Difficulties, defects and disappointment

This page covers common problems, hopefully helping to recognize them, understand them, and, sometimes, address them.

The subtropical jet are located around 30°N (which happens to be the latitude of the observatory). The jets streams vary in height of four to eight miles and can reach speeds of more than 450 km/h at the core. When a jet is above the observatory, the weather is usually clear, somewhat windy near the surface, and the seeing not only degrades but also becomes extremely variable, to a point where having a consistent focusing solution becomes impossible, at least at Moana’s resolution of .5 Arc sec/pixel. At that point everybody but the short refractors operating at low resolution gets shut down. The video above shows 1mn frames looping, taken by Moana. Not only are the star bloated, but the bloating varies by over 30% from frame to frame. Shorter exposures show even more variation.

Moana’s spider system, with double arms is extremely sturdy and rigid. If a bright star gets near the field, however, pronounced diffraction spikes may become visible. This has become a non issue thanks to Radon spike removal.

In some very rare cases, I have also had internal reflections on my frames. It is unclear so far if the cause is the moon, bright stars or both. The easiest solution is a Meridian Flip, as Moana is an asymmetrical design. A longer tube or “sun shield” would cause problems on windy days and baffles (including focuser baffles) may cause some vignetting.