Aligning mount

[Anthony1979]

Do you align your mount before or after setting up your scope... Just wondering 

[Alan64]

You want to level the mount, align its RA-axis to Polaris(NCP), then attach the telescope and balance it.

[Geoff Lister]

If you have an Az/Alt mount, again, level the mount first. This reduces any cross-coupling between the altitude and azimuth axes. If you have a GoTo mount, this reduces the corrections that the alignment calculations have to make, and so reduces the pointing error when the mount slews to the second alignment star.

Geoff

[Anthony1979]

Thanks

[Anthony1979]

I do all the alignment before putting telescope on but wouldnt it knock it out off alignment while setting up.... What is second alignment star

[Geoff Lister]

On 16/01/2020 at 08:18, Anthony1979 said:

I do all the alignment before putting telescope on but wouldnt it knock it out off alignment while setting up.... What is second alignment star

The setting of tripod/mount level and OTA horizontal takes out most of the errors. The GoTo alignment on 2 stars, centred in the eyepiece, should remove the final errors in your initial settings. The first star takes out most of the errors, and the second adjusts the final values.

Geoff

[Anthony1979]

Thanks 

[Geoff Lister]

When I bought my first GoTo system, the Skywatcher Skymax 127 with Synscan handset, I found it hard to get good alignment; adequate, but not as good as I would have liked. I think I now know what is needed.

In an ideal world, the azimuth axis (mechanical, or the digital model in the handset) has to be perfectly vertical. To get the mechanical axis vertical, the top surface of the tripod (or the Dobsonian base of my Skyliner) has to be horizontal. The bubble level glued into the Skymax's tripod is a good indication, but the one on mine was not quite level. I used a good builder's spirit level to get the tripod level, and marked the position of the bubble with a spot of Tipp-ex, about the same size as the bubble. Now, tripod leveling is a simple game of "hide the bubble". Provided that I do not kick the legs, attachment of the mount and OTA does not seem to shift the bubble.

At power-up, the mount's digital model starts at zero (level and pointing North). I usually try to start with the mount's altitude pointer at zero and the OTA pointing roughly North (compass, or if it is dark, Polaris and then lower the OTA back to zero).

Assuming that your tripod/mount/OTA is not "perfectly" level, the azimuth axis will be leaning over, and this will affect altitude accuracy. It will be accurate at 2 points, 180 degrees apart, and most inaccurate at 90 degrees to these points. However, at this stage, you, and the digital model, do not know where these points are. The digital model, assuming that you have set up time/date/location correctly, does know where the important stars are. The manual slew, and final eyepiece centring on the first star will give the digital model a fairly accurate azimuth and altitude calibration, including the component of the azimuth axis "lean" in that direction. If you choose a second alignment star close to 90 degrees in azimuth from the first, and preferably with a different altitude, the digital model will have calculated the direction and magnitude of the "lean", and so can compensate for it when using tracking or GoTo.

It may be my imagination, but I have found that, if I can minimise the time duration between starting the slew to the first star and centring the second, the GoTo accuracy is better. By using the brightest stars, easily visible from your observing location (not too high, but high enough to be well clear of houses, trees and fences), it is possible to speed up the process. A bit of prior homework (I use "Stellarium" on my PC) to identify the 'best' stars, is worthwhile. I went to the next stage, and produced a table of 14 stars by the 12 months of the year, and for the middle of each month, at dusk, I selected 3, 4, or 5 stars with good separation, and noted their rough direction and altitude. For an Az/Alt mount, Polaris is just as valid as any other star.

The above is for Az/Alt mounts; EQ mounts are not so reliant on a level tripod, but substitute that for good polar alignment.

Geoff