Sunday, April 13
Bright comet, C/2025 F2 SWAN, was discovered from SWAN images of the Solar and Heliospheric Observatory (SOHO). I observed this comet for the first time at predawn on Monday, April 7 JST. The comet was 8th magnitude, 5f in diameter, tail length of 7f and 3 in degree of concentration (DC) at 4:03 JST when the altitude was 5 above the horizon, while 8th magnitude, 6f in diameter, 15f in length and 3 in DC at 4:20 JST with the altitude of 9. Those values were estimated by intuition not formal using 46 cm 97x with Ethos 21 mm, and I did not report to any institutions like COBS. The comet is expected to reach 4th magnitude, and it already has a wonderful tail.
There has been active exchange in information on the comet via comets-ml before the ground-based observation was made. First of all, Michael Mattiazzo, Australia, wrote gThere is a pretty obvious comet visible in SWAN comet tracker mapsh, and he requested to confirm at April 1.517 UT. Mattiazzo said it was visible on March 23 and the magnitude was 11+. Some tried to confirm the object both visually and photographically, but reports were negative. Qicheng Zhang, US, succeeded to observe the comet using 40 mm refractor as R magnitude of 12th on April 02.51, however, the position was 2.5 off from the orbital elements calculated by Rob Matson. Michael Jäger, Austria, observed the comet on April 03.13479 UT photographically, with the magnitude of 9 to 10.
According to CBET the comet was reported by Vladimir Bezugly (Dnipro, Ukraine) on March 29, followed by Michael Mattiazzo (Swan Hill, Vic., Australia) on the same day, and Rob D. Matson (Irvine, CA, USA) on March 31. Bezugly is a person who revealed that the fast-moving comet-like object I detected last year as potentially a Chinese satellite spewing fuel. He discovered SWAN comet in the past.
Positions of C/2025 F2 SWAN. A: Predicted position based on the orbital elements calculated on April 2, 2025, by Horohisa Sato. B: Calculated on April 6, 2025. B is 2.9 west of A. Produced using Guide 9.
I tried observing C/2025 F2 on April 2.82 UT based on the orbit calculated by Hirohisa Sato that did not include data of Qicheng Zhang, but failed. On the next day, April 3.82 UT, I could not again. The cause I could not spot it was, firstly, tried to observe during the session and searched only }1 of the predicted position, secondly, the ephemeris included 2.9 of error (it was reasonable considering calculation based on SWAN images), and lastly, I received huge amount of emails from comets-ml and did not read one from Michael Jäger received on April 3.18 UT that included the positions he measured.
I did not regret very much that I failed to confirm the comet, because I am a comet hunter not an observer. Nonetheless, it is a little embarrassing if I had spent several minutes more to search wider area, I would have been the second observer of the comet from the ground in the world next to Qicheng Zhang.
As described in CBET, Guoyou Sun, Xingming Observatory, China, found the comet was in CCD image taken by 0.11-m f/3.6 refractor on March 26.93 UT. The coma was 30h and V magnitude of 14.0 to 14.4 without a tail. Mattiazzo estimated from SWAN images that the magnitude was 11th, and the discrepancy from the observation at Xingming Observatory was very large. A comet hunter Tetsuo Kudo, Kumamoto, said on March 31 the comet was only recognized as a haze on the photos, while on April 4 it was 10th magnitude. Those observations indicate that the comet was bright from the beginning on SWAN images but that it was dark from the ground-based observation. I think one of the reasons that initially the comet was undetectable from the ground, i.e. visible to infrared, was the darkness in comparison with SWAN images, ultraviolet.
Takaaki Oribe, Saji Observatory, Tottori, observed the spectrum of C/2025 F2. Oribe showed the comet is gas-rich and less dust. I think the comet looks dark using Night Vision (NV), because NV is less sensitive to gas-rich comets, although, unfortunately, I could not observe C/2025 F2 using NV, since my NV is out of order and under repair.
As for C/2025 F2, it was almost impossible to discover either visually or photographically from the ground before the discovery from SWAN images.
Sunday, March 30
I look back my comet hunting session in February and March. I did it five times in each month. In the February sessions, I used Night Vision (NV) twice and Ethos 21 mm three times, while in the March sessions, NV three times and Ethos twice. NV was employed only nights with moonlight. Having said so, I sometimes use NV under poor transparency. These days, I use both eyepieces depending on the sky conditions.
C/2023 A3 Tsuchinshan-ATLS, which was bright and long-tailed comet on the western sky at dusk last year, rose on the eastern sky at predawn before I knew it. I noticed this comet was on the east at predawn after the comet hunting session one day at the end of February. Just at the moment, I had an opportunity to contact Hideo Nishimura, a comet discoverer. He said he could not observe the object, because clouds continued to appear on the eastern sky every time he tried to observe.
On the other hand, it had been cloudy and rainy at my observing site, and finally, I had a chance of observation on Friday, March 7 local time. The comet was estimated as12th magnitude, apparent diameter of 1f with a tail, using 46 cm reflector Ethos 21 mm 97x. The visual data obtained with Ethos 21 mm was the same as NV & Paracorr, 37x, but the object was easier to detect using NV. I thought the comet was discoverable during the comet hunting session using each eyepiece, although it was difficult.
On Sunday, March 9 local time, using Ethos 21 mm, the comet fated to 13th magnitude, apparent diameter of 0.2f, degree of concentration (DC) 2, and I managed to noticed it. With NV the magnitude was 13th, 0.1f in diameter, DC 8. It looked almost stellar but the existence was obvious. I observed the comet at around 4:15am local time with the altitude of only 17 above the horizon. After that, I took the break during the session and trained my 46 cm reflector at C/2023 A3 at 5:13am local time when the comet rose 29 above the horizon. However, there was no difference in appearance.
IC1295, planetary nebula (left) and NGC6712, globular cluster (right). The image is 1.44 wide.
https://aladin.cds.unistra.fr/AladinLite/
All deep sky objects identified during the session on Sunday, March 9, were N26, NGC6712, IC1295, M11, and M75. Among those, this was the second time in nine years I observed IC1295. The object is a planetary nebula seen in the same field of view with NGC6712, a globular cluster. I observed it for the first time at 3:02am on May 18, 2016, at the site in Niigata Prefecture with the altitude of 950 m with the air temperature of 5C, and the object was 44 above the horizon. Although by the time, I observed NGC6712 tens of times, I had never noticed a comet-like object in the vicinity of the cluster, and in a moment, I doubted it as a new comet. Nonetheless, unfortunately, the object was on the atlas. Since then, I tried observing the nebula and aimed my telescope, but I have never succeeded. Although the conditions of this time, IC1295 being 29 above the horizon, air temperature of 0C, the altitude of the site 850 m, were nothing special, I could finally encounter this planetary nebula again thanks to good transparency.
Whether the object of interest is visible or not is dependent on the atmospheric conditions at the time, and even if one can see it, the way it looks greatly different. On top of that, the impression of the object one observes changes depending on onefs state of mind at the time. Discovery of a comet is also dependent on atmospheric conditions and mental state.
Sunday, February 9
It had been cloudy and snowy due to strong cold wave, but finally, we had a stable clear sky. This was the first comet hunting session in February. It was clear sky and windless with the air temperature of -10C. The record low throughout my comet hunting history circa three decades! Having said so, I felt chillier when temperature is plus a few degrees with strong wind.
Shogo Utsunomiya, a comet hunter living in Mimami-oguni, Kumamoto, told me that an air temperature reaches -15C at his observatory. He said he bought a down jacket but he had never worn it, because the walls of his observatory protect him from winds and kept warm.
NGC6633, an open cluster in the constellation of Ophiuchus. The image is 60f wide.
The Digitized Sky Survey copyright © 1994
I often searched along the ecliptic mainly Scorpius and the western part of Sagittarius last month. There are many open clusters and spent a lot of time to identify. This time, I began with M14, a globular cluster in Ophiuchus, more northernly the region, using a glass eyepiece (Ethos 21 mm, 97x). Although there are less globular clusters in the area, many stars shine in the FOV as it overlaps with the summer Milky Way. Numerous open clusters are sparkling like a blooming flower. The most impressive open cluster I observed this time was NGC6633.
Generally, the colder the amount of water vapor in the atmosphere decreases making transparency high, the better the star visibility is. Today, stars shined beautifully, and the observing site was covered with pure white snow as Minoru Hondafs poem.
May the starlight be purely transparent
The flowers beautiful in spring
Water flow crystal clear in summer
Breezing refreshing in autumn
and
Pure white snow fall in winter
I take a decent measure to protect against cold during the comet hunting session, and I do not feel chilly up to the air temperature of -5C. However, under the temperature lower than that I feel my fingertips cold. On the other hand, it is inefficient to use gloves with heater, because it requires more time and effort for preparation.
The time when I completed the session, Scorpius shines splendidly on the east. The season when Scorpius rises in the eastern sky at predawn, the coldness reaches its peak as Tsutomu Seki wrote in his publication.
Sunday, January 12
I went on a comet hunting everyday Wednesday Jan. 1 through Monday Jan. 6 thanks to good weather at the beginning of the month. Among those days, the forecast predicted clear sky on Thursday, and I drove to the observing site. However, it was cloudy at the site and there was no expectation to recover, and I returned back to my home. On Sunday, soon after I started comet hunting clouds began to cover the sky, and I searched only 20 min in total.
Observation site with slight snow cover. The photo was taken after the session on Saturday, Jan. 11.
Yesterday, Saturday Jan. 11, it was cloudy around my house, however, the Super Computer Weather predicted recovery of the weather. Unexpectedly, soon after I left home for the site, snow started to flicker. I did not care and drove around 10 min, but snow began to fall in earnest. I parked my car to confirm the radar image using my smart phone, but no snow clouds. I headed to the site believing the forecast. At 3:45am local time, I arrived at the site and it was snowing but I could see some stars through thin clouds. There was slight snow accumulation there. I decided to wait for the recovery of the weather.
At 4:15am, it stopped snowing and starry sky began to open up. I set up my telescope and was ready to observe at 5:00am. In summer I am ready to observe within 30 min after arrival, but in winter it takes 45 to 50 min: wearing polar jackets, pants and boots, preparing disposal chemical heater for my back and toes, slow motion due to low temperature. The thermometer indicated the air temperature of -8C. It was windless but the coldest predawn chill this winter.
When I lived in Niigata Prefecture, I observed at the site with the altitude of over 900 m above sea level in summer, while 430 m above sea level in winter because the roads are closed due to snow. The record low of the air temperature at that time was -8C. The coldest record during comet hunting session after moving to Kumamoto was -9C at the present observation site with the altitude of 850 m.
On Saturday before the comet hunting session, I observed Omega Centauri that rose 4 above the horizon with Ethos 21 mm, 97x. Although it was slightly dark because it was still low altitude (9 at the meridian), the cluster contains a great number of stars and impressive enough as the most beautiful globular cluster in the entire sky.
I started sweeping the sky from Ώ Librae, Zubenelgenubi at 5:04am. The deep sky objects I encountered were NGC5796, 5897, a pair of galaxies 5915 and 5916, M4, M30, M62 and M19. When I spotted NGC5915 (magnitude of 12.3), I identified it referring to SkySsafari 7 Pro, a digital map. On the map, a darker galaxy, NGC5916 (magnitude of 13.2), was shown in the south of NGC5915 and I noticed in the FOV.
NGC5915 (center) and 5916 (lower center). The image is 30f in width
The Digitized Sky Survey copyright © 1994
Double stars looked beautiful since a glass eyepiece was used for the session instead of Night Vision that produces black and white images. The most impressive one during the session was ΐ Scorpii. The primary star was golden and the secondary light blue. The scintillation deformed them and it looked like they were smiling and dancing.
I stayed at the site after the comet hunting session that ended at 6:21am, 30 min after the start of the twilight, to observe C/2024 G3 ATLAS. The comet rose at 6:29am, but there were clouds at the low altitude and I could not observe the comet. The clouds moved slowly, and I continued observation. At 6:41am, I found the comet with a tail 0.1 long using 7x50 mm binocular. The altitude of the object was 1.2. I could not see it with naked eyes. The comet was visible for a while disappearing in and out of the clouds. I could not aim the 46 cm telescope at the comet because grasses at the site hided the comet. While I was taking down my telescope, the sun rose at the direction where the comet rose. I visited
the site hundreds of times, but this was the first time I saw sunrise here. It was so refreshing to watch the sunrise after observing the bright comet.