Sony SXV-M5C user manual Taking and using a flat field

Handbook for SXV-M5C Issue 1 August 2004

read-out noise will become dominant. About 30 seconds is a reasonable minimum. Now go to the ‘Multiple Exposure Options’ and selec t a number of exposures to take. You can also select to average the images, rather than adding them, and there is a ‘Alternative Slew Mode’ available, which uses the c orrelation of image areas, rather than a single star. This mode can be better in dense star fields.

Another option is ‘Auto remove dark frame’. This is advisable with S&S images, as the slewing will mis-register the images with a single dark frame that is applied to the finished sequence. To use this option, you will need a dark frame, taken with the same exposure time as a single image from the sequence. This is stored on drive C with the name ‘dark.def’

Now click on ‘Take Photo’ and the sequence will beg in.

Taking and using a flat field:

Flat fields are images, which display only the variations of illumination and sensitivity of the CCD and are used to mathematically modify a wanted image in such a way that the errors are removed. Common flat field errors are due to dust motes on the camera window and vignetting effects in the optical system of the telescope. Dust motes act as ‘inverse pinholes’ and cast out-of-foc us images of the telescope aperture onto the CCD chip, where they appear as shadow ‘do- nuts’. Most optical systems show some vignetting at the edges of the field, especially when focal reducers are used. This causes a brighter centre to show in images, especially when there is a lot of sky light to illuminate the field.

If dust motes are your main problem, it is best to clean the camera window, rather than to rely on a flat field to remove the do-nuts. Flat fields always increase the noise in an image and so physical dust removal is the best option. If you have serious vignetting, first check whether the optical system can be improved. The most likely cause of this problem is trying to use too powerful a degree of optical compression with a focal reducer and you might want to try moving the camera closer to the reducer lens.

If you really do need to use a flat field for image correction, then it must be taken with care. It is most important that the optical system MUST NOT be disturbed between taking your original images and taking the flat field. Any relative changes of focus and rotation etc. will upset the match between flat field and image and the result will be poor correction of the errors. The other necessity for recording a good flat field is a source of very even illumination for the telescope field. This is surprisingly difficult to achieve and many designs of light source have appeared in the literature and on the Web. These usually consist of a large wooden box, containing several lamps and an internal coating of matt white paint, which is placed over the objective of the telescope to provide an evenly illuminated surface. These can work well, but I prefer a simpler method, as follows:

Most imaging sessions begin or end in twilight and so the dusk or dawn sky can provide a distributed source of light for a flat field. However, using the sky directly is likely to result in recording many unwanted stars, or patches of cloud etc., so a diffuser needs to be added to the telescope. An ideal material is Mylar plastic draughting film, obtained from an office supplies warehouse. It is strong and water resistant and can be easily replaced if damaged. Stretch a piece of the film loosely

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