Truss tube lengthI i

[dark star]

I have nearly finished a truss dobsonian for my 14 inch mirror. I am having a lot of problems getting it to focus on the moon. I am planning to check the measurements from the focuser to the secondary mirror and the secondary mirror to the primary mirror. Obviously I have miscalculated somewhere. I will check the focal length of the mirror again but am pretty sure I have this correctly. i meant to leave the truss tubes around 2 inches two long to begin with.

When I rack the focuser all the way out I can just get focus on some leaves on a nearby tree.

Does this mean I need to increase the focal length to get focus at infinity or reduce it? I am sure this is obvious to those with more experience!  I could not focus on the moon gradually moving the focuser all the way in, I also tried with a barlow and a high power eyepiece.

I was definitely pointing at the moon as if I looked through the focuser without any eyepiece I could clearly see it reflected in the primary mirror.

David

[Sporadic Dobstronomer]

You could try using a piece of greaseproof paper, tissue, frosted glass, or any other semi-transparent screen to catch the image of the moon and see where it comes into focus.  Once you find the position of good focus, you will know exactly how much you need to alter the length of your trusses.

You can do a simplified rehearsal: if you point the telescope slightly away from the moon, you can catch the image in front of the telescope to get some feel for what to expect when you do the proper test. (For this first go, you want the moonlight to only just miss the diagonal or you won't get a sharp image at all)

Shame about the weather for the next several days...
 

[dark star]

I think I have figured out why my calculations of the length of the truss tubes are off, I made a very simple error. I forgot that the truss tubes do not go straight up to the secondary cage but are at angle! This means that they should have been longer than I thought. Hopefully someone can confirm that if with the focuser all the way out  the telescope just focuses on a leaf on a near by tree I need to increase the focal length.

If I make another truss dobsonian I will take the time to make a scale drawing and hopefully avoid silly mistakes like this.

It is not a major disaster as the way I have connected the trusses to the mirror box is by bolting them to some long peices of angle (possibly steel) which are attached to the mirror box with screws. I can just move the angles further out of the mirror box and so increase the focal length. The angles are pretty strong so I don't think it will be a problem. I will post some pictures in a while once I have improved the telescope a bit. At the moment it more or less works but looks a bit of a mess! As this is my first attempt at making a telescope I am happy that it works at all! But it needs improving.

David

[Ags]

Yes, it sounds like you need to make the tube longer.

[dark star]

So I should hold something like greaseproof paper in front of the focuser with no eyepiece in and see where the moon comes to focus, assuming it does come to focus I will know how much I need to increase the focal length? Thanks a lot, that's a great idea!

It is a pity about the weather forecast, but telescope making (and mirror making) is teaching me patience!

David

[Sporadic Dobstronomer]

If you can focus on close things but not distant things that suggests your tubes are too long rather than too short.

My guess is that the moon will be sharp when the paper is placed between the focuser and the diagonal.  But the real answer is to try it and see.

You can also try catching the image of your tree on the paper as another experiment to get the feel of how to do this .
 

[dark star]

Actually, I tried to make the focus length a few centimeters too long in my calculations, as you can't make the truss tubes longer but can make them shorter.

So despite my mistake with forgetting the truss tubes are at an angle I may have still made them too long. I will try moving the truss tubes down in to the mirror box around 3 centimeters and see what happens (they are resting on the sides of the mirror box at the moment).

David

[cassiewoofer]

One way to see if you're too short or long is remover the focusser and just hold the eyepiece in your hand. Getting the thing pointed towards the moon can be easy enough by just getting a bright mirror whilst looking through the hole for the focusser. I had this problem with my 12" truss dob. Though the mirror is mounted in a tube section so some adjustment could be done here rather than at the truss tubes.

Keith