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palerider
member
Reged: 06/19/08
Posts: 69
Loc: Rancho Cucamonga, CA
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I have a couple of questions about the index marks for the RA and Dec axes on my CG-5. First, I assume they provide a starting or reference point for aligning the mount since the book says to make sure these two indexes are lined up before you start the Alignment procedures. Or, are they something more? Second, I've read quite a few comments that these indexes can be off by several degrees. How important is their accuracy and is there a simple way to check to see if they are correct?
I've only had the mount for about a month and already the mylar tape (or whatever it is) is beginning to peel off the mount (could ya get any cheaper than that, Celestron?). I've taped them down for now. I also made a small alignment mark on the mount for each of them in case they actually fall off. But, there's gotta be a better way.
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jrcrilly
Refractor wienie again
Reged: 04/30/03
Posts: 22342
Loc: NE Ohio
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Quote:
How important is their accuracy and is there a simple way to check to see if they are correct?
What's important is to always use the same initial position. The system software monitors the user's adjustments to the first alignment star and fudges that in for subsequent alignments. Thus, no position is better than any other - so long as it's consistent. Wherever the marks are is fine if you use them every time.
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John C
Urban Observatory
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Fred1
scholastic sledgehammer
Reged: 09/19/07
Posts: 767
Loc: Garnet Valley, Pennsylvania
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For my CG5 I used 2 strips of yellow electrical tape 1/2" wide across the axis motor housings roughly where the index markers are. I then slit the tape with a razor along the crease in the tape where the motor housings meet. The yellow tape is much easier to see and align than the stock index markers are.
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Fred
Celestron 8" SCT w/Starizona Feathertouch Microfocuser and Orion 9x50 RACI FinderScope;
Stellarvue Nighthawk Aplanat w/Orion 6x30 RACI Finderscope
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palerider
member
Reged: 06/19/08
Posts: 69
Loc: Rancho Cucamonga, CA
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Thanks for the info guys. I appreciate it.
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EWhytsell
super member
Reged: 06/15/08
Posts: 150
Loc: North Central Ohio
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If you still have the marks in place you could always hit it with a sharp chisel. I've been meaning to do that to mine because your right the stickers are pathetic. A punch mark like that would be very easy to feel in the dark to if you needed to realign.
Evan
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Classic C8 Super Polaris upgraded to CG-5 mount
Canon 40D unmodded
Edited by EWhytsell (08/18/08 09:52 PM)
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palerider
member
Reged: 06/19/08
Posts: 69
Loc: Rancho Cucamonga, CA
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Quote:
If you still have the marks in place you could always hit it with a sharp chisel. I've been meaning to do that to mine because your right the stickers are pathetic. A punch mark like that would be very easy to feel in the dark to if you needed to realign.
Evan
I thought about doing something like that or some other way to make permanent index marks on the housings. I didn't out of fear that I might crack the housings if I gave them a whack with a hammer and chisel. Aren't they made of cast aluminum or something similar? Tell you what, you do it first and let me know how it works out. Laffs.
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 1950
Loc: Massachusetts
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To be able point toward the initial aligment stars, the scope needs to know where it is (lat, lon), the time, and what direction the scope is pointed. The index marks set the scope to a known pointing direction.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED refractor
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WadeH237
sage
Reged: 02/24/07
Posts: 326
Loc: Snohomish, WA
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It's my guess that the marks serve two purposes:
1. So that the mount knows approximately how to point to the alignment stars.
2. So that the mount knows when the RA axis is horizontal with respect to the tripod so that it will stop tracking at the meridian and not risk running the OTA into a tripod leg.
I doubt that the index marks affect pointing accuracy at all after the alignment and calibration stars are set. I also suspect that it doesn't matter if the marks are off by a few degrees (other than the above conveniences.)
These are just suspicions on my part. It would probably be easy to test by starting out way off of the index marks, but I haven't bothered to try it.
Thanks,
-Wade
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http://www.faintfuzzy.net
Stuff
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Patrick
Postmaster
Reged: 05/16/03
Posts: 6776
Loc: Franklin, Ohio
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Quote:
First, I assume they provide a starting or reference point for aligning the mount since the book says to make sure these two indexes are lined up before you start the Alignment procedures. Or, are they something more?
You are correct that the indexes are designed to put the scope in the 'start' position before going to the first alignment star. Unfortunately, sometimes the index marks are in the wrong position and therefore the start position is wrong. Regardless of where the index marks are located, the start position is with the OTA pointing towards Polaris and the counterweight shaft pointing straight down as you look at it from the front. That's how you can check to see if the index marks are in the right place or not.
When I first got my scope I was very concerned about the index marks, but I never use them any more. I just put the scope in the start position by eye. As John noted, the start position is not that important...it's just used to get to the first alignment star. By the time you go to your second alignment star plus a couple of calibration stars, the computer should have captured the info it needs to give you good goto's.
Patrick
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Discovery 10" f/6 Split Tube Dob
Celestron C6 SCT
Denk Binoviewers
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