URANUS AND NEPTUNE
Large apertures and long capture times reveal the ice giants’ secrets
They may be large bodies, but the ice giants Uranus and Neptune are also very distant and this makes them significantly more difficult to image usefully. Large apertures work best but the quality of the atmosphere you’re imaging through also plays a very important part. With an apparent disc of typically just 3.7 arcseconds across for Uranus and 2.2 arcseconds for Neptune, it doesn’t take much atmospheric unsteadiness to hide any detail in these far off planets.
Their great distance also makes them fairly dim for the process of high-frame-rate imaging. However, if your goal is just to capture any belts or bands visible on their globes, long capture times are perfectly acceptable as long as the technique is documented with the image. Capture times running to several tens of minutes are not uncommon. Upping the camera sensitivity and capturing frames at a relatively slow frame rate is a viable technique here. A useful tip for focusing is to image a nearby star before you start your capture run and pre-focus accurately on that. This assumes you can find the planet again after focusing has been completed!
For Uranus, long-pass or infrared-pass filters tend to be the most useful, combined with a mono highframe-rate camera or an IR-sensitive colour camera. A large aperture of at least 10 inches (250mm) is recommended, together with an R+IR filter. The Baader RG 610 delivers a relatively bright image for instruments at the smaller end of the recommended size range under average seeing conditions. For larger apertures above and including the 12- to 14-inch (300 to 350mm) size range, a Baader RG685 filter can be used under excellent seeing conditions.
Bright spots on the globes sometimes occur but the small disc sizes and unsteady atmospheric conditions sometimes make it hard to determine if they’re real. In which case, make several captures separated by, say, 15 minute intervals. Processing the captures then turning them into an animation will help determine whether the spots are consistent, and therefore real.
The freeware WinJUPOS can be used to provide important ephemeris and analysis tools when imaging both Uranus and Neptune
An amateur observation of Neptune’s moon, Triton, occulting a star. Note the characteristic central peak as the star’s light was focussed by the moon’s atmosphere
Uranus (left) and Neptune