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The 20" F/3 MX Telescope
Part 2 - Observing in Missouri

Article, photos by Mike Lockwood, Owner, Lockwood Custom Optics

A beautiful day for a drive

AFTER multiple snowfalls in central Illinois, it was nice to head southwest to central Missouri. By the time I was crossing the Mississippi, the temperature was in the upper 50s and climbing. It was a happy, sunny, breezy day in St. Louis, and about an hour later I pulled into Bob Kirschenmann's soggy driveway and met up with him and Rick.

We talked for a few minutes, enjoying the warm day, and then we unloaded the 20" F/3 parts from Rick's truck. Bob had cleared snow from a patch of ground earlier in the day, but by now most of it had melted anyway, and the spot had dried out nicely thanks to Bob's foresight. I was shown how the assemble the telescope (I'd never actually assembled a Starmaster scope before).

Scope assemblyBob, owner of a 20" F/4.3, is surprised by the ultra-short truss poles on the 20" F/3 MX.

The 1.25"-thick 20" mirror and cell were very easy for me to lift out of the box and place in the back of the telescope because they were just not that heavy. We collimated using a laser, sight tube, and finally an autocollimator.

How short it is!

Bob was quite amazed with how short the trusses are - his 20" F/4.3 scope is much taller. Bob is 5'5" tall, and Rick and I are both around 6'2". For Bob, the 20" F/3 is the perfect height. For Rick and I the eyepiece height at the zenith requires bending over just a little bit. If I want to bring it up a little, I can just put some blocks under the bottom board. A 22" F/3.3 will be perfect for Rick, but the 20" is about the largest I can fit through my door, and I will be proud to say I own an F/3 telescope.

Bob wonders why he got the ladder Rick and Bob check collimation (Left) Bob wonders what size F/3 scope he could observe with using his trusty aluminum ladder - I'm guessing a 40" F/3 would work!

(Right) Rick and Bob check collimation on the 20" F/3 MX as the sun sinks lower and dinner time approaches.

Bob wonders why he got the ladder Rick and Bob check collimation (Left) Bob Kirschenmann is 5'5" tall, and the eyepiece height is perfect. Note his stylish green boots! (Right) Rick is taller, and he can sit on a tall chair and observe any object in the sky, as can I, if the chair is tall enough.

We headed into town for dinner. After re-hydrating and re-fueling, we returned to Bob's under a darkening sky with a not quite first quarter moon and Venus stacked in the west. However, before we left I got Bob to take a picture of me with my new telescope. It's so windy that my hair is blowing all over the place, as you can see. This breeze continued into the night.

Mike with his new one-of-a-kind telescope Me with my new telescope. The wind is blowing (look at my hair) and the sun is shining in my eyes, but I'm ready for a memorable night of observing.... after some dinner, of course.

The wind was strong and gusting up to 20-30 mph, and this made the rather warm night feel fairly cool. The moonlight made it easy to see, and we pulled the shroud all the way down to keep the interior of the telescope dark. This makes the telescope more susceptible to being blown around. This happened a couple times with the drive disengaged but never with the drive engaged. The worst effect we saw was some shaking of the image during the strongest gusts.

Pushing the limits - astronomical myth-busting

My first look through the telescope was at a random part of the sky using a 17mm Ethos. I moved the scope around and after finding some brighter stars noted no significant aberrations around the field. If there are/were any, the seeing was having a larger effect. Stars were round and showed subtle colors, thanks to the large aperture. The background sky was surprisingly dark given the moonlight.

It was a bit hard to believe, but there it was - F/3 WORKS! The myth that fast telescopes can't perform is completely blown away, and now lies smoking, smoldering, and twitching in the dirt.... uh... I mean mud.

Ending the disinformation about "thin" mirrors

One manufacturer's literature claims that a thick mirror guarantees no astigmatism. Many people take this to mean that thin mirrors can't be made without astigmatism. Well, as a professional optician who made the 1.25"-thick 20" F/3, and as amateur builder of more than ten telescopes, I maintain that neither statement is true. A "thin" mirror can be made just as good as a thicker one by a careful optician, and a "thick" mirror can be given terrible astigmatism by a careless optician.

The simplest way to disprove statements of the type that say something can't be done is to provide a counterexample proving that it can be done - and the 20" F/3 is my counterexample.

No matter where we looked in the sky, the stars in the field stayed round. The experience was the same for Rick's previous viewing sessions with the 20" F/3. Yet another myth busted, lying lifeless beside the road like an unlucky flattened squirrel. My counterexample just ran it over! I hope the manufacturer above updates their literature.

Making a mirror thin has a wonderful positive side effect - the mirrors actually cool off in a reasonable time. The many lucky owners of 16.5" F/3.7 FX telescopes already know about the round star images and fast cooling times of those telescopes.

So, I ask those who own or have used a 14.5" or 16.5" FX to spread the word around, and help set the record straight regarding the 1.25"-thick mirrors. Tell your friends, observing buddies, etc., and post online about how quickly they cool and how well they perform. I work very hard to make sure every mirror meets my standards, and I'm going to need your help to get the truth out there because the myth we have busted with the 14.5", 16.5" and now the 20" F/3 has been perpetuated for quite a number of years.

Memorable observing with our feet on the (muddy) ground

We aligned on three stars and then we were off to look at M35, M42, and M78. All looked quite nice despite the moonlight. Venus was about half illuminated. The moon was detailed and BRIGHT, and the whole thing fit in the 17mm Ethos with lots of room to spare. The whole moon was as sharp as the seeing would allow, with no blurring near the edge of the field. Outstanding.

I stood up from the observing chair and backed away from the telescope, blind in one eye from the brightness. I was pleased that I did not have to descend a ladder to let someone else observe, because that's tricky when you're moon-blind.

As we were dialing in other objects, Mr. Kim Colter arrived. As the owner and frequent user of a couple of larger instruments, he would be our impartial observer, for a total of four experienced observers.

We again went back to M42, and the view was very good. M46 was beautiful with the central star of the imbedded planetary nebula easily visible with direct vision. Nearby M41 was beautiful, too. Both would look much better at Chiefland and WSP in a few weeks, I surmised.

The moon was still up, though, which meant it would improve later on. The Horsehead showed with an H-Beta filter. We moved up to the California Nebula, and I assumed we wouldn't see much. However, there it was, taking up a few fields, some detail and the edge of the nebula clearly visible with the filter - by far the most enjoyable view I have had of that object to date.

The 2" O-III filter went into the 17mm Ethos, and we set off to see Thor's Helmet. It showed excellent detail with faint extensions revealed. I put a 1.25" O-III in the 13mm Ethos and liked the view a bit better. The Sculptor galaxy was too low to see.

By this time we were all blown away by what we were seeing, in a telescope that popular convention said would not work.

So what else could we do? Let's break some more rules! We decided to blast away at another myth.

Scope assembly

Exit pupil - who cares?

We broke out some of the low power eyepieces. I put the 41mm Panoptic in. M42 was nicely framed - the whole thing. The entire Pleaides fit in the field, as did the Beehive. In went the O-III filter again, and we slewed to the Rosette Nebula. There it was, the whole thing in the eyepiece. The secondary shadow was not visible in the star images, and the view was nice. Higher power shows more detail in portions of the nebula.

So, the next time you have the urge to use an eyepiece that yields an exit pupil size greater than 6 or 7mm, the commonly accepted size "limit", DO IT! Don't listen to the guy who calculates the exit pupil in his head and tells you you're foolish to exceed the "limit" of 6 or 7 mm.

While stars will get dimmer due to light missing your pupil, extended objects (non-stellar objects) will stay about the same brightness because their light is concentrated into a smaller area. The result? You get beautiful wide-field views without dragging another telescope out with you. Try it, you'll like it!

Beer and the wait for moonset

We took a break to wait for the moon to set and to enjoy some tea, tasty German beer, and snacks. We were still amazed at what we were seeing, but we were all seeing it, and our considerable collective observing skills could not be wrong. We listened as Kim commented on the quality of the images and how impressed he was. This was unprompted feedback from a fairly unbiased observer, and he was impressed. Bob commented on the ease at which he saw color in M42. I commented that F/3 was not the lowest we could go with a visual telescope, and I was not sure where the limit really was. I guess another experimental scope will have to be built.....

It gets better

We emerged into a darker outside world, with the now-dim moon lying on the horizon. The Orion-Canis Major region was noticeably more populated with faint stars, and the winter Milky Way had popped out. It was still windy, and the seeing was still poor, but M42 looked better than I can remember it anywhere. For quite some time I studied the many levels of light and dark nebulosity in M42 and M43. The others followed. The Horsehead was even more evident with and H-beta filter, and M46 was quite beautiful. Thor's Helmet was examined again with slightly higher power. Nebulosity was evident in the Pleaides with the 41mm Panoptic.

The double cluster was quite beautiful in any eyepiece. The color in the various stars was evident and striking. Cruising around and down into Cassiopia I stumbled across the ET cluster, which I couldn't recall observing for a while.

We turned to M51. At this point some high haze was drifting in, and there was some light pollution in this part of the sky. The view was OK. M81 & M82 fit in one field, as did M65, M66, and a companion NGC.

At this point, with high haze moving in, we decided to pack up.


For some reason there were lots of cars driving by Bob's house that night, so we were never truly dark adapted. That made the observations, particularly those of low-contrast objects, just that much more impressive. Getting this telescope under a dark sky is going to be a fantastic experience, and I hope that happens soon.

In summary, we busted four major astronomical myths. Here is what we have seen to be TRUE:

  • 1) A 20"x1.25" mirror is NOT too thin - it works well, cools fast, and is much easier on your back! We did not see any astigmatism.
  • 2) F/3 is NOT too fast for visual use - with a Paracorr and Ethos eyepieces, it give superb images! I am not sure where the limit lies.
  • 3) You CAN exceed the size of your eye's pupil and get enjoyable images!
  • 4) You CAN have fun observing in strong breezes - a 20" F/3 is compact and wind resistant!

This experience has left me wondering where the lower limit for visual observing is. For a commercial telescope, F/3 requires careful collimation adjustments and some practice, but it is quite doable for the motivated observer. Any faster, though, is uncharted territory.

So, pending some more observing with it in Florida, I think I'm comfortable offering the 20" F/3 MX as a product. It won't be cheap, given the work required to make a good F/3 primary, but it will be unlike anything available on the market. That's my kind of product.

If you are interested in a 20" F/3 or 22" F/3.3, give Rick a call. That will let us know there is interest.

- Mike Lockwood, Lockwood Custom Optics

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