About Skylight Observatory
About the Photographer
Jared Willson is an American amateur astronomer and astrophotographer residing in the San Francisco Bay Area. On many clear Friday and Saturday evenings he can be found on the observatory deck at Chabot Space and Science Center, sharing his passion for the night sky and for astrophotography with anyone who cares to look and listen. In addition to his volunteer work at Chabot, Jared owns a remote telescope that is hosted in Rowe, New Mexico under dark skies. This remote site provides relief from the light pollution of major urban centers, and allows Jared to produce very deep images of the night sky that simply wouldn’t be possible under urban skies.
About the Equipment
For his outreach work at Chabot, Jared generally uses a 110mm apochromatic refractor on a compact Harmonic drive mount. This combination is an ideal compromise of good light grasp for faint subjects, a wide field of view, and compact size for easy transport and setup. The entire setup can be carried from the car in a single trip, including tripod, battery, mount, telescope, computer, tablet, and camera.
While small telescopes like the 110mm refractor are great for portability and ease of setup, they simply don’t have the light grasp and resolution of a large reflector. So, the remote site houses a 305mm diameter aperture Riccardi-Honders astrograph. No need for nightly setup and polar alignment, since the system is housed in a roll-off roof observatory that automatically monitors the weather conditions to know when it’s safe to open for observing. At f/3.8, the 305mm Riccardi-Honders is quite a bit faster than most similarly sized telescopes, allowing for fairly wide fields of view despite the aperture. This focal ratio is an excellent match to the modern trend in astronomy cameras of providing very small pixels. The remote telescope sampling rate allows for 0.69 arc seconds per pixel, small enough to capture all the resolution the telescope is capable of providing under most observing conditions.
The mount that provides tracking of the night sky includes absolute encoders that, when coupled with a precision pointing model, ensure tracking accuracy is as very high. The mount does not require any closed-loop guiding for exposures of five minutes or less, despite the roughly seventy pound payload of telescope and camera equipment.
About the Image Acquisition Software
Experienced astrophotographers will tell you that the most important part of getting good data is having a mount that can track accurately. Both of the mounts I use are capable of very accurate tracking, but the AP-1100GTO AE has a trick up its sleeve. It can keep track of all the various aspects of mount performance, from flexure of the optical tube to atmospheric refraction as the air density changes, from errors in polar alignment to imperfections in the mount’s worm gear. It does all this using a combination of absolute encoders, which monitor the right ascension and declination axis with extremely high precision, and Astro-Physics Command Center software.
With APCC as the baseline for mount control, it is possible for the telescope to track accurately enough on its own that guiding is not necessary for even five minute exposures, no matter where in the sky the telescope is pointed. On nights of good seeing (steady air), the combination of APCC, absolute encoders, and a good pointing model will produce five minute exposures with RMS guide errors under 0.4 arc seconds, even with guiding disabled.
Aside from the mount, all hardware control is through NINA. This is a feature rich, open source software package that allows for easy inclusion of third party utilities. It provides a powerful and intuitive scripting tool that can be used to setup even very complex, multi-panel observing sessions that include: connecting to the required hardware, waiting till the target is in an appropriate location, waiting for twilight to end, slewing to the target, cooling the camera, plate solving the target, refocusing based on image sharpness, time, or temperature changes, looping through various filters, providing filter offsets to the focus point, etc. In addition to the core functionality, I utilize several bolt-ons that improve the quality of my data. First is Hocus-Focus. This utility allows me to determine the point of best focus to an extremely high degree of precision and accounts for things like different star profiles on either side of focus (common for telescopes with large central obstructions), different hyperbolae on either side of focus, etc. I also use an Astro-Physics toolset to automate the creation of declination-arc high precision pointing and tracking models for a given target, Discord Alert to notify me of errors or warnings during an imaging run, and a Remote Copy tool to move my images from the remote computer at the observing site to my home Network Attached Storage device.
Astro-Physics 110mm Refractor
This small refractor is prefect for travel, outreach, and wide field imaging. It has a flattener/reducer which provides a focal length of 550mm and a focal ratio of f/5. It is coupled to a full frame one-shot-color camera to allow full color images in real time. The RST-135E mount is extremely light weight and requires no counterweights due to the high torque of the Harmonic drive motors. The high-precision encoders in the mount allow for unguided images of up to two minutes, and routinely allow RMS guide errors under 0.5 arc seconds. That results in images that are limited primarily by the seeing conditions and diffraction, not by the guiding.
Astro-Physics 305mm Riccardi-Honders
Most of the images on this website were created using the Astro-Physics Riccardi-Honders pictured above. The Riccardi-Honders is a very unusual optical design that works well for a wide variety of subjects. Unlike most reflectors, the primary mirror on the Riccardi-Honders is a Mangin mirror meaning the reflective surface is on the back surface of the mirror rather than the front surface. This allows the mirror itself to act as both a lens and a mirror since light is refracted on its way through the mirror substrate as well as on its way back out. In addition to a spherical secondary at the front of the telescope, there is a singlet field lens in the focuser draw tube that acts as a flattener. There is also a meniscus lens at the front of the telescope that acts as a corrector as well as a support for the secondary mirror. The design of the Riccardi-Honders allows for very fast, well corrected optics with a wide imaging circle without requiring any aspherical surfaces. There are, however, a few disadvantages. First is that the mechanical tolerances are extremely tight, not unusual for fast optics. Even though no aspherical optical surfaces or exotic optical glass are required, large optical tubes are still quite expensive. The second disadvantage is that the primary mirror can’t be made of a zero expansion material such as fused silica quartz or Astrositall. While optical glass like BK7 can provide the appropriate optical qualities, it does not allow for stable focus with falling temperatures. As with most refractors, constant attention to accurate focus is required. Since all mirrors and lenses are fixed inside the OTA, there is little risk of differential flexure which makes it possible to guide with an external guide scope rather than an off-axis guider, should I need to guide. I tend to use guiding only for longer exposures.
The OTA is mounted on an Astro-Physics 1100GTO AE. This mount can easily handle the approximately eighty pound payload of the OTA, camera, and dovetail. The absolute encoders provide precise careful monitoring of the exact position of the telescope at all times, as well as a safety net should the telescope ever lose its “home” position.
The camera is a QHY600PH monochrome unit coupled to a large filter wheel. Due to the steep light cone from the f/3.8 optics, 50mm x 50mm square filters are required to ensure complete coverage of the 42mm diagonal sensor. The camera contains an IMX455 sensor which provides low noise, a large full well capacity, good quantum efficiency, and a 3.76 micron pixel size that is very well matched to the 1,124mm focal length of the telescope. The combination of the telescope and the camera provides 0.69 arc second sampling which allows for resolution to be seeing rather than diffraction or camera resolution limited on all but the best nights.