Alignment question.

[DroneSaif]

Hi,

I have been reading quite a few posts here and elsewhere about the trouble with getting the alignments too the stars accurately.

I don't understand why alignment is a problem. It seems to me (perhaps naively ) that with the accurate date and time, GPS satellites and electronic compass, the mount software for goto mounts should be able to automatically align to Polaris, or to whatever star/planet the viewer wants, without the need for the alignments. 

I don't yet have a telescope or a mount and am looking to get enough info to get the one that's good, so perhaps the reason why I am not clear on this. 

What am I missing ?

Edited August 9, 2020 by DroneSaif

[Cornelius Varley]

3 minutes ago, DroneSaif said:

Hi,

I have been reading quite a few posts here and elsewhere about the trouble with getting the alignments too the stars accurately.

I dint understand why alignment is a problem. It seems to me (perhaps naively ) that with the accurate date and time, GPS satellites and electronic compass, the mount software for goto mounts should be able to automatically align to Polaris, or to whatever star/planet the viewer wants, without the need for the alignments. 

I don't yet have a telescope or a mount and am looking to get enough info to get the one that's good, so perhaps the reason why I am not clear on this. 

What am I missing ?

The reason the mount needs to be star aligned is so that the handset's internal model of the night sky can be mapped onto the real world sky. Slight variations in time, date, location and polar alignment are reduced by star alignment, the handset knows roughly where the stars are in the sky based upon the time etc and then star alignment makes the necessary adjustments to the internal model.

[Jiggy 67]

If you were out walking in the wilderness with a map, you would need to orientate yourself with the map, the map would be useless unless you fitted it to the ground you’re walking in. A GoTo is no different, it has the map but needs to be orientated to sky to be able to use the map. 

[teoria_del_big_bang]

To the best of my knowledge there are no mounts for ameteur use that have any motors on the adjustments needed to align to Polaris ( or more accurately to align the Centre of rotation of Ra to the rotation of the Earth).

The only motors are on the Ra and Dec axis themselves. The adjustments needed are mechanical movements with leveling the mount. then the angle of the tripod to the ground needs adjusting so this is placing the tripod at the correct approximate  angle then using some fine adjustment screws to get it correct and also using some fine adjustment to get the Declination angle correct (although this is dependent on where you are on the Earth Latitude wise usually some very small adjustments are needed each time you move the mount which again is a mechanical thing using some adjustment screws.

 

The only motors are on the rotation of Ra and DEC themselves which are not the adjustments needed to align the Ra axis correctly.

Now I am not saying this is not possible to make it automatic in theory but for one, the cost of putting motors on the adjustments will be expensive and for another how accurate is the GPS system currently ? Probably not as accurate as we can achieve currently using this manual alignment method.

After polar alignment the 2 or 3 star alignment again is just another very fine adjustment used to get it even better.

 

Steve

[Gfamily]

Although the GPS can easily determine the position of the telescope on the Earth's surface, it needs to know how the telescope is pointing relative to North and also whether there is any tilt to the telescope mount. 

There's no issue with knowing the position of the stars and planets on the sky, but it's the alignment of the telescope here on earth that needs to be confirmed. This is similar to the way the Satnav in your car doesn't know which way you're pointing until you start moving. 

And for astro, ideally, this would be to within a small fraction of a degree. 

 

 

Edited August 9, 2020 by Gfamily

[jambouk]

There is no built in feedback system in most mounts / telescopes, so the mount has no idea if it is actually pointing at the correct thing or not. The user is the feedback system. Many issues are related to user error. A system is only as good as the data it is supplied with. Give it duff data and your output will be duff to the power 4.

You also have to remember polar alignment is totally different but as important as star alignment if using GOTO on an equatorial mount.

One could argue why so many cars crash, when they are built so well these days with sensors to indicate when the tyres are going flat or the engine needs oil - all well and good but there is still a human behind the wheel and they are prone to errors.

James

 

[CraigT82]

Mounts also have clutches than can be undone to move the mount manually without using the motors. The software has no idea if the clutches have been undone and mount moved or not. 

[Cosmic Geoff]

Further to the above, some mounts are much easier to align than others.  An equatorial mount requires a polar alignment (mechanical) to be performed before the star alignment (electronic).  An alt-azimuth GoTo mount just requires the star alignment.

Then there are plate-solving accessories, of which Celestron's 'Starsense' is the leading example, which image and recognise sections of the night sky, and proceed to automatically perform the star alignment.

GPS, either built-in or as an accessory, will substantially ease the alignment process by automating the entry of date/time/location, a tiresome procedure which newbies in particular often get wrong.

WiFi schemes in which the user's tablet or phone replaces the handset, or a scheme where a laptop replaces the handset, are becoming increasingly common. Whether this helps or hinders usability is a matter of personal choice.

Finally, the GoTo software packages from different manufacturers differ significantly.  IMHO the 'Nexstar' system is easier to use than the 'Synscan'.

So all GoTo systems are not the same - it is far easier and quicker to align, for instance, a Nexstar alt-az mount with GPS than a Synscan EQ  mount without GPS.

[AstroMuni]

2 hours ago, DroneSaif said:

I don't understand why alignment is a problem. It seems to me (perhaps naively ) that with the accurate date and time, GPS satellites and electronic compass, the mount software for goto mounts should be able to automatically align to Polaris, or to whatever star/planet the viewer wants, without the need for the alignments. 

Besides the ones you mentioned there is one other parameter - direction of polar axis. You are theoretically close to achieving alignment if the Polar axis as shown in a diagram in post above is close to the true Pole. With these parameters the mount should be able to get close to objects. But as we are looking at these objects with high magnifications the field of view (FOV)is quite narrow and this needs to be more precise. Otherwise what tends to happen is that the object is close by but not visible in the eyepiece 😞

Once the object is in our FOV, then comes the business of tracking this object as it moves across the sky...If the mount is not properly aligned, as the mount attempts to follow this path, it slowly but surely becomes out of sync with the object. If you are a visual observer you could gently nudge the scope to bring it back into view. But if you are attempting astrophotography over several minutes/hours, this becomes harder to achieve and you start seeing smear marks on the output.

HTH in your understanding of this issue 🙂

[DroneSaif]

Hi all,

Thank you for humouring me with these answers and they are very insightful. 

If I can summarise the answers.

1. The accuracy of the GPS as they stand today is not good enough for this. Best I have seen is 5 meter radius, but I have seen discussions where 1m is possible in open space. 
2. Accuracy if Compass is also not good enough, to know the direction the mount was pointing to. I suppose a gyroscope may be more accurate, but I think using triangulation 3 GPS antenna 2 meters apart, might do the same job. Again, accuracy of GPS may be an issue. 
3. Amateur mounts do not have motors to for RA/Earth Axis alignment, thus requiring human input, which is also inaccurate.
4. Accuracy of the mechanical movements of the motors are too low. And position information is lost if clutch is used. In think the AZ-GTI from sky watcher is able to track despite having a clutch, but yes motors may not move accurately enough.
5. All the above meant that accuracy to the level needed on a very narrow FOV scope, is not currently achievable even for visual observers, much less for astrophotography usage. I suppose a small error will be magnified simple due to the distance involved.

thank you for greatly increasing my understanding of the problem.

 

 

 

 

[Gfamily]

In fact, your point #1 isn't really an issue, moving the scope a few metres in either direction isn't going to make a difference. 

Point #2 is more of a realistic limitation. Yes, in principle, an electronic compass could assist with finding magnetic north, but the system would then have to apply the correction for the magnetic declination, which itself varies by location and over time.

Given #2, #3 is not really relevant, and #4 is more a guiding / tracking issue than an alignment issue.

But enough of the theory, how are you finding things in practice 😀

[DroneSaif]

1 hour ago, Gfamily said:

 

But enough of the theory, how are you finding things in practice 😀

Ahhh but sir... I don’t have a Telescope yet. Right now I am just using a camera and a tripod. 😁😁
 

i have been looking around to find a mount and scope that makes life easier, this the need to understand.