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My astronomy project:
Design a 12" f/4 Newton telescope



  1. Optimize for a Paracorr-2 3
  2. Zoom in ray trace at focuser

1: Optimize for a Paracorr-2 3"

Now after have got some understanding how to built a Newton telescope for visual use I shall try to fit in a Paracorr-2 3" coma corrector in the focuser to transform the 12" Newton to an astrograph.

Don't forget I'm new to this and do mistakes!

With data from Televue I can do this (strange drawings, how could they mix inch and mm in the same drawing?).

  • Important figures:

  • Distances from incoming flange to focus are 5.1" + 0.96 mm or 130.5 mm
  • Outside diameter, 3" or 76.2 mm
  • Barlow effect = x1 ?

We don't want the coma corrector to protrude inside the telescope tube.

Design a Newton telescope with the Newt-Web tool, 12 inch f/4

I have changed most of the figures in the specification. Normally when you buy a mirror they are made in inches, a 12" mirror is 304.8 mm. But you should always mask the rim, it's very difficult to make a mirror perfect all the way out to the edge. I have set the effective diameter to 300 mm, but maybe 295 mm could be a better effective diameter.

The secondary mirror I have chosen to a 4". The built length of the coma corrector is very long and make problems. I have let it protrude 10 mm inside the telescope tube. The telescope tube inside diameter I have let be very tight around the ray, important in this case because a wider tube have let the focus point to be adjusted more farther away from the secondary mirror and that cause problem. But it is difficult to make a tube with that precision and center it perfect around the incoming ray. Just my thoughts, have never built something like this.

Design a Newton telescope with the Newt-Web tool, 12 inch f/4

Here is the ray trace, as you see is the inlet in the focuser narrow and it will protrude 10 mm inside the telescope tube that make it even worse. The inlet lens in the coma corrector must of course be smaller then the inside diameter of the focuser, maybe at maximum 67 mm lens diameter.

Design a Newton telescope with the Newt-Web tool, 12 inch f/4

The performance page say that the 100% illuminated area will have a diameter of 38 mm, but I shall have a full frame sensor and that has a diagonal size of 44 mm. We will still get the sensor illuminated but with vignetting. The size of the secondary mirror is 34% of the primary mirror. Not good but it can't be very much smaller without we get serious problem with the vignetting.

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2: Zoom in ray trace at focuser

Design a Newton telescope with the Newt-Web tool, 12 inch f/4

A look the ray trace around the focuser show that this will not work without vignetting. There is some space around the red ray, but remember this coma corrector now protrude 10 mm inside the telescope tube, then the lens in the coma corrector will be to small to fit the red edge ray. The illuminated diameter in this ray trace is only 38 mm, I need 44 mm. We must have a part of the ray outside the 100% field, maybe not the 75% ray, more like 85% to 90% I guess. That make consequences and we must increase the telescope tube inside diameter, then we have to move the focus point even farther away from secondary mirror, more problem!

Here is the new design file if you want to do some test by your self:

Newt-Web design page:

This was the best I could get with the 3" focuser and Paracorr-2 3" coma corrector. Next test will be how to adapt for a 3" Wynne coma corrector, they perform much better at the edge with better correction. But the drawback is that Wynne correctors are even longer then the Paracorr corrector. I think it will be much worse, but first a test to see how it behave before I say to much.

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