DIY Dob Mount for SW 200P

Posted by Paul McKay

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…?

“Martian” Potatoes are shooting up!

Posted by Andrew Thornett

The potatoes I am growing a la “The Martian” book/film style using Martian Regolith stimulant (sort of) given to me by RAG last year on my 50th birthday are now shooting up. As per the book I had to find something to mimic the Martian soil (earth from under the foundation’s of the new Peter Bolas Observatory at Rosliston Forestry Centre was given to me on my birthday for this purpose – the Regolith simulant) and an alternative to human faeces for organic matter which is probably not legal and certainly rather disgusting if used in my back garden….manure from the local garden centre was pulled into service here.

Andy

Catching up on Images

Posted by Rob Leonard

It’s been something of a hectic week, so it was nice to have a chance this weekend to download some pictures from last week, when Andy and James were round, and do a bit of processing.

Firstly- the best of the moon photos. These were mostly captured with my Meade ETX105 Mak, which I’ve mounted on my HEQ5 mount, because the original mount is not reliable. The pictures were taken with an ASI224 camera through a 2x Barlow- which gives a focal length of 2.8m and a hard time focusing! Each image is a 30second avi that I  stacked using Autostakkert, and then the multiscale process in Pixinsight for the sharpening. This seems to work in the same way as wavelets in Registax.

This is one of Sam’s of the Mare on the Eastern Side:

This is one that James took centred on the Cuvier crater:

This is one of mine – I loved this view, because you can just see the summit of the central mountain in what I think is the Walther Crater, upper left. This view was stunning in the eyepiece. If I’ve identified it correctly then this little spec is 4.1k high- i.e. it’s a similar size to Mont Blanc sat on a crater floor that’s about the same size as the entire English Midlands.

Finally, on the moon, from a bit later in the week is Montes Jura, which is the prominent range towards the top of the picture. This feature is 3800m high and 422km long- it’d be a pretty substantial mountain range here on earth.

Next up were some globs, which are a great target when the sky’s not properly dark. I took these with the 130pd-s, and again used the ASI224, but taking 20 second .pngs rather than .avi’s. The mount was unguided (the ASI224 is usually my guide camera) and even over 20 seconds there were some wobbles so when I stacked it I set DSS to only take the best 50%. Each imaging run was 30 minutes, so these are effectively a 15 minute exposure and all use darks and bias frames. I took flats as well, but between M92 and M3 a big lump of something landed somewhere on the image train which meant they could only be used on the last 2 images.

First up was M13:

Next was M92. I’d normally crop and re-size this to present it better, but I wanted to make it comparable to the M13 pic above, so all the settings are exactly the same, although my processing has yielded a slightly different background colour. You can clearly see M92 is a bit smaller, but I think it’s also neater and more compact.

Next up was M3- again with the same parameters:

Finally, I had meant to go over to M5 and get another glob, but went for M64, the Black Eye galaxy. I took this with exactly the same parameters as the globs above (mainly because I was having fun doing visual and didn’t want to faf around with settings). I’m quite pleased with this; at the Practical Astronomy Show earlier in the year Dr Robin Glover (author of Sharpcap) gave a talk about how, for CMOS cameras, long exposures are not necessarily needed, or, indeed, optimal. This has enormous potential, because, by stacking loads of shorter sub exposures instead of a small number of long ones, it greatly reduces the precision (and therefore cost) needed for an AP setup. Well, that’s all well in theory, but I’ve had a go at M81 and NGC2903 using this method and the results were pretty pants! In this case, the exposures, at 20s were a bit longer, and the gain a bit lower and I’m really pleased with the outcome. I mean you can see what it is, which is more than can be said for the other 2 I’ve had a go with.

 

Next stage of putting new shingles on roof of our log cabin/observatory in Lichfield

Posted by Andrew Thornett

Rhys and I addressed the back of the log cabin today. In a previous post I described how the roofing felt on the observatory was blown off in recent high winds and how I started the process of re-roofing the cabin with shingles. Ed Mann has meant me a ladder which is an absolute God-send for this work as the cabin is 2.4m high and I am a bit short! My own step ladder is not up to the task.

My son Rhys, also member of RAG, is somewhat taller than me and this really helped today as we successfully shingles all but one row and the apex of the back of the observatory together.

NB for anyone wondering we were not allowed to have an observatory or dome in our garden (household rules!) And in any case there are a lot of trees at bottom of garden which obscure views of sky and this was where the observatory had to go….so instead the log cabin has double doors and a ramp to allow the 16 inch Dobsonian on castors to roll out on to the lawn.

This follows from previous post:

New shingles on front of our observatory in Lichfield 11/5/2019

Andy

Schroters Valley and the Gruithuisen Domes.

Posted by Roger Samworth

Looking at the Moon from the “window-sill observatory” last night, the lunar phase looked suitable to observe Mons Rumker. Unfortunately the exact phase and unfavourable libration precluded this, but as compensation there was a nice view of Schroters valley and the Gruithuisen domes. Better images of the domes than the last time I tried! (https://roslistonastronomy.uk/more-domes)