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Collimating / Aligning my RedCat 51

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#1 StevenBellavia


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Posted 08 December 2020 - 12:31 PM

I have about 100 hours into collimating and aligning my RedCat 51 using a custom made artificial star.

This is the third RedCat from William Optics, and though much better than the first two, it still had a non-flat field.  It is better now.

An image summary of the collimating process, along with final uncropped images, can be found here:




and here:




The scope is incredibly sharp, and has excellent color correction, but flatness of field did not start off well. (2 out of 3 is not bad though).
I deliberately used an open star cluster (NGC 7789) in a rich star field, as this is the acid test for any scope.
I believe it is now outperforming its $748 price tag.


The "procedure":


1. Assuming your scope is collimated, that is a real or artificial star forms concentric diffraction rings at intra or extra-focus, then you only need to adjust the tilt plate*.

2. Use an artificial star, 10 meters away, and get the best possible focus on center.

2. Then move the scope to place that star into the 8 other positions (four corners, top, bottom, left and right).

3. Find which of the 8 is the worst, and change the focus until it is good in that position, noting which focus direction improved it. Because of the inversion, that is the opposite side (or direction) for that position.

4. Measure the gap at the four locations on the tilt plate, loosen all the screws just a tiny bit, and then make the adjustment (approx. 0.2mm) in one of the four locations, re-tighten all the screws, and measure again to make sure it has been achieved* 

5. Take another 9 images, and repeat steps 1 through 4 until you are satisfied with the star shape and size in all 9 positions. Be prepared to spend many hours doing this.



 - If you do not get concentric diffraction rings on center, you will need to collimate the front lens group.  Once done, it should not change by a noticeable amount, regardless of the tilt plate adjustments.

 - At each of the four locations on the tilt plate, the center screw is "pulling" (reducing the gap) while the two screws on either side are "pushing" (increasing the gap).  Always loosen the two outer screws first and tighten last.

 - 3 points define a plane, so the fourth location on the tilt plate should only be made snug and not overtightened.  (Actually, none of the screws should be over-tightened.  Remember this is aluminum, not steel.  Go gentle)




P.S.  Also note, in the attached image. location "d" should have measured 0.19mm, not 0.25mm , to be in the plane of a, b and c.  So does that imply I am "warping" the plate?  Maybe.  I may leave "d" and change "a" to 0.10mm, which also forms a plane.  I am not necessarily done yet.

Attached Thumbnails

  • WO_RedCat_Tilt-Adjustment.png

Edited by StevenBellavia, 08 December 2020 - 01:03 PM.

  • tkottary, wcoastsands, km4304 and 2 others like this

#2 joeytroy


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Posted 12 January 2021 - 11:57 AM



Very nice sir! Thanks for sharing going to bookmark this is I ever run into issues! I always am on the Astrophotography forum so I would have never seen this. Time to keep a look out lol.gif

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