The Telescope Mirror Coating

----- Original Message -----

From: coating_customer

To: 'alcoat'

Sent: Thursday, July 27, 2006 1:13 PM

Subject: RE: Aluminizing

Hi Bob,

Thanks for your prompt aluminizing service on my telescope mirrors.

I finally got around to installing, collimating, and testing them, and while the images are much improved they still fall far short of their original acuity. Stars de-focus into pillow-shaped squares instead of concentric rings.

I’d appreciate your advice.

( Editor note, mirror had been in a house fire. )

The problem you describe is not typical

of a coating problem. Could it be caused
by bad collimation?
Coating problems might more likely result in
loss of contrast.

Be sure when star testing to get something comfortable

to sit on so your head is always aligned the same

way for each comparison. Otherwise you will have a

problem with two unknowns. If your eye glasses have astigmic

correction then when your head is tilted the axis of astigmatism

will not be correct because your eyeball will rotate

relative to your head ( and glasses ).

Try rotating the eyepiece first since that is the easiest,

also try a different eyepiece since there might be

some unknown damage to the eyepiece.

If you are fairly certain the problem is not in the eyepieces
then look at a double star, e Lyre for instance, and note the
diameter of the abberated star image compared to the
dark separation between the stars. If the dark separation
increases relative to the abberated image with higher power
then the higher power eyepiece is of higher quality or the
aberration is in your eye.

Try defocusing the eyepiece inside and outside of focus.
How does the image compare inside and outside of focus?

Check the diagonal support vanes to see if they seem
normal.

Try blowing some air into the tube with a hair drier
set on cold and watch the aberration to see if it settles back
into the same pattern.

Make a drawing of the problem and tape it to the scope

then put an index mark on the primary and rotate

it 90 degrees and test again after re collimating.
If the diagonal is of unknown vintage and you have another diagonal,
even if it is the wrong size, try with that.

On a Newtonian scope the things above are the
most probable things that can account for your problem other

than some type of atmospheric refraction near

the horizon.

The old Amateur Telescope Making books talk about warped
mirrors but I have not found any of these yet. Maybe modern
glass is better annealed.
Removing an old coating with a polishing lap is a good way
to remove a stubborn coating such as a coating with a chrome
or nichrome primer coating but could easily cause the aberrations
described.

Bob Fies

http://home.covad.net/~alcoat/

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Woo-hoo! We finally finished, after 2.5 years, and the telescope is working!

As a word of warning to those still in process, though,
do not get lazy in your polishing.
Our mirror has a powdery look, and is not as shiny as ones I have seen in the past.
Bob said this was because we had insufficiently polished the rascal.
Now, I am regretfully considering re-polishing, but you can spare yourself
this anxiety by doing it right the first time.

Happy holiday wishes to all,
Walt

-----Original Message-----
From: dobson@.com
On Behalf Of Kenneth F
Sent: Saturday, December 16, 2006 9:01 AM
To: nancy@.com; John Dobson telescope making class fall 2006;

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Hi Bob,

I thought you might be interested to see the results of knife edge tests before and after you coated my mirror. It looks like, if there was any change, it is within the measurement error. All the numbers are in mm and referenced to zone 5.

------------------------------------------------------------------------------------------------------------------------------------

1 0.11 0.19 0.17 -0.05 0.19

2 0.81 0.71 0.71 0.67 0.63

3 1.15 1.15 1.12 1.06 1.05

4 1.48 1.41 1.42 1.47 1.47

5 1.96 1.96 1.96 1.96 1.96

So it looks like the coating is very uniform!

Regards,

Charles

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Hello Bob,

I received the mirror and diagonal and they were in fine shape. Thank you for the coating service. Great job!

Season's Greetings.

Jerry