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Does Alt-Az on a wedge = GEM?

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#1 Cobalt5120

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Posted 08 July 2009 - 06:44 PM

Isn't that really all a GEM is anyway?

RA Axis = Azimuth.
Dec Axis = Alt.

Tilt the Alt-Az base so the angle from level = your latitude and then rotating azimuth is the same as rotating the RA axis on a GEM.

Is there more to it than that?

#2 jrcrilly

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Posted 08 July 2009 - 06:49 PM

Isn't that really all a GEM is anyway?


No. An alt/az mount on a wedge is one of many kinds of equatorial mount. A German EQ mount is a specific type of EQ mount.

#3 Luigi

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Posted 08 July 2009 - 07:04 PM

The term alt-az refers to the orientation of the axes with respect to the the horizon. As soon as you tilt it, it isn't an alt-az. If a GEM is oriented to the polar axis it isn't equatorial and some can be rotated to become an alt-az mount. Bottom line is alt-az and equatorial refer to axis orientation, not mount design.

#4 Cobalt5120

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Posted 08 July 2009 - 07:33 PM

Bottom line is alt-az and equatorial refer to axis orientation, not mount design.


OK, that clears it up. But then that leads to the next question.

I've read that a lot of goto alt-az mounts have excellent tracking of a star, but if one wants to take astrophotos using that mount, then although the primary object stays in the center, field rotation becomes a factor. Based on the answers above, if any alt-az mount is properly "tilted" to be polar aligned, then it really will work just as well as an equatorial for photos...except for the fact that certain equatorials are designed for fine adjustments via auto-guiding. Other than the autoguiding feature, they would be equivilent.

Would that be a correct analysis?

#5 jrcrilly

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Posted 08 July 2009 - 07:37 PM

Based on the answers above, if any alt-az mount is properly "tilted" to be polar aligned, then it really will work just as well as an equatorial for photos...except for the fact that certain equatorials are designed for fine adjustments via auto-guiding. Other than the autoguiding feature, they would be equivilent.


Most such tilted alt/az mounts can also be autoguided. Some of them track better for imaging than inexpensive German EQ mounts. A GEM has no inherent superiority over a wedged forkmount for imaging. The reason so many imagers select German EQ mounts is the flexibility of changing optical tubes.

#6 Cotts

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Posted 08 July 2009 - 08:08 PM

Conversely to the above, an 'Alt-Az' mount would be an 'Equatorial' mount at either pole of the Earth (if Astro-travel is your cup of tea....) and would track objects without any field rotation issues although any tracking motors would have to be reversible if you had trips planned to both poles!

An Alt Az mount at the equator would have to be mounted sideways so that its former azimuth axis would be pointed to the northern (or southern, doesn't matter) celestial pole. Its former altitude axis would then become the Declination axis but you could only access half the sky because Altitude axes only have a 90 degree range of motion and you'd need 180...

Pedantic lunacy I know but you never know about some people's travel plans!!

#7 Larry F

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Posted 16 July 2009 - 05:44 PM

For years, Celestron and Meade sold fork-mounted scopes to be mounted on a wedge. You never bought the scope without the wedge. Those were (and still are) polar-aligned, non-field rotating scopes, perfectly capable for astrophotography. The only problem, at least in the Celestrons, was that there was no power on the declination axis (which worked off a tangent arm and screw rather than a worm gear) and only constant speed on the (AC-powered) RA drive motor (in fact, even though the scope was fork-mounted, the two axes were called Dec and RA, not Alt and Az, indicating the expectation that the wedge would be used). Orion manufactured a device called the Accutrak which did a number of things: it converted DC to AC in case you needed to run the scope off your car battery, had a 12-volt output for a guider reticle, had differential speeds for sidereal, lunar and solar, and it was able to speed up and slow down the two axes (you had to buy an aftermarket motor drive for the dec axis) using a joystick. I don't think this setup can be autoguided, at least without additional electronics to convert the autoguider outputs to inputs to the Accutrak controller, which is probably not a digital device. It was meant for the old-style of visual autoguiding, where you painstakingly kept a guide star, observed at high power, in the center of a crosshair.






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