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CG-4 mount

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#1 seiffmann

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Posted 03 March 2021 - 10:52 PM

Hi all.  I'm hoping someone can help me figure out how the declination works on my, new to me, CG-4 mount.  Just got it a few days ago from the local astronomy club.  The declination has two pointers or arrows above the degree ring.  One arrow is cast and the other one has a metal tape piece with an arrow.  Both are pointing down to the ring.  The ring goes up to 90 degrees and repeats 4 times. 
When I have one arrow set at 90 degrees the other one is at 82 degrees or so.  I've looked both online and
in the owners manual.  Nothing explains why this is so.  New to the hobby and would like any suggestions or explanations.  Thanks.



#2 rblackadar

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Posted 03 March 2021 - 11:33 PM

I don't know why you have two markers, you only need one. The correct marker should point to 90 when the scope is lined up with the R/A axis; that's the declination of true North.  When you rotate on the Dec axis so that the scope is pointed toward the E or to the W, the reading should be 0. (That's why there are two zeroes.) If you go beyond zero in either direction, you are pointed to a negative declination, i.e. south of the celestial equator. (Which of course is perfectly OK to do even if you live in the northern hemisphere! Rigel and Sirius are in the south, for example. There'll be a limit to how far south you can go, though.)


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#3 rblackadar

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Posted 04 March 2021 - 12:33 AM

Just some more info, just in case my previous reply wasn't complete enough for you.

 

Declination is like latitude on the Earth, it goes from zero at the (celestial) equator, up to 90 at the North Pole. Or, going southward, zero to -90 at the South Pole. You should imagine there are minus signs on all the numbers to the south.

 

That accounts for two of the four zero-to-ninety quadrants on your setting circle, and if it weren't for the peculiar design of the equatorial mount, that would be all you need. But as you will discover when you use the scope, if you follow a star as it travels across the sky from east to west, when it gets to the meridian, your counterweight shaft will start to go above horizontal and the scope may hit the tripod, etc... you have to do what is called a meridian flip, where you rotate the shaft to the opposite side (west) and flip the scope over in Dec as well. When you've done that successfully, the pointer should be at the same Dec (the star has only one Dec coordinate) but it will be pointing to the other side of the setting circle, i.e. to the duplicate range. That is, you need numbers on both the E side and the W side -- they are duplicates.

 

It's a bit of a challenge to learn how it all works, but I think it's worth it; the EQ mount gets you really in tune with how everything moves in the sky. But don't get too hung up on nitty gritty details -- get set up, get pointed (at least approximately) to north, and start observing! If you can identify constellations pretty well, you really don't need the setting circles anyway; I haven't looked at mine in a couple years.

 

And welcome to CN. It's literally cloudy tonight, which is why I'm here rather than out observing.


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#4 Redbetter

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Posted 04 March 2021 - 12:47 AM

The ring position for 90 seems to match the one with the black arrow on a shiny metal plate.  I hadn't really noticed the other on the opposite side before; because while it is cast, it is not painted.  I am guessing the second one is vestigial from some prior incarnation of the mechanism.  The ~8 degree offset matches the offset angle of the slow motion drive/gear drive holder position of the saddle.  


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#5 seiffmann

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Posted 04 March 2021 - 09:24 AM

Thanks for the reply's to my question(s).  I do wish that Celestron would respond to my emails and put an

end to it tho.  I believe that using the one marker is plenty and that star hopping would be the way to go.

I'll just get out there and start using it and get some experience using it.

Again, thanks for responding.

S



#6 radiofm74

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Posted 04 March 2021 - 01:30 PM

I confirm: don't even look at the cast, white painted one. The one you're supposed to look at is the black arrow. 

 

As for the pros and cons of using circles, you'll find plenty of differing opinions around here. The hard fact is that the circles on the mount are too small to be really precise to a tee. On the other hand, if you have a wide enough FoV at the eyepiece, you don't need them to be very precise. 

 

I use them – most of the times successfully – when star-hopping is not practical. Living in a light-polluted area where many of your stepping stones are not visible in the finder, that can happen. So learning how to use them is not redundant in my view (and not too hard either). 


Edited by radiofm74, 04 March 2021 - 01:32 PM.

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#7 dmgriff

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Posted 04 March 2021 - 05:28 PM

A quick trick for getting the ota in the ball park, so to say, is to set the declination and lock the axis. Sweep through the right ascension axis.

 

Good viewing,

 

Dave


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#8 PeterAB

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Posted 05 March 2021 - 06:31 PM

Hi,

 

The reason why the declination pointers are the way they are is because of a design/manufacturing error.      Both of the pointers should point to the same number.     My CG-4 has the same fault.     The pointers are 9 degrees off.     I just use the pointer that it stuck on the worm gear assembly and ignore the cast in pointer on the other side.

 

I don't use the setting circles very often.    They are too small to be accurate.    Sometimes they do help me get a reference star in my finder scope field.     I wouldn't expect to center an object in the eyepiece.

 

Peter


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