Finder Scope

[Eldest_Sibling]

Round January of 2020, perhaps some time before, I was wanting a finder-scope for my blind-as-a-bat 127/1900 Maksutov...

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I say "blind" for it will be used with manual mounts. A go-to mount is practically out of the question, as I have too many trees upon the lay. In addition, a suitable go-to mount for said telescope would cost more, much more, than I am able or willing to pay.

I didn't want an 8x50 or 9x50 RACI finder-scope; no, out of the question.

50mm is really too small, dim, not to mention the narrow view of the integrated erect-image diagonal. Then, an 80mm is too large and bulky. That leaves something along the lines of a 60mm, or a 70mm even.

There is the Orion(of California) "70mm Multi-Use Finder Scope", a 70mm(actually 63-66mm) aperture at f/4...

https://i.imgur.com/GpbhFAw.jpg

But that one is not what it seemed to be, in the beginning. There have been complaints about the helical-focusser, the lack of thumb-screws to secure eyepieces/diagonals, focussing issues with this eyepiece and that diagonal, poor optics/astigmatism, et al. In addition, specifically, a 32mm Plossl will not come to focus.

I had taken a hard look at that one, but fortunately, in hindsight, I ran from it, leaving it in the star-dust.

No, the only thing that was going to do it for me was a 70/300, a 70mm f/4.3, achromatic-refractor.

There is a veritable sea of them, albeit a small one, on eBay and elsewhere, under this suspect marque and that, the listings oft with glamorous sales-pitches, full of hope, and promises of many merry respites under the stars...

https://i.imgur.com/QSGRSqc.jpg

But I was leery, suspicious of those. Does this one come with a 1.25" visual-back, then, does that one come with a glass, or a plastic doublet; or, the three Fates forbid, a singlet?

To avoid all of those uncertainties, I purchased the most expensive one I could find...

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...the Barska 30070-225 "Starwatcher", and for US$50.

But there's a catch, quite a few actually; for one, you can't use it fresh out of the box.

[Eldest_Sibling]

The exhortations...

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I can't use a solid refractor. If it's solid, how might one be expected to see through it?

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The highest power, stated twice on the box, 225x, just so happens to be the exact same magnification that I was able to achieve with one of my 70mm f/12.9 achromats, and far more easily I might add.

Very well, I will hold them to that.

The kit came rather well-equipped, albeit with refuse...

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But after all, in the end, I was after just the telescope itself...

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I find my efforts, when working on astronomy equipment, most rewarding in the renovation of entry-level telescopic OTAs from overseas, those mass-produced. My goal has always been to make them the very best that they can possibly be. Indeed, fortune has been on my side in that the vast majority of the objectives of these telescopes have proved to be, at least, of very good quality. I, personally, cannot re-figure mirrors, let alone lenses, therefore I am somewhat dependent, but I can do practically anything and everything else associated therewith, from fore, to aft.

[Eldest_Sibling]

The achromat, its anatomy, exploded...

The first thing to do was to remove a most proprietary mounting-interface...

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The focusser was easy enough to remove...

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The bull-horn of a dew-shield pulled right off...

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The cell for the doublet was not secured to the OTA with screws, but with glue...

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It was held by cement rather than the usual "super glue"...

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I wasn't surprised in the least by that. I can incorporate screws and nuts easily enough.

The paint where the cell attached to the OTA was damaged, so I removed it...

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Happily, placing a neodymium magnet adjacent to the OTA revealed that it is of aluminium; the lighter the better...

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[Eldest_Sibling]

There was something rather odd, strange about this telescope upon its arrival. I took only one snapshot prior to its renovation, through one of the bundled eyepieces inserted into the provided star-mirror diagonal, and during the day...

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That is what was so odd. It's a 70mm f/4.3 achromat, but I can't really see any false-colour within that image.

After a time, I had learned that this particular configuration of an achromat, and quite probably many others if not all of them, arrived as a 30mm f/10 instead.

Hmm, what do we have here...

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A baffle, a field-stop in this instance, and utter madness in the factory having installed it; or rather in having slyly done so? It was easy enough to remove...

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Then, to make the stopping-down even worse, the draw-tube of the focusser is much too long, strangling the entering light...

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Worse still, the draw-tube contained a baffle of its very own. I could kick myself for not having photographed it, prior to removing it...

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Then, on top of that, just inside the focusser's housing, a wall arose round the draw-tube's run...

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I had cut that back, too. I had also cut the draw-tube down...

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I was all set, and to enjoy a full 70mm aperture at my disposal, or so I had thought.

[croweater]

Hi Alan. I'm enjoying your efforts to make a silk purse out of a cows' ear. :P
Cheers, Richard.
ps. How is your Meade mount that you did up performing?

[Saturnine]

Hi Alan.

Enjoyed your story, having done much the same several years ago. Wanted a larger diameter finder for my 12.5" dob and saw the adds on Ebay for the 70 X 300mm refractors so bought one for $45, at the time
.
Worked out, as you have done, that the baffles were useless, reducing the effective aperture to what I worked out, to about 40mm and the included eyepieces and barlow were better off in the bin and were only the .945" type, 24.5mm I think. Did as you have done and cut the length of the drawtube by half and carefully machined out the plastic at the eyepiece end to accommodate an 1.25" correct image diagonal. Removed the tripod mounting interface as well and made up some 80mm ID rings and standard finder sized dovetail.

Didn't see any need to remove the objective, so the glue etc didn't concern me but painting the inside of the dewshield with flat black helped reduce stray light. All up, for less than $100, including the 1.25" diagonal and 32mm plossel I ended up with a quite serviceable 9 X 70mm finder with a field of view of about 5 deg. At that magnification false colour is not a problem and the field is sharp almost to the edge.

[Eldest_Sibling]

Oh, I haven't taken that mount outdoors yet. I've been working on, not just that mount, but on other things as well.

But never fear, as that mount-head is now light-years away from the way it was upon arrival, and I will be updating when the time comes.

[Eldest_Sibling]

Thanks for your adventure as well. As for my own, there's more to come, much more.

[Eldest_Sibling]

The doublet proved to be of glass, after all...

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...and with a coarse, rough, textured, approximately 1mm thick spacer-ring...

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That will be replaced with tabs.

At least one of the lenses are coated, on one side, the front of the crown-element, I expect, I hope.

[Eldest_Sibling]

The focusser is really the only mechanical component of a refractor, and therefore most critical to the telescope's collimation. With those of entry-level kits, like this one, and of others I have, the draw-tube is not necessarily square to its run within the housing, nor is the draw-tube slop-free. At a mid-point, or when racked fully outward, at least, the draw-tube wiggles, wobbles.

I had given the original focusser the works. Strips of PTFE as bearings for the draw-tube's run...

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Bronze bearings for the pinion-gear's shaft, even...

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Alas, it was to be my "Feather Touch" or "MoonLite" -esque or -ish focusser, but in the end it didn't pan out, not to my complete satisfaction. The main problem was simply my having to cut down the focusser in the first place. It was inexcusable for the factory to have included a long draw-tube for a short achromat. Also, the draw-tube's run seemed defective, uneven.

I went ahead and used it as is, and for testing for the proper spacing of the doublet. I use HVAC-type aluminium tape for spacers, at 0.09mm in thickness...

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I layer large pieces of the tape, one on top of the other, depending on the thickness required, and cut the spacers out from that; for example, this edge-blackened doublet from a 70mm f/12.9 achromat...

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[Eldest_Sibling]

With the proprietary mounting-interface removed, the optical-tube had two holes left behind...

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If I'm not going to strip the optical-tube of its paint, I insert set-screws into the holes, epoxying them in place, but in this case, I wanted the tube blank, with no marque upon it. So, I filled the holes by epoxying thin disks of aluminium on the inside...

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...then filled the depressions left on the outside, and all with J-B Weld steel-reinforced epoxy...

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The OTA was painted. After a bit of trial-and-error, this is actually the second coat of paint...

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It takes well over a week before it can actually be handled and used.

[Eldest_Sibling]

This bull-horn of a dew-shield...

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No.

I made a slimmer shield of aluminium flashing, for roofs and other. I also cut off the bulk of the original shield...

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The aluminium shield was epoxied to the remainder of the original dew-shield, the portion that slips over the lens-cell. I then cut a thin strip out the original shield, at the finished end...

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...and epoxied it within the new shield at the front, to strengthen, to make it rigid...

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Where the new shield joined the retained portion of the original, I had this to fill, all round...

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Filled with the epoxy and smoothed...

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[Eldest_Sibling]

The dew-shield, continued...

Much better, more in scale...

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I masked off the outside of the shield, and painted the inside with matte, chalkboard black...

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I didn't paint the tapered portion of the shield. I simply took 100% acetone and "painted" it, which restored its glossy finish...

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I masked off the interior and the tapered portion, then applied the finish...

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[Eldest_Sibling]

Once the first coat of paint on the optical-tube had cured, I flocked the inside...

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Later, I had painted the tube again, as the first painting had flaws.

The retaining-ring for the doublet blackened, and dulled, where required...

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The lens-cell itself was not blackened, no need, but certain areas were dulled just in case...

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The lens-cell, with the doublet installed and retained, completed...

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I've yet to blacken the edges of the doublet. That will come later after I'm done testing for the spacers.

[Eldest_Sibling]

Fortune does indeed favour the foolish, namely myself.

One day, somehow, some way, I landed on AliExpress, and most happily discovered this...

https://i.imgur.com/o1qNFjm.jpg

The focusser was practically identical to my own, with a shorter draw-tube, and without the original's shortcomings, I hoped.

I promptly ordered it. What did I have to lose? I was desperate for a replacement. I sat here, utterly amazed and dumbfounded that it was available in the first place. It's as though these sellers knew just how inappropriate, shoddy, the original focussers of these short 70mm achromats were, and are still.

It didn't take too long to arrive...

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A chance of a lifetime, a new beginning...

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I didn't get the solid, knubby knobs for focussing...

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...but who cares. Instead, I got two, count 'em, two thumb-screws for the visual-back...

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...with large knobs. Even the name-brand entry-level kits don't have those.

The old one came with just one, and I had been fighting it.

In addition, no need to cut back the draw-tube's run...

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...like I had to do for the old one...

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[Eldest_Sibling]

The new focusser, continued...

Out with old, in with the new...

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There was no baffle within the new, shorter draw-tube, either...

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...as there was within the original, longer one.

The visual-back accommodates my diagonals easily, but most importantly, my fattest eyepiece...

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Not that I'm going to be able to observe straight-through with this achromat, à la Japanese, although I might be able with those of shorter focal-lengths. I've already tested a 32mm Plossl straight-through, and I had to hold the eyepiece about an inch out and away from the visual-back in order to come to focus. Hmm, I could attach a short extension.

[croweater]

Hi Aan. I have a Parks 25mm Kellner (made in Japan) from the early 90's. It's is surprisingly good. :)
Cheers, Richard.

[Eldest_Sibling]

My own came with my Parks 8" f/5 OTA in 2003 or '04. Did you get yours with your Parks telescope?

[croweater]

Yes Alan. Also a 10mm plossl which was very good.

[Eldest_Sibling]

The time came to renovate the new focusser. Whilst it was quite good upon arrival, the draw-tube with negligible slop, it was still not up to my standards. I removed the draw-tube, and discovered these within its run, and as bearings or shims...

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They straddled plastic ridges which act as permanent, molded bearings for the draw-tube, to centre same.

At first, I thought that they might be of woven metal...

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But they appear more like woven fibre-glass, after I examined them more closely. You can see within that image how the "fabric" has been damaged, and where it had straddled a ridge.

In any event, I had never seen anything like that being used as such. I've always been used to seeing these...

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...corrugated plastic strips, which bear an eerie resemblance to the paper-trays of cherry-rolls or coffee-cakes. I have always removed and replaced those.

Life's too short, so out they came. This time round, I left the plastic ridges within the run alone. I had ground them down within the original focusser, and to make way for the strips of PTFE.

These are the screws for attaching the original focusser to the optical-tube...

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You can see how I've X-ed them out. They're like sheet-metal screws, but for plastic. They work okay, and there are three pre-drilled pilot-holes for them from the factory, round the new focusser's flange, but I wanted better to secure the focusser. Common and stainless steel hardware...

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I had to drill new holes into the new focusser's flange to match the position of the holes of the optical-tube. Three #6 common-steel nuts were sanded all over, and the new holes abraded then scored from the inside...

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The nuts were then epoxied into place, the epoxy sloping up and surrounding the sides of the nuts, and secured until the epoxy hardened...

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You can see the factory's pre-drilled holes within that image.

[Eldest_Sibling]

The interior of the focusser's housing was sanded down, then washed...

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These are the bearings for a pinion-gear's shaft, and from the original focusser. That area of the new focusser is the same...

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But there are no bearings there. I don't see any, do you?

New bronze bearings were created for the pinion-gear's shaft, the plastic "bearings" to be underneath sanded with a drill-bit wrapped with sand-paper beforehand...

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The bronze bearings were then epoxied into place, a pinion-gear shaft placed over them, then that secured to where the knobs would not rotate...

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Voilà...

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Since this was the second time I've ever made these, they turned out better than they did the first time, and for the original focusser.

[Eldest_Sibling]

After the bronze bearings were installed, I could then blacken the interior of the focusser's housing, including the forward-facing rim of the flange, and all round with two coats...

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Only the shadow knows.

The draw-tube's run, cleared of its shims, and cleaned/de-greased with 91% isopropyl-alcohol...

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[Eldest_Sibling]

The materials for collimating focussers...

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The clear tape is easily had locally. The aluminium tape can be had locally as well, although not as easily, and from heating & air supply-stores.

Although, I had to order the PTFE, as it's a bit more exotic, and I got my own from ePlastics in California...

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It was rather economical, for 12" x 12" sheets, in thicknesses of 0.005", 0.010", 0.015", and 0.020". Thus far, I've only used the two thickest in this madness.

The aluminium tape acts as a base for the PTFE. One, two, three, or more layers, each on top of the other, can build up the original substrate to the thickness required. Examples of past projects...

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The method works especially well with focussers of metal, like a proverbial charm...

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Most well-collimated those two were, afterwards.

The layering of the bearings...

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[Eldest_Sibling]

Pre-cut strips of 0.015" PTFE(Teflon), aluminium tape, and a roll of Scotch-brand, clear, double-sided tape...

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The thickness of the thickest ridge within the run is at about 0.46mm. Those strange shims, which overlaid the ridges, were at a negligible thickness. The thickness of the bearing materials combined is at about 0.54mm, and so to clear all of the ridges, as though they're no longer there.

Note how the strips are considerably longer than the run itself. You have to have something to grab onto, and to guide the strips of each inside the run, slowly, carefully, and accurately; or near enough. For the clear tape, I take a toothpick with one end rounded off, and run that end along the length of the tape, pressing it down gently, about two or three passes. You don't want to press the clear tape down with the fingers. I trim off the excess of each material flush from the outside, as each one, one at a time, is applied.

If mistakes are made, simply rip it all out and begin again. The PTFE will always be re-usable; the tapes, not so much.

All surfaces in between each material must be dirt, grease, and oil free, just before joining. I handle the tapes as little as possible. When I apply and rub down the aluminium tape flat within a run, with my fingers and thumb even, I then have to clean, de-grease the surface afterwards, as I had just mucked up same. The backsides of the PTFE strips are cleaned and de-greased as well, although the bearing sides get cleaned in the process, too. But once I press the PTFE down flat, with my fingers fresh from having had lunch, or other, I'll want to clean that again, and before installing the draw-tube.

The bearings for the draw-tube of the new focusser, completed...

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Two strips of PTFE were positioned adjacent to the sides, rather, of their respective ridges, and one in between two ridges, centred.

The racking in and out was not bad before, not at all really. Now, it's even tighter, yet O so smooth, and with no binding or slop whatsoever, even when the draw-tube is racked fully outward; superlative.

Some may wonder as to how well the PTFE will stick to the clear tape, and whilst operating the focusser, over time, in the long run. One day, I applied a strip of the clear, double-sided tape onto my glass-topped desk.

I then laid a strip of PTFE on top of that and pressed it down. When I lifted the PTFE, straight up, perpendicular to the tape, it lifted off most easily, too easily even. However, when I pulled the PTFE strip parallel to the tape, with considerable force mind you, it did not move in the least, let alone detach...

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It is the latter characteristic that applies, parallel, to these focussers.

Some may also wonder as to the longevity of this method, particularly that of the clear tape. I've seen very old applications of clear, one-sided plastic tape, and where the plastic part of the tape had fallen off long ago, leaving behind the ancient adhesive, a dark stain, long dried out. Yucks.

But that's just the point. That was over a long period of time. Hence, I feel that the clear tape used in this method might last 10, 15, 25, 35, 50 years even? Who knows, really, but if it fails, you simply do it all over again. The benefit absolutely, most definitely outweighs any potential detriment.

The first sign indicating when it's time to replace the clear tape would be when one of the PTFE strips slips outward a bit from the run, eventually slipping all of the way out and falling onto the ground; and, as fate might have it, during a critical observation.

Here, the new focusser, and the new dew-shield, are attached to the OTA, but not secured, not quite yet; glamour shot...

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Before, and after...

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[Saturnine]

Really enjoying your detailed article on modifying the scope, certainly a lot more involved than my own endeavours but similar enough. I was fortunate that in the last 20 odd years of my working life I worked in industries that used aluminium for machine design, construction and repairs and also lots of PTFE and teflon adhesive tapes, of varying thicknesses. Always was on the lookout for the offcuts etc that otherwise would have been tossed in the bin. Managed over the time to gather lots of handy materials for telescope mods and construction.
Things like relining the focuser body and reducing the slop really make a difference to these cheap scopes. Was using a modded 70 X 400mm Barska as a guide scope last night with no problems and it cost less than $100 and just a bit ( lot ) of tinkering.

[Eldest_Sibling]

A precise collimation becomes increasingly important as a telescope becomes shorter, and shorter still. I can't think of a single, practical telescope shorter than this 70/300 achromat. Can you?

Collimation is not as critical for telescopes with longer focal-lengths, but it most certainly is for those shorter, and how.

Therefore, I have to do everything possible, leaving no stone unturned, in collimating this 70/300 achromat. As it sat, before the new focusser arrived even, it was already set as a most capable finder-scope, or even a "Moon" telescope, for viewing the Moon only, albeit casually at best, perhaps.

The focusser is the only mechanical part of a refractor, a collimatable lens-cell being somewhat of an exception. Consequently, it is the collimated state of a draw-tube within its focusser that will either make, or break, a successful observation. This is every bit as important with reflectors, too.

With this new focusser, the greater the expectation in being able to use this achromat in a stand-alone capacity, for observing in its own right, and not attached to any telescope as a finder.

A while back, I had gotten "tube rings" for this wee achromat...

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Since it will be used as a finder, although not exclusively, I had to get those type of rings, a finder-bracket.

But when wanting to place the telescope onto one of my smaller, or larger, mounts, it must remain within those rings, and for use as a finder when desired. All of my mounts, large and small, feature Vixen-type saddles. I'm not going to swap the telescope back and forth between two types of rings as a result, hence, I needed an adaptor...

Red-oak, two pieces glued and clamped together...

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Red-oak shaped, formed, to my specifications...

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Red-oak enclosed by plates of aluminium and bronze...

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The metal plates and the oak were scored beforehand...

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The ends of the oak were enclosed with thin, 0.005" thick aluminium, almost foil-like.

I did not use J-B Weld epoxy to attach the metal plates, but a premium construction-adhesive instead, polyurethane-based, and superior to so-called "Liquid Nails"...

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The aluminium base-plate for the adaptor was secured further to the oaken core with screws...

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Done...

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If you look closely, there on right within the last image, you will see that the base-plate does not span the entire width of the rings' dove-tail bar. That ensures that the base-plate will not interfere whilst clamping the assembly.

[Eldest_Sibling]

Thus far, I've got more than US$100 invested in this one, but not much more. I don't expect any further spending. I haven't really kept track. The telescope kit was $50, and the finder-bracket another $50. I did buy a $5 rattle-can of paint in addition, and some screws. I'd say another $10 to $15, perhaps, in the end.

Congratulations on the 70/400.

[Eldest_Sibling]

When I first began to test for the spacing of the doublet, I had not yet discarded the original spacer. On a whim, I hand-held the achromat one night, whilst attempting to observe Jupiter. I saw Jupiter like this, this being nigh to a virtual sketch, and with bad flaring off to one side...

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That's when I realised that I needed to get this telescope onto a proper mount.

Incidentally, the achromat loves its new bar-adaptor. The telescope, here on my Astro-Tech(GSO) "Voyager I", with its old focusser still, can even be slid upward, still secured, to accommodate larger eyepieces, and barlows...

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I did test the original spacer once I got the telescope onto a mount, knowing full well I was going to replace it anyway, and Jupiter still exhibited a plume of light rising up from the top of its "head". The views were terrible overall.

Thus far, I have tested the doublet's spacing at varying thicknesses, and with three, proper spacers instead. This is how I create the spacers, and illustrated here for this doublet...

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At 0.27mm, the views weren't so good. I can't recall what I saw exactly, as they were that unsatisfactory. Jupiter still had its flaring in any event. I soon found that those thicker were required. To confirm just how bad that original, 1mm thick spacing-ring was, I chose same, or near enough; 0.99mm to 1.08mm, about 12 layers of tape...

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I marvelled upon the sight of the steep curves of that doublet. Indeed, one might eat a serving of something or other, and in using the convex surface of the flint-element as a bowl.

The images were then, quite sharp, and much to my surprise...

...but the flaring had not gone away. :tasdevil:

However, the plume of light was tighter, perhaps shorter, dare I say sharper, than with the original, 1mm thick spacer-ring.

For kicks and giggles, I made those at 2mm, which was disastrous. An altered afocal-shot of Jupiter, altered to match what I saw live...

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I then knew that round 1mm was the sweet-spot, in so far as sharpness at least.

Next, I made spacers at 0.76mm. The flaring all but vanished. However, I am unsure if the images at 0.76mm are as sharp as those at about 1mm. At 0.76mm, the images were indeed sharp, during the testing for the draw-tube's final length even, with a wide range of eyepieces and barlows, but I need to be certain, so I will need to install the 1mm spacers to test again.

Should I prefer sharper images with obvious flaring on smaller, brighter objects, and over those not quite as sharp with minimal flaring, or the reverse?

In that the flaring on smaller, brighter objects had decreased, at 0.76mm, and down from 1mm, I want to try 0.50mm spacers next. I'm most curious as to what I might see at that thickness.

I haven't tested the spacing of the doublet with the new focusser yet. I'm going to test again, with the 0.76mm spacers, which are still installed, and to see if the new focusser makes any difference. Then I will test at 0.50mm. I will be close to a final result after that, then I can finally blacken the edges of the doublet.

Thus far, it looks like it's going to be 0.76mm.

[Eldest_Sibling]

For testing the fate of the draw-tube, all I needed was an object at infinity...

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...the Moon, with the new focusser and the 0.76mm spacers, through my GSO 32mm "Super Plossl"(it's not all that super, really), and at 9.4x...

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I had my old Minolta DiMAGE F100, its shutter, set at 1/45th of a second when I took that shot, as the Moon was much brighter than that. From that, I know I'm getting the full aperture, or near enough.

I tested with the following items...

Celestron star-prism diagonal
SVBONY SV188P star dielectric-mirror diagonal(99% reflectivity, albeit alleged)
Antares 2x and 3x barlows
32mm, 20mm, 16mm, 12mm and 6mm eyepieces

Why, I had even tested the Vixen 6mm "NPL" Plossl combined with the 3x-barlow, at 150x. The view of the Moon was not bad at that power, not at all. It didn't make my jaw drop, but one of my socks did fall down a little ways towards the ankle.

But I've had one of my 70mm f/12.9 achromats up to 225x, on Polaris, and I could still make out the star's first diffraction-ring, albeit not razor-sharp. The Moon would take even more power besides, and still look pretty, hence I will expecting that from this 70/300, not that I'll realise that of course. In that event, I may contact Barska.

I can't wait to get these and other of my telescopes onto my new EQ-5, and with its wee 9V motor whirring away in the night.

Incidentally, Polaris is one of my most favourite targets, as it doesn't move.

The draw-tube was tested with each eyepiece, with both diagonals, prism and mirror. I made my measurements here...

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All measurements were shortest with the star-mirror, but it is with the star-prism that I will be using the telescope stand-alone.

Although, as an optical-finder, I want to use a star-mirror. I will only be using the 32mm(9.4x), perhaps the 20mm(15x), with the star-mirror, in that capacity.

The farther the draw-tube racks inward into the telescope, the more likely the end of the focusser will threaten to cut off the short, fat light-cone formed by the fast-doublet.

The shortest measurement was with the 32mm, at about 10mm, and with the star-mirror...

283027

That's not good, I'm thinking. Will the end of the draw-tube slice into the light-cone?

Perhaps I could just shave off the end, about 2mm or 3mm off, flush with the end of the flange, and be done with it...

283029

The longest was with the Tani 20mm Erfle, at 35mm, with the star-prism...

283028

No problem at all, and I wouldn't have to touch the draw-tube.

If I used star-prisms for both, problem solved, no need to cut off the draw-tube, but I just got this SVBONY star-mirror, and I want to make use of it.

I haven't tested my 90° and 45° erect-image diagonals yet, during the day, for birds in trees, ships at sea, nor at night. That testing will be forthcoming.

In that I will using this for observing, stand-alone, I wanted to transfer the specs-label from the old focusser to the new. I heated the label with a blow-dryer, whereupon it pulled off quite easily. I then placed it on a piece of glossy backing-paper from the flocking...

283030

I can re-apply it with clear, double-sided tape, or spray-adhesive, likely the former.

I began the renovation of this achromat almost two years ago. Apparently I wasn't in any hurry, but I will be finishing it, at last, and relatively soon.

[Eldest_Sibling]

My environs finally "dehydrated", since the rains, and to where I could resume testing.

On All Hallows' Eve, in the latter part of the afternoon, I tested the draw-tube terrestrially...

283054

A neighbour across the way was having some sort of work done, for the past week or so, and with heavy equipment. The target, indicated by the yellow arrow, a backhoe-loader or excavator, was the farthest target I could find. The following erect-image diagonals were tested initially with the GSO 32mm "Super Plossl", and at 9.4x...

Parks 45°; I think I got closer to eye-lens there, inadvertently zoomed slightly, compared to the other two...

283055

Celestron 90°...

283056

GSO 90°...

283057

All of the last three images were sized, and sharpened to match the live view, only; no cropping, brightening, or contrasting.

Again...

283058

...and with the 32mm Plossl at focus...

Parks 45°: 4.5mm, with the visual-back barely racked outward. That was somewhat surprising. That will definitely threaten to cut, slice, into the light-cone...

283059

If I were to cut that flush to the flange, 7mm off, I could be done with it.

Celestron 90°: 21mm

GSO 90°: 12mm

I have to wonder as to what accounts for the considerable discrepancy between the two 90° erect-image diagonals with the 32mm Plossl inserted, 21mm versus 12mm. Are they not "kissing cousins"?

I then tested the Tani 20mm Erfle(15x), the UO 16mm Konig-II(19x), the 12mm Konig-II(25x), and the Vixen 6mm "NPL" Plossl(50x). Incidentally, no barlows were used, not for terrestrial observations, and with only 70mm of aperture(!). All gave satisfactory views, save for the 6mm Plossl with all three diagonals...

283060

But then, I inserted the 6mm TMB-clone planetary...

283061

Wow! What a difference; again, the images sized and sharpened only. During the live view, I could not see the full field-of-view through the Vixen 6mm all at once, but I could with the 6mm TMB-clone, of course.

Still, 50x, for terrestrial use with a 70mm aperture, isn't going to get it; much too dim, and forget about a 4mm(75x).

The testing for the draw-tube is completed.

[Eldest_Sibling]

On All Hallows' Eve, at night, Jupiter was in the southern sky, shining brightly. I tested the doublet's existing spacing, at 0.76mm, again, and with the new focusser; no change of which to report. So, I hurried back indoors, and swapped out the spacers with those at 0.52mm all round, half of that of the original spacing-ring.

With the 32mm Plossl, at 9.4x, the flaring on Jupiter was quite evident, but with the 20mm Erfle, at 15x, the flaring diminished somewhat.

By the time I got to the 12mm Konig, at 25x, the flaring vanished, not a trace; the same as I went up further in magnification.

I have to wonder as to the sweet-spot. Is it at 0.40mm, or 0.60mm?

An out-of focus Jupiter exhibited a perfectly circular ball of light, and well-defined round the edge even. When I used an erect-image diagonal to view Jupiter out-of-focus, and for that RACI -esque or -ish experience, I could see the diagonal's Amici-line even, there in the center, vertically.

During the testing, I'm began to think that the Celestron-kit erect-image diagonal is slightly better than the GSO, and that I had purchased separately...

283072

That's not good. I have been somewhat curious as to why Agena Astro, of California, still has them in stock, whilst all the other GSO diagonals are sold out, and until next year. Incidentally, Agena Astro is the only, the sole vendor for GSO-branded products in the U.S.

Unfortunately, the blackening of the doublet, the completion, will have to wait, and until the Moon comes round again, then to test the 0.52mm spacing further. I have a good recollection of what I saw of the Moon at the 0.76mm spacing, and I must compare the two.

[Saturnine]

Hi Alan

You're producing quite a detailed account of the mods and testing of the various diagonals and eyepieces but are you keeping the same indexing of the 2 objective elements when you change spacers and have you tried different index positions to see if that has any effect on the flaring, just curious.

[Eldest_Sibling]

Do you mean like having two 0.76mm spacers, and one 0.52mm; along those lines?

If so, I have thought about that already, although I've never attempted it. I suppose with this one, there would be good reason to consider it.

[Saturnine]

Indexing means rotating one element in relation to the other not the thickness of the spacers varying. Sometimes there is an optimal position of one element in relation to the other because of wedge, the lens, either one, may be slightly thicker on one side. Might only be .002" but it could affect the refracted image
Don't know about the cheap telescopes but with most quality lens, they are usually tested by rotating one element in relation to the other until the optimum test readings are found and then a pencil line is drawn on the edges of the 2 lens elements to denote the best position for them.
When removing lens from the cell, their positions should be checked to see if there is indexing marks, pencil line, on the edges and if not mark a line before separating the 2 halves so they can be positioned the same way when returning to the cell. It may be worth investing a little time and experimenting to find the best alignment of the lens, considering the trouble you have gone to with the mods already.

[Eldest_Sibling]

Ah, rotation...

Thank you, Jeff, for explaining all of that. That of my lone apochromat is indexed, and in the past I have wondered as to those of achromats. I imagine that they might benefit from that as well. But do most, if not all, bother with that of achromats? In testing varying thicknesses of spacers, I have reduced the flaring, considerably.

I would love, dearly, to remove every last trace. I am not seeing any flaring now, at 25x and higher.

Then, it is not entertaining to me at all to watch Jupiter transiting across the field-of-view at 9.4x, with a 32mm Plossl; so very small they are, and my 32mm is not a Tele Vue. I do have a Tele Vue 40mm Plossl, and purchased when I didn't know any better. The view through both simply does not do it for me. Until most recently, I have preferred medium-to-high powers, particularly those highest, and possible per a given aperture. However, I'm going to give the lower powers a bit more "love", and with a 102mm f/5.9 achromat I purchased recently.

Although, getting back to the 70/300, I do get a bit giddier at 15x with a 20mm, and the flaring does seems less obtrusive, but it's still present.

This, a best simulation of what I saw, at 9.4x...

283135

I will be researching indexing further. I could prevent the elements from rotating in future, once a sweet-spot is found. Thank you again.

[Eldest_Sibling]

For better or worse, 'til before the next appearance of Halley's do we part...

283147

"No! Stop!! Don't do it!!!"

I'm afraid that my schizophrenic friend will just have to cope.

Incidentally, I have not been diagnosed as such.

"No, he hasn't."

Shut up.

The mark has been described...

283148

Can you see it?

"Hold on. Let me get my glasses."

283149

Does that meet with your approval?

"You're mad."

So are you. <ahem>

Firstly, I had to square the table to the spindle of the drill-press, with that protractor, and that hammer...

283150

Ready to cut, and right on the mark...

283151

The draw-tube rested upon wax-paper, an aluminium sheet underneath that, and for ease in rotating whilst cutting...

283152

Oh no! I didn't secure the diamond-wheel! It dropped, whilst spinning, and I cut too much off, into the rack even!

:tasdevil:

[croweater]

Murphy's law Alan :eyepop:

[Eldest_Sibling]

My bad; rather, my friend's bad. "It wasn't me!" Yes, it was.

That image was of the old draw-tube from the old focusser. The diamond-wheel was secured after all...

283168

Nigh precisely 8mm was cut off the end of the draw-tube...

283169

The cut-off portion there is 7mm in length, or height, as the diamond-wheel is at about 1mm in thickness.

The rim at the end of the draw-tube was sanded and rounded...

283170

The draw-tube was masked off...

283171

I took a shot of the original interior, and for posterity...

283172

The interior of the draw-tube was re-blackened. After I removed the tape and paper, I roughened the mirror-like end of the draw-tube's rack, and blackened it...

283173

Unfortunately, the camera would not focus well...

283174

It looks as though the end of the rack still has its chrome, but no.

[Eldest_Sibling]

Come again?

[Eldest_Sibling]

Yea, this, and to greet the incoming light...

283175

The position of the draw-tube, at its farthest inward travel when using an eyepiece, and practically flush with the housing's flange...

283176

The position of the draw-tube, at its farthest outward travel when using an eyepiece, and with no threat to the stability of the draw-tube within its run...

283177

Even when the draw-tube is racked fully outward, where no eyepiece is used, the draw-tube still exhibits no slop whatsoever, as tested...

283178

The touche finale; again...

283179

Voilà...

283180

I had used said tape.

The new focusser is completed.

Incidentally, I've decided not to use the flat and lock washers that I had gotten with the screws for securing the focusser to the optical-tube...

283181

Just the three screws are quite enough to secure the focusser...

283182

But I will need to shorten all three, as I don't want them to protrude past the surfaces of the nuts epoxied on the inside of the mounting-flange.

Presently, it's a bit cold, and wet, outdoors. Although, in a few days I should be able to resume testing of the doublet.

But, will the Moon be out by then? I can't win for losing.

[croweater]

Whatever can go wrong will go wrong Alan

[Eldest_Sibling]

But that's just it, Richard. Nothing had actually gone wrong. Although, that's not to say that nothing can go wrong from here on out.

[croweater]

No worries Alan. I think you'll be right mate. It's pretty obvious you know what you are doing.
Cheers, Richard