Help resolve design error on Newt-Web Software

[Nitin]

Hello Guys, I am trying to build my own Newtonian (150F8).

First I worked out a design on https://stellafane.org/tm/newt-web/newt-web.html

Inputs (mm): (Sorry, Forum doesn't allow me to attach my .newt file)

OPTICS: Primary=150, F/R=8, Secondary=32(next size available=38mm, gives vignetting error)

TUBE: TubeInsideDiameter=190, Thickness=5 (PVC tube available nearby), Focuser2Front=100, Mirrorface2back=100

FOCUSER: FocuserMinHeight=75 (suggested by software), SpareTravel=6.5(suggested by software), AdditionalHeight=0, FocuserDiameter=31.75(1.25")

This design has reasonable 11mm "100% illuminated diameter" and 21% obstruction

Now the problem. I received my 1.25" focuser yesterday and boom, its height is more then 75mm, its 120mm.

Changing FocuserMinHeight from 75 to 120, "100% illuminated diameter" goes down to 4.5mm

Diagonal is 32 mm, next size available is 38mm which gives vignetting error "Vignetting at focuser of 75% ray".

Focuser is made of plastic, and it seems it can be cut to 75mm, but it will reduce the focusing movement to 65-70mm. Is that enough ?

Can anyone suggest any other solution ?

[Peter Drew]

I would stick with the 32mm diagonal if you can to retain the performance of the F8 mirror. If you can reduce the height of the focuser by cutting without introducing any mechnical issues then that would be worth doing, 65-70mm travel should be plenty. If you use a zoom  or parfocal eyepieces then a couple of millimetres is adequate! 

[ronin]

Only guess work but could you not move the primary up the tube by 40mm, so moving the focal plane out at the focuser by the same amount. Or if not constructed reposition the secondary and focuser 40mm downwards.

The secondary would be illuminated more but I would expect that a secondary bigger then necessary was selected and should therefore still be big enough for the light cone, especially as the mirror is f/8.

I would not "cut" the focuser.

Not sure about: "Secondary=32(next size available=38mm, gives vignetting error)"

I have always assumed vignetting was caused by a tube restricting the view, a mirror is not a tube and a bigger one will block more incoming light but not as vignetting.

[Nitin]

I have not started constructing (Primary and Secondary will take 2 weeks to arrive).

The software doesn't take the length of tube or relative position of primary/secondary on tube, etc.

All the inputs required are mentioned above, and then the software calculates approximate positioning as output.

Also, it gives out Vignetting errors of 2 types (https://stellafane.org/tm/newt-web/newt-help-scope.html#Vignetting):

1. As mentioned by you at the Tube aperture.

2. At the point focuser is attached to tube, caused by focuser itself.

Probably a mirror of 38mm would require a 2" focuser instead of 1.25". This part is way out of my budget.

Will explore ways to reduce the focuser length for now.

[Peter Drew]

Regardless of what the computer predicts. provided that the mirrors are good, the collimation is correct and you can get a focus, you will be pleased with the reults of your project. 

[Laurie61]

Hi, I have put your numbers ? into newt and it looks OK. It is the focus point above the tube rather than the range of focus adjustment that will affect the performance. 

[Nitin]

Laurie, I see you used 75 mm Minimum Focuser Height.

Problem started when I increase that to 120 mm.

So when I try and cut focuser back to 75 mm, will the shortened 60 mm travel left be enough for all eyepieces to come in focus.

Also if the shortened focuser travel is not enough, will an extra tube (between focuser and eyepiece) of around 50 mm serve the same perpose ?

[Laurie61]

As long as the focuser can position an eyepiece at the focal plain of the scope it should be OK, if using the same design eyepieces they usually all come to focus at or around the same position so 10 -15 mm either side of focus point should be enough focuser movement.  If needed you could use a 50 mm extension tube.