Prime Focus Instrument™ for Wide Field Astronomy
When Obtaining a Wide FOV A Short Focal Length is Desired:
The Field Of View (FOV) is determined by the effective focal length and the size of the detector.
ATLAS OTA and LEO Scope™ and LEO Comm™ are examples of optical systems purposely designed to obtain a wide FOV. When adapting already existing optical systems, there are typically two solutions for expanding the FOV.
The first solution is to change the secondary mirror to provide a focal ratio of perhaps F/5 to F/8 with an F/3 primary mirror but then the central obscuration becomes large. A second solution is to build a focal reducer for an existing optical system to reduce the effective focal length to provide a usable FOV. However, this becomes very complicated, expensive, and requires considerable space.
DFM has reinvented a better option, developing a prime focus instrument position. This solution can provide an effective focal ratio just a little slower than the focal ratio of the primary mirror. The system described here uses a 1.2m parabolic F/3 primary mirror. The prime focus field corrector design is simplified by allowing the effective focal ratio to become F/3.1.
Prime Focus Instrument™ (PFI™)
Many 1m class telescopes were made in the 1950's and 1960's. The telescopes tended to have a Cassegrain optical system with a parabolic F/3 primary mirror and an effective Cassegrain focal ratio of about F/13. These older telescopes were primarily used for single star photoelectric photometry and had a limited Field Of View (FOV). Additionally, the use of modern detectors (CCDs) with their physical small size limits the FOV to perhaps 5-10 arc minutes.
A Prime Focus Instrument™ (PFI™) retrofitted to these older telescopes can allow these telescopes to be re-purposed to perform wide field astronomy such as multi object photometry, super nova patrol, variable stars, planetary transits, occultations, orbital debris searches, asteroid searches, etc. With a PFI™ the FOV can be increased to 1° or more.
Good Image Quality is Desired:
Any wide field telescope will require a field corrector. A Ritchey-Chrétien (R-C) optical system has astigmatism and field curvature, which will need to be corrected at some FOV and desired image quality. Typically, the older telescopes are a classical Cassegrain with a parabolic primary mirror. The field corrector tends to have negative spherical correction so they are simplified when used with a R-C primary mirror, which is a hyperboloid. It is reasonable to design a field corrector for a F/3 parabolic mirror with 3 all spherical elements.
Example: A Prime Focus Instrument™ for the 1.2m Kryonerion Telescope
The Goal: Bring a dinosaur telescope back to life!
This example is used to convey some of the various attributes and design intricacies of the Prime Focus Instrument™ by DFM Engineering. For more information or for a custom implementation of the PFI™ for your institution, please contact us.
The project goal was to modernize and upgrade the 1.2-m F/3 - F/13.6 Kryonerion telescope to start searching for asteroids by looking for flashes on the Moon's surface. After studying the possibility of building a focal reducer to provide a 30-arc minute FOV on a fast readout (30 frames/second) sCMOS camera, the decision was made to implement a dual purpose prime focus instrument.
The instrument can support time resolved imaging of the Lunar surface in two colors and support wide field direct imaging and photometry. There can be up to three cameras installed on the instrument. A sCMOS camera for the Short Wave Infra Red (SWIR), a second sCMOS camera for visible Lunar imaging, and a cooled and integrating direct CCD camera.
Selecting the Lunar imaging or the direct imaging modes is motorized and remote controlled. This allows interleaving Lunar observing and other science without performing an instrument change and highly increases the efficiency of the telescope. Wide field observing can be performed when the Moon's position in the sky is too low to be useable for asteroid searches.
There is a provision for a filter changer for the direct imaging mode and for an autoguider.
Optical System Details
The 1.2m telescope parabolic primary mirror (F/3) was used with field correctors. The corrected direct imaging optical system produces a F/3.1 focal ratio. The Lunar imaging optical system includes a focal reducer and produces a F/2.8 focal ratio with SWIR and visible channels with a 30 arc minute field.
The first and second field corrector elements are common to the direct imaging focus and to the Lunar foci.
The Lunar imaging optical system has its own #3 field corrector element followed by a dichroic beam splitter and SWIR and visible doublets.
The Lunar imaging doublets are necessary to provide the focal reduction and to provide the same effective focal length at the 2 colors.
Field Corrector Diagrams: Lunar Imaging and Direct Imaging Optical Paths
Not shown in this example, but described below and included in the optical design are the filters, Lunar camera filters and windows, the science filter, and the science camera window.
Spot Diagrams:
The circles are 40 μm in diameter (2.5 arc seconds for the Lunar images and 2.2 arc seconds for the science camera).
The images are essentially seeing limited over the entire field.
A full science FOV of 1.4° is supported by the design.
Spot Diagrams for the 3 Foci
Direct Imaging Camera (Option):
The optical design contains provisions for direct imaging using a cooled and integrating CCD camera.
The design includes a filter and an optical window for the CCD dewar.
The direct imaging filter is not used for Lunar observing as there is a dichroic filter/beam splitter and other filters located just in front of the sCMOS lunar cameras.
The direct imaging filter may be placed anywhere between element #2 and element #3.
Direct Imaging Optical Path Diagram
Camera Selector Mechanism:
The 3 cameras mount to the camera slider plate. The slider plate is driven by a motor and a lead screw.
The motor is controlled by the Telescope Control System (TCSGalil™).
Position is measured by counting the motor encoder steps and using the limit switches.
The slider plate can move 5.25 inches (133mm) and allows a space of about 6.3 inches (160mm) square for the science camera.
The camera slider plate and drive mechanism have been custom configured for the 1.2m telescope example.
In this configuration, the camera systems are mounted on a two position motorized slider that positions the direct camera and its #3 field corrector element or positions the Lunar cameras and their optics on the optical axis.
Camera Selector Mechanism
Filter Changer (Option):
The filter changer box is located on the outside of the Optical Tube Assembly (OTA) and consists of a filter cassette box that is driven up or down to position the filter.
A selector rod inserts or withdraws the desired filter on rails that run from outside of the OTA to the prime focus instrument.
The filter cassette box contains up to 8 filters mounted in individual cells. The filter is withdrawn when used in the Lunar observing mode.
Filter changer box with filter cells partially removed
Auto Guider (Option):
Provisions for future mounting of a pick off mirror and an auto guider camera were designed into the system. The DFM Engineering TCSGalil™ supports auto guiding.
For more information on the Auto Guider feature, contact DFM Engineering.
Focus Mechanism:
The optical system is focused by moving the focus plate that carries the camera slider plate and the cameras. The motion is driven by (4) precision lead screws. The lead screws are geared together with a timing belt (a belt with teeth) and a phase adjustment is provided at each drive pulley. A geared servo motor moves the focus plate and a linear travel potentiometer provides very accurate focus position information.
The focus travel is ± 2mm. The F/2.8 beam requires positioning the Lunar cameras' optical focal planes very close to the nominal design focal plane position. The science camera must also be mounted to position its optical focal plane very close to the nominal design focal plane position.
The focus position resolution is better than 5 microns. The TCSGalil™ will control the focus motion and provide the position readout.
Prime Focus Instrument™ Mechanism
For more information about the Prime Focus Instrument™ or the ATLAS Optical Tube Assembly (asteroid tracker) project and how to order an instrument for your institution, contact DFM Engineering.