SXVR-H16 handbook - Starlight Xpress

Handbook for the SXVR-H16 Issue 1 June 2009telescope to provide an evenly illuminated surface. These can work well, but I prefer asimpler method, as follows:Most imaging sessions begin or end in twilight and so the dusk or dawn sky canprovide a distributed source of light for a flat field. However, using the sky directly islikely to result in recording many unwanted stars, or patches of cloud etc., so adiffuser needs to be added to the telescope. An ideal material is Mylar plasticdraughting film, obtained from an office supplies warehouse. It is strong and waterresistant and can be easily replaced if damaged. Stretch a piece of the film looselyacross the aperture of your telescope and point the instrument high in the sky, to avoidany gradient in the light near the horizon. Now take several images with exposuretimes adjusted to give a bright, but not overloaded, picture. Averaging flat fieldtogether is a good way to reduce their noise contribution and so recording 4, or more,images is a good idea.To use your flat fields, they must first have a dark frame subtracted. Although thismay appear to be unimportant with such brightly lit and short exposures, there is the‘bias offset’ of the camera in each image and this can produce an error in the finalcorrection. As we are mainly interested in the bias, any very short exposure darkframe will give a good result. The dark subtracted images should then be averagedtogether before use.After the above procedures have been executed, the flat field will be ready for use.Load up your image for processing, subtract the dark frame and then select ‘Applyflat field’ in the ‘Merge’ menu. The result should be an image with very little sign ofthe original artefacts.********************************************************************The accessory portsThe SXVR-H16 is provided with two ports for use with accessories. The Autoguideroutput port is a 6 way RJ11 socket, which is compatible with the standard autoguiderinput of most telescope mounts. It provides 4 active-low opto-isolator outputs and acommon return line, capable of sinking a minimum of 5mA per output. This socketmay be used for telescope control if the SXVR-H16 is employed as an autoguider, butis primarily intended to be the control output for the optional add-on autoguidercamera head, available for use with the SXVR-H16.18