Levelling alt-az mounts for goto operation

[Zermelo]

In the spirit of ... "there is no question too basic" ...

I've been experimenting with my setup (SW 150i on Star Discovery mount) to see what might improve the consistency of the pointing and tracking accuracy.

One aspect is the initial levelling, which seems to be the subject of some debate. I read that for EQ mounts, it's fine to be only roughly level since it's the subsequent polar alignment that really counts. Then for an alt-az mount, some say again that a rough level is sufficient because the star alignment (I have tried both 2-star and 3-star) will build a sky model that compensates for any (small) differences from level. Others say that it's important to level as well as you can for good alt-az operation.

So far, I've played safe and followed the latter advice. Neither the tripod or mount in question has a built-in bulls-eye level (though apparently they're not always accurate anyway) so I use a mini bubble level (I checked it against an engineering level first). My preference was to level the mount when attached to the tripod, since I usually struggle to locate the bolts and suspect that I'm disturbing the tripod level in so doing. Unfortunately, the Star Discovery mount has a very "organic" rounded shape that will admit no levelling device, so I did end up having to use the top of the tripod (I didn't trust the accessory tray to keep a consistent position each session).

Another concern I had was that the weight of the OTA, when mounted afterwards, would disturb the level alignment, so more recently I switched to levelling the OTA itself when it was all assembled. I reasoned that the point of levelling was that the OTA itself should start level (i.e. it rotates parallel to the plane of the horizon), so measure that rather than the tripod or mount.  I first lock the dovetail so that the altitude scale reads 0 degrees. Then I use the mini level along the OTA axis only, and adjust the tripod legs to get horizontal readings with the OTA pointing in two perpendicular directions (this involves a number of iterations). Then I do the star alignment with the goto.

The conundrum I discovered recently was this. Having adjusted the tripod so that the OTA was level when pointing north and west, I tried checking it by flipping it 180 degrees to point east. It was significantly off level.  Given that I trust the spirit level, this suggests that the mount's azimuth axis is not aligned properly with the dovetail, i.e. not perpendicular to the dovetail's "zeroed" position.

Q: does this analysis sound right and, if so, can the goto's sky model still compensate for this when I do the star alignment? Or would it mean that, while I might get higher local accuracy in a particular area of sky by using additional pointing accuracy enhancement operations, it would not be possible to get the same level of accuracy simultaneously across the whole sky?

[Cosmic Geoff]

My experience is that the leveling of an alt-azimuth mount is only critical if you are using a one star or solar system align. (I am not sure if the Star Discovery mount does a solar system align at all - with Nexstar it means aligning on one planet or the Sun - same basic method as aligning on one star.)

[Zermelo]

19 hours ago, Cosmic Geoff said:

My experience is that the leveling of an alt-azimuth mount is only critical if you are using a one star or solar system align. (I am not sure if the Star Discovery mount does a solar system align at all - with Nexstar it means aligning on one planet or the Sun - same basic method as aligning on one star.)

Yes, that does tally with one set of opinions I've read (levelling not critical unless aligning on only one target).

[ScouseSpaceCadet]

With the AZGTI I assemble mount and tripod then check level via the mount bubble level, then add and balance the OTA. Once complete, level the OTA north by pointing it in the direction of polaris, then step back a bit and check the OTA is level against the house bricks, or rear garden wall.

Job done, gotos in the fov of a 24mm 65° eyepiece every time and tracking is fine.

One point regarding alignment, acquire and centre the alignment stars in a 25mm then swap to a 12mm (or 10mm if that's all you have) for fine tuning before accepting.

North Level Alignment is always used, unless I'm keeping things super simple and utilising Point and Track.

[Zermelo]

8 minutes ago, ScouseSpaceCadet said:

centre the alignment stars in a 25mm

yes, I use a 12mm illuminated reticle for alignment

 

8 minutes ago, ScouseSpaceCadet said:

check the OTA is level against the house bricks, or rear garden wall

I must check if the horizontal plank fence I put up is still level, that would be useful 😀

[ScouseSpaceCadet]

Just now, Zermelo said:

yes, I use a 12mm illuminated reticle for alignment

 

I must check if the horizontal plank fence I put up is still level, that would be useful 😀

'Keep things simple.' is my motto. 👍

[PEMS]

For either variety of mount then do the levelling as best you can in the time available.

If you are on an EQ mount then yes the polar alignment should take care of it, but to set the latiutude and so get Polaris in a reasonable position the level status of the mout comes into play. If your level is out by 5 degrees then the latitude scale is out by 5 degrees and a very good chance that Polaris is not in view and you spend more time locating Polaris then just doing a good job of levelling the mount.

Consider it as 2 minutes to level the mount or 5 minutes to find Polaris with a poorly levelled mount. So maybe you do not need to but it makes sense to do so.

On an Alt-Az mount level is more relevant. People think it doesn't matter and that the software will take care of it all. The software might but the better everything is set uo the better the corrections applied by the software will be and the more accurate the Goto will almost certainly be.

I found that Wilkinsons sold a very simple cross level, plastic, cheap. But it worked well. Put on the mount base and level that, put on the front of the OTA and level that. One thing I immediatly noticed was my idea of level was a lot different to reality. People are not generally that good at determining level. Easy a 5 degree difference and consider these days that if your car timing went 5 degrees out the EMU would almost certainly put a red warning up and stop the car.

And I have seen it said by someone on here: Levelling everything well makes it look as if you know what you are doing. Even if you really do not.

[Zermelo]

22 hours ago, ScouseSpaceCadet said:

22 hours ago, Zermelo said:

yes, I use a 12mm illuminated reticle for alignment

 

I must check if the horizontal plank fence I put up is still level, that would be useful 😀

'Keep things simple.' is my motto.

 

I had another go last night.

I adjusted the OTA in two directions using the spirit level, but also varied the dovetail angle away from 0 at the same time, until I had it level all the way around. Then I compared the OTA against the fence planks, and it looked good both ways (I have fencing on two adjacent sides of my patio/observing location, so it's actually quite convenient).

During the session, the pointing accuracy and tracking were amongst the best that I've had so far. I'll need to experiment more, but it looks like I put up fencing more accurately than I level scopes.

 

[ScouseSpaceCadet]

5 minutes ago, Zermelo said:

 

I had another go last night.

I adjusted the OTA in two directions using the spirit level, but also varied the dovetail angle away from 0 at the same time, until I had it level all the way around. Then I compared the OTA against the fence planks, and it looked good both ways (I have fencing on two adjacent sides of my patio/observing location, so it's actually quite convenient).

During the session, the pointing accuracy and tracking were amongst the best that I've had so far. I'll need to experiment more, but it looks like I put up fencing more accurately than I level scopes.

 

Fencing and brick work as scientific aids. Great. 😄

To be honest I picked up the tip from Astronomy and Nature TV on Youtube. Iirc it was an AZGTI review.