New focuser too short - Need Advice - (newtonian)

[starman-scott]

Hi, I have changed the focuser on my newtonian but its too short.

I realise that I can just add an extension tube to it but I’m not sure if that is the best solution.

I also seem to have have a stray light problem (I’m getting a hazy image on a 5mm eyepiece).

A third problem is that I find it quite cumbersome that the focuser has to stick out so far (155mm+eyepiece).

I wonder if I might solve the stay light problem by extending the main telescope tube instead of the focuser and therefore getting the eyepiece closer to the secondary. And also maintain more precision in the focuser by not extending it.

• As it stands the focuser would have to come out 265mm from the centre of the secondary mirror in order to focus.
• The secondary is 50mm wide.
• From primary to secondary is 950mm approx..
• The primary is 200mm

Thanks for reading. Any advice will be much appreciated.

 

Edited November 1, 2020 by starman-scott

[andrew s]

You need to find where the stray light is coming from first. If it's light entering the top of the tube and getting into the focuser your idea will make it worse. You need to look at the overall layout and see what's best. There is a web based program called NEWT that can help with checking the design.

Regards Andrew

Edited November 1, 2020 by andrew s

[starman-scott]

14 minutes ago, andrew s said:

You need to find where the stray light is coming from first. If it's light entering the top of the tube and getting into the focuser your idea will make it worse. You need to look at the overall layout and see what's best. There is a web based program called NEWT that can help with checking the design.

Regards Andrew

thanks do you have a link to NEWT

[andrew s]

Try here Regards Andrew 

[jetstream]

15 hours ago, starman-scott said:

Hi, I have changed the focuser on my newtonian but its too short.

Excellent move, you will improve illumination or you can reduce your sec size.

15 hours ago, starman-scott said:

I also seem to have have a stray light problem

This will come from many sources. Obviously the tube must be painted flat black or flocked. The spider assy must be flat black- the sec edges painted black too.The mirror cell assy must be flat black. Paint the bevel on the primary.

Shine a bright light down the tube and see where there are any reflections or brightening.

IMHO, the primary must be moved down, lengthening the tube here. You can then add a foam shield up front. The inside of the focuser draw tube must be flat black...

I have a 200mm f3.8 that needs a longer tube- a project for later.

[JamesF]

What make/model newt is this, out of interest?  Needing the eyepiece to be 155mm from the tube seems a bit extreme.  I'm wondering if perhaps it's a newt that's primarily intended for imaging?

And how much more outward focuser travel do you need compared with what you already have?

James