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12.5inch Obsession review

Posted: Thu Mar 29, 2007 10:52 pm
by Canopus Lim
10 months back I said I would do a review on the 12.5inch Obsession if I have the time. Here it is! :)

Without shroud and fully assembled: ... 197mn2.jpg

With shroud and fully assembled: ... 200fk6.jpg

Flocked primary light baffle cover I made: ... 199on7.jpg

Mirror box and rocker box on car seat. UTA and truss tubes in the car boot (not shown):

The "Control panel" - UTA with Argo Navis DTC, Telrad with Astrosystem Dew shield, JMI DX1 focuser and customed fabricated laser mount:

In 2004, while I was in USA for internship, I bought my first dob. It was an Orion XT8, 8 inch traditional dob at a price way below that of an 8inch SCT. At that time I was having my internship and that scope was all I could afford given my small pay check. I bought the scope after doing a lot of research on dobs and found out that it is a highly recommended commercial dob at a cheap price. The performance of the scope is not bad for the price. It is simple to setup, takes only 5min. The optics has no zonal or turn edge problems and it probably just meets the diffraction limit 0.8 strehl ratio criteria. The motion of the scope is not that smooth, but it is very stable unlike tripod mounted telescopes. I used the scope extensively in USA. I attended bi monthly star parties in a few locations in North California. I was at San Jose then, so I traveled about 2h to the dark sites. Although the journeys were tiring, I felt it was worth it. I saw many objects with the Orion XT8 and they were pretty nice. I also looked through a 12inch dob and was blown away by the resolution on M22. When compared to corresponding SCTs, the Orion XT8 beats it optically. During the Mars watch at Republic Poly in 2005, that scope could match or beat majority of the scopes there except for the 8inch Portaball which has a better mirror and that scope was probably one of the cheapest scopes there!

It was because of this good experience with the Orion XT8, that in 2005 I started thinking of buying another dob. What I liked most about dobs are the simplicity in setting up, finding targets and simplicity in usage, ability to modify, good optics, wider field compared to Cassegrains, mechanical stability and great price point. I wanted a scope that was also portable too. The Orion XT8 though it is simple to setup has a disadvantage which is its size. The car that I drive does not allow the seat to be folded down and hence it requires the tube to be placed in the boot or to be placed on the seat. That would translate into bigger volume taken up in the car. Therefore I decided that my next dob would have to be a truss tube design which is more compact; one that can fit in taxis or any small cars. I also wanted a large scope that offered a significant improvement in light gathering and optical quality than the Orion XT8.

After doing research on the internet, I came out with a few sources that made truss scopes. They were T-scopes, Starmaster, Portaball, Teleport, Obsession, Nightsky etc. However, all of them had different strong points and weaknesses. For example for price, the T-scopes were the cheapest but their quality in built and optics were not. For Starmaster, the mechanical quality was about the same as Obsession but the price was much more expensive than an Obsession. For those with Zambuto mirrors, the waiting time for the scopes were very long and not to mention the price difference. Also there was the size of the scope that played a part. Obsession for one had the lowest profile mirror box (the largest component). Also Obsession looked great (hey…. Scopes are like cars.. they need to look nice other than performance). Obsession had fast delivery and the workmanship was good (as many people talked about the great workmanship). The optics was noted to be excellent and was interferometer tested (most accurate testing and not many optics companies do that because of the high cost in these machines). The reviews said that the motions were buttery smooth. So with all these positive reviews or testimonials on the internet, I decided to get the 12.5 inch Obsession. Did these reviews or testimonials live up?

I ordered the 12.5inch Obsession in Feb 2006 and got the scope in May 2006. Accessories I added were dew heater, Obsession light shroud and Argo Navis DSC. I ordered it with the JMI DX1 focuser. During that period, I waited a few months for the OMI Torus mirror and picked one with 0.97 Strehl ratio tested by interferometer. For strehl ratio tells how good the optics is compared to the ideal (in engineering ideal means almost impossible to achieve) telescope of that design. An ideal scope would have strehl of 1.0. Hence a 0.97 strehl mirror means it is near to a perfect Newtonian by design. For scopes having 0.95 strehl and above, it will give excellent star test and excellent contrast and sharpness on planets. Note that many manufacturers quote strehl but they use different testing methods. The most stringent test is actually through interferometry as it measures the full surface of the mirror and tests hundreds of points. Therefore a 0.97 strehl on an interferometer is actually very good. Other methods will usually give more optimistic strehl numbers. However, this is a test of the quality only. Mirror makers can make good mirrors without interferometer testing. Also, the OMI Torus primary mirror has enhanced 96 percent reflectivity coatings and 98 percent secondary mirror coatings. I feel this will aid in faint DSOs at the limit of visibility and mainly I bought this scope for DSOs. The Obsession dob is like an ‘XLT’ dob.

When I got the scope, it came in 2 big crates. The scope was well packed and it was really difficult to take it out from the foam that wrapped it perhaps too snuggly. I had to install the primary mirror, the altitude bearings and a few other things. It was quite straight forward. The only thing I did not like was installing the secondary mirror. I had to stick the dew heater onto it and I centre doted the secondary with a small dot of probably 0.3mm size which is too small to impact performance (through my optical simulation). This is to aid in the laser collimation. After installing the parts, I balanced the scope with my heaviest eyepiece (Pan 35). I used a container and put sand inside it, using 3M velcro (removable type) to tape it on the UTA (Upper Tube Assembly). It was fairly easy to do the balancing but just requires time and effort to do so.

The built quality is very good. I liked the colour of the plywood and the protective polyurethane coating and maple stain on the scope is nice. The telescope motion in Altitude and Azimuth is buttery smooth. If I polish the bearings with car wax, it becomes so smooth that I literally can use one finger to push the scope up and down. There is a brass name plate with the Obsession logo and my name on it. It is very cool. Setting up of the scope is a one man job. It takes 15min to setup and collimate the scope. I like the fact that the scope is a tool-less setup. Collimation just requires my Howie Glatter laser with Barlow attachment. Collimation is very simple with the scope as there are 3 big sized primary mirror collimation knobs and the secondary mirror has its own knobs which are arranged in x and y directions too. No tools to fiddle about in the dark. With my barlowed laser, I can get good collimation all the time and that is why it is good to invest in a good laser. I did star tests and the star test was excellent. It corresponded very well with my Zemax (optical design software) simulation of a 0.97 strehl mirror Obsession. The inside and outside focus diffraction pattern was similar with a slight hint of spherical aberration (expected) seen by one side of focus being very slightly softer in brightness. The rings were well defined and evenly bright meaning good spherical correction and smoothness of figure. There are no zones or turn edge. There is also zero astigmatism with the mirror (indicating good mirror cell and rigidity of the mirror). All in all, it means a well figured mirror and good mirror cell.

So how does it actually perform on the field? It performs very well. For all objects it gave me the best view I have ever seen. Jupiter has many bands with lots of details like blue colour festoons, satellites that are prominent discs, GRS with spiral detail inside it, small oval storms, many belts, sharp shadow transit etc. There was once when the seeing suddenly became perfect that Jupiter looked so sharp and the details were hair thin instead of a blur. The thing about large aperture scopes are that because of its high resolution, images are affected a lot by the atmospheric seeing. Hence, to get the best out of the image requires patience to wait for the seeing (which changes every few seconds) to be good. When at moments of good seeing, I saw lots of details that otherwise a small scope no matter what quality can never see. Also, I saw Red Spot Junior and GRS almost colliding. It was memorable as the Red Spot Junior was touching the GRS at the edge. After continuing observation, I observed the Red Spot Junior shifting position. It was cool. For lunar, the sharpness with my T6 Nagler eyepieces was good. As the scope does not track, it really felt like orbiting the moon at high magnification and huge apparent field. If you have seen Star Wars, you will know the effect of the X-wing fighters flying through Death Star’s canyons…it had that kind of effect looking at the moon. I also observed the craterlets of Plato and it gave very good resolution on it. The 5 largest craters were resolved as a crater unlike seen in smaller scopes which just show a speck of white. Just by hand tracking the telescope, I observed 11 craterlets inside Plato; a 5 inch apo will probably only see 5 of them as reported by other people. I probably could see more if I had an eyepiece of shorter focal length to achieve higher magnification instead of using a Barlow and if there is tracking. For Saturn, the best view was when it was near opposition this year. Saturn was sharp and the Cassini Division was inky black and sharp. The C-ring was so prominent and nice. The rings had nice shadings and Encke minimum and Encke division was seen for the first time. There was a lot of banding on the globe itself and 5 to 6 of Saturn’s moon were easily seen.

There were a few opportunities to go to Mersing to observe DSOs. Unfortuntely in 2006, out of 3 trips, only 1 trip had acceptable skies. During that trip, I brought my Argo Navis DSC but could not use it as it was having problems. Actually the most frustrating thing about my setup was the Argo Navis DSC; I do not need to set it up actually but the DSC aids in finding objects automatically by pushing the scope till the numbers count to zero. The set I received had power problems and after exchanging many emails with Gary of Argo Navis for a few months and sending the set back twice to Australia (Argo Navis is an Australian company), I received a Argo Navis DSC set that was working. Also, I did modifications to reduce the setup time for the Argo Navis on my Obsession. I did it by using cable-tie to tie the encoder wires onto one of my truss poles so that whenever I setup my scope, I just need to plug in the cables. Also, for the tangent arm (the arm holds to the altitude axis to make sure it turns), I fabricated an adaptor to allow mounting the arms easily once setting up. After these modifications, it is really much easier to setup the Argo Navis DSC. The Argo Navis DSC is the best DSC (Digital Setting Circle) out there. It has many functions and operates very smoothly like an Ipod. It only has a click wheel (like Ipod way of operating but it is made of the same knob as those found in electronic test equipment) and 2 buttons. It is a very simple and elegant design. The last Mersing trip I used it, and it really helped in pointing the objects especially faint ones like galaxies. Operating it was a breeze and the most powerful application was the Tour Mode which allows objects of different magnitudes, types to be found in defined constellations or section of sky (in degrees). It reduces my time looking through my Palm PDA for targets and hence preserves my dark adaptation better. It also allows me to spend more time observing objects instead of finding.

Ok.. now on the DSO performance. Simply put aperture rules. Brighter galaxies have details like spiral (if spiral galaxy), dust lanes and core variation. It was cool. This is the first time I saw so much details in galaxies. For stars, with my 17mm Nagler giving 93x and 0.9 degree true field, it is nice. Stars are so fine that they looked like dust. For star clusters, this is really the eyepiece as it gives a panoramic view of star clusters, stars and globular clusters among stars. With higher magnification, globular clusters are resolved very well and they do not look like faint smudges like in small telescopes. At 93x, some stars in globulars are resolved. Nebulae also looked nice. The dumb-bell nebula with the surrounding stars was the nicest thing I saw in my 17mm Nagler. The nebula looked as if it was floating among the stars. Even from my house, despite the real bad light pollution, I could see globular clusters and they even looked better than a small scope in a dark site.

Lastly, the good thing about this scope is its alt-az motion and damping time. The movement is fluid and balanced on both axes. This means one axis is not overly smooth compared to the other which makes it harder to hand track an object due to varying amount of energy required to push in different axes. Also, the motion has to be uniform without bumps when moving and this scope does not have any bumps. Tracking at 440x or 600x is not an issue and I have done that frequently. It is easy to track at 260x with my 6mm Circle T ortho and you know the ortho has a very small AFOV. Damping time is less than 2 seconds for 600x magnification. For lower powers of 300x or less, it is even easier to track. When compared to my Microstar mount motion, the Obsession motion is much better and the damping time is way ahead. I have not seen an alt-azi mount that can do that. However, note that if one is very critical, it would still be better to have a tracking platform to ensure that the eyepieces will perform best (on-axis). For casual viewing or for viewing DSOs at magnification less than 300x, the scope motion is more than sufficient.


What I like:
- Great looks. I like the big altitude bearings!
- Nice finish. I like the colour of the wood and the quality of the finish
- No tools setup and collimate
- Compact size. Easily fits in a small car
- Curve spider and well figured optics from OMI. No diffraction spikes. Sharp, good contrast and high resolution images.
- 15min setup and collimate. I timed myself.
- Fast delivery
- Good and quick response from Dave Kriege (Obsession Telescopes) and James (OMI). Dave Kriege answered all my questions and I asked many.
- Reasonable price for the quality. It is not cheap but it is not the most expensive.

What I dislike:
- If someone accidentally knocks the scope, it can go out of collimation because the Primary mirror is displaced. The collimation will remain if it is not bumped.
- It does not come with a bag for the truss tubes and cover for the secondary mirror

Improvements I did to further enhance the performance:
- 9V battery container for the dew heater and attached to the spider
- Attaching the DSC encoder cable and power cable onto one of my truss tube and fabricating an adaptor to allow the tangent arm to be mounted onto the rocker box for easy attachment of the tangent arm to the altitude encoder when setting up.
- Flocking of the UTA with high quality Protostar Hi-Tack flocking paper that can adhere to the Kydex light baffle on the UTA. Making a cardboard cover that is strengthened with aluminum sheets and flocked with Edmund Optics flocking paper. This cover goes over the birch plywood primary mirror baffle. This makes the primary mirror baffle dark black and this cover is attached with 3M Velcro (removable) so that I can clean the primary mirror baffle. This allowed my scope to operate in my house garden which is surrounded with many street lights. Improvement in contrast can be seen when I turn on my barlowed laser. The primary mirror spot is better defined and less stray reflection seen.
- 12V battery placed on a tray that is mounted onto my rocker box with 3M hooks

All in all, it is a nice scope and a keeper. The Obsession telescope lives up to the praises it receives from reviews and testimonies. I hope to keep it for a long time if possible. I call this scope Obby (for Baby Obsession in short because it is the smallest of all Obsession telescopes.). Next improvement is to have a tracking platform for critical observation and if in the future I cannot drive or do not have time to go to Mersing, I may just try out video astronomy.

Posted: Fri Mar 30, 2007 9:39 am
by Tachyon
Thanks! I'm printing it out to read over the weekend.

Posted: Mon Apr 09, 2007 2:22 pm
by rlow

Posted: Mon Apr 09, 2007 7:42 pm
by starfinder

Hey Canopus Lim, congrats on getting your review posted on Cloudy Nights! That is a very good write-up. It's one fine scope too! I savoured the views through it at Mersing last month.

Posted: Mon Apr 09, 2007 9:35 pm
by acc
I really like the nice woodwork on the Obby!

Posted: Mon Apr 09, 2007 11:28 pm
by Canopus Lim
Actually I did not expect it to be published because the CN never reply me anything. I did some modifications to the review to suit the Americans..especially their american english. Also I had to add some other things as required under the guidelines.

and yup, I like the wood work. :)

Posted: Tue Apr 10, 2007 11:23 pm
by reis
Gosh... Canopus... I din know ur [glow=blue]Obby[/glow] was such a treasure until i read ur review and comments from the rest!

Real glad that i had the opportunity to view jupiter thru it last week! and haha... feel it too! :lol:

Although without tracking, it was incredibly smooth to "move". This was something i din expect... given that the entire height was taller me!! :roll:

Posted: Wed Apr 11, 2007 9:24 am
by Tachyon
Reis - I'm afraid you have been contaminated with large apertures. Henceforth, your expectations for smaller scopes will be unfulfilled, and you are doomed to wander from star party to star party to hunt for the elusive 'better' scope than what you had seen on your first night out.

I have warned beginners NEVER, EVER to go near anything larger than an 8-incher, but some would not listen!

But fear not, although there is no cure, soon, there will be a new 15-incher (check the other thread on the community project) to satisfy your yearnings. Until then, I suggest the solution is to visit Hubble-Site and browse the pictures.

You have been warned! :twisted:

Posted: Wed Apr 11, 2007 10:26 am
by VinSnr
reis wrote:Gosh... Canopus... I din know ur [glow=blue]Obby[/glow] was such a treasure until i read ur review and comments from the rest!

Real glad that i had the opportunity to view jupiter thru it last week! and haha... feel it too! :lol:

Although without tracking, it was incredibly smooth to "move". This was something i din expect... given that the entire height was taller me!! :roll:
There are a lot of things that people still do not know about large aperture scopes.

The best for me is that for these scope, you can DIY or upgrade it anyway you want. You can add, you can change, heck you can even drill on them. Now try drilling an APO.

Posted: Wed Apr 11, 2007 1:35 pm
by rlow
But fear not, although there is no cure, soon, there will be a new 15-incher (check the other thread on the community project) to satisfy your yearnings.
Ironically, thats coming from that horse's mouth. :)