The Altair Astro 0.5x focal reducer reduces the focal length of your telescope or guide scope by 50%, in most applications.
 
The amount of focal reduction can vary slightly with spacing from the camera sensor. By moving the focal reducer inwards and outwards from the sensor you can get a variation of about 40% - 80% reduction.
 
By reducing the F-Ratio of your telescope to half it's normal value, you can drastically reduce the exposure time of your autoguider, or astro video camera such as the Altair GPCAM.
 
The larger field allows you to gather more light per frame and to fit objects onto the small sensors found in most astro video applications.
 
The 0.5x reducer has the following benefits:
 
1) Increase the number of guide stars available in your autoguider field.
2) Allows you to increase autoguiding frame rate.
3) Allows video astronomers to use a shorter exposure duration to pick up fainter objects.
4) Get full solar disc on your camera sensor.
5) Get a larger field when lunar or planetary imaging.
 
Attaching the Altair 0.5x focal reducer: The Altair 0.5x reducer has a standard 1.25" filter thread both sides, and can be stacked with filters. It can be screwed into most 1.25" camera nosepieces or eyepieces directly, and because the barrel is 1.25" outer diameter, it can be inserted with the nosepiece into any standard telescope pushfit focuser with a 1.25" aperture.
 
Optical compatibility: Works with any telescope. This reducer doesn't correct for off-axis field aberrations which are common to all optical designs and it can make them appear more apparent at the field-edge, so it isn't recommended for flat field CCD imaging with large sensors, or use with wide field eyepieces. Coma may appear to be increased on fast Newtonians with a focal length shorter than F6 because of the wider field, as will other completely normal off-axis aberrations. However, this isn't an issue with autoguiders or planetary webcams because of the small sensor size - in fact aberrations are rarely detectable. Astro-video cameras are barely affected because the target of interest is in the centre of the field and on-axis sharpness is very good with this high quality little lens system so it's a good choice for video astronomy. Solar images remain very sharp and in focus so this focal reducer is ideal for solar imaging to get the full solar disc on your sensor (especially with Daystar Quark) or to remove "Newton's Rings" artefacts.

Spacing data: (credit to Mike Wood for extensive testing with F10 scope)
 
Distance from sensor (mm)    focal reduction factor    F10 scope
38                                                    0.6                         f4.0
40                                                    0.58                       f4.2
47                                                    0.5                         f5.0
54                                                    0.43                       f5.7
60                                                    0.37                       f6.3
67                                                    0.3                         f7.0
73                                                    0.23                       f7.7