Test Optic / t :

/ t . :

a e g : T r ' . ' f "

Scatterplate

HeNe Source

Figure 1.10: OSL Scatterplate Interferometer During Surface Testing

profilometric testing. It was demonstrated in a very simple form by Dr Kim. [Kim. •18). th a t profilometric testing could ultimately be useful in the very early si ages of optical production. However, the resolution th a t can be obtained using profilo­ m etric instrum ents can be compared to the resolution obtained from optical tests. [Church, 16). The size of optics which can be measured using profilometric methods is dependent upon the nature of the equipment used. Prior to the project discussed in this thesis, two simple contact profilometers were developed by [Kim, 48). These instrum ents are summarised below.

1.2.0 . 1 C o n ta c t P ro filo m ete r M odel 1

T he aim of this profilometer was to assess the profile of milled surfaces, generated using the 8 ft milling machine at OSL. In the construction of this profilometer. a Im optical flat provides the reference plane for measurements. linear velocity differential transducer, (LVDT), [Hordeski, 40], provides the probe transducer used to m easure the height of a surface under test, with respect to the reference plane provided by the optical flat. Such systems are also discussed by Wills-moren and Leadbeater, [Wills-Moren and Leadbeater, 81). These height measurem ents are then converted into a profile.

Carriage, (Aluminium)

Optical F lat

Optical F lat Support 150mm

1060mm LVDT Probe Optic

Side View

/

End View

Figure 1.11: Contact Profilometer Model 1. [Kim, 48]

T h e repeatability of the LVDT transducer system is about l ^ m , which is within th e accuracy required for milled surfaces. This is especially true if it is considered t h a t up to 20^m of glass material can be removed during loose abrasive lapping. However the LVDT must be calibrated to measure profiles to within an accuracy of 10//m. During profile m easurement, the LVDT has a continuous motion across the optic surface. Readings from the LVDT are sent via a RS232 serial link to a PC. To move the LVDT carriage assembly, across th e test surface, the 8 f t m achine’s quill was used. The quill is fitted with an X position. Moiré Fringe encoder. From the work carried out by [Kim, 48] a LVDT positional accuracy of 18/^m could be achieved for profile measurements. The key limitation with this profilometer is the

non-linearity associated with the LVDT. This is especially true if the LVDT is used over a large range.

1.2.0.2 C ontact P rofilom eter M od el 2

When the surface quality of optical surfaces approaches the standard where polishing can be started, it is necessary to obtain profile information to a higher resolution. A second profilometer was developed to obtain higher resolution profiles, which could also be used as an additional cross check on the profiles taken with the previous model 1 profilometer, as well as providing information during the early stages of polishing.

The second profilometer developed at OSL was also of surface contact type. An Invar rod formed the support frame for a series of ten LVDT probes, positioned at precisely determined locations along the length of the Invar support rod. Each LVDT could be pre-set to a height which conforms to the mirror sag, to the order of I m m . This was done by placing the profilometer on an optical flat, and using slip gauges under each LVDT to set the height. This m ethod reduces the dynamic range used for each LVDT to a much smaller value than compared to profilometer model 1 and therefore reduces the effects of non-linearity by the LVDT. This is the key advantage to profilometer model 2 over model 1.

This profilometer was designed to stand on the mirror surface using two support legs at both ends of the Invar rod. The separation of the legs was 830mm. The LVDTs used in this profilometer had better repeatability and lower therm al effects th an the LVDT used in profilometer model 1. Typically, the worse case total error, estim ated to be of the order of ±5//m , [Kim, 48]. Figure 1.12 shows a schematic diagram of this profilometer.

1.2 .0 .3 L im itation s o f th e E x istin g P rofilom etric S y stem s

The main aim of these profilometers was to provide a feed back mechanism during the milling process, by providing surface form information. This information could then be fed back into the milling process, therefore enabling mechanical errors in the 8 f t machine to be compensated.

Control Electronics

58mm

Side View

865mm

Knife Edge

LVTD Probe Instrum ent Support Arm

Invar Rod

End View

Figure 1.12: Contact Profilometer Model 2, [Kim, 48]

The two existing profilometers, at OSL, were both capable of producing acceptable results to feed back into the milling process. However both show limitations in both accuracy and dynam ic range. Profilometer model 1 has a m axim um range in X- position of Im . This is the size of the optical flat used as a reference. Also th e X encoder used to obtain LVDT positional information does not cover the full dynam ic range of 2.7m, which is the m axim um capacity of the 8 f t machine. Therefore, if a larger optic has to be produced, a new reference fiat and X encoder m ust be used to cover the size of these larger mirrors. However, for profilometer model 2, larger optic diameters can be accommodated by simply replacing the existing Invar rod

with one of longer dimension.

The dynamic range in the measurement direction, associated with model 1, is tied to the range of the LVDT transducer. If optics with sags greater than the dynamic range of the LVDTs are to be figured, then profilometer model 1 cannot be used without modification. Profilometer model 2 overcomes this problem by allowing the vertical positions of the LVDTs to be adjusted to conform with the mirror sag, although both profilometers suffer from non-linearity in the LVDTs.

The contact force between the probe and the optical surface in both model 1 and model 2 profilometers was of the order of tens of grams. When very smooth surfaces are to be measured, low contact forces are required to prevent surface damage. The accuracies obtained from these profilometers was also low. Although, for the measurement of profiles during the milling and early polishing stages their perfor­ mance is adequate. However for fine figuring and surface quality measurements, these profilometers are incapable of producing the high measurement accuracies re­ quired. Therefore, to obtain the higher accuracy profile information, required in later production stages, a different additional instrum ent must be used.

1.3

D e fin in g A N e w In str u m e n t

The aim of this project is to develop a new profilometric instrum ent to work in the area of large optical production at OSL. The specifications required for this new instrum ent are defined by the optics likely to be measured. To quantify these specifications a study was performed using the dimensions of a specific telescope optic. The optic used to quantify the specifications for this project was the Gemini, [Osmer, 59], telescope project secondary mirror. The Gemini telescope project is a collaboration between 6 countries, to build two 8m telescopes, one based on Mauna Kea in Hawaii and the other based at Cerro Pachon in Chile. A profile of the secondary mirror for the Gemini project can be defined by the following expression, [Augustyn, 3].