Polar Alignment Question

[Philip Benson]

Hopefully I won't be deafened by the cries of 'Oh no not another question about polar alignment'!

Last night I was helping a friend test out his new HEQ5 mount with v4 Synscan handset.   We sited the tripod on his paved patio and then set about levelling with a clinometer app I have installed on my phone.  This gives a generously sized circular bubble level which seems entirely fit for purpose.

A few minutes later the tripod was adjusted to better than 0.1degrees in all directions.   Unfortunately we could see the NCP directly so we had set the tripod with the azimuth adjuster as close to north as we could estimate.   The mount was then attached, the latitude set to as close to 51.5d as I could see and the procedure for a 2 star alignment followed. 

After centring Arcturus and Vega we got the message 'Alignment Successful' and then aimed at Saturn with the handset.  Slew completed we inserted a 25mm eyepiece but no sign of Saturn.  Next I suggested following the polar alignment routine provided by Synscan.   Once again after aligning on Vega and Arcturus Ichose the latter as the alignment star.   After adjusting the mount manually in alt and azm as per the routine I ended up with the latitude scale at 54 degrees.  That is some 3 degrees off what I knew it should be (51d 35') 

Since we made sure that the tripod was definitely level at the outset I was surprised by the apparent error with the latitude scale.  I know the scales shouldn't be taken as gospel but surely a 3 degree error is a bit too much?!?!

[Bizibilder]

The latitude scales on SW mounts is more for show than any accuracy - 3° out is not bad!  Believe the handset alignment and the goto's should be OK.  I must admit I use EQMOD but I would suggest that with the handset alignment you choose your alignment stars reasonably close to your target.  Aligning the tripod top "level" will make little difference - it is the polar axis alignment that is critical.  The main body of the mount and tripod are simply there to keep the thing attached to the nearest planet   !!

Will the handset allow a three star alignment?  If so then you will get better goto's using that - the more alignment points you have the better the "map" held by the computer and therefore the better alignments when you goto.

[jambouk]

The labels on the mount may not be accurate. You will be best to set it up where you can see polaris, and after aligning the polar scope with the RA axis of the mount in daylight forst, get Polaris in re cross hairs of the polar scope and note the elevation on the labels on the mount. Whatever the labels say, if polaris is in the cross hairs, you have your mount set correctly for your latitude to within less than a degree. Note this is not how you polar align but can be used to get a rough polar alignment.

Levelling the mount to that degree of accuracy is unnecessary. If it looks roughly level, that is sufficient levelling.

Even if the handset says "aligent sucessful" the goto cab be [apparently] miles out for numerous reasons, in nonparticular order:

Telescope has a very long focal length.

Poorly balanced set up.

Lots of cone error.

Mount/sxope gets knocked/moved.

And lots more...

It's a steep learning curve, but an interesting one.

James

[Owmuchonomy]

If it is GoTo accuracy you are after use the 3 star alignment and make sure one of the stars chosen is the opposite side of the meridian to the others.