Changes

The 5x6 fiber bundle supports were designed with two 5x3 bundle halves offset such that the center of the front face of the middle column in each bundle half would sit on the magnetic focal plane for a &beta; angle of 12.0^o. This angle was selected as a compromise that would allow coverage through the photon energy range of 10 - 5.6 GeV. <i><u>As a side note</u> - If required and finances permit, the 17 bundle supports can be easily redesigned for a different &beta; angle. This redesign would take less than an hour of CAD work, with a manufacturing turn-around time in as little as two days. Costs are estimated to be around $2k. A CAD drawing of a new bundle support design already exists, which incorporates updated locations of the threaded holes for mounting the clamps that keep the bundle straps in place. The best time to replace/modify the bundle supports, if so desired, would be during fiber replacement. This way the new fibers can be mounted to the new bundle support outside the tagger hall, before ever making it to JLab. A conservative time estimate for changing the TAGM fiber configuration would be approximately two days (16 hours).</i>

The 5x6 fiber bundle supports were designed with two 5x3 bundle halves offset such that the center of the front face of the middle column in each bundle half would sit on the magnetic focal plane for a &beta; angle of 12.0^o. This angle was selected as a compromise that would allow coverage through the photon energy range of 10 - 5.6 GeV. <i><u>As a side note</u> - If required and finances permit, the 17 bundle supports can be easily redesigned for a different &beta; angle. This redesign would take less than an hour of CAD work, with a manufacturing turn-around time in as little as two days. Costs are estimated to be around $2k. A CAD drawing of a new bundle support design already exists, which incorporates updated locations of the threaded holes for mounting the clamps that keep the bundle straps in place. The best time to replace/modify the bundle supports, if so desired, would be during fiber replacement. This way the new fibers can be mounted to the new bundle support outside the tagger hall, before ever making it to JLab. A conservative time estimate for changing the TAGM fiber configuration would be approximately two days (16 hours).</i>

Each bundle has a "pivot point" that when placed on the focal plane, provides the optimal y-displacement from the focal plane for each fiber column in that bundle without encroaching too close to the tagger magnet window. If the bundle &beta; angle &ne; 12^o, then the 1^<u>st</u> & 6^<u>th</u>, 2^<u>nd</u> & 5^<u>th</u>, and 3^<u>rd</u> & 4^<u>th</u> fiber column pairs will have the same magnitude offset as one another from the focal plane in Y, but with opposite signs, see Figure ???. This is all provided that when the bundle support is mounted on the parallel railing system the tagger magnet's focal plane passes through the midpoint between the front and rear bundle halves (the so called pivot point).

Each bundle has a "pivot point" that when placed on the focal plane, provides the optimal y-displacement from the focal plane for each fiber column in that bundle without encroaching too close to the tagger magnet window. If the bundle &beta; angle &ne; 12^o, then the 1^<u>st</u> & 6^<u>th</u>, 2^<u>nd</u> & 5^<u>th</u>, and 3^<u>rd</u> & 4^<u>th</u> fiber column pairs will have the same magnitude offset as one another from the focal plane in Y, but with opposite signs, see Figure 17. This is all provided that when the bundle support is mounted on the parallel railing system the tagger magnet's focal plane passes through the midpoint between the front and rear bundle halves (the so called pivot point).

[[Image:popsicle_stick_fiber_angle_top_view.png|center|thumb|800px|Figure 18: Sketch showing the "pivot point" that should always sit on the focal plane to provide the optimal fiber alignment. The green line running through the pivot point represent the placement of the focal plane for a bundle support with &beta; = 12^o, while the red line shows the placement for &beta; > 12^o.]]

[[Image:popsicle_stick_fiber_angle_top_view.png|center|thumb|800px|Figure 18: Sketch showing the "pivot point" that should always sit on the focal plane to provide the optimal fiber alignment. The green line running through the pivot point represent the placement of the focal plane for a bundle support with &beta; = 12^o, while the red line shows the placement for &beta; > 12^o.]]

As shown in Figure ??? by the red and green lines, if &beta; &ne; 12^o then the front face of the first column of SciFi no longer sits on the focal plane. This offset must be accounted for. The Excel spreadsheet starts by finding the optimal first bundle support crossing angle (&beta;_avg) and places the front center of the first column of SciFi on the X_FP location corresponding to E_{&gamma;_o}. Depending on &beta;'s departure from 12^o the pivot point will be displaced from the focal plane. The spreadsheet calculates the (&Delta;x, &Delta;y) needed to return the pivot point to the focal plane and keep the first fiber column's longitudinal axis on the E_{&gamma;_o} electron's path. The spreadsheet's title for this is "Pivot Point Move" and the description is detailed below.  

As shown in Figure 18 by the red and green lines, if &beta; &ne; 12^o then the front face of the first column of SciFi no longer sits on the focal plane. This offset must be accounted for. The Excel spreadsheet starts by finding the optimal first bundle support crossing angle (&beta;_avg) and places the front center of the first column of SciFi on the X_FP location corresponding to E_{&gamma;_o}. Depending on &beta;'s departure from 12^o the pivot point will be displaced from the focal plane. The spreadsheet calculates the (&Delta;x, &Delta;y) needed to return the pivot point to the focal plane and keep the first fiber column's longitudinal axis on the E_{&gamma;_o} electron's path. The spreadsheet's title for this is "Pivot Point Move" and the description is detailed below.  

<i><u>NOTE:</u> &beta_avg is used to determine this displacement. While the E_{&gamma;_o} electron's &beta; angle differs slightly &beta_avg and would keep the column's centerline on the proper electron path, this discrepancy is so small that it does not come close to the TAGM machining parts' tolerance. Additionally, the sixth column's &beta; angle has the same magnitude difference from the bundle support's &beta;, but with a different sign. For these reasons &beta;_avg was used.</i>

<i><u>NOTE:</u> &beta_avg is used to determine this displacement. While the E_{&gamma;_o} electron's &beta; angle differs slightly &beta_avg and would keep the column's centerline on the proper electron path, this discrepancy is so small that it does not come close to the TAGM machining parts' tolerance. Additionally, the sixth column's &beta; angle has the same magnitude difference from the bundle support's &beta;, but with a different sign. For these reasons &beta;_avg was used.</i>