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The Bintel BT-302 (12") Dobsonian - A Review and a Few Modifications
Submitted: Thursday, 13th December 2007 by Scott Tannehill

In March 2007 I bought a Bintel BT-302 (12”) Dobsonain from Bintel in Melbourne, Australia. This scope is made by Guan Sheng Optical (GSO) in Taiwan.  I paid $899 AUD (~ $720 USD at the time) and that price included several basic eyepieces, a single speed 2” Crayford focuser, an 8x50 finder, a front tube cover with built in aperture mask, and a cooling fan.

I’ve owned the scope for nine months and have put it through its paces. I will first review the telescope, and then describe a few modifications to improve function.

GSO markets this same scope under their label in addition to selling it to Bintel. GSO also makes (I’m pretty sure) the Zhumell Dobsonian available in the United States. The Zhumell and a Bintel scopes are identical except for the paint and logo. Of note, GSO also manufacturers many of the components of the Meade Lightbridge telescopes.

Figure_1-_Assembled_Telescope_[800x600].JPG

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Figure 1 - Assembled Telescope

Like any Dobsonian this scope has two parts: the optical tube assembly (OTA), and the mount.

Optical Tube

The tube is rolled steel and painted gray. The more recent scopes are white. The ends are protected by trim which provides a finished look.

The altitude trunnions are plastic cylinders projecting out opposing ends of the tube, and are shaped to slide down into the comparably sized opening at the top of the sideboards and onto Teflon pads. The trunnion and Teflon pads form the altitude bearing assembly.

The trunnions on my scope are about 6” in diameter, a bit small by ATM standards. The trunnions are not perfectly engineered. When I move my tube from zenith to horizon, the tube translates about 5 mm across the sideboards. I assume this is from slight asymmetry or some related issue. Aside from slightly complicating my altitude encoder installation this has absolutely no impact on function and is not even detectable unless you are looking closely.  I’ll note that my 18” Obsession does the same thing, but just 2 mm worth of side-to-side translation.

Spider and Secondary Mirror

The spider is a four-vane model with Phillip’s head collimation screws. It is quite serviceable, no serious flaws, but use the right sized screwdriver on these screw heads because the screw heads are not terribly hardened and you can strip them easily. 

As delivered the vanes were appropriately tight.  There is enough length in the vane’s mounting thread for small adjustments of the vanes, if you want to offset your secondary away from the focuser to achieve so-called fully offset collimation.

The secondary mirror assembly attaches to the spider by a center bolt. This center bolt threads through a hole in the center hub of the spider and into the secondary holder below. The head of the bolt does not sit flush on top of the spider. Rather it can pivot or wiggle in this position, meaning the entire secondary holder below can also pivot about. This is intentional; it provides the variable positioning for collimation. Over the center bolt, between the bottom of the spider and the top of the secondary holder, slides a compression spring. This spring provides the dynamic tension for secondary mirror collimation.  While the bolt itself is holding the secondary to the spider (but not rigidly), the spring is pushing it back, and the expansion force of this spring dynamically “locks” the secondary in position while still providing enough leeway to fine tune the position during collimation.  When you need to move the secondary up or down the bolt, to position it correctly under the focuser, you tighten or loosen the center bolt, respectively.  Moving it up compresses the spring and moving it down relaxes the spring.

Figure_2_-_Secondary_[800x600].jpg

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Figure 2 - Secondary Spring

Why the operational details on the spider assembly? Because in my case, with the secondary situated correctly under the focuser, there was too little ‘sproing’ in the spring to keep the secondary mirror from flopping about as the collimation bolts pushed down on the secondary holder. This led to touchy collimation: I’d turn a secondary collimation screw slowly and my collimating laser spot wouldn’t move at first, then it would unpredictably jump 2-3 cm across the face of the primary mirror.  The fix ended up being fast and cost about 15 cents (discussed later).

The clips holding the secondary mirror in place were, surprisingly, not too tight, unlike many economy Dobsonians where the clips pinch the secondary mirror and perturb your star images.

Primary Mirror Cell and Cooling Fan

The primary mirror cell is very good and I noted no disabling features. I had only two concerns: the screws that secured the cell to the base of the tube, and the primary mirror collimation springs.  Both seemed a bit under-engineered for their task. The screws mounting the cell to the tube were small and short, considering the weight of the mirror. The screw head is only slightly larger than the hole, and the thread isn’t that long.  My instinct was to add a small washer under the screw head, to distribute the pressure across a larger surface area of the tube, but the screws are too short for that. I’ve not had any fall out over time, but I check them regularly and re-tighten a few which seem to loosen over time; at some point I’ll look into a bit longer screw with a washer.

Like the secondary mirror spring, the primary mirror collimation springs were too soft. This spring floppiness made primary collimation a bit erratic: the mirror and inner cell would clunk about with altitude changes – and not hold collimation, obviously – unless all three springs were near maximal compression. As discussed below I easily remedied this by changing over to heavier springs from Bob’s Knobs. Others have used heavier springs purchased at local hardware stores with equal success.

In addition to the collimation bolts there are three locking bolts which are designed to push against the inner frame of the cell and secure the mirror in that position. That is, the position after turning the last collimation bolt, dynamically balanced against the force of the springs pushing the mirror ‘up’ and the collimation bolts holding it ‘back.’  These locking bolts work well, but they definitely do not lock the mirror exactly in the same place. This is easily confirmed by checking your primary mirror collimation as you tighten a locking bolt. If you lightly tighten each bolt the same amount the collimation drift is minimal, but it does drift and if you are fussy you will have to deal with this. It is easy to over-tighten these bolts, but tighter will not incrementally improve your collimation ‘lock’ and I have to believe that super-tightening these bolts can’t be a good thing for the cell, either.

I removed my mirror to add a reflective center mark so I could use an autocollimator. In the process, I noted that the mirror clips were not too tight. This is (or was) another common problem, leading to pinched optics and decreased image quality (Checking the Optics of your GSO Dob). One should be able to slide a single piece of copy paper between the mirror and the clip pad. In the past some mass-produced scopes had mirrors vise-tight in the clips, leading to a characteristic pinched optic appearance on a star test. Not so, here. Thank you, GSO. Or, for all I know, Bintel intercepted the problem after taking delivery. In any event, I was expecting this problem and it did not manifest.

You need a screwdriver to collimate this scope. In fact, two screwdrivers: a Phillip’s head for the secondary collimation screws, and a regular screwdriver (or a thin coin) for the primary bolts. Why haven’t all the vendors added finger-friendly knobs for all collimation bolts?  Bob from Bob’s Knobs is happy, I’m sure. To any manufacturers reading this, if you are going to ask your customer to use a screwdriver to collimate, at least make it the same style screwdriver for both ends.

The cooling fan with this scope is a nice addition, especially the pre-wired battery back and plug in the back of the cell. As designed this fan is meant to help cool down the mirror before observing. Mounted as it is, it causes too much vibration to use while looking through the eyepiece. Being a fan of fans for Newtonians, I modified this to make it usable continuously without introducing vibration (GSO Dob Fan Project).

Optics

There were no obvious scratches or flaws on the mirrors. The coatings looked good.

I’m no expert on optical quality, but I have a Televue refractor and an Obsession Dobsonian back home in the U.S., and when correctly collimated and under good conditions this mirror showed no obvious astigmatism, spherical aberration, or edge defects.  The stars are nice points, not as pinpoint as my Televue but very nice. However, what I know about these flaws I learned from forums and internet sites, so I look forward to having someone skilled at interpreting these things look thru my scope and advise me.

Focuser and Finder

The stock focuser was a single speed Crayford.  I upgraded to the Bintel (GSO) dual speed 10:1 focuser.  It’s worth the money because the fast focal ratio of this f5 scope – the steep light cone, in other words – makes for a very narrow focus sweet spot. Because of vibration when you focus, it can be a bit hard to identify the best focus position, and the 10:1 focuser really helps. I note on their website that Bintel now provides the dual-speed 10:1 focuser as standard on the stock scope. 

I will add that the provided finder, an 8 x 50 straight-through version, was not great optically. I disassembled the finder to check for lens stacking perturbations, but concluded it was a simple case of get-what-you-paid-for.  A few friends with identical units have no complaints about their finders, so mine could be an isolated QA slip-through. I didn’t have the energy to return the finder or exchange it because I only use a Rigel Quickfinder and my digital setting circles to find objects.  This finder now serves mainly as a counterweight for the tube.

Mount

The mount and ground board are particle board (i.e., glue- or chip-board) covered with a non-descript black plastic laminate.  Assembly is easy with the provided hardware. The part of the sideboard where the tube trunnions sit is often called the cradle or simply the top of the sideboards. The sideboards and the front board are the same smooth black laminate-covered particle board. 

These sideboards flex in and out a bit if you squeeze them together. This is not a disabling feature, just another manifestation of the economy of design.  It works fine.  But, I will point out that any such flexion tends to wear on the joints and fittings. If it moves a bit now, it’ll only move more in the years to come as those joints experience more wear and tear.  And the associated bolts can loosen up over time. After several months of use I went over all the mount bolts and found several that needed tightening. Remember the first Blues Brothers movie?  Near the end of the movie Jake and Elwood exit their abused and exhausted vehicle, and turn to watch it spontaneously deconstruct, parts leaping off the car until all that remains is a pile of junk.  Replace that car with your well-used Dobsonian. It’s dawn after a long productive night of observing.  Your mount groans a bit and then everything spontaneously disassembles.  So tighten those bolts now and then, eh?

All the Teflon pads are real virgin Teflon, but are small and affixed with a single center-mounted and barely countersunk staple that can – and did in my case – work its way up and scratch the bearing surface, fortunately with no functional consequence.  Others have commented on this staple problem, and everyone should periodically check all pads and tap down any rising staples with a small screwdriver or awl-tip tool.

The laminate on the rocker bottom is the business end of the azimuth bearing.  On the GSO the undersurface of the rocker is the same as that covering all other surfaces of the mount. It’s a fine texture (almost smooth) soft black plastic laminate.  Smoothness is not a good choice because two smooth surfaces create a slight vacuum when pushed together.  Think of two panes of flat window glass stuck firmly together; even without any glue or tape, they don’t easily pull apart.  There are other forces at work at the molecular level (so I’ve read) but this cohesion contributes largely to the friction and initial sticky friction – termed stiction – that is quite noticeable when first initiating movement. More expensive Dobsonians use a slightly cobbled laminate surface (such as Ebony Star) which prevents this vacuum adhesion phenomenon and reduces friction and stiction.

Friction and stiction are related issues, but different. I think most Dob users would agree that, as we use the terms, friction is the resistance you feel as the scope is moving, and stiction is that initial stickiness just when the scope first starts moving, often jumping a bit at first because the force to overcome the static (at rest) friction is greater than the force needed to balance dynamic friction (i.e., the force needed to keep a moving scope in motion).

Of the two, stiction is more annoying. You try to nudge the scope a bit, it resists, you push harder, and it suddenly jumps and overshoots your target.  Furthermore, when the scope is aimed at zenith (in “Dobson’s Hole”) this problem gets worse because your lever arm – your mechanical advantage in moving the scope in azimuth – is smaller. It takes even more force to start a vertical scope moving and this leads to even more over-shoot when the scope does start moving.

Of all the features of the Bintel BT-302 Dobsonian out of the box, the azimuth stiction and friction were the most problematic, in that order. Hand guiding at high power was almost impossible. I gave the azimuth motion of this GSO Dobsonian a C grade due to this azimuth stiction.

Moving on to the altitude motion. Tension springs are included for the altitude bearing. These squeeze the tube down onto the mount sideboards harder than gravity itself would provide. This increases friction and stiction.  The intent is to prevent too-easy altitude movement of a tube which is not perfectly balanced. The tube balance varies with tube altitude position because the axis of rotation is not in the exact center and there is more weight at the mirror-end. A scope perfectly balanced at 45 degrees will be top heavy when horizontal, and bottom heavy when near vertical. The tension springs add friction to prevent tube drift in such situations. The down side is that it increases both stiction and friction.  At low to medium powers, this isn’t too bad. But at high power it was very hard – but just barely possible – to hand guide the scope over 250X with the compression springs attached.  Overall I give the altitude motion features on the stock scope with the compression springs attached a B minus grade.

Lastly, I will mention that many owners of this type of scope have advised that the particle board material can absorb water through the seams of the laminate, and expand or warp. Some advocate coating all seams with a silicon sealant to prevent this. At a minimum try to avoid letting water sit on the mount for long periods. I haven’t had mine long enough to be able to comment on this phenomenon, but I have seen some grizzled GSO Dobs in the field and noted what did, indeed, look like some expansion of the particle board under the laminate. This may be more a reflection of hard use than any flaw of design, by the way.  But there seems little harm in coating your seams with something that will repel water.

Review – Conclusion

The Bintel BT-302 Dobsonian is great value and a wonderful telescope for those who can handle the weight. I congratulate GSO for bringing affordable aperture to the amateur community.

I would buy this unit again in a heartbeat, as the price offered is more than fair, all things considered.  It is a great way to get large aperture for an unprecedented low price. I give the economy of this scope – the aperture ‘bang for the buck’ – an A grade.

My overall grade is a B grade: great value, overall very good function, with just a few non-disabling flaws of function. My primary issue here is the mount, which reflects a clear imperative of economy of design over premium performance.

Specific design elements that could be improved, in order of importance, include the following:

  1. Azimuth friction and stiction
  2. Altitude friction and stiction when compression springs are used
  3. Primary mirror collimation eccentricities due to floppy springs and need for screwdriver
  4. Secondary mirror collimation eccentricities due to floppy spring, and need for yet another type of screwdriver

Modifications

1. Azimuth friction and stiction

Armour All to the base helped initially but after about 2-3 hours the stiction returns. I think it gathers dust and gums up. 

I tried milk jug washers, also called pivot pads. These are thin washers around the azimuth bolt which off-weight the ground board pads and decrease the friction of rotation.  They are traditionally made by cutting out washers from a plastic milk container and placing them over the azimuth center bolt between the rocker bottom and ground board.  The addition of these milk-jug washers helped a bit but I didn’t appreciate the magnitude of benefit that others have noted, even when I’d added so many that the rocker wobbled, which wasn’t acceptable.

After reading up on the subject I choose to add Ebony Star to the undersurface of the rocker box, and larger Teflon pads to the ground board. There is fair consensus that the best material for the rocker bottom is Wilson Art’s Ebony Star™ line, or a similar texture laminate.  Ebony Star is an attractive black and silver surface with a pimply texture that minimizes the friction and stiction when placed atop Teflon pads. 

I know many who have added Ebony Star to their mount themselves and I envy them their skills.  As for my skills…well, I paid a local kitchen laminate store to glue Ebony Star onto the bottom of my rocker. They did an excellent job, trimming the edge and routering out the holes in the surface for bolting on the sideboards, thereby preserving my ability to disassemble the unit in the future.

Figure_3_-_Ebony_Star_[800x600].jpg

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Figure 3 - Ebony Star

At the same time I added larger Teflon pads from www.Astrosystems.biz.  These are nicely made, with a counter-sunk center mounting hole for ease in screwing them to the ground board. The pads come in three sizes: 1, 1.5, and 2.5” square.  I bought a set of each size, since I had other projects to use up the leftover Teflon. I attached the Astrosystems pads onto the base in the same location of the originals (just over the hard rubber feet on the bottom of the ground board) and tested the motion.  There was still stiction and friction evident as I worked through all three sizes, ending with the largest size pads (2.5” square).  This is larger than the conventional ATM formula would predict based on my scope’s weight, but I found it the largest size pad offered the best motion, although still not optimal in my opinion.

I had also bought a pivot pad made of Teflon from Astrosystems. This is the high end version of the milk jug washers.  I didn’t really notice that a Teflon pivot pad was any better than stacked milk jug washers. There was still a bit of stiction, even after I had optimized the thickness of the pivot pad.  It ended up being the Teflon pivot pad plus 2 milk jug washers, any thicker and the rocker box wobbled.

You should chamfer all new Teflon pads (including the pivot pad) so the edge of the pad is not a sharp 90 degrees. This is a quick and easy task with any file.  Unless beveled off, this sharp 90 degree edge could cut into the surface moving above it. Also, before screwing the pad down you should drill a guide hole and make sure no fragments of the laminate curl up around the mounting hole as you screw the pad down, or the Teflon won’t lay flat against the ground board. A little scraping with a wood chisel may be needed.

After these maneuvers (Ebony Star upgrade, using larger Teflon pads and a custom pivot pad) my azimuth motion was noticeably better. But there was still just a touch of stiction.

I bought some small roller bearings but I found that the cobbled surface of the Ebony Star made rollers useless. The scope chattered roughly atop the rollers and was a step backward in function.  I should have tried the rollers before gluing Ebony Star to the base.

Lastly (and I regret not thinking of it earlier) I moved my azimuth pads from the outer circumference, where they a mounted just over the feet of the ground board, farther inward toward the center bolt. On other forums I’ve heard the best location was 60% of the distance from center to outer edge.  On the undersurface of my rocker box are eight holes for mounting the side boards to the rocker bottom. My kitchen laminate installer had routered out the mounting bolt holes in the Ebony Star and Teflon moving across these perforations might catch on the edges of these holes. But I found a circumference on the Ebony Star that ran just inside these mounting holes which was coincidentally exactly 60% of this center-to-edge distance, and put the pads there.

Figure_4_-_New_pads_[800x600].jpgFigure_5_-_New_Pad_Travel_[800x600].jpg

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Figure 4 - New Pads

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Figure 5 - New Pad Travel

After moving the pads in this way and remounting the rocker box I needed to go back and thin out the washers because the rocker wobbled. I guess that makes sense, since I changed the flexure dynamic of the rocker bottom by moving the Teflon pads inwards.

Success!  Now the azimuth motion is truly premium.  Even moving it near the zenith is a pleasure, and I would rank this as comparable to my Obsession in quality.  I also note very little difference now whether I use the 1.5” or the 2.5” pads.  I’ve left the 2.5” pads on because I’ve used the smaller ones for a different project.

Looking back I wonder if I could have avoided the Ebony Star step and just used larger pads placed more centrally to begin with, but now I’ll never know. The Ebony Star is a popular upgrade and I’m sure it helps greatly. I just can’t report on the impact of a larger Teflon pad and more inward mounting in the absence of Ebony Star because I moved my pads inward after I upgraded to Ebony Star.

2. Altitude friction and stiction when compression springs are used.

A friend cut and attached Ebony Star strips to the trunnions. That is an ideal way to reduce friction and stiction.  I didn’t have the skills to try this. Instead I simply chose to not use the compression springs. The down side, of course, is that the scope’s balance is fussier.  I use Velcro mounted weights which can easily be added or removed to the tube ends to achieve near-perfect balance. After this I can effortlessly hand guide at 300X magnification.  I find myself using just one of two eyepieces during the night, and I am usually working at 45 degrees or higher, so I quickly learned what weight I needed for a given eyepiece.  In fact, because I found there was still a touch of friction, I used a simple ATM technique and moved my altitude Teflon pads closer together, down the circumference of the cradle about 1.5 cm each. This reduced the friction and stiction enough to meet my goals.

3. Primary mirror collimation eccentricities and need for screwdriver for collimation.

This fix was fairly easy. I bought replacement knobs and stiffer primary mirror springs from Bob’s Knobs.  Of note, these knobs and springs are the same specifications as for the Meade Lightbridge scopes.  This was an easy change-out, and you don’t even need to remove the cell. Just do one spring and collimation bolt at a time. After this, collimation was a breeze.  My comment regarding the locking bolts above still holds, however.

4. Secondary mirror collimation (due to floppy or too-short spring, and need for a different type of screwdriver)

I could have changed to a longer or stiffer spring, but instead I found my tool kit contained some washers of the correct size to fix this problem. The washers were just barely larger diameter than the springs and still fit over the bolt and into a recess on the bottom of the spider. I used the washers as spacers, placing them over the bolt at the spider end. By stacking the right number they had the effect of increasing the compression of the spring for a given length of bolt. With more compression, there was more “push back” by the more tightly compressed spring, and as the collimation bolts down on the secondary holder, the spring provided more dynamic back-pressure to prevent the secondary from clunking about bolts.

As for the need to use a screwdriver for secondary collimation, this I corrected by replacing the screws with Bob’s Knobs secondary collimation bolts. Now I can easily turn the bolts with my finger.  Secondary collimation is now easy, smooth and predictable.

Other Modifications

This and most Dobsonian scopes are easily modified to accept encoders; I added encoders using a dedicated kit from Wildcard Innovations and the installation was fast and easy.  I also added flocking to the upper tube to reduce scattered light, and a ring baffle above the mirror to reduce stray light and help direct air flow from my customized fan across the mirror face.  This ‘tinker-ability’ is one of the great features of Newtonians in general and Dobsonians in particular.

Summary

Overall the Bintel BT-302 Dobsonian is a great value and a very good scope.  Connoisseurs might find the stock telescope’s azimuth and altitude motion and collimation mechanics less than optimal, but overall this is consistent with the telescope’s cost.  These minor flaws of function can be tweaked out with relative ease, as described here and on other forums.

Again I will congratulate GSO on making large aperture telescopes available at a low price, and I thank Bintel, Australia, for good service before, during and after the sale.

Review by Scott Tannehill (Tannehill). Discuss this review on the IceInSpace Forum.

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