Finder Scope

[Eldest_Sibling (Alan)]

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



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...



...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 (Alan)]

The exhortations...



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



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...





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



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 (Alan)]

The achromat, its anatomy, exploded...

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



The focusser was easy enough to remove...



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



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



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



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...



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

[Eldest_Sibling (Alan)]

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...



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...



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...



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



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...



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



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



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

[croweater (Richard)]

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

[Saturnine (Jeff)]

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 (Alan)]

Originally Posted by croweater

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

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 (Alan)]

Originally Posted by 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.

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

[Eldest_Sibling (Alan)]

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





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





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 (Alan)]

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...



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



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...



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...

[Eldest_Sibling (Alan)]

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



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...





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





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



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

[Eldest_Sibling (Alan)]

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



No.

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



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...



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





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



Filled with the epoxy and smoothed...

[Eldest_Sibling (Alan)]

The dew-shield, continued...

Much better, more in scale...



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



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



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

[Eldest_Sibling (Alan)]

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



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

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





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



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



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

[Eldest_Sibling (Alan)]

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...



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





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



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



...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...



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

[Eldest_Sibling (Alan)]

The new focusser, continued...

Out with old, in with the new...





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



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

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



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 (Richard)]

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 (Alan)]

Originally Posted by croweater

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

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

[croweater (Richard)]

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

[Eldest_Sibling (Alan)]

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...



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...



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...



...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...



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...



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...



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



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