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Learning about my CG-4

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#1 SporQ


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Posted 12 September 2013 - 04:10 PM


I thought I would post a few questions regarding my Omni XLT CG-4 EQ mount.

Both the DEC and RA axis have pointers that are cast, along with the stick-on DEC pointer and Vernier scale for RA. The cast DEC pointer is about 9 degrees offset from the stick-on arrow and the cast RA pointer is just to the right of the Vernier, by the set screw knob.

Just curious:

1. Why are there two different pointers on each?

2. Why does RA get a Vernier scale and DEC does not?

3. Why does the RA have a knob for locking the scale? It even has a hole to prevent it from moving at all when set near zero.

4. While I understand how to use the RA vernier scale, what is the other scale on the tapered end of the RA scale? It goes to 12 hours and has "E 20 10 0 10 20 W" on it.

Sorry for not providing photos, but it is a work day and that means not much time. Most questions are not important, but I do want to learn about using the second scale for RA.



#2 Eddgie



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Posted 12 September 2013 - 05:02 PM

I can only guess at why they have stickers along with the cast pointers, and that guess is that the stickers are more readable than the cast pointers.

Ok, on to your other questions, with this in mind. You can use either pointer you choose for each axis, but pick one of the other. If you pick the stickers of course that most likely confirms my thought that they are more legible and more accurate.

But here we go.

First the Dec scale. it is fixed because Declination never changes (well except for procession but that is millennially slow to change).

Anyway, 45 degrees north or 45 degrees south is always the same night after night, and if the mount is polar aligned, once the Dec scale has been calibrated, it should never need to be touched again.

And it is in all likelihood, not fixed. If you look carefully, there should be a recessed hex head screw that is used to calibrate it.

RA changes from night to night though as the sky changes seasons, and this scale needs to be able to move.

To use this scale, you would point to a calibration star of known right ascension and then loosen the RA circle and rotate it to line up to the correct hour/minute for the calibration star. Now if you are not accurately polar aligned, as you move away from this position, it becomes less accurate, but now if you have the right ascension for a nearby star, if you point the scope at it, they should match. Or of course if you are trying to locate a faint object, you would point the scope to the RA indicated on the circle.

Why can't the circle be fixed? Well, suppose that you locked it down to a star that is at 6 hours of RA tonight.

Now, leave the mount and come back in six months and point to a star in exactly the same point in the sky and what you will see is that the circle still reads 6 hours of RA. The sky at that is now in that position has moved 12 hours so you are looking at 18 hours rather than 6 hours that your mount shows.

The Declination though will not have changed. If the star was at -20 degrees, it is still at -20 dec.

So that is why you have to be able to move the RA circle. Every night, the position of each object in the sky moves a bit west for the exact same real time vs the sidereal time. You have to be able to sync the RA circle ever night.

Ok, how do you calibrate the DEC.

Well, to do it right, you need to be pretty accurately polar aligned. After you are pretty well aligned, go to a star with a known Dec and use your hex head wrench to loosen the Dec circle, position it so that the pointer is matched to the number on the scale for the Dec of the target, and tighten it. Not it is permanently fixed and should never have to be moved again.

But RA has to be set every night for it to be accurate. Hence, there is a knob.

Last question. The other scale usually plays two roles. First is that it is the opposite of the top scale and that is because the Aussies are upside down.

Don't tell them this. It freaks them out. They are in denial for sure, but facts are facts.

And because of this, they have to point to the bottom celestial pole, and well, that means that not only are they upside down, but they are backwards too.

Its kind of funny. Like that movie where the kid says "They're dead, and they don't know they are dead!"

I have many good Aussie friends though, so I try not to make them feel uncomfortable about it, but none of them ever read these forums..

And there is often a second role for this. Some polar finders have instruction on how to use the polar finder and this scale to do a polar alignment.

I have tried to read the great American Novel "Gravity's Rainbow." It is the densest, hardest to follow written work I have ever encountered with one exception. That exception is the instructions on how to use that scale to make a polar finder work to do a polar alignment.

It is as if Thomas Pynchon wrote them and was just plying with your mind to see how far he could push you until you were reduced to great, sobbing tears.

Ignore it. It will help you preserve your sanity.

And likewise, don't try to read Gravity's Rainbow. I think it is impossible to finish and you may injure yourself trying.

#3 T1R2



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Posted 13 September 2013 - 12:43 AM

You shouldn't have to move the Dec. scale, as long as you point the counterweight shaft straight down in front of the north leg and the ota is parallel with the polar axis when you dial in to 90* then your good to go. If your setting circles and you don't have a polar alignment scope then it will always be a little off, but it will still get you in the general area of your object, and a little slewing around in the area will usually find what your looking for if its able to be detected visually.

ps. use the little sticker arrow on the Dec. gearbox side, that's the one you use.

#4 SporQ


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Posted 13 September 2013 - 08:30 AM

Yes, I understood all this about how to use it. That second scale is what I was wondering about (and still am). Thanks.

The scales that are printed on the shiny surfaces are pretty cheesy. They should be black with white tick marks, just like the Vernier. It is very hard to compare the white and black lines with a red nightlight. A black RA knob with white markings would be a great upgrade.


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