Since Daylight Saving Time (DST) is just a week away for most of the USA (you guys in Arizona and parts of Indiana are lucky :-), it bears reminding (and for new comers, warning) that the ASIAIR does not understand the concept of DST. They don't practice DST in China (heck, they have only a single time zone for the entire country!).

So, you need to go to your mount and turn off the DST setting. For some mounts that run on GPS and hell bent on setting the UTC offset to include the DST offset, you may need to also change the local time. The UTC Offset needs to be the UTC Offset for Winter (Standard) time and the local time on the mount should also show the time as if you are still in Winter (Standard) Time. If you don't know what the UTC Offset should be, go check Google now, so you will know what to set in your mounts a week from now.

If your don't do this, the coordinate system on ASIAIR and the coordinate system on the mount will be different, and the mount may not properly rotate during polar alignment. It may do an erroneous meridian flip when you perform a GOTO, not only pointing to the wrong location, or even drive your telescope into the tripod or pier. It will almost certainly also not perform Meridian flips (both automatic and manual) when a star crosses the Meridian.

The east-west position of a star in the sky is determined by its Right Ascension (RA). The east-west hour angle (HA) of your mount (the mechanical angle of the polar axis relative to the Meridian) is determined by the mount's motors. They are related by:

RA = LST - HA , where LST is the Local Sidereal Time.

The Local Sidereal Time can be computed from the Local Time, the Local Longitude and the Local UTC Offset.

Notice that a star crosses the meridian (when HA = 0) when the star's RA coincide with the Local Sidereal Time. This is how Longitude used to be determined accurately in the past by using a transit telescope located in Greenwich, England; everyone else checks when the same star crosses the meridian locally with their own transit instrument, and then figure out their local Longitude by comparing with the time the star crosses the meridian at Greenwich.

If Local time is wrong, or if UTC offset is wrong, or if Longitude is wrong, then the Local Sidereal Time is wrong. As a result, the HA (mount's angle) and RA (star's angle) will be off (usually by the 1 hour "Spring Forward" or "Fall Back").

Anyway, be prepared to make the corrections when DST arrives. Just remember that the ASIAIR assumes DST is always off at all times of the year, so you need to also turn off the DST on your mount, and for some mounts, you also need to adjust the time and UTC offset so that the mount thinks it is living in Winter (Standard) time.

Not all other countries changes to DST this coming weekend. Just make the above precautions when you switch to DST.

Chen