I bought my 8″ Dob in June 2003 from Teleskop Service, a German online dealer. It’s a good value for money telescope, but as you might expect, it can be improved. This page describes some of the improvements I’ve made.
After a year or so of having to “nudge” the scope to track objects at high magnifications I decided I wasn’t going to get any better at it, so I bought tube rings and a dovetail bar to mount the scope on my recently acquired EQ-6 equatorial mount. I was able to do this without removing the Dob altitude bearings, so could use the scope in either EQ or alt/az mode. However, the EQ-6 mount’s tripod is a little too high, and reaching the eyepiece was not always comfortable.
Improving the mount
I don’t think it’s unfair to describe Sky-Watcher telescopes as “made down to a price”. Fortunately the optics are of good quality and most of the compromises appear to have been made elsewhere. This offers plenty of scope for tinkerers like me to make some worthwhile improvements.
It didn’t take long for me to discover that the Sky-Watcher’s azimuth movement is not as smooth and precise as I’d like. One reason for this is easy to see, and easy to cure.
The diagram shows a cross section through the azimuth pivot before modification. (Actually, it should be the other way up, according to the telescope’s assembly instructions, but I don’t think it matters.) The ground board and rocker box bottom board (brown) are joined by an M10-1.5 bolt (light blue), fitted with washers (dark blue) and a “Ny-Lock” nut (yellow). A PTFE spacer (green) keeps the boards apart and a plastic sleeve (red) covers the bolt threads to allow smooth movement.
In the classic Dobsonian design (see the San Francisco Sidewalk Astronomers’ plans) the pivot bolt is a wood screw, screwed into the ground board (which is a double thickness of plywood). The rocker box bottom (also double thickness plywood) then rotates about the fixed bolt. If the hole in the rocker box bottom is just the right diameter, there is no free play.
In the Sky-Watcher design the pivot bolt is not fixed to either board. Furthermore the boards are made of single thickness chipboard. The result of this is that there is too much lateral play, i.e. the rocker box can move sideways relative to the ground board. This might not seem to be important, but I have fitted setting circles and there is no chance of setting azimuth to an accuracy of 1° if there is 3 mm of lateral play in the pivot.
The next diagram shows my modifications to the azimuth pivot. The new components are a nylon block screwed to the rocker box bottom (white), an aluminium block screwed to the ground board (gold) and a locknut (purple). The aluminium block is about 40mm square and about 6mm thick. I made an M10-1.5 tapped hole in the centre and holes for fixing screws (No. 8 woodscrews) near the corners. It is important to try and get the tapped hole nice and “square” to the block surface. The nylon block is about the same size and also has holes for fixing screws near the corners. However its centre hole is 10mm diameter and is not tapped. It should be a good fit on the unthreaded part of the bolt.
The pivot should be assembled as shown in the diagram (but without the locknut) before fixing either block to its board. As you tighten the bolt move the rocker box around and try to keep it centred on its range of lateral movement. Tighten the bolt firmly to clamp the nylon and aluminium blocks in place, then fix them to their boards with wood screws. (Make sure that the screws you use are not long enough to go all the way through the chipboard!) Finally, slacken off the bolt until the top washer is just loose and then fit the locknut to stop the bolt loosening or tightening as you turn the scope.
Although I made the above modification to reduce lateral play, I was surprised and delighted to discover that it has reduced “stiction” in the azimuth pivot. A further improvement was made by putting two shims cut from overhead projector film between the ground board and the PTFE spacer to reduce the load on the main azimuth bearing pads. Now it is easier to make small “nudging” movements of the azimuth.
I know there is some controversy over putting setting circles on a Dobsonian, as one is supposed to find objects by “star hopping”. However, I don’t live at a dark sky sight, and in summer it doesn’t get very dark anyway, so why shouldn’t I have the option? Using them is not compulsory.
Azimuth setting circle
I made a setting circle from clear acrylic sheet plastic, about 2mm thick. I used a remnant from when I fitted secondary double glazing several years ago. I cut it to the shape shown in the figure using a router with a “beam trammel” fitting, having drilled a small hole in which to locate the trammel’s centre point. The majority of the circle is 218mm radius, to fit within the azimuth bearing pads, and a small sector is 260mm radius, to allow the circle to be adjusted when in use. A central hole of 30mm or more diameter is required to make room for the azimuth bearing’s PTFE spacer.
I made a scale for the setting circle with a computer drawing program (called xfig) and laser printed it onto overhead projector film. I had to do this in four A4 sheets, so I included alignment marks to aid joining them together. The scale is 190mm radius, with divisions every degree and numbers every 5°. (Download the azimuth scale pdf file.) I fixed the scale to to the acrylic circle with some adhesive sheet used to mount photographs.
The setting circle rests on the telescope ground board, and is held in place by three 1.25 inch steel washers. I drilled an off-centre hole in each of these, countersunk to ensure the fixing screws don’t interfere with the azimuth movement. I fixed each washer near an azimuth bearing pad, then carefully adjusted their positions so that the setting circle could turn freely and the scale was concentric with the pivot.
The setting circle is read through a 35mm diameter hole drilled in the rocker box bottom, on the observer’s side near where the side panel joins the base. I laser-printed a single straight line onto OHP film and cut it to size before fixing it to the underside of the rocker box with double sided sticky tape.
Altitude setting circle
My altitude setting circle is also made from acrylic sheet. I cut a circle about 185mm in diameter, with a 25mm hole at the centre, then cut off the lower third. This shape fits on the rocker box side board (observer’s side) and has clearance for the tension handle. It is held in place with three wood screws, each of which is fitted with a large washer to spread the load. I made extra large clearance holes in the acrylic sheet to allow the circle’s position to be adjusted. I made a scale in the same way as for the azimuth circle, but only a quadrant (0-90°) is needed, and it all fits on one A4 sheet. (Download the altitude scale pdf file.)
Before attaching the scale I made and fitted the pointer. This is folded from sheet aluminium about 3mm thick. It has a complex shape that is hard to describe. It is fixed to the telescope’s side bearing (on the larger part near where it meets the tube) with two self tapping screws. It then bends up and around to clear the rocker box side board. It ends in a pointer made of the same acrylic sheet. I scratched a straight line on the underside of the acrylic and used a black marker pen to highlight it.
Adding a fine focuser
Having bought a Borg #7315 helical focuser to use with my 4″ Mak I discovered that it can also be used on the Dob. The Dob’s 1¼” eyepiece holder can be unscrewed to reveal a male T-thread for camera mounting. Inside this T-thread is a female 36.4mm thread which accepts the Borg focuser. Unfortunately, the Borg focuser is about 15mm longer than the Sky-Watcher eyepiece holder, so there is a danger of not being able to reach focus with some eyepieces. My worst case example is the 14mm ScopeTronix eyepiece (with built in Nikon Coolpix camera adapter), for which I had to rack the main focuser all the way in. However, the fine focus adjustment and smooth motion of the Borg focuser make the scope a lot easier to use, particularly at high magnifications.
I’ve done a few simple things that should improve the contrast of the telescope, particularly in the presence of off axis light sources such as street lights. I didn’t want to dismantle the scope, as it still had the manufacturer’s collimation which I assumed was probably better than I could do. The following measures were all done with the mirrors in place, but I did clamp the tube in the horizontal position to avoid any accidental damage to the primary mirror by dropping something on it.
Looking down the telescope tube from the front it appeared that the paint is not very light absorbing at shallow angles – there is some “shine” at the mirror end. I reduced this with a couple of sheets of “Funky Foam Fun” which you can buy in many colours from artists materials shops. I bought the black (obviously) foam in 18″ by 12″ sheets. I cut the sheets to size then joined them end to end with sticky tape (on the back – the front face needs to be left clean) to form a cylinder that just fits inside the telescope tube. I then squeezed this cylinder past the secondary mirror spider and right to the far end of the tube. It has enough spring to stay in place next to the primary mirror.
Looking into the focuser (without an eyepiece) shows similar low angle shininess on the focuser draw tube. I used another cylinder of Funky Foam to suppress this. The final improvement is to blacken the plain glass side of the secondary mirror, where it can be seen through the focuser. I did this, very carefully, with a black marker pen.
Last, but not least, I’ve collimated the telescope. I should have done this sooner, but didn’t pluck up the courage until six months after buying it. I used a collimating eyepiece from ScopeStuff and Nils Olof Carlin’s FAQ about collimation. I am amazed at just how much improvement collimation has made, and it’s really not that hard to do.