Ronchi star testing filter update.


I have been notified by the parcel service that my Baader SC filter can be picked up. Do other countries have such a sophisticated parcels/package services? When you buy something online you give your email address and telephone number to the vendor. These are then used to automatically contact you by SMS or email when the items is waiting at a local package/parcel outlet or post office. Far more reliable and user friendly than waiting in all day for the post person and then not get the package. Or they didn't bother to ring the doorbell and took it back to the sorting office. Almost every village has a collection center so they are usually within easy reach and particular outlet options are suggested depending on one's home address and can be specified when ordering. A further option is to have the item delivered directly to one's place of work. 
The absence of a green filter made my results of Ronchi testing achromats somewhat difficult. The multi-coloured images showed how each colour is brought to a different focus. This tended to soften the Ronchi bands.

After much reading online I placed an order for a Baader Solar Continuum filter. This transmits only a narrow band in the green. Hopefully, and according to reviews, it aids the testing of achromats. It also helps to bring out the Solar surface detail provided an IR filter is also used in tandem to stop reported leakage in that area of the spectrum. 

I must add the usual safety note here that the Solar Continuum filter is NOT a safe, solar heat rejection filter on its own. It still passes more than enough heat and light to instantly blind any user who has not fitted a full aperture safety solar filter as well. These solar safety filters are readily available in film or glass form. The very popular Baader AstroSolar Safety Film filter reflects away almost all of the heat and light before it enters the telescope. This is vitally important to avoid blindness or sharply focused heat which can rapidly cause a fire or damage the instrument's optics through rapid overheating. There is still enough light to safely see the Sun's image visually in white light and to safely take pictures. 

The Baader Solar Continuum filter appears mirror like from both sides. It comes, well packed, in a drawer type of case for protection.

Only when one looks directly through the filter can the strong green cast be seen. I held it up to my compact camera and took a snap of its case to show its effect. Note how the printed spectrum on the packaging is now all but invisible. 

I added an IR block filter to my order in case I wanted to do more solar imaging.

This will be dependent on having a driven mounting which can easily manage the 7" f/12 refractor. The Fullerscopes MkIV proved inadequate and the VFO drive box died.

Click on any image for an enlargement.


2" shaft mounting Pt.26 Wormwheel options.


Having the wormwheels finally available meant I have to seriously consider their placement. The large 11.5" RA wormwheel could be placed above or below the polar axis housing bearings.

Placing it at the top under the axis joint cylinder looks smart but increases the cantilever above the top [Northern] PA bearing by 45mm. To make it into a "boater" hat.

While placing it at the bottom shortens the distance between the bearings by an equal amount but limits overhang to a minimum. I don't believe that torsion loads on a 50mm [2"] shaft are worthy of serious consideration. So won't let that sway my decision about wormwheel placement.

By allowing the RA wormwheel to rest on the plastic cups on top of the studs any extra flexure for the Dec overhang will be reduced. With the top flange bearing reversed there is no bare shaft above the bearing. Though that makes a compressed bearing housing much more difficult. I shall have to reverse the top bearing back to 'normal.'

A Teflon/PTFE disk could be fitted between the wormwheel and cylinder to provide even less flexure. The wormwheel and cylinder effectively become one unit. Carrying the applied loads directly from the top [North] bearing into the declination bearing housing without reduction in cross section. Russel W Porter would approve.

The Tollok bush is a delight to work with and instantly releases its tenacious grip on the polar shaft with half a turn of a single extractor screw. That is, providing the tension screws are all slacked off first. Likewise, the large studs allow rapid adjustment of bearing spacing and shaft projection to try different arrangement in only a few seconds. The bearings are also captive on the shaft via their grub screws. Which gives considerable freedom to adjust friction.

It all makes for a simple and almost effortless assembly and break easily down into its separate parts without special tools. I say "almost" because the weight increases with each additional part. The Polar axis assembly, as seen here, is already reaching just beyond my comfort level [at 55lbs] to carry in and out of the workshop. Naturally this goes with the territory of building a heavy mounting to support larger and longer telescopes in particular. Otherwise, why bother?

The drive worms are easily attached via sturdy alloy plates threaded over the four studs and tightened with the existing nuts. The worm support plate could be used to further increase load spreading against the end of the [still potential] plywood housing.

I thought I might seek out a square tubular metal option to compress between the flange bearings but have drawn a complete blank. My usually metal outlets had only plate or small profiles in aluminium.  No combination of my larger and heavier alloy profiles suits the spacing of the studs nor the size of the bearing flanges. These suit a 8" diameter round pipe but a 5"x5" square PA tube makes far more sense. A square, steel tube of that size, in any reasonable wall thickness, would probably need a crane to lift it.

Click on any image for an enlargement. 

2" shaft mounting Pt.25: 60-50mnm wormwheel sleeve adapters.

Since the wormwheels had been supplied with 60mm bores, instead of the 50mm I ordered, I had to make some brass adapter sleeves. This was very time consuming since so much material had to be removed from the only stump of brass bar I had left in a suitable size. This required a diameter and length which would provide two 45mm long adapters plus enough material for parting off and something to hold firmly in the 4-jaw chuck. Though I had a number of options to ensure concentricity I chose to bore and finish turn both sleeves without ever removing the brass bar from the chuck. 

I found a stump of yellow brass 65mm diameter and made a start on making the packing sleeves. First I center drilled the stump and took a cleaning cut. Then fitted a center and 'clocked' the piece in the 4 jaw chuck. The chuck jaws were tightened securely to avoid losing concentricity. Then I ran a 10mm drill in the tailstock chuck deep enough to give clearance for the nose of a boring bar.

In the picture above I have reached 45 mm bore and will continue deepening and widening the bore until I can part off two sleeves for the wormwheels. In fact I worked on the bore of only one adapter at a time due to the depth matching my medium length boring bar. That concludes tonight's effort.

Next day I finished boring and turning the first adapter sleeve. By sheer luck I was able to use the smaller wormwheel as a gauge to sneak up on the correct diameter. In between I had used a vernier caliper to reach the correct diameters. I also had my original, short stump of SS shaft which could be used to fine adjust the bore size.

Finally I parted off the first sleeve and started boring the second. This went much more rapidly because I could now trust my measuring tools to rough out to get close to finished size. Once that was finish turned to size I could part it off with the lathe in the slowest back gear. You would not believe how much swarf was left in the lathe's tray!

Now came the marking for the three radial holes in each sleeve for the adjustable clutch pads. These consisted of short lengths of ~8mm diameter nylon rod. Each of these is pressed against the axis shaft by a stainless steel grub screw to allow fine adjustment of friction/slippage so that the telescope can be pointed anywhere in the sky without having to retract the worms.

Instead of marking the radial holes with a marker pen I chose to tighten the grub screws straight onto the brass sleeves. This left clear imprints from the hollow nose of the screws which could then be carefully center punched. After using a small drill to ensure an accurate starting point I used a series of larger drills to open out each hole. Then all it remained to do was open out each hole with a tapered broach to aid entry for the nylon plugs. The alternative would have been to open out all the holes to 9mm which was slightly too large.

I have deliberately made the brass sleeves slightly too long so that there is a linear thrust bearing surface if needed. Should I later decide to make them flush with the wormwheels the sleeves are easily removable by backing out the grub screws. Meanwhile the nylon clutch plugs locate the sleeves securely in the wormwheels so that they do not rotate independently of the wormwheels. It is obviously desirable that the only bearing surfaces are formed by the close fitting brass sleeve and nylon plugs on the axis shaft. There is no slop either internally or externally on the brass sleeves to cause any eccentricity. Which would obviously badly affect the tracking.

I have been notified by the parcel service that my Baader SC filter can be picked up. Do other countries have such a sophisticated parcels/package services? When you buy something online you give your email address and telephone number to the vendor. These are then used to automatically contact you by SMS or email when the items is waiting at a local package/parcel outlet or post office. Far more reliable and user friendly than waiting in all day for the post person and then not get the package. Or they didn't bother to ring the doorbell and took it back to the sorting office. Almost every village has a collection center so they are usually within easy reach and particular outlet options are suggested depending on one's home address and can be specified when ordering. A further option is to have the item delivered directly to one's place of work.

Click on any image for an enlargement.



2" shaft mounting Pt.24: Wormwheels arrive.


Two months after ordering the wormwheels they have arrived. The first set were made with a 1.25" bore so I had to wait another month while another set was made. I ordered nominal 11" and 8" diameter wormwheels with a 50mm bore and matching worms and received 11.5" and 8.75" wormwheels [293mm & 222mm] with 60mm bores. Tooth count is 287 in both cases. This matches a 5rpm synchronous motor drive. Though stepper motors are possible and actually more popular these days.

I was told that the machinest is a gentleman in his mid 80s and now reaching the end of his desire to continue after two decades of producing these wormwheel sets in up to 14" diameter. The 14" are now discontinued. My own wormwheels are numbered 567 and 568 which is rather a lot of anything made to this level of precision.

The worms used to be made in stainless steel but these are now of brass. If I outlive them [as I approach 70 years of age] that will be a remarkable bonus!

They arrived in a very large box with masses of protection even including pipe insulation wrapping the circumference of the wheels.

I had no real idea of the true dimensions until they arrived so have postponed any serious progress on the mounting. I shall now have to make brass sleeves to match the 60mm bores to my 50mm stainless steel shafts. Making smaller holes larger is impossible if accuracy is to be maintained. Sleeving to make the central holes smaller is possible with care.

Note how the "threads" on the worms are coarser for the larger wheel. This makes obvious sense because the number of teeth on the wheel does not change while the circumference grows considerably.

The worm housings are simple U-channel profiles in sturdy cross sections to avoid flexure. The worms are held in journal bearings pressed into the webs of the channel section and fixed securely with tiny, SS grub screws.

 A view of the 'teeth' on the larger wormwheel. The thread on the worm engages over much its flanks to avoid play and ensures low wear in the wormwheel teeth.

Each wheel bush has three radial holes to take stainless steel grub screws with nylon pressure plugs. The latter press against the mounting axis shafts to provide a very simple clutch to allow the telescope to be moved easily, independent of, and without disengaging the worms.

The overall finish is very acceptable but the teeth could do with a scrub with a fine wire brush to remove some slight debris from the machining process. The vendor recommended lapping with plain oil rather than using any abrasive or even metal polish. His argument was that polishing particles would tend to bed into the materials and continue to cause wear over time.

Click on any image for an enlargement.



Another [10"] OTA lash-up.


It has long occurred to me that I could use four of the builders straight edge profiles to build a very stiff and light, four sided, altazimuth OTA. 

The images show a full sized mock-up using recycled rings largely held together with cord.  I have a sheet of 12mm birch plywood in stock to make smarter new rings.

The main problem with my earlier spar-type OTA was the torque forces around the doubled beam from the heavy 10" mirror's considerable offset. The beam did however suit a German equatorial mounting at the expense of some potentially odd eyepiece angles. 

The 2 meter, 6'6" long cardboard tube was simply far too heavy and awkward for comfortable handling. This four spar and rings OTA assembly is much lighter and readily provided spaces to grip it securely for transport to the observing site.

Having spars on all four sides immediately increases the stiffness of the 'tube' assembly. The spars have their own considerable depth to resist bending loads. 

The spars are also widely separated. Which naturally increases their 'moment.' So that bending of the OTA is greatly resisted in all planes.

My plan is to throw together a Dobsonian to get the 10" mirror back in action after far too long a hiatus building the 7" refractor and building mountings.  I don't think I ever once had the beam OTA correctly collimated due to flexure. The Dobsonian design provides the minimum possible ground clearance. Which obviously suits the very tall 10" f/8 OTA. The lower the eyepiece the less I need stepladders at modest object altitudes. A Dobsonian has no eyepiece access problems since it avoids odd tube rotation angles.

The spars suit clamp-on altitude bearings to achieve balance without permanent cosmetic damage to the spars. This is useful if I should ever rethink the design in the light of experience.

There are no torque loads applied to the new OTA. The mirror mass and secondary are both at the tube's center of gravity axis. I have simultaneously got rid of the thermal mass of the large and heavy aluminium saucepans and the aluminium plate of the primary mirror cell. I shall use a simple birch plywood cell for primary collimation.

The focuser will be fixed to the cardboard tube between two spars in the secondary cell to minimize the distance from the optical axis. This location will automatically apply a 45° angle to the focuser base for comfortable viewing at any tube angle.

The tube rings have no need to be made of heavy plywood if the short cardboard tubes are glued to become solid units with their pairs of rings. The cardboard tubes will provide some protection against dew and stray light. The plywood rings will each have four rectangles cut out to closely match the spar's outer dimensions. With the rings separated by the length of the cardboard tubes there is considerable natural resistance to any leverage applied by the spars to the ring assemblies. The reverse is also true.

No serious spar clamping is necessary due to friction in the rectangular cut-outs of the rings. In fact it is very difficult to move the cells along the spars with a quite normal tolerance sliding fit. Even with only two spars in rectangular cutouts I needed a rubber hammer to move the rings over and along the spars. Simple screws driven through the edges of the rings will easily suffice to fix the cells and allow easy, later dismantling if required.

I am trying to think how I could use a router to mass produce the 16 rectangular holes without wielding a hand saw. I have several bearing-guided, router bits which might work with a suitable template. Oddly enough I have never made or used a router template until now. Previous attempts to cut the holes with a coping saw were rather 'clumsy.' Requiring considerable coarse filing to smooth things out but left ragged edges from the birch plywood splintering.

I ended up using heavy brass plates with added barbel weights and G- cramps [C-clamps] to "box in" the router base plate as I cut each rectangle. An initial small drill helped to 'sharpen' the rounded corners left by the 1/4" 6mm router bit. Next time I shall make a square base plate out of Tufnol or plastic to better confine the cutter. The rounded base of the Bosch router does not slide perfectly straight against an edge. Nor does it work well inside a template. 

Click on any image for an enlargement.