OTA dent - collimation mystery...

[michaeldurban]

Well hello and welcome to my very long (and annotated) help request.

I have made a bit of a...boo-boo....a major one.

Was tweaking my SW 10" flextube and decided (why???) to re-center the secondary.

So off I went, loosening one spider vane and tightening the other.

But..it didn't really work..

Finally I had achieved a respected center point for my secondary (using the phillips-head

as a reference point).

I then set out to collimate the whole thing.

So first I centered the secondary under the focuser tube..all fine.

Then I made it nice and round, removed the yellow paper blocking the primary,

and set out to tilt the secondary to get a nice reflection.

Now...normally, this would look something like this in my scope:

The middle (shiny-white) circle is a reflection of the cheshire faceplate, onto which

the (red) center spot is centered (not show, blocked out by light.)

Or..as shown in the drawing (exagerrated)..

( 1. cheshire faceplate     2. prim.mirror center spot  )

As you can see, the offset is very noticeable and collimation is good for viewing.

After the whole spider-vane thingy I noticed the following during collimation....

(have a look at my drawings below, please don't measure exact center and such as I made

quick illustrations to strengthen my problem..)

This is a sketch during collimation -the tilt of the secondary-

First thing I noticed was the 'lack of' offset  .(not well shown in this drawing,

it was actually even more towards the right, almost in the center)... there seemed to be much less of it..

Secondly I noticed that (after centering the secondary under the focuser tube and making

it nice and round) my reflection in the primary was way off ! I could even see two mirror clips

which (as SW 10" owners know) you never see during collimation.

In any case, I soldiered on.

It took much more effort to get the cheshire crosshairs alligned onto the reflection of the

cheshire faceplate..

My suspicions were aroused...something was not right....

even when I first acquired my scope I 'mastered' cheshire collimation in a month..

I blocked the primary with a sheet of paper, and checked the 'roundness' of my secondary..

Way off !! It was a rugby ball !!

I tweaked and tweaked and finally -after one hour- I had the crosshairs aligned and the mirror was

round(-ish)...

Now to get the red center spot under the crosshairs..

A simple case of tuning the primary mirror ..

I turned the three knobs to center the spot and..

To my surprise not only the center spot moved but also the reflection of the cheshire faceplate!!!

I could center the red spot, but it threw out my secondary alignment..

Then I clicked...

The spider vanes..

I removed the top ring of the SW (the heavy white ring) to get to the spider.

To my absolute horror I saw that on either side there were small dents in the OTA..

I removed the spider..to look at the dents (about 1-2 mm deep) on the left and right side..

The top ring won't even go back on, as it doesn't fit anymore..

So..

Remove the dents, center the spider and try again?

I'm at a loss...and in pain...a lot of pain..

Hope you guys have some tips for a very stupid -and careless- astronomer..

[Stridor]

Oops!!!!

I`m sure some helpful soul will be along shortly to help you out.

[Steve Ward]

Can't quite fathom why you'd remove the ring to get to the spider ... ? ... but that's by the by ... can you provide some pictures to illustrate the problem , will make life easier if we can see the problem ... 

[Moonshane]

you need to adjust the secondary with the top ring in place or you may find the strain will pull the tube out of round with the results you described. I'd loosen the spider adjusters and put the top ring back on, then adjust them again.

unless a lot out, the spider will not affect the collimation greatly.

[michaeldurban]

hello..

I only removed the top ring to completely remove the spider.

I just checked, the dents can be pushed out with my thumb...

still..the question remains, what is happening with the collimation of the primary?

Another thing..how tight must the spider vanes be?

I know people talk about the 'ping' effect..the sound..

Should the four vane-adjusters be finger tight? ..or more?..do I need pliers?

Before this whole debacle the vanes were very tight, so much so that when

I would hold the 'stalk' of the secondary and move it it wouldn't even show

any significant shift when looking through the focuser tube..

I'm thinking of strengthening the upper OTA where the spider is..

[Moonshane]

you seem to be confusing the secondary and primary tilts.

the tilt of the secondary (three secondary adjusters) is used to centre the red donut with the cheshire cross hairs

you then centre the dot (central hole in the Cheshire) with the donut by adjusting the primary

other than these two steps your collimation  looks ok (assuming the images should be round not oval - they look oval to me).

[Moonshane]

ps adjustment of the spider should be done with no tools I'd say. just finger tight.

[michaeldurban]

maybe I am confused...

(is it the crosshair on the center dougnut?)

It was a late night...

I removed the indentations..

will put some washers between the OTA and vanes..

Thank you giys for the replies...

will let you know how it went

[michaeldurban]

you're right Moonshane..(and thank you very much..)

I aligned the secondary with the reflection of the cheshire instead

of aligning it with the center dot..

I can't believe I confused the two..has never happened before !

It appears that all my mistakes described above are as a result of stupidity, lack of sleep

and repetition..

Thanks again people...

this is why I love SGL !!!

[Moonshane]

I think we have all been there! glad you got it sorted.

[michaeldurban]

I feel so stupid...

can we delete this thread please..

[Moonshane]

we probably can delete but surely threads like this help others in the same boat? we're all mates here and there's no shame in making daft errors as we all do that too

don't we learn that way?