# Presentation: 533821

## Analysis of Filter 

***Transmission Uniformity Specifications ***

**Huan Lin**
**Experimental Astrophysics Group**
**Fermilab**

## PanSTARRS Filters from Barr

***from B. Bigelow***

***Similar to DES filters:***
***570 mm size***
***10 mm thick***
***Fused silica substrates***
***Data on griz filters available***

***PanSTARRS i-band filter***

## PanSTARRS Filter 

- The red dots are the positions where the filters were evaluated.
- 9 *radial* points every 1”, last point at 9.5”
- I’m calling these positions 1-11, from center to edge
- Position 7 is used as reference
- Eight *azimuthal* points

- side 1

- side 2

*** ******adapted from M. Schubnell***

##  i band

-  i band

***from B. Bigelow***

##  i band

-  i band

***from B. Bigelow***

##  g band

-  g band

***from B. Bigelow***

## Filter Transmission Uniformity Analysis

**Give filter specifications to vendors using upper and lower absolute transmission envelopes, similar to PanSTARRS filters **
**DES photometric calibration requirement is 2%;  assign 1% error budget component to filters to account for spatial non-uniformity in filter transmissions**
**Test sets of filter curves fitting within absolute envelopes in order to specify transmission spatial uniformity requirements**
**Use galaxy SEDs (E, Sbc, Scd, Im) from Bruzual & Charlot GISSEL package:  CWW SEDs extended using theoretical models to the UV and IR **
**Calculate fractional flux differences, vs. *****average***** of all test filter curves, for 4 galaxy SEDs over redshift ranges relevant to main optical spectral features: 4000Å break, [OII] 3727 and [OIII] 5007 lines**
**Also account for transmission variations due to changes in incidence angle over focal plane **
**Use galaxy analysis results to define “fraction envelopes” on transmission uniformity**

## E: < 3200 Å feature

***E: 4000 Å***

***Im: [OII] 3727 Å***

***Im: [OIII] 5007 Å***

## Example Absolute Transmission 

- Average filter transmission required to fit within absolute transmission envelopes

- Example Absolute Transmission
- Envelopes for i-band Filter

## Example Transmission Uniformity

- (“Fraction”) Envelopes for i-band Filter

## Slide 11

## Slide 12

## Status

**Derived filter transmission and spatial uniformity envelopes, based on relative photometric calibration requirements applied to galaxy spectra**
**Contributions of “envelope” and “incidence angle” effects are about the same, for adopted 0.84% fractional flux cut used to define acceptable envelopes, and for Barr/PanSTARRS n****eff**** values**
**Current acceptable envelopes should lead to < 1.2% fractional flux difference for galaxies**
**Vendor responses to filter RFI (see M. Schubnell talk, docdb #1635) indicate it is too expensive and/or difficult to meet our current uniformity specifications, basically about 3% transmission variation over flat parts of filters**
**Will try using color terms (which are no longer avoidable) and see if non-uniformity of Barr’s PanSTARRS filters can still be acceptable for DES**
**Derive approximate relaxed uniformity specifications**

## Revised Filter Transmission 

**Use measured filter transmissions at different radial positions for PanSTARRS filters**
**Use position 7 as reference transmission;  it’s approximately the median (see next slide)**
**Also use DES filters with gradients applied to derive results more directly applicable to DES, as PanSTARRS filter bandpasses differ in detail**
**Use Pickles stellar library, with 131 spectra of wide range of stellar types, to derive transformations between the magnitudes at different filter positions**
**Use quadratic fits:  e.g., g - g0 = a + b(g0-r0) + c(g0-r0)****2**
**Use reference colors g-r for g,  r-i for r,  and i-z for i and z**
**Use same galaxy SEDs (E, Sbc, Scd, Im) as before **
**Also consider SN Ia “Hsiao” templates (via John Marriner) at -7, 0, +7, +14 days vs. maximum**
**Apply color transformations from stars to galaxies and SNe and look at residuals vs. redshift and color**
**Aim for 1% photometric errors as acceptable for the filter contribution to the total 2% error budget**

- Revised Filter Transmission
- Uniformity Analysis, with Color Terms

## Stars

***Stars***

## Stars

***Stars***

## Stars

***Stars***

## Galaxies

***Galaxies***

## Type Ia Supernovae

***Type Ia Supernovae***

## Bad

***Bad***

## Good

***Good***

## Ok

***Ok***

## Slide 23

## Tilt by gradients of up to  10% center to edge

***Tilt by gradients of up to ************ 10% center to edge***

***Shift cut-on and cut-off by up to     ************ 50 Å***

***Original g-band filter in black***

## Galaxies

***Galaxies***
********* ******10% gradient filters***

## Galaxies

***Galaxies***
********* ******5% gradient filters***

## Gradients from filter center to edge

***Gradients from filter center to edge***

***Red points indicate <1% flux errors for galaxies***

## Red points indicate <1% flux errors for galaxies in i and z

***Red points indicate <1% flux errors for galaxies in i and z***

***Red indicates <1% flux errors for Y***

***Blue indicates <2% flux errors for Y***

***Gradients from filter center to edge***

## FWHM vs. central wavelengths

***FWHM vs. central wavelengths***

***Red points indicate <1% flux errors for galaxies***

## Cut-on vs. cut-off wavelengths (half-maximum points)

***Cut-on vs. cut-off wavelengths (half-maximum points)***

***Red points indicate <1% flux errors for galaxies***

## Cut-on vs. cut-off wavelengths (half-maximum points)

***Cut-on vs. cut-off wavelengths (half-maximum points)***

***Red points indicate <1% flux errors for galaxies in i and z***

***Red indicates <1% flux errors for Y***

***Blue indicates <2% flux errors for Y***

## Slide 32

## Slide 33

## Atmospheric

- absorption

- Y4KCam z

- Transmission

- Wavelength (Angstroms)

- DES z , Z , Zs

- (Gross exaggeration)

- Y4K z     = 0.0025
- DES z     = 0.0031
- DES Z   = 0.0036
- DES Zs  = 0.0011
- DES Y   = 0.0009

*Slide from Douglas Tucker*

## Slide 35

## Galaxies

***Galaxies***
***Color term residuals for 2-50%  less transmission in the H******2******O band***

## Slide 37

## Slide 38

## z/Z, Y Filter Summary

**Recommend wider z (8500-10000 Å) over Z (8500-9200 Å)**
**Using z improves S/N about 20-50% vs. Z, for redshift 1-1.5 elliptical galaxy**
**Impact of water band variability**
**Residual photometric zeropoint fluctuations larger for z vs. Z, but estimated to be only ~0.3% from analysis of CTIO 1m data;  is also taken out using relative photometry calibration**
**Color-term residuals are also small, even for exaggerated water band variability **
**Y filter bandpass **
**Recommend 9700-10200 Å to cut off before CCD QE drops below ~20%**
**Extending red cutoff to 10300 Å (e.g., better matches LSST y) only gives S/N gain < 10% for z > 1 ellipticals**
**Recommend less stringent Y filter requirements giving 2% flux errors (instead of 1%), to make filter easier to manufacture**

## Conclusions

**Keep same absolute transmission envelopes**
**Relaxed transmission uniformity specifications possible, by using color terms based on stars, and applying to galaxies and SNe Ia**
**About 2-3 times less stringent than original ~3% uniformity specification when color terms not allowed**
**Uniformity specifications to keep filter error contribution to < 1% (< 2% for Y)**
**Filter****	****Center-to-Edge Gradient****	****	****Cut-On****	****Cut-Off**
**g****	****	****	**** ******** 5%****	**** ****	****	****	**** ******** 30 Å ****	**** ******** 15 Å **
**r****	****	****	**** ******** 7%****	**** ****	****	****	**** ******** 30 Å ****	**** ******** 20 Å**
**i****	****	****	**** ******** 5%****	**** ****	****	****	**** ******** 25 Å ****	**** ******** 20 Å**
**z****	****	****	**** ******** 9%****	**** ****	****	****	**** ******** 20 Å****	**** ******** 25 Å**
**Y****	****	****	**** ******** 6%****	**** ****	****	****	**** ******** 20 Å ****	**** ******** 20 Å**
**z and Y bandpasses as described on previous slide**

## Extra Slides

## DECam Filter Wavelengths 

**TO.15**** **** Filter transmission requirements:  > 85% in [*****g, r, i, z*****]**
**Table 3  Filter Transmission Requirements**

- DECam Filter Wavelengths
- and Transmission Requirementsfrom DECam Technical Specifications (document #806)

| filter  | CWL (nm)  | FWHM(nm)  | Transmission |
| --- | --- | --- | --- |
| g  | 475  | 150  | 85% |
| r  | 635  | 150  | 85% |
| i  | 775  | 150  | 85% |
| z  | 925  | 150  | 85% |

## Slide 43

## DECam Filter Specificationsfrom DECam Technical Specifications (document #806) and DECam Filter RFI (document #1045)

**Substrate dimensions:** 620 mm diameter
**Substrate material:** fused silica
**Minimum clear aperture:** 610 mm diameter, concentric with substrate diameter
**Total thickness:** 13 mm
**Surface parallelism:** 30 arc-seconds or better
**Bubble class: **0 (req. 0.1 mm2 per 100 cm3; goal 0.03 mm2 per 100 cm3)
**Maximum bubble size: **0.5 mm diameter
**Striae:** requirement of 30 nm, goal of 20 nm
**Pinhole limits: **None with D > 60 microns; no more than one of the size 30-60 microns
- 	over an aperture of 50 mm, nor more than a combined equivalent area of
- 	30 microns per 10 mm region of an aperture.
**Surface quality**: 80-50 scratch-dig
**Durability:** Mil-C-48479 (moderate abrasion)
**Surface quality**: Filter edges are to be sealed to prevent moisture incursions into
- 	the substrate
**Radioisotope limits in filter substrate**: U<0.8 ppm, Th<2.5ppm, K<0.03% (by weight)
**Radioisotope limits in filter coatings**: U<80 ppm, Th<250ppm, K<3% (by weight)

## DECam Filter Specifications (cont’d)from DECam Technical Specifications (document #806) and DECam Filter RFI (document #1045)

**Operating focal ratio:** f/2.9
**Optical thickness:** All filters shall have the same optical thickness, ±0.10 mm air equivalent.
**Transmitted wave error in 125 mm diameter sub-aperture:** < λ/4
**Beam Angles of Incidence: **0 – 4 degrees for f/2.9 beams (maximum range 0-12 deg)
**Coatings:** All air-glass interfaces are to be anti-reflection (AR) coated.
- 	Predicted performance of the AR coatings should be provided with the quotation.
- 	Filter substrate edges to be sealed against moisture penetration.
**Transmission:** Specified by absolute transmission envelopes vs. wavelength,
- 	and spatial uniformity specified by “fraction envelopes” (see later this talk)
**Normal operating temperature:** -5C to 27C
**Survival temperature range: -**56C to 40C
**Normal operating humidity range**: 0-60% (dry nitrogen environment)
**Survival humidity range:**  0-100%
**Normal operating elevation: **2200 m (7220 ft)
**Normal operating pressure: **570-590 Torr
**Operational lifetime: **10 years

## Top Level Photometric Calibration Science Requirements (version 6.5 draft of document)

**S-16****  The magnitudes of an object may be calculated to within 2% by convolving the spectrum of the object with the system response curves. This requirement assumes that the spectra are spectrophotometrically calibrated and that the system response curves are absolute.**** **
***This is the total photometric calibration requirement***
**S-17 **** The magnitudes vary only by –2.5 log *****f*****2****/*****f*****1****, independent of position in the final map to within 2% (1% enhanced goal), where *****f*****2****/*****f*****1**** is the ratio of photon fluxes. This is to be true in *****g, r, i, z***** individually.**
***This is basically the relative photometric calibration requirement***
***We’ll focus on this***
**S-18****  The magnitudes have an absolute zero point that is well-defined and known to 0.5%. The magnitudes will be on the natural instrument system.**
***This is basically the absolute photometric calibration requirement***

## Proposed Relative Photometric Calibration 

**S-19  ****Uncorrected nonlinearities** due to imperfect shutter timing and nonlinear CCD/amplifier gain shall be less than 0.3%, measured as the peak error between shortest and longest exposure times, and between the faintest and brightest unsaturated stars.
**S-20  **The **aperture correction** shall have an internal rms error no bigger than 0.6% for any CCD and seeing between 0.8” and 1.5”.
**S-21 **The rms photometric errors due to **imperfect flatfielding** (including errors in removing the ghost image of the night sky and removing other stray light sources) will be no worse than 0.84%.** **
**S-22  **The rms photometric variations due to spatial changes in the **shape of the system optical transmission** (telescope, corrector lenses and coatings, and filters) will be no worse than 0.84%.
**S-23  **The rms photometric variations due to spatial changes in the **shape of the CCD QE vs. wavelength** curve will be no worse than 0.84%.
**S-24  **The rms photometric errors due to imperfect removal, using the global relative photometric calibration solution, of temporal and spatial changes in the **atmospheric transparency and extinction**, will be no worse than 0.84%.
**S-25  **The rms photometric errors due to imperfect corrections for **astrometric and other distortions** on the focal plane (including those due to the optical design and to the CCD “glowing edges”) will be no worse than 0.84%.
**Kept 1st two requirements at 0.3% and 0.6%**
**Leaves remaining 1.88% for last 5 terms, divide by sqrt(5) to get 0.84% per requirement**

- Proposed Relative Photometric Calibration
- Science Requirements

## SNe Ia

***SNe Ia***
********* ******10% gradient filters***

## SNe Ia

***SNe Ia***
********* ******5% gradient filters***