An Experiment - a 14.5" F/2.55 Newtonian

By Mike Lockwood, Lockwood Custom Optics

After making a 20" F/3 mirror and seeing how well it performed
 under steady Florida skies, what could I possibly dream up next?

14.5" F/2.55 OTAProject 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 pose with the 14.5" F/2.55

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-TexAl tests the 14.5"

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 some moreAl approves!

Al tests it some more.... and he gives his verdict of the system performance and makes my day.

John Joseph and 14.5"

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.

The long and short of it

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.

Al, John, and me

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......

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