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Friday 10th August 2007
Some time ago I was asked to make a
6.9 inch Zerodur F11 aplanatic mirror for a Newtonian telescope for Simon
Briggs. This project was a refreshing change from the large fast mirrors I am
usually involved with. Simon has given permission for me to post the test report
he sent me.
Test
Report on the 6.9” F11 paraboloid for my aplanatic Newtonian project
My design
brief was simple to produce a high resolution small to medium aperture telescope
which was transportable from our house to the garden, for the visual observation
of double stars at our temporary residence. The prospect of a 6”+ refractor of
sufficient quality was daunting and not being in a position to permanently mount
such a refractor with a tube length likely to be in excess of 8’. I chose to
build a Newtonian reflector as I had a suitable disc of Zerodur ready for use.To
produce the best results when observing and measuring double stars, a Newtonian
reflector has to have both a small central obstruction and as flat a field as
possible.To achieve the flat field I require, I calculated the focal length of
the mirror required for my proposed telescope to be 70” or greater. I chose to
have the 6.9” diameter mirror figured to 75” focus. On recommendation I
contacted John Nichol to see if he could help. He was very accommodating and
showed real interest in my project, he could offer at a very reasonable cost to
grind and figure the necessary paraboloid. Keen to start the project I
contracted him and sent him the order to process my blank.John’s contact was
excellent by keeping me up to date with regular e-mails, the work was achieved
and in very reasonable time, the mirror was ready to receive its coating. As the
above work was progressing I constructed the telescope using an oversized tube,
to allow for cooling to be achieved with minimal optical effects, together with
a low profile focuser, I calculated that a small diagonal of 0.9” would be
sufficient to illuminate the required field. I ordered a 1” Quartz secondary
from Protostar USA. The mirror arrived right on cue and a month after its
arrival, I had a fully operational telescope.
Optical
Tests
The mirror
arrived very well packaged and sealed ready for use. I left it a month before
use to allow the coating to harden. Once this time had passed I fitted the
mirror into its cell and the telescope was ready for use. At last after another
month, a clear night!! The telescope was taken outside and allowed to settle for
2 hours together with a selection of high end eyepieces.
Simply put
the results were stunning, intra and extra focal star images were of textbook
quality and time after time my TMB Monocentric, Clavé and Zeiss Abbe Ortho
eyepieces showed a real advantage over anything else I tried. For the first time
I have in my possession a telescope that could get very close to and be at the
edge of theoretical limits of double star separation for it’s aperture!
A real
reference this telescope could be used very effectively, for the assessment of
eyepiece quality, for a while this was a distraction as I continually rifled
through my eyepiece collection, the differences in eyepiece quality was larger
than anticipated with the University Ortho’s in my collection a surprise doing
very quite well only bettered by the three types described earlier.
Conclusion
I am an
optical fusspot but I now have a telescope in my possession that I find a real
pleasure to use matching any 6 or 7” refractor I have used and embarrassing many
short tube telescopes twice its aperture. As a result I can without hesitation
recommend Nichol optical to any fellow Atm’er, or John I am sure would make you
an excellent mirror to upgrade any imported Newtonian to produce first class
results both visually and for astro imaging.
Simon John
Briggs FRAS
Lowestoft
Suffolk
August 8th 2007
I got back today from
rural France where the telescope now lives. First light was
on Sunday, about a year after buying the mirror from you.
The views were really good and I am very pleased. I
expected it to be excellent on faint galaxies but was also
very impressed with the tightness of the stars across the
field of view; excellent on globulars! Please don't
hesitate to use me as a reference if you wish.
Best Regards,
Jonathan Burton (19.5
inch mirror and flat)
And a comment from Frank Rossennell, Denmark (two 10 inch mirrors
re-figured) "The dealings with Nichol
Optical went smooth without any problems whatsoever.
The final cost was what we had
agreed upon to begin with without any extras.
The mirrors came back well wrapped and
once installed , the diffraction images
inside and outside focus are textbook
quality."
Hello John,
I
have fitted the
mirror and flat to
my telescope. I
tested it on Saturn
and the new moon. I
am very pleased. The
performance is first
class. I used a 6
mm. Orthoscopic
eyepiece which would
give about 250x. I
was observing
through thin cirrus
stratus, and the
image was very good,
I could not use a
higher magnification
due to the seeing
conditions. Even
though it was only
twilight the four
stars of the
trapezium in the
Orion nebula were
clear/sharp and very
well defined.
Thank you again
John I am very well
pleased.
The mirror was
extremely well
packed when it
arrived here and the
physical size (300
mm) is identical to
my old mirror, so it
slotted straight in.
I will
most certainly sing
your praises at the
next meeting of
our local
astronomical society
( The Shropshire
Astronomical
Society).
With
very best regards,
Douglas Renton-Cooper
(30cm F5 Mirror and
Flat) Feb. 2006
Hi John,
We got the scope set up and after a week of cloud
there was a break in the weather,
we got a few hours of clear skies. The seeing wasn't to
good but it wasn't bad high up,
I put in a medium strength eyepiece and looked inside
and outside the focus & got round disc's
on both sides, we left it all to cool down for a couple
of hours & the nights viewing involved Saturn, the
Orion nebula (which my dad has never seen before & he
nearly fell of the steps) & Mars
(which I have never seen before) & some globular
clusters etc. Higher magnification eyepieces were
inserted on each object, which was always followed by ooooh look at that, I don't think I have ever seen
Saturn looking that good before, she really did look
like a Jewell, with a crisp image
even with this seeing.
So its a big thumbs up for this one, and if later on I
feel like a bigger mirror which is lets face it likely,
I will give you a call,
although it would have to be a cassegrain next time!
Thanks again and I hope you have clear sky's
Tim
Greener. (16 inch mirror and flat March 2006)
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