Edited by Dave Cook, 24 January 2021 - 11:15 PM.

Cave-Astrola 8" f/7 Model B Deluxe restoration / upgrade
#51
Posted 24 January 2021 - 11:13 PM
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#52
Posted 24 January 2021 - 11:26 PM
Today the polishing goodies showed up. I got sampler packs of the Chemical Guys polishing compounds and their foam polishing pads. They actually have 7 different levels of firmness on the pads (overkill anybody??) but I figure the 3 pads in the smaller sampler pack should be fine. The pads have standard hook-and-loop on the back.
Man, I hope you're wearing a *really good* respirator that fits well.
Metal polishing can be a real mess, and pose a serious inhalation toxicity danger.
The polishing compounds can be hazardous themselves, and many older aluminum alloys actually can contain a substantial amount of lead, and/or other toxic metals you *definitely* don't want to breathe
I'm likely "preaching to the Choir" about the above, but I simply care, and thought it worth saying, if even only for the benefit of others reading the thread.
But yes, on another note: your work is looking great Dave
Edited by Thomas_M44, 24 January 2021 - 11:29 PM.
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#53
Posted 25 January 2021 - 01:03 AM
Actually this stuff is all for polishing the paint, which produces next to no residue compared to the sanding (everything done outdoors btw with my 3M cartridge respirator). The Chemical Guys auto paint polish is probably too fine to be very effective on metal. And yeah, the aluminum in Cave's castings is not the modern stuff, no telling what's in it. I've been pretty careful about not doing anything that would make a lot of fine aluminum dust. Rocketry folks know that very finely ground aluminum (we're not talking machining chips here) is actually a viable propellant additive, and having a lot of it floating around is an *explosion* hazard. Powdered aluminum below a certain size is actually a regulated product because of that.
Another thing people should know about older telescopes is that lead based paint was not banned until 1978, near the very end of Cave Optical production. It's a fair guess that the original paint on many older mounts (Cave, Parks, Criterion, etc) would have lead content. A good reason to prefer the Citristrip, which doesn't throw dust into the air, vs wirebrush or sandblasting.
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#54
Posted 25 January 2021 - 07:45 PM
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#55
Posted 28 January 2021 - 08:32 PM
#56
Posted 30 January 2021 - 02:36 AM
And tonight's big push...rotating ring mounting success! Positioning them on the tube left a bunch of black marks but no real scratches - everything polished off easily. I ended up drilling out the holes #27 for a closer clearance fit. The nut holders work perfectly and it actually wasn't hard to get everything aligned for smooth operation. I did need to get my daughter to wield the screwdriver while I reached down the tube with the nut holders. The magnetic holder idea was a fail because I forgot stainless nuts aren't very magnetic.
I think the outer rings look great finished in polished aluminum instead of painted grey like they were originally.
No idea what I'm going to work on tomorrow. I need to polish the tube end rings, shop for a longer travel focuser, make nut holders for the finder and tube counterweight brackets, figure out where to put the Vixen bar, work on the mirror cell, dismantle the dec axis...it's a target-rich environment.
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#57
Posted 02 February 2021 - 06:40 PM
Some progress made on things I mentioned in the last post. I got one tube end ring polished, worked on the dec shaft some with a Scotch-brite pad, did some focuser shopping (no decisions yet), modified the nut holder design to have another parameter for tilt on screws that aren't normal to the tube, printed and read up on the OnStep control system.
My focus for the next few days is going to be on getting the OTA reassembled, and to take it out at night a few times to enjoy the optics before fully demolishing the EQ head.
Link to the improved nut holder in OnShape: https://cad.onshape....952ab4f69cde398
It's fully parameterized now, you can generate a new one just by plugging in the various dimensions. For #hexsize you should use the exact dimension of the next nut across parallel sides; that will give you a mild press fit. The depth should be 5-10 mils more than the actual thickness of the nut.
#58
Posted 04 February 2021 - 11:35 PM
You will love the images through this scope. Well worth the time and effort you have put into the restoration.
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#59
Posted 05 February 2021 - 07:12 PM
Well, I'm going to have to wait a bit for the quasi-first-light view thru the OTA - realized I have to take the outer rotating rings apart again for some measurements and modification to add the Losmandy rail on one side and a dual-width saddle on the other. Along the way I did get the finder rings and old William focuser mounted with freshly printed nut holders; #10 and #12 sizes with 5Ëš toe-in on the screws. Between the nut holders and machining the feet, the finder brackets are non-wobbly for the first time ever.
On a really good note, I just scored an in-stock AT 80ED from our CN sponsor. That was on my shortlist for the guide/aux scope. I've been checking for moderately priced 80 APO doublets every few days for a month and a half with no luck until now....pulled the trigger in about 60 seconds.
#60
Posted 13 February 2021 - 09:09 PM
- The non-concentricity is real. The 1.5" saddle bore is laterally off-center by about 0.10" in the casting. The walls of the rim are about 0.587" on one side and about 0.684" on the other. This can also be measured on the other side by looking at centering of the bolt hole.
- There is a smaller ~1.23" bore that extends a little beyond the main 1.5" bore. The shelf between the 1.5" and 1.23" bores forms the bearing surface for the end of the shaft. Due to the sloppy fit of the bore, this little surface completely defines the relationship between the dec shaft and the tube, and has to be clamped very tightly with the 1/2" bolt. This surface is not even remotely flat, so it was not done with a boring bar. It is also non-concentric with the main bore, for which my best guess is that a regular 1.5" drill was used. The good news is that the slope of the shelf looks to be uniform.
- On the other side (not shown), the clearance hole for the 1/2-13 bolt is counterbored only about 0.30". The standard head height of a 1/2" hex bolt is 0.50", so the bolt protrudes about 0.20". That works OK because the saddle arms reach upward, so there is clearance between the top of the bolt and the tube.
- There's no good way at all to statically measure on the casting whether the tube is on the plane perpendicular to the dec axis, because there is no machined reference surface defining the tube position on the saddle. The curved saddle support surfaces are wildly inaccurate so you can't use them as a reference.
- There is just about exactly 1.00" of shaft engagement in the saddle, and the shaft is 3.5 to 4 thousandths undersize. I think I mentioned before that that's a pretty coarse fit in the machinery world and is not considered a good location fit, which would have the shaft somewhere between a half and 1.0 thou smaller than the bore.
None of this probably matters too much for visual use, except that it makes offsetting from one object to another using the setting circles not very accurate (and I have memories that this was the case). After seeing how this is all set up, I imagine there could easily be a degree or two of perpendicularity error.
The second image is a CAD drawing I made showing the idealized original Cave shaft interface, to help me design the new one. You can see all the dimensions in the left column. As usual the OnShape CAD file is public here: https://cad.onshape....59b590c09b4186f
I'll publish the new design after I finish up a bunch of details. It's going to use brass setscrews onto a ground flat on the shaft to get a much more definite location, along with a proper 1.5" spec 304 stainless shaft (+0, -15 microns), and maybe double the engagement, which will also create room to use a tucked-under dec motor with pulley drive.
Edited by Dave Cook, 13 February 2021 - 09:13 PM.
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#61
Posted 14 February 2021 - 10:36 AM
Very cool. Thank you for posting all of this.
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#62
Posted 15 February 2021 - 02:36 PM
- The non-concentricity is real. The 1.5" saddle bore is laterally off-center by about 0.10" in the casting. The walls of the rim are about 0.587" on one side and about 0.684" on the other. This can also be measured on the other side by looking at centering of the bolt hole.
Great stuff! Thanks for posting!
I'm not sure any of these non-concentricities / offsets in the saddle will affect anything. Probably the most important alignment will be the orthogonality of the RA and Dec axes. I haven't taken mine apart, but I assume the Dec. housing / RA shaft joint is probably similar to the casting / machining on the saddle.
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#63
Posted 19 February 2021 - 12:15 AM
The offset doesn't actually hurt, but I think the tube-to-dec-axis orthogonality may be kinda far off in the original setup. So many things lack precision that there's really no way to tell. Visually it affects star hopping over considerable distances, as the polar alignment will never seem to be very good.
Anyway I'm working on making it better <evil grin>.
Face milling the new dec shaft interface block with the big fly cutter.
Oh yeah, my AstroTech AT80ED aux scope showed up from Astronomics. (Don't shoot me for posting a non-classic scope but imma gonna bolt it to the Cave, so there). I couldn't resist sticking it on a tripod and taking it outside for a look at a few things. I practically fell into the Panoptic 19; I think it's better than my eyeballs.
Edited by Dave Cook, 19 February 2021 - 12:34 AM.
#64
Posted 24 February 2021 - 04:48 PM
Got back around to the mirror clips. Machined the slots and wet-sanded the factory mill marks off the extrusion. They've joined the stack of things ready for the Electric Homer Bucket of Anodizing . I've recently collected all the bits n bobs needed to anodize and dye small aluminum parts...weirdest Amazon order ever with lead sheet, ping pong balls, lye, an aquarium air pump, titanium wire...probably got me onto half a dozen watch lists Hoping to try it out over the next couple of weekends.
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#65
Posted 26 February 2021 - 07:35 PM
Edited by Dave Cook, 26 February 2021 - 07:46 PM.
#66
Posted Yesterday, 03:05 AM