SBI Optics

The optical design of the SBI balloon instrument features of a 30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, followed by a filter wheel holding a set of neutral density filters with different attenuation factors and/or band-pass filters depending on the mission specific science objectives and requirements. The Dall-Kirkham design was chosen to provide inexpensively the required long focal length with a compact package necessary for a balloon flight. The telescope focus can be adjusted by moving the secondary mirror along the optical axis by means of a motorized actuator. The telescope resolution at 0.28 μm is 0.2 arcsec and at 2.6 μm it increases to 2.2 arcsec. However, the detector pixel size is 2.86 × 2.86 arcsec per pixel therefore also in the NIR most of the point spread function peak lies within one pixel. This guarantees a nearly constant resolution over the entire portion of the spectrum measured by our bolometer.

Calculations and measurements indicate that bare (uncoated) Pyrex primary and secondary mirrors coupled with an Inconnel-coated neutral density filter with fused quartz substrate provide both the appropriate attenuation and the best spectral flatness over a 0.28 μm to 2.6 μm spectral range. This range accounts for over 96% of the total solar irradiance and most of its percentage variability. This graph shows the predicted transmission of the optical system. The transmission curve varies by only ±7% over this range and is limited at short wavelengths by a local minimum in the reflectivity of Pyrex at 0.25 μm and at wavelengths greater than 2.6 μm by absorption features in the fused quartz.

The design includes a filter wheel. For SBI-1 and SBI-2 we have selected different filters depending on the scientific requirements of each mission:

SBI-1: It holded a set of four Inconnel coated neutral density (ND) filters with attenuations: ND1.0, ND1.15, ND1.3, 1nd ND1.5, as well as a 10-nm interference filter centered at about 390 nm. The ND1.3 filter was the nominal filter used for the broadband imaging while the ND filters with higher and lower attenuation factors were used for calibration measurements. The 10-nm filter is centered above the CaKII line, which is particularly suitable to image the bright faculae and the enhanced network. This filter provided a record of the location of these magnetic features during the flight.

SBI-2 & SBI-3: A ND1.5 will be the nominal filter that will be used when recording the bolometric images. A ND1.3 that was folown previously with SBI-1, will be used for inter-flight comparisons. A 100-nm band-pass filter centered at 750 nm will be used to take full-disk color temperature measurements near the peak of the solar emission spectrum. A 10-nm band-pass filter centered at 670 nm will be used for calibration purposes. We will compare mosaics recorded in flight with images taken with the same filter on the ground with the McMath Solar Observatory at KittPeak. A step-wedge neutral density filter divided into 6 pie wedges of ND values 1.3, 1.55, 1.77, 1.98, 2.2, and 2.42. This filter will be used for post-flight photometry calibrations.

All the materials used for the telescope are vacuum compatible, i.e. they do not out-gas continuously when exposed to vacuum, to prevent contamination of the optics. The telescope tube is made of carbon fiber, which exhibits very low thermal expansion. The tube, the filter wheel , the detector and all other electronic coponents installed in the vicinity of the optics, have been vacuum baked for 24 hours to eliminate residual outgassing that may contaminate the optical surfaces during the flight. The secondary actuator motor, used to focus the image, is vacuum prepared and the focus mechanism is lubricated with a vacuum compatible lubricant.

Optical Design