ST8 CCD Camera

The ST8 CCD Camera is the main instrument available on the telescope. It is usually used with the f/6.3 focal reducer/field flattener and gives a field of view of ~18x12 arcminutes with a pixel scale of 0.805"/pixel (measured). This is a good match to the median image quality of about 2--2.5-arcseconds, typically giving stellar FWHMs of ~2-3 pixels.


Current filters are;
Filter Focus Notes
Clear 0  
B -50 Custom Scientific
V 0 Custom Scientific
R 0 Custom Scientific
H-alpha -250 Custom Scientific 4.5nm
SII -800 Baader 7.5nm
OIII -800 Baader 7.5nm
H-beta -800 Baader 8nm
Grating-200 -950 200lpmm grating (slitless spectroscopy)
Grating -950 100lpmm grating (slitless spectroscopy)

Standard Star of the Month

Please observe one of these stars whenever you observe with the ST8 on the Wetton. The monthly star should be visible at the beginning of the night in the months named. Images can be taken in evening twilight, and guiding shouldn't be necessary for these. The observations will take ~3--5 minutes. Sequence files exist for these targets.
January/February SA94-242 02 57 21 +00 18 39 20s 12.03 11.73 11.55
February/March SA96-36 04 51 42 -00 10 09 20s 10.84 10.59 10.46
March/April SA98-653 06 52 05 -00 18 18 20s (TBC) 9.54 9.54 9.54
April/May SA100-162 08 53 15 -00 43 30 10s (20s in B?) 10.40 9.15 8.50
May/June SA102-1081 10 57 04 -00 13 12 20s 10.56 9.90 9.53
June/July SA104-461 12 43 06 -00 32 18 20s 10.19 9.71 9.42
July/August SA108-551 16 37 48 -00 33 05 20s 10.88 10.70 10.60
August/September SA111-773 19 37 16 +00 10 58 10s 9.17 8.96 8.84
September/October SA111-773 19 37 16 +00 10 58 10s 9.17 8.96 8.84
October/November SA112-275 20 42 36 +00 07 20 20s 11.12 9.91 9.26
November/December SA114-750 22 41 45 +01 12 36 20s 11.96 11.92 11.89
December/January SA92-342 00 55 10 +00 43 13 20s 12.05 11.61 11.34
A full list of Landolt stars, with finding charts, can be found at

Sensitivity and zero-points

Rough zeropoints (1ADU/s at zenith) are;
Band Zeropoint
B 19.2
V 19.6
R 19.8
Uncertainty is ~0.1 magnitudes.

There is an exposure time calculator available here:


20100901: Measure of extinction in clear filter of 0.35mag/airmass.

20110324: Extiction measured by a fit to B,V and R data on M3 taken over the range 1.35-1.05-1.35. Clear evolution of magnitude with time as well as airmass (increasing haze/humidity?), so fitted a plane to time vs airmass vs mag. Result is;
Band Extinction
B 0.67 mag/airmass
V 0.33 mag/airmass
R 0.32 mag/airmass

Dark Current

The camera is Peltier cooled to reduce dark current, and can typically reach temperatures ~30 degrees below ambient temperature (hence ~-15C in summer and -25C in winter). It is advisable to run the camera as cold as possible to reduce the effects of dark current (see attached figure) and detector cosmetics (significantly more bad pixels appear about -10C). Below -15C, dark current will not contribute toward the noise in the image.

Read-noise and Gain

Read-noise is 18.5e-. Gain is 2.65e-/ADU (measured 20101020). The gain is a function of flux level, and it is not clear how this will affect photometry.

Linearity & Good Data Range

The detector does display significant non-linearity of ~2% towards the detector full-well depth (60000 ADU). Linearity is better than 0.5% up to ~35000ADU.

Above 48000 counts, the detector begins to display correlated noise, and it is best to avoid going above this level with data. Initial tests suggest you can correct the linearity to better than 0.2% over the 0-48000ADU range by applying a 3rd order polynomial to the data, using the form;

ADU_Corrected = ADU_raw*1.00104 + -1.7324E-07*ADU_raw^2 + 7.4081E-12*ADU_raw^3

You can use the following IRAF procedure to do the correction:


Filter wheel not moving

2009/09: There is a problem with the filter wheel not moving, even though the software thinks it has changed position. The problem seems to be most common at relatively low altitude (<45 degrees), so is a big worry for dome flats and standard star observations. The filter wheel will be checked and cleaned to try and fix this problem. In the meantime, check the sky background after changing filter to see if it has changed.

2012/01: Seems to have been a problem with the filter wheel not coming back to exactly the same place, particularly in the V-band filter. Not clear what the problem is/was...

Detector icing over

If the detector is cooled too fast, and/or the dessicant needs recharging (see below), the ice can form on the detector, giving a mottled appearance ( example below). This should not affect the detector electronics, but makes observations impossible. To remove the ice, warm the detector up to ~0C (or higher if necessary) until the ice clears, and then slowly take the detector temperature back down again. You will have to go down in ~5C steps ever 10-20 minutes. Keep a check for ice forming again.


Recharging the ST8 dessicant plug

The desiccant in the ST8 camera needs to be recharged roughly once per year (usually done at the end of summer). To do this:
  • Unscrew the brass desiccant container from the rear of the camera and remove the O-ring.
  • Plug the resulting hole by screwing in blanking plug (in the equipment cupboard in the dome). Finger tight is adequate. Don't put a wrench on it.
  • Heat the dessicant container in an oven at 350°F (175 deg C) for 4 hours. The solder used to seal the can melts at 460 degrees F, so be sure to stay at least 50F degrees below this number.
  • Replace the desiccant container into the rear of the camera, being careful to reinstall the O-ring and insure that it does not get pinched.
  • Expect the camera to take an hour or two to reach the frost free state.

-- FraserClarke - 07 Dec 2016
I Attachment Action Size Date Who Comment
PWTlincor.clcl manage 1 K 31 Jan 2011 - 11:26 FraserClarke IRAF linearity correction routine
SA100-162.JPGJPG SA100-162.JPG manage 135 K 21 Feb 2012 - 14:56 FraserClarke SA100-162 finder
SA102-1081.JPGJPG SA102-1081.JPG manage 121 K 21 Feb 2012 - 14:57 FraserClarke SA102-1081 finder
SA104-461.JPGJPG SA104-461.JPG manage 124 K 21 Feb 2012 - 14:57 FraserClarke SA104-461 finder
SA108-551.JPGJPG SA108-551.JPG manage 106 K 21 Feb 2012 - 14:57 FraserClarke SA108-551 finder
SA111-773.JPGJPG SA111-773.JPG manage 144 K 21 Feb 2012 - 14:58 FraserClarke SA111-773 finder
SA112-275.JPGJPG SA112-275.JPG manage 94 K 21 Feb 2012 - 14:58 FraserClarke SA112-275 finder
SA114-750.JPGJPG SA114-750.JPG manage 130 K 21 Feb 2012 - 14:58 FraserClarke SA114-750 finder
SA92-342.JPGJPG SA92-342.JPG manage 118 K 21 Feb 2012 - 14:55 FraserClarke SA92-342 finder
SA94-242.JPGJPG SA94-242.JPG manage 160 K 21 Feb 2012 - 14:29 FraserClarke SA94-242 finder
SA96-36.JPGJPG SA96-36.JPG manage 118 K 21 Feb 2012 - 14:56 FraserClarke SA96-36 finder
SA98-653.JPGJPG SA98-653.JPG manage 143 K 21 Feb 2012 - 14:56 FraserClarke SA98-653 finder
ST8_icing.jpgjpg ST8_icing.jpg manage 175 K 01 Oct 2010 - 00:05 FraserClarke Example of icing on the detector
st8_darkcurrent.pdfpdf st8_darkcurrent.pdf manage 13 K 25 Aug 2010 - 18:17 FraserClarke  
Topic revision: r29 - 07 Dec 2016, FraserClarke
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