Author Topic: equatorial mounts confusing !  (Read 131 times)

viogreetnifi

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Re: equatorial mounts confusing !
« Reply #30 on: February 02, 2018, 05:41:50 PM »
Gunny, all scopes have two axes of movement. For your XT8i dob, the base of the scope lets you rotate it 360º in azimuth... and the optical tube can tilt up/down to angle adjust the angle above the horizon.

The equatorial mount is tiled so that the lower axis is parallel to Earth's axis.

So imagine you could take your Dob to the North Pole... at that point, an "alt az" mounted scope effective IS "equatorially mounted" because the main axis of rotation is actually parallel to Earth's axis of rotation (but this only works at the north pole.)

Move 1º away from the North pole, and you could imagine that you tilt the base of the Dob by just 1º to compensate (tilted toward the North pole) and now the scope's axis of rotation is parallel to Earth axis of rotation again.

If we pick a city like Indianapolis... it's roughly 40º North latitude (I rounded to the nearest whole value). 40º north latitude (40º north of the equator) is 50º away from the pole... so if you tilted your dob on a 50º angle (toward the north pole) then it's axis would be parallel to the Earth's axis. Your alt/az dob would be "equatorially mounted" (btw, this is the premise of the equatorial "wedge" that is available for many alt/az mounted scopes).

The magical thing that happens at this angle is that rotating the main (lower) axis (called the "right ascension" axis) will cause the optical tube to move in perfect East/West direction. And tilting the optical tube up and down will move in a perfect North/South direction. So you can think of the Equatorial axis as moving in perfectly orthogonal North/South vs. East/West directions that exactly match the globe.  If you're on an alt/az mount and trying to follow a star as the Earth rotates, you notice you have to nudge the scope toward the west and also nudge the optical tube up or down (depending on if that object has crossed the meridian on the sky yet).

Once you get your head around this orientation ... it's easier to use.

For your equatorial mount, there's a scale on it that indicates the latitude angle... you set that angle to match YOUR latitude on Earth. E.g. if you live 40º north latitude, then you adjust that angle to 40º (the scale is already reversed so you don't have to do the math of subtracting your latitude from the North pole).

The mount needs to be pointed north (for northern hemisphere observers). Basically if you could imagine drilling a hole through the center of the RA axis... you'd see the north star.

Your Equatorial mount probably also has "setting circles". Those are the scales you see on the RA axis and Declination axis.

The declination axis is the easier one... it goes from -90 (scope is pointed at the south pole) to +90 (scope is pointed at the north pole) to everywhere in between. If it reads 0º then it's pointing to stars that are straight out above the equator.

But the RA axis is confusing... first there are TWO scales, not one... one for northern hemisphere and one for southern hemisphere. Secondly, that setting circle is meant to be rotated. The scales are usually not high quality (unless you buy a fairly high-end mount) and I find them horrible to use.

My advice... don't use them.  But "in theory" you could point to a well-known star then rotate the RA setting circle to the RA coordinate of that known star. Now when you want to find an object and you don't know where it is... you can look up it's RA value and (with the setting circle locked) move the scope in RA to the RA value of the object you want to observe. Then move the declination to the Dec value of the object. The object should then be in your field of view.

But what I find is these scales are small, don't have fine enough graduations, and worse... they often slip when they should remain fixed to the scope as you rotate the scope in RA. Using them is frustrating.

BTW, the RA scale does have a "0" point in the sky. The position of the sun at the moment when it crosses from the southern hemisphere into the northern hemisphere during the spring equinox marks that "0" point. It's referred to as the "first point of aries" because back when it was established... it was at the extreme corner of the constellation of Aries. But after a couple of thousand years of precession of the Earth's pole... it now points to a location in Pisces. (it's still called the "First point of Aries" even though it isn't really in Aries.)

Keith Dixon

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Re: equatorial mounts confusing !
« Reply #31 on: February 09, 2018, 02:28:41 AM »
Thanks for convincing me to stick with alt/az mounts!

tricapenup

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Re: equatorial mounts confusing !
« Reply #32 on: February 09, 2018, 04:58:36 AM »
Quote
Thanks for convincing me to stick with alt/az mounts!

Come on over to the dark side penguin

You will love high power tracking, and you can leave your scope for fifteen minutes, come back and just
turn one knob to re-aquire your target

Nothing beats a nice alt-az for just sweeping around the sky.

Gabriel Green

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Re: equatorial mounts confusing !
« Reply #33 on: February 09, 2018, 10:23:53 AM »
Wow. I've been using GEMs forever and never found them complicated in terms of motion, setup, polar alignment, tracking, or go-to.

But all this talk of learning to use the manual setting circles... ouch. All these years and I've never tried them!