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Leveling a mount doesn't matter.

Astrophotography Equipment Mount Polar Alignment
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#1 psugrue

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Posted 25 April 2021 - 01:00 PM

OK I will make some statements and you guys tell me if I am wrong, please.

 

Leveling a mount is not that important because although it might make PA a bit more interesting using say Sharpcap, once you get near perfect PA, the mount doesn't "care" or "know" if it's level.

 

You CAN NOT do a sloppy PA and expect PHD guiding to take up the slack in your bad PA because even though PHD can keep the cross hairs nuts on, you are then subject to Field Rotation.

 

Kind Regards,

 

Patrick


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#2 Synon

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Posted 25 April 2021 - 01:09 PM

Correct, you could bolt it to the side of a barn and it wouldn't know the difference so long as PA is accurate. 


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#3 jdupton

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Posted 25 April 2021 - 01:09 PM

Patrick,

 

   You are correct on both counts.

 

   A very badly leveled mount will only mean that polar alignment methods may become iterative and have to be done a couple of times to home in on the PA. Once you have PA, the leveling of an EQ Mount will not matter. (It might matter for an Alt-Az mount because the GoTo algorithms could use the leveling of the mount as a key parameter for sky mapping.)

 

   Poor PA may mean that even with perfect guiding, you may experience field rotation about the guide star. The higher declination the target is, the worse it will be. Poor PA may be hard to spot at the celestial equator.

 

 

John


Edited by jdupton, 25 April 2021 - 01:09 PM.

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#4 rj144

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Posted 25 April 2021 - 01:09 PM

It matters for GOTO I think.  And, I think it does matter for PA because the tracking "circle" (circle made by the rotating mount) is not aligned properly.



#5 bobzeq25

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Posted 25 April 2021 - 01:18 PM

Well, you don't want the mount to fall over.  <smile>

 

It doesn't matter, either for polar alignment or for GOTO.  Some polar alignment methods are a bit more difficult (because adjusting one thing changes the other a bit), with my PoleMaster it makes no difference at all.


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#6 Synon

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Posted 25 April 2021 - 01:26 PM

It matters for GOTO I think.  And, I think it does matter for PA because the tracking "circle" (circle made by the rotating mount) is not aligned properly.

Negative on that, the position of the base relative to the central axis is irrelevant. You could literally mount a GEM on the side of a wall and GOTO's would work just as well with good PA. The tracking "circle" you mention is what polar alignment is, PA can be achieved without regard to levelness or angles or anything else.


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#7 rj144

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Posted 25 April 2021 - 01:32 PM

Negative on that, the position of the base relative to the central axis is irrelevant. You could literally mount a GEM on the side of a wall and GOTO's would work just as well with good PA. The tracking "circle" you mention is what polar alignment is, PA can be achieved without regard to levelness or angles or anything else.

Yeah, now that I thought about for another minute, the axes are aligned, so the center of each circle is aligned and in the same plane.


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#8 Stelios

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Posted 25 April 2021 - 01:44 PM

It doesn't matter *ONCE* you've got PA nailed, but it will take you longer to nail PA as alt adjustments will throw out az adjustments and vice-versa. 

 

As for field rotation, for that to be noticeable at the relatively short (1 to 6 minute) exposures currently in vogue due to CMOS cameras PA error would have to be quite large. Like half a degree or something. 


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#9 ravenhawk82

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Posted 25 April 2021 - 03:04 PM

The only impact I've seen leveling make is how quickly I can get it polar aligned. I use an ASIAir which has a great PA routine to point where you need to go. When everything is leveled properly I can get pretty close to the software specified adjustments right away using the angular measurement markings on my mount. If it's not leveled very well I just adjust blindly and see where it ends up pointing until I hone in on the right spot. In the end they both get the RA axis aligned in the right place but one way is much quicker and easier. 


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#10 kathyastro

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Posted 25 April 2021 - 03:27 PM

Correct on both counts.  Levelling makes it easier to get a good PA, but is not a requirement.


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#11 AhBok

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Posted 25 April 2021 - 04:47 PM

I think the idea that leveling is required for PA comes from the days of drift aligning. It is a bit (a lot for some) easier to drift align when the mount is leveled. Otherwise, as Bob wisely stated, you don’t want your mount to fall over.
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#12 GR-Amateur

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Posted 25 April 2021 - 08:32 PM

If PA method followed is through polar scope only (no other aid) then leveling of the mount is very important while first step is to align polar scope’s reticle with the mount (it almost never is). At this stage mount is supposed to be leveled... The leveling error in this case is transferred to the PA. If somehow we have a good PA then leveling doesn’t matter any more.
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#13 BQ Octantis

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Posted 26 April 2021 - 02:55 AM

When your only PA references are gravity, a compass, and your latitude (like when you setup well before nightfall), you better believe leveling the mount is critical. I do this all the time if I'm shooting planets or the moon before nightfall (or when I'm shooting the sun).

 

Once I can see BQ Octantis (through any aperture attached to the mount), I can see directly whether the mount's RA axis is on the SCP. At that point, leveling the mount becomes irrelevant.



#14 GR-Amateur

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Posted 26 April 2021 - 08:43 AM

Setting Polaris at polar scope’s reticle (with mount’s ALT-AZ bolts) only makes sense when reticle is leveled otherwise we are introducing an error. In other words, you can have a non leveled mount but you cannot have a non leveled polarscope for this procedure.

 

Using polemaster, sharpcap ... and or any other aiding method, leveling of the mount is irrelevant, at least a small(*) one.

 

(*) in order not to affect mount limits, parking position etc or in other words, star alignment can correct all problems but hanging the mount on the wall might damage your scope, camera, dog, garage door etc

 

 

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  • 45BB7367-658F-4162-AEBD-00653D4DCD18.jpeg

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#15 TelescopeGreg

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Posted 26 April 2021 - 02:16 PM

If you are using a camera-based software-driven polar alignment tool, the software can compensate for whatever non-levelness there is in the mount's position.  What RA offset remains after alignment will affect the mount's initial GoTo, but that is corrected for by a star alignment or plate solving.  For this method, being level is only a nice thing, not critical.

 

However, as GR-Amateur notes, not everyone uses a software-driven polar alignment.  The AVX mount's polar scope was "dumbed down" to eliminate any and all possibly confusing techno-visual stuff, leaving one with the bare essentials.  That's probably adequate for casual visual work, but quite inadequate for DSO AP.  The issue is orienting the mount's RA axis such that it exactly matches the orientation of the sky.  Since there aren't any references within the scope's field of view, they resort to cartoons, which one has to somehow orient (while standing on their head) to match the sky.  Yeah, right.

 

I do a polar alignment using the "The Elf" method, where one sets the mount's RA orientation such that it's parked with the 6 - 12 line in the polar scope to be exactly vertical.  This is set during the day, either by rotating the polar scope in the mount and locking it there, or indicating where to set the mount's RA axis clutches with tape.  I use tape.  Then at night, turn on the mount and tell it go GoTo Polaris (northern hemisphere) and the mount will magically move the RA axis such that the indicated position in the scope for Polaris is exactly where it should be.  Use the mechanical adjustments to put the star there, and you're done.

 

With this method, it is very important that the mount is absolutely level, otherwise that initial state will be off, which will make the polar alignment off as well. 

 

For reference, and perhaps some entertainment, this is what the AVX polar scope's reticle looks like:

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  • Celestron Polar Scope.jpg


#16 ks__observer

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Posted 27 April 2021 - 04:30 AM

For PA i think you need to be [edit] leveled E-W.

PA requires that RA axis of mount be parallel with the Earth's rotational axis.

If the mount is not balanced EW how do you get PA?

It might look polar aligned momentarily in Sharpcap but I don't think it will be polar aligned.


Edited by ks__observer, 27 April 2021 - 07:11 AM.


#17 T~Stew

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Posted 27 April 2021 - 06:11 AM

For PA i think you need to be balanced E-W.

PA requires that RA axis of mount be parallel with the Earth's rotational axis.

If the mount is not balanced EW how do you get PA?

It might look polar aligned momentarily in Sharpcap but I don't think it will be polar aligned.

What do you mean by 'balanced'? Balancing generally refers to offsetting your scope weight with a counterweight on opposite side of the RA axis. That really has nothing to do with polar alignment, but helps the RA motor run easier, improves tracking, etc.

 

Think about the mounts RA being parallel with Earth's rotational axis... now zoom out so your in space looking at the planet... from different points on the Earth the base/tripod will obviously vary as we travel around the globe and up and down, the tripod will be pointed in all sorts of different directions yet all can be aligned to be in parallel, its why the mount has alt & az adjustments so you can point it in any direction (well some mounts may have limitations of course).

 

Some methods may depend on a level mount, if you are not doing the standard visual alignment with the NCP/SCP. Like if you are aligning with a compass bearing and latitude value for example, that is assuming your base is level. But if you can see the NCP/SCP and align to it, it does not matter how you got there as long as you are aligned, your axis will be parallel to Earth's, and that is the goal yes?

 

The big difference in being perfectly level means the alt & az adjustments will be precise and expected. If your mount is say at a 45º angle then moving your alt adjustment is not going to just move the altitude, it will move both altitude and azimuth by about the same amount. So it will require much more fiddling to get it right.

 

I like to explain it like an Etch A Sketch, anyone remember those? You have two knobs, up/down and left/right, much like alt-az adjustment. If the Etch A Sketch is perfectly level the up and down will go straight up and down. But now shim one side of the Etch A Sketch up a little, now if you try to make a straight line up, you'll have to move both knobs, one a lot, the other a little, to get a straight line up. When you have software like ASIair pro that tells you to move a certain number of arc-seconds up/down, and a certain number left/right, its much easier when your up/down, left/right adjustments do just what they are suppose to and not skew the other adjustments in the process. Just makes it a little easier is all. waytogo.gif


Edited by T~Stew, 27 April 2021 - 06:13 AM.

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#18 ks__observer

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Posted 27 April 2021 - 07:15 AM

I like to explain it like an Etch A Sketch, anyone remember those? You have two knobs, up/down and left/right, much like alt-az adjustment. If the Etch A Sketch is perfectly level the up and down will go straight up and down. But now shim one side of the Etch A Sketch up a little, now if you try to make a straight line up, you'll have to move both knobs, one a lot, the other a little, to get a straight line up. When you have software like ASIair pro that tells you to move a certain number of arc-seconds up/down, and a certain number left/right, its much easier when your up/down, left/right adjustments do just what they are suppose to and not skew the other adjustments in the process. Just makes it a little easier is all. waytogo.gif

I guess you are saying guiding will correct out the error as it will nudge in both the RA and Dec.



#19 BQ Octantis

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Posted 27 April 2021 - 08:29 AM

The setup error of the mount is in pitch (N-S), roll (W-E), and yaw (azimuth). The pitch error is compensated for mostly by the latitude adjustment. The roll error is compensated for in calibrating the RA setting circle (or doing a 1-star align with the GoTo). The yaw error is absorbed in the azimuth adjustment. If the starting errors were small, the cross products of the rotation matrices should be small; larger errors will result in an azimuth change induced by a latitude (altitude) adjustment and an altitude change induced by an azimuth adjustment during alignment. Regardless, all the cross-products get absorbed as small variances in the latitude, azimuth, and RA offset errors required to achieve polar alignment and calibrate the RA clock angle.

 

Once polar aligned and the RA clock calibrated, all the errors are fixed and do not change at all with tracking. With the two RA axes (i.e., the mount's and the Earth's) aligned, there is no further offset to account for in tracking. The mount tracks around its RA axis alone. The residual polar alignment error will trace out a sine wave in DEC error with an amplitude of the polar alignment error and a period of one sidereal day (23h56m); whereas the effect on RA tracking will be swamped by periodic error. The source of the residual polar alignment error is completely irrelevant. This is why leveling is irrelevant once polar alignment is complete.

 

BQ


Edited by BQ Octantis, 27 April 2021 - 08:43 AM.


#20 kathyastro

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Posted 27 April 2021 - 08:50 AM

I guess you are saying guiding will correct out the error as it will nudge in both the RA and Dec.

No, he is saying that getting it polar aligned requires adjustments in both az and alt, but, once you get it aligned, there is no error for guiding to correct out.  At least no more than if the mount were level: no PA is ever perfect.


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#21 bobzeq25

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Posted 27 April 2021 - 11:47 AM

 

 

The big difference in being perfectly level means the alt & az adjustments will be precise and expected. If your mount is say at a 45º angle then moving your alt adjustment is not going to just move the altitude, it will move both altitude and azimuth by about the same amount. So it will require much more fiddling to get it right.

Not with a PoleMaster.  You get real time feedback about where the mount RA axis is moving with regard to the North Celestial Pole, you basically just overlay two targets.  Even if one adjustment affects the other, it's trivial to get the two things aligned.



#22 audioengr

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Posted 27 April 2021 - 01:12 PM

I believe it depends on how smooth and accurate your Declination drive is compared to your Right Ascension drive, and the effects of counter-weights and masses hanging on the OTA.  Guiding control may be smoother and less jerky with mostly RA movement and almost no Declination movement.  With non-level mount, it will continuously have to compensate in one direction of declination.  This is probably more of an issue with ALT-AZ mounts on a wedge.

 

I verify my levelness by doing polar alignment and centering a star on my screen and then shut down the mount.  Then wait 2-3 hours, power the mount back on and manually slew the OTA using only RA movement to the same star.  If it is not still centered, I'm not level.


Edited by audioengr, 27 April 2021 - 01:16 PM.


#23 Der_Pit

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Posted 27 April 2021 - 03:57 PM

I believe it depends on how smooth and accurate your Declination drive is compared to your Right Ascension drive, and the effects of counter-weights and masses hanging on the OTA.  Guiding control may be smoother and less jerky with mostly RA movement and almost no Declination movement.  With non-level mount, it will continuously have to compensate in one direction of declination.  This is probably more of an issue with ALT-AZ mounts on a wedge.

Not true.  Dec drift comes from PA error.  If you are correctly polar aligned, there is no Dec drift, no matter how un-level your mounts base plate is....

 

The only reason I see for very good leveling is if you intend to drift-align.  That will be a real pain if there is crosstalk between Az and Alt corrections.


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#24 kathyastro

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Posted 27 April 2021 - 04:56 PM

With non-level mount, it will continuously have to compensate in one direction of declination. 

No it won't.  You are confusing being out of level with not being polar-aligned.  The continuous compensation in declination only happens if you are not polar aligned.  As has been amply indicated in this thread, the mount does not have to be level to have a good polar alignment.



#25 xiando

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Posted 27 April 2021 - 05:18 PM

The real question is, do you mind leveling your mount? I have a torpedo level in my kit for the purpose, and it is done almost without even thinking while setting my tripod, just prior to installing the mount head, on the theory that a leveled mount will in general result in better balance and less strain in gearing (regardless of whether that's "true" or not).

 

Personally, I think it's just "good practice", but some folks around CN seem to almost sneer at the idea, as if it was a bad idea, offering up what amounts to advise to NOT level it. For me the leveling takes all of about a minute. 

 

So if you do mind, then don't, and if not, do, and just ignore the dude or dudette you go to star parties with or setup with who mocks you for it. It's kinda like cleaning your eyepieces. I just breathe on them and wipe them off with a chammy. Gasp! And I don't rebalance each time I setup. I have my optical train permanently assembled (sans maintenance), and have a "stop" mounted to my carry rail, so when I drop the optical train onto the mount, it's already balanced (within reason) and whatever residual is left is handled by my guiding software. we all have our own style and quirks. 


Edited by xiando, 27 April 2021 - 05:24 PM.

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