Topic 8: Calibrating your system Arne Henden Director, AAVSO

Steps Inspect for cosmetic quality Dark current measurements Gain and readnoise Linearity Scattered light Residual Bulk Image Best to do in warmth of house!

Cosmetic quality Don’t trust spec sheet, look for yourself Bias/dark will tell you about low-level traps/blocked columns Flats will tell you about weak column, point defects

ND5 bias

ND5 flat

Typical Flatfield APASS U16m ASA N8 astrograph 25% vignetting Corners outside corrected image circle Cosmetically beautiful

Dark Current Primarily to look for particularly hot pixels You CAN create a bad pixel map, interpolate across them for sky backgrounds Some hot pixels can be particularly bad (LED defect) Often can improve with lower temp

APASS-South LED camera defect Grew over several nights Required replacement of sensor (under warranty) Kodak took 4 months to deliver sensor to Apogee

Gain and Readnoise 1 Use Janesick’s method Take two light-box flats Form mean_flat1, mean_flat2 (average of central pixels) Take two bias frames Form mean_bias1, mean_bias2 Form flatdif = flat1 – flat2, sigma_flatdif Form biasdif = bias1 – bias2, sigma_biasdif

Gain and Readnoise 2 Gain = [(mean_flat1 + mean_flat2) – (mean_bias1 + mean_bias2)] / [sigma_flatdif**2 – sigma_biasdif**2] Readnoise = gain * sigma_biasdif/sqrt(2) Gain in e-/adu, readnoise e- Iraf obsutil/findgain Example: sonoita

Linearity and full well Can be done with lightbox, or with LED blinker Find exposure that does not saturate sensor; perhaps 30sec Take ramp of exposures from 0sec to 30sec Plot mean of central region

SRO STL-1001e linear to 65K ADU (actually doesn’t sample full well)

APASS KAF16803 linear up to 65K ADU; only samples approx ½ full well

SXV-H9

Nofs tek1k

Scattered Light Need to do on-telescope First, look through telescope WITHOUT camera – can you see sky, reflections? Next, try examining what the camera sees

Pinhole Camera Read Grundahl & Sorensen A&A Sup Ser 116, 367, 1996 Edmund pinholes 20-50micron Smaller pinholes = better resolution Greater distance twixt pinhole and sensor = bigger magnification

Pinhole images

Pre/post baffling to remove scattered light

Pinhole arrangement

Another pinhole image

temp

Raster scans Photometric (clear) night Take 27 images: one with bright star centered, 25 in a 5x5 pattern, last one with bright star centered Compare photometry at each position wrt. Center Example: Sonoita

Raster scan with problems

Raster scan with no flatfielding

Raster scan after problem fixed

Residual Bulk Image Common with front-illuminated sensors Particularly bad with KAF Worse for red light (watch out for autofocus programs!) Examples from R. D. Crisp

M19 300sec Z

Field after M19

temp

Photon mean free path

temp

RBI retention

Traps last a long time

RBI elimination

RBI flush results

Shutter errors Two types – non-photometric shutters (like blade), and exposure timing errors Photometric shutters usually remove shutter vignetting (SBIG is an example)

Linear/curtain photometric shutter

ST-402 shutter using the filter as shutter

SBIG 8300 shutter

Blade shutters

Shutter error elimination Take long-exposure flat Ratio with short-exposure flat to create shutter flat After regular flat-fielding, divide by shutter flat to further correct illumination