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Webster D18

Posted by Rob Campbell   02/04/2012 08:00AM

Webster D18
Review of the Webster Telescopes D18

The Webster Telescopes D18 is a relatively compact and light-weight 18" truss-tube Dobsonian. Made in Detroit, MI. The scope contains a 1.5" thick f/4 Zambuto primary and an Antares secondary. As standard, Webster supply their scopes with a 2" Feathertouch focuser, rip-stop Nylon shroud, Telrad, and transportation handles. The following optional items were added by Webster: ArgoNavis, Servocat with wireless hand-pad, powered groundboard, cooling fan, Astrosystems secondary heater. In addition, I added an Astrocrumb filter slide (the traditional version) and a Dewbuster for eyepiece and Telrad heaters. I use the scope with a Paracorr 2, Panoptics, Radians, a 7 mm Nagler, a couple of 82 degree Explore Scientific eyepieces, and even the odd shorter focal length Plossl. All told, the D18 weighs in at about 95 lbs, the same as the Obsession 18" ultra-compact. The telescope owes it's light weight in large part to the relatively thin primary mirror. However, weight savings by Webster also play their part. Eyepiece height is 68.5" at the zenith.

I will go through each feature and sub-system of the telescope, describing any notable positive or negative attributes it may have. I like modifying things and would have been disappointed if there was nothing for me to play with. True to form, I have made some alterations on the D18 and I will discuss what I changed and why I did so. I have done a little ATM in the past and modified every scope I've owned, so the Webster is no exception.


Overall I am very happy with the instrument. The Zambuto primary produces excellent images with superb planetary views. The scope is easy to collimate using Webster's mirror cell and the Astrosystems secondary holder. Viewing through an autocollimator, one sees that alignment is retained throughout the elevation range. Webster set up the drives and electronics so tracking and GoTo work accurately without requiring any tweaking. The telescope is about as small and compact as it could be. I picked up from Webster in July 2011, having placed the order in December 2010. I'm writing in Feb 2012, so how does this elegant and well-designed telescope stack up after half a year of of use?


Setting up the OTA is fast and easy since the truss pole hardware works well. Although the D18 weighs only about 10 lbs more than an Orion XX12, about 80% of the weight resides in the mirror box. Whilst I can physically lift the mirror box myself, it's not something I like to do regularly and I wouldn't want to carry it any distance. Thus, moving the D18 around requires the wheels.

With a reasonably high observing chair I get seated observing throughout the elevation range. My only quibble is that the 9 o'clock position of the focuser means that I'm almost crouching whilst sitting on the ground to observe at elevations under 30 degrees. Whilst I don't do that very often, some objects never rise above 30 degrees and I want to observe them with ease. The 9 o'clock position was chosen to ensure that the eyepiece doesn't get high too quickly as the scope is raised towards the zenith. This is a reasonable design choice and, although I could easily rotate the UTA by 20 or 30 degrees, I currently have no plans to do so.


The rocker is seriously low-profile and well built. A 12 Volt battery connects conveniently to the ground board via 3-pin audio connector located in one of the feet. The Servocat install is very neat and well thought out. The rocker itself is painted black on the inside and coated with Roadex on the outside. The wheel-barrow handles screw into the rocker via sturdy steel eye-bolts. Since the Roadex is black, it's pretty hard to see the holes in the dark. Consequently, the Roadex can get a little detached (un-stuck) around these holes as one fumbles in the dark to screw in the eyebolts for the handles. I have therefore reinforced the area with large washers. The process of screwing in the eyebolts now has a very solid and positive feel to it.

Mirror box and finish

The scope's colouring of unstained wood and anodised black aluminium creates a minimalist feel, which I like. The anodizing is very tough indeed and is a deep black colour. The mirror box is the centrepiece of this telescope. It is beautifully and solidly built with dove-tail joints. The finish on the mirror box is excellent and very smooth. The mirror box cover and some areas of the wooden portions of the UTA are slightly less smooth, which is a pity. The mirror box must have been in situ when the azimuth encoder arm was attached to the rocker, as there is a shallow hole drilled into the mirror box at the location corresponding to the encoder's attachment screw. Nothing I can't fix with a little wood filler, but it would obviously better if I didn't have to do this.

Stability and motions

The scope feels stable and smooth in both azimuth and elevation. There is a good balance between static and dynamic friction. Despite being light for an 18" scope, the D18 handles very well. Star-hopping from object to object is a breeze. All bearings were coated in Turtle Wax by Webster, but I cleaned off the wax as I prefer the motions this way.

The large aluminium elevation bearings contribute to the scope's stability and good balance characteristics. In places the drive-side elevation bearing material does not adhere well to the underlying aluminium. This doesn't appear to have any negative consequences but at some point I ought to renew the contact cement in these regions. The non-drive bearing material is well cemented to the aluminium.

The scope balanced perfectly with all my eyepieces apart from the heavy 2.2 lbs 30 mm Explore Scientific. Oddly, I noticed that under colder conditions the scope would also fail to balance with my 27 mm Panoptic. I wanted balance with all eyepieces and considered adding counterweights. Eric supplied me with instructions for securing the counterweights discretely inside the mirror box, retaining the stock look of the scope. Although I experimented a bit with an elastic ("virtual") counterweight located inside the rocker box, I eventually adopted Eric's counterweight suggestion. I chose this route because adjusting the tension correctly for the elastic counterweight proved hard.

Truss hardware

The trusses attach to the outside of the mirror box, slotting in to low-profile aluminium blocks built by the Moonlite company. Having the blocks on the outside contributes to the overall compact size of the scope. The truss poles have an outer diameter of one inch and were cut to the correct length by Webster. Inserting the poles into the blocks is easy because they form pairs, being clamped and hinged together at their upper ends (you can make this out in the first photo of this review). The hinge incorporates a brass T-bolt which secures each truss pair to an L-bracket located in the UTA. The clamp arrangement is shown on Webster's site. The clamps can be loosened and moved up and down by an inch or two along the pole, which may be useful if you're having in-focus problems (I didn't, by the way). The only down-side to the clamps is that they can slip along poles even when tightened. I plan on lining the junction between the clamp and the pole with thin rubber to increase friction and stop the sliding. For those who are interested, Eric has made a video showing the set-up process of the D-series telescopes.

The 4 pairs of truss poles are labeled, so that you can always put the same pair of poles into the same pair of truss blocks. Not only does this contribute to consistent collimation between sessions, but it's actually necessary to place the poles into the correct blocks: My UTA L-brackets have been machined by hand and are not identical. If the poles are in the wrong locations it can be impossible to mount the UTA without adjusting the clamp positions on the poles. Of course you only notice this if you put the poles into the wrong blocks.

Optics and optical housing

As mentioned above, the OTA fulfills it's two main functions with flying colours: It holds collimation well and houses the optics correctly to provide excellent views. The optics star test very well and provide both stunning planetary and deep-space views. Both planetary and deep sky views have impressed other seasoned observers. No more needs to be said on this score.

The mirror cell is minimalist, providing plenty of ventilation. You only get two collimation bolts, but this is all that's needed and they work well. The mirror is supported by a cable sling and cooled with a by a single fan. I asked Eric Webster whether adding extra fans would speed cool-down time. He said he had experimented with this and found no significant difference when extra fans were added. However, a fan system which would blow air over the mirror's front surface would help to remove the boundary layer during cool-down and ward off dew after cool-down. I may add these in the future, but it is not high on my priority list.

The padded stainless steel contact-points on which the mirror sits can rust following exposure to dewy conditions. Temporarily, I am painting these with oil and at some point will remove them and coat them with clear nail varnish. Similarly, the paint-job on the steel mirror cell would benefit from being tougher, as I can see thinning patches which will eventually need touching up to avoid rust.

The secondary holder does a good job of supporting the secondary mirror. When sufficiently tight, the 4 collimation bolts prevent flexure across different elevations and allow the scope to maintain good collimation. After measurement and calculation, I chose to switch the provided 3.1" secondary for a 3.5". Eric very kindly sent me the larger and more expensive mirror and holder at no charge, even though he didn't feel the switch was necessary. Carl Zambuto tested the secondary before I received it, also at no charge. So excellent customer service from both parties.

Argo and Servocat

Webster set up the electronics so I expected the scope to track and GoTo "out of the box." This was indeed the case, although on the second night the azimuth roller (which physically links the motor shaft to the ground board edge) came loose, causing the motor shaft to which it is attached to slip. In other words, the motor would turn but the roller which links it to the ground board didn't turn with it. This resulted in loss of azimuth motion. It was only a ten minute job to re-attach the roller, but it did involve removing the motor from the bottom of the rocker box. Since then, I have had consistently accurate GoTo and tracking.

I found that actuating the clutch levers became easier and smoother after I applied a few drops of oil into the hinges. I highly recommend this to other ServoCat users.
I opted for the wireless ServoCat hand-pad. Although this accessory costs $200, it simplifies set up since no wire has to run up to the UTA. I use self-adhesive magnetic tape to secure the controller next to the focuser for easy removal. Occasionally the controller fails to connect with the main unit, but this is resolved by cycling the power.
The Argo is situated conveniently on an aluminium control stalk. This stalk is a mixed blessing, however. Firstly, it adds bulk and needs to be removed to get the scope out of my kitchen door or into the car. Webster have made this simple by mounting the stalk using the same clamps employed for the trusses on their larger scopes. Nevertheless, this un-clamping adds to set up time. Secondly, the stalk is located alongside the elevation clutch lever for the ServoCat. This makes engaging and disengaging the Servocat slightly harder than it needs to be. Not a problem if one only does this a couple of times a night, but it may be an issue for those who anticipate doing this often. Since I enjoy star-hopping, I moved the Argo to the back of the mirror box, securing it with self-adhesive Velcro. This position is slightly hard to get to, but I don't use the GoTo much so I don't find it problematic.

The only thing that concerns me with the tracking is that the azimuth roller may be mounted a little too far out. I say this because in one or two areas the groundboard edge is scuffed (my doing) and the roller loses traction in those patches. The scuffing appears minor and I wouldn't expect loss of traction so easily. If the roller were mounted 1/16 of an inch further in, this slippage wouldn't happen. Although the tension on the roller can be altered, I can see no way of adjusting its position when engaged. Re-painting the scuffed patches of the ground-board serves as a fix for the problem.

Electrical connections

Power is connected to the ground-board, which is very convenient. A 12V distribution box is located at the inside of the rocker box. The primary cooling fan can be controlled here. A thick phono cable runs 12V power up a truss tube and to the UTA, where it is used to power the secondary heater. The secondary heater automatically switches on when mirror temperature is within 2 degrees of ambient. Heater activity is indicated by a small red LED located on the far side of the focuser. I have a Dewbuster which I mounted on the UTA and use to power an eyepiece heater and Telrad heater. On my first night with the scope I experienced a short in the fan cable which required me to re-wire it. Not a big deal, but it could have been prevented if the wires had been independently insulated with heat-shrink tubing. After a few months use I got fed up of the UTA phono cable and set up the scope so that 12 V power is delivered via two of the truss tubes. I very much enjoy the simplicity of this modification. It's a quick and cheap alteration to perform so I recommend it to others. You can find a description of the process at this link: Click here for more about this subject. -Ed.

Light shielding and baffling

The scope has a rather rough black painted finish inside the tube. This helps to decrease internal reflections and potentially boost contrast. However, the back of the mirror box is open and quite a lot of light can get in that way. I therefore baffled this region using rigid FlockBoard from Protostar. The mirror is now presented against a nice black background.

The shroud itself works well at excluding light but, in common with many shrouds, will need a second layer if you're observing in an area with significant direct lighting or with the moon up.

The downside with ripstop Nylon is that it leaks in very dewy conditions. Three or four hours into dewy night sees large droplets forming on the inside of the shroud. These the drip onto the primary, which is no fun at all. I considered a few solutions and eventually went with a Lycra shroud from Heather. This gets wet but it doesn't drip. Also, it fits the OTA more closely and looks better. A slight disadvantage of the Lycra shroud is that it has to be in place before the UTA is connected to the truss tubes. This issue can be dealt with by storing the shroud bunched up around the UTA; still, it can be awkward, so there are times when it's easier to use the Webster shroud.


Observing with the D18 is easy and a lot of fun. Optically, things are excellent. I generally star-hop to targets and flick in the clutches when I want tracking. I can do this all night and it never feels like a chore. I love the fact that I can sit throughout the elevation range and don't need a ladder.

Both Eric Webster and Carl Zambuto have been easy to reach on the phone or by e-mail and it's always pleasant to talk to them. Customer service by both Webster and Zambuto has been very good. I should also point out that I have had similar excellent experiences with both Astrocrumb (the filter slide) and DewBuster (the dew heater controller).
At the prices charged by premium telescope manufactures, a customer's expectations are sky high. Does the scope meet those expectations? After over 6 months use, I feel the answer is "yes." This really is a telescope I want to keep and certainly I have no plans to ever sell it. It feels like a lifetime purchase, which is what such an instrument ought to be. The views are stunning, the mechanics work as they should, it's easy to use. There were some items I had (or chose) to modify, but this is part and parcel of owning a telescope. That said, some of the things needing attention early on (e.g. fan wiring) should have been caught by Webster during testing.

Overall, I'm happy with the D18 and would have made the same choices if I were given the chance to repeat the buying process. I would recommend this scope to others looking for a "lightweight" large Dobsonian.

Rob Campbell - February 2012, NY