Paul McKay

DIY Dob Mount for SW 200P

After hearing about the rapid set up and simplicity of a Dobsonian mount for casual observing, I thought I would investigate how to get one as an alternative to my EQ5. Strange that these mounts cannot be bought separately, except at Orion Optics UK, where I was quoted a high price. This set me on the DIY route.

I decided that I wanted the capability to adjust the tube axially (for balancing) and rotationally (for comfotable viewing position), as with the EQ5.I also wanted easy transfer of the tube between Dob and EQ5 (no tools needed). After a week of research, I settled for a hybrid that included a features from this article: http://www.scopemaking.net/dobson/dobson.htm, The Sky at Night articles in Dec 2014 and Jan 2015 and the Orion Optics design. Originally, I was going to design the rings and dovetail bar to be interchangeable, but when a set became available I settled on a separate ring set for each mount.

I won’t go into detail about the build/assembly but show various stages in pictures. The main stages are; 1.mods to the ring set, 2.cutting, shaping and painting, 3.bearings and the 4.optional brake. Anyone who wants more detail please contact me.

1. Modify ring/rails assembly.

Trunions: PVC 160mm pipe plugs (Buildbase, Newhall, Swadlincote). Protect bearing surface with masking tape. Locate centre and fix to bar with 1/4″UNC fasteners (Pugh & Sanders Ltd Burton on Trent).

Shape and fit 2nd ‘rail’ from 10mm plywood. Fix to rings with 1/4″ csk head screws. Locate trunion on centreline in same position exactly as other trunion.

 Trial fit completed ring/rail assembly to scope

2.Cutting, shaping and painting frame

I used 18mm mdf for the base and sides and 10mm plywood for the front, back, rail and accessory tray. Use plastic fixing blocks and screws to hold everything together. No adhesive needed. Take basic dimensions from the article referenced above, except width of front and back, noting that alt bearing box is not needed and friction brake needs to be included.

Mark out parts using trammel to draw circular base.

Cut with jigsaw and smooth with rasp and glasspaper

To obtain width of front and back, measure distance between trunion flanges and add 10mm.

Use plastic fixing blocks to assemble, drill through upper base and screw to frame, bolt to lower base, trial-fit scope assembly. If all goes pear shaped, use as a ‘lazy-susan’ coffee table!

Trial fit 3 feet 120° apart.

Use jigsaw and ripsaw to cut holes to reduce weight and improve appearance.

Smooth all edges with rasp and coarse glasspaper, particularly the upper curve of the front cut-out to be used for carrying

Hang from washing line for painting – 2 coats minimum. Have a coffee between coats!

3. Bearings

For altitude bearing use two 2mm thick ptfe sheets, drilled and countersunk in centre for small csk head screw.

 

 

 

 

 

For azimuth bearing use 3 Magic Glides (Wickes) spaced 120° apart within 300mm circle .

Use M10x60mm carriage or ordinary bolt and M10 Tee Nut (Amazon or ebay) inserted upside down for pivot in lower base. Tighten so it will not fall out or turn when M10 Nyloc nut is tightened.

 

For upper bearing use 12″ vinyl record (grooves make for low friction). To form a good bearing for the bolt in the upper base use a brass10-15mm reducer plumbing fitting (Wickes) drilled out to 10mm. Secure bases with oversize washer, spring washer and M10 Nyloc nut. Tighten only enough to take up slack.

Small spacers are needed to prevent sideways movement of scope assembly. Spacers are squares of ptfe fastened with small screw and spring washer fitted between side and flange of trunion. Trial fit to to gauge the spacer size and position of spacers.

 

4, Friction Brake Feature – Optional

This feature prevents the scope moving if the assembly becomes out of balance, although there is the option to slide the tube axially.

Attach another strip of ptfe to top of curved section of brake. Attach small hinges between brake and side using small 90° brackets to allow screwing into face of wood – mdf will split if screwed into edge! Attach a ‘Brighton sash window catch’ (satin chrome finish from Screwfix) such that it can be released to allow the scope to be lowered into place and tightened to stop movement or lock the scope.   Fit accessory tray to front and hooks to sides for clipboard, glasses etc. Extension legs can be used if elevation is low or if the ground is long wet grass. To make carrying more comfortable, fit a length of 12mm soft clear plastic hose cut lengthways to upper curve of the front.

I had great fun making this but have used it only briefly to observe the Moon and was pleased the way it moved…but I still like the fine control provided by my EQ5 control cables. Now how can I add this feature to the Dob…?

Fixing Extra Shoe to SkyWatcher 200P, with screws!

Hi All

After some discussion with Andy T on the benefits of a laser pointer for finding objects, I decided to get one of these. The laser and the bracket are yet to arrive but the extra shoe needed to mount it to the tube, ordered from Harrison Telescopes, arrived in 3 days. This is now fitted with the M4 countersunk screws and black nuts supplied. I will outline the method I used and tools needed, for comparison with Andy’s adhesive-based method (10 March) so you can decide which to use.

1. Make sure you think hard before you decide on the location; drilled holes are permanent. I placed mine about 20mm from the finder-scope shoe, to match the gap between it and the focuser base.

Tools needed

2. Attach masking tape to the area where it is to be attached.

3. Rest the tube horizontal up against a firm support with the focuser aperture above the area to be drilled, to prevent swarf/cuttings getting in. Also, I put newspaper directly under the drilling area to catch any cuttings and masking tape along the inner edge of the stiffener on the end of the tube. Time taken in preparation is well worth it. See the photo below. I would not advise doing this task with the tube in the mount.

4. Mark lines on the masking tape and use the shoe as a template to mark the locations of the two holes.

5. Check that the holes will clear the reinforcing plate (if fitted) inside the tube used for the finder-scope shoe.

6. Tubes are made of thin steel, work-hardened by the rolling process, so sharp drills are essential. Start with a small size drill, say 2.5mm and work up, in steps of 0.5mm, to 4mm diameter. This minimises the force needed to break through initially and subsequently to increase the hole size. Small drills break easily so do not apply too much force, have only a short length of drill protruding from the chuck and try to align the drill radially to the tube.To make sure the drill chuck could not touch and mark the tube, I pushed a rubber tap washer onto the drill, masking tape alone is not thick enough. Make sure you are in a comfortable position and able to control the pressure applied by the drill.

7. After drilling one hole, loosely attach the shoe and check the marked location of the 2nd hole.

8. If the 2nd screw will not insert, increase the hole size of one hole, or both if needed, to 4.5mm. Mine were fine with 4mm diameter.

9. Remove masking tape and the paper inside the tube and attach the shoe. I used a small spanner (shown in photo) to hold the nuts while tightening. Take care not to shear the screws as they have a small cross-section and not to scratch the black paint inside the tube.

Hold nut with small spanner
Job done

Once the bracket and laser arrive I will post a photo of the finished assembly, soon I hope!

Have now fitted bracket and laser but not used it yet. It was quite a loose fit into the shoe dovetail (left in view below) so needed a small aluminium shim. The SW finder bracket is not a brilliant fit either but at least it tightens up before the clamp screw runs out of thread. I think it looks the part at least (see below).

Total outlay for all 3 parts was about £25, all bought separately off ebay.

Paul

Astrophotos without tracking.

I have been asked to share some of my astro photos taken over the last 18 months, some with just my tripod mounted Canon 60D with a 200mm telephoto lens and some with this camera at primary focus on my EQ5 mounted SW 200P.

This was taken a few days after the clouded-out lunar eclipse of 21 January. I was a bit disappointed about the eclipse but I got a good shot in the end! Its a general wide angle view of the garden stacked (using simple star-trail stacking software) over 7 zoomed-in shots, taken 2.5 minutes apart, of a waning setting moon.

This is about 3 hours worth (!) of 30s exposures giving an apparent rotation of 45 degrees. This was one of my GCSE projects to evaluate the length of Earth’s sidereal day by extrapolating the measured angle of rotation and exposure time. I am still amazed at how well these shots reveal the star’s colour. I used PhotoImpression to get a negative of the image, printed it at A3 and drew construction lines from which to measure the angle of rotation. I did this with 3 separate dates to get an accurate figure. A lot of work!

 

The photo below is a 30s exposure of an aircraft flying left to right in front of Orion while banking away to reveal its wing lights. Taken just after a star trail session, so timing was pure luck but I have since noticed these planes always follow the same path from East Midland Airport.

Talking of Orion, this is its nebula taken through the SW 200P with a single shot of 1s exposure. The telescope’s fast F5 rating registers the faint nebulosity with quite short exposure times so the short star trails do not mess up the detail too much. Just needed some slight colour enhancement from MS Picture Manager. I must try this again with some manual tracking until I get motorised drives for the mount.

If you only have one telescope, the Skywatcher 200P, with adaptors for attaching a DSLR, has got to be high on the list.

DIY Polar Scope Illuminator for EQ5 Mount

I recently bought a 2nd hand Sky-Watcher Explorer 200P on an EQ-5 mount off AstroBuySell UK.com and was pleased to discover that it came with a polar scope. For the unitiated, these are used for aligning the polar axis (that’s the extra one that alt-az mounts don’t have) to the north celestial pole so that objects can be tracked by moving the scope in the Right Ascension (RA) plane only. Alignment requires sighting Polaris through the polar scope so that it aligns with a specially engraved reticule inside the polar scope eyepiece. The problem is that when its dark, the reticule markings cannot be seen so have to artificially illuminated (but not too much otherwise it swamps the stars!)…by a polar scope illuminator. Of course, you can buy one at £23 but reviews of them were very mixed so I researched how to make one, after all it’s only a fancy dim torch how difficult can it be?

The description below is really a prototype (with help from utube) as I tried various options while making it. The bought ones fit on the eyepiece but mine fits on the inside the hole in the mount at the ‘objective’ end.

Parts Needed – plastic

From Screwfix, Wickes or Discount Store, Swadlincote

  • 32mm plastic equal tee with compression joints –– about £3.
  • 32mm socket plug – £1.20
  • 40mm socket plug – £1.20
  • 32mm PVC pipe – £2.40 for 3m – we need about 250mm! Try to find an off-cut.

Parts Needed – electrical

From RS Potts Babbington Lane, Derby

  • Small red LED
  • Small rocker switch
  • AA double battery holder
  • 1W rated resistor
  • Small connectors (3)
  • Low voltage cable
  • Insulating tape, or earth wire insulation
  • M4 screw, nut and washer
  • 2 x AA batteries

I had some of these already but I bought the LED, resistor and battery holder for a total of 94p.

Method – refer to photos

Cut the pipe into 2 pieces: 100mm for battery/switch compartment and 50mm for inserting into mount. The longer piece and the branch stub of the tee need to be cut to fit the rocker switch, making sure it faces downwards for easy access when looking through the polar scope. Cut away the flange of the 40mm plug to form a neat end for the battery compartment.

For my mount, I needed to reduce the diameter with a rasp/coarse emery for it to fit snugly inside the hole in the mount. This was a pain by hand but would take only minutes in a lathe.

I made a support (12mm x 150mm but length depends on your mount) for the wires to the LED from a 150mm length of pipe and bolted it to the bottom stub of the tee with M4 screw/nut. Tape wires to the support to keep them out of the field of view.

The 32mm plug is just a cap for the top plug when not in use. My photo shows the branch of the tee curving upwards but it’s better to arrange it curving downwards (remember mine is a prototype!).

Wire up the battery compartment, switch, resistor and LED (polarity is important for the LED). Carefully measure the lengths of wire needed to avoid excess. Soldering is better but I used small plastic connectors. Use tape or insulation to cover any bare wire connections.  Fit the batteries and test. If all ok, carefully thread the wired assembly into the tee piece and your ready to try it out. The support and position of the LED may need to adjusted/bent to avoid it shining directly into the polar scope.

You have a polar scope illuminator for about £10. At the next opportunity, I will attempt to take a photo of the view through the polar scope when illuminated and add to this post. Feel free to ask questions. To finish I would like to hear details from anyone who uses a 90 degree viewfinder on their polar scope, its a long way down to the eyepiece without one.

Illuminator Fitted to EQ5 Mount
Finished Assembly
Exploded View

Wake up to Venus and the Moon

My first shot of Venus at 7.40am on Friday 30th November through my Skywatcher 200p with a Canon EOS60D. I assume the blurry image is due to it being quite low and that the planet is always cloud covered.

Just to check it was not any fault of the optics I took a few shots of the Moon high in the sky at 3rd quarter, always a fascinating photographic subject. I particularly like the way the Sun is catching the peaks of the Apennine range.

Morning astronomy sessions seem to suite me more than late evening ones but there is always the race against time before the Sun rises. I need to set everything up the night before!

Many thanks to Lee for checking and collimating my scope and explaining a few of the basics to me on a busy evening last Friday.

Moon through SW 200P

Hi All Having just acquired my first telescope I was keen to try it on the easiest of objects, the Moon. To a relative novice, the detail visible was truly awesome. I am not set up with T ring and adaptors yet to connect the Canon DSLR but could not resist putting my compact Panasonic TZ4 to the eyepiece, hence to low photo image quality. My only criticism of the EQ5 mount is finding and reaching the RA control knob around the bulk of the 200mm tube. Control cables are now on the Christmas list.

Big thank you to our chairman, Andy for his advice and guidance on checking out a secondhand telescope.