Skywatcher focusser extension tube question

[ukskies]

Hi everyone,

I have today washed the mirrors on my 2nd hand 12" flextube Dob, no dramas and no scratches, all went according to plan, they are now gleaming.

After reinstalling the mirrors I attempted collimation. I am new to reflectors but I began checking the secondary alignment through the focusser tube. Having got this to my satisfaction, it looked round and i could see the primary mirror, I decided to try the laser in the inch and a quarter adaptor. This is where my problems began.

The laser wasn't striking the middle of the secondary, not even close, I had fitted both 2" extension tubes that I had with the scope to the focusser and when I tried rotating the assembly in the skywatcher focusser the laser described an arc on the secondary. something clearly was out so I first checked the laser, it was fine, when I rotated it in the adaptor it stayed in the same position on the secondary. this leads me to the extension tubes and the adaptor. I spent an hour and a half patiently adjusting this then that before I packed away for today.

I've never collimated a scope before but I have engineering experience so I'm able to grasp the concept of what is required. 

I have modified the original skywatcher focusser by flatting the bottom of the draw tube and reducing the slop to zero, it's not the best but it is very solid now with no slop.

The above is background for my questions.

Firstly, does anyone happen to know how much extension the 12" flextube needs to achieve focus please? In addition to the standard drawtube that is.

Secondly, do most people fit better adaptors and extensions to these, ones that centre the eyepiece every time? If not, how do you all collimate?

[ukskies]

Hi ,

Does no one on here know how much the focusser needs to be extended to reach focus? As I bought a used scope with no eyepieces and wish to order everything at once, this would be very useful information for me right now.

I'm reasonably convinced that the problem I am experiencing lies with the 1.25" adaptor, the one supplied with the scope doesn't seat securely and can be fitted in the 2" extension tubes at different angles quite easily.

A picture of the tubes I received with the scope might help perhaps.

The one on the right is a 2" 50mm ext tube the one in the middle is a 2" 47mm ext tube.

Edited February 29, 2020 by ukskies

[Geoff Lister]

7 hours ago, ukskies said:

Does no one on here know how much the focusser needs to be extended to reach focus?

Hello Gary,

I have the 250mm (10") version, and have not needed an extension tube with any of my 1.25" eyepieces. I do need a 40mm extension if I use my Meade 2" 56mm Plossl eyepiece.

7 hours ago, ukskies said:

I'm reasonably convinced that the problem I am experiencing lies with the 1.25" adaptor, the one supplied with the scope doesn't seat securely and can be fitted in the 2" extension tubes at different angles quite easily.

The 1.25" adaptor supplied with my 'scope fits directly into the rear of the focuser drawer tube.

 

The 2 eyepiece adaptors - both have the same diameter tapered flange, and are clamped in place by the 2 thumbscrews (roughly 9 and 1 o'clock positions in above photo)

Geoff

 

 

Edited February 29, 2020 by Geoff Lister

[ukskies]

Hi Geoff,

Thank you, that's very helpful information. Your adaptor obviously sits flush and thus square into the 2" tube.

Mine stops when the clamp screw touches the 2" tube and there is no flange to keep it square to the tube.

Another picture. It's the same direct into the draw tube.

Edited February 29, 2020 by ukskies