My astronomy project:
Design a 24" f/2.75 Newton telescope

Page:

• 24" f/2.75 Newton, ver 1.01
• Optimize for a 2" focuser, ver 1.02
• Optimize for a 3" focuser and 2", ver 1.03 and 1.04 * (You are here!)

Part:

1. Optimize for a Paracorr-2 3"
2. Optimize for a Paracorr-2 2"

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.

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

• http://www.televue.com/ Paracorr 2
  (Paracorr-2 with 3" house)

• 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 protrude inside the telescope tube.

I have changed most of the figures in the specification. This was very tricky to get this to work. The coma corrector is very long and impossible to not protrude to inside of the telescope tube. As a compromise I let it protrude 20 mm inside the telescope tube. Still very hard to not get vignetting from focuser and now a too big secondary mirror.

The secondary mirror is a bit oversized, that's good if we have to mask the rim one mm or so. But with 30% size compare to primary mirror this is not a good construction. But maybe nothing to do about it if we want a big 100 degree field in the eyepiece.

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 20 mm inside the telescope tube, then the lens in the coma corrector will be to small to fit the red edge ray.

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

• Design file Newton 24" f/2.75 ver 1.03
  (Download, unpack and upload to Newt-Web page)

Newt-Web design page:

• https://stellafane.org/ tm/newt-web/ newt-web.html
  (Newton telescope Web design tool by Kenneth H. Slater, math and physics by Dale Keller)

This was the best I could get with the 3" focuser and coma corrector. I will give the 2" coma corrector another chance.

2: Optimize for a Paracorr-2 2"

Disappointed with the bad fitting with the long 3" coma corrector I now give the smaller and shorter 2" coma corrector a new try. It's the Paracorr-2 2".

With data from Televue I can do this.

• http://www.televue.com/engine/TV3b page.asp?id=61 Tab=_phot
  (Paracorr-2 with 2" house)

• Important figures:

• Distances from incoming flange to focus are 85 mm plus some extra mm we don't get information about
• Outside diameter, 2" or 50.4 mm
• Barlow effect = 1.15

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

Now I could decrease the size of the secondary mirror, two reason for this. The coma corrector is shorter but still I need it to protrude 10 mm inside the telescope tube, better then before with the 3" corrector. The other reason is that it has a 1.15x Barlow effect. This mean we can lower the demand on the field of the telescope, about 30 mm / 1.15 = 26 mm given that we use the same 17 mm 100 degree eyepiece.

We increased the power fro the telescope and lower the angular field because of the Barlow effect. Even in this case the secondary mirror is a bit oversized.

Here we get even worse problem now with the smaller 2" focuser, the coma corrector can never handle the incoming ray, we get vignetting.

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

• Design file Newton 24" f/2.75 ver 1.04
  (Download, unpack and upload to Newt-Web page)

Newt-Web design page:

• https://stellafane.org/ tm/newt-web/ newt-web.html
  (Newton telescope Web design tool by Kenneth H. Slater, math and physics by Dale Keller)

Conclusion:
None of this two coma correctors work very well together with this telescope. Must find one that is of a shorter design to get rid of that problem. And Televue also say that this coma correctors only will be good down to f/3.