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Focus Reducer Spacing PDF Print E-mail

The information on this page page has been gathered from from many sources such as Russel Croman's white paper on proper placement of focal reducer for RC scopes. My purpose is to capture best practices when I find them, and publish them here. Personal web sites come and go, so I don’t want to link to it and not find it at some future date.

The relation between the reduction factor R, the focal length Fr of the reducer and the distance D between the lens and the focal plane is:

R = 1 - D/Fr

The Meade/Celestron F/6.3 reducer has a focal length of about 230 mm. Therefore, the nominal distance between the lens and the CCD focal plane is about D = FR*(1 - R) = 230*(1 - 0.63) = 85 mm. It is only at this distance, that this reducer gives its nominal reduction factor of 0.63x.  The design point of the reducer is matched the the type of scope and the focal ratio.  If the reducer does not have the right spacing, it changes the reduction factor and starts introducing coma distortion.

The real point being made hear is that there is a correct distance from the CCD focal plane to the center of the Focal Reducer. This is not dependent on focal length of the scope or the aperture of that scope.  It is specific to the focal distance of the focal reducer. 

SCT scopes can not use image scale calculators commonly found on the Internet because the imaging train is fixed to the back of the scope, and the primary mirror is moved to achieve focus.  The primary mirror moves to bring the scope into focus, thus the focal length of the scope actually changes when focusing the scope.  A 2000mm focal length scope may actually vary from 2000 to 2250mm focal length over the range of focus.  When the focal length changes, the point of focus behind the scope also changes. 

There are two very important considerations on the SCT scope, the distance between the focal reducer and the CCD chip and where the focal reducer is placed relative to the  back focus point of the scope. The first distance is fixed as calculated using the above formula.   The scond calculation says that the focal reducer is placed in front of the focal point of the scope by some measure based on the reduction factor of the reducer. On refractors this is no a big problem, because the whole equipment train changes in unison with the focusing process.

The SCT has unique issues in that the focal length changes when the primary mirror is moved to bring the image in focus. My experience based on measuring the image scale shows a reduction factor closer to 7 when the imaging train is fixed to 85mm on my Celestron C-8 and the focal reducer attached directly to the back of the scope.

The distance between the camera and the reducer needs to be adjusted until the proper image scale is achieved.  According to my calculations the focal reducer need to be about 1/2 inch inside the scope to achieve the proper reduction.  In order to get the proper reduction I need to push the distance between the reducer and the camera closer to 95mm.  The point is, you have to measure to know the proper spacing.  The other thing I learned is the focal reducer needs to be as close to the back of the scope as possible to work.  This is true for both the C8 and the LX200 10"

The correct distance from the focal reducer to the CCD  focal plane based on image scale measurements, was closer to 95mm. My measurements are based on the center of the focal reducer not the leading edge. The center of the Celestron focal reducer is about 11mm from the back edge of the reducer.  If the reduction factor is not 6.3, the focal reducer is not placed properly within the imaging train and the scopes focal point.

Placement of Focal Reducer

Y=D/R

The focal reducer must be placed a distance Y in front of the native focal point of the telescope. This distance is simply the ratio of the working distance of the reducer to the reduction factor. 

 

Actual Working Distance

In practice it is difficult to achieve the exact working distance needed to yield the specified reduction factor.  Recalling that the reduction factor is just R = 1 - D/Fr, the above equation for Y can be rewritten as:

Y=(Fr*D)/(Fr-D)

For example, here is the calculation of the working distance for my setup:

Working distance is 85 mm with a Fr =231

Then:

Y=(231*85)/(231-85)

Y=134.48

or about 5.29".

Thus the reducer optics must be 5.29" in front of the native focal point of the telescope, according to Celestron the back focus is about 5.25", which puts the reducer in side the scope to place it properly. The distance from the base of the scope cell to the center of the reducer’s optics is about 30mm, which pushes the focal reducer back even further.  This offset has to be taken into consideration when calculating the proper position for the focal reducer. A more practical approach may be to place the reducer as close to the back of the scope, set the working distance from the CCD to the focal reducer, measure image scale and keep adjusting unto the proper scale is achieved.  In order to get the proper image scale, I actually had to move the CCD focal plane back another 10mm beyond the calculated distance. This is with the focal reducer directly attached tot he rear cell of the C-8.

As the working distance is increased the image size on the CCD chips decreases and vignetting occurs.  So it is possible to change the working distance and or reduction factor because of the equipment you are using.  There are consequences for not achieving the proper spacing and proper placement of the focal reducer.