Occultation Timing and Recording

Occultations are astronomical events where one object passes in front of another more distant object. This type of event allows astronomers to measure various characteristics of the foreground and background object such as the foreground object’s size, shape and atmosphere (if any), and the background object’s location precision and discover such properties as binary star systems.

I first entered in to the field of occultations when a request came through to record observations of a Pluto occultation. This was in June 2006. Since then I have attempted to record several Pluto and other Trans-Neptunian object occultations as well as asteroid occultations.

Recording occultations ideally requires high sensitivity, high frame rate (ideally video) camera on a large aperture telescope, providing maximum precision. I do not have a video camera suitable so make do with what I have, astronomical CCD cameras (QHY-5 and SBIG ST8-XME). Hence, I record occultations using two distinct methods:

  1. Single long exposure “drift-scan” methodology using a CCD camera. Read about drift-scan occultation recording.
  2. Multiple short, high frequency exposures using a CCD camera. Read about high frequency CCD camera occultation recording.

You can read more about how I record occultations using the above methods at the links above.

Which method of recording an astronomical occultation depends on the characteristics of the particular occultation:

  1. Drift-Scan:
    1. Occultations where the duration of the event is less than 50 seconds in length, including any error in the prediction required either side of the event. When events are shorter than 50 seconds the entire drift of the star from start to end can fit within the FOV of my ST8-XME detector on my 12″ LX200 telescope at f/10.
  2. High frequency CCD:
    1. Occultations where the duration of the event (including any error tollorance either side) is greater than 50 seconds. When the event is a long one (for example a 90 second Pluto occultation) the fact that my CCD has a slow frame rate of approx 2 frames per second becomes less of an issue.

Brightness of the target object also has a bearing on which method is used, but less so than the duration.

Recording occultations using CCD detectors as I do, directly to .FIT file on a PC through software such as CCDSoft is not recommended by occultation experts. The primary reason it is frowned upon is the timing accuracy which can be achieved. In my scenario I am relying upon the accuracy of the PC clock in terms of it’s correctness to the current time, and it’s precision over a period of time. On top of that, I am relying upon the software (CCDSoft) and camera hardware downloading and saving the .FIT file with the correct time embedded. I synchronise my PC clock with a time synchronisation program shortly before each occultation, ensuring an accuracy of the clock within 4ms. 4ms is a large error bar for occultations that may last only 1 second. Compare this to video with GPS time insertion which reportedly reaches much greater orders of magnitude of time accuracy.

Why use CCD if there are issues with timing accuracy? It is a case of working with the equipment you have and using that to the best of its ability. It is also a question of automation. I am not interested in being up recording occultations in-person at 2am in the morning, the fact is I have work and other commitments which make this impossible to do on a regular basis, I prefer to instead start from the outset on the basis of using automation. The current video recording equipment used for occultations doesn’t permit automation, you can’t insert the use of video to record an occultation in amongst a night of supernova galaxy imaging. Also, swapping from CCD automation to manual video is problematic for a remotely controlled and automated telescope setup (not only do you need to physically change equipment but also recalibrate various aspects such as focuser calibrations and perhaps pointing calibration or weight balance).