Home position on mounts.

[miguel87]

1 minute ago, Robindonne said:

I agree with all the answers.  And maybe im to confused to see your points.  Or maybe im confused about the polaralign methods using asiair or sharpcap.  Im really just starting astronomy for some months. Im more experienced with making wooden  stairs on my cnc machine.  And if my axes are not calibrated perfectly the straight line im planning to route through wood will always need some movement of my second steppermotor.  
 

So to understand all of your answers.  Polaralignment can never be done accurate through the main scope?  

It could in theory but why worry when you can just use the mount and be done with it rather than worrying how well aligned your main scope and mount are, marking home position etc etc.

Align mount. Done.

[Robindonne]

17 minutes ago, miguel87 said:

It could in theory but why worry when you can just use the mount and be done with it rather than worrying how well aligned your main scope and mount are, marking home position etc etc.

Align mount. Done.

So rebuy a polemaster and forget about asiair polar alignment? I hate the idea of bringing a laptop and still believe a good polaralignment is possible in some minutes with a solid homepositioned mainscope and without kneeling or using a polemaster and its laptop.  

Edited April 28, 2020 by Robindonne

[CraigT82]

10 minutes ago, Robindonne said:

So rebuy a polemaster and forget about asiair polar alignment? I hate the idea of bringing a laptop and still believe a good polaralignment is possible in some minutes with a solid homepositioned mainscope and without kneeling or using a polemaster and its laptop.  

Have you heard of cone error? It can be quite difficult to have the main scope optical axis exactly aligned with the mount's RA axis, especially with moving mirror scopes.

[Robindonne]

9 minutes ago, CraigT82 said:

Have you heard of cone error? It can be quite difficult to have the main scope optical axis exactly aligned with the mount's RA axis, especially with moving mirror scopes.

Aah.  Thats i think some missing info in my head.  And that error can occur in both directions of course.  So my fixed homeposition is not even possible when changing scopes?

[CraigT82]

8 minutes ago, Robindonne said:

Aah.  Thats i think some missing info in my head.  And that error can occur in both directions of course.  So my fixed homeposition is not even possible when changing scopes?

No I dont think so, even changing primary collimation on a newtonian will introduce cone error

[Robindonne]

On 28/04/2020 at 18:23, CraigT82 said:

No I dont think so, even changing primary collimation on a newtonian will introduce cone error

Thanks. Thanks all answers but this cone error reason answer in particular.  
 

To make a happy end on this conversation a last question.  If you somehow manage to reduce cone error to almost zero. Let say by calibration in whatever form. Turning a newtonian or refractor in its rings or some other methods for other scope types.  And having a perfect polaralignment.  Will it reduce the amount of help movements of the second motor for accurate guiding?   Or will their always be slight(minimal) differences in starpositions? I mean a minimal off the equatorial axes movements.

Edited June 30, 2020 by Robindonne

[carastro]

This may answer some of your problems, including cone error.

Carole

 

[Grierson]

54 minutes ago, Robindonne said:

So rebuy a polemaster and forget about asiair polar alignment? I hate the idea of bringing a laptop and still believe a good polaralignment is possible in some minutes with a solid homepositioned mainscope and without kneeling or using a polemaster and its laptop.  

Why? If you have the ASIAIR the Polar Alignment process in the software is more than capable of producing the required accuracy. I can see little or no difference between using the PoleMaster or the AIR or indeed SharpeCap for that matter!

Edited April 28, 2020 by Grierson

[CraigT82]

13 minutes ago, Robindonne said:

Thanks. Thanks all answers but this cone error reason answer in particular.  
 

To make a happy and on this conversation a last question.  If you somehow manage to reduce cone error to almost zero. Let say by calibration in whatever form. Turning a newtonian or refractor in its rings or some other methods for other scope types.  And having a perfect polaralignment.  Will it reduce the amount of help movements of the second motor for accurate guiding?   Or will their always be slight(minimal) differences in starpositions? I mean a minimal off the equatorial axes movements.

In theory yes. But difficult to achieve in practice. Depends on the resolution you're imaging at (or focal length if visual). Some expensive mounts don't require guiding at all, they use sky models and encoders to know exactly where they are pointing. 

[Robindonne]

13 minutes ago, carastro said:

This may answer some of your problems, including cone error.

Carole

 

Ah great vid.  Very stupid i skipped this episode because the whole “home-position-method” with levels was from this guy.  So tiny adjustments can be made to reduce cone error.  Thanks again for the link

[Robindonne]

On 28/04/2020 at 18:41, Grierson said:

Why? If you have the ASIAIR the Polar Alignment process in the software is more than capable of producing the required accuracy. I can see little or no difference between using the PoleMaster or the AIR or indeed SharpeCap for that matter!

No well than im happy with your advice and just plan my future set without paying much attention to the whole polarscope/polemaster.  I think if i never got to know asiair Or sharpcap my answer would also be: polaralignment can only be done with a polarsope.  Thx

Edited June 30, 2020 by Robindonne

[Robindonne]

13 minutes ago, CraigT82 said:

In theory yes. But difficult to achieve in practice. Depends on the resolution you're imaging at (or focal length if visual). Some expensive mounts don't require guiding at all, they use sky models and encoders to know exactly where they are pointing. 

Thx.  Im aware the scale we are using with astronomy is not imaginable with earth sized calibration possibilities.  That I underestimated i think.  My questions were mostly raised due to all these graphs?? Of guiding accuracy.  It looks like their is always adjustment of the tracking needed and the less adjustment the better of course.  But getting a straight line without adjusting is just not possible it seems. Thank you again for the info.  Ill never have to ask it again after this chat. 

Edited April 28, 2020 by Robindonne

[IanL]

You're confusing three different concepts:

1. Polar alignment refers to the alignment of the RA axis of the mount so that it is parallel to the Earth's axis of rotation. Neither the mount's home position nor the scope's cone error have any bearing on this.

- You can polar align using the mount's polar scope. This is subject to a number of cumulative sources of error; how well the polar scope and reticule are aligned to the mount's RA axis, and your ability to precisely align Polaris on the reticule over several steps to find the appropriate hour angle. Usually this can be done well enough for visual, but unlikely you'll get within more than a few arcminutes of the pole unless very experiences.

- You can't really use the main scope for this job, as it is subject to cone error, and you do not need to centre on Polaris, but position Polaris in the field of view such that the North Celestial Pole is centred (assuming no cone error).

- Much easier is to use a Polemaster or something like Sharpcap. These don't rely on any precise alignment of anything. They take an image of the sky through the camera (dedicated or through your main scope respectively). You then rotate the mount approximately 90 degrees in RA and a second image is taken. By plate solving both images, the software can determine the centre of rotation in the image, and therefore exactly where the mount's RA axis is pointing (not the camera or scope, the axis). You then adjust the mount until the axis is pointing at the correct position in the sky, very precisely determined by repeated imaging and plate solving. You can achieve a few arc seconds of accuracy very easily, the limit being the mechanical stability of the mount and the coarseness of the alt-az adjuster threads.

2. The home position is only important for mounts that don't have absolute encoders (such as many Skywatcher mounts). The mount controller (handset, EQMod, whatever) only knows where the mount is pointing by counting the number of steps the RA and Dec stepper motors have been told to move. This is unlike an absolute encoder which will usually start from a defined index position (found automatically using a sensor), with the encoders counting the rotations of a given gear axis electronically and reporting back to the controller the encoder position, which the controller converts in to RA and Dec.

The problem with stepper counter control is that you have to know where you are starting from. There is no absolute position as such, so the controller can only determine that a given axis has moved X steps (and thus degrees) clockwise or anticlockwise from wherever the mount was pointing when it was switched on. Thus the need for a home position, so that the mount can assume a given start location and go from there. If your home position is off a bit, you'll be correspondingly far off after your first slew to a target. You can of course correct this by moving the mount on to target and syncing the handset or EQMod which updates its pointing model to eliminate this error slightly.

3. Cone error also induces slews to be off target since the handset will of course assume that the scope is pointing at the Celestial Pole (not Polaris) when in the home position. Again the pointing model can and will compensate for this as you do more corrections and syncs, but it is desirable to get a good home position and tune out any cone error mechanically to make life less difficult for the visual observer. For an imager, just use plate solving once you are polar aligned; there are numerous free plate solvers and most capture software supports one or more of them, and it literally saves hours when the software finds the target unaided.

The challenge with having a set of home position marks made by the manufacturer is perhaps that you can rotate the dovetail clamp by 90 degrees to accommodate a side-by-side bar? I don't see any reason why they couldn't put fixed index marks at 90 degree intervals around both halves of the RA and Dec Axes to be honest, it would save 10 minutes with stickers and fine-tipped pens, but I suppose there would be additional work to adjust everything so they were properly aligned and represented the actual home position.

[Robindonne]

18 minutes ago, IanL said:

You're confusing three different concepts:

1. Polar alignment refers to the alignment of the RA axis of the mount so that it is parallel to the Earth's axis of rotation. Neither the mount's home position nor the scope's cone error have any bearing on this.

- You can polar align using the mount's polar scope. This is subject to a number of cumulative sources of error; how well the polar scope and reticule are aligned to the mount's RA axis, and your ability to precisely align Polaris on the reticule over several steps to find the appropriate hour angle. Usually this can be done well enough for visual, but unlikely you'll get within more than a few arcminutes of the pole unless very experiences.

- You can't really use the main scope for this job, as it is subject to cone error, and you do not need to centre on Polaris, but position Polaris in the field of view such that the North Celestial Pole is centred (assuming no cone error).

- Much easier is to use a Polemaster or something like Sharpcap. These don't rely on any precise alignment of anything. They take an image of the sky through the camera (dedicated or through your main scope respectively). You then rotate the mount approximately 90 degrees in RA and a second image is taken. By plate solving both images, the software can determine the centre of rotation in the image, and therefore exactly where the mount's RA axis is pointing (not the camera or scope, the axis). You then adjust the mount until the axis is pointing at the correct position in the sky, very precisely determined by repeated imaging and plate solving. You can achieve a few arc seconds of accuracy very easily, the limit being the mechanical stability of the mount and the coarseness of the alt-az adjuster threads.

2. The home position is only important for mounts that don't have absolute encoders (such as many Skywatcher mounts). The mount controller (handset, EQMod, whatever) only knows where the mount is pointing by counting the number of steps the RA and Dec stepper motors have been told to move. This is unlike an absolute encoder which will usually start from a defined index position (found automatically using a sensor), with the encoders counting the rotations of a given gear axis electronically and reporting back to the controller the encoder position, which the controller converts in to RA and Dec.

The problem with stepper counter control is that you have to know where you are starting from. There is no absolute position as such, so the controller can only determine that a given axis has moved X steps (and thus degrees) clockwise or anticlockwise from wherever the mount was pointing when it was switched on. Thus the need for a home position, so that the mount can assume a given start location and go from there. If your home position is off a bit, you'll be correspondingly far off after your first slew to a target. You can of course correct this by moving the mount on to target and syncing the handset or EQMod which updates its pointing model to eliminate this error slightly.

3. Cone error also induces slews to be off target since the handset will of course assume that the scope is pointing at the Celestial Pole (not Polaris) when in the home position. Again the pointing model can and will compensate for this as you do more corrections and syncs, but it is desirable to get a good home position and tune out any cone error mechanically to make life less difficult for the visual observer. For an imager, just use plate solving once you are polar aligned; there are numerous free plate solvers and most capture software supports one or more of them, and it literally saves hours when the software finds the target unaided.

The challenge with having a set of home position marks made by the manufacturer is perhaps that you can rotate the dovetail clamp by 90 degrees to accommodate a side-by-side bar? I don't see any reason why they couldn't put fixed index marks at 90 degree intervals around both halves of the RA and Dec Axes to be honest, it would save 10 minutes with stickers and fine-tipped pens, but I suppose there would be additional work to adjust everything so they were properly aligned and represented the actual home position.

No you’re right about polaris and the celestial pole.  I mis-typed that word.  I know a good alignment means polaris is always off center, off the real celestial pole.  But based on so many factors a polarscope can be off alignment with the mount and I think also a polemaster can suffer minimal off alignment with the mount.  Due to attachment on paint etc.  Thats exactly why i believed a polaralignment can be done with the mainscope.  But encoders or no encoders, the scope should be pointed exactly to the celestialpole for this mainscope polar alignment.  Thats why i wondered why the can not mark the mount of factory.  But all made sense when cone error was mentioned.  A home position is just not possible, at least not for the reason i think i need it: asiair polaralignment.  
 

The guidescope will have to correct these small errors.   I do wonder if someone ever  accomplished a zero adjusted guiding. I mean No corrections? And how they did it

Edited April 28, 2020 by Robindonne

[IanL]

Again, yes a polarscope can be misaligned and will cause issues. A Polemaster does NOT depend on being well aligned though. The software determines the centre of rotation in the series of images, so any misalignment is automatically taken to into account. (Same for Sharpcap). That is why they are superior at alignment.

Yes you are correct that if you have a perfect polar alignment, an accurate home position and zero cone error the main scope should be centered on the NCP. In fact the easiest way to check your cone error is to take an image of the NCP (once polar aligned and homed). Plate solve it and you can see how much cone error you have.

You can get to (or close to) zero guiding with a high-end mount, e.g. a direct drive mount with no gears. For normal mounts you still need to guide as the gear train will have backlash and periodic error in the in order of arcminutes, and no amount of polar alignment will overcome that for exposures more than a few tens of seconds.

Edited April 28, 2020 by IanL

[Robindonne]

16 minutes ago, IanL said:

A Polemaster does NOT depend on being well aligned though. The software determines the centre of rotation in the series of images, so any misalignment is automatically taken to into account. (Same for Sharpcap). That is why they are superior at alignment.

You mean asiair and sharpcap? Or polemaster and sharpcap? Ive never used the polemaster because i sold it before i new how great it is and what it does.  But how it takes multiple images i dont know. I thought it was stuck to the mount.  And didnt move at all.  I hope you meant asiair and sharpcap because asiair is what im planning to use for my polar alignment.  I now only use it to connect my mount with SkySafari.  Photographing with my incompatible dslr is not possible.  Also polaralignment is not possible for me right now.  But my very near future plans to upgrade some gear, mount and camera, are based on planning to align with asiair.  

Edited April 28, 2020 by Robindonne

[IanL]

I don't know about the ASIAir but I assume it uses the same process. Basically you take an image, rotate the mount 30 (polemaster) or 90 (sharpcap) degrees in RA and take a second image. The software identifies the corresponding stars in both images and uses geometry to work out where the centre of roation is (i.e. where the RA axis is pointing on the sky). Using plate solving it works out where the NCP is in the image and directs you to adjust the mount until the NCP and centre of rotation match.

Edited April 28, 2020 by IanL

[ApophisAstros]

3 hours ago, Robindonne said:

Oh and maybe a second question related to my first.  A lot of tutorials tell you how to determine the home position of a mount. By leveling in both directions.   What is wrong with this other method: polaralignment through the polarscope, and after that, aligning the mainscope on polaris?   Shouldnt this “alignment of both scopes”, the polar- and mainscope not give a more accurate homeposition than leveling the axes?

When i am polar aligned i can see Polaris in the scope and guidescope without moving it , are you the same.

Roger

[miguel87]

The level of accuracy being discussed here is insanely unnecessary.

Is the OP doing advanced astrophotography or visual observing?

[Robindonne]

3 minutes ago, apophisOAS said:

When i am polar aligned i can see Polaris in the scope and guidescope without moving it , are you the same.

Roger

Well i meant pointing the polarscope on polaris itself( off polar-alignment) and then center polaris in the eyepiece of the mainscope to align the two.  And to determine the home position.  Ive never tried it.  tonight perhaps.  

[Robindonne]

8 minutes ago, miguel87 said:

The level of accuracy being discussed here is insanely unnecessary.

Is the OP doing advanced astrophotography or visual observing?

No not at all actually.  Just starting this addiction. Amateur astrophotography i mean.  But always wondered why mounts dont have a home position mark.  Excepts for some meade mounts i looked at.  And was also wondering what is causing the happiness when people look at guide graphs that show minor adjustments.  Why not be happy when its straight and un-adjusted.  But a lot of these questions are answered and showed me the reasons and the need of guiding instead of just tracking

Edited April 28, 2020 by Robindonne

[miguel87]

2 minutes ago, Robindonne said:

the need of guiding instead of just tracking

There is no need unless you are doing long exposure astrophotography.

[ApophisAstros]

4 minutes ago, Robindonne said:

Well i meant pointing the polarscope on polaris itself( off polar-alignment) and then center polaris in the eyepiece of the mainscope to align the two.  And to determine the home position.  Ive never tried it.  tonight perhaps.  

But when you align to another star or 2 it will change the mount anyway so no point moving Polaris to the centre before that as its a good idea to align near your target.

Roger

[miguel87]

1 minute ago, apophisOAS said:

But when you align to another star or 2 it will change the mount anyway so no point moving Polaris to the centre before that as its a good idea to align near your target.

Roger

What on earth are you talking about?

Sorry, no offense.

[Robindonne]

3 minutes ago, apophisOAS said:

But when you align to another star or 2 it will change the mount anyway so no point moving Polaris to the centre before that as its a good idea to align near your target.

Roger

Thx and I understand.  Just to be sure polar aligning with my mainscope would be safe and (almost) as good as with the polarscope

Edited April 28, 2020 by Robindonne