Untitled Document

OCCULTATION BY TITAN OF 28 SGR 3rd JULY 1989

Observations that follow are a result of a video recording of the above
event made by members of the SALFORD ASTRONOMICAL SOCIETY from SALFORD OBSERVATORY. Read the description below first and then click the image on the right for a 9 second movie
(a 969KB avi) showing Titan's central flash. The digital watch in the foreground displays time in minutes and seconds since the start of the occultation.

DATE: 03.07.89
TIME: 22h 17m 40s to 22h 44m 50s GMT ( To Speaking Clock ).
PLACE: Salford Observatory, Chaseley Fields, Salford, M/c, GB.
LONG.: 02o 17' 47" W
LAT.: 53o 29' 38" N
ALT.: 70m

INSTRUMENT: 0.47m Newtonian Reflecting Telescope.
F.L. 2.56m / f5.64, P.F. Mag. x101.

EQUIPMENT : PULNIX TM36K CCD Camera ( NO FILTRATION ),
525 Lines, B&W, Battery Operated 12V DC.
JVC HR2200EK VHS Portable Video Recorder,
625 PAL Colour System, Battery Operated 12V DC.

CONDITIONS: Clear, Cloudless, Urban Mist & Haze low down,
Light Pollution South from Manchester & Salford.
VZM about 4.5mv at Zenith, Saturn initially in tree
tops at beginning of observing/recording session.

OBSERVERS: Mr J. K. Irving - Video Recording.
Mr A. O'Sullivan - Vis/Audio Recording 105mmCAT/x40.
Mr C. Jordan - Visual 20x80mm Binoculars.
Mr T. Wakefield - 105mm O.G./x48 Finderscope.
Mr C. Lawson - 50mm/x24 O.G. Finderscope.
Mr S. G. Holt - 10x50mm Binoculars.
Mr W. Mathews - 150mm Newt. Spec.
Mrs E.Mathews - Video Monitor, Dome.
Mrs K.Redford - Video Monitor, Dome.
Mr G. Singh - Video Monitor, Dome.
Mr G. Ellicot - Video Monitor, Office.

Other members who helped in the observations were Mr T. Brown for fabrication of adaptors, Mr R. Haslam for video clock inserts/titles.
Members of the Society began arriving at the Observatory from 20h GMT. Equipment was set up and tested to 22h then at 22h 17m Video
recording commenced until after the event at 22h 45m.
Normally an RCA HC2000/U ( ULTRICON ) camera would have been used with the option of a three stage EEV Image Intensifier.
However on this occasion we were in the process of evaluating the above CCD and although not ideal for our deep sky pursuits it was
decided to use it based on the previous night's work indicating adequate sensitivity for this event. It should be noted that the camera
images into the near IR and hence is well suited for the red gK4 28 SGR.

Due to unforeseen camera/recorder incompatibilities, timing difficulties have shown up on the recording, see over for details. In
hindsight an MSF receiver should have been used, and a digital recorder would have been better than the above analogue one,
as the event was much more detailed than we had expected. Copies of the original recording are available.

DISCUSSION OF THE OSERVATIONS AND TIMING ERRORS

Following the recording of the observation, it was decided to add a clock to a copy tape of the video image for single frame timings.
However it was noticed that the video clock against the audio markers was progressively drifting out and amounting to about four
seconds over the time period of interest ie 22h 38m 00s to 22h 44m 50s.

Further investigation, and discussions with a video engineer revealed that the cause was the 525 line frame rate which appears
to have been weighting the video recorder's servo system to a slightly faster running speed during record, and resulting in slower
speed on playback. It appears to be a constant error which can be calculated out. Using the full available recording, the error was
determined:

VIDEO TAPE STOPWATCH COLUMN COLUMN TIME PLAYBACK RATIO OF
AUDIO TIME OF 1 2 DIFF TIME FOR COLUMNS
MARKERS PLAYBACK (Secs) (Secs) (Secs) 1s ERROR 3/4
h m s m s
22 17 40 00 00.00
25 10 7 34.03
33.86 450.0 454.0 4.0 113.5 0.9919
32 55 15 24.16
24.02 915.0 924.1 9.1 101.6 0.9902
36 05 18 36.18
36.21 1105.0 1116.2 11.2 99.7 0.9900
36 20 18 50.50
50.50 1120.0 1130.5 10.5 107.7 0.9907
38 00 20 31.21
31.14 1220.0 1231.1 11.1 110.9 0.9910
39 15 21 47.32
47.21 1295.0 1307.25 12.25 106.7 0.9906
39 40 22 12.48
12.21 1320.0 1332.3 12.3 108.3 0.9908
40 10 22 42.57
42.52 1350.0 1362.5 12.5 109.0 0.9908
42 20 24 53.85
53.83 1480.0 1493.8 13.8 108.2 0.9908
44 05 26 40.48
40.28 1585.0 1600.3 15.3 104.6 0.9904
44 20 26 54.99
55.05 1600.0 1615.0 15.0 107.7 0.9907
44 50 27 25.01
25.02 1630.0 1645.0 15.0 109.6 0.9909

AVERAGE = 107.3 0.9907

Therefore it appears that every 107 frames an extra frame must be added to maintain 'real time'. Alternatively stopwatch time can be
recorded then multiplied by the constant 0.9907. Also because of personal equation errors on the video and in determining the above,
two error times must be added to any determinations. A combined personal equation value of 0.5 second was used for event timings.
Next a very simple but laborious procedure was used to obtain a light curve of the event. Using a good quality monitor, the image of the
star was recopied with the ultricon camera to a much larger scale along with a stopwatch image. Each alternate frame/star image was
then measured direct from the monitor screen using an ORP 12 photoresistor coupled to the resistance bridge of an AVO 8 multimeter,
maximium image brightness was determined on a arbituary linear scale and it was this value that was taken and plotted to give the
curves shown.

Although crude, with no differentiation and subtraction of sky background taken into account the major events of the disappearance and
central flash can be seen, resolution = 1/25 sec. Reproducibility was excellent ( <+/- 0.1 units ), most data points ( 2015 ) were measured
twice over the 5hrs it took! Hence no reappearance data yet.

TIMINGS OF MAJOR EVENTS

Five very distinct events occurred on our observations;

1) Disappearance - initial deep minima 22h 39m 22.8s
2) Disappearance - Final flash 1s wide 22h 39m 54.1s
3) Central Flash - 2 secs wide at 22h 42m 07.5s
4) Reappearance - First flash Visual 22h 44m 10.8s
5) Reappearance - Final deep minima 22h 44m 45.5s

Using the 38m 00s audio/video marker
and applying the derived error corrections of t=(t+0.5)*0.9907 the corrected
times were obtained. So much detail was seen at disappearance and reappearance,
it is difficult to convey these fluctuations in light by comments only, see
plot of disappearance.

DISAPPEARANCE COMMENTS--CENTRAL FLASH COMMENTS--REAPPEARANCE COMMENTS

39m 22.8s gone 0.6s 42m 06.8s brightening 44m 03.4s faintly ?
25.1s dip 0.4s 07.5s maxima 06.3s faintly ?
26.3s fade 1.2s 08.6s gone 07.6s faintly ?
27.9s back 10.8s back vis.
28.3s fading flickers 11.8s gone
30.6s minima 13.3s flash
31.2s maxima 13.8s gone
31.4s fading 14.4s flash
33.7s gone 1.7s 15.1s flashes
36.4s maxima 20.6s maxima
37.4s minima 21.0s gone
38.7s maxima flickers?
40.0s gone 25.9s B maxima
43.1s back 1.2s 26.4s gone
43.8s gone 27.4s B maxima
49.5s back 1.4s 28.4s gone
50.1s gone 29.5s B maxima
54.1s back 1.0s 29.9s gone
54.8s gone 30.7s maxima
flickers 31.4s gone
Titan ? 32.7s VB maxima
40m 24.3 last flicker ? 33.6s gone
34.8s VB maxima
35.4s gone
36.3s maxima dip
37.3s VB maxima
37.8s dip
38.1s flash
38.3s gone
38.5s VB for0.2
42.0s dip gone
44.0s dip
45.0s full Bness
45.5s dip gone
BACK ON

Terms used are V=very, B=bright,
maxima=brightest image in rise, minima=faintest image of fade, dip=quick short
fade which recovers, fade=slower fade to minima, gone=not on video, back=back
on video. Flickering throughout the occultation was suspected intermittently
and Titan is just visible on most of the tape where the star is not showing.

Ken Irving - 2 Dell Avenue,

Swinton M27 2TT
British Isles 03.07.89
JKI