Re: [ATM] Ultimate Collimation

["Bruce MacDonald" <brumac@gmail.com>]

John,
I'd like to see this in action before I deliberately remove any of my
mirror's coating, even if it is in a spot that does not matter :)

However, I can vouch for one thing:
I centre-spotted my primary using a reinforcement ring, and that was
good.  When later I came to question the original placement of the
ring, I found the exact centre of my mirror, and placed a single tiny
black dot there.

Using the autocollimator with the black dot there made a big
difference.  When approaching "perfect" collimation, I see ghostly
images of the ring-and-dot, which when they converge tells me that my
collimation is "as good as it gets".  Somehow, I find this much easier
than ever I did with the ring alone.  Could be just me.

Also, I know what you mean by wiggling the autocollimator flashing the
circle.  I take that to mean that at that point, the collimation error
is less than or equal to the mechanical limits of my focuser.

The downside of the above is that it must be done with a reasonable
amount of light.  I've been able to do it within about a half hour of
sunset, but after that it becomes increasingly difficult.

--BM

On 9/2/07, johncdeitz@comcast.net <johncdeitz@comcast.net> wrote:
> Hello Gang: Try this for the ultimate in Newtonian Collimation-
> 1) remove reflective coating in center of mirror, about 1/8 inch
> 2) if mirror cell is solid provide hole through cell at center
> 3) shine a light at the hole from behind the cell (I use an LED with variable brightness so as to preserve vision- this fits in snug hole at center of the back of mirror)
> 4) using an autocollimator eyepiece, with the optics roughly aligned, you will typically find two spots of light from the LED (or flashlight etc.). Tweak the mirror to bring these two into one and you will see an additional reflection that is quick to change position- this is very sensitive method! Bring all three head-on and your view is flooded with light as with conventional autocollimator. You will find enough play in the autocollimator that his spot will wink on and off as you play with the autocollimator in the focuser.
> This method offers the following advantages:
> 1) you can follow the whole action continuously as the cell is tweaked, unlike with the conventional use of autocollimator
> 2) can be performed in suboptimal seeing with accurate results
> 3) no problem with off-set.
> 4) can be performed without ruing night vision so you will not hesitate to repeat.
> 5) can even be done by one person!
> John C. Deitz
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