Setting Circles

[cuivenion]

Hi the Dec setting circle on my EQ2 shows 0-90 degress four times. If an object has a dec of 33 degrees how do I know which 33 degrees on the circle to point to?

[cuivenion]

Also -20 degrees which has me very confused. There's no -degrees on the dec circle.

[D4N]

Firstly the setting circles on an EQ2 are mostly ornamental, don't expect to find anything with them! Learn to star hop

Secondly you need to understand what Dec is. Try and think of it as celestial latitude. At the North celestial pole the Dec is 90, on the celestial equator it is zero, at the South celestial pole it is -90.

So with the Dec at 90 turning the RA will do nothing but rotate the field of view, moving the Dec 5 degrees either way will set it to 85 regardless of which way you go. So you can see they are really two pairs of 0-90.

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[cuivenion]

Firstly the setting circles on an EQ2 are mostly ornamental, don't expect to find anything with them! Learn to star hop

Secondly you need to understand what Dec is. Try and think of it as celestial latitude. At the North celestial pole the Dec is 90, on the celestial equator it is zero, at the South celestial pole it is -90.

So with the Dec at 90 turning the RA will do nothing but rotate the field of view, moving the Dec 5 degrees either way will set it to 85 regardless of which way you go. So you can see they are really two pairs of 0-90.

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Ok I understand that. So if an object is -20 degrees dec I need to move the scope 110 degrees south from the polaris (north) position?

[jamespels]

Ok I understand that. So if an object is -20 degrees dec I need to move the scope 110 degrees south from the polaris (north) position?

Correct. This will take you all the way to the celestial horizon and 20 degrees beyond it. Assuming your score is reasonably polar aligned with 90 lining up with Polaris (unless you are in the Southern hemisphere when you won't be aligning on Polaris anyway!), this will leave your scope pointing just below the Southern horizon at this time of year. Of course, if you lowered the scope on the northern side of the mount, it would end up pointing about 30 degrees below your northern horizon. The object you are considering on this -20 dec will actually follow a circle sound the Earth's axis of rotation, through both these points, once every 24 hours.

[cuivenion]

Awesome thanks for that. I understand that the setting circles aren't very accurate but I thought I try the out as a learning exercise. As I'm still learning my way round the sky they should still get me in the general area of targets.

[D4N]

Ok well with any luck yours are better attached than mine were on my EQ2, they would slip making them useless.

Learning the coordinate system is certainly a good idea though.

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[cuivenion]

Yeah tried using the setting circles the other night. Not accurate at all. It's also to difficult to align the mount with the accuracy needed to use them. Managed to find M31 via star hopping though so fairly pleased with that.