Comet Hyakutake (C/1996 B2) Photometry obtained with 20-cm, f/2 B-S camera, V filter and ST-6 CCD.

1996 Feb. 18.15UT: V=7.8, Elliptical coma ~14'x16' DC=7. Delicate tail ~0.3o in ~PA 280o.
1996 Feb. 27.15UT: V=6.7, Elliptical coma ~20'x25' DC=7.  Faint, diffuse tail ~1o in PA ~290o.
Further 3-min R image shows similar ~1o fan-like tail in PA ~270-290o.
1996 Mar.  1.16UT: V=6.3, Elliptical coma ~24'x28' DC=7. Comet has ~0.7o diffuse tail in PA ~285o.
1996 Mar.  3.17UT: V=5.9, Elliptical coma ~24'x28' DC=7.
1996 Mar.  6.16UT: V=5.6, CDia=~20' DC=7. Moonlight.

1996 Mar. 21.10UT: m1=1.5, CDia=~60' DC=7. Tail ~20 deg long in PA ~260o 
Star-like c. cond. clearly visible with naked eye. Very good conditions!
1996 Mar. 27.10UT: m1=0.4, CDia=~60' DC=7. Straight tail extending for ~45deg, until Beta CVn.
Limiting mag. 6.5. Comet and stars defocussed with spectacles.

In 15x80B the coma appears of distinct greenish color with star-like nuclear 
condensation. With the same instrument, straight narrow jet ~3deg long was 
visible, spanning from the comet c. cond. in PA ~145deg.
1996 Apr. 05.80UT: m1=2.9. Tail about 6 deg.

Naked eye estimate, using the convex lens (comet appears as a stellar source)
1996 Apr. 5.80UT: m1=2.9, CDia=~15' DC=7. Straight tail ~10deg in PA 45deg. (7x50B)
1996 Apr. 6.82UT: m1=3.1, CDia=~12' DC=7-8. Tail ~11deg in PA 44 deg. (7x50B)

1996 Apr.  7.82UT: V=2.8, CDia=~20' DC=8-9. Tail length at least 14 deg (frame edge).
1996 Apr.  9.82UT: V=2.8, CDia=~20' DC=8-9. 
Mosaic frames were taken around  Apr. 9.82UT in order to determine the full length of 
comet tail. It may be traced up to 22o from the comet nucleus. Straight, narrow tail of
rather uniform surface brightness is present from the nucleus distance ~10o to 22o. 
Take a look at Hyakutake images page.
1996 Apr. 15.81UT: V=1.9, CDia=~10' DC=8-9. Tail ~15deg in PA 40 deg. 
1996 Apr. 17.80UT: V=2.4, CDia=~10' DC=8-9. Tail at least 15deg in PA 37 deg.     
1996 Apr. 18.82UT: V=2.6, CDia=~ 8' DC=8-9. Tail at least 15 deg.
1996 Apr. 19.81UT: V=2.8, CDia=~ 8' DC=8-9. Tail at least 15 deg. 
1996 Apr. 20.81UT: V=2.7, CDia=~ 8' DC=8-9. Tail at least 15 deg. 
1996 Apr. 21.81UT: V=2.7, CDia=~ 6' DC=8-9. Tail at least 15 deg.

1996
Mar.     23.02UT,  1.0, 1.7  deg, 20 deg  Breuil-Cervinia, Italian Alps (2100m)
         24.06,    0.8, 1.9  deg, 30 deg 
         25.00,    0.5, 2.0  deg, 40 deg 
         28.13,    1.1, 65'       60 deg  (Monte Generoso)
         29.14,    1.3, 55'       45 deg
         30.14,    1.5, 45'       35 deg 
         31.14,    1.6, 40'       30 deg
Apr.      6.82,    2.2,  -         9 deg
          7.82,    2.2,  -         9 deg
          9.82,    2.2,  -        15 deg  (San Marco Pass)
         12.82,    2.2,  -                (Monte Generoso)
         13.82,    2.2,  - 
         14.82,    2.1,  -        18 deg 
         15.82,    1.8,  -                ( Milano)
M. Zanotta writes: "In 25x150 binoculars, amazing colors were visible inside the coma. 
The brighter parabolic envelope was yellow/green, the outer diffuse halo fluorescent blue
and the tail was of white color".
Apr.     16.82,    1.8,  -        18 deg/gas  12deg/dust (San Marco Pass)
In 25x150B easy streamers and dark lines in the gas tail.
Apr.     18.82,    1.7,  -        10 deg  (Mount Mottarone)
         19.82,    1.7,  -        10 deg  (San Marco Pass)
         19.82,     -    -        22 deg tail in PA 41 deg  (7x42B) 
         20.82,    1.9,  -                (Milano, 7x42 binoculars).
         21.82,    1.9,  -                (Milano, 7x42 binoculars).
         24.81,    1.8,  -                (Milano, 7x42 binoculars).

1996 Mar. 3.18UT: 10-min wide-field CCD exposure, taken  with the Zeiss 2.8/180mm lens and
narrow-band H2O+ filter centered at 620nm (FWHM=10nm) shows ~3o delicate ion tail in PA ~293o.

1996 Mar. 10.958UT: Two co-added wide-field CCD exposures of 12 min total duration, taken with 
the Zeiss 2.8/180mm lens and narrow-band H2O+ filter at 620nm (FWHM=10nm), shows at least 
~4deg ion tail in PA ~285o. There is conspicious disconnection in tail, between the nucleus 
distance 1.0-1.4deg. 

1996 Mar. 20-21: Series of wide-field images were taken at the Crni Vrh Observatory in very 
good conditions with the 180mm and 65mm lenses, CCD and narrow-band filters. The mosaic 
image, taken on 1996 Mar. 20.94UT with the H2O+ filter, centered at 620nm (FWHM=10nm) 
shows numerous streamers in the sector PA 245-275o, some of them extending up to ~7o from 
the nuclear region. Also there is conspicious disconnection in ion tail at nucleus distance 137'. 
Further mosaic frames taken around 1996 Mar. 21.09UT display at least ~20o complex ion tail 
structure with kinks, clouds and knots of material along the main tail stream in PA 260o. There 
is a bifurcation at nucleus distance ~7o.

Additional imaging with the same lenses and red continuum filter, centered at 647nm 
(FWHM=10nm) shows ~7o broad, diffuse and slightly curved dust tail in PA ~250o.

1996 Apr. 6: Series of narrow-band CCD images were taken at the Crni Vrh Observatory in very 
good atmospheric conditions. The 90mm f.l. frames, obtained in the light of singly-ionized 
water ions shows   numerous streamers, extending up to ~5deg from the nuclear region. Mosaic 
images revealed several  knots of material along the main tail stream. The ion tail total length 
on 65mm f.l. images, is about 15 degrees. A remarkable kink is present at nucleus distance 
6.0deg. At this point,  the tail PA is turning from 44deg to 50deg. Additional monitoring with 
red continuum filter display  ~8 deg broad dust tail.

Reports from the Italian and Slovene observers for the Mar. 23-25 nights

Antonio Milani writes: I observed from a dark site at Mt. Grappa (1650 m), Italy on Mar. 24-25. Clouds ended around 1400-1500 m and the top of the mountain was clear. The cloud layer covered nearly all the light from the nearby plane, allowing to have a very dark sky. I estimated m1= +0.2, coma diameter nearly 2 deg. With a strong condensation the tail was easy to see for 40 deg but with averted vision it was detectable at least for 50 deg. A wide angle photograph by Eraldo Guidolin shows at least a 60 deg. tail. The tail had a very smooth wavy "S" shape and showed a noticeable evolution during the night. 

With binoculars (and little telescopes) the inner bright part of the tail close to the nucleus was really bright and narrow. Three jets were also seen with binoculars in the sunward direction. Observing the tail visually through a blue and a yellow filter, I did not notice substantial differences; CO+ emissions are strong but H2O+ must be present too. The photograph obtained by Eraldo Guidolin (telephone communication) show a very rich and complex structure in the plasma tail. 

There was a strange effect: looking at the comet one had the impression it was moving. It was not the really motion of the comet but an illusion probably due to the shape of the comet. I was wondering if a similar effect in the past lead to the common idea that comets move in the sky fastly during the night... 

Roberto Haver observed the comet near Rome (high humidity conditions). On Mar. 23th morning, he estimated m1= +0.5 and 28 deg. tail with the naked eye. He had also clearing on 24-25 and estimated m1= 0.2 and saw a 42 deg tail with the naked eye. 

Gabriele Vanin observed on the nights between 23 and 25 from Passo Falzarego near Cortina. He estimated m1 = 0.5 and 57 deg. tail with the naked eye. 

Ales Arnsek observed the comet from a dark site at Rakitna (Slovenia) during the 1- hour clear period on 1996 Mar. 24 evening. At 22h30UT, he estimated the comet m1=0.5, coma dia. ~1 deg. The comet had some 50-60 deg tail, spanning down to Gamma Vir. The limiting naked eye mag. at zenith was 6.0.