1. Topic 8: Calibrating your system Arne Henden Director, AAVSO arne@aavso.org

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

3. 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

4. ND5 bias

5. ND5 flat

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

7. 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

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

9. 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

10. 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 060220

11. 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

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

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

14. SXV-H9

15. Nofs tek1k

16. 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

17. 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

18. Pinhole images

19. Pre/post baffling to remove scattered light

20. Pinhole arrangement

21. Another pinhole image

22. temp

23. 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 060531

24. Raster scan with problems

25. Raster scan with no flatfielding

26. Raster scan after problem fixed

27. Residual Bulk Image Common with front-illuminated sensors Particularly bad with KAF-09000 Worse for red light (watch out for autofocus programs!) Examples from R. D. Crisp

28. M19 300sec Z

29. Field after M19

30. temp

31. Photon mean free path

32. temp

33. RBI retention

34. Traps last a long time

35. RBI elimination

36. RBI flush results

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

38. Linear/curtain photometric shutter

39. ST-402 shutter using the filter as shutter

40. SBIG 8300 shutter

41. Blade shutters

44. 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