Does a mount have to be perfectly level?
Posted 30 June 2013 - 02:42 PM
Posted 30 June 2013 - 03:07 PM
If you had a portable setup, depending on the design of your polar scope, you might need to level it in order to polar align but that does not appear to be your case .
You probably will permanently drift align anyway correct?
Some people will tell you that the mount will work better if it's level because it's better balanced. That is a good point but you seem to be close enough.
Posted 30 June 2013 - 03:12 PM
I posted the same question about 4 years ago and the general consensus was leveling is absolutely NOT necessary for proper polar alignment, but having a mount that is properly leveled will greatly facilitate and expedite polar alignment. So while it is not necessary, it is recommended, especially for AP.
Posted 30 June 2013 - 03:35 PM
Your mount is permanently on a pier, so you don't really need to worry about re-doing the polar alignment.
Posted 30 June 2013 - 03:49 PM
Posted 30 June 2013 - 04:15 PM
Dan - just curious why you were unable to get the bubble centered. Can you somehow shim if necessary?
Posted 30 June 2013 - 05:43 PM
Yesterday I was out helping a friend level his iOptron iEQ45, and we got it pretty close. Does it have to be perfect, though? Why or why not? This is a GEM and is permanently mounted to an 8" diameter pier. See the attached photo for a depiction of how close we got it.
GEMs? No. In fact, alt-az go-to systems don't need to be perfectly level, either, though in their case it can help them land closer to the initial alignment stars...
Posted 30 June 2013 - 05:52 PM
+- 5 degrees?
+- 1 degree?
+- 0.5 degree?
+- 10 arc minutes?
+- 1 arc minute?
+- 30 arc seconds?
+- 1 arc second?
"Perfectly level" could be said to be far better than these. In reality within a degree is easy, just looking at the mount may be OK. getting to better than 6 arc minutes might be achievable with a builders level. Better than this will need surveying instruments.
What effect does not being level have on alignment or polar alignment? I think that being level hardly matters for alignment and goto and that more or less level - within a degree or two - will be OK for polar aligning.
Getting to a degree
Posted 30 June 2013 - 06:14 PM
If you are drift aligning you want to be as close as level as possible so that your adjustments in alt won't affect your adjustments in az. How close? I don't know. My guess would be less the one degree in either direction, however perhaps someone could do the math to better advise you.
Posted 30 June 2013 - 07:38 PM
cosine of 10 degrees = 0.984807753, sine = 0.173648178
cosine of 01 degrees = 0.999847695, sine = 0.017452406
The sine (side opposite on a right triangle) is the relevant value for this discussion. It tells us how much the orthogonal axis "cross-couples" to movement on the other axis.
For instance, if you adjust the azimuth axis by one degree and the mount is out of level in azimuth by 10 degrees, the axis will move in elevation by 0.17 degrees. That is a substantial amount.
If you are within 1 degree of level, the cross-axis movement is 1/10th as much, or 0.017 degrees.
To me, this says get within a couple of degrees of level for visual and less than a degree for astrophotography and your life will be simpler when aligning the pole. On the other hand, you can just iteratively adjust both az and elevation and you will converge on accurate polar alignment pretty quickly.
Posted 30 June 2013 - 09:54 PM
A tilt error in the north-south direction is equivalent to the telescope being exactly level at some slightly different latitude.
A tilt error east/west is equivalent to the telescope being level at a slightly different longitude and/or a constant error in the local siderial time (if you calculated that).
The mount will compensate for these errors automatically.
The only issue anyone may face is when they are relying on precise coordinates of the site location (eg GPS) for computerised tracking using an altaz mount, but the telescope is not level.
Posted 30 June 2013 - 11:18 PM
Posted 30 June 2013 - 11:53 PM
Posted 01 July 2013 - 01:52 AM
As has been said, having the mount very close to level can help you to achieve an accurate polar alignment. This is because polar alignment is achieved by adjusting the mount in altitude and azimuth - and if the mount is not level, a change in altitude also affects azimuth and vice-versa. Even with this, it's still possible to achieve polar alignment, but you may require more iterations of adjustments.
Posted 01 July 2013 - 07:40 AM
Alas, mount manufacturers do not state level sensitivity, usually quoted in mm/m (or in/ft). Bubble levels as provided on mass-produced equipment are quite poor, and are generally accurate enough to indicate which way is up.
Further innacuracies arise when the level fitted to the tripod are not parallel to the mount head base. I've seen some bubble levels in mounts that were several degrees off level. I get the feeling that assemblers feel that the bubble level is purely cosmetic and just make sure the installation looks neat, rather than accurate.
All this means that you should use an accurate third-party level to check things. I use a Starrett machinists level. On my alt-az, I place the OTA horizontal, check it with the level, and then rotate the OTA in azimuth, checking the level around the 360 degrees of azimuth.
I don't use go-to, but I have been playing recently with encoders and push-to aiming on a Astro-Tech Voyager. I'll fly in the face of "conventional" wisdom and say that the accuracy of your push-tos (and by implication, go-tos) are critically dependent on the level of your rig during and subsequent to alignment. Get it right, and my targets are always withing 0.25 degrees of expected. Get it wrong, and I could be over a degree out over the course of an evening's viewing.
Some people may be OK with that.
Posted 01 July 2013 - 09:14 AM
IMHO yes, if you want precise pointing and tracking. I have seven permanent GEM's and all are dead-level. I used an Engineer's level then checked that with a GPS and found both good. Once a drift alignment was done, I can get within 30 arc secs or so of the target.
Nope. Polar alignment is not affected at all by level. Think about the geometry of the situation and the surface of the Earth.
Nor is go-to affected. Polar alignment can affect go-to accuracy on some mounts, but not level.
Some mounts need a fairly good polar alignment for accurate go-to, but some like the Celestrons do not. How do I know? One night one of my fellow club members, a novice, accidentally polar aligned on Kochab instead of Polaris. His tracking sucked, but his go-to was fine.
Posted 01 July 2013 - 11:58 AM
Posted 01 July 2013 - 01:03 PM
Alas, mount manufacturers do not state level sensitivity
Takahashi does. Their EQ mount manuals (the question that started this thread refers to an EQ mount) clearly state that the mount does not need to be level. Their tracking performance is legendary so they clearly care about performance.
Regarding alt/az setting circles: most are primarily dependent on orthogonality - the axes must be square to each other. Most alt/az mounts equipped with setting circles have no provision for leveling their bases (think dobs) and work quite well off level.
Posted 01 July 2013 - 03:56 PM
Even tho I now use an AP mount with the RAPAS, I also have an NJP and I think that the TAK polarscope is still the golden standard. It has a built in bubble level that is aligned with the reticle inside the scope. All you need to do is make sure that the bubble is level and then align the reticle with polaris. Whether the mount is level or not is completely irrelevant thanks to this clever design.