Topic 8: Calibrating your system

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-1001elinear to 65K ADU(actually doesn’t sample full well)

13. APASS KAF16803linear 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