1998 Holography at the HHT


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HHT 1998 Holography Run


 

Map Results


 

We are using the LES-9 satellite, provided by the Air Force and MIT Lincoln Laboratory. This satellite has a 37 GHz transmitter, which the Air Force let us use for 6 hours a day. The satellite normally supports communications (most notably to the south pole research stations).

Previous holography runs are summarized for 1997 and 1995.

Here we present log entries and representative individual holography maps (given by scan number). Each map covers the range of surface errors shown (early maps are of +/- 50 microns only). The total size of the maps (in pixels) is also given. The holography maps are made by measuring the signal from LES-9 as the telescope is scanned in a raster pattern. Along a raster line the telescope is scanned at a constant rate in either the azimuth direction or the elevation direction. In principle, the direction of the raster lines should be immaterial. But to check for errors in the analysis, we made some maps in the azimuth direction and others in the elevation direction. (There was no noticeable difference between the two kinds of maps.)

There are 3 rings. The center ring has 12 panels and the outer two rings have 24 each. Each panel has several positions around the edge or in the middle where it can be adjusted. The goal is to adjust the panels so that the RMS deviations from a perfect surface are less than 15 microns.

There is a quadrapod that supports a secondary reflector. It casts a shadow along diagonal lines. We get no information about panel positions here, but they don't matter since the light that hits them strikes the quadrapod legs. The legs also produce diffraction lines parallel to them that confuse the measurement. We have blanked out the shadowed region plus about an additional foot on either side in the maps below.

 

Animated GIF of progress This animated GIF shows the daily progress of the 1998 holography session. For this animation we have used averages of azimuth maps for 09/11-09/14, individual azimuth scans for 09/15-09/16 and el-az averages for 09/17-09/30. There is one frame per day starting September 11, 1998. The scale is +/-50 microns. The last entry is the average of the 8 maps made September 24-30, 1998. These maps were made after the last panel adjustments were done.
 

We present below a sample of our maps to illustrate how the the holography results changed with time.

Map

Log Entry

Scan

Scale +/- 50 Microns
 
Day 2
September 10, 1998
 

 
scan 4050, 50 microns First full map of 1998.

RMS of ring 1 = 8.0 microns
RMS of ring 2 = 12.5 microns
RMS of ring 3 = 24.8 microns

The average map at the end of the last holography session in 1997 gave 8.5, 9.6, 21.6, and 16.3 microns, respectively. So over a year, the inner ring of panels held their positions. Ring 2 shifted slightly, but still exceeds the accuracy goal. Ring 3 has gotten worse.
Scan 4050
Sept 10, 1998
(65 by 65 map)
Overall RMS of map = 18.8 microns
 
Scale +/-50 Microns
 
Day 3
September 11, 1998
 

 
average 4153-4172, 50 microns After a few adjustments of the worst panels (5 and 9 o'clock), we realized that these were the panels that were jammed against each other in 1997. Most of the deterioration was probably due to those panels releasing over the year. We overshot the adjustment at 5 o'clock when they came unstuck from each other, again.
Average Map 4153+4172
Sept 11, 1998
(65 by 65 map)
RMS's by rings: 9.4, 17.3, 20.3; overall RMS = 18.0 microns
Scale +/-50 Microns
 
Day 6
September 14, 1998
 

 
average 4357,4390, 50 microns Changes to panels in rings:

Panels near 9 o'clock are still giving trouble. The pattern of dots around the circumference are probably not real, as they come and go.
Average of Scans 4357 and 4390
Sept 14, 1998
(65 by 65 map)
RMS's by rings: 9.4, 11.1, 19.2 microns; overall RMS = 15.2 microns
 
Scale +/-50 Microns
 
Day 10
September 18, 1998
 

 
Average of scans 4661 & 4684, 50 microns The adjustment goals have almost been reached. We are now being limited to the extent that successive maps, made without making any adjustments, fail to repeat themselves exactly. We are trying to understand the source of this limitation. If we understand it and can avoid it, we may be able to refine this antenna adjustment procedure even further.
Average of scans 4661 & 4684
Sept 18, 1998
(65 by 65 map)
RMS's by rings: 11.5, 10.0, 16.2; overall RMS = 13.3 microns
 
Scale +/-50 Microns
 
Day 14
September 22, 1998
 

 
Average of scans 4935 & 4963, 50 microns It's hard to beat this map, although we may have just been lucky that the variations we see between what should be identical maps happened to cancel each other in this average. We'll make only a few adjustments more and then take a few more maps to try to average out the map-to-map variations and estimate to what extent we can trust the maps.
Average of scans 4935 & 4963
Sept 22, 1998
(65 by 65 map)
RMS's by rings: 6.6, 6.7, and 16.0 microns; overall RMS = 11.8 microns
 
Scale +/-50 Microns
 
September 24-30, 1998
 

 
Average of the last 8 maps made in 1998.  No panels were moved 
during this time. Average of last 8 maps. The inner two rings are essentially perfect, as the individual panels are only supposed to have an RMS of about 7 microns.
RMS's by rings: 7.9, 7.5, 15.4 microns; overall RMS 11.8 microns
 
Scale +/-30 Microns
 
September 24-30, 1998
 

 
Average of the last 8 maps made in 1998.  No panels were moved 
during this time. Same as above, but displayed on a +/- 30 micron scale. Many of the remaining errors would be impossible to remove as they lie in between the adjustment screws.
RMS's by rings: 7.9, 7.5, 15.4 microns; overall RMS 11.8 microns
 
Scale +/-25 Microns
 
September 24-30, 1998
 

 
Animated GIF of the difference of each of the last 8 maps and
the average of them From this animation, one may get a better idea of which features come and go, and which were consistent between maps. They are the result of subtracting the average map from each of the maps that went into the average. The RMS's of these difference maps give an estimate of the measurement accuracy (as opposed to how well the panels were aligned).
Difference maps.
Sept 24-30, 1998
(65 by 65 map)
Mean RMS's by rings: 3.2, 4.0, 8.1 microns; overall RMS 6.2 microns.


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Last updated: 11/08/11.