Why are Equatorial mounts better for AP

Started Nov 22, 2012 | Questions
Hotphotons
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Why are Equatorial mounts better for AP
Nov 22, 2012

I have always heard equatorial mounts are better for astrophotography but have never seen an explanation as to why. A computerized and motorized alt azimuth type mount can still track a celestial object so why is it not good for AP?

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PC Wheeler
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Re: Why are Equatorial mounts better for AP
In reply to Hotphotons, Nov 22, 2012

Field rotation. The line-of-sight will track with a Alt-Az mount but the star field will rotate about the line of site.

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RustierOne
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Re: Why are Equatorial mounts better for AP
In reply to Hotphotons, Nov 22, 2012

Hotphotons wrote:

I have always heard equatorial mounts are better for astrophotography but have never seen an explanation as to why. A computerized and motorized alt azimuth type mount can still track a celestial object so why is it not good for AP?

PCwheeler's comments are quite correct. To help understand why, one needs to understand how the stars appear to move across the sky due to Earth's rotation.

An observer located on Earth's north pole would see the stars appear to rotate around a point directly overhead, where the Earth's rotational axis intersects the sky at the north celestial pole. An alt-azimuth mount at that location would function quite well since its vertical (azimuth) axis is aligned with the Earth's rotational axis. Tracking a star would take place by just turning the mount's vertical  axis. There would be no field rotation since the mount's rotating axis is aligned with Earth's rotational axis.

An observer located at a mid northern latitude (say 30°) would see the north celestial pole elevated 30° above his northern horizon. An alt-azimuth mount located at this latitude would have neither of it's axes directed at Earth's rotational axis. Tracking a star would require turning on both of the mount's axes. While the star would be tracked quite well, the field would rotate around the star due to misalignment of the axes.

In contrast, an equatorial mount located at 30° latitude is tipped 30° so that it's polar axis is aligned with Earth's rotational axis. By rotating just it's polar axle, the mount is able track a star with no field rotation.

I hope this explanation helps in understanding why an equatorial mount is best for long exposure astrophotography.

Best Regards,
Russ

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Hotphotons
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Re: Why are Equatorial mounts better for AP
In reply to Hotphotons, Nov 23, 2012

Thanks to both of you for the explanation. Seems obvious now that I know.

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RustierOne
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Re: Why are Equatorial mounts better for AP
In reply to Hotphotons, Dec 4, 2012

Hotphotons wrote:

Thanks to both of you for the explanation. Seems obvious now that I know.

Equatorial mounts are wonderful devices invented some centuries ago. If you could view an astronomer on Earth from some location in space, his telescope on an equatorial mount would appear to be stationary - pointed at a star. But the Earth would be rotating under his feet. So our goal with long exposure astrophotography is to have the telescope stationary and not rotating. From our perspective on the (rotating) Earth, the telescope is moving to keep up with the stars. But in reality, the stars and our telescope are really stationary. That's quite a revelation for many.

Best Regards,
Russ

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JimP
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Re: Why are Equatorial mounts better for AP
In reply to RustierOne, Dec 5, 2012

If you were just interested in visual astronomy (the old eye up against an eyepiece), then the Alt/Az mount is very convenient.  Simple to align new scopes with electronics.. just set it up, turn it on, aim at 2 stars and you are tracking the sky.  Your scope will find and follow Jupiter all night long.  Even though the field around Jupiter is EVER SO SLOWLY rotating, you won't notice and it doesn't matter.  You can even do short exposure photography (under 30 second exposures) with negligible field rotation.  Also, in Alt/Az mode, the eyepiece is always in a convenient location for viewing.

If you are serious about long exposure astrophotography, then, like the previous fine gentlemen noted, you'll need to switch to equatorial mount (or use a wedge on an Alt/Az mount). Then you can track for LOOOOONG periods of time and the entire field stays nicely aligned.

The downside of an Equatorial mount is that you have to closely align the mount to the polar axis and do an electronic alignment.  Adds some tedium and time, although newer scopes have electronic assist for polar alignment as well (e.g. Celestron's All Star Polar Alignment).  Also, the eyepiece can get into locations where a visit to a chiropractor is required the following day.

Clear skies,

Jim

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RustierOne
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Re: Why are Equatorial mounts better for AP
In reply to JimP, Dec 5, 2012

JimP wrote:

If you were just interested in visual astronomy (the old eye up against an eyepiece), then the Alt/Az mount is very convenient. Simple to align new scopes with electronics.. just set it up, turn it on, aim at 2 stars and you are tracking the sky. Your scope will find and follow Jupiter all night long. Even though the field around Jupiter is EVER SO SLOWLY rotating, you won't notice and it doesn't matter. You can even do short exposure photography (under 30 second exposures) with negligible field rotation. Also, in Alt/Az mode, the eyepiece is always in a convenient location for viewing.

If you are serious about long exposure astrophotography, then, like the previous fine gentlemen noted, you'll need to switch to equatorial mount (or use a wedge on an Alt/Az mount). Then you can track for LOOOOONG periods of time and the entire field stays nicely aligned.

The downside of an Equatorial mount is that you have to closely align the mount to the polar axis and do an electronic alignment. Adds some tedium and time, although newer scopes have electronic assist for polar alignment as well (e.g. Celestron's All Star Polar Alignment). Also, the eyepiece can get into locations where a visit to a chiropractor is required the following day.

Clear skies,

Jim

Thanks, Jim.

Those are some good points worth noting by all. That last one explains why my back is sometimes out-of- wack after a night of observing.

I would add that with an equatorial mount not perfectly aligned to the celestial pole, the field of view will also slowly rotate around the center, perhaps even slower than with an alt-azimuth. And you're right about the convenience of alt-az on visual astronomy. It can be a lot less complicated than long exposure astrophotography and provide enough interest for a lifetime.

Best Regards,
Russ

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RustierOne
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Re: Why are Equatorial mounts better for AP
In reply to RustierOne, Dec 29, 2012

RustierOne wrote:

JimP wrote:

If you were just interested in visual astronomy (the old eye up against an eyepiece), then the Alt/Az mount is very convenient. Simple to align new scopes with electronics.. just set it up, turn it on, aim at 2 stars and you are tracking the sky. Your scope will find and follow Jupiter all night long. Even though the field around Jupiter is EVER SO SLOWLY rotating, you won't notice and it doesn't matter. You can even do short exposure photography (under 30 second exposures) with negligible field rotation. Also, in Alt/Az mode, the eyepiece is always in a convenient location for viewing.

If you are serious about long exposure astrophotography, then, like the previous fine gentlemen noted, you'll need to switch to equatorial mount (or use a wedge on an Alt/Az mount). Then you can track for LOOOOONG periods of time and the entire field stays nicely aligned.

The downside of an Equatorial mount is that you have to closely align the mount to the polar axis and do an electronic alignment. Adds some tedium and time, although newer scopes have electronic assist for polar alignment as well (e.g. Celestron's All Star Polar Alignment). Also, the eyepiece can get into locations where a visit to a chiropractor is required the following day.

Clear skies,

Jim

Thanks, Jim.

Those are some good points worth noting by all. That last one explains why my back is sometimes out-of- wack after a night of observing.

I would add that with an equatorial mount not perfectly aligned to the celestial pole, the field of view will also slowly rotate around the center, perhaps even slower than with an alt-azimuth. And you're right about the convenience of alt-az on visual astronomy. It can be a lot less complicated than long exposure astrophotography and provide enough interest for a lifetime.

Best Regards,
Russ

It comes to mind that one area of astrophotography not requiring an equatorial  mount is lunar (and possibly planetary) imaging. Since the Moon is so bright, exposures are brief - field rotation is no problem. Even multiple exposures can be stacked with Registax to capture brief instants of good seeing. Registax can handle field rotation in the later versions. So even a Dobsonian mount can function quite well in this regard.

This was pointed out by someone in the Sony NEX forum (whose name I forget), before we had our astrophotography forum.

Best Regards,
Russ

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