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CG5 GoTo Mount Alignment Question

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#1 asteroid7

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Posted 15 September 2009 - 01:46 PM

Are the following statements true: One does not need to
put Polaris in the eyepiece FOV, just point one's telescope in a rough direction to the North Pole. Putting Polaris in the eyepiece only makes for a closer first star, that is to say, your telescope is not too far from the first alignment star.



Rich

#2 Eddgie

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Posted 15 September 2009 - 04:00 PM

You are correct. One does not have to be pointing directly at the north celestial pole to aligne a CG5 ASGT. One only has to aim the scope roughly north and have the elevation set to roughly the right lattidude.

And yes, putting Polaris in the eyepiece can help reduce the amount of error you get when slewing to the first alignment star.

To be accurate though, the mount isn't ever aimed at Polaris. For best polar alignment, the mount needs to be aimed at the North Celestial Pole, not Polaris.

But that is splitting hairs. Your statement is indeed correct. The telescope does not have to be aimed at EITHER Polaris or the NCP, though the closer, the better.

Also, the closer you aim, the less stars will drift out of the field. The Meade and Celestron scopes do NOT track in Dec, only RA.

So, if you do a rough alignment, you may find that at high powers, stars still drift out of the field of view. This is because the scope is only driving in RA and NOT in Dec, so the RA track of the scope may not actually be following the TRUE arc of RA. If your alignment is quite rough, the scope will still align and still center the target, but the minute it stops slewing, the star will start drifting from the center of the field.. How fast depends on how high the power is and how far off the alignment is.

For this reason, I personally find that putting Polaris or better yet, the NCP in the field is worth the small bit of effort if I am doing high power planetary observing.

#3 Falcon-

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Posted 15 September 2009 - 04:23 PM

The Meade and Celestron scopes do NOT track in Dec, only RA.



That is a valuable bit of information! I am in the process of ordering a CG-5 goto myself and so have been following this thread... I plan on doing astrophotography with my mount and, coming from a 30-year old RA clock drive driven mount, was very confused by various people saying I did not need to level or north point super accurately.

So if I understand this correctly:

- to get accurate Goto you do *not* need to spend huge amounts of time getting north-oriented, tripod level, and lat angle set correctly. The mount is smart enough to compensate.
- as the mount does not track in Dec, but only RA, if you want your object to stay in view for a long time or you want to do longer exposure photography you *should* spend the time to get things set up as accurate is possible before turning on the mount and doing the align.

Is this correct?

(sorry to jump into the middle of your question Rich)

#4 asteroid7

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Posted 15 September 2009 - 05:17 PM

Thank you, Eddgie. No problem, Falcon.

Should I assume that to keep from the most drifting, it is best to put Polaris near the edge of FOV, if you have a true eyepiece fov of 1-1/2 degrees. And if so, which edge do I put Polaris on? Can the double of Polaris be of help to know what edge to put Polaris on?

Rich

#5 Patrick

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Posted 15 September 2009 - 08:58 PM

Should I assume that to keep from the most drifting, it is best to put Polaris near the edge of FOV, if you have a true eyepiece fov of 1-1/2 degrees. And if so, which edge do I put Polaris on? Can the double of Polaris be of help to know what edge to put Polaris on?



Forget about aligning the telescope to Polaris as described above! The important thing is to align the RA axis of the mount to the NCP. This may be a fine distinction, but it's important to differentiate between the telescope's optical axis and the mount's optical axis. All the discussion so far has made an incorrect assumption that the optical axis is parallel to the RA axis, when in fact it may not be. Since the telescope can move about 2 axis, it's not possible to accurately tell when the optical axis is parallel to the mount's RA axis by eyesight.

It's important to note that this is the reason mount manufacturers have supplied mounts with a polar axis scope rather than instructing users to aim their telescopes at Polaris (or the NCP). Even polar scopes can be misaligned, however. The tried and true process of drift aligning has been the traditional way of insuring that the RA axis is pointing directly at the NCP. With Celestron's All Star polar alignment routine built into their hand controller, it's now possible to get accurate polar alignment without drift aligning because they've built in algorithms to compensate for the mismatch between the optical axis and the RA axis.

I think it's also important to make a distinction between the need for accuracy for goto pointing versus tracking for astrophotography. For simple goto pointing the polar alignment does not have to be very accurate. Simply looking through the RA axis hole and placing Polaris in the center will work fine. The hand controller software will compensate for the cone error and will use both axis for object location and tracking. However, it's important to note that with polar alignment error the object in the eyepiece will rotate slightly. For visual work, it doesn't matter.

However, for astrophotography, field rotation does matter. That's why it's important that the polar alignment be as accurate as possible when imaging.

Regards,

Patrick

#6 David Pavlich

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Posted 15 September 2009 - 09:23 PM

The CG5 comes with a pretty good polar alignment routine built in, especially the newest version. Get it pointed north in the home position, the go through the PA routine as it is spelled out in the manual. You'll have great tracking to go along with the gotos.

David

#7 Eddgie

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Posted 16 September 2009 - 11:22 AM

Yes, you are correct.

HOWEVER. When I first get a mount, I DO set it up using the level.

The next thing I do with the CG5 and LXD (I have owned a couple of eacy), is to tear off the Declination Home Position index marks. These markes are slapped on by someone that doesn't even know what a mount does.

Usually, these index tabs for some ODD reason point the scope to the right of the true north axis of the mount. If you do a true NCP alignment and you always have to move the scope a lot in DEC to hit your first alignment star, this is most likely the reason why.

Anyway, I just peel off those index marks and visually align the axis of the OTA to the RA housing axis. I just eyeball it visually so that they look pretty close.

Now, I adjust the latitude scale so that the latitude is fairly close to correct.

In the future, I alwasy take the time to level the mount. It only takes few seconds.

Since I now know that the latitude is set more or less correctly, in the future setups, I just align my marks (I put on home-made marks for DEC), level the mount and move the east and west legs slightly (pivot on the north leg) to roughly align the mount to the NCP. I do all of this visuallly.

My first alignment star is almost alwasys within a 2 degree telrad circle.

If I just use the Telrad and a chart to put Polaris into the 2 degree telrad circle at the approximate hour position that the chart says Polaris should be at, my first alignment star will usually hit the half degree telrad circle.

This is on both my CGE and my CG5, and my LXD 55 used to do the same thing.

Now, I have tightened all of the gears in my mounts, and I have eliminated cone error as well.

AND, I have at least ONCE done an ACCURATE polar alignment to ensure my level and lattitude scale were hitting their marks, and I have yet to own a CG5 or LXD mount where the DEC home position indes marks were right from the factory, so I always ensure that I re-do those marks.

Bottom line, for visual use, I can set up and align in less than 5 minutes, and get perfect pointing and tracking so good that I can obsever at high powers for many munites and not have a star drift out of the field.

So, tweaking the mount and ensureing these things are all close (DEC index marks, cone error, level so that the lattitude is accurate) means that your alignment can be super fast. Doesn't have to start out perfect but it is EASY to get close enough that pointing and tracking are just about perfect for visual use.

For critical alignment though, the Polar Alignment routine is essential.

#8 Eddgie

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Posted 16 September 2009 - 11:25 AM

Oh, and the reason I eliminate the cone error is that I am (A:) lazy and don't like having to add a calibration star (yes the mount is supposed to save this, but it seem like all-sky pointing improves quite a bit if you eliminate cone error, making the calibration routine unnecessary) and (B:) I have trees in my western sky, so often, I cannot find a good calibration star.

I have adjusted all of my telecope/dovetials to eliminate cone error, so now I can put any scope on any mount and onlly do a two star alignment, and get good all sky pointing.

Just an FYI.

#9 rmollise

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Posted 16 September 2009 - 05:24 PM

Yes, you are correct.

HOWEVER. When I first get a mount, I DO set it up using the level.

The next thing I do with the CG5 and LXD (I have owned a couple of eacy), is to tear off the Declination Home Position index marks. These markes are slapped on by someone that doesn't even know what a mount does.


Don't matter pea turkey. It really does NOT MATTER where the marks are, as long as the OTA is pointed roughly north and the counterweight bar is down when you are on the marks. What is critical is that you start from the same marks each time. ;)

Levelling MIGHT help you come close to the initial alignment star (I've never noticed that it does), but will not do anything else. It is certainly not worth wasting time over beyond just adjusting the tripod until the mount's built in level is centered.






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