Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Jumped on a Cave

  • Please log in to reply
162 replies to this topic

#151 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 15 January 2019 - 07:44 PM

My 10" f/6.8 jittered the first time I put it all back together after taking it all apart to clean, lube and paint it.  I found a loose bolt holding the Dec housing to the polar axis and tightened it and then hit the wrench with a rubber mallet a few times.  You may have a loose screw somewhere.  wink.gif

 

The number of possible failures in Caves mount is limited.  My tube is 8" longer than yours and damps out after ringinginging.  You have to get all the backlash out and adjust the clutches.

 

If you swapped out the tube elements (focuser, spider, mirror cell, etc.) then it's a good chance your tube is not correctly balanced by that tube weight.

Someone had changed everything before i bought it as he gave me the old tube and Cave parts.  The mirror is Cervit and was refigured by Galaxy in 1988. The scope has a Novak spider and mirror cell , the focuser is the stock unit that Cave used and 2ndary is smaller than the stock version. Tube has great balance as it is. Guess it is really not a Cave anymore other than the focuser, rotating rings and finder.  Mirror is insane good and worth having a much better mount and will try to find a used non GO-TO AP 900 or 1200.


Edited by CHASLX200, 15 January 2019 - 07:47 PM.


#152 KentTolley

KentTolley

    Viking 1

  • *****
  • Posts: 523
  • Joined: 26 Mar 2012
  • Loc: Southern California

Posted 16 January 2019 - 10:19 AM

You could build a second Cave scope with the extra parts you have if you can fix the mount.  All you need is a Cave mirror.  



#153 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 16 January 2019 - 06:50 PM

You could build a second Cave scope with the extra parts you have if you can fix the mount.  All you need is a Cave mirror.  

They sold in my mega sale last week.  The buyer had a mirror laying around that worked out right for the parts and tube.



#154 photoracer18

photoracer18

    Mercury-Atlas

  • *****
  • Posts: 2559
  • Joined: 02 Sep 2013
  • Loc: Martinsburg, WV

Posted 16 January 2019 - 07:01 PM

One look at the Double double told me all i need to know.  I got a Zambuto in a classic tube.  The not so good, mount has lots of slop. The Sky Mirco won't go out far enough to focus many of my eyepieces.  If i remember there could be a slip in draw tube and will check when it is day time.

 

Scope is 95% collimated and the moon and Saturn looked good in the soup.  Ring and M13 were nice with great contrast.  Double double was just clean and sharp like in my Mak.

 

Scope will stay as it is, but the green is going and the mount will get a good working over. Finder is like new for being 42 years old. Odd it don't have a cross hair eyepiece and it is .965OD".    Not sure what i will do with the extra OTA.  Tube has some damage on the end, but would be fine for a F/5.5 or faster optic.

 

More pics of mirror specs and scope to come later.

Sky Micro and Sky Micro Giant have built in helical focusing draw tubes, not slip in ones.


  • tim53 likes this

#155 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 16 January 2019 - 08:04 PM

Sky Micro and Sky Micro Giant have built in helical focusing draw tubes, not slip in ones.

Mine has a slip in that pulls out of the threaded tube.  I can take a close up pic if you like. There is someone selling a 10" F/7 Cave with the same focuser on here and you can see a close up of the black draw tube that i am talking about.


Edited by CHASLX200, 17 January 2019 - 06:20 AM.


#156 Geo31

Geo31

    Voyager 1

  • *****
  • Posts: 11150
  • Joined: 28 Jan 2013
  • Loc: Willis, TX

Posted 17 January 2019 - 07:18 AM

Sky Micro and Sky Micro Giant have built in helical focusing draw tubes, not slip in ones.

The Sky Micro does have a tube that slips inside the helical tube.  The Giant does not however.


  • tim53 and TOM KIEHL like this

#157 Geo31

Geo31

    Voyager 1

  • *****
  • Posts: 11150
  • Joined: 28 Jan 2013
  • Loc: Willis, TX

Posted 17 January 2019 - 10:53 AM

Chas, if you decide to replace that Sky Micro, please let me know.  I'll buy it from you.



#158 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 17 January 2019 - 06:43 PM

Chas, if you decide to replace that Sky Micro, please let me know.  I'll buy it from you.

I will let you know.  I kinda like it and had them before.  I don't plan to make any changes to the OTA.  The mount is another story.  Even a Meade RG mount would be a big step up.



#159 Geo31

Geo31

    Voyager 1

  • *****
  • Posts: 11150
  • Joined: 28 Jan 2013
  • Loc: Willis, TX

Posted 17 January 2019 - 07:31 PM

I will let you know.  I kinda like it and had them before.  I don't plan to make any changes to the OTA.  The mount is another story.  Even a Meade RG mount would be a big step up.

OK, cool.  You had mentioned replacing it.  Only reason I responded.



#160 dhferguson

dhferguson

    Explorer 1

  • *****
  • Posts: 52
  • Joined: 21 Sep 2018

Posted 11 April 2019 - 01:47 AM

Cheers,

 

'Thought I'd pass something along for the OP. I just finished replacing the Cave drives with Byers drives on my Astrola mount with 1.5" axis. I want to discuss my findings related to the pier-mount attach point in some detail as I suspect this may be your problem.

 

Originally, the mount was mated to a casting that fit on the pier, let's call this casting "the cap." The top portion of the cap consists of two parallel vertical plates drilled through for a 3/4" bolt. The interior spacing between the two vertical plates is 7/8". The RA casting has a tab on the bottom that fits between the two vertical plates that is also drilled for a 3/4" bolt. The thickness of this tab is coincidentally 3/4". That means, in the absence of spacers, that there is a 1/8" void when the tab is mounted to the cap with the through bolt.

 

Cave supplied two spacers to take up this void, one on either side of the cap. One of these appears to be a relatively thin washer of teflon. The other, long destroyed on my mount, was if memory served a thin washer of bakelite. Together, they filled the void. Note, however, that both of these materials are relatively soft. This has two important ramifications. First, they have a certain springiness when the bolt is tightened. This will allow vibration of a certain small but definitely detectable amplitude. Second, they both can be damaged (and mine were) by the two retraining machine screws just above the 3/4" bolt on each of the two vertical plates.

 

OK, what not to do. First, DO NOT torque that 3/4" bolt tight without some form of spacers to fill the gap. You may stress the casting and crack one or both of the vertical plates. What if you skip the spacers but don't tighten the bolt much? In that case, you will experience a large-amplitude, relatively slow (~1 s) vibration that will take 8-10s to settle. Is this what you saw? Consider the frequency f of a pendulum of length x with gravitational acceleration g: f = 1/(2*pi) * SQRT(g/x). We have g = 980 cm/s^2 and x (the vertical height from the bolt center to the mount center of mass, which is about where the "tee" of the mount is) ~ 10 cm. We then have f ~ 1.6 s. Is this about what you saw?

 

I fixed this problem in combination with the latitude adjust issue. I made a 2" wide by 3/32" thick by 3/4" hole washer using a large, old washer buried in my spares collection but drilled out on a drill press (a drill press is mandatory to do this) using a spade bit. Yeah, I know you machinists out there will object but it worked. Incidentally, the standard 1/8" thick washer available in stores was too thick for me to slip in, hence the need for a 3/32' washer (it needn't be round on the outside). Then I attached an eye bolt to the RA housing plate and another to the pier, and connected these two with a small, stainless turnbuckle. I arranged the geometry of the turnbuckle to be parallel to the RA plate so as not to put any "push/pull" stress on the relatively small 8-32" machine screws that Cave used to bolt the RA plate to the RA axis casting (the one with the tab). I ca now both adjust the latitude easily and smoothly  while eliminating the side-to-side play of the RA axis between the vertical plates. The two set screws on each vertical plate are now superfluous. Incidentally, my time-to-settle dropped from 3 s to 2.5 s (at high power) from the original "soft" washers Cave supplied.

 

I sense your frustration, and I hope this solves your problem. Really, properly tuned, the old Cave 1.5" mounts are quite wonderful and very stable.

 

Happy observing always,

 

Don


  • KentTolley likes this

#161 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 11 April 2019 - 05:11 AM

Cheers,

 

'Thought I'd pass something along for the OP. I just finished replacing the Cave drives with Byers drives on my Astrola mount with 1.5" axis. I want to discuss my findings related to the pier-mount attach point in some detail as I suspect this may be your problem.

 

Originally, the mount was mated to a casting that fit on the pier, let's call this casting "the cap." The top portion of the cap consists of two parallel vertical plates drilled through for a 3/4" bolt. The interior spacing between the two vertical plates is 7/8". The RA casting has a tab on the bottom that fits between the two vertical plates that is also drilled for a 3/4" bolt. The thickness of this tab is coincidentally 3/4". That means, in the absence of spacers, that there is a 1/8" void when the tab is mounted to the cap with the through bolt.

 

Cave supplied two spacers to take up this void, one on either side of the cap. One of these appears to be a relatively thin washer of teflon. The other, long destroyed on my mount, was if memory served a thin washer of bakelite. Together, they filled the void. Note, however, that both of these materials are relatively soft. This has two important ramifications. First, they have a certain springiness when the bolt is tightened. This will allow vibration of a certain small but definitely detectable amplitude. Second, they both can be damaged (and mine were) by the two retraining machine screws just above the 3/4" bolt on each of the two vertical plates.

 

OK, what not to do. First, DO NOT torque that 3/4" bolt tight without some form of spacers to fill the gap. You may stress the casting and crack one or both of the vertical plates. What if you skip the spacers but don't tighten the bolt much? In that case, you will experience a large-amplitude, relatively slow (~1 s) vibration that will take 8-10s to settle. Is this what you saw? Consider the frequency f of a pendulum of length x with gravitational acceleration g: f = 1/(2*pi) * SQRT(g/x). We have g = 980 cm/s^2 and x (the vertical height from the bolt center to the mount center of mass, which is about where the "tee" of the mount is) ~ 10 cm. We then have f ~ 1.6 s. Is this about what you saw?

 

I fixed this problem in combination with the latitude adjust issue. I made a 2" wide by 3/32" thick by 3/4" hole washer using a large, old washer buried in my spares collection but drilled out on a drill press (a drill press is mandatory to do this) using a spade bit. Yeah, I know you machinists out there will object but it worked. Incidentally, the standard 1/8" thick washer available in stores was too thick for me to slip in, hence the need for a 3/32' washer (it needn't be round on the outside). Then I attached an eye bolt to the RA housing plate and another to the pier, and connected these two with a small, stainless turnbuckle. I arranged the geometry of the turnbuckle to be parallel to the RA plate so as not to put any "push/pull" stress on the relatively small 8-32" machine screws that Cave used to bolt the RA plate to the RA axis casting (the one with the tab). I ca now both adjust the latitude easily and smoothly  while eliminating the side-to-side play of the RA axis between the vertical plates. The two set screws on each vertical plate are now superfluous. Incidentally, my time-to-settle dropped from 3 s to 2.5 s (at high power) from the original "soft" washers Cave supplied.

 

I sense your frustration, and I hope this solves your problem. Really, properly tuned, the old Cave 1.5" mounts are quite wonderful and very stable.

 

Happy observing always,

 

Don

The Cave mount will be moth balled or offer it to someone that needs parts. I had it out just now looking at Jupiter and even with a light weight 826 8" F/6 OTA the shakes are unreal.  I have a much better mount coming.  I will check into the spacers, as i don't remember my other mounts having them.


Edited by CHASLX200, 11 April 2019 - 05:18 AM.


#162 PiSigma

PiSigma

    Vanguard

  • *****
  • Posts: 2468
  • Joined: 14 Oct 2009
  • Loc: Indianapolis

Posted 11 April 2019 - 09:52 PM

On the 1.5" diameter shaft Cave mounts I've never seen spacers between the pier cap and RA housing. Yes, the 3/4" bolt should not be tightened much but the elevation is always held with set screws in the pier cap not the clamping force of the 3/4" bolt. Sometimes there is just one set screw on each side but usually there are two.


  • KentTolley likes this

#163 CHASLX200

CHASLX200

    Hubble

  • *****
  • topic starter
  • Posts: 18011
  • Joined: 29 Sep 2007
  • Loc: Tampa area Florida

Posted 12 April 2019 - 05:11 AM

I just have big washers on both sides.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics