I’ve very recently obtained an Antares 0.5 FR1 0.5X Focal Reducer. After some testing I’ve discovered it works quite well for reducing the focal length and lowering the focal ratio of my 90mm Maksutov. I have found that threading the reducer directly onto an eyepiece works well. I could not obtain focus by threading the reducer onto the diagonal nosepiece. I mainly intend to use it on a 102mm Sky-Watcher Skymax to improve its field of view and quicken its rather slow f/12.7.



However, I have a question. Does anyone know the exact amount this type of reducer actually shortens the focal length of the telescope? I’ve seen claims for cutting it by half (50%/0.5?) or reducing it by only 40%. As I couldn’t achieve focus with this particular reducer by placing it in the diagonal it will be used on any eyepieces employed by directly threading it onto the eyepiece draw tube. I’m not entirely sure if this is a factor in calculating the exact focal reduction or not.



All I have to go with are conflicting statements from sites advertising this reducer. This is from the site I purchased it from and is probably ‘blurb’ from Antares' own marketing department.

Very Useful Reducer. Can Be Used With Any Telescope

I won’t argue with its usefulness or efficacy, but it won’t let me achieve proper focus on a 3“ f/4 Newtonian reflector. However, it does work very well on my 90mm, f/11.1 Maksutov.

This superb 0.5x focal reducer has been designed to be used with a wide range of modern CCD cameras and eyepieces. The lens has a female 1.25" eyepiece filter thread and can be attached to camera and eyepiece nose barrels to achieve a 40% increase in field of view. Basically the lens has the effect of reducing the focal length of the telescope by 40%.

This is the bit I’m having difficulty with. An FOV increase of 40% is highly likely, but where does that figure come from? I only plan on using the reducer for visual. If the reducer is threaded into an eyepiece placed into a diagonal does it still cause a 40% reduction in focal length?

This super quality dual element lens system features fully anti-reflection coating on all air-glass surfaces for increased contrast.

It certainly looks good quality!

The exact degree of focal reduction achieved with any given set-up depends somewhat on the precise position of the FR relative to the imaging chip in the case of CCD cameras and the optics when used with eyepieces. Thus the judicious use of extension tubes and carefully positioned filters can squeeze a great deal of versatility out of these excellent quality observing and imaging aids.

Hmmm ... this just makes it even more confusing. Is threading it directly onto the eyepiece a default position? Can the modified OTA focal length actually vary in this position with individual eyepiece type and make? Will I stop asking these questions lol?

Great for visual use and an indispensable CCD imaging aid!

I think it could be great for visual use for me as it will give my 102mm Mak much more flexibility to view certain deep sky objects.



The big question for me is will this 40mm Plossl give 16.25x or 19.5x on my 1300 f/l Skymax with the reducer threaded directly into the eyepiece (as above)?

Does anybody know?