred on 1010nm with a bandpass of 20nm. He thought this might be better than just the 1000nm longpass on its own as it should block much more of the unwanted reflected sunlight; however he had never tried the filter so we didn't know if it would be of any use. As Phil was going to use his 508mm F/4 Newtonian he set about doing everything he could to reduce unwanted stray light, including making a magnetic cover for the primary inspection mirror which is permanently attached opposite the focuser, blocking the area around the outside of the primary mirror with adhesive foam, and repainting all internal surfaces with black chalkboard paint. Phil’s first attempt using only the Thorlabs FELH1000 on April 11th showed promise, the thermal signal was clearly visible but no discernible features were present. This was repeated on the 12th with the same result. The next tests on the 13th, 16th & 17th using 2x2 binning looked somewhat better but due to poor seeing no features were seen. These tests were done during the very early stages of the morning elongation of Venus, so it was still close to the sun and difficult to image. Looking at the images we realised that there was still far too much light reaching the camera sensor, Anthony had a Thorlabs 1050/10nm bandpass filter which Phil tested on the 18th without success. It seems there is no thermal signal at 1050nm. On the 19th Phil added the Semrock 835/170 filter to his system, stacking it onto the Thorlabs 1000nm longpass filter and making an effective narrowband filter 1010/20nm. This produced the best image so far and although there were numerous sharp diffraction spikes in the resulting data surface features were at last seen. Another test on the 23rd showed similar results. Features were visible but there was still a lot of interference from artifacts generated internally. This image from “Detection of Sub-Micron Radiation from the Surface of Venus by Cassini/VIMS” courtesy Kevin Baines & others. We have overlaid the bandpass of the combined thorlabs-semrock filters to show that we're recording close to the centre of the thermal emission peak at 1.01 micron.
On the 24th we decided to rotate the camera approximately 45 degrees every 15 mins so that the separate images could be combined to reduce the in-camera generated spikes. By chance one of these rotations produced a much smoother image with almost no spikes so the following day a test was done using only that alignment with excellent results, all camera orientations are now done in that position as shown on the 26th raw image. It seems that some of the internally created artifacts can be reduced to very low levels by choosing the orientation for the bright crescent of Venus. While we don't understand the precise mechanism for this improvement it is nonetheless very welcome, and may also work for others who are trying to image the same target. Further stray light reduction was also helped by placing several baffles comprised of black rubber washers of the correct diameter at each filter and a fibre washer at the top of the camera extension tube so only the F4 cone is visible to the sensor, the internal filter spacers were also flocked to prevent any reflections. The clear glass protective window in the camera was also removed to minimise the number of sources of reflections and scattered light. The barlow was removed as well and the system operated at his native focal length of 2000mm, ie the camera is at prime focus, further reducing scattered light. The final results are very good, with many features clearly visible in the more recent images. Animation by Javier Peralta (JAXA) overlayed with the surface elevation of Venus. (Note the circular effect at lower right is a side-effect of the projection and is not in the original data). Higher altitude surface features (green/yellow in the elevation map) show as dark features in the thermal emission image as they are cooler than the surrounding area. Animation by Anthony Wesley showing a bright spot, both images have been rotated, aligned and scaled to remove the changing angular size. 508mm (20”) Newtonian F4 Fullum Air-tech Mirror / Phil Miles & Anthony Wesley
We wish to thank Kevin Baines (JPL NASA) and Javier Peralta (JAXA) for their advice and Venus info. Christophe Pellier who’s previous Venus night side captures inspired us, also John Rogers, Marc Delcroix and Giuseppe Monachino. Phil Miles & Anthony Wesley Rubyvale, Qld, Australia