Ob Report 20.6.09 at Sedili Besar (15" Zambuto/Obsessio

rlow · Post by **rlow** » Wed Jul 01, 2009 6:34 pm

Observation Report 20 June 2009 at Sedili Besar with 15" Zambuto/Obsession scope

Prelude
This report was long overdue as the observation took place two Saturdays ago but as I had been busy working day & night last week to meet some project deadlines, hence I only manage to write it down last night after Kun Loong (Kamikazer) had already posted on the following observation trip on 27 Jul 2009.

Introduction
On 20 June 2009, I drove up to Sedili Besar, Malaysia for our regular monthly overnight observation session. Joining me for this trip are Andrew (Tachyon), his friend Darren, Liuming (apomania). Andrew brought along his 15” Obsession dobsonian powered by a 15”f/4.5 Zambuto mirror, together with a 13mm Ethos & the Paracorr. As usual, I brought along a set of Pentax XWs & some series of planetary eyepieces to supplement the scope. We just managed to fit in five persons and these equipment comfortably into my car .

Getting There
We set off at about 1.30pm and as the traffic was relatively light at the immigration checkpoints and on the Johor trunk roads, we arrived at around 3.30pm at Sedili main town and had our customary tea-break of roti chernai (roti prata) at the old coffeeshop besides the jetty. After that, we head up to Tanjung Sutera Resort for a 2-hour relaxing leisure break, where we saw a full 180 degrees rainbow that ended in the middle of the South China Sea! We also had our dinner there, and everyone had a huge coconut drink... ok my coconut was just the average size, but the other four coconuts were really enormous, much bigger than our heads! Unfortunately I didn’t take a photo of these coconuts.

Nightfall
After dinner, we head over to my regular observation site nearby, which is an open space next to the cliffedge 30 metres above the rocky seashore, with a scenic panoramic view overlooking the South China Sea in a Northeast direction. Compared to the earlier observation site discovered south of Sedili Kechil, this site has much less mosquitoes. Incidentally I was not bitten by mosquitoes at all for the last two sessions here after I applied on some insect repellent. Furthermore, we are made aquaintance with the land owner who lives nearby; she approves of us using this site for astronomical observation, and somehow we feel more secure and safe at this site. So while my wife settled down to chat with the nice old lady landlord, and Darren wandered around and catch fireflies, Andrew, Liuming and I set up the 15-inch scope for observation. The night sky was clear and naked eye limiting magnitude at zenith was about magnitude 6, but there was an overall slight haze which reduces the sky transparency down one notch to 8/10. The Milky Way was visible but somewhat diluted in intensity; the Coalsack was just barely visible to me. I noted that this faint haze has been around for many months. The seeing condition that night was quite excellent, about 9/10.

About the Scope and Optics
As expected, the Obsession has very smooth motion on both axis, and I had no problem keeping an object centred for observation even at 686x magnification. Although setting up the scope was a tool-free affair, we realised that adjusting the mirror sling was not so straightforward and require 2 nos. of wrenches. The wood stain and polyurethane finish used on the mirror box and rocker base was executed excellently. Fortunately this Obsession doesn’t contain a 2-inch thick primary mirror so there was no issue of thermal effects affecting the image right from the start. I did a quick star test and the in-and out-focused star image were similar symmetrically and uniformly illuminated, so I can conclude that the Zambuto mirror was potentially very promising. There was however one anomaly which I noted. Usually I would observe coma in scopes faster than a focal ratio of f/5 without a coma corrector; for example my 11” f/5 Woden mirror (strehl ratio 0.97) revealed images with slight coma (just discernable to my eye) in my Pentax XWs eyepieces without a Paracorr, and the coma was very obvious in my 114mm f/4 Orion Starblast. Yet somehow the few Pentax XWs which I tested on this f/4.5 scope seemed to show images which were well corrected without the usual coma at the edge even without a Paracorr. I am quite surprised and puzzled (and I was not the only one who noticed it) so I wondered if there was some Zambuto magic in it… hmmm.

One Slight Problem & How We Overcome it
Well, Andrew had a slight problem with his Rigel Quikfinder at his last observation session at Punggai, so he asked me to bring my Rigel Quikfinder instead. When we set up the scope, we realised that the Quikfinder base was not on the upper tube assembly (UTA), and I only brought the Quikfinder without any base, as I have two bases stuck onto my other scopes. As we are unable to find a temporary way to adhere the Quikfinder onto the UTA for fear of having the finder dropping off during the night, so we had to resort to using the 40 XW with a 1.6 degree true-field as a starhopping eyepiece to find objects. As Mooey would know from his last experience handling the 15” scope without a finder, zeroing onto objects with just a 1.6 degree eyepiece is no easy feat. Fortunately three of us coordinated and worked closely to overcome this issue. I would use a green laser to point at an object in the sky, so that Liuming can target the object with the laser. Meanwhile Andrew got behind the mirror box to direct me closer towards the lasered target while I view through the eyepiece to do telescopic starhopping to the laser beam and then onto the targeted object.

Objects We Observed
In this way, we managed to find quite a few objects in the 5-hour observation session: Saturn, Alpha Centauri, Omega Centauri, Centaurus A galaxy, Jewel Box Cluster, M65/M66/NGC3628 galaxy trio, Markarian’s Chain (10 galaxies starting from M84/M86), M104 Sombero Galaxy, Antares & secondary, NGC 6231 cluster & False Comet, NGC 6441 globular, M4 globular, M22 globular, M8 Lagoon Nebula, M6 Butterfly Cluster, M7 Ptolemy Cluster, M17 Swan Nebula, M16 Eagle Nebula, M11 WildDuck Cluster, M51/NGC 5195, M101 galaxy, M97 Owl Nebula/M108 galaxy, Izar (epsilon Bootis), M13 globular, Double-Double, M57 Ring Nebula, Albireo, Coathanger, NGC 6960/6995 Veil Nebula, Pickering Wedge, Dumbbell Nebula, Jupiter

Eyepieces Used
Throughout the whole observation session, we ended up using only three eyepieces:
(a) the Pentax XW 40mm giving 43x (TFOV 1.6 degrees) was our starhopping eyepiece
(b) the Ethos 13mm & Paracorr provided 151x, was our main eyepiece for deepsky observations
(c) the Pentax XW 5mm giving us 343x at exit pupil 1.1mm for high-powered views
The three of us also cooperated very well as we each hold on to a particular eyepiece (Liuming-40XW, Andrew-Ethos/Paracorr, Richard-5XW) and we pass our respective eyepieces around to the next guy when it is our turn to remove our glasses to observe.

The Highlight Of The Night
Andrew was hoping to see a spectacular fireball drop from the sky, but we only saw some sporadic meteors. For me, the memorable highlight for this session was the positive sighting of the M57 Ring Nebula’s central star. This elusive object apparently varies in brightness from 14-16 magnitude and its visibility is also hampered as it is embedded in the central nebulosity of the Ring Nebula. While sharp observers like Stephen O’Meara had seen it in a 9-inch refractor, some observers had difficulty or limited success with 18-inch scopes. For many years I had hoped to catch a glimpse of this elusive object, so now I can finally tick this item off my list of visually-challenging objects that were ‘conquered’. That night I saw, using averted vision, a sharp pin-point light for one second within the nebulosity at the centre of the Ring, this was at 343x magnification using the 5mm Pentax XW. There was also a star visible with direct vision just outside the ring. No filters was required or used. Then when we added a Zeiss Abbe 2x barlow to the 5mm XW to observe at 686x, I saw the central star again intermittently three times together with four other stars outside the Ring with averted vision, and this time Liuming and Andrew had also seen the central star. I was not able to detect the second star in the central nebulosity. When we viewed M57, it was only rising at about 45 degrees in altitude above the ocean at that time. At 686x, we were pushing the optics at 45x per inch, and I noted there was no image breakdown.

Homeward Bound
Around 2pm after observing the GRS and moon shadow transit on Jupiter, we realised that the secondary mirror had dewed up, and this has caused some light scattering around the images and reduction of planetary details. We did not bring along any heat packs, and we were quite exhausted anyway, so it was a great excuse for us to pack up and head for home.

Postscript
For those of you who were wondering what happened to my 15" f/4.5 Zambuto mirror which I had ordered earlier and which arrived together with Andrew's mirror, it is still spotless and fungus-free in its original packing box when I opened it a few times to admire its reflection.... Ok, the truth is that after patiently waiting for 12 months for a replacement mirror cell which never came despite several email correspondences promised by a particular US dob-maker, I have decided not to wait further and to move on. I will re-start on my 15" dob structure after the 22 Jul 09 eclipse trip and I target to complete this ATM project within this year. Certainly the great views I had observed through Andrew's 15" Obsession/Zambuto scope was very inspiring to me. Even though I had experienced it just those few hours, I could already feel the optics has great potential which I need to utilise in my personal quest to complete the rest of my list of 'visually-challenging objects'. I have been deeply motivated... yeah, I can't wait to complete my 15" dob and start observing with it too!

rlow · Post by **rlow** » Wed Jul 01, 2009 7:00 pm

Here is how we fit in the 15" Obssession (and 5 persons) into the Honda Airwave.

Here's the full-length rainbow over Tanjung Sutera Resort. That's Andrew catching up with the end of the rainbow at the ocean. Direction is facing East.

Here's the view from the cliffedge at Tanjung Sutera Resort facing the South China Sea in the Northern direction. Our observation site has a similar view.

Here are the extra-large coconuts!

rlow · Post by **rlow** » Wed Jul 01, 2009 7:02 pm

That's the 15" f/4.5 Zambuto mirror inside the Obsession mirror box

Here are the eyepieces which we brought but most of which we did not use: sets of XWs, ZAO-IIs, Pentax XPs/XOs, Supermonos, Nagler Zoom 3-6mm, TV 1.8x Barlow, Zeiss Abbe 2x Barlow, UltraBlock & O-III filters. Not in photo: 13mm Ethos & Paracorr.

rlow · Post by **rlow** » Wed Jul 01, 2009 7:16 pm

Here is Andrew, the proud owner of the 15" Obsession powered by Zambuto mirror.

Finally, here is our group photo (L-R): Liuming, Andrew, Richard.

Canopus Lim · Post by **Canopus Lim** » Wed Jul 01, 2009 11:43 pm

Cool finally the 15" is out for action! Haa. The reason why the stars are sharper off-axis in the 15" scope is probably due to the longer focal length and thus less field curvature. If I have time, I can verify this with the optics design software in office.

Hmm..strange, why do you need to adjust the sling? It came loose? It should not be something that needs to be adjusted when setting up the telescope.

jermng · Post by **jermng** » Thu Jul 02, 2009 1:23 am

Did Andrew buy an obsession from US or did Canopus sell his?? hmmm ...

Post by **weixing** » Thu Jul 02, 2009 10:49 am

Hi,
Finally, his 15" see some real star light... just wonder is it the 15" first light?? ha ha ha

jermng wrote:Did Andrew buy an obsession from US or did Canopus sell his?? hmmm ...

He buy the obsession structure and use his own magic zzzz mirror.

Have a nice day.

Canopus Lim · Post by **Canopus Lim** » Thu Jul 02, 2009 12:27 pm

jermng wrote:Did Andrew buy an obsession from US or did Canopus sell his?? hmmm ...

He bought his own Obsession structure. Mine is 12.5"... it cannot 'grow up' to 15"

If can, I wanna a 25". Hehe.

Hmm..I think it is not the scope's first light but maybe first light at Malaysia? I have looked thru his scope at his place before...but that's Singapore.

I have verified that field curvature in Newtonian depends only on the focal length. As the focal length is longer, the field curvature is less. Coma depends on the F/# and aperture. Bigger the aperture at the same F/#, less coma and it is due to the mirror being less curved in a larger mirror. Hence the eyepiece may look sharper off-axis because the field is flatter. (if the eyepiece is designed for flat field first). I remember the Pentax 30 and 40XW are flat field in design from the graphs posted online.

How sharp the eyepiece off-axis depends on a few things:
1. The eyepiece astigmatism and coma control
2. The eyepiece design itself..if it is designed for flat field or for curve fields. Some eyepiece are not flat field and will not work well in a scope with little field curvature.
3. The scope's field curvature
3. The scope's off-axis aberration (in Newtonian is only coma). In SCT's is a mix of astigmatism and coma. In aplanat refractors it is astigmatism only.
4. The observer's eyes (if his is aberrated with astigmatism and not corrected with glasses)
5. The observer's accomodation to the field curvature

Many factors.

Zephyrus · Post by **Zephyrus** » Thu Jul 02, 2009 10:48 pm

Hi rlow, very interesting and detailed report as usual. I really enjoy reading your reports, and I'd very much love to join you on your observation trips in future. Haha, I only own a very humble 5" maksutov and I've never seen the Milky Way in it's full glory. I've only been stargazing in Singapore's light polluted skies.

If you don't mind, do let me in when you go for your next observation trip again since I'd love to join you and have a look at the giant scopes, and also let my eyes have its "first light" in real, dark skies! I think I also need to go out and have a good observation session as I haven't actually gone out to stargaze ever since school started for me in April. That's a whole 2 months! I need to go get myself back the motivation to continue the hobby! Hahaha!

I'll just drop my contact here and maybe you can just send me an sms if you can grab me along on your next ob. 9one2eight1six4one. (:

rlow · Post by **rlow** » Fri Jul 03, 2009 1:58 pm

Thank you Zephyrus. I usually travel to Malaysia monthly (and sometimes twice) for overnight astronomical observation and we always welcome anyone who is keen to join us. Among the regular observers here, we traditionally joked that bringing beginners (or those who have not observed under dark skies) is a good omen for clear skies... haha, so it looks like you fit in that category perfectly! Anyway I have planned for the next Malaysia overnight session on Saturday 25 Jul 09, after we are back from the 22 Jul 09 China eclipse trip. I have vacancy available for this trip so if you are free to join us, do drop me a PM.