f/8 Guide Camera
Focus Map
Focus problem explanation
The 4 meter f/8 guide camera needs to be manually refocused after most telescope moves.
The reason the camera needs refocusing is that the focal plane the camera focuses on, has curvature of field. Manually refocusing the camera multiple times per night during field set-ups, ends up costing the astronomer a significant amount of valuable science time.
Solution
Automate guide camera focus in software, using one of the focus map solutions below. Guiding on stars that
are at nominall focus should increase overall data aquisition quality, as well decreasing field setup time for the
Telescope Operator.
Solving for a new probe locations focus - a working example
Here's an example of an implemented correction between probe moves.
Assume a starting south guide probe position of x = 70 , y = -70, and a starting best focus of 260 units.
The second south guide probe position will be x = 140 , y = -140.
Find the probe distances from the center of stage using the Pythagorean theorem.
guide probe position A
702 + -702 = (distance A)2 = 98.99
guide probe position B
1402 + -1402 = (distance B)2 = 197.98
Find the new focus value for the probe at position B, using the known distances, first focus,
and the correction constant with the following formula,
[ ( distance B - distance A ) x ( correction constant from graph ) ] + (focus value before move) = New focus value
Applying the above formula to our examples numbers gives,
[ ( 197.98 - 98.99 ) x (1.1124) ] + (260) = 370 units
* This solution assumes that the telescope operator will manually set the first value for the initial guide camera focus.
Check
For a guide probe distance of 197.98, the computed new focus value
of 370 units matches the expected value from the graph.
Focus map data
To obtain the focus map data set below, a field was selected rich in guide stars.
Sixteen unique guide probe camera focus values were then recorded by moving the guide probe from star to star,
along imagined radial spokes emanating outward, from the guide probes stage center, to the edge of field.
The graphs above and the focus problem solutions they provide, were generated from these 16 values.
|
|
|
|
|
|
|
|
|
|
|
|
| record |
probe x position |
probe y position |
camera focus |
probe distance |
|
| 1 |
-46.00 |
-63.00 |
251 |
78.00 |
|
| 11 |
48.09 |
-61.54 |
251 |
78.10 |
|
| 6 |
-19.50 |
-78.00 |
237 |
80.40 |
|
| 12 |
48.70 |
-86.40 |
250 |
99.18 |
|
| 2 |
-68.00 |
-74.00 |
272 |
100.50 |
|
| 7 |
-22.40 |
-105.00 |
264 |
107.36 |
|
| 8 |
-14.38 |
-136.00 |
292 |
136.76 |
|
| 3 |
-67.00 |
-121.00 |
275 |
138.31 |
|
| 13 |
92.00 |
-111.63 |
306 |
144.66 |
|
| 9 |
-8.49 |
-164.00 |
334 |
164.22 |
|
| 14 |
105.00 |
-138.00 |
348 |
173.40 |
|
| 4 |
-95.54 |
-148.00 |
334 |
176.15 |
|
| 15 |
98.00 |
-154.00 |
348 |
182.54 |
|
| 10 |
5.49 |
-188.00 |
366 |
188.09 |
|
| 5 |
-107.73 |
-163.00 |
375 |
195.38 |
|
| 16 |
111.00 |
-184.00 |
403 |
214.89 |
|
|
|
|
|
|
|
Bill Gillespie
Kitt Peak National Observatory
Observing Support Office
gillespie@noao.edu
Last update - Jan 02, 2003
;)