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How a GEM works


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As this has cropped up, I thought a primer type thingy would help, so here goes.

The German Equatorial Mount, or GEM, or EQ mount as we know it is designed to allow a person to carry on viewing an object without having to manually track it. It works like this.

The mount has a tripod which keeps the thing off the floor, nuff said about that.

The mount has two axes or bearings, which are able to rotate to locate an object in the telescope. These are like up and down for one axis and left and right for the other, but not quite. That's the clever bit.

When we look at the sky, all the stars appear to rotate around (for us northern hemisphere dwellers) a point very near the star we call Polaris (that's why it has that name, it's the pole star). So the clever bit with the equatorial mount is that the main axis is aimed directly at the centre of rotation of all the stars, that means that if the mount has a motor that drives the axis pointed at the centre of rotation, it will track anything, anywhere. The reason that an equatorial mount is so ungainly and awkward is that one axis HAS to be pointed at the apparent cantre of rotation of all the sky. The whole sky appears to turn around this axis, which is a reflection on the fact that the apparent rotation of the sky is caused by the Earth rotating around it's axis. If we line up one of the axes with the axis of the Earth,we can arrange for the mount to cancel out the earth rotation thing and the sky appears to be static in the telescope. this can only happen if one axis is lined up in the same direction as the earth's axis and is driven at the same speed, but in the opposite direction.

To get the magical tracking worked out, it is necessary to be able to point the driven axis so that the centre line of the axis is parallel with the rotational axis of the Earth. This is like being on a children's roundabout in the park really. If you look without moving your head or eyes, then everything seems to be moving. If you turn your head or your eyes in the opposite direction to the movement of the roundabout, then you can make everything appear to be static (which it really is). You will need to move your head or eyes at exactly the same rate as the roundabout, but in the opposite direction so that the roundabout's rotation is cancelled out.

Imagine a photographer's tripod set up on the roundabout set for a long exposure, and he wants to photograph something in the distance. He will have to counteract the fact that both he and the camera are rotating, or else the image will be blurred. If he pans the camera to keep the image framed, he is rotating the camera around a vertical axis which is parallel to the axis of the roundabout.

The axis of the roundabout is vertical so the photographer has to pan around a vertical axis to keep the subject framed, likewise we have to pan around the axis of the Earth's rotation. This happens to be a non vertical axis, so the easy way to get the axes parallel is to aim the axis at a point near Polaris (because somebody forgot to put Polaris exactly in the right place).

The polar alignment is crucial in getting the mount axis accuratley lined up with the Earth's axis so that the mount tracks the stars corrctly.

HTH

Captain Chaos

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Thanks for that CC - here's a few questions from someone who understands the GEM in theory but has never actually used a decent one. I daresay the practical experience won't be as cosy as it is on paper. Here goes:

1. Do more expensive GEMs eg EQ5, HEQ5 etc have more accurate setting circles with markedly better bearings etc than cheaper ones, or is it just that they are heavier and more stable?

2. How far from the pole star should the star that you choose for calibrating the mount be?

3. I read somwehere - probably on this forum - that you can only accurately move around the sky in 20 deg shunts so that you have to star hop to get to something a long way from the calibrating star?

4. Do you eventually get completely hacked off with doing things manually then cave in and get a goto scope?

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Thanks for that CC - here's a few questions from someone who understands the GEM in theory but has never actually used a decent one. I daresay the practical experience won't be as cosy as it is on paper. Here goes:

1. Do more expensive GEMs eg EQ5, HEQ5 etc have more accurate setting circles with markedly better bearings etc than cheaper ones, or is it just that they are heavier and more stable?

2. How far from the pole star should the star that you choose for calibrating the mount be?

3. I read somwehere - probably on this forum - that you can only accurately move around the sky in 20 deg shunts so that you have to star hop to get to something a long way from the calibrating star?

4. Do you eventually get completely hacked off with doing things manually then cave in and get a goto scope?

1) I don't do setting circles as the ones on the HEQ5 don't work - at all. All they do is jam up the mount and make it stop working.

2) For polar aligning, 90 degrees gives the biggest possible movement so go for that if you can find a star close to there. If you can find a star in the east, that'll be good. Go for stars on the celestial equator which means 90 degrees from polaris. Basically you remember the path of the sun during the day, that's the celestial equator.

3) 20 degrees sounds like a long way to "hop", you should be able to find a hopping star within less than that angle.

4) I got goto straight off because, even though I think I might be able to star hop now, I didn't know any stars when I ordered the HEQ5 so I have been goto from day one.

HTH

Captain Chaos

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Looking back, I'm so glad that I got goto because I can use goto to aim the 'scope, look down an eyepiece and see nothing. Because of the problems that many of us face, it's not possible for me to see anything more dim and distant than M31 so I could have been much more disappointed than I have been due to clouds. On many occasions I have looked down the 'scope out of interest to see the delights that await, only to be amazed that there's nothing there. Stick a camera on and an image appears, do it loads of times and I get a pretty picture. Maybe I could star hop and find something, but the automation takes away a load of hassle and makes this hobby doable, for me, from where I live.

If you can actually see M33 in your sky, then it's going to be different to my experience. AM in Arizona has all sorts of naked eye stuff that I can barely get an image of, but short of retiring to somewhere like that, I have to deal with fitting my hobby around my everyday life. For me, goto is brilliant, but others have different skies, different kit, different expectations.

Buy what you need to buy to get you to the place where you want to be in your hobby, don't forget the fact that it's a hobby, don't mess up your life by having a hobby that costs more than you can afford and enjoy what you have. Goto is expensive, but a divorce is massively more so.

Captain Chaos

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1) I don't do setting circles as the ones on the HEQ5 don't work - at all. All they do is jam up the mount and make it stop working.

Bit confused by this - I thought the whole point of an EQ mount was to find things by their RA/DEC coords -

if the setting circles don't work what's the point?

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Hi TBF. I could be wrong, but I think the point of a EQ mount is to be able to follow objects through the sky with the movement of only one axis. The whole point of the marked setting circles is to enable us to find things by their RA/DEC co-ords. Hope that helps matey :)

I s'pose one day I really should learn how to use the setting circles on my mount - just ain't got round to it yet. Well it has only been 2 years :lol:

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The whole point of the marked setting circles is to enable us to find things by their RA/DEC co-ords.

I coud've sworn I just said that? :?

I know what an EQ mount is for, its just CC's comment about settings circles has thrown me - why should the setting circles jam up the mount - I thought the whole point of them was to tell you what coords you are looking at?

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Me and fine lines eh Arthur? (Mr)

I do post some carp sometimes don't I? :lol: It's just that I have seen pics on the WWW of some old and some home made EQ mounts - lots o them didn't have setting circles. They are included on most(all?) mounts nowadays - but when you read the specs it's like setting circles have been added as an extra feature for us. Their use is not compulsary though is it? Not the whole point then - heh, which is unfortunately how it read to me :)

Sorry folks, will try and keep up in future.

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To sum up, are we saying that to get a decent EQ mount that actually does the job it was designed for,

i.e. locate objects by using their RA/DEC coords, you need to buy something a lot better than a HEQ5.

If this is true, its a bit of a blow as I thought a HEQ5 was a good quality mount.

Does anybody make a decent mount that won't break the bank?

PS

Hate to bang on SteelRat but you said this

"The whole point of the marked setting circles is to enable us to find things by their RA/DEC co-ords."

immediately after I said this

"Bit confused by this - I thought the whole point of an EQ mount was to find things by their RA/DEC coords"

The point is setting circles, eq mount, RA/DEC - they're all next to each other in my brain so semantically the two sentences are equivalent from my perspective. Here endeth the Cognitive Psychology lecture!!!

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Just fired up the PC for a quick 'fix' of SGL and found this thread....

I know someone who regularly uses the setting circles on his HEQ5 to place an object within a low-medium mag' eyepiece. Often, when I observe with him and we struggle to find a fuzzy using charts, he will-fire up Starry Night for the coordinates then find it using the setting circles.

Granted, the bigger the circles are, the easier they are to read but don't be put off using whatever you have got - before he had an HEQ5, he was using the circles on his EQ3, blissfully unaware that they were 'unsuitable'....

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Hate to bang on SteelRat but

Hi TBF :)

Like I said; 'I DO post some carp (followed if you hadn't noticed by 'sorry'). Just cos my body is letting me down and I have to walk with a stick, my brain of late seems to think it can behave the same way as the rest of me :)

Just ignore me matey... it works for ALOT of people :lol:

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Hate to bang on SteelRat but

Hi TBF :)

Like I said; 'I DO post some carp (followed if you hadn't noticed by 'sorry'). Just cos my body is letting me down and I have to walk with a stick, my brain of late seems to think it can behave the same way as the rest of me :)

Just ignore me matey... it works for ALOT of people :lol:

No worries. Pour yourself another brandy then have yourself equatorially mounted - this should compensate for the rotation of the living room :)

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Pour yourself another brandy then have yourself equatorially mounted - this should compensate for the rotation of the living room :)

Ahem, think I'd better have a word with the wife :lol: or is that straying off topic - sorry CC

By all accounts though TBF, the HEQ5 is a stonking mount. Am sure you'll get what you want from it. Lets have some clear weather soon so you can try it eh?

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What about those Orion dobsonian intelliscopes?

The SCS astro website says:

"In seconds, the IntelliScope's high-resolution, 9,216-step digital encoders pinpoint the object,"

What does this mean exactly, is it using some sort of pattern recognition to work out what part of the sky its looking at?

SteelRat: I'm not in a rush mate, this astro lark's a really slow burn thing for me - from the little I've done so far I know I want something easy to use so ultimately I may save up for a GOTO - I just know that there is often a massive gulf bewteen the way you think something's going to work and the way it works in practice.

| can see that there is a lot of potential for EQ mounts to be quite fiddly and cumbersome, not forgetting ofcourse that you'll be using it by torchlight.

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Just a couple points here;

1) The path the Sun traces across the sky is the ecliptic, not the equator.

2) Setting circles are indispensable for non-GOTO location of objects, regardless of size, although the larger they are and the more accurate the alignment, the better they'll work. (CC's malfunctioning circles notwithstanding. Star hopping is learned as you go, once you've located objects by other means. (I personally have a catalog of over 100 objects memorized by star hopping. Most of which I found with circles first.)

3) Polar aligning to reduce the amount of room spinning must be done with the galactic pole, not the Earth's pole. It's important.

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What about those Orion dobsonian intelliscopes?

The SCS astro website says:

"In seconds, the IntelliScope's high-resolution, 9,216-step digital encoders pinpoint the object,"

What does this mean exactly, is it using some sort of pattern recognition to work out what part of the sky its looking at?

The encoders merely measure the angular position of the Dob. mount, no pattern recognition there at all. The intelliscope is a push to system i.e. no motors, it knows how much you have pushed it via the encoders so it can tell you which way to push it to find stuff.

Captain Chaos

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The encoders merely measure the angular position of the Dob. mount, no pattern recognition there at all. The intelliscope is a push to system i.e. no motors, it knows how much you have pushed it via the encoders so it can tell you which way to push it to find stuff.

Captain Chaos

How does it know where it is to start with?

Do you just point it at a couple of known stars to start it off?

I've just read the website blurb, it says

'Setup is a simple two-star alignment. '

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