Mirror layout for new tube - help appreciated

[martindale]

Hi guys

After many years of drilling holes in my old newt for different finder brackets, guide scope mount, new focuser, electric focuser, new mirror cell.

I've decided to treat myself to a new shiny tube.

My question is, is there a program/utility I could use to calculate the distance of the focuser/secondary from the main mirror.

Or, do I need to brush up on my trigonometry, and if so, where should I aim for to achieve prime focus?

Cheers.

[ronin]

For some reason I seem to think that the focal plane from the mirror should fall at the top of the tube that the eyepiece drops into.

I seem to recall this from doing something with a plossl and trying to work out where the primary image plane was.

I was looking at a refractor and diagonal but the optics would be the same.

In which case position the focuser central in it's travel and set up accordingly.

Mind you it is a question I have thought about and never seen a defined answer.

[Pan Euro]

This may be of help. https://stellafane.org/tm/atm/general/scope-calc.html

[Joseki]

Since you've already got your old tube, why not just copy that? 

[martindale]

Since you've already got your old tube, why not just copy that? 

When I installed a new mirror cell, the main mirror had to be closer to the secondary due to the lenth of the tube, and I can't remember how much.

Plus I'd just like to know I've got everything in the right place

[Joseki]

When I installed a new mirror cell, the main mirror had to be closer to the secondary due to the lenth of the tube, and I can't remember how much.

Plus I'd just like to know I've got everything in the right place

fair enough   I was just about to say +1 for the link Pan Euro gave but I'm not sure it points to the same Stellafane page, but we're probably thinking of the same thing ... anyway I've used "newt" which is a great way of avoiding trig.  https://stellafane.org/tm/newt-web/newt-web.html

[martindale]

Thank for the link ,it looks promising.

[JED-E3]

The major factor is the size of your diagonal mirror, but everything else is linked, like focuser height and ota diameter.

I found this calculator to be useful.

James.

Edit, just noticed someone else linked to the same calculator. oops. should read more carefully.

[Tony H]

I built a small (114mm) newt and as well as using the calculators listed above, decided to set the mirrors up on a bench with some clamps and cheapo pipes to hold everything in place.  I was then able to work out better where i should position the focuser before I took the drill to the carbon tubing I had acquired.

The calculators were a fraction off and I was quite pleased to have been able to "mock" collimate everything and do a daytime focus test before I started drilling holes.

For a tiny scope it performs very well and is better than my 130mm Meade goto for some things.

Just need to get it onto a better mount now.

[Pompey Monkey]

When I built my 6" F5 Newt as kid, I got a very big piece of paper (off a big roll, I think) and drew the optical path full-size.

Who needs computers or trig?

[martindale]

When I built my 6" F5 Newt as kid, I got a very big piece of paper (off a big roll, I think) and drew the optical path full-size.

Who needs computers or trig?

Cheers Pompey Monkey,

That sounds like a good idea.

Thanks everyone

[Merlin]

A method I used that worked to find where the focuser should be placed was to subtract from the focal length half the diameter of the tube plus about 75mm. It would be a bit less for a low-profile focuser.