Why it’s cool to cool. The mirror of a large Newtonian reflector is a large block of glass, a material which dissipates heat slowly but also has a significant thermal capacity. While the mirror cools down, due to variations in density, air currents are set up which can spoil the image. The larger the mirror, the longer a mirror takes to cool and as temperature may be falling continuously for many hours while observing, it may never reach a stable temperature…without a fan! Mirror mass increases with the cube of its diameter, so a 10” mirror is twice the mass of an 8” (1000/512) and 10” seems to be the size when fans are sometimes included in the standard telescope design.
Research the internet. (https://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-2/) shows only a small 12volt fan is needed, preferably with a high speed for initial cooldown followed by slow speed while the temperature falls during observing. This Youtube video (https://www.youtube.com/watch?v=SsvMs4HGRnk) was also useful, even though it’s quite slow and a bit long. Do not mount the fan directly to the mirror cell even though my telescope had tapped holes for this. Mount it on a baffle plate to stop air re-circulating from discharge to suction and it also reduces vibration. Although I used the resources above I added the following: an alternative 5.2v USB supply (for ultra low speed), an illuminated on/off switch to avoid leaving fan switched on and flattening the battery and I used the 3 mirror cell locking screws to secure the baffle instead of Velcro tape.
fan: ex computer 12v 102mm with integrated 3 speed control – RS Potts, Babington Lane Derby – £4,
baffle plate – 3mm black Perspex/acrylic machined to outside diameter to match recess in mirror end and with hole to match fan duct size – sheetplastics.co.uk – £22. Alternatively, use old 12” vinyl record, more cutting but much cheaper.
12v plugs and sockets: Discount Store Swadlincote High St. – £1.20 each
12v/5.2v USB converter – ebay – ? already had one
small illuminated switch – ebay – £2.40
small canister for switch – Discount Store Swadlincote High St. – 80p
12v re-chargeable battery – RAG member Bob Williams – contribution to Observatory Fund
various M4 and M5 screws, washes, nuts, low power cable, black adhesive tape – Discount Store, Swadlincote.
5. Procedure I’ll let the photos speak for themselves. Be cautious cutting acrylic because it splinters easily. Always support on rear side of cutting tool and use fast speed and minimum force. Peel off clear protective film only after all shaping is complete. To get the approximate positions in the baffle plate for the 3 locking and 3 collimation screws, I made a cardboard template to transfer the hole positions to the acrylic baffle plate, first attaching masking tape to the surface. Drill holes in the acrylic with a sharp 5mm drill and a 20mm hole borer for the locking and collimation screws, respectively. Route wiring so it does not shorten as telescope is moved and so switch and speed controls are handy. Fix to tube with black fabric adhesive tape. A bit crude, but could not think of a better way.
6. How does it perform? On low speed range (5.2v) its silent so unlikely to be any detectable vibration. Will only use high speed for initial cooldown, then swith to low speed while observing. Since fitting the skies have not been clear so will report back as soon as it’s been tested.
cardboard templatefan with 12v and 5.2v adaptor
trial fit on telescope
transfer positions from template to acrylic baffle
drill holes for locking and collimtion screws
drill holes for M4 fan screws
fit fan to a shiny baffle plate with nuts on the outside
trial fit and trial run
route cable and add the illuminated switch mounted in small plastic canister next to speed control, fix with black adhesive tape.
finished arrangement, red light on switch is brighter than it appears.
The night sky was predicted to be clear so it was time to find out what all the fuss was about with the new comet NEOWISE. I had researched its predicted position below the Plough and to the west of Capella but despite searching with 15×70 bins could not find it. I now realize it has dimmed considerably since it was last visible in early July. Frustrated, I called on back-up, the WhatsApp group of astrophotographers who quickly pointed me in the right direction (many thanks to all who responded), vertically below Dubhe, brightest star in the Plough. Through the bins it was unmistakeable, a bluish blob with a faint whitish tail. It was still quite light to the west but showed better as the sky darkened. It was then easy to find in the Dob-mounted Sky-Watcher 250 PDS with a 32mm eyepiece. To record the occasion of my first comet viewing I used a Canon 60D with ISO 1600 and 3.2 seconds, longer exposures gave star trails.
I tried to allow plenty of space in the frame for the tail but it still extends out of view. Not perfect, but I am pleased with my first comet photos.
Paul has come up with this ingenious solution to the problem of a cricked neck when looking through the polarscope on these popular Sky Watcher mounts.
The photos below show how the 90 degree viewer and fitting are assembled, noting the nut used to clamp it together.
Paul has very kindly made one of these for me using a second hand Nikon DR3 right angle camera attachment off ebay (approx. £6). He has made the hole in the plastic fitting a tight fit on the short thread of the DR3 such that no nut is needed. He has learnt how to carefully enlage the bore of the plastic fitting so it is a snug fit in the polarscope eyepiece…a bit tedious because it’s trial and error and must be done very carefully to ensure that the bore is not accidently made too large. Patience needed and lots of cups ot tea! The finished product is a tight fit and needs screwing onto the thread of the viewer. It should be a snug push fit onto the polarscope eyepiece. Paul used a 32mm Waste Compression End Plug costing £1.50 from “Discount DIY Store”, Swadlincote High Street, Swadlincote, Derbyshire, UK.
In use: Fit collar to viewer first, then push it onto polarscope eyepiece as far as possible – see photo. Try different angular positions if its too tight to go on. The collar should go over the eyepiece and over the polarscope tube as well.
To focus polarscope, Paul has found that he needs to unscrew eyepiece about 5mm so he fitted an 0-ring onto the thread for eyepiece to clamp against and keep eyepiece tight in the focused position.
Prime focus with Canon 60D attached to SW200p last night when Saturn was at culmination (15 degrees) but still low enough to give a blurry image. My first photo of this planet. Jupiter was even lower but with iso800 the 4 Galilaen moons show up well but planet massively overexposed. The nearby full Moon also reduced the contrast. Attempted eyepiece projection but gave up trying to get the fast moving image in the camera lcd. Have an HEQ5 Pro for collection from RVO on Monday which should help track images and eventually get photos of some faint DSO’s. Thanks to Andy T for the demo of his HEQ5.
Having owned this mount and scope for nearly a year, I am thinking of adding a GoTo/tracking function by purchasing the SynScan Pro V3 Upgrade kit but would first like to seek the views of any members who have already added this feature or been down this route. At around £300 it should be an excellent accessory and all the utube videos and internet reviews support this. Alternatively, is it possible to add just motor drives (at around £100) and control these with planetarium software, like Stellarium or Skysafari?
Any views, in favour or against are very welcome. Thanks.
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.
Hang from washing line for painting – 2 coats minimum. Have a coffee between coats!
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…?
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.
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.
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.