DDM85XL "bent pier" alignment

[nakbrooks]

I'm in the process of polar alignment.  Before I try and fine tune the alignment I need to make sure the pier is as near vertical as possible - that is easier said than done with the ASA equatorial ("bent") pier as it has no vertical or horizontal edges against which to use a spirit level for coarse alignment.

 

Would the following procedure work for coarse alignment or am I missing something:

 

1. Home Find then move the mount using Autoslew until Autoslew shows 90.0 deg in its Alt box and 180 deg in its H box (see image_1 attached).

 

2. Place a spirit level across the East/West horizontal surface of the OTA (see image_2 attached).

 

3. Adjust the pier until the spirit level indicates the OTA is horizontal East/West.

 

4. Repeat 2 & 3 for North/South.

 

5. Do normal fine polar alignment at nighttime.

 

If I can use this process to get the pier vertical to within 1 degree in the daytime then it will make final adjustment at nighttime much quicker because, with the DDM85XL and the "bent pier", alignment can only be done by adjusting the entire pier (the mount itself cannot be adjusted separately) and with a total weight of several hundred kg and 18 adjustment bolts adjusting the pier is not quick.

 

Nigel

[GeorgeCarey]

That would work if you were certain that the optical axis of the telescope was exactly parallel to the telescope plate on the DEC axis.

Also a very good polar alignment. But if you are adjusting the pier, what will that do to the polar alignment?

[nakbrooks]

Adjusting the pier is the only way of adjusting (physical) polar alignment on the DDM85XL/Pier combination.  The DDM85XL bolts rigidly to the pier and it is the pier itself that has to be aligned. In principle this is good, because there is one less potential source of misalignment.  In practice it is tricky because the weight of the whole assembly means that adjusting the pier is quite tricky.

 

There are 3 axes around which I need to align:

 

- East/West vertical alignment of the pier.  This is relatively straightforward as I can use a spirit level up the sides of the pier to ensure a coarse vertical alignment east/west.

 

- North/South vertical alignment of the pier.  This is the tricky one.  Because the pier is Z-shaped there is no vertical surface on the north or south sides to check whether it is vertical using a spirit level.  I can't see an easy way of doing daylight coarse north/south adjustment so will have to rely upon the normal nighttime polar alignment process for that.

 

- Rotational alignment in the z-axis.  This is also relatively easy.  I have a north/south line drawn on the observatory floor accurate to around 0.25deg (created using sun's shadow at noon during observatory construction) and there is a flat surface at the side of the base of the pier that I can set a laser against and check that the laser is parallel to the line on the floor.

 

[Just paused there to watch a gheko crawl across the observatory floor, first one I've seen this Spring here].

 

Having checked the pier alignment East/West as best I can with a spirit level and compared it to my earlier-described method it does seem there is a discrepancy of about 2.5deg.  i.e. If I check both across the telescope plate (East/West) and along the top of the counterweight arm these both agree, but differ by about 2.5deg from the pier vertical (as measured with a spirit level).

 

I guess I'll have to leave it at that for daytime coarse adjustment and continue tonight.

 

Nigel

[GeorgeCarey]

Adjusting the pier is the only way of adjusting (physical) polar alignment on the DDM85XL/Pier combination. 

 

 

 Doesn't that rather imply that if you bolt the DDM85 to the pier and then do a polar alignment, then the pier will have put itself into the correct position?

 

George

[nakbrooks]

Yes it does George.  But adjusting the whole pier assembly is a much more major task than simply adjusting the mount would be, so I was trying to do as much as possible in daylight.

 

On a related topic, I presume the Alt and H figures displayed by Autoslew are calculated using deltas from the home position found during home find.  Can I rely upon them as being absolute Alt and H degrees or can they vary from the true Alt/H and, if so, is there a way of syncing once I have obtained polar alignment such that an Alt of 90 is genuinely vertical and an H of 180 is genuinely horizontal.  Is this what the "Set New Home Position" command does (I've never found a decent description of that command)?

 

Nigel

[GeorgeCarey]

From what I can gather the 'Set New Home Position' command is an ultimate 'Sync'.

When you make a pointing model you start by syncing on a star. After the pointing model if you move to a new star it will probably be a bit off because the pointing model is doing all sorts of magic.

If you then centre that star and 'Sync' everything will be fine, but if you shut down then Autoslew will forget the Sync position.

So, after your first accurate Sync on a star, go to 'Set New Home Position' and Autoslew will then remember where it is pointing when you next switch on.

 

That seems to work for me. I did my last new home position in September 2014 and have not had to Sync since.

Edited April 9, 2015 by GeorgeCarey

[RamaSpaceShip]

Hi Nigel,

 

Can we assume that the place where you put the base plate of the pier was perfectly level?

 

If this is the case, then you have to measure very precisely the height of the pier above this base plate at four points around it, and you will have to adjust so that the measured heights at these four points are all nearly equal. If the difference between the measures is less than a millimeter, the angle error is less than half a degree.

 

If this is not the case, you can use a laser level to adjust precisely the level of the base of the pier.

 

If you cannot get a laser level, you can draw leveled reference marks on the base plate, and do the measures indicated above from these marks.

 

Hope this helps.

 

Bernard

[nakbrooks]

Just another quick question. I'm aware that, with a pointing model, Autoslew can compensate for less than perfect polar alignment, although field rotation may be an issue.

 

Just how good should my physical polar alignment be? Is +/- a couple of degrees OK or does it have to be within a small number of arc mins (for astrophotography and unguided exposures of, say, 5 minutes)?

 

Nigel

[lukepower]

Hi Nigel,

 

maybe I can help with my experience. Please note that my bent pier has no way of changing altitude, just azimuth. I found out soon that it was not precise enough (we are talking of about one degree too low), so I had to add some steel plates between the base plate and pier on one side to compensate for it. What I did were severeal iterations of:

• Take a few pointing exposures
• Let Autoslew calculate how far off the alignment was
• Add or remove steel plates as needed

I found out that I had to be quite close to the perfect position to not have trouble afterwards with the slews, especially when slewing from east to west. But this might be also caused by the long focal lenght (3550mm) at that time. Damn, it would hae been way easier with some tool to compensate for altitude. I ended to be within 2 arcminutes from the ideal position, and thats fine for me with subexposures of up to 20 minutes.

 

Best regards

Edited April 13, 2015 by lukepower

[nakbrooks]

Thanks Luke. I'm also 3600mm fl so looks like I need to get to within a couple of arc mins.

 

The ASA pier has plenty of adjustment, but what makes it complicated is firstly the weight and secondly the fact that, although the mount is equatorial, the adjustment on the pier is basically Alt/Az. It has 6 sets of bolts around the base at 60deg intervals that need to be adjusted in unison to tilt the pier left/right (E/W) or forward/back (N/S), plus a pair of bolts to rotate the whole pier about its vertical axis.

 

Nigel