Modernising the Towa 339

Transcription

1 Modernising the Towa 339 This scope was an ebay purchase in late 2009 but came with a few objective lens issues which were soon sorted by correct collimation. I was so impressed with the lunar/planetary views this scope gave that I thought it worth spending a little time and money to modernise it. I know, the purists out there will wail and howl at this prospect but I reckon these good scopes can be made into great scopes with a little modernisation. This can only mean it will get used a lot more; not a bad thing IMHO. Focuser The 1 st priority was the focuser. Although functional, especially with a homemade 1 ¼ adapter, the 0.96 focuser on this scope is looooonnnng suffering from draw tube droop despite fresh felt packing. Because the scope will be used for viewing fine planetary detail, the rack & pinion, again functional, needed to go. The fine focus adjustment of a dual speed Crayford focuser was what this scope needed. Because I was going to use the scope with binoviewers any new focuser scope needed to be capable of carrying the weight of these as well as supporting any extension tubes that would be required if I wanted to use ordinary eyepieces. Researching the current batch of upgrade focusers available for refractors I decided on the Antares for the 120 Skywatcher. Now this focuser IS NOT A DIRECT REPLACEMENT and will not fit into the 79.5 mm ID Towa tube but with a new adapter ring made specifically for the job it will. A quick word with my local machine shop and a new adapter ring was made. The new ring outside diameter is machined to match the outside diameter of the Towa tube avoiding unsightly steps in the scope lines. The Antares body and draw tubes are nowhere near as long as the original Towa, (I have seen one modification where the scope tube has been extended by 6 but this makes an already long tubed scope even longer). After a few calculations I estimated the set up would easily reach focus with binoviewers with the Antares fitted direct to the tube. But this arrangement would however extension tubes if viewing conventionally with eyepiece and diagonal. After marking, drilling and tapping the adapter for the three fixing screws the Antares was fitted to the tube using

2 stainless steel screws. Prior to fitting the focuser onto the new adapter I greased the 2 faces and the area where the locating grub screws would run. Finder scope. The Towa finder bracket fits directly onto the old focuser barrel therefore with the modification I would loose this facility. Aesthetically the long tubed Towa 30mm finder looks the part, (and works well), but a stumpy modern 50mm finder would look out of place on this f15 scope. Thus the original finder would be retained but with a new bracket. A rummage through my scope box found an unused SW 30mm bracket into which the Towa finder fitted perfectly. A finder shoe was then fitted to the scope tube just forward of the focuser. The resulting additional height of this bracket makes finder use very comfortable. For a future project I intend to make more traditional 2 part finder rings which will improve the overall look of the scope and hark back to the scopes classic routes. Tube Internals, Baffles/Flocking Initial inspection of the tube internals showed the surfaces were haphazardly painted with matt black paint; some areas, in particularly the baffle face, were hardly covered at all. A quick application of matt black paint ensured less internal reflections and improved image contrast. Towa fitted just 1 baffle in the main tube and 1 baffle on the end of the draw tube. Fitting the new focuser would mean loosing one baffle. Although I m sure the scope would function OK with the remaining baffle I wanted to cut out all glare and internal reflection as much as I could so decided the addition of at least one more baffle and possibly a further one in future would help considerably. Using Turbocad I plotted a ray trace diagram to calculate the size required for a baffle 4 from the focuser end of the tube. Between the baffles and between the new baffle and the focuser end of the tube I lined the inside of the tube with Protostar flocking material. This was also applied to the face of the new baffle and the internally for the first 8 of tube from the objective. Flocking a small diameter tube is not easy due to the difficulty in getting ones hands in to apply the material.

3 The new baffle was made on my mini lathe from 6mm aluminium plate suitably turned to provide a knife-edge. This was painted matt black before being fitted with the protostar material. The baffle was duly positioned as calculated and fine tuned by sighting up the scope from the focal point. Once in position the baffle was secured with three small dabs of silcon sealant; the flocking material either side also ensured the baffle was held securely. Focuser end baffle - position = 4 from focuser end of tube Hole size 1.27 I intend to fit another baffle at the objective end of the tube if the new extra baffle is successful. This will be positioned 8 in from objective end of tube with a hole size of st Light Testing 1 st light testing was carried out before the second baffle and flocking were fitted. Set up was initially with the scope on an altaz mount with I fitted two 2 extension tubes plus a 2 diagonal fitted to the focuser. The 1 st subject was Mars. A Vixen 8-24 Lanthanum zoom revealed superb detail with Syrtis Major standing out clearly. The atmospheric conditions proved very good allowing for more magnification, so the scope was swapped to a tracking equatorial mount. With the Vixen at 8mm (150x) the views were good. Image was useable in size, crisp and bright. Filtering provided an almost 3d view with the #12 orange/red filter. Blue brought out the polar cap nicely. With binoviewers fitted (no extension tubes required) focus was achieved with the draw tube approx ½ way. At 48x I could see a small very bright disc with Syrtis Major and the polar cap well defined. 120x improved the size and gave comfortable views with good detail but the image brightness suffered which was to be expected. I then used filters to find the best views. 2x Barlow with binoviewers required 4 of extensions. I initially fitted the 2 diagonal but swapped to an 1 ¼ to avoid overloading the new focuser. The views at 240x were a good size but much dimmer and when filtered reduced the brightness considerably, this was useable with yellow/red filters but the greens and blue darker filters were too dim. However, 96x gave a very nice brighter more useable image for working with filters. As an experiment I fitted my 30mm Vixen WA 2 eyepiece (unbarlowed) to see what a few DSOs would look like and to test how the scope stood up to 2 wide angle

4 eyepieces. M42 was better than I had ever seen through this scope, contrast was excellent giving some very nice details in the fainter regions of the nebula. The trapesium split nicely into the main 4 stars. There was no sign of vignetting at all and very little coma towards the edge of the field of view. M81 and 82. Both galaxies stood out nicely in the field of view and mottling was evident in the central region of M82. 2 nd Light ( Post extra baffle and flocking) Similar atmospheric conditions to the 1 st light test provided an ideal opportunity to perform a comparison test with the new extra baffle. Contrast was markedly improved on Mars helping tease finer detail on the disc, also the background sky appeared to be much darker. M81 and M82 were decidedly easier to pull out of the darker background, similarly fainter M42 showed a little more nebulosity than before. The Moon; what can I say but wow! The benefits of the extra baffle and flocking was evident from the first glimpse. Viewing a February 1 st quarter moon over 2 nights I was amased at how much detail this scope was bringing out. Tiny craterlets just popped into view, sharp shadows across craters and mountains all made possible by the ability of this scope to handle magnifications of 300x plus without image degradation. Conclusion The re blacking of the single baffle in the tube helped considerably with the contrast in the eyepiece and I would recommend this cheap and easy modification be a priority for anyone with this scope, the benefits are well worth a little time. The flocking and extra baffle has increased contrast further making this a worthwhile next step improvement. The focuser works as it should providing, fine focus adjustment for when teasing out delicate planetary detail. With unbarlowed binoviewers the scope tube length is perfect, resulting in a manageable draw tube extension of ½ way out reducing stress on the focuser mechanism. Using single eyepieces requires up to 5 extension tubes and the focuser ¾ extended. However a 2 filter wheel, plus 4 of extension tubes and an 1 ¼ diagonal the stress on the focuser is less than using a 2 diagonal and extension tubes, I use this configuration quite a lot when pushing the magnification higher than my binoviewers can handle. One day I intend to make a 6 extension for the main tube which can be inserted between the focuser and tube to allow use of single eyepieces without long extension tubes. Altogether a very satisfactory outcome. This modification has taken a good telescope and made it into an excellent telescope. I was amased at just how well the scope

5 performed on the fine detail of Mars. The ability to use modern quality eyepieces and the lack of focuser play/droop has improved the views through this scope considerably. Final words Is the modification worth it? You bet!