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polar alignment vis-a-vis dec axis w/ no drive

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#1 telescopemullet

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Posted 01 May 2011 - 09:31 AM

My Optic-Craft mount has RA drive but it lacks dec drive. I think it is aligned pretty well but I am getting drift in dec so I am not sure if it is an alignment issue or a lack of drive issue. Objects stayed centered in RA but over time drift out of view in dec, all I have to do to recenter is move the scope in dec and it goes right back to center of the eyepiece. The question, is the drift due to the lack of a dec drive or is the scope not aligned enough?

Thanks in advance.

#2 ColoHank

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Posted 01 May 2011 - 03:21 PM

That sounds like an alignment issue. If the mount were precisely drift-aligned so that the RA axis was exactly parallel with the Earth's axis of rotation, an RA drive alone would suffice for accurate tracking. A properly polar-aligned scope doesn't need a declination drive.

#3 telescopemullet

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Posted 01 May 2011 - 05:43 PM

Okay, thanks! Now that I know it's alignment, is it just RA as you mention in your post, or both? I figured it was the dec since that is the part of the mount that I move to bring object back.

Sorry for these noob questions, this is just the first time I have really used one of these type of mounts before.

#4 ColoHank

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Posted 01 May 2011 - 10:49 PM

Assuming you are located north of the equator, and if your mount is accurately aligned, then your telescope will be pointed at the celestial north pole when declination is set precisely at +90°. Polaris, however, is actually located about 3/4° from true north, so if you have been sighting on it alone, and foregoing further refinement (such as drift alignment) to orient your mount, your alignment will have an error of 3/4°, and tracking of objects will reflect that error. Perhaps that explains why your scope appears to slowly drift off in declination. If you are precisely aligned on the celestial north pole (and not simply on Polaris), you should be able to track objects located at any declination value for prolonged intervals using only your RA drive.

#5 Eddgie

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Posted 02 May 2011 - 08:41 AM

I think ColoHank is correct in his first post, but he mentioned "Drift-Alighment" and my guess is that you are not famaliar with this.

Drift Alignment is used to tweak a mount into final, perfect polar alighment. I will include a link to the instructions.

Before you read it though, let me first say this. For general observing, where you just want tracking, it is not at all important to have perfect polar alignment.

Most of the time, if your aligbment is even pretty close, and if you aren't doing observing at 250x or more, the amout of drift typically just isn't enough to worry about because you may not find yourself looking at objects long enough for them to drift from the field.

If you are imaging and doing long exosures, then drift alignment is much more important.

So, the question would be "Do you really NEED to drift align for general observing?" The answer might be "No".

I use Go-To scopes and if the alignment is not pretty close, I get drift (most Go-To scopes don't track in Dec, so if your alignment is not perfect, the target will drift out of the field), but honestly, even when doing planetary oberving, it is rarely a problem (though I do try to do a decent rough alignment). For most deep sky observing it is NEVER a problem unless I leave the mount unattended for a while and the target disappears totally from the field.

Anyway, here are the instructions for drift alignment. DO this and your problem will be solved...

http://www.backyarda..._alignment.html

#6 telescopemullet

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Posted 02 May 2011 - 09:36 AM

Hey thanks for the info!!! This is entirely for visual and I am trying to get the alignment nailed as I am using setting circles so I figured the more accurate the alignment, the more accurate the circles will be.

In terms of where the alignment is now, using a 15mm Axiom. Saturn takes about 20-minutes to drift out of the FOV entirely, so I am close but I'd like to nail this. I'll look at the drift site, thanks again!

#7 Donnie D

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Posted 02 May 2011 - 10:05 AM

Starizona has a great tutorial on how to drift align that just gets to the point. (just my humble opinion)
I don't remember seeing anyone mentioning a polar scope. Does your mount have a PS or can be fitted for one?
If you use a Polar Scope (PS) then remember, it is NOT polaris that you want to center, but it is the N.C.P that you want to be centered. Polaris is actually 44' (or 1/2°) from the NCP. Another very successful method that I use is adding in the star Kochab. It just so happens that if you extend a line starting at Kochab and dissecting Polaris and continue the line, it will "intersect" or terminate at the NCP. Most all PS'es are in reverse so be careful there too.
That 44' can make a difference - some may argue.

After performing a near perfect Polar align, then yes, I would progress to the Drift align. I am re-visiting this method myself as I need to fine tune my scope tracking.

The drift align really only involves two major components. Finding one star in the "south" within about 5° of the meridian and celestial meridian. In the Northern H, it is usually due south about 65° to 70°.
Then finding one star in the east about 20° to 30°.
The tutorials will explain what to so with these stars.

Donnie


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#8 telescopemullet

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Posted 02 May 2011 - 10:36 AM

Thanks all. No polar scope on this rig.

I knew about Polaris not being the true pole, the problem is there is a huge tree that blocks the pole area just enough that I can't view Polaris when the tree is fully flush with leaves, so I have to do a bit of guessing.

Again, thanks for the information.






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