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 8" F4 Imaging Newtonian, as well as Synta Skywatcher, Orion USA and GSO Newtonians.
(You can order the coma corrector with the Altair Astro 8" F4 Imaging Newtonian for a significantly reduced price!)
Features of the Altair Astro Coma Corrector:
Perfect accessory for Altair Astro 8" F4 Imaging Newtonians!
Can be used in Photo or Visual mode.
2" Push-fit adapter has two clamp screws to reduce flex in the imaging train.
M48 threaded barrel to attach your Nikon / Canon DSLR adaptor, or T-Thread adaptor directly. (Gives less Vignetting compared to M42 type T-Thread with 6mm extra diameter).
Removable 2" non-marring compression ring fit with two thumbscrews.
- 75mm optimal back-focus requirement.
- 4 Lens elements in 2 optical groups.
All optical surfaces are fully multicoated to improve contrast.
Very robust and nicely machined unit with a high quality finish.
General use: The coma corrector has a standard 2" push-fit barrel for use with eyepieces. It can also be broken down into "photo mode" by unscrewing the upper push-fit section to reveal a M48 thread, which allows you to attach your Canon / Nikon DSLR M48 adapter and various M48 format extension tubes (available separately in the drop-down menu below). The extension tubes can be used to ensure that the distance between the rear lens cell and the CCD plane is about 75mm which is the optimum distance for best performance. In the images opposite, the Altair Astro Photo-Visual Coma Corrector and Field Flattener is firstly shown assembled in "Visual mode" with 2" push fit adapter attached, and below that in "Photo mode" with the 2" push-fit adapter removed. Finally, it's shown with a Canon EOS M48 to DSLR adapter in place.
Q: 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 particularly 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 reduction when using the coma corrector (a huge reduction!), and will exhibit a flatter field too, due to additional lens elements designed to cancel out field curvature which is another optical effect which most telescopes also exhibit.
Q: How do I get the best optical performance? The optimum distance from the rear lens of the coma corrector to the imaging focal plane is approximately 75mm, which can be achieved using our fixed and variable extension tubes. A tolerance of 2-3mm can be allowed without significantly affecting visual or photographic performance.
Q: How do I get the correct 75mm spacing with my camera? Remember you can check the distance on your camera by looking for the focal plane marking (a flat incised line often found on the camera body), or in the instructions if you have an astro CCD camera. Most DSLR cameras have a 44mm (Canon EOS) to 46.5mm (Nikon F-Mount) back-focus distance from the rear lens element to the film plane. If you have a Nikon F-Mount or Canon EOS type DSLR, then you need our 17-23mm Variable M48 Extension Tube which is linked-to at the bottom of the page. This lets you adjust the spacing for optimum results when used with an M48 to DSLR adapter which is also available separately, or in the drop-down menu below. This works out because our M48 to DSLR adapter adds 10mm of distance, so for Canon EOS, adjust the variable adapter to 18.5mm and for Nikon F-Mount, adjust it to 21mm. Use a ruler to judge the distance to the film plane - most cameras have an incised line on the casing to make this easier.
Q: Why should I buy the variable adapter and the fixed 10mm extension tube? Using the variable adapter in conjunction with the 10mm allows you more flexibility to use other CCD cameras with 10mm less back-focus than a DSLR.
Q: I already have a T-Thread to DSLR adapter, can I use it?
Yes you can with an M48 to T-Thread adapter, but the extra adapter will probably cost the same as an M48 to DSLR adapter so we don't see the point. Our M48 to DSLR adapter adds 10mm, so that means you can use the 17-23mm variable adapter for Canon EOS (adjust to 18.5mm) or Nikon F-Mount (adjust to 21mm). If you add another adapter our variable adapter might not bring things to the correct distance. Also we like to keep the aperture as wide as possible, in order to minimise any change of "Vignetting" or light fall-off at the edge of the field. Vignetting isn't going to ruin your photos by the way, and it's common in all optical systems, but minimising it does improve things. Converting to M48 can also be useful anyway, because it's the same thread standard used for all 2" astro-filters...!
Q: I have a CCD camera which has a T-Thread nosepiece, how do I connect it up? The principles are the same as for DSLRs, but you will need to introduce an M48 to T-Thread adapter at some point in the optical train, and we suggest doing this as far back as possible. In tems of spacing we therefore suggest that you use either the 17-23mm Variable M48 Extension Tube or the 10mm M48 Extension Tube or if you have similar back-focus to a DSLR then both stacked together to give you ranges of either 17-23mm OR 27-33mm. If you need more distance then you could add another extension tube - best to contact us with your exact back-focus requirement, and we'll help.
Q: What other effects does the Altair Astro Coma Corrector have on the telescope optics? The coma corrector increases the focal ratio of the telescope by a negligible 10%. In practise this is undetectable visually, and irrelevant photographically. Furthermore, this design allows for better illumination, and field flatness.