In the scramble for wider, richer fields, the focal ratios of Newtonians have been evolving downwards. It is not unusual now to be able to purchase fast telescopes well below a previous ‘norm’ of about f/6; off-the-shelf instruments can be obtained with f-ratios as low as f/5 and lower. However these f-ratios can come with inherent characteristics that must be dealt with, particularly regarding imaging requirements.
This unusual lens from ASA (Astro Systeme Austria) makes rich-field telescopes even richer, whilst correcting aberrations. Thus, a wide field telescope is able to grab even more ‘space’ in a single frame. With a reduction ratio of between .75 to .73, the examples quoted are that an f/4.7 becomes f/3.4 and an f/4 is corrected to f/2.9. By interposition of this lens, owners of an already abbreviated f/3 can expect an f/2.2 as a result. As imagers will already know, these reductions mean wider real fields and shorter exposures (or deeper shots with the same exposure).
Allowing comfortable chip coverage of cameras employing Kodak’s 8300 sensors, it can be persuaded to flood almost all of the APS-C (except for the corners). The device fits a two-inch focuser, offering a rear T-thread fitting.
For more information, contact Modern Astronomy www.modernastronomy.com