Imaging

Testing the new blocking filter to reduce Z-band thermal leak. (on-going, no news) -------------------------------

In July 2012 we received a new filter replacing the out-of-spec KG4 filter we had ordered from Custom Scientific. The optics supervisor at NDC Infrared Engineering (who made the Z-filter for us) suggested KG4 and BK7 as two possible ways to reduce the red leak. The 5mm BK7 reduces the thermal leak by only  25however, out of specs with only 5expected to have  70and since they do not produce it anylonger, he suggested to send us at no cost a 0.5mm KG2 filter as replacement. KG2 was installed in NOTCam in October 2013. Very thin and difficult to handle. Domeflats with lamps ON and OFF were made to check throughput and thermal leak, scaling them to K-band. Preliminary results show that the throughput of KG2 at 830-930 nm is much better than for KG4, but this needs to be properly quantified. The thermal blocking seems to be no better than the 25properly determined.

Done:

- some more data of standard fields were taken, but due to the not fantastic performance, this has been given a lower priority

TBD:

- Summarize all these blocking filter test results and add them to the page: http://www.not.iac.es/instruments/notcam/staff/ZYfilters.html

NOTCam flat field stability (on-going, updated) --------------

The best way to obtain imaging flats with NOTCam is differential twilight flats. The differential mode cancels both the thermal component and the dark. In principle, one could use differential dome flats with the lamps on and off, but at the NOT we have no flat field screen and the inside of the dome is highly structured (and daylight leaks through the hatches). The flat field stability has been checked on a few master flats taken with the WF camera and the JHKs filters. Investigation of ratio flats in periods where NOTCam was mounted several days in a row suggest that stray light is a problem (see also NOTCam Calibration plan v2.0 from Nov-13). There seems to be a larger difference between consequtive nights due to pointing on the sky than if the same blank field is repeated.

This points to scattered light and different light gradients. We decided to re-install the WF-camera imaging mask, a square mask to be put in the aperture wheel to limit stray light in the system. Because the aperture wheel has only 4 large slots (+ 8 small) we decided to remove the 64 micron slit in order to find space for this mask. The mask was tested once in 2003, but not for twilight flats. On the dark sky it made little improvement and was removed to make space for slits. Re-installing mask October 2013.

It has been confirmed that even < 10 degree from the full moon, the use of the WF imaging mask shows marginal if any difference in the backgrounds in JHKs filters.

Done:

- Skyflats on various nights and various blank fields using the mask have been obtained regularly from Nov-13 to Aug-14.

- Preliminary results show that there is no big differences or advantages in using the mask compared to not using the mask. This shows that the mask does not properly shield the stray light.

TBD:

- Summarize the analysis (when recovering lost disk).

WF-camera photometric surface (on-going, updated) --------------------------

The photometric response over the FOV may not be flat even after having divided by the nightly flat field. For many IR arrays the so-called photometric surface correction (or illumination correction) is needed, believed to originate in scattered light in the system. Modelling this requires photometric conditions and repeated measurements of a standard star on at least 25 positions of the array. In 2012 Janz, Laurikainen, Lisker et al. spent part of their photometric nights in March 2012 to obtain illumination correction data for the H-band, and present a model they use to correct their NOTCam data in Janz et al. (submitted Oct. 2012) together with a similar model for NICS/TNG. The most pronounced effect of the surface currection is a vertical gradient over the array with a  10array. It is not entirely clear whether this photometric surface correction varies with time. http://www.not.iac.es/instruments/notcam/calibration.html#illu

Done:

- All data has now been obtained for both the Ks band and the J band.

TBD:

- Analyze all data sets to produce models for J and Ks.

The Z-filter we have has a severe red-leak. The optics supervisor at NDC Infrared Engineering (who made the Z-filter for us) suggested KG4 and BK7 as two possible ways to reduce the red leak. The 5mm BK7 reduces the thermal leak by only $\sim$25%, but has a 90% throughput in the Z-band. The 1mm KG4 we got was, however, out of specs with only 5% transmission at 830-930 nm, while it was supposed to have $\sim$70% transmission. The provider thought this was probably due to old age, and since they do not produce it any longer, they suggested to send us at no cost a 0.5mm KG2 filter as replacement.

The KG2 filter has now been installed. It is very thin and difficult to handle. Dome-flats with lamps ON and OFF were made to check throughput and thermal leak, scaling them to the K-band. Preliminary results show that the throughput of KG2 at 830-930 nm is much better than for KG4, but this needs to be properly quantified. The thermal blocking seems to be no better than the 25% reduction we get with BK7, but this also needs to be properly checked.

The best way to obtain imaging flats with NOTCam is differential twilight flats. The differential mode cancels both the thermal component and the dark. The flat field stability has been checked with a few master flats taken with the WF camera and the JHKs filters. Investigation of ratio flats in periods where NOTCam was mounted several days in a row suggest that stray light is a problem. There seems to be a larger difference between consecutive nights due to pointing on the sky than if the same blank field is repeated. This points to scattered light and different light gradients. We decided to re-install the WF-camera imaging mask, a square mask to be put in the aperture wheel to limit stray light in the system. Because the aperture wheel has only 4 large slots (+ 8 small) we decided to remove the 64 micron slit in order to find space for this mask. The mask was tested once in 2003, but not for twilight flats. On the dark sky it made little improvement and was removed to make space for slits. Full tests will now need to be made to see if this mask reduces stray light. Also, more extensive tests will be made to check the stability of the flat fields.