New dobbie mount for 8" f/4 OTA

[mental4astro (Alexander)]

Hi all,

I took a little break from the 17.5" remake to try out a few techniques with making a dobbie mount for an 8" rich field scope. Peter also has a coma corrector which will be necessary with such a fast scope.

Peter (aka Hickny) and I purchased a new 8" f/4 OTA from Bintel this week that was lacking a mirror cell (cheap scope consequently). This is to be a grab-n-go scope which we shall share. You will see in the pics how small and 'cute' it looks, .

We made a new cell for it, same design as the 17.5", with massive springs to double as collimation springs and locking mechanism with their brute strength. The pics show the cell just before we attached the lateral braces. The mirror sits on three silicone pads. The three white tabs seen in the pics are door stops we used to act a feet to rest the OTA on.

Today we installed the cell and made the mount.

We used 15mm C-D pine ply to reduce cost, and we like the knotty look of the material. The alt bearing is made out of a 100mm PVC plumbing flange like in the 4.5" dob mount I made last year.

We cut out a series of holes throughout the rocker box panels to reduce some weight and act as handles. I pay tribute to Galileo with all the scopes I build by making a pattern with holes which resembles the image Galileo saw of Saturn through his little telescopes, a 'ball with two ears, or lobes either side' on the front of the rocker box.

We used 3mm Teflon and cut it to size to follow the "15lb per sq inch" rule. Ended up using bugger all Teflon. The altitude bearing is silky smooth, and the azimuth bearing I'll be glueing the textured laminate to the under side of the rocker box tomorrow.

We designed this scope specifically to be as compact as possible. The rocker box is designed to accomodate the scopes brace wholly within it for easy stowing.

The height of the rocker box is also configured to set the scope at an easy chair sitting height for the observer. With the relatively small height travel between zenith and horizon of the scope, this was an important consideration.

This design also allows this scope to be eq. mounted should the need ever arise as the brace only gently squeezes the tube in place. Foam strips lie between the scope and the brace.

I've also included a pic of the 4.5" scope too.

It should be noted that the tools we used to make this mount are readily available: Cordless drill, jigsaw, power drill and circular saw. Nothing speciallised, and easy to make in as a weekend project with the kids.

Mental

[lacad01 (Adam)]

Nice work mate How is the box portion fixed to the OTA, is it just a matter of the box claming around the tube or is there some other method as I couldn't see any screws?

[mental4astro (Alexander)]

Hi Adam,

The inside of the brace box is 1mm larger than the diameter of the OTA. With the foam between the two, the box is screwed shut around the OTA. Loosen one side of the box and there is enough give to slide the tube up or down for rebalancing. Simple.

Glued the textured laminate yesterday, waxed it with car polish and reassembled the mount this morning. Very smooth, .

[mental4astro (Alexander)]

Several layers of marine grade varnish later, all the painting is finished.

Had first light a few nights ago from home. I was astounded at how much brighter the skyglow is through an f/4 scope. Yes the coma is significantly more apparent, as to be expected. The coma corrector helps, but we will need a better quality long focal length eyepiece to make the most of the coma corrector.

I took it to 390X on Mars. Other than the poor atmospheric conditions, those moments of clear seeing revealed a clear crisp disk with an easily observable polar cap and dark green thin rim surrounding.

A quick scan of the is amazing. Such a wide field of view encompasses so much detail. A definate dark sky scope this one. I can't wait. Now, is the clouds would clear... .

I'm very pleased with this little scope. Works well and looks a treat!

Mental.

[mental4astro (Alexander)]

OTA modification

Its been a while since this new dob was made and I've done a small modification to it. As I use this scope a lot from home, the ambient light is a real pain in the neck reflecting off the white tube. I had some foam rubber left over from the stuff I used to wrap around the poles of my 17.5", so I used it to cover the top half of the 8" OTA. I cut out the necessary openings to accomodate the focuser, finder shoe and spider nuts. The white markings were done to position these openings.

The foam rubber sheet is wrapped around the OTA and only glued onto itself, not the tube. The holes that the various components poke out from hold the thing in place so it doesn't slip. I made it a snug fit which helps too.

The result works really well to tone down the reflections, . An added bonus was the insulative effect it had on the secondary mirror, protecting it from dewing up for a longer time than before. A bit of a surprise that was. I might consider making a dew shield for it out of thicker foam, much like the ones used on SCTs.

This stuff weighs bugger all, so it doesn't upset the balance.

Some food for thought.

[mental4astro (Alexander)]

Hi all,

A few nights back I used this scope to try out a few different eyepieces. I placed her on the lawn in my backyard and found that the mount was surprisingly stiff. Thing is, the lawn could have done with a cut. That ment as this the legs of this scope are quite short, the ground board as a whole was in contact with the lawn. The result was that the whole mount became a bit of a slog-fest to use rather than its normal silky smooth action to overcome the mushyness of the lawn.

Another thing I noticed was that this scope was just a bit too low to the ground making it difficult to use whilst seated.

The solution to both these problems was to make a tripod for it.

I took some inspiration from the tripod for "A scope like Alice" by Ronald Ravenburg. I liked the simple tapered shape of the legs. All I needed to do was decide how long to make them, and cut them out of a length of pine 4X2 I had. A scrap bit of ply, half a dozen monster screws and a bit of play with a mitre saw and a custom tripod is made. All stuff I had at home too. The only money spent was on a pack of platic furniture glides nailed into the tips of the tripod.

As the vertical travel of the eyepiece with this scope is around 300mm, a sitting in a chair is a very comfortable proposition now, rather than on one's knees.

I had a bit of a play with it on the lawn to gauge how it now responds - not an ounce more of effort needed to move her than when she's on concrete, .

There was another change done to the rocker, done some time ago. The original angular spacing of the Teflon pads was done as suggested for large DIY dobs, some 30degrees. Because the scope centre of gravity is so close to the geometric centre of the OTA, it ment that changing between large to small eyepieces the centre of gravity shifted to the extent that a large magnet was needed to act as a counter balance.

The solution was to recut the cradle and replace the Teflon pads at a much larger angular separation. Now I can insert a puny 6mm Plossl, swap it for a chunky 30mm 2" EP and no magnet is needed to re-establish the balance. Easy! Sure the action in altitude is a little firmer, but after waxing the PVC bearings, the action is again very smooth, and no backlash either.

This is such a sweet RFT to use!

[omegacrux (David)]

Great looking setup Alexander
When I had mine I pointed it to the zenith then stood up measured the difference between my eye and the focuser
then made a table that hight 350mm I think , made a huge difference no having too stoop
Didn't go as far as you but that looks real nice

David

[tlgerdes (Trevor)]

Now you need to make a stool so the small kids can see too.