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.

Filters

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.

Month	Star	RA	DEC	EXPTIME	B	V	R
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 http://www.cfht.hawaii.edu/ObsInfo/Standards/Landolt/

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: http://observatory.physics.ox.ac.uk/etc.php

Extinction

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: PWTlincor.cl

Problems

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