1.0 Overview of Field Rotation in UnISIS
UnISIS is installed at the coude focus of the 100-inch Hooker telescope on Mount Wilson. The telescope is of classical cassegrain design with interchangeable secondary mirrors and a tertiary fold-mirror to bring the light to its various foci. As the UnISIS optical benches are fixed with respect to the rotation of the earth and the telescope tracks celestial objects, field rotation must be compensated prior to the light reaching the imaging cameras.
While it would be advantageous to correct for the imaging field rotation at the entrance port of the telescope, the UnISIS laser guide star projection system would have to then fire its laser through the field derotator. This would introduce 3 additional problems: 1) it would make the optical systems more difficult to align; 2) it would require hardened optical coating to be used in the derotator greatly increasing costs, 3) since every photon is precious it introduces 3 extra reflections that are unneed in the projection optics. As a result the location for the field de-rotator was chosen to be directly in front of the imaging cameras. An alternative solution would have been to rotate the cameras, the advantage being no extra reflections, however, the cost of a rotator sufficient to support the physical size and weight of the cameras was prohibitive.
The rotation corrections required for the highest resolution work at the egdes of the field of UnISIS are determined in the remainder of this paper. As such, towards the center of the field of view, or as pixels are binned to larger sizes these criteria and limitations can be relaxed somewhat.
2.0 Angular Scale of Field Rotation
The length of a sidereal day is 23 hours 56 minutes 4 seconds, that is the time it takes for the rotation of the earth to bring a fiducial star to its same point in the sky on one day as the next. This translates to 86164 seconds of time ( 23*3600+56*60+4). The rotation of the earth over this time covers 360o or 1296000 seconds of arc. Thus the sidereal tracking rate (STR) is 15.04 seconds of arc per second of time (1296000/86164).
The importance of rotation on the scale of an imaging camera can be assessed by knowing the angular size of one pixel with respect to the rotation axis, presumed here to be the center of the imaging chip. The primary UnISIS imaging camera is a 2048x2048 CCD with 15mm square pixels. The angular extent of one pixel as measured 1024 pixels from the center of the CCD is a = arctan(1/1024)*3600 = 201 seconds of arc.
This implies that the field rotation at the outter edge of the imaging chip will see a 1 pixel shift in the position of the image in 201/15.04 = 13.4 seconds.
3.0 Compensation of Field Rotation
In most cases it will be more than adequate to keep the image rotation to within 1/10th of a pixel. This is offered without proof and will be demonstrated at some point in the future, but will serve as a target value for the moment. This translates to approximately 20 seconds of arc of rotation on the sky. Thus a compensating effect for the field rotation must be introduced every 1.3 seconds of time.
4.0 The Field De-Rotator Optical Parameters
The Field De-Rotator can be most simply thought of as a Dove prism, that is a chunk of glass with 2, 45o angled ends, sort of like an elongated 45-45-90 prism. Of course, a prism, being a big chunk of glass, would introduce a considerable amount of light loss and so will be made using mirrors, but the principles are similar. One interesting fact about a Dove prism is that the image rotates twice as fast as the prism and this will directly effect the operation of the field de-rotator.
5.0 The Field De-Rotator Mechanical Parameters
UnISIS makes use of several Newport Instruments motion controllers, translation and rotation stages. These are particularly convenient systems to use as they are highly accurate, durable and elegant, but they are definitely not for the "shallow of pocket". The motion controller available for use in our system is an MM3000 and the rotation stage is an RTM160PP -a stepper motor based rotation stage capable of 360o movement and the has a resolution of 0.001o .
This rotator can provide 360000 steps over the full 360o range of rotation or 3.6 seconds of arc per step. As a result, our earlier criteria of keeping the image rotation to within 1/10th of a pixel can be easily met and rotation can be controlled down to a best of ~1/56th of a pixel with this motor. However, this is accounting only for the motion of the rotation stage. There is an additional correction for the optical property of the Dove prism where the image rotates twice as fast as the prism angle. As such the best correction we will be able to maintain is 1/28th of a pixel when de-rotating the field while tracking at the sidereal rate. In otherwords, each resolution step of the rotator is 7.2 seconds of arc on the imaging chip.
The above image shows the UnISIS field de-rotator in position in front
of the imaging cameras. The entrance mirror (1) is shown in the center
of the steel ring, the edge of the second mirror (2) is visible immediately
above it and the edge of the third mirror can also be seen.
6.0 Controller Software
The controller software was written in National Instruments LabWindows CVI and couples directly to the UnISIS RS-485 actuator control network. The following functionality was determined to be required:
Basic Control Algorithm Loop:
Basic functionality is in place in a program called drot.c constructed using the LabWindows/ CVI programing language. Basic hardware/ software implementation has been operationally tested. Next time telescope time is available the rotation direction will be determined and preliminary images can be taken. Interactions/ conflicts etc with other software on the system needs to be tested and worked out.
8.0 Future Needs
Several other control programs are required to run on the MM3000 at the same time. To accommodate this, the De-rotator software uploads and compiles an executable routine to the MM3000 to provide the derotation. This program then runs continuously and updates are obtained as needed on the windows interface. What is needed is a modification to operating philosophy of the De-rotator such that it provides the astronomical advantage of always having north in the same position on the imaging ccd. Having north aligned on either a row or column of pixels will greatly simplify the data reduction process and make the identification of the field considerably easier. Thus code will be added to the De-rotator to determine the appropriate offset based on the starting RA and DEC positions at some time in the future.