Updated 2010 Dec 31

Successfully recording the time (UT) of disappearance (D) and reappearance (R) of star provides information on the position and size of the occulting object. With more observations the asteroid shape can be deduced. Detection of D and R with a telescope can be made by eye (visual) or with electronics (Video / CCD). Electronic detector/recorder combinations will produce a permanent record of the event for re-play and further analysis. Reliable timing is important, so a UT time signal is recorded with the observation. The techniques described here have been used successfully by many observers. Predictions of possible asteroid occultations near you can be obtained from internet resources (EAON) and for the UK here> http://www.maidenhead-astro.net/astocc/.

Reminder
There are small errors in the orbit of an asteroid and the position of the star. This results in a likely shift in the track away from the observer, who then sees a shorter occultation or "miss". Or indeed the shift may work to an observer's advantage if he/she is not in the track but the shift is towards the observer. See this graphic by David Dunham. A miss (or no occultation) is a negative observation and should be reported.

1. Geographical Coordinates
2. Which Time Signal ?
3. How bright is the star?
4. Stopwatch or chronometer
5. Audio Observation and tape analysis
6. Reaction Time
7. Video
8. Video / VHS Tape
9. Video accuracy
10. Fast Frame Rate
11. Integration time
12. Video Time Overlay
13. Digital Video
14. Video Grabber VHS to PC
15. DV tape to PC
16. Video Grabber and calibrated clock
17. Drift Scan and Manual drift scan with DSLR
18. Primary Time signals sources
19. Secondary Time sources.
20. Detectors
21. Video mode DSLR
22. Recording

1. Observer's Geographical Coordinates
GPS is the preferred longitude and latitude datum for an observer, referred to as WGS84. This has superceded the Ordnance Survey datum OSGB36. The two coordinate systems refer to the same spot on Earth, but the co-ordinate origins are different. Read the Wiki. If you have OS coordinates, the OSGB36 to WGS84 conversion can be done online at http://www.nearby.org.uk/coord.cgi. Alternatively you can read them off Google Earth using the cursor to an accuracy of about 5 meters. I have checked my carefully surveyed observatory's OSGB36 coordinates and the mathematical conversion to WGS84 matched the Google Earth location exactly (to +/- 0.1"). After May 2000 the deliberate inaccuracy introduced into GPS caused by "Selective Availability" was switched off, so that GPS now provides sub 10m accuracy.

Height above mean sea level is also required when reporting events. I give heights to the nearest 10 m above MSL. Look up http://www.streetmap.co.uk 1:50000 for height information, or use GPS.

Map (left): The contours and spot heights are still usable

2. Which time signal?
Timing accuracy is crucial. There are some time signals that should be avoided such as from TV channels and un-calibrated computer clocks. Some phone networks (e.g. Virgin - thank you to Andrew Bate for this information) can produce a time signal, but accuracy might be unreliable. Primary time sources are listed in section-18. These are generated from atomic clocks and transmitted to telephone land-line (in the UK), Radio signal, and GPS equipment. In practice one of these time signal is used to synchronise another clock which could be electronic or electromechanical with a constant rate, and this is used at the telescope. A recent development is the use of GPS as the time source and this is rapidly becoming the preferred method for video occultations.

3. How bright is the star?
Stars undergoing occultation by an asteroid are quite often fainter (typically 9-12th magnitude) than those of lunar occultations. Occasionally some are brighter; and there are two events (D and R) to be recorded in succession. The interval (or chord length) can range from 0.1 sec to 30 seconds. If the asteroid and star are of similar brightness the magnitude drop (dM) is small. Under these circumstances (dM < 0.7) an observer might not see the event, or his/her reaction time may be longer (up to 1 second). Short occultations (less than 1 second duration) are subject to larger timing errors and uncertainty because the duration is close to the observers reaction time. ( Reaction time can be negated by using video Section-7, or Drift Scan Section-17). The magnitude drop is part of the prediction and an important consideration that will influence the method of observation.

4. Stopwatch and Chronometer
Most visual observers will be familiar with the “stop watch and telephone pips” method of timing. Start the watch (or multi-function chronometer) at the instant of occultation and stop the watch at a known second provided by a telephone or radio time signal. (Note: Use BT land-line in the UK). Lunar Occultations are often timed by this method.

A lap mode setting can record an interval while the watch still runs.

5. Audio observations and tape analysis
One solution to recording events in quick succession is to use an audio tape recorder with a UT clock tick or radio time signal superimposed.
Remember to announce a minute marker at the start and end of the recording. Analysis involves counting all the seconds. This takes time, and the process needs to be double checked, but the record is permanent and can be re-evaluated. Those who have Lunar grazing occultation experience will already be familiar with this method of recording. Some observers have digitised the audio which makes analysis easier.

The author has used a camcorder video ( Sony TRV 22E) as an audio recorder while using the internal clock as a time overlay (synchronised to UT). Prior to this he constructed an MSF 60 Khz receiver on a 4x1 inch Veroboard with the help electronics students who tuned the aerial.

Diagram (above) drawn by John Toone and reproduced from The Salford Astronomer (1977)

6. Reaction time (reaction delay or Personal Equation)
An observer's Personal Equation (PE) is subtracted from the recorded time. PE is the largest uncertainty in timing by visual means and depends on physiological factors. In the end PE may only be an educated guess for a difficult observation e.g. 0.5 +/- 0.2 sec. For clear events (bright star, good conditions) reaction time can be in the range 0.2 to 0.3 sec depending on the observer. There are methods of estimating reaction time. The dropping ruler method, software simulation or on-line apps. The author has used a stopwatch to time an asteroid and lunar occultation off a TV monitor, and compared it to the same event on Video. The reaction time is consistent for bright stars by direct vision. The estimated times of fainter events required greater PE correction (0.5-1 second). Estimated errors should be realistically reported.

7. Video.
An integrating video camera has become the detector of choice for asteroid occultations. Watec120N, 120N+ and Mintron 12V6HC are used by amateurs with excellent results. The camera has other applications such as meteor detection, or as a finder, or as an auto guider. Video produces a permanent unbiased record without reaction time uncertainties. The exposure can also be adjusted to suit conditions (magnitude drop, instrument aperture, seeing), so it is a versatile piece of equipment.

The Watec902 H camera is also used extensively (and costs less) but does not have an integration function. (It runs at 50 or 60 fps). In the US the Supercurcuits camera is available with similar sensitivity.

8. VHS video tape
VHS is a simple option to record from the video camera. It will also record a time overlay by inserting a time and date generator such as this unit from Voltec into the video signal. The result is a permanent record of the event. VHS video can be digitised for analysis on a PC.

9. Video Accuracy
Video introduces a much finer degree of accuracy, with 0.02s time resolution now possible (One field = 1/50th second CCIR). Compare this to a visual observer who at best may achieve +/- 0.1s accuracy. Although you will find integrating video observations reported with errors larger than 0.2 sec, this is due to the integration time selected, and is not a reaction time. Also video does not suffer from "detector dead time". This is an significant advantage over CCD cameras used in sequential frame mode. The time taken to download a 16 bit CCD frame is about half a second (Depends on the computer specification and camera). The exact time can be deduced, but a short occultation might not be detected if it occurs while the CCD is down-loading the previous frame. With video there is not apprec delay.

10. Fast frame rates
Low light monochrome video has the ability to detect very short events, fringe patterns, non instantaneous events or other unusual phenomena like double stars. Some CCD camera used in sequential imaging can reduce the dead-time by setting a smaller region of interest.( I'm told ). Also the ATIC Titan is reputed to operate at 10fps or faster. A USB2 web cam (e.g. Image Source) can also operate at a range of different exposures (and frames rates). This is untested, and accurate time stamping is a difficulty.

USB web cams are a distinct possibility, although the writer has not seen any reported observations. Low sensitivity may be a limitation.The IMAGESOURCE DFK21 (colour with IR filter) does not have the required sensitivity. The author has tested one. A 10th mag star needs > 2 sec exposure in a 12"

11. Integration time using an integrating camera.
The aim of video observation should be to use the shortest integration time the telescope and weather conditions will allow. This will require some experimentation before hand. For example a 20cm instrument in good conditions can record 12th magnitude stars in 0.16 sec, and a 30cm to 13th magnitude. So an20cm can record most of the predicted events from EAON and IOTA using an integrating video camera, and fainter objects by extending the integration time. The author finds that a short integration time of 2 full frames ( 2/25 sec) helps to smooth out seeing ripple in bad seeing conditions. 10cm and 15cm instruments can be used with longer video integration times. A maximum of 1 second exposure should be considerd as giving the best compromise between detecting the event (s/n), and time accuracy.

Displayed on a monitor, video will produce a bright clear images that could be timed with a stopwatch, but it is preferably to record the event either on tape (VHS, Digital Camcorder / Video 8 ) or a computer hard drive. Whichever method is chosen, a time overlay is to be added, one which has been synchronised to a UT time standard.

12. Video Time Overlays
There are two choices, either obtain a time and date generator (TDG) with manually sync to UT, or a GPS time inserter.
GPS
The GPS system produces a time stamp on every video frame to a precision of 0.001 seconds. It is the convenience of analysis of a permanent record that makes this method so attractive. The author is aware of two commercial GPS units GPSBOXSPRITE2 by Blackbox and AME-TIM10 by Alexander Meier Electronik. Here is a Youtube demo of the TIM10. Here is a screen shot of the Sprite in use, and Youtube demo of the Sprite. Another GPS time inserter here on Youtube by GaPaJaWil (DCF77) time source - This unit needs further clarification.

The Kiwi-OSD system is no longer available commercially.

The IOTA-VTI is a new addition to video time insetion via GPS, and is designed to give best results for occultations. It can be purchased and it will be maintained for the future.

TDG
A Time and Date Generators (TDG) is reliable but it doesn't’t produce an exact time on each video frame. The TDG clock is manually synchronised to UT ( e.g. from telephone land line, or long wave radio) and to obtain the time of each frame, the user advances the video one frame at a time until the frame in which the disappearance is reached. The fraction of a second can then found. Single frame advance is available on most better video players. This is made easier if the video is digitised and analysed by software e.g. Limovie and TANGRA. Both these programs are free beta down loads. Here is the Voltec unit in use

13. Digital Video Recording.
The author uses a DV camcorder with analogue to digital conversion (AV/in) built in. There are several makes available second hand (Sony and Canon). The astro-video camera output is plugged into the camcorder AV/in for display and recording. Newer equipment might also do this. Older units can be found on EBay, e.g. SONY TRV 22E 33E. They run on there own batteries and are highly portable - Beware that tape heads need to be cleaned. A dirty tape head could spoil your observation. Remember to clean it .

Digital Video (DV) cameras capable of A to D conversion (known or reported to the author by David Dunham and others) are:

Sony: TRV22E, TRV33E, TRV480E
Canon MV600i , MVX25i
Canon ZR camcorder ( if PAL ), models 60, 65, 80, and 90 (and other "i" models not beyond 300i)

Canon MX2, Canon XL2, Sony VX2000, Sony VX2100, Sony PD 150 and Sony PD170 are camcorders with better quality than consumer level equipment. I'm a permanent user of VX 2100 and PD170 - writes Pawel Max Maksym (pl)

A comprehensive list is here http://www.4kam.com/camcorders_with_av-in_av_input.htm sent to me by Jan Manek (update 2011Sep13)

14. Video Grabber VHS to digital
Recording directly to VHS tape and viewing on a monitor or TV is straightforward to set up. This may be sufficient for your needs if the VHS player has single frame advance for analysis. Digitization of the tape can be beneficial for post analysis with Limovie. You might already have a PC with a suitable video card, but at the end of the day, you are going to analyse or archive the video data in a PC readable format (e.g. AVI, DivX). The simplest method is to invest in a USB Video Grabber (Analogue-to-Digital). They are supplied with software for video editing which you might not need, but it would be advisable to install as there may be a feature you need later.

The author has a USB Video Grabber that came without processing software. There are software tools available such as the freely downloadable VirtualDub. This will detect the Video Camera (or VHS tape AV-out) and digitise the signal. I select a file name and duration for the recording, set the frame rate to 25.000 fps and record. This could be done during the event provided your PC will record happily – test your system before the event and make any adjustments.

One point to note is that the bottom of the frame might not be digitised by the video grabber (The Author’s did not). This results in the TIME overlay at the bottom of the screen being lost. The answer is to put the time stamp at the top of the frame. The TDG or GPS time overlay unit should have the facility to re-position the text overlay. The Authors BlackBox Camera SPRITE2 GPS text overlay unit does not do this however. I'm sure its the cheap USB video converter that is the cause,

15. DV tape to PC
Currently the author transfers DV tape direct from his Camcorder by firewire to the PC which was bought with firewire installed. Connection is made to the PC and NeroVision Express3 SE is launched when the connection is made. This software sees the whole video frame.

16. Video Grabber and Calibrated PC clock
Some observers use a calibrated PC clock to time stamp the AVI obtained from VirtulDub. The Author uses free software Dimension 4 which applies a correction to the PC clock by connection to the internet. The displayed PC clock (Windows Date and Time Properties) is not more than 0.2 sec different to UT immediately after synchronisation, and over 24hrs the drift is +/- 0.5 s. Dimension 4 is using the tick.usno.navy.mil internet time service, and is set to synchronise on start up, and every hour. PC time has been used for Drift Scan Detection with good results. The Drift Scan method is dependant on control provided by specific software and is outside the scope of this primer, however there is manual DRIFT SCAN worth trying using a DSLR.

17. Drift Scan and Manual Drift Scan with DSLR
The drift scan method is described here by John Broughton. The occulted star is allowed to drift across the detector controlled by software.

Manual Drift Scan with DSLR
Fit a focused DSLR at the prime focus and track the star. Position the star near the East side of the frame so that it will drift across the longest side. Start the audio tape recorder with time signal. Then about 1 min before the event, simultaneously switch off the telescope drive and open the camera shutter. The sound of a “switch throw” or “shutter opening” should be audible and recordable. This marks the start of the exposure. Now close the shutter after a predetermined time (sound is recorded again). The length of exposure should made less than the transit time across the field, with the predicted time of occultation occurring near the centre of drift.

Example:
A 0.5 degree field of view has a drift transit time in minutes of
0.5 x 4 / Cosine (star’s declination) Equation (1)

Field calculation
To calculate the field of view of your camera + telescope combination use this example based on a typical Canon DSLR with CCD size of 13.8 x 20.7 mm:

Field of view in degrees where FL is the focal length of your telescope in mm is:
20.7 x 57.3 / FL Equation (2)

This is for the longest side of the camera frame. So for the Author’s 30cm F/4 with focal length of 1200mm, Field = 0.99 degrees

The start and end of the exposure (taped events) were recorded. The time and duration of an occultation can then be worked out from the trailed image. Its helpful to know the rate of sidereal drift in pixels per second because this will indicate the accuracy of the observation. The Canon chip is 3456 x 2304 pixels

Pixels per second for a Canon 350D and 1200mm FL telescope is:
3456 / [ Equation (2) x 4 x 60 / Cos(declination of the star) ] Equation (3)

= 14.4 pixels per sec (Star declination = zero)
and 12.5 pixels per sec at Declination +30

Experiment with the ISO settings so that the frame is not unduly overexposed as this will effect the measurement of the star trails.

18. Primary Time Sources (the most accurate)
1) Telephone (BT 123)
2) Audio radio reception (Many stations world wide, three are selected)
a. MSF (UK) 60 KHz long wave
b. DCF77 (Germany) 77.5 KHz long wave
c. WWV (Fort Collins US) 10,000 KHz Short wave

3) Global Positional System (GPS)
a. GPS receiver that contains a one pulse per second clock (1pps) and associated electronics to generate and display precise UT.

4) An observatory clock
a. You have access to a professional system perhaps.
b. A clock that displays UT based on radio reception (commercial or home made).

19. Secondary Time Sources (Clocks synchronised to UT with a measured time difference)
1) Atomic wrist watch – time automatically synchronised by a radio time signal. There might be a small internal delay in the display.

2) PC internal clock set by an internet time service. (Rate may vary slowly).

3) Other clocks with established errors ( i.e. “running fast / slow by:” ) and manually synchronised to a Primary time signal (see above):
a. Video Time and Date Generator (TDG)
b. Quarts crystal controlled analogue clocks (electromechanical)
c. Electronic metronome (pips).
d. Camcorder with an internal time display HHMM ss.
e. Stopwatch or multi-function chronometer started or stopped on a UT second.
f. Digital wrist watch

20. Detectors
1) Eyeball
Be comfortable, warm and preferably seated. You use the optimum magnification for your system and there is shielding from glare. (Moon, outside lights ). Sidereal drive recommended, but not essential. If not driven, select a 1degree field and let the star drift across. Start observing two to three minutes before the predicted time, so that the event occurs while the star is drifting slowly through the center of view of the eyepiece.
Having found the star with the finder charts, allow it to drift through and see how long it takes.

2) Video
Analogue camera needs VHS tape or AtoD conversion for recording.

a. Sensitive in low light e.g. Watec, Mintron, Supercircuits
b. Suitable for very accurate time stamps (eg TDG, GPS).
c. Integrating Video- can adjust the sensitivity at the expense of time resolution. (Watec, Mintron) e.g. adding 4 frames = 0.16s
d. Electron multiplication Video (EMCCD) e.g Raptorphotonics, high sensitivity at fast frame rate. (Few examples in amateur hands)

3) CCD camera
Digital camera with USB2 connection to a PC
a. Full frame sequence or region of interest (roi).
Use is dependant on Camera / software and computer. Time resolution and time accuracy tricky. Not detecting during down load time
Good for slow occultation (e.g.TNOs).
b. Drift Scan
Needs specific software and telescope control for best use.

c. Drift scan with DSLR
Possible to rig up a system with manual control (some examples on the internet).

d. USB web cam.
Low sensitivity and time stamp issues to overcome

21. Video mode of DSLR
Sensitivity and time stamp issues to overcome.
Suitable for manual start-stop and bright events? - untested.

22. Recording
1) Stop Watches
2) Tape recorder or Video recorder for audio observation
3) VHS tape
4) Camcorder Digital tape or SD card
5) Camcorder CD ROM
6) Computer Hard Drive (needs a video grabber)

Author: Tim Haymes
Assistant Director (Occultations)
Asteroids and Remote Planets Section
British Astronomical Association.

- Lunar Section Occultation coordinator.

Maidenhead,
UK
January 2011