Right Ascension Readings

[Philosophaie]

I think I have the calibration with the North Star down. I want to understand how the whole Right Ascension works. I know that the RA is the angle around the Equator and it starts counting at the Vernal Equinox Line.

So what is with the RA arc in the Equitorial Mount?

If the Mount, just the base and not the Telescope's settings, is rotated the arc is still there how does that work in reference to the value of the RA readings?

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Sky-Watcher 130mm EQ2 Newtonian

Reflector Telescope

-130mm EQ2 Reflector Telescope130mm (5.1 inch) diameter primary mirror made from grade-A glass and multi-coated for optimal reflection

-900mm focal length, f/6.9

-20.4 sq. in. light grasp

-307x maximum theoretical magnification and 1.08 arc-sec resolving power

-High-quality super Plossl fully-coated 25mm and 12mm 1.25” eyepieces provide clear, sharp images at magnification of 36x and 75x with a 52° apparent field-of-view and great eye relief

-1.25” 2x Barlow lens doubles magnification of the included eyepieces to 72x and 150x for up-close planetary and lunar viewing

-Red-dot finderscope with variable brightness makes locating night-sky objects easy

-Stable 1.25" rack-and-pinion focuser

-EQ-2 equatorial mount provides proper stability with dual slow-motion controls, 360° azimuth adjustment

-130mm (5.1 inch) diameter primary mirror made from grade-A glass and multi-coated for optimal reflection

-900mm focal length, f/6.9

-20.4 sq. in. light grasp

-307x maximum theoretical magnification and 1.08 arc-sec resolving power

-High-quality super Plossl fully-coated 25mm and 12mm 1.25” eyepieces provide clear, sharp images at magnification of 36x and 75x with a 52° apparent field-of-view and great eye relief

-1.25” 2x Barlow lens doubles magnification of the included eyepieces to 72x and 150x for up-close planetary and lunar viewing

-Red-dot finderscope with variable brightness makes locating night-sky objects easy

-Stable 1.25" rack-and-pinion focuser

-EQ-2 equatorial mount provides proper stability with dual slow-motion controls, 360° azimuth adjustment

[Merlin66]

Rotating the base in Azimuth (on an equatorial mount) should only be used to achieve polar alignment.

The RA "arc" is the path of the object across the sky from rising to setting. The rotation of the equatorial RA axis allows you to follow the object (tracking)

[Philosophaie]

On a Equitorial mount the RA dial goes from 0degrees or 0hours to 180degrees or 12hours maybe a little past that.

Does that mean that on that Equatorial Mount RA will Arc from Horizon to Horizon?

The RA dial on the Equatorial Mount will not give you the true RA Reading because it is not facing the Vernal Equinox or the RA=0 point on the Earth?

I think the true RA can be found by calculating the Local Sidereal Time from the date, time, longitude, latitude and time zone to find the offset RA from the one on the Telescope.

Can anyone toss me a website that explains this procedure?

[Merlin66]

The RA circle on an Equatorial mount is usually calibrated in Hrs.

ie 0 - 24 hrs. The circle can be unclamped and indexed to the marker(s). The fixed marker on the mount (meridian) usually shows the LST and the moving marker (on the shaft) the RA position of the telescope.

You can obviously set the meridian marker if you know the LST, but an easier way is just to point the telescope at a known star ( Vega etc) and set the moving index to the star's RA.

Google Equatorial coordinates for more detail.

Ken

[Philosophaie]

Now if I go to a portion of the sky and I know the RA and DEC of the star or object. I can recalbrate using these settings.

I will then have true RA if I change with the dials or the telescope not the base swivel?

[Merlin66]

Yes.

The azimuth adjustments are ONLY used to get the initial polar alignment for the equatorial mount. Once set properly they never need to be touched again.

Moving the telescope on the equatorial mount will move in arcs of RA and Dec.