Download Telescope mirror integration and alignment

Preliminary Design Review
Ultra Violet Imaging Telescope
Tests and Calibration of Optics
__________________________________________________________________
Part II - Telescope Integration, Alignment and Tests
S. Sriram
Document No. : UVIT-PDR-009-2-Version 1.0
March - 2006
________________________________________________
Indian Institute of Astrophysics
Bangalore-560 034
Abbreviations
ASTROSAT
ATAC
AZ
BS
CPU
FWHM
FUV
IIA
ICD
LEOS
MFD
MC
NUV
OSF
PMA
PV
PCD
RC
RMS
S/N
SMA
TIR
UHP
UVIT
VIS
Astronomical Satellite
Alignment Telescope& Auto Collimator
Azimuthally
Beam Splitter
Camera Proximity Unit
Full Width at Half Maximum
Far Ultra Violet
Indian Institute of Astrophysics
Interface Control Document
Laboratory for Electro Optic System
Mirror Fixing Device
Monochromator
Near Ultra Violet
Order Sorting Filter
Primary mirror assembly
Peak to Valley
Pitch Circle Diameter
Ritchie Criterion
Root Mean Square
Signal to Noise
Secondary mirror assembly
Telescope Interface Ring
Ultra High Pure
Ultra Violet Imaging Telescope
Visible
1
Contents
1.0 Introduction
4
2.0 Sub assemblies
6
3.0 Alignment requirement:
7
4.0 Mounting Sequence of telescope subassemblies alignment
7
4.1 Gantry Mount
9
4.1.1 GANTRY and ATAC alignment to the PMA
9
4.2 Secondary mirror assembly (SMA) integration and alignment
11
4.3 Effect of Gravity
12
4.4 Gadget for detector plane alignment.
12
4.5 Integration of the detector mount and its alignment
13
4.6 Primary mirror baffle integration
15
5.0 Interferometric Alignment
15
5.1 The principle of the interferometer alignment
15
5.2 The of Interferometric alignment
15
6.0 Integration of Filter wheel and Filter wheel motor
17
7.0 Telescope Focus determination
17
8.1Vacuum chamber setup
18
8.2 Setting up the line of sight inside the vacuum chamber
19
8.3 Collimator integration/alignment inside the vacuum chamber
20
8.4 Telescope setup and alignment inside vacuum chamber
21
8.5 Telescope focal plane determination
23
9.0 Photon number Calculation
25
9.1 Contribution to photon number from various parts from source to detector.
26
10.0 Tools Requirements:
27
11.0 References:
28
A1- Appendix
28
2
Figure/
Table
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Table 1
Table 2
Description
Optical Layout –FUV Channel
Optical Layout – NUV/VIS Channel
Schematic view of Gantry
Schematic view of the PMA and Telescope interface ring
integrated in the Gantry and is aligned with ATAC
Schematic view of the telescope tube integration on the TIR.
Schematic view Gadget used for finding the detector
mounting bracket perpendicularity to the optical axis (for
target T6)
Schematic view of integration and alignment of detector
mount.
The schematic view of the integrated telescope alignment
with interferometer.
Schematic view of Vacuum chamber.
Schematic view of alignment (colinearity & Tilt of the port)
of exit and entrance port of the vacuum chamber
Schematic view of collimated unit alignment/positioning
inside the vacuum chamber
Schematic view of the integrated telescope alignment
The schematic view of the setup for the integrated telescope
focal plane determination.
Illustration of focus variation with respect to the pinhole
position
Contribution to photon number from various parts from
source to detector.
Datas From LEOS (Ref. Doc. No. LEOS – AST – TR-02-0519)
Page
No.
6
6
9
10
11
13
14
16
19
20
21
22
23
24
26
28
3
Telescope Integration, Alignment and Tests
1.0 Introduction
The Ultraviolet Imaging Telescope (UVIT) is one of the payloads in ASTROSAT, the first
Indian satellite entirely devoted to astronomy. The Ultra violet Imaging Telescope (UVIT)
is a Twin Telescope system intended for providing the flux calibrated images of the sky at
spatial Resolution of about 1.5 arcs second at FWHM. The telescope functions in the three
spectral regions viz. Far Ultraviolet (FUV) region covering from 130nm to 180nm, Near
Ultraviolet (NUV) region covering from 180nm to 300nm and Visible (VIS) region
covering 350nm to 550nm. One of the telescopes is exclusively for FUV region and the
other for the combined NUV & VIS regions.
The telescope optics is same for both FUV and NUV/VIS regions. It is a RC system
comprising of concave hyperboloid primary mirror and convex hyperboloid secondary
mirror (Figure 1 and Figure 2). The system focal length is 4756.4mm with circular field or
radius 0.240.
In order to achieve the final intended image quality from the UVIT, it is necessary to
calculate the error contribution on the image due to various sources which can blur the
image and allowed tolerances required to control them. The error contribution on the final
image is from optics, mechanics and thermal.
The beam splitter in the NUV-VIS channel splits the beam from the NUV-VIS telescope
towards VIS detector and NUV detector. The introduction of the 45-degree beam splitter in
the path of the beam introduces aberration in the VIS channel and hence blurs the image
quality to 2.1arc sec.
The diameter of all three telescopes is 375mm and the net throughput is 15Sq.cm for FUV,
20Sq.cm for NUV and 25Sq.cm for VIS channel.
It is necessary to align and test the integrated telescope in the laboratory prior to flight.
These report summaries the various steps involved in the integration of the telescope in the
mechanical structure, alignment with sight telescope and fine tuning with interferometer.
Finally it describes about the focus determination test on the integrated telescope.
Integration, alignment and test procedure involves the following aspects:
4
Mounting scheme of the telescope integration
Alignment of sight telescope for establishing the optical line of sight
Coarse Alignment of individual components (sub assemblies)
Fine alignment of integrated telescope
Telescope focal plane determination
Before starting the telescope integration, each and every sub assemblies of the telescope
needs to be tested metro logically and it has to be ensured that the parameters of the sub
assemblies of the UVIT is within the specified tolerance. As we get optical and
metrological details of primary and secondary mirror assemblies (PMA & SMA) from
LEOS (see in the appendix), the above measurements need not be repeated for PMA and
SMA.
It is assumed that all the subassemblies of the UVIT is expected to be with in the specified
tolerance, the telescope final integration and alignment shall be started from the primary
end of the telescope.
The sequence of the integration of the UVIT telescope and its coarse and fine alignment are
described below.
As the finer alignment of the UVIT is proposed to be carried out with interferometer, the
pre-alignment (Coarse) of the individual components in the telescope structure shall be
carried out. The coarse alignment of the optics in the structure is shall be carried out by an
Alignment telescope cum Autocollimator (ATAC) with alignment targets.
5
Figure.1 Optical Layout – FUV Channel
Figure.2 Optical Layout – NUV/VIS Channel
2.0 Sub assemblies
Optical
PMA
SMA
Filters, beam splitters and corrector lens
Detector
Mechanical
Telescope Interface ring
Telescope tube
Secondary spider unit
6
Thermal compensation unit
Detector mounting legs
Detector mounting bracket
Filter wheel mount
Primary main baffle
Secondary baffle
Expected Tolerances on the sub assemblies are given in the appendix.
3.0 Alignment requirement:
The final image quality from the UVIT is 1.5” FWHM and in terms of RMS it is
0.84”RMS. The plate scale of the telescope is 43”/mm and is 24micron corresponds to 1”.
0.84” RMS corresponds to 20micron RMS and 400 sq. micron variance
To achieve the final image quality from the telescope, the telescope needs to be aligned in
the ground to the following requirements.
1. Primary and secondary mirror relative decentre < 0.1 mm, including effects of
gravity,
2. The two mirrors should have a relative tilt < 30", including effects of gravity
3. The separation of the two mirrors should be correct to 0.1 mm
4. The axial position of the detectors should be correct to 0.05 mm
5. The radial position of the detectors should be correct to 0.5 mm
6. Plane of the detector perpendicular to optical axis to 30”
4.0 Mounting Sequence of telescope subassemblies alignment
For the initial alignment of the telescope with ATAC and interferometer, the telescope sub
assemblies shall be mounted in the following sequence
1. Mount the GANTRY on the optical bench
2. Mount the Telescope interface ring on the Gantry
3. Mount the Primary mirror assembly on the telescope interface ring
Align the ATAC to the primary mirror (centered to primary mirror
and perpendicular to the primary mirror rear surface)
4. Mount the telescope tube on the telescope interface ring
-Check for any mechanical flexure due to the telescope tube
5. Mount the thermal compensator on the secondary spider unit
7
6. Mount the Secondary mirror assembly (SMA) on the thermal compensator
7. Mount the Secondary baffle to the secondary unit
8. Mount the secondary unit (4, 5, 6) on the telescope tube-mounting flange.
Now align the secondary with respect to primary mirror (tilt,
decentricity) with the help of alignment/autocollimator instrument.
The axial position of the secondary mirror has to be checked for the
focal position with in +/-1 mm with interferometer. LEOS will
provide this visible focal position and the axial position of the
secondary
9. Mount the detector supporting legs on the telescope interface ring (behind primary)
– Check for any disturbance in the alignment
10. Mount the detector-mounting bracket on the detector supporting legs.
- Check for any disturbance in the alignment
11. Mount the pre aligned alignment target/pinhole (refer section 4.3) on the detector
mount.
Now align the detector-mounting bracket (for decentricity and tilt
with respect to the mechanical axis) with respect to primary mirror
with the help of alignment/autocollimator instrument.
12. Dismantle the detector mounting legs (as a unit) from the interface ring
13. Mount the primary mirror baffle
14. Mount back the dismantled detector supporting legs to the telescope interface ring
15. Mount the filter wheel motor
16. Mount the filter wheel
Now align the filter wheel unit (plane tilt with respect to the
mechanical axis) with the help of alignment/autocollimator instrument
In case of NUV/VIS telescope after the step 14 the following sequence shall be
followed for VIS channel
1. Mount the beam splitter unit
2. Mount the corrector unit
8
3. Then step 15 and 16
In case of NUV/VIS telescope after the step 14 the following sequence shall be
followed for NUV channel
1. Mount the corrector unit
2. Then step 15 and 16
4.1 Gantry Mount
Figure 3. Schematic view of Gantry
It is proposed to have L type Gantry, as shown in figure 3, to assemble the telescope in
horizontal direction on the optical bench in the clean room environment. The GANTRY is
made of SS material tubes and C channels. The C channel is the bottom element of the
GANTRY and some rollers are attached to it. Using the roller the GANTRY can move to
the desired position on the optical bench with the test object. The design and analysis of the
GANTRY will be made by IIA mechanical team.
4.1.1 GANTRY and ATAC alignment to the PMA
9
Mount the Gantry on the optical bench (Approximately 6 meter length by 1.5 meter width)
about 3.5meter distance from the edge (length side) of the table. Clamp the Gantry on the
optical bench rigidly. Mount the telescope interface ring on the Gantry using the mounting
holes provided in the Interface ring and the Gantry. To find the center of the primary mirror
a target (from LEOS, with cross mark) has to be mounted in the bore of the primary mirror
with suitable mechanism. Now mount the PMA in the interface ring with special care.
After the PMA is mounted on the interface ring, the alignment telescope cum auto
collimator (ATAC) shall be mounted on the optical bench and it has to be aligned to the
primary mirror. For this mount the ATAC on a optical rail and keep the rail on the bench at
the distance of ~3meter from the primary as shown in figure4.
Optical Bench
Telescope Interface
Ring
Optical Rail
PMA
Position 1
Gantry
ATAC
Position 2
Target
Schematic view of alignment ACAT with respect to primary
mirror rear surface.( position1 for perpedicualrity and Position2 for colinarity)
Figure 4. Schematic view of the PMA and Telescope interface ring integraed in the Gantry
and is aligned with ATAC
Move the ATAC away from the center and look at the rear surface of the primary mirror.
As the primary rear surface is polished to lambda / 4 finish, it will act as a auto reflector for
ATAC. Now look at the image of the reticule of ATAC (Auto collimator mode). Adjust the
ATAC until the reflected image of the reticule exactly coincides with the ATAC reticule. If
this is done then the primary mirror is said to be perpendicular to the ATAC axis. Mark the
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position (Position 1) of the ATAC on the optical bench and this will be useful for
rechecking the alignment.
Move the ATAC on the optical rail and bring it to the center of the primary mirror. Now
focus the ATAC (Alignment telescope mode) on to the center of the target mounted in the
primary mirror bore. Once it is done then the ATAC is said to be aligned to the optical axis.
The position (Position 2) of the ATAC can be marked on the optical rail for further needs.
Recheck the primary mirror alignment with respect to the ATAC at both position 1 and
Position 2 by moving the ATAC on the optical rail. Finally position the ATAC in the
perpendicularity check position (position 1).
Bring the telescope tube (with the help of Grain) and mount it on the TIR as shown in
figure 5. Now release the grain. As the ATAC is looking the perpendicularity of the
primary surface, in case of any mechanical flexor (due to the free ended telescope tube) the
ATAC can measure it and
if the flexor is more than the optical tolerance
spatial attention should be given??
Optical Bench
Optical Rail
Telescope Tube
Position 1
Position 2
Target
Schematic view of alignment ACAT with respect to primary
mirror rear surface.( position1 for perpedicualrity and Position2 for colinarity)
Figure 5. Schematic view of the telescope tube integration on the TIR.
4.2 Secondary mirror assembly (SMA) integration and alignment
11
Before starting the secondary mirror assembly integration in the structure, the secondary
mirror assembly (SMA) has to be mounted on the thermal compensator and the thermal
compensator has to be mounted on the spider unit. Now mount the secondary baffle in the
secondary unit. Now mount the SMA (with the spider, the thermal compensator and the
baffle) to the secondary end of the telescope tube.
Once the secondary unit mounted in the free end side of the telescope tube, the mechanical
flexor should be measured from the ATAC, which is kept at the position1.
If the
flexor is more than the optical tolerance spatial attention should be
given??.
Now move the ATAC to the position 2 and focus it on to the target in the bore of the
primary mirror and check for any disturbance. If there is no disturbance, then remove target
from the primary mirror hole.
Focus the ATAC on the secondary mirror and look at the image of primary mirror and
image of the secondary reflected from the primary mirror. In the focal plane of the ATAC
one can see the multiple image. Now tune the secondary mirror in such a way that all the
images come to concentric and centered on the ATAC reticule. If this is done then the
secondary is said to be aligned with respect to the primary mirror.
4.3 Effect of Gravity
If there was any mechanical flexure (bending) in the structure due to gravity, it would have
been corrected by adjusting the secondary mirror. That is the decentricity against the
bending. But to the see the effect of gravity on the structure, the telescope has to be rotated
by 180 degree about the mechanical axis and recheck the alignment of secondary. If there
was any error in the first case, it would have been doubled in this case or otherwise the if
there is no bending then the secondary does not demand any further adjustment after the
telescope is rotated by 180 degree.
4.4 Gadget for detector plane alignment.
12
The requirement of the perpendicularity of the detector plane to the optical axis is better
than 0.5-arc min. and location of the center to the optical axis better than 0.5mm. For doing
this test, a mechanical gadget with reflecting target with centering mark (cross hair) has to
be made. As shown in figure 6, the mechanical gadget is a reflecting flat holder with the
mounting holes PCD matching with the detector mounting holes PCD. The tilt (parallelism)
between the mount face to the reflecting target should be better than 10” and the accuracy
of the centering mark with respect to the gadget PCD should be better than 0.1mm.
Before mounting this target in the telescope for checking its alignment, it has to be pre
aligned and checked with ATAC or with interferometer or with some mechanical device
(gauges)
Ref. Flat
Surface Mechined
and kept parallel to
ref.flat surface
Mounting hole PCD eqivalent to
Dector PCD
Figure 6. Schematic view Gadget used for finding the detector mounting bracket
perpendicularity to the optical axis (for target T6)
4.5 Integration of the detector mount and its alignment
After the primary and secondary mirrors are aligned to the optical axis, (the colinearity and
perpendicularity), the detector plane has to be aligned to the optical (both detector axially
centered, with in 0.5mm and perpendicular to the optical axis, <30”) axis as shown in figure
13
7. For this alignment, the ATAC has to be in the position 2, looking at the secondary
mirror.
Mount the detector supporting legs (three legs) to the TIR and then mount the detectormounting bracket. Mount the mechanical gadget on the detector-mounting bracket as shown
in figure 5.
Optical Bench
Detector
Supporting
Legs
Detector
Mounting
Bracket
Gadget
Schematic view of alignment of detector mounting plane with
respect to the optical axis
Figure 7. Schematic view of integration and alignment of detector mount.
Now using the ATAC (in Auto collimator mode), check the perpendicularity of the flat and
if needed, adjust the detector-mounting bracket for its tilt by shimming method. Once the
tilt is corrected, the reflecting flat has to be centered to the optical axis (defines by ATAC).
For this again the detector-mounting bracket can be adjusted but not the
target in the mechanical gadget.
In this stage the detector-mounting plane can be checked for the telescope focus and for any
tilt in the plane using interferometer. The tilt in the detector-mounting plane can be
corrected by spacers/shims. When adjusting the detector-mounting plane with
shims/spacers special care should be take for any further tilt due to shims and spacers.
The requirement of on body of Camera Proximity unit (CPU)
14
Flange of CPU shall be parallel to photocathode with in 0.2mm and the parallelism would
be calibrated with an accuracy 0.02 mm Body of CPU shall be coaxial with the imaging
field within 0.5 mm per axis, and the alignment shall be calibrated with an accuracy 0.2
mm. Window of the detector has a wedge < 3'.
These parameters should be incorporated during the alignment of the detector-mounting
plane to get the final intended image quality.
4.6 Primary mirror baffle integration
After mounting and aligning the detector-mounting bracket, the primary baffle has to be
mounted in the structure. First remove the target in the primary mirror bore and remove the
detector supporting legs form TIR with out disturbing the ATAC. Now look at the
secondary alignment with respect tot the primary mirror to ensure any disturbance in the
integrated system.
Now insert the baffle from the focal plane region of the telescope
(to be checked
with telescope structure team) and mount it in the structure.
After mounting the baffle now mount the detector supporting legs back in the structure
(accuracy of repeatability of location better than 10micron) and check for any
misalignment with ATAC and the reflecting reference in the detector-mounting bracket.
5.0 Interferometric Alignment
The fine-tuning of the telescope mirrors and different types of aberrations can be measured
with Fiezo Interferometer (ZYGO corporation, USA).
5.1 The principle of the interferometer alignment is as follows:
Generate an f/12 (or faster) beam from the interferometer and illuminate the primary mirror
thru secondary mirror. The collimated beam from the primary is brought to (Auto
collimation) focus and made to coincide on to the focus of interferometer. The interference
15
between the beam, reflected from the object (telescope) and the interferometer itself is
analyzed. The various Zernike coefficients / PV/ RMS will give the measure of different
aberration/misalignments in the aligned telescope.
5.2 The of Interferometric alignment
Mount the interferometer on the XYZ stage, as shown in figure 8, and place it infront
(about 790 mm distance from the head of the interferometer) of the target in the detectormounting plane (where the expected telescope focus is supposed to come). Replace the
reflecting target with a 0.5mm diameter pinhole (size 0.5mm initially and even smaller size
can be used later for fine adjustment). Adjust the XYZ stage in such a way that the focused
beam (f/12 beam with the help of the Reference Sphere) from the interferometer falls on the
pinhole.
Optical Bench
XYZ Stage
Reference
Flat
Interferrometer
Schematic view of alignment of intgegrated telescope with
interferrometer
Figure 8. The schematic view of the integrated telescope alignment with interferometer.
Now load the reference flat (size 18 inch diameter) on its mount (Tip/tilt) and place it
behind (about 500mm) the telescope secondary unit. Adjust the reference flat so that the
reflected beam from it finally falls on pinhole. When the reflected beam falls on the
pinhole, the fringes will appear on the interferometer screen. If the return focused beam
16
does not fall on the pinhole exactly, adjust (tilt) the reference flat so that the focus comes on
to the pinhole.
Now take an exposure of the fringes and analyze the interferrogram and measure the
misalignment of the telescope mirrors and then tune the secondary mirror of the telescope
accordingly.
For adjusting the secondary tilt/decentricity, shims of size of few microns have to be used.
After every itteration the secondary is tuned, alignment should be checked with the
interferometer. When the distance between the primary and secondary is adjusted to get the
best wavefront, it will shift the focus. Hence the pinhole has to be adjusted to bring it the
right focus.
6.0 Integration of Filter wheel and Filter wheel motor
Once the telescope mirrors are aligned, the filter wheel motor can be mounted and then
filter wheel. Rotate the filter wheel to the empty slot position. Now again check the
alignment of mirrors with interferometer (to ensure that the telescope alignment is not
disturbed). Now the telescope FUV channel is said to be aligned and it is ready for focus
test inside the vacuum chamber.
The integration and alignment of telescope will be done in 100Class clean room
environment. Special attention to be given in
Contamination
Vibration stability (during Interferometric test)
Thermal stability
Air turbulence
7.0 Telescope Focus determination
After alignment of the telescope its focus (location of the detector with in 0.05mm) has to
be determined For the FUV channel this experiment should be done inside the chamber.
The arrangement for this focus determination test is shown in figure 11.
17
8.0 Collimator:
It has been proposed that the collimator is a stand alone, pre-aligned and tested unit and
when the source is at the focus of the collimator it gives collimated beam of 430mm. At the
focus the collimator a pinhole turret is mounted on a XYZ stage (part of collimator), Z is
for adjusting pinhole position along the optical axis to determine the telescope focus. The
turret has got different size pinhole (10, 20, 50, 100micron) and one at a time can be
brought to the position of the collimator on-axis focus. If it is needed the collimating
position of the pinhole can be checked with the help of interferometer. Or in case if there is
any disturbance in the collimator alignment, it can be tested interferrometicaly as done for
the telescope before moving into the vacuum chamber. The collimator is shared between
LEOS and IIA for UVIT alignment purpose.
The wavefront quality needed for the UVIT focus test is only lamda/10 wave and it has
been simulated and checked in Zemax optical design software which shows that the focus
determination for UV by our method( finding focus of collimator at visible by
interferometer and UV focus of UVIT in vacuum chamber) is as good as would be obtained
by a collimator with no wavefront errors. The collimator needs the thermal compensation in
the structure to keep the focus.
8.1Vacuum chamber setup
The detail of the chamber is explained in a Vacuum chamber document and ref. will
be given latter.
The vacuum chamber (schematic is shown in figure 9) for the test of focus determination is
placed in the clean room (1000 class) environment. The length of the chamber is 5meter
and 1meter in diameter (Approx). It has got door opening at its both ends. The chamber is
placed in the clean room such that one door can be opened in the 10000-class clean room
and the other can be opened in 100-class clean room. The camber has got various ports for
pumping, viewing, source and detector, venting etc., as shown. The source port (entrance
port for collimator) is in the 10000-class clean room and telescope side port (exit port for
telescope) is in the 1000 class clean room. In side the vacuum chamber, the bed is also
18
made for the telescope and the collimator. Bottom of the bed is welded to the chamber.
Over the bed an intermediate plate of same length of two numbers is mounted –one is for
telescope and other for collimator.. Over the intermediate plate, two pairs of rails are fixedone pair for the telescope and the other for collimator. The integrated collimator on a bed,
which has got rollers at the botton, can be moves over the rail inside the vacuum chamber.
Similarly the integrated telescope on the Gantry can also be moved over the rail inside the
chamber.
Reinforcement
Feedthrough
Port
DN100CF
Vent
Valve
Reinforcement
Gauge Port
DN35CF
Gauge Port
DN35CF
Hinge
Door
View Port
DN100CF
Elcetrical Port
DN200CF
Pumping Port
DN250CF
Reimaging Optics
Port DN200CF
Pumping Port
DN250CF
Pumping Port
DN250CF
Reimaging optics
Port DN200CF
Monochromator/View
Port DN100CF
Pumping Port
DN250CF
Elcetrical Port
DN200CF
Hinge
Door
Mounting Flange
Vacuum Chamber/ Ports
TopView
Figure 9: Schematic view of Vacuum chamber.
8.2 Setting up the line of sight inside the vacuum chamber
Before putting either collimator or telescope inside the vacuum chamber, mechanical
center, tilt of the entrance port where the monochromator is to be mounted, has to be
checked and then the line of sight has to be established. This can be accomplished by the
ATAC and couple of alignment targets.
The tilt or wedge of the entrance port should no be more
than 5 arc min.
19
Mount a reflecting Target T1 on the entrance port of the chamber, as shown in figure 10,
and place the ATAC infront (about 5meter distance) of the target T1 and align it to the T1
center and tilt.
(The accuracy of centericity of the target better than
100 micron and tilt between the mounting face and the reflecting
surface should be better than 30-arc sec) Now mount another target, T2,
similar to the target, T1, in the exit port and align it with the ATAC. The target T2 shall be
used for positioning the integrated telescope to axis of the collimator when it is put inside
the vacuum chamber. Open both the doors and close them again and check the colinearity
of the ports again for ensuring that there is no disturbance.
10000 Class
Clean Room
100 Class
Clean Room
ATAC
Target T1
Target T2
ATAC
Schematic view of alignment of Entance port and
Exit port alignment.
Partition Wall
Figure 10. Schematic view of alignment (colinearity & Tilt of the port) of exit and entrance
port of the vacuum chamber
8.3 Collimator integration/alignment inside the vacuum chamber.
Accuracy of alignment/positioning requirement of the collimator
inside the vacuum chamber is such that the pinhole in the
collimator (collimator axis) has to be centered to the entrance
port to 100 micron and the tilt of the collimator should be better
than 1 arc min.
The integrated collimator with the pinhole unit shall be moved inside the vacuum chamber
from the 10000-class clean room as shown in figure 11 (Details of handling and shifting the
collimator is to be worked out). After moving the collimator inside the chamber, it has to be
20
positioned and locked on the rail. The alignment of the collimator, tilt and decentre with
respect to the line of sight, can be done with Autocollimator which is placed infront of the
target T1. For tilt adjustment the cross mark in the rear surface of the secondary mirror (to
be demanded from vendor) and for the colinearity the collimator pinhole
shall be utilized. For finding the tilt of the collimator to the axis defined by the chamber
ports, the ATAC has to be mounted in the exit port side (about 3 meter distance from the
port) of the vacuum chamber and the target T2 and the secondary rear surface mark can be
used for it
10000 Class
Clean Room
100 Class
Clean Room
ATAC
Target T1
Target T2
Collimator
ATAC
Primary
mirror
Secondary
mirror
Collimatorpinhole
Schematic view of alignment of collimator
pinhole and the tilt of the collimator
Partition Wall
Figure 11. Schematic view of collimated unit alignment/positioning inside the vacuum
chamber
Once the collimator is placed/positioned inside the vacuum chamber, mount the
monochromator in the entrance port of the chamber and mount the source to the
monochromator. Put the source on and tune the monochromator to visible spectrum (say
550nm) and look at the collimator pinhole through the viewing window of the chamber.
Adjust (move) the collimator along the axis (Z- focus direction) towards or away from the
monochromator until the focused beam from the monochromator falls on the pinhole. Since
the pinhole position is pre aligned to the collimator on-axis and to its focal plane, the
collimator will give the parallel beam.
Once the collimator is aligned inside the vacuum chamber the chamber and the
monochromator can be evacuated. This is just to check the gas load inside the chamber.
21
Before putting the integrated and aligned telescope inside the chamber, the chamber can be
purged with UHP nitrogen.
8.4 Telescope setup and alignment inside vacuum chamber
The best focal plane of the telescope can be determined with the collimated beam inside the
vacuum chamber in the FUV region. The required accuracy of finding the location of the
detector plane is better than 50 micron. The telescope has to be shifted inside the vacuum
chamber for finding its focus.
The positioning accuracy of the integrated telescope
inside the vacuum chamber is that its tilt to the axis of
the chamber or collimator should be better than 3 arc
min and the decentricity should be better than 5mm
The integrated and pre-aligned telescope over the Gantry mount is shifted inside the
chamber over the rails. (the details of handling and shifting the telescope is to be worked
out).
Once the telescope is moved inside the chamber, it has to be aligned (tilt & decentricity) to
the axis of the collimator. The
tilt of the telescope should be with in
3 arc min for on axis focus test and the decentricity
should no be more than 5mm. The alignment of the telescope can be done
with the autocollimator, which is already set to the axis of the collimator as shown in figure
12. Procedure for checking the tilt of the telescope to the collimator axis is very similar to
the perpendicularity test of detector mounting bracket in the telescope, which has been
described earlier. After all the alignment the telescope can be locked to rail for avoiding
unnecessary disturbance.
22
10000 Class
Clean Room
100 Class
Clean Room
ATAC
Target T1
Target T2
Collimator
Integrated Telescope
ATAC
Primary
mirror
Secondary
mirror
Collimatorpinhole
Schematic view of alignment of integrated
telescope to the collimator axis with ATAC
Partition Wall
Figure 12. Schematic view of the integrated telescope alignment
8.5 Telescope focal plane determination
The required accuracy of finding the location of the
detector plane is better than 50 micron
Once the telescope is aligned to the collimator axis for on-axis focus determination, the
gadget in the detector mounting bracket and the target can be removed. Mount the detector
carefully in the detector mounting bracket. With the detector, the backend electronics and
high voltage unit also have to mounted inside the chamber as shown in figure 13 Do all the
electrical cabling and connect them to the electrical feed through in the chamber wall. Now
the chamber can be evacuated to 10-5 mbar for the focus determination test.
23
10000 Class
Clean Room
Filter Wheel
100 Class
Clean Room
Monochromator -Vacuum
chamber Interface
UV source
Collimator
Integrated Telescope
ATAC
Primary
mirror
Secondary
mirror
Detector
Collimatorpinhole
Schematic view of focus determination of
integrated telescope
Partition Wall
Figure 13. The schematic view of the setup for the integrated telescope focal plane
determination.
Turn the filter wheel to the position of MgF2 filter to come in the optical axis of the
telescope and tune the monochromator to mid wave band (????) of the MgF2 filter. Tune
the collimator turret to 20 micron pinhole and expose the detector for 100 msec ( if the
photons are not sufficient then increase the exposure time) and find the FWHM of the
image of the pinhole from the detector.
Now adjust the pinhole using the XYZ stage along the z-direction from –2mm to +2 mm in
steps of 50 micron from its original position ( it has to be recorded) . In each and every step
record the image and find the FWHM.
24
Var
thro iation
ugh of im
foc
us age si
ze
FWHM
FWHM
(minimum)
Best Focus
Pinhole position (micron)
Figure 14: Illustration of focus variation with respect to the pinhole position
Plot the graph (as shown in figure 14) between the distance moved by the pinhole and the
FWHM at the pinhole’s corresponding position. From the graph the position of the pinhole
for best FWHM can be found. This pinhole position subtracted from the original pinhole
position is offset (z-offset negative or positive) of the detector from its best focal plane.
Once the detector offset is found, the vacuum in the chamber can be broken with UHP
nitrogen purging. Now open the door (telescope side) and remove all the electrical cabling
to the electrical feedthrough.
To adjust the detector location for its best focus, it has to moved to the edge of the chamber.
Now dismantle the detector form its mount carefully. Make a shim of thickness equal to the
z-offset and put it over the detector mount and mount the detector back with out disturbing
the other elements. (Process of making and procedure of introducing shims is to be
worked out in details)
Now do all the electrical cablings, close door and evacuate the chamber again. Repeat the
process of finding the detector best focus and find the z-offset again and apply it the
detector. Thus the detector best focus can be found with in the required accuracy.
25
The focus test has to be done for onaxis (with in 3arc min) for finding the axial
position of the detector and at +/-0.24 degree in four positions to check tilt of the
detector.
For finding the focus at different field, the telescope can be tilted by means of its tilting
mechanism.
9.0 Photon number Calculation
VM-502-X Monochromator from Acton research
Monochromator F/ratio
4.5
Monochromator focal length 200mm
Monochromator resolution 0.04nm with 10 micron slit width
Monochromator dispersion 8nm/mm with 600 l/mm Grating
Monochromator Wavelength coverage 30 nm to 550nm
Source size at its focus
1.5mm X 2mm (Uniform source)
Number of photons reaching the detector per sec= Flux * Wavelength *time/Planks Const
*c
N = φ*λ∗τ / h*c
26
9.1 Contribution to photon number from various parts from source to detector.
Source
Source strength (spectral
Irradiance) mW/cm2/sr/nm
Monochromator input slit
(fully open) 1mm by 2mm
(mW/cm2/sr/nm)
Monochromator Efficiency
(mW/cm2/sr/nm)
Monochromator Exit slit 10
micron by 2mm –F-ratio 4.5
Spectral Band 8nm (mW)
Flux in 450mm collimated
beam
(mW)
Contribution
3
Net effect
3
66%
1.98
1%
0.0198
0.01*2*(1/4*4.5*4.5)*8
4X10-7
Due to F-ratio
[4.5/13]2
Due to mirror
Reflectivity
1.2X10-8
12%
25%
50% of 50%
Flux collected by 380mm
71%
2
aperture(380/450) (mW)
Reflectivity due to mirrors
50% of 50%
25%
(mW)
PMT efficiency (mW)
10%
With 10micron slit width
(mW)
Number of photon at 150nm 1 Sec Exposure
in
S/N
1 Sec Exposure
With 10micron slit width
S/N With 5 micron slit width 1 Sec Exposure
S/N calculation with 5micron pinhole
Number of photon at 150nm 1 Sec Exposure
in
S/NWith 5micron pinhole
1 Sec Exposure
S/N With 5 micron pinhole 10 Sec Exposure
8.5 x 10-9
2X10-9
2X10-10
2X10-10
151000
380
275
375
20
60
Table 1.
27
10.0 Tools Requirements:
Alignment telescope/autocollimator (ATAC)
1 Nos:
Accuracy:
5 arc sec
Short focus range
500mm
Optical rail with carrier
1 Nos
Length
1 meter (Rail)
Length X Width
150mmX 100 mm (carrier)
Reticules with circle
10 Nos
Target mount (custom made)
10Nos
Concentricity accuracy
better than 50micron
Tilt
better than 1 arc min
Reference flat
1Nos
Reference flat mount(custom made) 1Nos
Target mount holder(custom made) 10Nos
Monochromator and its accessories 1Nos
UV Source & order sorting filter
1Nos
Shims of different size (thickness)
Vacuum chamber and its accessories
•
Pumps and controls
•
Electrical feed through
•
Mechanical feed through
•
Targets for collinear test
•
Vacuum gauges
Computer for instrument control
28
11.0 References:
1. Interface and Control Document (ICD) , Doc. No. LEOS –AST-TR-02-05-19
2. Tolerances and Error budget, Document No. : UVIT-PDR-009a-Version 1.0,
March 2005
A1- Appendix
Table 2. Datas From LEOS (Ref. Doc. No. LEOS – AST – TR-02-05-19)
Critical Dimensional Data of Optical elements
Sl.No Optical
OD
Tolerance ET
Tolerance CT
Tolerance Wedge Circularity
Element
1
Primary
380.0 +0.01
47.0 +0.00
-0.5
2
Secondary 140.0 +0.01
41.90 +0.01
-0.5
0.02
.010
.02
-0.5
25.0 +0.00
-0.5
0.02
26.5
-0.5
+0.01
-0.5
Critical Dimensional Data of Mechanical elements
Sl.
Mechanical
Wedge
Planarity
Circularity
No.
element
mm
mm
mm
degree
degree
degree
1
MFD for
Perpendicularity
mm
degree
0.02
0.003
0.02
0.02
0.003
0.01
0.009
0.005
-
0.02
0.003
0.005
-
0.02
0.02
0.003
0.005
-
Primary
2
Ring for
Primary
3
Gluing
Fixure
4
Sec. Cell
0.01
0.004
0.005
-
0.02
0.01
0.004
5
Sec. Gluing
0.01
0.004
0.005
-
0.02
0.01
0.004
fixure
28