Pier and tilt questions : )

[Rusty2]

Hello again !

Due to ongoing frustration with saturated grass and mud , the possibility of a sinking tripod and the constant hunt for Octans when using tripod legs I bought a pier mount .

After I bought the pier I started wondering about "leveling" the pier while the mount is attached and came across 2 astronomy guys saying that it wasn't necessary .

https://youtu.be/AOeLnecOMeg?t=375

Having a think about it , I'm assuming this is true because the mount only rotates on the azimuth axis while adjusting the azimuth knobs . If the mount is polar aligned , then the mount rotates off the polar axis , not the azimuth axis , right ?

My second question relates to earths tilt . If earth tilts 23.5 degrees and is continually tilting , how come astronomers don't keep adjusting for this tilt , or do they ?

Any thoughts would be appreciated : )

[MattT]

A level pier makes it easier to get the alt close to perfect at the start. Then its just the az axis to get right.
It’s just as easy to pour a level bit of concret as a non level one!

Never used a ‘rat cage’ as my pier was level and stayed that way for the 6 years I had it. Octans was behind trees for me so drift aligning was my method to find the SCP.

[Rusty2]

I appreciate your thoughts Matt : )

The new pier isn't a "rat cage" .

[DarkArts]

It's your RA axis that aligns to the SCP - that's the only one that matters because you're only counteracting Earth's rotation through the combination of SCP alignment and RA rotation at a rate that's equal and opposite. Whatever method you use to get your RA axis to point to the SCP is up to you, as long as it stays fixed there once alignment is achieved. In other words, your Az and El planes don't need to be horizontal and vertical respectively - once you've got SCP alignment, their job is done - just lock them down tight.

It may be more practical to have the mount's Az level to make El adjustments easier. However, IMHO, the real practical consideration (nothing to do with actually getting SCP alignment) is balancing a heavy weight on a pier. If the pier is tilted, then the weight and lever-arm will create a force due to gravity that may cause more tilt over time. For this reason, IMHO, it's more important to level the base of the pier rather than the top (and to use a stable foundation). But most people don't complain about this, and I haven't owned enough piers to say it's a real problem.



The SCP is a projection to a point in space based on the Earth's south pole (the rotational axis). Your SCP alignment stays aligned with the Earth's axis even if the axis itself moves, so you don't need to adjust it.

Precession of the Earth's axis is measurable (apparent motion is ~50 arcseconds per year) and the main cycle is long (~26,000 years). Other axial motions - nutation (due to gravitational forces) and polar motion (due to Earth variables) are very much smaller - a few arcseconds for nutation and a small fraction of an arcsecond for polar motion.

Professional astronomers, star charts, planetarium programs and mount controllers do take these motions into account from what I understand. The epoch setting in a chart or planetarium program, or your mount's handset/controller, is needed for coordinate calculation.

Atmospheric refraction and mount mechanics are probably greater sources of pointing error on any given night. Those, and error in SCP alignment in the first place, are why we have tracking.

[Rusty2]

Yes , I thought so .


I didn't think about that . It's a SW AZEQ6GT + a SW Esprit 150 ED + a SW guide scope + a canon EOS , it's heavy .



Aha , it didn't occur to me that once aligned , no more was needed but now it sounds obvious .




I had no idea , thank you for taking the time to explain to me : )

[ReidG]

As a matter of good engineering it is almost always a good plan to start by doing your best. Get it as vertical/level as you reasonably can and make it easier for the software in your mount to do its job. Mounts and their software are not all equal. The software will inevitably not be totally perfect either.