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My first night woes


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Well where to start it was my first night with my new gear, Skywatcher Explorer 250PX OTA on the EQ6 Mount anyway I think I messed up the following: Balancing, aligning the finderscope, pointing the scope to Polaris, and then inputting my coordinates into the Goto and of course star alignment – putting all that aside I was amazed when I manually pointed to the moon and will be out again tonight if conditions are right.

There are a few questions (as always).

I’m pretty sure when I was using the direction arrows on the Hand Controller Left was Right and Right was Left (hope that makes sense)

I cant for the life of me understand how the scope can rotate through 360 Deg ?, I cant believe I would have to manually turn the whole thing 180 Deg to view the other half of the sky

I’m a beginner so please go easy on me

Regards

John B

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Hi John, and welcome.

There's loads of information on how EQ mounts work here: Setting Up an Equatorial Mount - McWiki -- they will happily point to any part of the sky by swinging around the two rotation axes. Of course, as you've got a goto scope you don't need to pursuade yourself it'll work: just set it up and ask it to point at things and you'll see that it points at the whole sky quite happily!

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I cant believe I would have to manually turn the whole thing 180 Deg to view the other half of the sky

The issue here is that if you allowed the scope to rotate 180 degrees around the polar axis the tube would have to pass through the space occupied by the tripod/pillar.

You soon get used to it.

Work the western half of the sky in the first half of the night, and the eastern half in the morning hours. That way you'll have to reverse the mount once per session.

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Thanks with regards to

"The issue here is that if you allowed the scope to rotate 180 degrees around the polar axis the tube would have to pass through the space occupied by the tripod/pillar."

Yes i see the logic here but am still a bit confused due to the following statment which suggests err not moving the mount

"If you draw an imaginary line through the mount at a right angle to the counterweight shaft, you define the polar axis. This part of the mount will be aligned to point to the North Celestial Pole (NCP), and it does not move, even when the telescope is moving in the other directions in which it can"

Sorry to be a pain

John B

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John,

I don't know if this will help or not, but imagine a simple camera tripod. This lets you move the camera in altitude (point it up and down) and azimuth (point it side to side). Just moving the camera around in these two directions lets you point it practically anywhere. So the tripod is in the three-dimensional world, but moves on two axes to look everywhere. The third axis never moves - it just points directly forwards and backwards.* (This would be much easier with a diagram!)

An equitorial mount is pretty much the same as the camera tripod, but this third axis, the one that never moves, is tipped up so that it points at the north and south celestial poles. But the same principle applies: it moves in two dimensions and that lets the telescope point pretty much anywhere.

Yeah, reading that back I'm guessing it won't help...

(* yes, I know a lot of camera tripods will rotate on this axis! I'm trying to simplify things...)

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This part of the mount will be aligned to point to the North Celestial Pole (NCP), and it does not move

Ah, but it does move ... it stays pointed in the same direction but rotates to follow the motion of the stars. The point about an equatorial mount is that this rotation is at a constant rate and, if the alignment is perfect, it is the only motion necessary to follow any particular star.

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Pictures would help, I see what your all saying in that the scope will move up and down and left to right with the third axis being lined up North South and static. Where im getting confused is that the scope is long and so it can only rotate before it want to pass through the mount either by going left or right, so for viewing a diffeetn area of sky the actuly mount will need to be moved and then i struggle to see the polar agliment

I guess I may still not be making sense ?

John B

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Think of it with the polar axis pointing straight up (as it would have to be if the scope was at the north pole). You can point the scope at any part of the sky by setting the altitude on the "cross axis" then rotating the polar axis until the object is in the field of view. No need to move or realign the mount. Similarly with the polar axis at an angle (at it would be with the scope somewhere else) you set the "declination" which is the celestial equivalent of latitude & rotate the polar axis to the correct "right ascension" (celestial equivalent of longitude). Just as you can specify any point on a globe uniquely by latitude& longitude, you can specify (or point at) any point on the celestial sphere by declination & right ascension. The polar axis sets RA, the cross axis (the one with the scope & counterweight which rotates with the polar axis) set Dec.

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Its what you are thinking that not making any sense... and its incredibly difficult to explain how a GEM mount copes with pointing at the entire sky without moving the entire tripod.

Instead of thinking about left/right/up/down, start thinking of the Axis its rotating around

The axis it moves around as the sky moves overhead is called Right Ascension or RA.

The axis that makes the telescope move closer to or further away from the north pole is called declination or DEC

Say you are looking at something in the west, so the scope itself is on the east side of the mount looking over the top of the mount at the western sky. As the night progresses, the scope will track the sky, and get closer and closer to being at the top of the mount as it moves in RA. If you leave it going, it will end up pointing at the ground and enetually bump into the mount.

Now, when the scope is at the top position, turn the scope 180 degrees in DEC, so its pointing to the east, and let the scope move over the top and onto the west side.

Its now pointing at the other side of the sky, the scope isnt going to bump into the mount for a LONG time, and you didnt have to move the tripod in any way.

This video on YouTube may help you understand how the GEM mount copes with the flip from east to west and vice versa.

YouTube - Meridian Flip

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This may be a little late if you've already got your head around it, but here's how I resolved the same issue in my own mind.

Once I had the scope nicely balanced in both RA and DEC, I then released both clutches on the mount and tried to manually slew it to random points in the sky by physically moving the OTA with my hands. It took me a few pondering moments to understand how the mount had to move in order to achieve certain positions, but once done - it just kind of 'clicked' in my head.

If you want to try this, just be aware of a few things:

  • Ensure your OTA is nicely balanced on the mount in both RA & DEC before doing it.
  • Make sure everything's tight, specifically:
    • The primary mirror in it's cell. You don't want it dropping out when you try a particularly unusual angle! :headbang:
    • The weights on the countershaft. The thought of them falling off onto a toe - just OW!
    • Any accessories on the OTA, like EPs or finders. Don't want to damage them.. Might even be better to remove them.
    • The rings! Nothing hurts more than the combined physical damage of an OTA landing on your foot and the simultaneous fiscal damage to the OTA itself..

IMO, it's a good thing to explore, as it also helps you to understand where a mount collision might occur for your particular setup - and therefore when you might need to hit that emergency stop button whilst it's slewing..

HTH.

Mike.

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