Collimation Woes

Hello All,
Right. Near enough at the 'Last Chance Saloon'! Going global !
I got the the Tak. collimating eyepiece and crosshair tube set from OPT yesterday to try and finally collimate a new'ish ( under a year ) 2.8 ED180 that I bought 'used' a couple of months ago. It had a pedigree of taking several published pictures and I picked it up without having a peek through it and when, after arriving home that evening, I had a looksee, it was woeful. The stars were smeared right across the field. In fact unrecognizable as such. The seller naturally enough pleaded total innocence. No matter, I had all the right tools: crosshair sight tube, Cheshire, a couple of lasers ( one, a LaserMax ) and a modicum of common sense. I thought.
Every tool ended up contradicting each other of course. Outside, on the one mag.4 night that's happened since I got it I couldn't get it together at all. The upshot being that at the end of December I ordered the Tak. collimating kit from OPT. Yesterday: joy ! No slop or play at all. Although the instructions are in Japanese of course, the drawings follow the time honoured sequence of events to line everything up. And yet, how is it that when I put the corrector back in the focuser and look across the valley here to some leafless trees on the skyline, I get to see more or less what I saw before ? Aside from a bit of kidney beaning, one side of the image will be sharp'ish and the other hazy.( Using a succesion of Naglers and Pentax's. 1.25" ) And so on. I would suggest it still isn't close to being anywhere near lined up. Stars,(given the opportunity!) I feel sure will still display as streaks. I'm given to wonder why?
Here's a pic. I snapped an hour ago which approximates pretty closely what I'm seeing in the collimating routine : sky end to the right of picture, and crosshairs rotated to line up over the spider.
The question is: does anything jump out at you as an obvious malady?
Cheers,

Bruce

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The right side of the picture looks vignetted. The lower left vane is longer than the upper right, but this may be the vignetting. Not sure why it would look like this if it were collimated, unless the primary is not centered in the tube or something like that.
Have you measured the spider vane from tube to center stud to see if they are all equal in length. How about the mirror to the tube, is it centered?
Could the focuser somehow be not collimated and askew?

>The stars were smeared right across the field. In fact unrecognizable as such.

What do you mean "smeared across the field." What camera were you
using?

>Aside from a bit of kidney beaning, one side of the image will be sharp'ish and the other hazy.
>( Using a succesion of Naglers and Pentax's. 1.25" ) And so on. I would suggest it still
> isn't close to being anywhere near lined up. Stars,(given the opportunity!) I feel sure will
> still display as streaks. I'm given to wonder why?

The F2.8 ratio is going to severely test most eyepieces. You are not likely to
get sharp images.

> Here's a pic. I snapped an hour ago which approximates pretty closely what
> I'm seeing in the collimating routine : sky end to the right of picture, and
> crosshairs rotated to line up over the spider.

It looks like a case of optical axis not being on the mechanical axis.
This leads to the focal plane being tilted, so star images are not sharp
across the field. See attached. Use your LaserMax and I think you
will see that the rays are not coming out of the front of the scope
centered on the tube axis; the laser spots will all be shifted to one side.
It will look perfect with the crosshair sighttube, however; sight tube or
single spot laser misses this type of error.

This can be caused by having focuser tilted (what I show in the
drawing). There is another varient which can occur if the
diagonal mirror is too far / too close to the primary, or too far /
too close to the focuser. Maybe best plan is to send it to TNR and
let them mess with it. Once you get in this situation, it is
difficult to get back to proper collimation.

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A note to let you know what I discovered tonight.
I wouldn't expect this to be the cure to the hassle I've been having, but, I finally got around to loosing off the primary's retaining ring and extracting the mirror for a looksee.
A really nice cast and machined al. cell was revealed with six cork faced floating pads to centre the mirror within. Mirror supported from underneath by three cork faced 40mm dia. al.pads floating on the three dome ended set screws that are adjusted from without.
Two of the pads appear to be stuck to the underside of the mirror and the other was at liberty to roam. How it effected its release I can but wonder. The photo shows to what effect. The dimpling is the effect of the dome ended set screw which ought to have been acting within the centre hole. Not good. :C
From the off I was given to wonder about why one of the set screws was so obviously higher than the other two, but couldn't picture the mechanical arrangement within. Now I know.
I expect I'll give the mirror a good scrub in the sink before replacing it tomorrow.

Bruce

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