Home Contacts People Research Education Institute Resources Popular Library Webmaster

The telescope cannot reach any altitude in the sky. The dome slit puts a lower limit because its lower edge is higher than the lowest part of the tilted main mirror. The telescope also have two physical limit stops which will cut power. However, before that the control software will stop the telescope. The reason for the lower limit is main mirror and telescope truss protection, and the upper limit is field rotation stability. The table show the different limits depending on equipment used. "Physical" refers to the software limit.

The telescope is focused by pushing the secondary mirror towards or away from the telescope. The scale is directed towards the main mirror in steps of 0.1mm. The zero point can be changed in the software but the default is to have zero in the outermost position i.e. all focus values should be positive. The temperature affects the focus which value should be reduced with decreasing temperature. Different filters also affects the focus. However, the biggest differences depends on which tube sections are used. The available focus range in F10 is about 13cm. However, the telescope optics has a specific optimum within this range.

NO DATA PRESENT

Since the camera has a specific dynamical range a single exposure cannot be arbitrarily long. The saturation time is depending on the flux from the source, which filter is used, the background flux, CCD temperature, and the aperture. There is also a shortest exposure time of about 0.12s set by the camera. However, the camera has a shutter delay which increases the shortest exposure time depending on the level of accuracy.

The camera shutter is a rotating metal plate (propeller) which either blocks or passes the light to the CCD. The reaction/speed of this shutter is slightly uneven so that the time a pixel is exposed is not exactly the same as the exposure time, and true exposure length can be different depending on the location on the CCD. The recommendation in the table is based on a model with a constant difference between the true time and the exposure time in the image header. Although the error will always be there it is eventually overtaken by other uncertainty sources.

Although the CCD is generally linear i.e. the detection of light is directly proportional to the incoming flux it is not necessarily so in the CCD:s whole dynamical range.

Counts lower than the recomended will produce CCD noise larger than wanted accuracy. Counts higher than recomended will start to give nonlinear effects larger than ordinary CCD noise. Shifting the CCD temperature will shift the count rates by the order of 2% per 10°C.

Impurities in the CCD can change the pixel sensitivity when the pixel is exposed. The effect can last for a long time which e.g. can effect measurements of faint objects exposed in the same place as a previous brighter object.

NO DATA PRESENT

The factor that will control the exposure time length in the end is the saturation time for the background.

In addition to the photon noise in the light from the object and the background, the CCD have a number of sources which include variations in DARK, BIAS, and pixel sensitivity. These in turn depends on temperatures on the CCD and ambient, changes i the supplied voltage, and detected photons.

NO DATA PRESENT

If the camera is rotated 180° then the gradient in one direction would cancel the gradient in the opposite direction. A problem is that the total flux will also variate during this time and the gradient change. However, this difference nearly disappears if the images within each rotation are normalized prior to the combining. Hence the following recommendation which generally is better than ~1000ppm deviation between different directions.

1. Point the telescope to the opposite size of the sky from the Sun in AZ, and and ALT 40-70°.
2. Use the mosaic routine to take at least some 3x3=9 images (but not too many since time is short). The counts should preferably be some 10,000 ADU (but not near saturation)
3. Rotate the camera 180° in the angle relative to the telescope.
4. Repeat in case of several filters. Generally take U and B band when the sky is bright followed by I, V, R and Open when it is darker.
5. Remove BIAS/DARK from all frames.
6. Normalize all flat images so that the image average is 1.
7. Median combine the images for the two rotation sets into two master images.
8. Normalize the two mater images and combine them into one mean image. This is the flat to use.

NO DATA PRESENT

An almanac can be found in the ICC:s start menue (lower left corner). Look under Programs->Observing->Documents->Almanacs.