With the objective of designing a telescope system for the Gig-E, a system of two lenses is implemented. A rough schematic of the telescope system is attached. Variables in the system include the focal lengths of the two spherical lenses(f1, f2), distance between the lenses(t), distance between the test mass and the lens combination(u), distance between the other lens and the sensor(v). Also the size of the object to be desired ranges from 3’’ which is the size of the test mass to 1’’ which is approximately focusing on the beam spot implying that the required magnification ranges from 0.06089 to 0.1826 (since the sensor image circle size if ¼”)
The lenses are selected to be 2” in diameter so as to ensure sufficient collected power.
Going through the focal lengths available, namely 50, 100, 150, 200, 250 mm, and noting that the object distance would be within the ranges of 1500 to 2500 mm, plots of various accessible u and v for different values of t were obtained. This optimization was done to ensure the proper selection of the lenses. Additionally, a sensitivity analysis was performed and plots depicting the dependence of magnification on the precision limiting measurements of u (1 mm) and t (5 mm) were obtained. (These were scatter plots quantifying the deviation from the desired magnification ranges). The plots depict the error term induced on the magnification if there was an error in measuring the distance between the lenses as 5mm and if the precision in measuring the object to lens distance by 1mm.
The telescope design might be limited by spherical aberrations and coma, which might be resolved by either using aspherical lenses or by increasing the f-number (typically with an f number around 5 or 6). The use of aspherical lenses particularly parabolic lenses was considered, however this was found to be quite an expensive route.
Analyzing the plots and taking into consideration the restrictions of the slotted lens tubes, the precision in measurement of the distances, a 150 mm- 250mm focal length solution is proposed. With a diameter of 2”, the f number is computed to be 2.95 and 4.92. With this combination and the object distances lying between 1500 to 2500 mm, the image distance to the sensor varies between 51 to 100mm. So a slotted lens tube controlling the distance between the lenses would be required.
I also considered a combination of focal lengths 250mm and 250mm, as then both of the lenses would at least have an f number of 4.92. The results for this combination are also attached. The image distance from the lens combination is about a 100 to a 140 mm. However, this would require much longer slotted length tubes thereby adding to the cost of the system. The number of accessible u-v points is the same as that for the 150-250 combination.
I am still trying to search for a much more concrete way of quantifying aberrations.
Since the f numbers of the lenses in the proposed design with biconvex lenses are a little less than 5 and the conjugate ratio(that is the ratio of object to image distance) is greater than 5, I explored the use of plano convex lenses, but with the same focal lengths, the accessible u-v range is restricted with the planoconvex rather than biconvex lenses.
On Friday, I had a discussion with Gautam and Steve about the hardware that is the cylindrical enclosures for the camera and the telescope and we examined two such aluminum cylindrical enclosures. One of them was the one being currently employed for the cameras. The dimensions were measured and the length was found to be 8’’ and an outer diameter of 26 cm within an error of 0.5 cm.
The other enclosure was longer with a length of 52 cm(±0.5 cm), outer diameter of 10”(±0.1”) and an inner diameter of 23.7cm(±0.1cm). Pictures of these enclosures are attached.
Both of these enclosures have internal optical rail to mount the camera and the telescope system. Depending on the weight of the telescope system(that includes the weight of the slotted lens tubes, the lenses), it might be more efficient to clamp the telescope system itself on the rails with the low weight camera mounted on the lens tube.
I also went around to get an idea of distance of the GigE from the test masses. This was just a step to verify if the object distances were really in the ranges being taken into consideration, that is between 1500 and 2500 mm. I also tried to cross check the measurements with the CAD drawing of the 40m. However, as I have been informed, the distances in the CAD version are not updated.
The distances from the optic to the CCD detector would range from around 75.1 cm for MC2, 94.01 cm for ITMX, 97.21 cm for ETMX, 117.19 cm for ITMY and 88.463 cm for ETMY. The illuminator for the ETMY was disconnected, so Gautam helped me access the manual lamp control to enable me to take measurements.
The values for ETMX, MC2 and ITMY are subject to an error of ±1’’. Due to a lot of obstructions, the values for ETMY and ITMX may be subject to a lot more error. Even so, these distances are clearly less than 2 meters, prompting me to run the simulations again and verify that the chosen combination is still useful.
As for the slotted lens tubes to mount the 2” lenses, the following options are available on the Thorlabs catalog. CVI and Edmunds do not seem to offer much of the stackable lens tubes.
SM2L30C is a lens tube onto which the optic can be mounted without the need of a spanner wrench. It also has a length of 3”. However, it has a rotatable slip shield which can be rotated open as and when the access to optic is required. However, there might be a slight compromise with rigidity here.
SM2L30 is a lens tube with internal thread depth of 3”, the optic can be mounted using spanner wrench and a retainer ring. The optic cannot be accessed from both ends of the tube here.
SM2M30 is a lens tube with no external threads, therefore lens tube couplers would be required to stack the tubes. The optic is accessible from both ends here though.
Considering the merits and demerits of all these available options, the use of SM2L30 might be considered as it provides a quick and efficient way of stacking multiple lens tubes. As for accessing the optic from both sides, using multiple tubes helps overcome the problem and still ensures that we are able to access a number of separation distances as per requirement.
Thorlabs also offers an internal C to external SM2 adapter so that the lens tube could be fixed onto the C mount of the camera.
I would be examining the use of 1" diameter lenses for the eyepiece as suggested by Rana, as that might give us more flexibility.
I examined the use of a single lens system for the available range of focal lengths, for the required magnification and found that a focal length of at most 100 mm would be required to sufficiently cover the object distance range. This would greatly compromise with the f-number and hence lead to a lot more spherical aberrations.
Therefore, a two lens system would be more useful to implement. Using an eyepiece of 1” puts an additional constraint on the system such that the separation between the lenses must now at least equal or be greater than half the image distance from the first lens to ensure that no light from the light cone is lost. This is clarified in the schematic. The image from the first lens in absence of the second lens would form at point A, subtending an angle θ. In order to ensure that no part this light cone emerging from the first lens is lost, the second lens must be placed at a distance atleast v/2 from the first lens.
A combination of 125mm focal length 2” diameter objective with a 250 mm 1” eyepiece covers the required range of object distances (650mm to 1500 mm). Increasing the focal length of the eye piece increases the minimum object distance accessible to 700 mm.
A glance at the accessible u, v points shows that all magnifications are not possible at a given object distance. To image the entire surface of the test mass, a distance of at least 1.25m is required from the objective, while a beam spot of 1'' diameter can be imaged easily at upto 1200 mm from the objective . This holds true even for the 150-250 mm biconvex 2" lens combination proposed earlier.
If this sounds reasonable, we could proceed with ordering the lenses.
- Started zoom stream; thanks to whoever installed it!
- Spent some time trying to understand how anything we did last thursday lead to the sensing matrix change, but still cannot figure it out.
- Tracking back on our actions, at ~10:30 we ran burt Restore with the 08:19/.*snap and in lack of a better suspect, we blame it on that action for now.
# ARM locking??
- Reading (not running) the scripts/XARM/lockXarm.py script and try to understand the workflow. It is pretty confusing that the result was to lock Yarm last time.
- It looks like this script was a copy of lockYarm.py, and was never updated (there's a chance we ran it for the first time last thursday)
- *Is there a script to lock the Arms?* Or should we write one? To write one, we first attempt a manual procedure;
1. No need to change RFPD InMTRX
2. All filters inputs / outputs are enabled
3. Outputs from XARM and YARM in the Output matrix are already going to ETMX and ETMY
- Maybe we can have the ARM lock engage by changing the MC directly?
4. Change C1:SUS-MC2_POS_OFFSET from -38 to -0, and enable C1:SUS-MC2_POS_OFFSET_ON
5. Manually scan MC2_POS_OFFSET to 250 (nothing happens), then -250, then back to -38 (WFS1 PIT and YAW changed a little, but then returned to their nominal values)
- Or maybe we need to provide the right gain...
6. Disabled C1:SUS-MC2_POS_OFFSET_ON (back to nominal state)
7. Look into manually changing C1:LSC-XARM_GAIN;
From the command line using python:
>> import epics
>> ch_name = 'C1:LSC-XARM_GAIN'
>> epics.caput(ch_name, 0.155) # nominal = 0.150
- Could be unrelated, but we noted a slow spike on C1:PSL-FSS_PCDRIVE (definitely from before we changed anything)
- Still nothing is happening
8. Changed the gain to 0.175, then back to 0.150, no effect... then 0.2, 0.3 ...
- Stop and check SUS_Watchdogs (should not have changed?) and everything remains nominal
- Revert all changes symmetrically.
- Could we have missed enabling FM1?
- Briefly lost MC lock, but it came back on its own (probably unrelated)
- Wrap it up for the day. In summary; no harm done to our knowledge.
For the arm locking, the "Restore Xarm (XARM POX)" script from the "IFO_CONFIGURE" MEDM screen should get you there (I just checked it and it works fine). It is worth getting a hang of the PDH signal chain (read what the script is doing and map it to the signal chain) so you get a feel for where there may be offsets, saturations, what the trigger logic is etc. The LSC overview screen is supposed to be pretty intuitive (if you think it can be improved, I'd love to hear it but please don't change it without documenting) and there are also the webviews of the simulink models (these are RO so feel free to click around, for the LSC the c1lsc model is the relevant one).
- Today we spent the morning shift debugging SUS input matrix diagonalization. MC stayed locked for most of the 4 hours we were here, and we didn't really touch any controls.