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Adding digital setting circles to a Cave GEM

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#1 Dean Norris

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Posted 05 May 2009 - 02:25 AM

Can I add optical encoders to a GEM mount where the right ascension axis does not protrude past the RA gear on the clockdrive? When I added this clockdrive to the mount I barely had enough axis length to add the clockdrive so I'm wondering if it is even possible to add an encoder on this axis? This is a standard Cave mount that I later added a Cave clockdrive to. As you can see in the photo the RA axis is short barely enough to mount the clockdrive. Now I'm wondering about digital setting circles too. Any ideas on how or if this can be done?
Thanks, Dean

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#2 Charlie Hein

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Posted 05 May 2009 - 06:33 AM

A good machinist should be able to add an extension to the end of the RA shaft that would give you enough room to mount the encoder.

#3 tim53

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Posted 05 May 2009 - 09:24 AM

Another option would be to put a DC servo motor with motor encoder in place of the original AC motor. In the Sidereal Technology servo goto kits, the pointing info comes from the motor encoders (though you can build them with shaft encoders on the mount, too).

-Tim.

#4 DAVIDG

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Posted 05 May 2009 - 09:53 AM

You can purchase inexpensive plastic gears or timing belt pulleys and bore them out to fit your shafts or for the RA axis, drill and tap a 1/4" hole in the center to bolt a gear onto the back. Stock Drive Products has many offerings in gears and pulleys. http://www.sdp-si.com/ I use timing belt pulleys and toothed belts with my encoders and have them geared 2:1 for higher resolution.

- Dave

#5 Dean Norris

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Posted 05 May 2009 - 03:07 PM

Thank you gentlemen for your good ideas. I wonder which method would produce the most accurate results with the encoders? I'm assuming that digital setting circles are more accurate than the manual ones? I'm not ruling out setting circles instead of the digital ones.
I included a pic of my mount showing the RA axis in the RA gear. You can see the axis inside the RA gear. A little over 20 years ago a friend and I did tap the RA axis to mount a setting circle. I didn't complete the project at that time because of major life changes. A move, change of a job and becoming a father to name a few which began a hiatus from astronomy for 18 years. Thanks again for taking the time to reply. Dean

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#6 DAVIDG

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Posted 05 May 2009 - 04:11 PM

Analog circles read to about 1/2 a degree and require that your mount be polar aligned. Digital circles system read out to 0.17 degree or less. With 12 bit encoders the resolution is 0.08 degress and by gearing them 2:1, half of this. What is realy nice is that they do not require accurate polar alignment, since you can choose a two star alignment mode. These systems have a data base of objects built in, with a Guide mode which gives you visual feed back on how much to move each axis and in which direction so the selected object is in the eyepiece. You'll go from viewing a handful of objects in an even to many times this. Also you'll start observing objects that you won't have tried for before.
I have also seen all the planets out to Saturn and many bright stars in the daytime using my digital circles.

- Dave

#7 Dean Norris

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Posted 05 May 2009 - 04:47 PM

Thanks Dave for your reply. It sure looks like the encoders are the way to go. I'm not sure of the assembly for this RA axis encoder etc.. It will require some figuring out on my part. If I have any questions I'll post them here. Dean

#8 Lynnblac

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Posted 06 May 2009 - 12:15 AM

Hi Dean, Take a look at the instruction manuals for the retro fit DSC kits on the JMI website. You will get some ideas from this. Just get a longer R.A. shaft if you need one, it is held in by a couple of set screws. Avoid machining cost, to many $$$. You can not use motors with encoders built in because you do not have a motor on the dec axis. Most likely you will end up using pulleys and toothed belts. Lynn

#9 Dean Norris

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Posted 06 May 2009 - 12:56 AM

I will check it out JMI's website. Thanks for the tip Lynn.
Dean

#10 Tom and Beth

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Posted 06 May 2009 - 09:18 AM

Hi Dean,

I have the same mount, but sold off the old drive in favor of an 8 inch Byers.

Anyway. Someone already mentioned the Servo Drive upgrade
http://www.bbastrode...nouncement.html
and there's quite a few people who have already done Cave Astrola Mounts.

Or you could use a belt drive system off of the two axis to turn an encoder. This also gives you a way to gear up the resolution.

Bear in mind that I'm still working with analog setting circles on my mount....so just passing on what limited thoughts I've had.

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#11 rwiederrich

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Posted 06 May 2009 - 02:34 PM

Great ideas...but I was looking at it another way.

Could you not assemble a means to read the RA axis from the outside via an encoder afixed to the body of the RA assembly and let it run agains the body of the DEC aspect of the RA axis?

Simply put...where the RA axis rotates against the RA body that bolts to the pier...there at that rotation point...could you not afix an encoder to read the RA axis there instead of at the end of the shaft?

Rob

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#12 Dean Norris

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Posted 06 May 2009 - 10:33 PM

Tom, you were able to mount the Byers clockdrive with a short shaft on the RA axis? I would consider upgrading my clockdrive if I could use a Byers on my mount. I bet the Byers tracks real well being 8 inch gear. My clockdrive has a 4 inch gear.
Rob, Thanks for your sketch and your interesting idea I'll have to investigate further to see if your idea would work. I think it's great that your thinking outside the box with your creative idea.
Dean

#13 Tom and Beth

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Posted 06 May 2009 - 11:52 PM

The Byers has a 2 inch bore, and so the adapter was machined to slip over the 1.5 inch shaft and then drive gear over that. IIRC it added an inch or so.

Since this pic I've added a 7 inch Mathis gear to the Dec Axis, but haven't done the motor yet....always something taking priority over "Frankenmount"

BTW, servo motors and their built in encoder REALLY would make a mount like this a poor man goto.

#14 rwiederrich

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Posted 07 May 2009 - 04:14 PM

Tom, you were able to mount the Byers clockdrive with a short shaft on the RA axis? I would consider upgrading my clockdrive if I could use a Byers on my mount. I bet the Byers tracks real well being 8 inch gear. My clockdrive has a 4 inch gear.
Rob, Thanks for your sketch and your interesting idea I'll have to investigate further to see if your idea would work. I think it's great that your thinking outside the box with your creative idea.
Dean



Well Dean...the housing of both portions of the RA body rotate against each other(well the aft end that attaches to the pier head doesn't)...but you know what I mean.

If the forward portion rotates around the aft portion...then there must be a way to encode that movement. Its the same movement along the entire RA axis.

I see a simple encoder that reads the motion of the forward RA/DEC axis. when the forward portion rotates an encoder can read its motion by a rubber wheel attached to the encoder motor...riding on the surface of the rotating forward portion.

If not that..then some kind of belt system can be devised.

Look at its motion and think of a slick way to get the job done. It doesn't have to involve replacing the RA axis or welding or bolting an extension to it.

Good luck with it.
I'll be watching to see what you come up with. :grin:

Rob

#15 Charlie Hein

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Posted 08 May 2009 - 06:26 AM

Could you not assemble a means to read the RA axis from the outside via an encoder afixed to the body of the RA assembly and let it run agains the body of the DEC aspect of the RA axis?


Sure looks do-able to me.

#16 rwiederrich

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Posted 08 May 2009 - 01:40 PM

Could you not assemble a means to read the RA axis from the outside via an encoder afixed to the body of the RA assembly and let it run agains the body of the DEC aspect of the RA axis?


Sure looks do-able to me.


Indeed..since the linear aspects of both section of the RA axis can be accessed...and that they are relative to one another at either end....why not be *Nifty* and not have to rework your entire mount for a *typical* encoder location.

The encoder registers motion and that motion can be calculated into what ever it needs to be calculated into.

I'm no expert on encoders...but two moving surfaces(Or one moving around another) can be encoded. :grin:

Now for the application and engineering feat of accomplishing it. :smirk:

It isn't that hard really.

Rob






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