[LVAS] Turned Down Edge

[Fred Rayworth]

All,

 

I was asked a question about turned down edge, and decided to answer it here.

 

Some of you may have heard that many if not most commercial mirrors have at least a slightly turned down edge. What does this mean? It means that not all of the light cone is going to come together at one spot. That wonky edge is going to throw some of the light away from the focal point, fuzzing the overall image just a bit. That affects contrast and focus. A Ronchi grating is a great way to see turned edge. One way or the other, a solution given many times is to mask the edge of the mirror in by 1/4" to 1/2". It can be done with flat black paint, press-on sticky paper, whatever.

 

What does this do? It decreases the size of your mirror by 1/2" to 1". But it also eliminates all that scattered light that the bad edge is throwing everywhere. There is also another bonus. It effectively increases the focal ratio. The longer the focal ratio, the sharper the images become and the less coma you have to deal with.

 

For instance, your mirror is a 10" f/4 and you mask 1/2" of the edge, effectively making it a 9". Your 40" focal length will give the 9" mirror a focal ratio of f/4.44. That slight increase in focal ratio takes away a bit of coma and thus slightly flattens and sharpens the image.

 

Keep in mind that this will not work if your mirror is astigmatic, has severe spherical abberation, is pinched, etc. However, if a turned edge is your only problem, this could turn your mediocre scope into a slightly smaller excellent scope.

 

Another feature of increasing the focal ratio is that if you DO have spherical abberation, that little stretch will make the abberation less severe. The longer the focal ratio, the more tolerant the center of the mirror is to error, up to a point. 

 

The larger the mirror, the more you can afford to cut out without making a significant difference in light gathering power. However, if you have a severely turned down edge, you could lose 2" to 3" of aperture just to get rid of it and then it is time to return the mirror and request a new one.

 

In the example above, you will likely see little difference between a 10" and 9" mirror except that it may be sharper. If you have a 16" mirror and have to mask it down to 14", them are fightin' words!

 

Fred

 
 		 	   		  
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