Focal reducers

Home Forums Spectroscopy Focal reducers for Alpy! Focal reducers

David Boyd


My comment to Steve was in relation to his f/7 apo refractor. For an f/10 SCT, I think you will definitely need a focal reducer with the Alpy.

The issue of focal length and positioning of various makes of focal reducers is a bit of a minefield. I have seen several figures quoted for the focal length of the Meade 0.63x focal reducer and even that there are two versions on the market, one made in Japan and one in China with different focal lengths. I have no idea if that is true. In my experience, the best approach is to believe nothing and measure it yourself. To get a reasonably accurate value, put the focal reducer at the back of any scope, point it at a distant object on a bright day and measure the distance from the position of the lens in the focal reducer to the point where the image is sharp. Call this F2. Do the same without the focal reducer and measure the distance from where the lens was in the focal reducer was to the new focus. Call this F1. It will be larger than F2. The focal length of the focal reducer is then FL=(F1*F2)/(F1-F2). If you don’t get a sharp image without the focal reducer you may need to rack in the focuser a bit and try again.

The focal reduction factor RF you have just measured is F2/F1. It will vary as you rack the focal reducer in and out. Every focal reducer is designed to operate optimally at the position where the reduction factor RF is the quoted value, 0.63 in the case of Meade. If the measured focal length is FL, the optimum distance from the lens in the focal reducer to the image plane, F2, in this case the slit of the Alpy, is given by F2=FL*(1-RF) = 0.37*FL. You may need to insert spacers to get as close as possible to this optimum spacing.

As a general comment, you want to keep the Alpy as close to the scope as possibly to reduce the possibility of flexure so go with the focal reducer with the shortest focal length provided there is enough space between the focal reducer lens and the Alpy slit to achieve the optimum spacing. Needless to say you should also use a high quality achromatic focal reducer in a situation like this.

Hope this makes sense. It is based on my experience and has worked for me.