Five months ago on ZenMarket, I outbid four others for a Vixen D=80mm F=640mm short tube (that was once a Super-Polaris-FL-80S) with focuser and finder, but missing both the objective lens and a lens cell. The amount I bid was a fair price for just the sage green 6x30 swivel finder that accounted for my interest in the first place, but after delivery and putting the finder to use on another Vixen Super Polaris classic refractor, I started wondering what I could do with the left over Vixen short tube and focuser . . . .
Being aware that Vixen uses such 90mm O.D. telescope tubes for both its 80mm aperture and 90mm aperture refractors, and having been reading in the Refractors forum praise for the SVBONY SV48P short tube achromat (D=90mm F=500mm), I set my sights on finding a 90mm achromatic doublet for my left-over Vixen short tube. On the Ali-Express web site, I found a merchant [Ruite Professional Optical Trading] who offered a modestly-priced airspaced O.D. 93mm [Clear Aperture 90mm] F=600mm [**] achromatic doublet, as well as a cheap (in every sense of the word) molded ABS plastic lens cell. I ordered both.
A couple months later I had the lens in the cell, and after replacing the supplied "spacer ring" with three foil spacers I could see under CFL illumination the bulls-eye pattern of Newton's Rings, well centered with the lens elements paired outside the cell, but the sloppy fit in the cell made Newton's Rings elusive. To address this, I cut shims from thin plastic that I placed at 3 places around the circumference of the lens, in between the outer diameter of the lens and the inner diameter of the cell, but even so, the lack of parallelism and/or flatness of both the plastic lens cell's register that the rear surface of the flint element was resting on, and/or the plastic threaded retainer ring on top of the crown element, put the Newton's Rings bulls-eye pattern significantly off-center in the aperture with the glass installed in the cell. Not to be deterred, I cut wide cylinders of cardboard and layered these to increase the O.D. at the front of the tube sufficiently for a snug fit inside the rear section of the Ali-Express lens cell, which I then wrapped with masking tape to hold it for a look at the achromat's optical performance.
My daytime tests visually --- and with photos taken using afocal projection from eyepieces into an iPhone's camera lens, accessible in a Google Drive folder here:
https://drive.google...IzH?usp=sharing
--- were nevertheless rather encouraging, despite the significant false color visible, as expected, for a 90mm f/6.67 achromat.
* * * * *
It was at this point that I private-messaged Sean (norvegicus) here on CN to ask if he could design and 3D-print a cylindrical adapter to replace my rolled-up cardboard strips and secure the Ali-Express molded ABS plastic cell onto the front of the Vixen FL-80S tube. A schematic cross-section of the notional adapter I initially had in mind is shown in solid-blue at the bottom of this drawing:
Sean pointed out some practical difficulties (a polite way of saying impossibilities) associated with 3D printing this adapter in one piece as originally conceived, and although female threads could be printed around the inner surface of the adapter to match the threads on the end of the Vixen tube, there were also a number of challenges associated with how the adapter could best be attached to the Ali-Express molded ABS cell (whose cross-section is shown hatched in the bottom half of my drawing above). Exchanging messages off-and-on and mulling things over for the first couple of weeks, it became increasingly clear to each of us that designing around the cheap Ali-Express lens cell was making the task more difficult instead of easier, and overall things would be much better -- both design-wise and performance-wise -- if we abandoned that and set about designing a custom lens cell that could be 3D printed in its entirety. Conceptually, this would have the cross-sectional shape shown in magenta in the upper half of the drawing above.
A week or so earlier, Chris (Kasmos) here on CN contributed several posts to the "What did you do to your Scope/Mount Today?" thread, describing his work to replace the spacers on a Unitron 114 objective lens (see: https://www.cloudyni...9#entry12708143), and what caught my imagination was his concluding remarks about how the three screws Unitron employed to secure the lens retaining ring onto the front of the cell could, with a "soft touch", be used "to get the [Newton's] rings centered" in the objective. Having struggled with off-center Newton's Rings with this 90/600 lens in the Ali-Express lens cell, inspired by Chris' remarks I was keen to suggest to Sean that we adopt the Unitron approach of securing the front retaining ring with three screws, instead of threading it into the front of the custom cell, since the capability to fine-tune the centering of the Newton's Rings bulls-eye pattern in the lens aperture was a very appealing idea. The retaining ring's cross-section shown in purple at upper left above notionally represents this approach.
* * * * * *
With this fresh-start concept thus agreed upon, Sean and I then collaborated closely over the next month-and-a-half (!) as he developed and refined the detailed design in his [new] CAD tool ... his insightful comments + ideas and his 3D-printing fabrication experience were really valuable throughout --- and naturally I'm very grateful for his many contributions and his patience, not to mention the many hours of his time.
Here is Sean's 3D CAD rendering of his final design for the retaining ring (note the three pads that contact the glass, to be positioned above where the lens spacers are located between the elements of the doublet ... i.e., the glass is to be installed and clocked such that the lens spacers are located under these tabs : - ) --
... and here a cutaway cross-section through Sean's final design of the full lens cell assembly, that includes the retaining ring again at the top --
In Sean's lens cell assembly's cutaway cross-section, the blue areas are cavities (the doublet lens occupies the widest cavity, while the cavities above and below the widest one are for the telescope's light path and -- in the lowest, teal-color section -- for the end of the Vixen tube as well), while the tan sections are the cylindrical walls of the cell the retaining ring. Note the assembly consists of three separate pieces (from top to bottom, these are the retaining ring, the main section of the cell containing the objective lens + the female threads that mate with the end of the 90mm O.D. Vixen short tube, and finally at bottom a tapered conical 'rear trim' piece). All three pieces are held together by two sets of three screws (only one screw of each set appears withing the tan cutaway wall section, as the other two screws in each set are positioned at +/- 120 degrees around from the single screw shown ... see post #2) .
Actual photos to follow in the next message . . . .
-- Jim
[**] EDIT: I didn't mean to gloss over the differences between the well-regarded F=500mm (f/5.56) 90mm SVBONY SV48P OTA and the F=600mm (f/6.67) 90mm objective lens chosen here (indeed on Ali-Express Ruite does offer an f/5.56 90mm lens also). But it's just that in the case of the left-over Vixen FL-80S tube + focuser I was starting with here, both the objective lens *and* the focuser body are threaded onto the opposite ends of the tube, so shortening the focal length much beyond the original 80mm f/8 (F=640mm) length of the original FL-80S lens was not an option, short of machining new male threads around one end of the aluminum tube.
Also, close inspection of the top CAD drawing above (my notional 2D meridional layout) will reveal slanted lines representing the paths of marginal light rays from the edge of the rear surface of the lens to (far off-screen to the right in the CAD view) the edges of the focal plane inside the focuser drawtube. Including these in my CAD layout helped me determine where to resposition the Vixen short tube's three internal light baffles --- originally designed and positioned for the D=80mm F=640 (f/8) lens) so as not to cause vignetting when used with my D=90mm F=600mm (f/6.67) lens instead. The internal light baffle closest to the objective lens can be seen in the view of the CAD layout above, far over on the right side.
Edited by jkmccarthy, 28 July 2023 - 02:41 PM.
