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The Altair Astro Photo-Visual Coma Corrector will reduce coma in most fast Newtonians, as well as acting as a field flattener, and is optimised to work with the Altair Astro and GSO 8" F4 Imaging Newtonians.
You can order it with the Altair Astro 8" F4 Imaging Newtonian for a significantly reduced price!
The coma corrector is shown assembled in "Visual mode" with 2" push fit adapter attached, and in "Photo mode" with the 2" push-fit adapter removed.
Removing the upper push-fit adapter reveals a male M42 or T-Thread, which allows you to attach your Canon / Nikon DSLR adapter - so you can fit most CCD cameras out of the box while minimising Vignetting. Most T-Thread to camera adapters introduce about 10mm of spacing but additional spacing my be required for the optimum 75mm required from the rear Coma Corrector lens to your camera CCD plane. Remember you can check the distance on your camera by looking for the focal plane marking (usually a flat incised line usually found on the camera body), and most DSLR cameras have a 56mm backfocus from the body to the film plane.
What is coma and why do I need a Coma corrector? Coma is an optical effect whereby stars at the edge of the field of view in Newtonian telescopes become slightly elongated. It's particulary evident in Newtonian telescopes of fast focal ratios when used for astro imaging with a larger CCD Chip or DSLRs, or for visual use with a wide angle eyepiece. Coma is most apparent in telescopes with an F-Number of less than F6, and the Altair Astro coma corrector is designed to work best from F3 to F6. An F4 telescope will perform like an F8 one in terms of coma (a huge reduction!), and will exhibit a flatter field too, due to additional lens elements designed to cancel out field curvature.
Optical performance and correct spacing: The Altair Astro Coma Corrector increases the focal ratio of the telescope by a negligible 10%. In practise this is undetectable visually or photographically, and the design allows for better illumination and field flatness correction. The optimum distance from the rear lens of the coma corrector to the imaging focal plane is approximately 75mm, which can be achieved using various 3rd party female to male T-Thread extension tubes. A tolerance of 2-3mm can be allowed for when fine-focusing without significantly affecting visual or photographic performance. Here's a link to using an 8" Imaging Newtonian with a similar coma corrector.
Features of the Altair Astro Coma Corrector:
•Perfect for GSO / Altair Astro 8" F4 Imaging Newtonians!
•Can be used in Photo or Visual mode.
•M42 threaded barrel to attach your Nikon / Canon DSLR adaptor, or T-Thread adaptor directly.
•Removable 2" non-marring compression ring fit with two thumbscrews.
•75mm optimal backfocus requirement.
•4 Lens elements in 2 optical groups.
•All optical surfaces are fully multicoated to improve contrast.
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