SW Go2 Alt-Az only 2 star align - why not 3?

[Fraunhoffer]

I'm a bit puzzled why the ALT-AZ handset configuration on a SW Dobsonian go-to only offers 2 star alignment, yet the EQ mount versions offers both a 2 and a 3 star alignment.
Isn't the 3 star more accurate - so why not on the ALT-AZ mount (Dobsonian) aswell ?

Which then makes me think - how accurate does the levelling on the Dob mount need to be?
Puzzled.

[vlaiv]

No idea how important 3 star vs 2 star is for accuracy, not if there is any difference between Alt-Az and EQ in needed stars for alignment, but maybe usage of the scope could provide some clues?

AltAz is not meant for imaging, hence there is no great need for alignment precision as one won't be working with small FOV and accurate finding / tracking of objects. Maybe rationale is that for visual, people will use different eyepieces - wide field ones once scope moves close to target and then adjust position with hand set or higher magnification views? So it could be the case that 3 star alignment is offered on EQ mounts for moments when you need greater precision - like when working with relatively small FOV (largish focal length and smaller sensor size) - so that you can still be on target after slew?

[Fraunhoffer]

I wondered that, but a 12 inch dob at F/5 will have approx. 1500mm focal length and a Fov of around 1 deg with 30mm ep.
Then I thought maybe if the base wasn't exactly level there could be elevation errors between the alignment points as the axis of rotation might be in slightly different places.

Maybe im just thinking too hard 🙂
I was just surprised it didn't come up in the menu and yet PE correction is.

[vlaiv]

Indeed, interesting question. I need to think a bit deeper on how alignment stars correct different issues, like mount not being level and with EQ - polar alignment error.

[Geoff Lister]

23 hours ago, Fraunhoffer said:

Then I thought maybe if the base wasn't exactly level there could be elevation errors between the alignment points as the axis of rotation might be in slightly different places.

Yes. This is my understanding of the process:-

With a perfectly level mount, and the altitude (elevation) set at (say) 45 degrees, a full 360 degree azimuth rotation of the mount should describe a circle at exactly 45 degrees to the horizon. However, if the mount was tilted slightly (say) towards the south-east, that circle would fall to below 45 degrees in a south-easterly direction, gently rising to above 45 degrees in a north-westerly direction.

At power-up, the Az/Alt mount does not know where it is pointing, so the handset sets its pointing software registers to zero; equating to the OTA level and pointing to the North Pole. It works out any new position by counting pulses from the motor encoders on the 2 axes, multiplied by the gearbox ratios.

When you select your first alignment star, the handset calculates the correct azimuth and altitude of that star, based on the time, date, and location that you have entered. When you centre the first star, the software can now put the "correct" Az & Alt values in its registers. When you select the second star, the mount performs an automatic slew to point the OTA towards the location in space where that star "should" be if the mount was perfectly level. The software then records the difference between its "guess" and the true position, when you have centred the star and hit "enter". It uses these differences to make a best estimate of the mount's tilt.  If the 2 stars have an azimuth difference of about 90 degrees, and an altitude difference of at least 30 degrees, this makes the sums easier, and the "estimate" more accurate.

It is much easier to get the mount level with a spirit level, than it is to get the OTA pointing to true (not magnetic) North. With the mount level, the calculations are likely to be more accurate, to give you a better GoTo position. It also means that the "guess" for the second star is more accurate, and so easier and quicker to find and centre. If your mount comes with a small bubble-level, it is worth checking this against a good spirit level, as I have found that the one on one of my mounts was a bit off. I marked the proper bubble position with a small blob of Tipp-ex, and that seemed to speed up alignment for future sessions.

I am sure that there are other forum members who have a more detailed knowledge of the alignment process, but I hope the above explanation makes sense. I use Az/Alt Goto, but I would assume that a 3-star alignment helps to eliminate the extra polar alignment errors.

[Fraunhoffer]

Thank you for the detailed explanation.
I found the metal bits affect the compass, so Ill have to mark the fence where north is and put a little sight on the tube when using it at home.
Im in the process of making a small platform with adjustable feet to get the base level.

 

[Geoff Lister]

3 hours ago, Fraunhoffer said:

Im in the process of making a small platform with adjustable feet to get the base level.

My 250PX mount does not have adjustable feet. I mounted a decent bubble level on the base. To level it, I turn the bottom of the base so that the bubble is towards the direction between 2 of the feet, and insert a metal wedge under the third foot to centre the bubble. I used a metal wedge, because it was handy, but a wooden one would work fine. Once level, I rotate the upper part of the base to point north, extend and fit the tube, and power up.

 

3 hours ago, Fraunhoffer said:

I found the metal bits affect the compass, so Ill have to mark the fence where north is and put a little sight on the tube when using it at home.

Yes, this is a problem, but the first star alignment will sort out any compass error. The important bit is the level mount.

Geoff