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Anyone have a 12.5 Cave astrola with Guidescope?

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#1 dgreyson

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Posted 11 December 2012 - 11:10 AM

I am restoring a 12.5 Cave-Astrola and someone had taken off the guidescope, finder and counterbalance and cut down the tube to make it an f 4.5. Likely it was an F5 or 6 originally. This lost all the original hole positions and just to make it interesting, they seemed to randomly drill various and sundry holes in it and painted it smurf blue.

I am in the middle of trying to bring it back to a little more of an original configuration. I have the Finder, Guide scope, focuser and weight adjustment rod.

Using a protractor, dividers and Calculus or suchlike, I could probabubly figgure out the spacing of all four items from exsisting partial side photos. but it would be much easier if I could see a picture from the front of the scope showing all of them in relation. That way, I could just use a protractor and get it pretty close to original.

Could someone please post a photo looking straight at the back of the spider / down the mouth of your scope showing what the angular positions of the finder, guidescope and counter balance are to the focuser? thanks.

Everyone just shows side photos or the back of the mirror cell. And the ones who do happen to show the front end dont have a guide scope and a counterbalance or they stuck the guide scope on the top of the rotating ring cage, which is no help.

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#2 Datapanic

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Posted 11 December 2012 - 01:47 PM

It's best to do the math and get it right instead of eyeballing it or using another setup as an example because the weights of the finder, focuser, guidescope and tubeweights and their distances from the optical center are probably going to be different and you'll end up with a tube that looks nice but is not balanced and not fun to use. Also, you have to consider the weight and balance of the entire OTA along the longitudinal axis, including the mirror, all the above and even the tube.

With the scope completely balanced, you only need minimal friction on the clutches on both axis so that the motors will move the tube and the OTA itself can be manually pushed in any direction with hardly any force. Otherwise, you'll end up constantly adjusting counterweight and tube weight everytime its pointed to a different area of the sky.

The math for balancing the tube can be found in the November 1958 Sky and Telescope "Gleanings For ATM's".

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#3 tim53

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Posted 11 December 2012 - 01:54 PM

Looks like your "calculator" also comes from the 1958 S&T! :grin:

#4 Ducky62

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Posted 11 December 2012 - 02:00 PM

:funny: :funny: :funny:

Looks like your "calculator" also comes from the 1958 S&T! :grin:


:roflmao: :roflmao: :roflmao:

#5 gelkin

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Posted 11 December 2012 - 04:36 PM

:funny: :funny: :funny:

Looks like your "calculator" also comes from the 1958 S&T! :grin:


:roflmao: :roflmao: :roflmao:

You guys should know by now, :4 only tools from the period of the scope being restored may be used during restoration. :rules: :roflmao:

#6 dgreyson

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Posted 11 December 2012 - 04:36 PM

Doggone! I didnt know how to read in 58 and anyway, back in the day, I couldnt leave the back yard to go get a copy of S&T even if I knew what money was. No wonder that slipped by me! Thanks for the head's up!

what I figured to do was adjust the place where the counter balance mounted a little, to balance out the other three items.

#7 dgreyson

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Posted 11 December 2012 - 04:48 PM

Hmmm, here is the gist of it:

In any equatorially mounted telescope there are three axes about which
moments have to be determined. It is important which order these are
considered. They are as follows:
i) radial equilibrium about the tube axis
ii) longitudinal equilibrium about the declination axis
iii) declination equilibrium about the hour axis

If the telescope tube is not in radial equilibrium it will not be possible
to adjust the system cg to the intersection of the declination and hour
axies, for all possible attitudes of the tube assembly. Radial balance is
the most awkward to achieve, and is crucial. Radial out-of-balance is the
most common cause of the complaint, "My telescope will not balance in all
positions."

To determine the radial momenta about the tube axis, measure the offset of
all the non-symmetric tube furniture (i.e. rackmount, finder, guide 'scope,
accessory mounting brackets &c). Ignore symmetric items, i.e. the primary
and secondary mirrors & cells, the spider, cradle rings &c. Weigh, or
calculate the weight of each item. Adopt an arbitary reference plane,
transverse to the tube's mechanical axis. A useful reference plane might be
in the direction of gravity when the tube is horizontal, pointing due south.
Call this plane X-X'. Prepare a scale diagram depicting the tube profile,
and each weighed item. Measure or estimate the angular distance of each
item, anti-clockwise, looking down the front of the tube from the reference
plane.

The radial momenta of each item will then be:

Moment = radial offset x weight x cos(angular distance from X-X')

Always measure the angular distance from the same reference, using 360°
notation, in the same anti-clockwise direction. Repeat the procedure using a
reference plane rotated 90° clockwise. Call this plane Y-Y'. Sum the momenta
about each plane.

As an example, I performed the calculation for my 10-inch f/10 Calver, with
the following results:

Moments about X-X'

rackmount = 12" x 7lbsf x cos270° = 0inlbs
lower counterwights = 8".75 x 56.68lbsf x cos270° = 0inlbs
Cooke 2-inch finder = 12" x 7lbsf x cos330° = 73inlbs
mounting bracket + 400mm lens = 9".75 x 11.8lbsf x cos0° = 115inlbs
mounting bracket + 150mm lens = 9".75 x 10.8lbsf x cos180° = -105inlbs
upper counterweight = 8".75 x 22.42lbsf x cos90° = 0inlbs
4-inch Guide 'scope = 10".25 x 7lbsf x cos135° = -51inlbs
Ottway 2-inch finder = 9".25 x 2.625lbsf x cos210° = -21inlbs

Moments about Y-Y'

rackmount = 12" x 7lbsf x cos0° = 84inlbs
lower counterwights = 8".75 x 56.68lbsf x cos0° = 496inlbs
Cooke 2-inch finder = 12" x 7lbsf x cos60° = 42inlbs
mounting bracket + 400mm lens = 9".75 x 11.8lbsf x cos90° = 0inlbs
mounting bracket + 150mm lens = 9".75 x 10.8lbsf x cos270° = -0inlbs
upper counterweight = 8".75 x 22.42lbsf x cos180° = -196inlbs
4-inch Guide 'scope = 10".25 x 7lbsf x cos225° = -51inlbs
Ottway 2-inch finder = 9".25 x 2.625lbsf x cos300° = 12inlbs

Sum of momenta about X-X' = 11inlbs
Sum of momenta about Y-Y' = 387inlbs

To calculate the weight (W) and orientation (A°) of a counterpoise necessary
to bring the tube into radial equilibrium, we must adopt a suitable radial
offset for the counterpoise (e.g. r = 8".75) and solve the following
simultaneous equation:

i) 8".75 x W x cosA + 11 = 0
ii) 8".75 x W x coa(A+90°) + 397 =0

divide (ii) by (i) (cos(A+90) = -sinA)

hence 8.75.W.sinA/8.75.W.cosA = tanA = 387/-11 = -35.182
(sinA positive - 2nd quadrant)
therefore A = 91°.63

subst. A in ii) hence 9.75.Wcos181°.63 = -387
& solving for W W = 44.25lbsf

To check the calculation, determine the resultant sum of the radial momenta:
about X-X' = 8.75x44.25cos91°.63 + 11 = 0inlbs
about Y-Y' = 8.75x44.25cos181°.63 + 387 = 0inlbs

Where the counterpoise is located along the length of the tube is arbitary.
It is its orientation with respect to X-X' which is important. I decided to
split the counterpoise into two equal weights, placed either side of the
dec. axis.

Once you have determined the radial equilibrium, calculating mometa about
the declination and hour axes is a matter of simply weight x axial distance.


Oh, ok. But I still need the starting positions of the finder and guide scopes to the focuser in order to do the calculations. Still need the picture anyway.

#8 JWW

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Posted 11 December 2012 - 04:49 PM

Maybe this will help;

http://www.pbase.com.../image/75800556

http://www.pbase.com.../image/71889754

-JW:

#9 dgreyson

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Posted 11 December 2012 - 05:12 PM

Nah, those are both Sideways views, but thank you. I dont think anyone has ever taken a photo looking at the spider end of a scope with all four components mounted. The best i've see has 3 of the four. Dont remember which of you owns this scope but I want something like this, but with the guide scope also. My best WAG guess is that the guider is the same distance from the focuser as the finder

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#10 Ham Radio

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Posted 11 December 2012 - 07:50 PM

WooHoo I have 3 of those old slip sticks. I still pull them out once in awhile just to see if I remember how to use them!

Had to use them in the Physics class and before we got the NEW technology of TI-30 calculators. We still had to use the slide rules for tests though.

#11 gelkin

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Posted 11 December 2012 - 09:45 PM

How about mounting the guide scope rings to the rotating rings? Taking it off the OTA.

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#12 John Higbee

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Posted 11 December 2012 - 11:13 PM

I have a 12.5" f/6 with guidescope...but the guidescope is mounted on the tube rings, not the tube. Look at the string I started awhile back in this forum (another Cave 12.5" reporting aboard) - it has a "down the throat" shot that may help, as well as a lot of other exterior shots.

If you like, I can measure mounting holes from the front of my tube for you, but as Dan already cautioned, "your mileage may vary" from mine with respect to optimum balance. John

#13 dgreyson

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Posted 11 December 2012 - 11:29 PM

:question: I could but the catalog for my cave shows it mounted up front. I dont think the 60 x 700mm guide scope I have would fit as its pretty short but I have a unitron 128 OTA that should go there. If I cant get a measurement or a picture for the finder to guidescope distance I may have to rethink my plan.

#14 Datapanic

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Posted 13 December 2012 - 10:22 PM

When I ordered my 8" Cave, it was recommended to mount the guidescope on the rings, mainly to avoid the problems of proper balance. On a larger scope (12.5"), just the finder and the focuser with an eyepiece in it is going to need about 6.5 lbs of tube weight. Throw on a guidescope, and it's going to be even more - maybe 10 - 14 lbs!

Anyway, to answer your positioning questions, on average, the angle between the focuser and the finder is usually 45 degrees - depends on the size of the apparatus. I truly believe that Cave eyeballed the assembly of their scopes and when it came to the position of the finder and guidescope, it was a location that worked well ergonomically. Since the Horsetrail Cave needed a new tube, I was free to do this and I physically placed the finder in a position that was comfortable to use without my face being in the focuser.

In JWW's links, the first photo out of the catalog of the 12.5" Cave with the guidescope nearly 180 degrees from the finder - well, that thing would be a nightmare. The guidescope rings Cave made were much shorter so that the weight of the assembly was closer to the center of gravity and thus requiring less tube weight which also affects the amount of weight on the Dec Shaft. The goal was to get the weight as close as possible to the optical center so that the minimum amount of tube weights would be required to balance it. I think some days Cave's employees put together a scope with whatever was laying around in the shop. Of course the earlier models had much better quality than than mid 70's to the end...

Anyway, think about it. Functionality is the goal and Tom Cave probably wouldn't mind if you made the OTA a little better than it was when it was made. Each Cave is a little bit unique and the end result of making it a joy to use is the goal :)

#15 dgreyson

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Posted 13 December 2012 - 10:36 PM

I think youre right Dan, I was getting confused because every cave I looked at looked a little different to me. the focuser to finder and focuser to guidescope distance seemed to vary. I figured it was the sideways camera angle but now it sounds like they really were just all different.

I will tape the guide scope and brackets to the tube in the front position and see where that and the counterweight balances out, if it dosent work then plan B is the top of the rings position for sure.

#16 Datapanic

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Posted 13 December 2012 - 10:53 PM

While you have everything off the tube, it would be good to record how much each piece weights so that you have that info for reference. Balancing the tube is a little difficult, but I've done it on 3 different Cave scopes so far. There is no sweet spot to the balance because eyepieces and cameras all weigh in at different amounts and those values affect the counterbalance. But, there is a zone that provides the most usable area with the least amount of fiddle :)

#17 vanderpol

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Posted 05 January 2013 - 02:57 PM

Greetings,
I measured my 12.5 F6 and quickly sketched the positioning;

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#18 dgreyson

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Posted 05 January 2013 - 07:04 PM

Awesome, exactly what I wanted. Thank you. I am in the process of stripping and then patching extraneous holes in my tube at the moment. So everything is off it and I will weigh the components and perform the radial vector balance equasions mentioned earlier and see where that puts me.

#19 vanderpol

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Posted 07 January 2013 - 12:49 AM

No problem; on thing Steve Dobbs noted is that, at least on mine, the secondary is undersized when used with the long travel rack & pinon. He suggested pushing secondary forward and using a low profile focuser to give more uniform field illumination. You may need extensions as well depending on your optics.

#20 bremms

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Posted 07 January 2013 - 12:06 PM

Can't really push the secondary forward with out moving the focuser. To get enough focus shift for a low profile, the spider would need to go forward as well. Instead of cutting up the tube just put in a slightly larger diagonal

#21 bremms

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Posted 07 January 2013 - 12:28 PM

Is this the one that was for sale in FL? Where are you in SC?
I'm in Columbia and just joined MAC

#22 dgreyson

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Posted 07 January 2013 - 05:01 PM

LOL, I think I saw you at the last meeting as I am also a member of MAC, I live in Lexington. Yes, I drove down to Palm Beach FL and picked it up one weekend. The Scope was mostly complete but was missing the guidescope and one of the finder rings. Someone had apparently removed the guidescope and use it's rings to mount the finder. I have bought all the parts that were missing now and have been waiting for a good day to fix the paint job on it. In photos it looked good but up close it was gouged up and the latex paint was peeling off the oil base undercoats.






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