Testing KDP crystals Jan 2016

In collaboration with the NTE team, it was decided to test KDP/DKDP crystals and their potential as thermal emission blockers. These crystals, KDP (potassium dihydrogen phosphate) and its deuterated analog DKDP, are described in e.g. Xu et al. 2012 (PDF file) , and have the unique property that they can block emission redward of ~ 1.6 micron while they have a transmission of > 90 % from optical to ~ 1.4 micron. They can therefore serve to block thermal infrared emission for filters with unwanted red leaks. The NOTCam Z-band filter is such a filter.

For test purposes it was decided to purchase small KDP crystals from United Crystals, and 3 pieces arrived as follows:

One of the practical challenges with these crystals is that they have a low tolerance to humidity. Their properties deteriorate with exposure to water vapor. Wheeler et al. 1999 (PDF file) studied the severity of exposure to environmental effects such as water vapor, and conclude that water is absorbed into the porous coating and etches into the surface of the KDP. From their graphs we estimated that 3-4 hours of exposure to 55-75% RH would be ok.

Mounting test units in the aperture wheel of NOTCam

On the 15th of January two of the crystals were installed in two small slots of the aperture wheel of NOTCam.

The vacuum sealed packages were opened in the last minute, and the crystal were exposed to ambient air (measured to have RH ~ 32%) during about 1 hour before NOTCam was put on the vacuum pump. The crystals were uniform and transparent.

NOTCam was pumped for 5 days, cooled with the PTR for 24 hours, then filled with LN2.
48 hours later NOTCam reached operating temperature and was mounted on January 23rd.
Domeflats ON-OFF were taken to estimate throughput and thermal blocking efficiency.
No tests were made on the sky due to clouds.

Images of domeflats through Y-band and KDP/DKDP

Y-band domeflats with the KDP (left) and the DKDP (middle) in the aperture wheel. The cross-section (right) is across the DKDP.

There is a profound structure seen in the KDPs and the throughput varies by a factor of two across their surface. This structure looks the same through all filters tested.

Visual inspection of KDP/DKDP through entrance window

The KDP (left) and the DKDP (right) in the aperture wheel seen through the entrance window.

The same structure is seen by visual inspection of the crystals (see above photos obtained by Carlos).
This means that their appearance have changed since they were mounted.
At first, it was believed the filters had cracked due to thermal stresses during cooling down.

Approximate KDP/DKDP transmission

Disregarding the structure over the KDPs, where the throughput varies by a factor of two, the average throughput, measured over the whole surface was estimated at several wavelengths using ON-OFF domeflats to correct for thermal emission leak (Z).

Average throughput of KDP and DKDP measured from domeflats ON-OFF.
Unit Z
(0.889 µm)
(1.020 µm)
(1.079 µm)
Pa gamma
(1.094 µm)
(1.211 µm)
(1.250 µm)
Pa beta
(1.287 µm)
KDP 68 % 70 % 69 % 68 % 64 % 71 % -
DKDP 68 % - - - - 73 % -

KDP/DKDP transmission in Z-band measured on standard star

On Feb 23 2016 we finally got a short test made in clear sky on standard star AS26-0 (FS23, M3-193) which is a Z_MKO standard used by UKIDSS at Z = 13.500 +- 0.015 mag (UKIDSS DR4). Observations were made as 5-point dithers on this star, using frame 4 4. The reductions were made skipping the 1st of every 5-image loop (due to reset-anomaly), flat-fielding using the Z-band differential twilight flat, shifting and combining the 4 individual images into one final image.

Below is shown Z filter only (left), Z+KDP (middle), and Z+DKDP (right) of the ~ 2' central field of AS26 (N up and E left).

The optical quality is bad, and the PSF is distorted in one or the other direction depending on the structure of the KDPs just in front. In the combined images this is somewhat smoothed out.

In the background of the two KDP images one sees a remnant of the reset anomaly as a broad vertical band (brighter or fainter). In addition, the narrower slightly tilted stripes are due to pick-up noise, often seen in very low-background images.

The throughput measured on the standard star varies strongly over the KDPs, with an average value of about 70%. This agrees with the estimate found from domeflats. The throughput varies by a factor of two across the highly structured surface, thus transmission from 50% to 90%. Expected throughput is 90%.

KDP/DKDP thermal blocking in Z-band measured on standard star

The sky background was measured using the 4/5 dithered images subtracting a dark obtained in the same mode (i.e. dframe 4 4. While the 16 second integration through the Z-band gives a count level of 4720 adu, when inserting the KDP/DKDP the sky background reduces to 47/55 adu, a factor of 100. This brings us to a sky background of ~ 18.2 mag/square arc second through the Z-band, which compares well with the WFCAM/UKIRT background, and shows that both of the KDPs block the thermal emission efficiently.

Structure disappeared when warming up!

Amazingly, when opening up NOTCam on March 7th to take out what we believed were cracked crystals, they had regained their original appearance, i.e. no cracks or structure seen, the crystals had again become completely transparent. To protect the crystals from humidity, they were unmounted and stored in a dry environment. When closing NOTCam again, only the KDP was re-mounted for further testing. NOTCam was pumped from March 8 until March 15 when the PTR was started. During the slow cooling down of NOTCam we checked the KDP appearance. See photos kindly obtained by Joonas, Tapio, and Pedro below:

The KDP photographed through the entrance window at different times during the cooling down: at -73 degrees (left), at -79 degrees (middle), and at -108 (right). These numbers are the temperatures measured at the center wheel section inside NOTCam. The exact temperature at the KDP in the aperture wheel is difficult to know.


  1. The average transmission is lower than the expected 90%, but this is likely due to the spatially highly structured appearance of the crystal. The transmission varies across the surface by a factor of two.
  2. The cause of this structure in the crystals is believed to be due to thermal stresses.
  3. The optical quality is strongly affected and can not be properly evaluated.
  4. The thermal blocking efficiency is, however, promising. Confirmed on sky.
  5. The structure in the crystal disappears when warming the KDPs up again.
  6. On the latest slow cool-down, the structure seems to come back at a temperature of around -80 degrees and definitely by -100 deg.

Comments to Amanda

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