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WIYN One Degree Imager

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Filter Changer

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Demonstration Filters

Sloan g’ r’ and i’ delivered from Infinite Optics Johnson U on order from Barr Filter below is a r’ - 146 mm demo of the 400 mm filter process

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IO g’r’i’ filters

Infinite Optics Filters

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Infinite Optics Experience: Summary

IO is new to the filter business, but has a lot of experience with complex and exotic thin film designs (e.g. broad band AR coatings)

Small company in southern California, surviving largely on diverse commercial contracts (e.g. colored lighting displays for Disneyland)

Very interested in the astronomy filter and AR coating business Did the AR coatings for PanSTARRS corrector The WIYN filters probably cost IO 2-3 times what we paid (typically

$5000 per filter) for their investment in engineering and testing Capable of producing good filters, but still developing the art Should be kept in the loop for RFPs

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Strengths of IO

Filters are extremely uniform across the field – only in the very corner can a transmission deviation be seen

Filters do not suffer loss of performance at high angles of incidence – though the usual “blue shift” is unavoidable (i.e., good for fast beam like Blanco and non-telecentric optics)

Excellent red leak suppression – critical for DEC’s red detectors Adequate in-band performance

Worked hard to achieve transmission and reduce wiggles Meets the out-of-band spec

Their AR coating work (as opposed to filters) was very fast (3 weeks) and high quality, as they have been doing that for years

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Weaknesses of IO

Slow to deliver as they learn about filter manufacture Some of the filters were “domed”, up to about 10 waves across the

146 mm clear aperture These were later replaced by filters that were good to 1-2 waves. Partly, this is WIYN’s problem by requiring thin filters (10 mm)

Small shop – can be distracted by other contracts unless large dollars are involved

Larger filters are still an unknown item – they purchased a large chamber that can do the job (as well as AR coating of large optics) but it has not been turned on yet.

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Plots of the IO r’ filter

Filter sampling points – p8 is in the far lower right beyond p7

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Specifications: Bandpasses

We may wish to modify the y filter since our detectors are not so red sensitive

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Low Resistivity Detector QE

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High Resistivity Detector QE

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Total ODI Throughput

Includes 3 mirrors, optics and coatings, low resistivity CCDs, no filter

Blue edge is defined by ADC glass and coatings, red by CCDs

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More Specs: Out of Band Blocking

Outside the main bandpasses, the following maximum transmissions cannot be exceeded. Between 300 and 1200 nm: filter transmission must be less than 10-2

(1% maximum transmission) outside the filter passband. Between 300 and 1200 nm, the total out of band light cannot exceed

0.25% of the total in-band light.

That is: The integral of the transmission blueward and redward of the passband must be smaller than 0.0025 times the integral of the passband transmission. We define the limits of the integral at the passband as the point where the filter transmission is 0.1% (0.001)

This definition is more meaningful from an astronomy standpoint than a simple peak violation, and is easier for the vendors to meet.

See full WIYN specs at: http://www.wiyn.org/ODI/filters.htm

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Testing

Filters need to be tested! For acceptance About every 1-2 years as they age (drift to the blue)

PanSTARRS has not tested their Barr filters – only witness samples. Kitt Peak’s “Coude Feed” telescope can serve as test facility; it is very

rarely used, and KPNO will allow us to use it Provides a perfect platform for filter testing – it is essentially a giant

spectrograph with an f/13 sampling beam. Need the following:

XY stage to obtain spectra through filter at any spatial position Rotational stage to obtain spectra at various incidence angles – allows

simulation of f/3 beam Red-sensitive CCD to check for red leaks (can be cosmetically poor) Software to automate process

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PanSTARRS Filters – g’

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PanSTARRS Filters – r’

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PanSTARRS – Good Enough?

Spatial variations are a concern Red leaks become serious at 1100 nm – is that okay?

Both DEC and WIYN can put a second filter in parallel to block leaks

But, changes optical design optimization and reduces throughput

Cost seems acceptable: $60-80K Delivery times seem acceptable: 1 year

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Next Steps

WIYN wishes to initiate RFP in 3-4 months Does DEC wish to coordinate?

Multiple filters of same type are always cheaper Must agree on specs

WIYN complement: 8 filters g’r’i’z’ – minimum first batch Narrow band: 500.7 and 656.3 nm (5 nm FWHM) – second

batch Then, Y and U bands – third batch

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Internal Ghosting

Multiple ghosts – extreme example Seen with g’ and i’, but not r’ Example at f/3; at WIYN f/6, not as bad Evident only around bright stars Diameters indicate separations of ~1 mm Filter are 10 mm in laminated layers:

g’ has 5 ~1 mm layers r’ and i’ have 4 layers All have external covers, 2-3 mm

LESSON! need specification for internal ghosting

Has been seen occasionally over the years with interference filters at Kitt Peak