After making a 20" F/3 mirror and seeing how well it performed
under
steady Florida skies, what could I possibly dream up next?
Project
Conception
After a very successful Winter Star Party where I showed countless
attendees views of Saturn, the moon, and deep-sky objects through my
one-of-a-kind 20" F/3 MX Starmaster telescope, I decided that I needed
to try to place a lower bound on the practical speed of visual
Newtonian
telescopes.
With the great success at F/3, I looked at a fairly compact instrument
of smaller aperture to serve as a test bed for various eyepieces and
correctors. John Pratte had a spare 14.5" blank, 1.38" thick,
so
that decision was easy. (It's a little bit ironic that this
mirror is thicker than the 20" F/3 mirror!)
Wanting to keep central obstruction below
35% and preferably closer to 30%, I hit upon a focal ratio of about
F/2.5-2.6. This meant a 4.5" m.a. flat was necessary to
illuminate the center of the field, with a 31% obstruction, and this is
exactly what I used (after testing/refiguring it, of course).
The mirror actually went fairly smoothly (both in terms of the work and
the optical surface), and it was done by mid-summer
of 2009. The telescope started to come together as John
completed
the
mirror cell, truss poles, and spider, and as the mirror came back from
coating. John knew the mechanical requirements for the parts,
and
I knew he would build the most solid components that he could.
In July, I received a prototype Paracorr that was intended for use with
faster instruments than the original Paracorr. I tested it
with a
variety of eyepieces in my 20" F/3, with excellent results - the new
corrector provided significantly better coma and field curvature
correction at F/3. This was very good, and I sent a report to
Al
Nagler.
A few weeks before Okie-Tex 2009, I started construction.
Baltic
Birch was my chosen material for all wooden telescope parts - the
mirror box, rocker box, bearings, focuser board, and secondary cage
rings. The tubes between the secondary cage rings are
aluminum,
1.25" in diameter. The aluminum truss poles are 1" in
diameter,
and remain in a connected "bundle" after the scope is
disassembled. This is very convenient, and it was a feature
requested by me because I have used it on other telescopes.
John
figured out how to do it quite nicely.
First
Light
First light took
place in
early September, and was conducted with the OTA only sitting on a
blanket in my driveway. The
focal plane was in about the right spot, and all eyepieces focused.
Initial testing showed that the prototype
Paracorr was still
working very well at F/2.55. I also learned that I had gotten
fairly close on the collimation.
This was promising optically and mechanically.
I checked the size of the secondary mirror by putting my eye at the
focal plane - I could just see all of it in the offset secondary
mirror, so it was perfectly sized in my judgement. I also
defocused a star and looked for the shadow of my finger as it was
brought into the light path at the front of the secondary cage, and it
appeared at the proper places, confirming that the mirror was catching
all or very close to all of the light cone. Thus, I had no
motivation to go up to a 5" flat, since it was not needed.
I told Al Nagler
that things were looking good. We were eager to meet at
Okie-Tex
in
a week or two and give things a serious shakedown.
With the OTA
complete, I found
the balance point and decided on the
diameter of the bearings and height of the rocker box. The
rocker
has a 3/4" thick bottom board, 1/2"-thick side boards, and 1/4"-thick
front and back boards, so it is quite light.
The fully
completed telescope
debuted on September 9,
2009 - I guess 9 is my lucky number, because it was 9/9/09.
In my overlit driveway, I assembled the scope for the first
time.
It sat on my equatorial platform and
the 24"-high triangular wooden stand that I made to go under the
platform in
order to elevate my 12.5" F/12.5 Cassegrain to a convenient height for
use. As it turns out, the 14.5" F/2.55 OTA is shorter than
the
Cass OTA, and the stand/platform/scope combo is the perfect height for
me. You can see the whole "stack" in the photo at right,
along
with me, the happy optician/owner.
My testing quickly showed that the new Paracorr was correcting nearly
all of the coma in a 13mm Ethos field, with almost no field
curvature! This was fantastic. The 17mm showed some
coma at
the edges of the field, but was still very good. I had the
spacing for the 8mm Ethos set wrong, so I instead turned to the 9mm,
5mm, and even 3.5mm Type 6 Naglers. Yes, the seeing was good
enough to use the 3.5mm for a bit, but I mainly used the 5mm.
I spent some time tweaking the Paracorr position to minimize spherical
aberration. This paid off - Jupiter looked spectacular!
My friend John Stone stopped by and was similarly impressed.
Jupiter was making its 1.5-hour appearance between the two large trees
that lie to the south of my driveway, and the seeing, amazingly enough,
was cooperating. We looked at many belts,
lots of detail within the equatorial belts, and some subtle shadings
near the poles.
It was unreal, doing planetary observing with a 14.5" telescope
from my driveway, looking over my garage, pretty much the worst
observing site it would be ever be used at.
As Jupiter slid behind the large tree just after midnight, I
disassembled the scope and packed for Okie-Tex. We left the
next
evening, driving through Thursday night to get to the dark skies of
Camp Billy Joe, near Kenton, OK.
The
14.5" does Okie-Tex
Our first night at Okie-Tex was spent slewing the scope around and
trying to view as quickly as possible so the other people that were
around could have a chance to see the object before the sucker hole
closed or moved on. Despite this, Al Nagler and Rick
Singmaster
were impressed.
Over the next six days, countless people viewed with the telescope and
so far as I could tell, they were all quite impressed. On one
evening a spirited group including James Mulherin and Jimi Lowrey
enjoyed the telescope for at least half an hour.
As soon as the telescope was uncovered, event attendees appeared -
smiling, looking puzzled, walking around it, asking questions, and just
generally not believing what they were seeing. Most said
they'd
be back later to see if it really worked, and most did return and had
their doubts erased.
On two evenings,
Al Nager
commendeered the telescope to test the
Paracorr and eyepieces by focusing on portions of the ridges adjoining
the camp (see photo at right). We looked
at bushes, a
large cross, pink flamingos (an
Okie-Tex tradition), and branches and blades of grass, all up on the
ridges. Al was very happy with how his corrector was working.
Eyepiece Testing
The new 21mm Ethos is a fantastic eyepiece, especially at F/3.3 in the
22" Super FX Starmaster that Rick Singmaster brought along.
You
can click
here to read about the 22" F/3.3 at Okie-Tex.
However, at F/2.55 the 21mm shows some coma in the outer parts of the
field. Despite this, it is still an enjoyable view, with a
field
1.9 degrees wide! Scanning the dark nebulae of the Milky Way
with
that eyepiece in my 14.5" scope was a religious experience for me, and
one that must be repeated as soon as possible!
Correction improves with the 17mm Ethos, and is superb with the
13mm. All of the high power eyepieces we tested - 6, 8, and
10mm
Ethos, as well as the T6 Naglers, worked very well.
As for the new Paracorr, intended for mirrors faster than F/4, it
simply works.... very, very well. Al is going to have some
trouble
getting his prototype back! Hopefully it will be available to
fast-mirror users very soon.
Collimation - not a problem
Collimation was done in two stages. First, I used my red-dot
laser collimator to align the secondary and roughly align the
primary. I collimate this laser periodically, and this is
necessary to get good accuracy. Then I use an autocollimator
to
tweak the primary's alignment. If I was careful about
aligning
the secondary, this two-step procedure would nail the
collimation. The only drift I experienced was due to not
having
the spider vanes tightened up sufficiently. That was quickly
remedied.
So, with proper tools and care, I could nail the collimation of an
F/2.55 telescope in less than five minutes. So, to anyone
that
says you can't collimate such an instrument, I am here to say that they
are wrong.
More photos of the scope from Okie-Tex 2009 follow.
My thanks to Al Nagler for making the new Paracorr and the Ethos
eyepieces - this really made this telescope practical and useful.
Also, thanks to John Pratte for his excellent work on the mirror cell,
spider, and truss pole bundle.
IN CONCLUSION - yes, the secondary mirror is large enough, yes the new
Paracorr makes a big difference and I highly recommend it for sub-F/4
mirrors, and yes, an F/2.55 telescope can provide quality views of
planets.
-Mike Lockwood, Lockwood Custom Optics
Al tests it some more.... and he gives his verdict of the system
performance and makes my day.
John Joseph, the new owner of Starlight Instruments, says, "Yeah, I
made part of this." Without that FeatherTouch I wouldn't be
focusing nearly as easily. That fine-control knob truly does
move
with a feather's touch, and this is vital at F/2.55.
John Pratte, on the ladder, shows of his fine 12.5" F/8.6 Newtonian
while I keep my feet on the ground. We couldn't resist this
photo-op.
Finally, here's the group that made the telescope useable - Al Nagler
at left, John Joseph in the middle, and I am on the right.
Ironically I am wearing my Winter Star Party t-shirt, and that is
probably where this telescope will make its next public appearance.
Suddenly a 30" F/2.8 telescope seems like a good idea......
Project
Conception
