Obsession 18" Classic Dobsonian

When planning for my backyard observatory I have decided that my CGE1400 will be permanently mounted on the pier and used mostly for imaging. Therefore I needed a portable visual scope with a reasonable aperture for deep sky observing. After a short research I decided for 18" Obsession - the aperture is reasonably big and I can still observe without a ladder, at least in most of the positions. Dave Kriege of Obsession Telescopes has an excellent reputation assuring that I will get to notch scope.

I have ordered my Obsession equipped with Argo Navis digital setting circles, ServoCat GoTo system, nylon shroud, Telrad, 80mm finder, dew heater for the secondary mirror and Dave's counterweight system. Separately I also got TeleVue 13mm Ethos to be used as primary DSO eyepiece. I chose OMI optics, and I am not disappointed: it is excellent. The quality and workmanship of Obsession is truly outstanding, they really deserve their reputation.

With 80mm finder, Telrad and 13mm Ethos on the top of the scope there is no chance to balance it without some kind of counterweights. Initially I just used Dave's system consisting of 3 aluminum tubes mounted underneath the main mirror. I filled the tubes with sand, but it was too light, and in the end I had to use lead shot. In order to complement the regular counterweights that cannot be easily adjusted I bought RCW/FCW system from Markless Astronomics (Charlie Starks)which is easily adjustable: the counterweights can be moved up and down easily (see picture below). Works excellently and exactly as advertised. Also, quality of Charlie's products, instructions and packaging (all separately labeled) is outstanding. I have also bought his stalk (to keep Argo Navis and ServoCat hand pads easily accessible) and truss power system (to power dew control system without hanging cables).

The views from this scope are fantastic! My observatory limiting magnitude is 6.24 and it helps too, but even when there is haze or Moon in the sky this scope is really the best. Very easy to use: roll it out to the pad, remove handles, attach stalk, connect cables (and power), uncover mirror - and I am ready! On a good night M51 looks almost like on pictures (except for color of course), M101 shows its ghostly arms with no problem, and major globular clusters resolve the stars to the core. Dream visual scope, it is . I need a small stepladder (with two steps) to reach eyepiece when pointing close to zenith - I am 6' 1" tall. This is not a big problem, but sometimes I miss the fact that I could sit down on a chair when using my big C14 SCT visually. I can still use a variable height Orion chair with the 18" Obsession, but the regular chair is too small for most targets. I bought CatsPerch Summit observing chair which is MUCH higher, and it works well, especially if I am looking at objects that are close in the sky for an extended period of time. Usually it is the case since I like to examine few targets in more detail, which is most of the time better mode of observing than hopping all around the sky. In order to feel comfortable sitting on the very high CatsPerch Summit chair I added a plastic stepstool so I can easily adjust position and step up and down from the chair.

Argo Navis and ServoCat are a very good investment too. Argo Navis is easy to calibrate and has a sizable object database (especially regarding deep sky) while ServoCat easily moves the scope to the target and keeps it there. You can disconnect ServoCat at any moment and reconnect at will. I usually calibrate Argo Navis manually (it is just faster) and then engage ServoCat for pointing and tracking. It does save a lot of time compared to star hopping, especially for fainter objects, instead of looking after the object I can look at the object. And good dark nights are rare here, so I it is a good idea to maximize observing time. Definitively it is the best accessory one can buy for an Obsession.

I also own Mallincam Hyper Plus color camera and I use it with my Obsession. Surprisingly it came to focus without any problems as it is (no Barlow or reducer), but it worked at high magnification with a small field of view of about 11'x8'. Tracking was not that good at this magnification reducing maximum integration time to a few seconds only. Reducing focal ratio was in order, but unfortunately neither my Meade f3.3 reducer nor my Celestron f6.3 reducer came into focus with the Mallincam and Obsession. I did some research, asked for help online (Yahoo groups for Mallincam and Obsession users) and it turned out I need to shorten the trusses. I have ordered a new set from Dave Kriege which was 1.5" shorter than the original one. With these trusses Mallincam came to focus with MFR-3 reducer and two 10mm extensions, effectively reducing focal length by a factor of 0.55, the field of view is now 20'x15' and image scale is similar to TeleVue Ethos 13mm. Mallincam works very well in this configuration - see Mallincam page on my site for more details.

I use the short trusses all the time, they help with my bioviewer as well. I have Burgess Optical Binoviewer and I use it with two Orion Expanse 20mm eyepieces. In order to come to focus with original trusses I had to use 2x Barlow, rendering magnification too high for most objects. With shorter trusses, I use 1.6x Barlow lens (SBW-1.6x OCA for binoviewer from Siebert Optics), and field of view is much better. For regular eyepieces I use 2" eyepiece extender from Orion.

As much as I like Argo Navis system it has two (small ) disadvantages, at least as compared to Celestron computerized scopes. Switching between objects and/or choosing an object is more cumbersome - one needs to choose with the dial, which is harder than just punch the numbers on Celestron hand pad. The worse is that the number of stars, double stars and variable stars in Argo Navis database is much smaller than in Celestron's. I used to prepare star lists using SAO catalog numbers and rarely I encountered a situation that the star was not there an I had to execute RA-DEC goto command using star's celestial coordinates. There is no RA-DEC goto command on Argo Navis, and moving an Alt-Az mount into exact RA-DEC position is not easy - even when reading RA and DEC on the hand pad in RA-DEC mode. The solution I chose is to use a handheld PDA (Dell Axim v51x) connected to the ServoCat/ArgoNavis via Bluetooth with a planetarium software capable of using observing plans.

Both ArgoNavis and ServoCat support computer control via a serial cable, so the natural way of using Bluetooth connection is to use an AirCable Bluetooth serial adapter AirCable Serial3. As the planetarium software I chose Astromist: it is a powerful planetarium program, and my astro planning program AstroPlanner can export plans to Astromist. I had some problems connecting it all together, but in the end I got a workable solution.

First, two rightmost dip switches on AirCable must be in off position - it selects speed 9600 baud and hardware handshake off.  Connecting Bluetooth to Dell Axim is a very smooth, plug-and-play experience. Choosing defaults there works fine most of the time, one thing that is not obvious is that establishing a relationship to AirCable device is not enough, one needs to click on the next tab on the config screen and link this device to a COM port - it won't happen automatically. It all needs to be done only once, each subsequent connection is fully automatic. I needed to use a pin to authorize Bluetooth to AirCable connection, the default one is 1234.

I used ServoCat serial cable supplied with the system and I connected AirCable directly to it. AirCable was set up in Axim as COM7. With Astromist telescope driver set to ServoCat I was able to get the coordinates the scope is pointing to, but I wasn't able to transmit anything to the scope, i.e. when I moved the scope Astromist followed, but when I tried to transmit a new goto point nothing happened. Then I connected AirCable directly to ArgoNavis box (port 2) and selected LX200 driver in Astromist. I also selected Meade protocol on port 2 in ArgoNavis setup. This time it worked much better. Astromist was getting the coordinates from the scope, and it was able to transmit goto coordinates to the scope. When a new object was selected and telescope icon clicked ArgoNavis immediately displayed offsets by which the scope needs to be moved. Of course it didn't initiate a goto, for that I needed to press goto button on ServoCat handpad.  

 I am quite happy with the results. I am now able to use AstroPlanner plans with my Obsession and I can use the Astromist sky charts to navigate manually if needed. The need to press goto button on ServoCat is not a problem, I have it connected on a very long cable (stereo extension works perfect), and it is light, so I can keep it near the eyepiece at all times. At worst I can invest in wireless ServoCat handpad. BTW, AirCable does not need any null modems or gender changers, it plugs directly to ArgoNavis and ServoCat cables, and with dip switch configuration on-on-off-off (left to right) it works just fine.  Based on other people experiences I think the ServoCat connection behavior is caused by a problem with Astromist ServoCat driver, but in the end I got a very good workable solution.

The most used eyepieces are 28mm UWAN giving 1.21 field of view and TeleVue Ethos 13mm with 0.63 (38') field of view (remarkably similar to what I was used to with Orion Optiluxe 40mm on C14). I also use Orion Expanse 20mm with my binoviewer which yields approximately 0.4 (24') field of view with 1.6 Barlow required for focusing. For planets I use Radian 10mm or Lanthanum 7.5mm - seeing in the Northeast is not good enough for any higher magnification.