Precise timing of astronomical events

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Version Francaise (Original by Jean-Paul.Godard !at! noos.fr April 2004)

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SECURITY : Your are NOT ALLOWED to use any element of this procedure if you don’t fully understand before the risk of permanent blindness linked with direct sun observation.

DON’T Look directly at the sun. DON’T POINT ANY optical instrument to the sun.

 What you will need

·        Ephemeredes of the event
To avoid missing the right time

·        A DV (Digital Video)recorder DV VCR

·        A compatible device for observation
(an amateur telescope with adaptor,…)

·        A  GPS handheld station or a DCF77 Radio driven Clock
(GPS to get position and Reference time. Radio driven clock gives only reference time. A GSM phone is a way to get speaking clock  or network time but precision is less !)

·        Laptop PC (PIII 500Mhz) with linear video editor program

·        Some training
(to repeat without hesitation : have a personal checklist)

Don’t buy all these devices. Friends may provide you with some of them.

Principle :

DV VCR records the event. Clock signal (DV) Is recorded and give a way to relative timing. Difference « Dt » between GPS reference clock and DV relative clock may be obtained afterwards by analysis of the pictures of  GPS digital clock/ DCF77 Radio driven Clock (pictures dated by DV signal from VCR).

DV VCR records first a clip from GPS (/DCF77 Radio driven Clock) screen and then the event. Difference between the 2 time sources will be determined at home at the time of measurement analysis. Precise determination of  important phases of event is also made at this time. Selection between different video frames at this moment is crucial to avoid observation effects (Black Drop during transit, Baily’s beads  during eclipse)

SECURITY : Your are NOT ALLOWED to use any element of this procedure if you don’t fully understand before the risk of permanent blindness linked with direct sun observation.

About Precision

Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
Signal multi-path — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Some think that GPS screen may display time with some delay due to calculations. This source of error is linked with older devices. You may prefer a DCF77 Radio driven Clock in case of doubt.

The  method on the field

Following hypothesis apply

·        DV VCR time drift is acceptable during observation : see Measure, Prevention and Verification of Clock drift

·         

Step 1 : Adjust Time Zone (VCR and GPS)

 

Step 2 : Acquiring GPS Clock

 

 

Capturing GPS Clock

Capturing GPS Clock and DCF77 radio controlled clock may be usefull to check GPS accuracy

 

Step 3 : Acquisition of the movie of the event

SECURITY : Your are NOT ALLOWED to use any element of this procedure if you don’t fully understand before the risk of permanent blindness linked with direct sun observation.

PUBLIC OBSERVATION may lead to unwarned people to look directly at the sun.

 SONY PC120 over ETX70 (altazimuthal drive)

with William Optics DCL4337 adaptator

Sun filter is not pictured here. Put your’s in front of ETX

DON’T Look directly at the sun. DON’T POINT ANY optical instrument to the sun.

Step 3bis : (Re)Acquisition of  GPS Clock (Optional, if Step 2 has failed, or to improve precision)

Step 4 : Processing Movie on a laptop computer (PC or MAC)

As  amateurs movie maker (You have a VCR, So you Are), you know that you can edit your DV movie on a PC/MAC. Non Linear edition software are provided (Moviemaker on PC) or freeware (Virtualdub) may be used if they can give back DV clock signal (to calculate  Dt). Plugins exist to display DV time signal recorded on tape/disk.

If GPS display don’t show fraction of second…

 

A partial view Pinnacle studio9 screen during movie analysis frame by frame

 

 

 

Step 5 : Analysis of Event Frames

 

Mercury transit on May 7,  2003 (I should have imagine the procedure for that time !!!)

 

Prevention and Verification of Clock drift

·        Have a brand new battery (The little one which is in charge of device permanent memory)

·        Have the standard Battery fully charged at the beginning, to avoid exchange.

You can measure precisely the VCR clock Drift by repeating step 2 many times with some hours (days) intervals.

SECURITY : Your are NOT ALLOWED to use any element of this procedure if you don’t fully understand before the risk of permanent blindness linked with direct sun observation.

happy transits !!!

 

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