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Evaluating the lunar observations on April 28^th 2024 at Calanais I

Introduction

On tis web page, the ground proofing of the 3D scenery of Calanais I site within Stellarium is progressed from the ground proofing on March 31^st 2024. On March 31st 2024 the azimuth difference was found to be 0 +/- 0.01deg, which is equivalent to around 0 +/- 3sec.

A few levels might need to be reiterated:

• Determining the azimuth (rotation of vertical axis) of the 3D laser scan
  
• Determining the tilt (rotation of East-West axis) 3D laser scan to give the apparent altitude
  
• Determining if the whole 3D laser scan has to be vertically and East-West transformed with respect to Cnoc and Turso's DSM.

These three levels are hopefully enough to get a good South viewing skyline (from the end of the avenue: near stone 8 and 19). More rotations/transformations could be needed, but that cannot be determined with the present camera location.

Steps to ground proof the Calanais I 3D scenery

On April 28^th 2024 some 9 relevant photos were taken (by E. Rennie) to record the Moon going over/through the Calanais I site. These photos are being used to ground proof the Calanais I 3D scenery in Stellarium for the three mentioned levels.

The following steps have been used for the preparation

• Camera
• FUJIFILM; Digital Camera X-T3
  
• The EXIF data (Create Date and Shutter Speed) of the photos has been used to calculated: a photo's NTPtime (= [EXIF Create Date] + Offset).
• Photo were taken with this workflow
• Place camera on tripod as near as possible near stone 8 and facing towards the circle, facing the circle (line E). The lens is, in this physical world, at a height of 113cm measured from the local ground. Zoom, so that some 10 stones are visible in the photo. Keep zoom and position of camera during the whole session the same. You migth need to rotate the camera.
• Timing
  
• For each photo, with its NTPtime, of the touch moment is compared with the time in Stellarium when the Moon also is at the touch moment: Moon in Stellarium.
• Tools
  
• The 3D scenery of Calanais I (version: callanish1_Readjusted20230821) and added convergence_angle in the ini-file of: -3.83588°) in Stellarium (version 23.2) has been used.
• PSP (Coral Paint Shop Pro X9) was used to layer Stellarium screen grabs on the photos

Photos taken on April 28^th, 2024

Here is an overview of the photos used in this evaluation:

Next iteration

Experiences

Overall accuracy

We have been able to position the 3D laser scan properly within the 3D scenery and the Moon's path. The accuracy is around 1arcmin, which is close to the accuracy of the 3D laser scan (0.5arcmin). No major improvement in precision looks to be possible.

Sequencing of adjustments

The best sequence of adjustments, by reducing interdependencies, of rotations and transforms is:

• 3D laser scan Rot_Vert
• 3D laser scan Rot_EW
• Cnoc an Turso DSM Trans_EW
  
• Cnoc an Turso DSM Trans_vert
  

Determining remaining rotation and transform

Of course there are still a transform (Cnoc an Turso's DSM in North-South direction: Trans_NS) and rotation (3D laser scan around North-South axis: Rot_NS) possible; these are not yet investigated as these can't really be determined using the camera location at stone 8. One would need a camera location at stone 33 or 23 (and times of resp. set or rise events of a celestial object).
A realistic Trans_NS can't really be determined (as the effect of this is marignal on celestial directions).
The rotation Rot_NS will not have effect on the Cnoc an Turso's DSM, more on the positioning of 3D laser scan with the celestial object.

Artefacts in the 3D laser scan

A few stones in the 3D laser scan have some triangulation artefacts (these were found as they were handy for aligning the celestial object: red areas); stone 32 (error some 2arcmin: due to possibly wrong normal of some triangles); stone 26 (error some 2 to 3arcmin: missing left side top and small bit on the right side top); stone 27 (error some 2 to 3arcmin: missing left side top); and stone 28 (has a mushroomed top).

These errors make up a part the accuracy of the whole 3D scenery (expected to be around 1 arcmin), so they are not that serious except if these missing stone contours are used for defining 'touch' moments. It would have been nice on this observation day to know the contours, as with a low Moon this precision would have been handy.

It looks that Stone 49 is smaller (some 4 arcmin) in the 3D laser scan than in reality (using a photo).

Acknowledgments

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