How easy is it to find TYC 1187-01197-1 from Laval – a 11.7 mag star? For some people maybe it is easy, but for some others not so. Well, the first good news is that our target will be very-very close to the mag 12.5 asteroid Arachne, so the combined magnitude of our target (star & asteroid) will be 11.3 mag.
The standard way to find a star is the star-hopping method. Normand has prepared some very good star maps for this task. But what if the weather is not perfect and the sky is not very clear 1 hr or 1/2 hrs before the occultation? And the current sky conditions make it difficult to locate the target star even for large telescopes ?
In the occultations community we have an « emergency method » that we can use to overcome these targeting problems. It is called « the pre-point stars » method and it is mostly used in cases when there are clouds coming and going before the event or when the target star is near the East (has just risen above the horizon) at the time of the occultation and so we have very little time for star-hopping.
Pre-point stars is a list of bright stars that earlier in time, in certain times, will pass exactly or very close at the position of the target star at the time of the occultation. This means that we can point our telescope at one of the bright pre-point stars, let’s say 15 min or even 1 hr before the occultation, center the pre-point star in our field of view, turn off the motor (if it is not a dob) and then just wait. At the time of the occultation our target star will pass in this same field of view!
In the following list you can see pre-point stars for the Arachne occultation on November 8th at hrs 20:18:51 (local time at Laval). For example, 23 min and 43 seconds before the occultation time, at 19:55:04, we can center at a 6.5 mag pre-point star (HIP-1057) secure our dobsonian telescope or turn off the motor of our Schmidt-Cassegrain and wait. HIP-1057 is an easy to find star, less than 2° away from γ-Pegasi, Algenib. One minute or so before the occultation TYC 1187-01197-1 will enter our field of view. Hopefully the sky will have cleared and we can observe the occultation! We can turn on the motor a little earlier, if we wish, and make a minimal star-hopping to find the star in time.