Deep wide field imaging

5. Wide field science

6. Lab demo

NearIR performance

Appendix A: Fundamental limits

Appendix B: Design parameters

Appendix C: Source Code

Robotic DSLR imaging system

Astronomical Optics course

Site map

Show content only (no menu, header)

The images acquired to perform the astrometric measurement are also suitable for a wide range of astrophysical investigations thanks to the combination of a large field of view, high image quality and very good PSF stability.

5.1 Loss of sensitivity due to the dots

The effect of the dots on the telescope primary mirror on general-purpose wide field imaging is evaluated in this section. The nominal design previously detailed is adopted here (Telescope diameter, D = 1.4m observing a m_V = 3.7 star. Dots covering 1% of the primary mirror area).

Light lost to the dots

The light lost due to the dots is the sum of the light directly absorbed by the dots, and the light they diffract out of the PSF core into a wide halo (this light is diffracted out to large distances, and is considered lost for science). Both quantities are equal to the fractional area of the pupil occupied by the dots. In the nominal design chosen, the total photometric loss is therefore 1%+1% = 2%. This loss is small compared to the sensitivity gain offered by adopting an unobstructed pupil, as the photometric sensitivity loss due to spiders and central obstruction in an on-axis telescope design would be larger than 2%.

Additional background due to light of the central star diffracted by the dots

The central star's diffraction spikes extend over most of the wide field image. Although their contrast relative to the central star is faint (approximately 1e-7 to 1e-8 along the spikes), the central star is much brighter than other sources in the field.

Photon noise from the additional diffracted light

percentile values:
1  percent      (->vp01)     8.289936886285431683e-06
5  percent      (->vp05)     1.078251261787954718e-05
10 percent      (->vp10)     1.285495000047376379e-05
20 percent      (->vp20)     1.676283682172652334e-05
50 percent      (->vp50)     3.233937604818493128e-05
80 percent      (->vp80)     9.268130088457837701e-05
90 percent      (->vp90)     3.057720605283975601e-04
95 percent      (->vp95)     1.185552333481609821e-03
99 percent      (->vp99)     4.347435384988784790e-02

Over 50% of the field, the additional diffracted light is less than 26 ph per day per pixel. The table below shows how, for a mV=3.7 central star, the light from the central star compares with the zodiacal light background. The 50 percentile line shows that the median diffracted light surface brightness is 0.03% of the zodiacal light brightness.

percentile values:
1  percent      (->vp01)     7.657130481675267220e-05
5  percent      (->vp05)     9.959437011275440454e-05
10 percent      (->vp10)     1.187367670354433358e-04
20 percent      (->vp20)     1.548325817566365004e-04
50 percent      (->vp50)     2.987077459692955017e-04
80 percent      (->vp80)     8.560654241591691971e-04
90 percent      (->vp90)     2.824311610311269760e-03
95 percent      (->vp95)     1.095054019242525101e-02
99 percent      (->vp99)     4.015577137470245361e-01

Over 95% of the field, the additional light introduced by the dots on the primary mirror is less than 1% of the zodiacal background.

Residual errors after subtraction of the spikes