Introduction:

Note: This page is intended as a very brief introductory guide only to using video to observe occultations.

Observing minor planet and other occultations visually, while still being useful, does also have some disadvantages. In particular, visual occultation timings are subject to the reaction times of an observer. These can vary widely, from a few tenths to several seconds, depending on the experience of the observer and the conditions under which the observation is being made. With occultations involving the smaller minor planets, errors in timings can often be a significant fraction of the observed occultation length, leading to large discrepancies between the lengths of adjacent observed chords. In addition, the centres of the chords obtained by visual observers are often significantly offset from each other for the same reason. While visual observations remain useful and important, these factors do limit the accuracy of data obtained by visual means.

If the human eyeball is replaced by some other method of recording the event - e.g. a video camera, CCD detector in drift-scan mode, or a photoelectric photometer - reaction times and other errors associated with visual observing can be eliminated. The accuracy of the data can be increased by an order of magnitude or more.

Using a low-light video camera to replace the human eyeball at the end of a telescope has proven to be the most cost-effective way of increasing the accuracy of occultation timings. For this you need:

• A video camera capable of recording in extremely low-light conditions;
• A VCR or digital video recorder to capture data from the camera;
• Some means of placing accurate timing information onto the video record;
• A monitor so you can see the event as it happens.

Camera:

During 2004 and 2005 the RASNZ Occultation Section has been facilitating the purchase of low-light surveillance video cameras for occultation work, on a bulk order basis. These cameras have been distributed throughout Australia and New Zealand. In addition to observing occultations, the cameras can also be put to a variety of other non-occultation uses.

The camera purchased is the monochrome PAL KT&C 350BH. The NTSC version of this camera is available in the U.S. as the Supercircuits PC-164C, and this has been widely adopted in the occultation community because of its extreme sensitivity (0.0003 lux). In practice, observing under good, dark conditions, observers with a 20 cm telescope can easily record stars to 11th magnitude, and in a 30 cm telescope to below mag 11.5.

The RASNZ Occultation Section has been able to purchase the KT&C 350BH cameras in bulk at a cost of $NZ 280.00 each. These cameras come with automatic gain enabled, which means that the camera automatically becomes more sensitive or less sensitive depending on the brightness of the subject. Auto gain may not matter for many occultations where one is looking at a faint star just above the sky background. However should an observer wish to record a minor planet occultation where a very bright star is contained in the field, or to record a lunar occultation where the proximity of the bright limb might cause the gain to be turned down, any fainter stars within the field will be lost. To overcome this the RASNZ Occultation Section has been offering a modification to the camera to allow an observer to boost the gain manually. The cost of the modification is $NZ 45.00.

Because the cameras also needs to be mounted to the telescope, a 1.25" drawtube adapter is also available at a further cost of $NZ 10.00.

KPC-350BH Camera with Manual Gain modification and 1.25" Drawtube Fitting

[NOTE: As at April 2008 we are out of stock of the KT&C-350BH cameras and there are no immediate plans to acquire more. It's unlikely another order will be placed unless we get a minimum of 10 firm orders. If you wish go on a waiting list please email Graham Blow, but there is no certainty that an additional order will be placed in the near future.]

Recording Equipment:

Traditionally, a VCR has been used to record the output of a video camera. For this almost any VCR can be used, but it is advantageous for the VCR to have a few specific features.

Because a recording on a VCR will usually be analysed by playing the tape back very slowly (to pinpoint the video frame where the occultation took place), it is highly advantageous that the VCR have the ability to step through a tape one frame at a time. Since each video frame is made up of two fields - an "odd" field and an "even" field - a VCR that will allow individual fields to be viewed will give the highest time resolution. However not all modern video cameras have pause/still or single frame advance functions, let alone the ability to examine each individual video field.

More recently, some observers have been using video capture devices to stream the output of the video camera directly into a computer. Research is still being carried out in this area, although it appears that most video capture devices are unable to capture every field because of the high data rate. This results in every second video field being thrown away, which will lead to a loss of time resolution in the data. There has been much discussion about this on the RASNZOccultations and KIWI-OSD Yahoo discussion groups.

Once data has been transferred to a computer, LIMOVIE software can be used to extract the occultation lightcurve. Frame-by-frame lightcurve data can then be placed into a CSV file, which can immediately be read, and graphed, using MS Excel.

An alternative to recording on a VCR or directly into a computer is to connect the video camera as an external source to a camcorder.

Timing:

Placing timing information onto your video record can be accomplished in several ways:

• Record a time signal (e.g. Shortwave station WWVH) onto the audio track of your video record. For this you will need a microphone input on your VCR (although not many modern VCRs have these). If you are using a camcorder as a recording device this will be much easier. Note however that with only audio on your tape you will only be able to play back the tape at normal speed and estimate the occultation time to perhaps 0.2 second.

• Use a Loader Beeper Box. After syncing the box to a time signal you can either plug the external LED output from the box directly into the audio-in of your VCR (which produces an audible click on tape), or hang the external LED over the front of your telescope tube to ensure that a flash is recorded on tape every second. If using this latter method you will later be able to play back the tape one frame at a time and count the number of video frames from the last time signal flash to the frame containing the occultation. This provides a very precise method of timing.

• Use an on-screen display (OSD) such as the KIWI-OSD available from PFD Systems. This OSD will overlay millisecond accurate time determined via GPS signals onto every frame of your video record.

More information on all the above can be obtained by joining the RASNZOccultations and KIWI-OSD Yahoo discussion groups.

Monitor:

While a standard TV can be used, a small black and white monitor will allow you to see the occultation much more clearly. Such a monitor is the Jaycar QM3409 available from Jaycar Electronics in Australia and New Zealand.

Useful Links: