Modified 10" GSO Dob
Submitted: Friday, 25th January 2008 by Sab Szalai
Having used a 4.5" newtonian for 7 years, I contracted a case of aperture fever. I needed a bigger scope. That being said, I have greatly enjoyed the little newtonian but at the same time I wanted aperture to work with - those faint fuzzies were just too faint and too fuzzy!
Recently a unpassable deal popped up on a used 10" F/5 GSO dobsonian reflector from David (Dave47Tuc). I had the cash and it was an easy drive to Dave's to pick her up. The scope had many nice mods, so it was a 'now or never' situation, so I did what I had to do.
In my opinion, a 10" dob provides an excellent compromise between size, weight and portabilty. Being only a 172cm tall shortstuff , a 12" beast would be awkward for me top carry around, especially through the 2 doorways, kitchen, dog infested deck and 4 stairs I have to navigate through to reach the backyard. The 10" f/5 size also allows an observer to be comfortably seated in all tube orientations. The light pollution in my backyard means I'll be travelling to dark sky sites so portabilty is a concern for me. A 10" f/5 can easily fit across the back seat of a car.
The telescope features the GSO 10:1 Crayford microfocuser, which is a nice touch. The f/5 focal ratio means a steep light cone is entering the eyepiece so the microfocus option is invaluable especially at high magnification. A cooling fan is mounted on the back of the mirror cell and the rear of the mirror itself is exposed to allow circulation of air and aid in cool down time. The finderscope is a good quality 8x50 right-angle, correct-image unit with a two-point adjustment system working against a spring-loaded bolt, which makes finder alignment very easy.
Dave has made some mods to the telescope during his time with it. These are described below:
Fit and Finish
The build quality of the scope is generally very good. There's no plastic parts to be found, on this baby it's metal all the way - the way it's supposed to be! After dealing with flimsy plastic parts on my DS-114, having metal components is a major relief. The tube is made of rolled steel with a silver finish. The mirror cell is well designed allowing air to circulate freely around the mirror and it features a built in fan to aid cooling. The base is made of particle board and painted black. Care must be taken not to expose the particle board to excessive amounts of moisture as it can swell and warp.
The scope moves smoothly in altitude, with no "stiction". The motion is probably a bit stiffer than what I'd like but that could be easily remedied by placing the altitude bearing pads slightly lower down. The azimuth motion is stiffer and suffers from some stiction when the scope is aimed near the zenith. The issue is the smooth underside rubbing against the smooth teflon creating a 'vacuum' effect, similar to when you rub 2 slabs of glass together. While this is a common problem with commercial dobs, it can be easily fixed and you don't need any carpentry or cabinet making skills. A sheet of Ebony Star Laminate glued on the bottom of the rocker will provide smoother motion and is something I'll address down the road.
Putting it through its paces
This 10" GSO proved to be a solid performer on all classes of deep sky objects. I observe from my backyard in suburban Melbourne, where NELM is usually around 4.5, ranging to 5 in excellent conditions. I will now give a rundown of the telescope's performance on a variety of deep sky objects with several examples to give you an idea of how the telescope actually performs.
Observing emission and reflection nebulae in this scope is a real joy. Even without a filter, M42 and the Eta Carinae nebulae can be traced to almost what is seen in photos. The views with an OIII or UHC filters are breathtaking. M42's core shows extensive mottling and the E and F components of the trapezium are very easy, even at only 73x. A filtered view of Eta Carinae is most memorable. The brightest portion exhibits considerable detail with numerous globules and lanes visible. The keyhole nebula is easy and the tiny Homunculus shows considerable structure in its lobes. The Tarantula is another glorious sight. A filtered view at 250x reveals a bright, complex core surrounded by swaths of fainter nebulosity that laces the entire FOV. At very high powers, up to 625x, the telescope can resolve the clump of stars at the core into individual components. Scanning the area at low powers reveals countless knots and fuzzies and is especially rewarding with a filter. On one good night, I have even successfully seen the T-shaped Bok globule within the reflection nebula NGC 1999. Using a UHC filter, large faint nebulae are easy prey for this dob. Objects such as the Running Chicken (IC2948), Rosette, GUM 39, Cederblad 122 and the Flame Nebula are all visible from my backyard with a low-power eyepiece. I can only imagine the views from a dark sky site!
A 10" reflector is large enough to start seeing detail in planetaries, making it viable to perform detailed observations of these objects. One of my favourite objects, NGC 1535 in Eridanus is excellent in this scope. Moderate magnification reveals a greenish ring surrounded by a larger faint oval haze. Using 625x, the scope revealed some structure within the center and the central star is crisply focused. At 625x!
47 Tuc is always a spectacular sight. Even at 39x, it is fully resolved. Views at 250x are incredible, not just the pinpoint clarity of the stars, but the resolution deep into the core. One excellent night, I pushed the dob to 625x and it was a memorable scene of tiny diamonds glistening in a delicate haze, with outliying stars streaming off across a black velvet background. The smaller NGC 362 is an attractive sight at moderate powers, showing resolution approx halfway to the core with the inner regions appearing granular. The tight, dense NGC2808 in Carina is also fully resolved at high powers.
I've had success splitting stars as close as 0.6" with this scope. The double B706 in Canis Major consists of two almost equal components of magnitudes 9 and 9.2. at 0.6" seperation. Using powers of 357x and 500x, a hairline gap between the two ruddy globes could be discerned in moments of good seeing. I've found that using very high power on tight doubles such as B706 is actually detrimental, despite many people's opinions, as any slight disturbance in seeing will instantly wash away the pair. In good seeing, I usually stay in the 300-500x range on extremely tight pairs like B706. This GSO dob can competently split the notoriously hard Sirius, in good seeing. I've seen Sirius B at powers as low as 156x, whereas at 500x and above it is distinct, if very tiny. I was ecstatic when I first spotted Sirius B, as it's one of amatuer astronomy's most famously hard targets. Yet I was sitting at the eyepiece, gawking at thetiny dot following its violently flaring and turbulent neighbour across the sky and thinking "this isn't that hard!"
Only Saturn is well placed for viewing at this time. This scope serves up bright, sharp images of the planet at 156x, with the Cassini division at the ringtips easy and the shadow of the rings on the globe is jet black and etched with hgh contrast. The planet's brownish cloud bands are easy to spot and in good conditions the polar shading is visible. One night of good seeing I could go to 250x and the Cassini division was better defined as were the cloud bands. Sometimes the planet's moons would appear so close to the planet itself it is like they're almost touching. The crepe ring is clearly visible in good seeing, especially at magnifications around 200x.
The optical performance of this scope has met, and in some cases exceeded my expectations. Deep sky performance is excellent and some of that no doubt stems from the flocked tube and dew/light shield. The scope's ability to handle high magnification surprised me. I was expecting to see nothing more than a blur at the 600x powers I was using but alas in good seeing stars focus crisply and inner detail in planetary nebulae shine through. My only gripe is the Azimuth motion, but it can be fixed with minimal time and effort. Now all that's left to do is get this baby out to a dark sky site to see her true colours!
Would I buy it again? In a heart beat!