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Cave-Astrola 8" f/7 Model B Deluxe restoration / upgrade

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#1 Dave Cook

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Posted 01 December 2020 - 09:56 PM

Hi!

 

I've embarked on a project to put my one-owner 1975 Cave-Astrola 8" f/7 back into operation.  This is pretty much my "one true telescope", bought when I was in high school by saving lawn mowing money for a couple of years, and it's still my only telescope of any size.

 

Its configuration:

  • Has the declination worm drive, no tangent arm
  • Dual power cords into the housing
  • Rotating rings
  • Steel pier tube, which I gather may be unusual for the date of manufacture
  • Gray and black original color scheme

At the starting point here, it's not quite stock...

  • Has a Meade add-on 60mm guide scope (I think purchased from Cave) from ~1977-1978
  • Extra counterweight rail to make up for the 60mm and cameras
  • Williams Crayford type 2" focuser replaces the awful original spiral one that you couldn't adjust without the OTA moving
  • Primary cell moved forward a bit less than 2" to handle the focuser height (secondary size is just barely enough now)
  • 50mm finder relocated a bit to accommodate the larger focuser base, and also because it toppled long ago and broke the finder mounting screws out of the tube. (ouch)

Overall it's in good shape, having been stored in benign indoor conditions for nearly its whole existence.  There's a little rust on the steel pier and chrome dec shaft, and some moderately corroded hardware, but that's about it.  The counterweight comes off the shaft fine despite a bit of rust there.  Oh, and the AC wiring scares me, we'll get to that...

 

Caveat:  this is not going to be a "put it back to factory original" project.  </me puts on asbestos underwear!!> Some things are gonna get modified to improve function and rigidity, sloppy parts are going to get machined, some things replaced.  I've a good mind to convert it to DC steppers to gain a useful slew capability.  It might get a new larger RA drive and "go naked", you have been warned.  Tube color is up for grabs but it will not be Pink. wink.gif

 

The "before" photos will come in the next post after I figure out the media album UI on this board.

 

-Dave

 


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#2 Dave Cook

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Posted 01 December 2020 - 10:13 PM

Here we go with the photos.  These are "not quite before" but close.  I started working on the parts and hardware replacement a month or so back but didn't assemble it for a portrait until now.

 
Overall view of Cave-Astrola 8" before most of the work
 
Overall view of 8" f/7 Cave
 
In this photo the steel pier has already been de-rusted, primed and painted with 2-part epoxy paint, and the legs are mostly stripped and already have new swivel-pad leveling legs.
Mount near start of restoration
 
Closeup of dec shaft, setting circle and counterweight.  I have a second, smaller counterweight somewhere but it's currently AWOL.
Counterweight and dec shaft showing minor rust
 
Focuser/finder area.  The finder is not shown but is in perfectly good shape.  There's a fiberglass patch under the gray area where the finder mounting holes got relocated after a crunchy accident 30+ years ago.  The patch has held up well but the paint color has aged very differently than the gelcoat on the tube.  Mistakes were made.
Top of OTA showing focuser, relocated finder holes, and fiberglass patch from 30 years ago
 
The dec motor, Hurst variety.
Dec drive motor

Edited by Dave Cook, 01 December 2020 - 10:14 PM.

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#3 ngc7319_20

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Posted 01 December 2020 - 10:56 PM

Looks great!!!

 

Maybe its just me, but I find stepper motors tend to introduce vibration and muck-up otherwise stunning planetary images.  I might be reluctant to convert this to steppers.  Maybe a better plan would be some sort of DSC that could be used in push-to mode?  Only my $0.02.


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#4 icomet

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Posted 02 December 2020 - 12:20 PM

So, you want to paint the tube a different color?

 

 

 

(missing a bunch of counter weights with this image)

 

Clear Skies.

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#5 icomet

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Posted 02 December 2020 - 12:35 PM

. . . and pink for the tube color wasn't to your liking. That's okay, it's already taken.

 

Clear Skies.

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#6 Dave Cook

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Posted 02 December 2020 - 03:24 PM

Hah, the Pink project was the inspiration for my comment :)  I do need to refinish the tube somehow, classic white is probably the leading contender but at this point it's a free choice.  Yummy drive on that unit, kinda of what I have in mind for mine, 6-7" worm wheel.  I'll want to make a mounting plate that rotates it 90 deg from the original orientation to maximize latitude adjustment if the drive ends up being 7"+

 

Motor types are a really interesting engineering issue.  Steppers are slightly buzzy, but there are a lot of things you can do to mechanically isolate, silicone mounting pad, soft motor couplers, etc.  Small servos might be another possibility, but they also tend to vibrate when holding position (my CNC mill being an instructive case) so the dec might be problematic, and overall would be quite a bit more expensive.  Push-to is indeed a valid option, but I've got to mostly eliminate the huge dec backlash for that to even be viable on this mount, and will still need shaft encoders.  Steppers have become cheap enough thanks to 3D printers and robotics that I can easily get a couple and test.  Simple enough to just strap one onto the drive mounting plate using various mounting methods and see how the vibe propagates.



#7 Dave Cook

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Posted 02 December 2020 - 04:15 PM

Project Status and Hitlist

 

Here's a summary of what's done and in the queue for the Astrola refurb project.

 

Done or mostly done:

  • Re-coat optics.  DONE by Ostahowski Optics.
  • Make and install Delrin tips for the finder and guidescope alignment thumbscrews.  DONE.
  • Replace cotton stuffing in the spider with a machined aluminum spacer + silicone cushions.  DONE.
  • Replace all hardware with non-corroding stainless or brass items. Change various washers to better sizes, and add washers that should have been there. MOSTLY DONE.
  • Replace spindly leveling legs with good swivel-pad ones.  DONE.
  • Fix original poorly drilled leveling leg holes (very non-straight, and very sloppy thread fit as too large a tap drill was used) by installing EZ-loks into drilled out caster holes.  DONE.
  • De-rust, prime (including the inside) and repaint steel center pier.  DONE.
  • Machine leg saddles with fly cutter to make them actually match the pier OD.  1 of 3 done.
  • Remove large amounts of casting flash and original grind marks from the legs.  MOSTLY DONE.

Upcoming:

  • Bore out counterweights 2-3 thou for easier fit on dec shaft.  They were pretty close tolerance from the factory, explaining why many get badly stuck.
  • Face mill the mirror cell back plate on both sides, and clean up the rough grind ridges on the edges.  Maybe anodize it.
  • Deal with incipient rust on the dec shaft.  Probably going to replace it with a higher quality one, cheaper than re-chroming it in California.
  • Strip and de-rust the counterweights.  Re-paint with epoxy primer so they won't rust any more.
  • Fix unsafe AC wiring.  Either re-do the AC with added grounds and 3-wire plugs, or convert to DC stepper/servo system.
  • Disassemble and clean drives and clutches.
  • Clean finder and guide scope optics
  • Add dew cap to finder - it has one but it's maybe 5mm long tongue2.gif
  • Decide what to do about the RA drive - favor installing a bigger and better one.
  • Reduce massive backlash in dec drive.  The worm has no meaningful bearings.  It may have to get replaced too.
  • Inspect/clean RA/dec bearings.
  • Machine saddle to accurately match tube radius, add 1/16" silicone pad strips.
  • See if saddle attachment interface can be improved.
  • Optional but likely upgrade - make solid aluminum base plate to allow use of other OTAs.
  • Go through the rotating ring system.  Add Delrin slugs if it doesn't have any.
  • Get encoders onto both axes
  • Fix non-flat and incorrect radius on the finder and guide ring bases with the fly cutter.  Maybe upgrade to rail mount.
  • Primer and paint everything else

Edited by Dave Cook, 02 December 2020 - 04:16 PM.

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#8 icomet

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Posted 02 December 2020 - 04:58 PM

I stayed with the original gel coat on mine.

 

Clear Skies.

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#9 icomet

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Posted 02 December 2020 - 05:02 PM

Clutch quick  adjustment knobs.

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#10 Dave Cook

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Posted 03 December 2020 - 07:14 PM

Some more recent work.

 

Big Suburban Tool fly cutter at work on the leg saddles.  It's a 7 lb beast; with a long bar I can cut maybe a 15-16" diameter saddle on this mill.

Fly cutter in action on leg saddles
 
Nice surface despite somewhat minimalist fixturing.  A horizontal mill would have been more convenient.  I had to take about 20 5-mil passes, meaning 1-2 hours per leg.
Machined leg saddle.
 
The result - perfect fit.
Leg saddle fit after machining
 
Array of tools being used for paint prep on the legs.  I'm eliminating all mold flash and overlap, and fixing a few noticeable casting defects - a 1/2" blob, a small sand edge collapse, and a place out on the end where a chunk of the casting cracked out when they originally drilled for the leveling screw.  I've got some SEM etching primer on the way.
Legs - paint prep tools

 


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#11 Dave Cook

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Posted 05 December 2020 - 08:44 PM

More photos of things that got done a little while back.
 
Cleaning the mirrors before re-coating by Ostahowski.  I used the standard soapy soak -> gentle wipe -> distilled water -> pure isopropyl sequence.  Terry O said that the original 45 year old coating was surprisingly free of blemishes, but was now very thin and you could shine a flashlight through it.  There was no sticker from Pancro coatings on the back, at least when I started this project.  If there ever was one, it fell off a long time ago.
Mirror cleaning before recoat
 
Boring the blank for the aluminum secondary spacer.  I drilled out a sequence of 3/8, 5/8, 3/4" on the mill using my rotary chuck, and then went to a boring bar on the mini-lathe.  That was outer limits for a 3" reach on a mini-lathe; I had to stop and make a couple of rigidity fixes along the way.  I left the thickness at about 0.100" so it would be plenty sturdy for the subsequent milling step without being too heavy.
Boring the secondary spacer
 
Cutting the diagonal face on the spacer with a 2.5" face mill.  Jury-rigged workholding setup using an angle block, my v-blocks didn't help here.  This might have been a good case for making some custom 45 degree vee softjaws for the vise if I were going to make more than one.  If I had a bigger lathe (soon...)  I would have instead made a longer bored blank and cut the diagonal on the Fein coldsaw, which would have been drastically faster.
Diagonal cut on the secondary spacer
 
Spider parts and the new spacer, shown with the massive wad of cotton it replaces.  Silicone pads (1/16" on each end) not shown.  I turned down the square end of the spacer a bit to adjust for very minimal pressure.  I also worked on the spider a bit to free up the studs at the ends of the vanes; I needed to grind off some vane corners to let them rotate more freely for installation/removal.  I'm not planning to repaint the spider except for possibly the outer shell of the secondary holder since the rest is almost pristine.
Secondary spacer finished!

 


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#12 apfever

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Posted 05 December 2020 - 09:09 PM

 

Some more recent work.

 

Big Suburban Tool fly cutter at work on the leg saddles.  It's a 7 lb beast; with a long bar I can cut maybe a 15-16" diameter saddle on this mill.

 
 
Nice surface despite somewhat minimalist fixturing.  A horizontal mill would have been more convenient.  I had to take about 20 5-mil passes, meaning 1-2 hours per leg.
 
 
The result - perfect fit.
 
 
Array of tools being used for paint prep on the legs.  I'm eliminating all mold flash and overlap, and fixing a few noticeable casting defects - a 1/2" blob, a small sand edge collapse, and a place out on the end where a chunk of the casting cracked out when they originally drilled for the leveling screw.  I've got some SEM etching primer on the way.

 

Going to a 'perfect' radius fit will create an indeterminate bearing between leg and pipe when torqued on at the bolts. A slightly undersized radius on the leg will let the leg seat on the outside edge of the mating flange. The original castings tend to have a gap area in the center line, and mate around...roughly 25% in from the edge on what I've seen of a few. A slight gap in the center area is the key idea. The legs on my current 8" Cave have some rough casting areas but the rough seams are central and not on the edges like the Meade RG.  If anybody wants to clean up the mating surface of a leg to a true radius, slightly under radiused is a good idea. A series of sanding belt grades placed on a smaller pipe would take down the aluminum legs fairly quick. 


Edited by apfever, 05 December 2020 - 09:14 PM.

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#13 Dave Cook

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Posted 06 December 2020 - 12:53 AM

Going to a 'perfect' radius fit will create an indeterminate bearing between leg and pipe when torqued on at the bolts. A slightly undersized radius on the leg will let the leg seat on the outside edge of the mating flange. The original castings tend to have a gap area in the center line, and mate around...roughly 25% in from the edge on what I've seen of a few. A slight gap in the center area is the key idea. The legs on my current 8" Cave have some rough casting areas but the rough seams are central and not on the edges like the Meade RG.  If anybody wants to clean up the mating surface of a leg to a true radius, slightly under radiused is a good idea. A series of sanding belt grades placed on a smaller pipe would take down the aluminum legs fairly quick. 

I'm aware of this and did set the radius slightly undersized to keep the contact patches to the outside.  You don't want it oversized at all.  Beforehand the outside edges did not all even make contact.  I could see as the milling progressed how uneven the original setup had been - apparently done by hand, there were significant errors both laterally and axially.  Now I can literally stand on the legs/pier and it doesn't budge.


Edited by Dave Cook, 06 December 2020 - 12:55 AM.

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#14 Dave Cook

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Posted 12 December 2020 - 06:27 AM

Got some more stills pulled from the videos I've been shooting.  Another thing I worked on at the beginning was making Delrin tips for the finder and guidescope brass thumbscrews.

 

Here's the initial state of things around the 8x50 finder.  The thumbscrew on the left has not been touched yet; the right hand one has been cleaned up with a Dremel wire brush.  I might give them a vinegar bath before the end.  Also note that the main tube has yellowed a great deal compared to the finder tube.  Ignore the grayish-blue patch on the big tube, that was a ham-handed repair from long ago.

 

Initial state of finder brackets and tube paint

 

First I needed to drill the thumbscrews to receive the tips.  I used a #30 drill (0.128).  In order to chuck them in the lathe without smashing the threads, I made this little aluminum fixture - just a piece of 5/8 round stock about 0.6" long, drilled and tapped for 1/4-20.

 

When putting the thumbscrews into this fixture I discovered that over half of them were bent enough to make them bind!  A couple were pretty hard to get in far enough.  For the ones that weren't bent I used a jam nut (as planned) on the back to prevent them from spinning out when drilled.  That almost made me buy new ones, but I liked the old brass, and they don't have any trouble in the thin mounting rings.

 

Fixture for holding thumbscrews in the lathe chuck
 
Spotting with a small center drill, followed by the #30.
 
Drilling finder adjustment thumbscrews for Delrin tips
 
Turning the Delrin tip.  It just has 2 cylindrical sections; the part that presses into the end of the screw is 0.139" diameter, and the diameter of the exposed tip is 0.175" to have enough clearance for the 1/4-20 threaded hole.  The 0.139" seems large for a medium press fit into a .128 hole, but at small diameters the Delrin gets squishy, and it's *very* slippery.  At 0.135" they would fall out easily.  I had to make a couple of prototypes to find the right size.
 
Turning the Delrin tips on the mini-lathe
 
Here's a finished thumbscrew with no-mar tip.
 
Thumbscrew showing new Delrin tip
 
And the whole batch, lined up for inspection.  Once I got going this just took a couple of hours.  It was fun to put some points on the board here before going on to things that are going to be a lot more work.
 
Finder and guidescope adjustment screws modified

 

 

 

 

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#15 Dave Cook

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Posted 14 December 2020 - 12:51 AM

On to the Park tube repair and repainting.  The outer finish is polyester resin, often known in the marine world as gelcoat.  Polyester resin is how fiberglass things were made before epoxy came along.  It still sees use due to lower bulk cost than epoxy, but it has some major drawbacks.  It's brittle (leading to the well known stress cracks around holes), darkens and yellows as it ages, has really noxious fumes, and surfaces exposed to air won't cure unless you use a wax additive or PVA to form a barrier layer.  I briefly used polyester resin for glassing rocket bodies a long time ago, but then epoxy showed up and I switched over for good.  My Park/Cave tube has really gone yellow and I have stress cracks around a majority of the holes.  So I have no appetite for putting any more polyester resin on it.  Fortunately, I don't think I will need to do that.
 
Someone around here probably knows details of the original build process for the tubes.  Mine has a single lengthwise seam lap about 1mm high, suggesting that they did the layup flat and then rolled it up with something wrapped around it to give the outside shape.  They were not painted; the dyed-in-the-mass poly resin was the outer surface.
 
It needs structural reinforcement around all the attachment points...the tube is too thin to withstand the loads from all the small-gauge hardware that was used.  That's actually going to be straightforward, in the form of 6oz glass patches on the inside, wetted out with West System epoxy.  I already did this once already 25+ years ago when I relocated the finder; the patch has held up fine, so I'm going to replicate that everywhere else there were (or will be) holes drilled.  I'm also looking to machine or 3D print some contoured backer plates to reduce stress on the tube.  (Bleedover here - we use tube backers with thin glass and cardboard rockets all the time). 
 
My finishing plan is to sand the poly down, eliminate the seam and cracks as much as possible, and then prime / paint gloss white with a 2-part epoxy system.  I have 40 year old things painted with that stuff that are still bright white, so I'm pretty sure this will effectively eliminate the fading and yellowing for as long as I'm around.
 
First things first...I need a tube cradle fixture that will hold the tube horizontally on a table and not let it spin.  Started by dicing up some 3/8" ply that I had hanging around.
Tube cradle fixture layout
 
Anti-spin done by adding some strips of the 1/16" silicone rubber sheet, glued down with clear silicone seal.  Good stuff.
Building the tube fixture cradles
 
The finished cradles.  The whole cradle build took about 25 minutes.
Completed tube fixture
 
Rotating ring removal.  The ever-useful deadblow hammer was essential for getting them started.  The rings are a pretty close fit, so I'm going to have to watch it on the paint thickness.  Before removing them I made temporary witness marks with a sharpie to indicate the front/rear and orientation, since they were match-drilled and have preferred orientations.  After they're off, I'll engrave permanent marks on the inside.  They were secured with a too-small number of too-small bolts.  The outer rings are missing some paint so they're in the "gotta paint this" pile now.
Removing the rotating rings
 
Removing the aluminum badge plate.  Gentle use of a heat gun and putty knife did the trick.
Removing the badge
 
It took a couple of hours to sand off the seam and stress cracks.  I used #320 on orbital sanders.  My old cheap Ryobi bit the dust halfway through - RPM drop, bad smell, bits of plastic flung out thru the vents, pad stops rotating.  Replaced it with a Bosch 5" unit that is twice as sturdy, half as loud, actually catches the dust, doesn't make my hand feel numb, and probably won't melt its own gears.
Sanding away on the tube
 
The sanded tube ready for fiberglass patches.  If you look closely you can see all the holes.  The HOLES...yikes.  As I was thinking about where to put the patches, it occurred to me that I need to revisit the location of some things on the tube like the guidescope (Dalek: "Moderniiiize!") and the rotating rings (too far aft when you are hanging more stuff up front).
Tube after sanding the original polyester resin

Edited by Dave Cook, 14 December 2020 - 12:57 AM.

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#16 jcruse64

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Posted 14 December 2020 - 11:38 AM

Awesome, you found a reason to buy a new tool, AND you're doing good work on this!!!


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#17 Thomas_M44

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Posted 14 December 2020 - 09:45 PM

Hi!

 

I've embarked on a project to put my one-owner 1975 Cave-Astrola 8" f/7 back into operation.  This is pretty much my "one true telescope", bought when I was in high school by saving lawn mowing money for a couple of years, and it's still my only telescope of any size.

 

Its configuration:

  • Has the declination worm drive, no tangent arm
  • Dual power cords into the housing
  • Rotating rings
  • Steel pier tube, which I gather may be unusual for the date of manufacture
  • Gray and black original color scheme

At the starting point here, it's not quite stock...

  • Has a Meade add-on 60mm guide scope (I think purchased from Cave) from ~1977-1978
  • Extra counterweight rail to make up for the 60mm and cameras
  • Williams Crayford type 2" focuser replaces the awful original spiral one that you couldn't adjust without the OTA moving
  • Primary cell moved forward a bit less than 2" to handle the focuser height (secondary size is just barely enough now)
  • 50mm finder relocated a bit to accommodate the larger focuser base, and also because it toppled long ago and broke the finder mounting screws out of the tube. (ouch)

Overall it's in good shape, having been stored in benign indoor conditions for nearly its whole existence.  There's a little rust on the steel pier and chrome dec shaft, and some moderately corroded hardware, but that's about it.  The counterweight comes off the shaft fine despite a bit of rust there.  Oh, and the AC wiring scares me, we'll get to that...

 

Caveat:  this is not going to be a "put it back to factory original" project.  </me puts on asbestos underwear!!> Some things are gonna get modified to improve function and rigidity, sloppy parts are going to get machined, some things replaced.  I've a good mind to convert it to DC steppers to gain a useful slew capability.  It might get a new larger RA drive and "go naked", you have been warned.  Tube color is up for grabs but it will not be Pink. wink.gif

 

The "before" photos will come in the next post after I figure out the media album UI on this board.

 

-Dave

 

I just love that telescope. 

 

I owned an 8-inch f/7 Newt for over 12 years as my primary scope. I only sold it (in 2005) due to grave illness and medical bills at the time. I'm determined to find or build another 180mm to 205mm f/7 Newtonian in the near future.

 

At f/7:  collimation is quite easy;  coma is essentially non-existent; most eyepieces work very well; depth of focus is relatively comfortable and forgiving.

 

From a reasonably dark location, it's surprising how much a better-quality 8-inch Newt can show.

 

With the RA drive polar mounting, I'm sure high-magnification lunar and planetary viewing is very enjoyable.

 

Nice telescope, and excellent refurbishment work --Kudos waytogo.gif

 

I'm eager to see how your renewed scope turns out.


Edited by Thomas_M44, 14 December 2020 - 09:52 PM.

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#18 Dave Cook

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Posted 17 December 2020 - 08:41 PM

In order to size and place the fiberglass reinforcing patches on the tube, I had to take a detour to build a balance moment spreadsheet.
 
Public read-only link to the Sheets file on Google Docs is: https://docs.google....dit?usp=sharing
Feel free to grab a copy of this if you think it's useful, there is nothing super fancy.
 
My main objective was to compute the balance point along the tube in various configurations, and compare to the center point of the rotating rings.  I have found that the factory location of the rings with only the 8x50 finder and the very light spiral focuser was within an inch of perfect balance.  However, with a more modern 80mm autoguider, a prime camera, and a heavier focuser, the rings will need to be moved forward about 3.0", even with a 2 x 1.9 lb tube counterbalance set (Cave original) added with the weights in the rearmost possible position.
 
A big constraint on moving the rings forward is that you lose ground clearance on the disfavored side when meridian crossing at the zenith.  This trades off against having to tighten the dec clutch to resist moving from the imbalance torque.  Quick tape-measure estimates suggest that I had 6-7" of clearance in the factory setup.  I've already lost nearly an inch to the new leveling feet, which sit lower than the stock casters + screws did (a good thing for stability though), so it seems uncomfortable to give up another 3".  Each 2-lb counterweight moves the CG by about an inch, so moving the rings up 2" and adding 2lb more is a viable option.  This weekend I may temporarily assemble the rings/tube/head/pier/legs to get a more accurate idea.  Fabricating a 4" taller pier is another possibility, but for now I'd like to avoid that.
 
Hmm, it does occur to me that making a pier top adjuster head that incorporates precision az/lat mechanisms might be a serious win here.
 
But anyway this info is enough to let me proceed with reinforcing the tube and filling in at least 12 holes that won't be used anymore.  I'll have to use a pretty big patch for the new location of the guidescope, since I'd like to put it back as far as possible without compromising the FoV, but I won't know where that is until I get it.  It's going to sit on an ADM Vixen style bar and 125mm rings, with radius blocks and shop-made tube backers.
 
A screenshot of the balance calculator is shown below.  This is just the bottom section; the computed CG can be seen at the bottom.
 
Tube balance spreadsheet snapshot

Edited by Dave Cook, 17 December 2020 - 08:48 PM.

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#19 apfever

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Posted 18 December 2020 - 02:02 AM

Wow.  I get mine together and toss it over my arms, then bring my arms together to get the center.  Toss a tube over any two items and bring them together to get the CG. It doesn't matter what the the two items individual coefficient of frictions are.  Static balance in one position isn't dynamic. The focuser and finder and such toss in imbalance at a different position.  I noticed my 8" Cave doesn't have a balance bar with weight. That would be nice, maybe opposite side of the tube from the focuser/finder cluster.

 

I love doing the math just for math sake sometimes. It's fun. I did this with the 12.5 O.C. on the Meade RG mount just to get a rough idea of where to start so it would be close when the OTA went on. I'm getting too old for brute force on that size.  

 

Dave, are you going to use the sheet to determine the rotating ring locations? My rings have 4 bolts each for 8 total holes in the OTA. They are small holes easy to patch. I have the extra complication of a factory original "Meade" 60mm guide. I have an extra balance weight set I'll use if the original ring location is an issue.  This would be a permanent mod to drill in balance bar brackets, but my set looks very Cave-ish.  No one would know. It might be a Cave set.



#20 Dave Cook

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Posted 18 December 2020 - 09:57 AM

Hi, sounds like you actually some components that are probably identical to mine.  As they originally shipped them, the Cave scopes didn't come with or need any tube counterweight, which I was able to verify with the calculator.  I had added the original Meade 60x700mm f/11.x long guide scope too.  Way back in the beginning I found that with that plus an SLR camera in the focuser, adjusting the dec clutch became tough because the dec axis wanted to move from the imbalance, but making it too tight made the dec motor struggle.  So I got the old Cave original tube balance bar and put it on, which helped a lot.  I'll post a photo of the bar and its brackets later this weekend.  I'm almost certain I bought those direct from Cave.

 

Back in the old days I did realize (as you mentioned) that the balance bar ought to go 180˚ around the tube from the focuser/guidescope,  because the rotating rings were also wanting to spin due to the unbalance *around* the tube as well as along the tube.  Fortunately I originally mounted the bar in about the right place and I won't have to move it.

 

Now I'm looking at adding even more weight up front, so I wanted to do the math for real before buying stuff that I might not be able to use if things didn't work out.  One thing I found in playing with the numbers a couple of nights ago is that if I want to go to the SkyWatcher Evo72, I'll need an additional weight on the bar to keep the dec axis roughly in balance.  Not something I want to do since it increases the moment of inertia and lengthens the damping time, but it's only ~5% of the OTA mass so not a drastic increase.

 

To answer your specific question, I have been able use the spreadsheet to figure out how much the rotating ring system should move for balance (about 3" forward), but there is a mechanical limit to how much it can be moved due to ground clearance.  I'm hoping I can move the rings at least 2.5".



#21 Dave Cook

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Posted 19 December 2020 - 03:38 PM

Got the fiberglassing done last night and this morning.  Here's a look down the tube showing all the internal patches.  It ended up being rather extensive; where there were multiple holes within a few inches of each other I just used a larger patch over the entire area.
 
The glass fabric was cut on an Olfa mat with a rotary cutter and a big wide fabric straightedge.  That's the best way I've found for cutout of small to medium sized chunks of cloth - minimizes handling and stretching, and you get a totally clean cut.  For really light fabric like the 1-2 oz stuff used in gliders it's about the only way that works.  Even for the heavier stuff (6 oz used here) it's way faster than scissors because you don't even have to mark the cloth before cutting.
 
I ended up running out of good 6 oz cloth halfway through and had to go grab some Bondo brand fabric at O'Reilly's.  It was pretty crappy - had some void gaps from being folded into a teeny bag and unraveled really easily.  It kind of worked but I won't go that route again - back to CST for the good stuff that comes rolled, and is actually less expensive.  Forgive me my sins, I was in no mood to wait days during the Christmas delivery crunch.
 
The fabric was tacked in place by dusting one side with 3M Type 77 spray adhesive.  There is a bit of an art to that; you need to avoid getting the adhesive on the corners where you are going to handle it as you position the fabric.  If you get adhesive on your fingers there will be comic scenes and profanity as the cloth keeps pulling off the tube when you try to take your fingers away...you will also be learning to clean the adhesive spray nozzle with acetone and a pin.
 
Tube with fiberglass patches tacked on
 
Here's what I used for the resin job - West 105/205 and a bunch of foam brushes.  The 205 fast hardener only gives you 8-10 minutes of pot life and you have to spend a little time forcing the resin into the coarse weave, so I ended up making batches of just 1 pump of resin and hardener.  Overall I did 5 batches to get all the patches done...took about an hour overall.  The only serious problem I had was the fabric pulling up off the tube around the big focuser opening - had a slight moment of panic but then pulled out an X-acto knife and made a couple of perpendicular relief cuts to allow the fabric to lie down.  Tomorrow after everything hits a full cure I'll trim the focuser hole and the other holes that aren't relocating, and put thickened epoxy in the holes that are getting closed up.  There are also some loose strands to trim off.  I learned a long time ago to just let everything cure and then trim - makes the strands much easier to cut.
 
Fiberglassing supplies

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#22 Thomas_M44

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Posted 20 December 2020 - 10:40 PM

 

Got the fiberglassing done last night and this morning.  Here's a look down the tube showing all the internal patches.  It ended up being rather extensive; where there were multiple holes within a few inches of each other I just used a larger patch over the entire area.
 
The glass fabric was cut on an Olfa mat with a rotary cutter and a big wide fabric straightedge.  That's the best way I've found for cutout of small to medium sized chunks of cloth - minimizes handling and stretching, and you get a totally clean cut.  For really light fabric like the 1-2 oz stuff used in gliders it's about the only way that works.  Even for the heavier stuff (6 oz used here) it's way faster than scissors because you don't even have to mark the cloth before cutting.
 
I ended up running out of good 6 oz cloth halfway through and had to go grab some Bondo brand fabric at O'Reilly's.  It was pretty crappy - had some void gaps from being folded into a teeny bag and unraveled really easily.  It kind of worked but I won't go that route again - back to CST for the good stuff that comes rolled, and is actually less expensive.  Forgive me my sins, I was in no mood to wait days during the Christmas delivery crunch.
 
The fabric was tacked in place by dusting one side with 3M Type 77 spray adhesive.  There is a bit of an art to that; you need to avoid getting the adhesive on the corners where you are going to handle it as you position the fabric.  If you get adhesive on your fingers there will be comic scenes and profanity as the cloth keeps pulling off the tube when you try to take your fingers away...you will also be learning to clean the adhesive spray nozzle with acetone and a pin.
 
 
 
Here's what I used for the resin job - West 105/205 and a bunch of foam brushes.  The 205 fast hardener only gives you 8-10 minutes of pot life and you have to spend a little time forcing the resin into the coarse weave, so I ended up making batches of just 1 pump of resin and hardener.  Overall I did 5 batches to get all the patches done...took about an hour overall.  The only serious problem I had was the fabric pulling up off the tube around the big focuser opening - had a slight moment of panic but then pulled out an X-acto knife and made a couple of perpendicular relief cuts to allow the fabric to lie down.  Tomorrow after everything hits a full cure I'll trim the focuser hole and the other holes that aren't relocating, and put thickened epoxy in the holes that are getting closed up.  There are also some loose strands to trim off.  I learned a long time ago to just let everything cure and then trim - makes the strands much easier to cut.

 

Very thorough job.

 

I like that wink.gif

 

Good information and suggestions too.


Edited by Thomas_M44, 20 December 2020 - 10:41 PM.


#23 Thomas_M44

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Posted 21 December 2020 - 05:18 PM

Hi!

 

I've embarked on a project to put my one-owner 1975 Cave-Astrola 8" f/7 back into operation.  This is pretty much my "one true telescope", bought when I was in high school by saving lawn mowing money for a couple of years, and it's still my only telescope of any size.

 

Its configuration:

  • Has the declination worm drive, no tangent arm
  • Dual power cords into the housing
  • Rotating rings
  • Steel pier tube, which I gather may be unusual for the date of manufacture
  • Gray and black original color scheme

At the starting point here, it's not quite stock...

  • Has a Meade add-on 60mm guide scope (I think purchased from Cave) from ~1977-1978
  • Extra counterweight rail to make up for the 60mm and cameras
  • Williams Crayford type 2" focuser replaces the awful original spiral one that you couldn't adjust without the OTA moving
  • Primary cell moved forward a bit less than 2" to handle the focuser height (secondary size is just barely enough now)
  • 50mm finder relocated a bit to accommodate the larger focuser base, and also because it toppled long ago and broke the finder mounting screws out of the tube. (ouch)

Overall it's in good shape, having been stored in benign indoor conditions for nearly its whole existence.  There's a little rust on the steel pier and chrome dec shaft, and some moderately corroded hardware, but that's about it.  The counterweight comes off the shaft fine despite a bit of rust there.  Oh, and the AC wiring scares me, we'll get to that...

 

Caveat:  this is not going to be a "put it back to factory original" project.  </me puts on asbestos underwear!!> Some things are gonna get modified to improve function and rigidity, sloppy parts are going to get machined, some things replaced.  I've a good mind to convert it to DC steppers to gain a useful slew capability.  It might get a new larger RA drive and "go naked", you have been warned.  Tube color is up for grabs but it will not be Pink. wink.gif

 

The "before" photos will come in the next post after I figure out the media album UI on this board.

 

-Dave

Hey Dave, have you recently tried any newer-production Edmund RKE's ?

 

Heads Up:

 

William Paolini  ( CN member BillP ) has been getting fantastic results with planetary and double-star observation in recent trials (including the Mars opposition) as compared even to his Zeiss Abbe Orthos and other premiums.

 

I've been shocked by similar results using mostly the 21mm RKE with a 5X TeleVue Powermate, and also the 12mm RKE with a 3X TV Barlow.

 

Bill has measured the clean AFOV as a full 50-degrees as opposed to the stated 45 degrees. Bill is mostly using a 2.5X Powermate with his RKE's I believe.

 

Here's the current thread in which we are discussing our RKE experimentation and experiences:

 

https://www.cloudyni...rke-collection/

 

A few complete 'mounted' RKE eyepieces (and also a few 'unmounted' RKE lens sets) are still available from Edmund Optics, but the near-future availability once current stock runs out seems doubtful:

 

https://www.edmundop...yepieces/12484/

 

If interested, these late-1970's designed eyepieces might be fun to mate with your refurbished 8-inch f/7 Newt  :-)


Edited by Thomas_M44, 21 December 2020 - 05:20 PM.


#24 Thomas_M44

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Posted 21 December 2020 - 05:21 PM

BTW:  I vote for RED wink.gif



#25 Dave Cook

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Posted 21 December 2020 - 07:49 PM

Red eh?  Since I like things that go to 11 (esp. if they say "Marshall" in script letters but that's a whole 'nother story), that's not implausible at all!

 

I will have to go look at those eyepieces.  I only have three or four, mostly the old style orthos that Cave sold back in the day.  It's definitely on my list to get a couple of good ones that can utilize the 2" focuser.  Of course thinking TeleVue but many things are possible.

 

It's just about time to go check out the conjunction. Wishing the scope was done already, at 0.1˚ spacing the view at 50-60x would have been great.

 

Here's a few recent photos.  First up are the original Cave tube counterweight parts.  Very surprisingly, it appears the 3/8 shaft may be stainless steel, there was essentially no corrosion.  Yesterday I hunted around and learned that no one makes counterweights with 3/8" bore anymore.  I will need one more so that will be a fabrication job.

 

Original Cave tube counterweight parts
 
Here are the legs after a wipedown with stripper post-wash (glorified MEK).  After this, no more touchy-touchy without gloves.
Legs ready for sanding and primer
 
Making some simple holder/stand fixtures for painting the legs.  All it took was an 8' 1x2 and 3 1/4-20 bolts and about an hour.  I used one of the 3/8 studs on the back of the legs to mount them to the 1x2 pieces.
Drilling the leg painting fixtures
 
Machine sanding and then hand sanding the legs to knock off the oxide layer.  The self-etch primer followed immediately, about 1 minute later.
Machine sanding the legs
Hand sanding what the machine couldn't reach
 
Spraying the self-etch primer.  It's pretty thin; to be followed by high-build epoxy primer.
Applying self-etching primer to the legs
 
Finally the row of primered legs!  I'm going to let this dry for a couple of days before spraying the epoxy primer.  Then comes the sanding.  Lots of sanding.  And Bondo.  And spot putty.
Legs with self-etch primer applied

 

 

 

 

 

 


Edited by Dave Cook, 21 December 2020 - 10:38 PM.

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