Book Mania

Saturday, August 23, 2014

Why You Should Get A Refractor Telescope

Yerkes Observatory 40 inch Refractor
Ok, maybe you shouldn't get a refractor, you'll never get one the size of the great Yerkes Observatory 40 Inch Refractor. But I'd have to say you should consider getting one, albeit a much smaller one. Let me tell you my experience with refractors to help you see why.

I've had a few interactions with refractors in my 5 decades of astronomical interest. I never owned one when I started, back in grade school and high school. I was intensely interested in astronomy for as long as I can remember. I read every book on the subject when in grade school (yup, both of em). But except for a couple of more or less toy telescopes, I never had one of consequence until high school.

Starting Out, No Refractor For Me

Not that I didn't want one. I wore out the pages on my Sky and Telescope issues, primarily trying to figure out which Unitron telescope to purchase. After every day-dreaming session, I'd always end up in the same place -- I could only afford their 2.4 inch Altazimuth model. The trouble was, all my books said I needed at least a 3 inch telescope for any serious viewing. Who doesn't want to be serious, right?

So my next option was to make my own telescope. I dug though every issue of each new Edmund Scientifics catalog, looking for objective lenses, aluminum tubes, focusers, and finders. It was always tempting to spring for all those parts, but once I added them up, I was again always running into a budget wall.

Even so, I bought many Edmund publications about telescopes and observing, like their informative Telescopes You Can BuildHow To Use Your Telescope, and even Photography With Your Telescope. Amazingly, they're still around, very inexpensive, and still good reads. Like with my magazines, I nearly wore out the pages in these paperbacks, planning and re-planning which telescope I was going to build, and what I'd see with it.

But all through high school, I never followed through on any refractor purchase or construction project. Big plans, little time, less money. I did lay my hands on a 6 inch Newtonian with my high school Principal's help, but never a quality refractor.

First Refractor Experience -- A Big One

However, my exposure to refractor telescopes changed when I went to college. I went to what was then a small college called Fort Hays Kansas State College. And sure enough, they had an observatory with what seemed to me a gigantic telescope. It was a 10 inch, long focus refractor built by Lohmann Brothers in the 1930's. The Lohmann factory built quite a number of similar sized instruments in that era.

The 10 inch refractor was a smaller version of the great Yerkes observatory 40 inch refractor shown at the article beginning. Though clearly smaller than the 40 inch, the 10 inch at my college was mounted on a large pier, was very long, and the eyepiece height above the floor varied greatly depending upon the target. There was a rolling set of steps that also worked as an observing seat. I could roll the contraption into position, and climb to the level that got me to the eyepiece.

I managed to get a lot of time on that telescope. Mainly because I expressed intense interest in it, and did a lot of begging to the head of the Physics department. I got to operate the telescope on open house nights, and for science classes. But I also got to use the telescope for my own clumsy efforts at lunar and planetary photography. It was an experience that I'll never forget.

Sometimes, I and a friend would just spend a couple of hours observing the intricate details of the moon that this behemoth made available. Even though by my junior year I had a 10 inch home constructed Newtonian telescope of my own (I purchased the mirror pre-finished for $100), it could not deliver the high contrast and exciting views provided by this great refractor.

The school now has a much newer telescope, a modern Cassegrain reflector mounted in its own obsbervatory. But the school had the foresight to keep the old, historic refractor in operation. And in a recent contact with a professor at my alma mater, I learned that on open house nights, the big line still forms to see the old classic refractor, and far fewer people line up to see the new instrument. There's something majestic and mysterious about those early 20th century refractors.

Next Refractor Experience - A Superb One

Still, in that college period, all of my own telescopes were Newtonian reflectors for budget reasons, and I still didn't own a refractor of my own. My next experience with one was during a summer when I got to work as a summer employee at the University of Arizona. Then I got to assist in using the Mount Lemmon Observatory 60 inch Cassegrain telescope. Now that was a telescope.

Okay, the 60 inch isn't a refractor. But on each side (left and right) of the big reflector was a 6 inch refractor mounted as a finder telescope. Yeah, you got it, 6 inch refractor finder telescopes.  With one on each side, one would be within reach no matter where the telescope was pointed. And on one occasion, when the telescope was between photographic targets, we used it to to take a peek at the moon. My first view was through one of the 6 inch finders.

It knocked my socks off. One of the best views of the moon I'd ever, or have ever, seen with a similar sized instrument. It hit me then what all the refractor praise was about from many enthusiastic amateur astronomers. Superior contrast, steady images, it all was there to see. The view through that 6 inch finder was far superior to those of my high school era procured 6 inch Newtonian.

Of course, I was observing then from about 9,000 feet. My old 6 inch reflector never saw through such a thin atmosphere.

Next Refractor Experience -- A Little, Big Surprise

A few years after that, a friend and I had our telescopes set up in my back yard. At that time I still had my Aperture Fever driven college era 10 inch Newtonian. My friend had set up a modest 3 inch refractor he'd constructed from surplus optics, similar in appearance and size to the Orion model shown at left.

When I looked through his refractor, I was stunned. The contrast of the image made Jupiter's belts stand out much sharper than even my 10 inch Newtonian showed. Sure, some details were visible through the 10 inch that the 3 inch couldn't match, but the 3 inch view was more eye catching, steady, and pleasing somehow.

I wanted one.

But years went by before I ever sprang for a refractor. When I did spring, I went for what I thought was a bargain: a Meade 90 mm f/10 refractor on an equatorial mount. How could I go wrong -- it was a big brand name. But alas, what I had bought was an imported telescope, made in China before China made high quality optics. I should have realized that the price was so low for a reason.

As an example of its low quality, I tried on many occasions to see  the Cassini Division of Saturn's rings with it. It wouldn't reveal the division. I since have obtained and used a Meade ETX 90 on Saturn that has easily revealed the Cassini Division, so the 90mm refractor just wasn't up to it even though the ETX 90 had no aperture advantage.

While I can't complain about the mount on that 90 mm refractor, the views through the instrument were much less than I'd hoped. Not at all comparable to my friend's 3 inch that was made from surplus optics. So I figured right then that I still couldn't afford a quality refractor, and still wasn't interested in building my own.

Bringing It Home At Last

Then some years later I joined a little astronomy egroup on the web. The group was dedicated to small refractors, 60 mm (2.4 inch) being their favorite. I read entertaining and almost unbelievable reports on views though various 60 mm telescopes.

It left me scratching my head. What about the sacrosanct 3 inch rule? I finally asked the group what they'd recommend that I try to obtain.

They listed primarily old brands, often of Japanese make, most of which were no longer produced. They were though readily available at garage sales and Ebay. The general sense was that in the 60's and 70's era, 60 mm refractors were not made to be toys, but serious introductory instruments. They were telescopes with good optics and solid construction. I looked on Ebay, and sure enough some were available. But not as cheap as I'd hoped.

One club member dug through their orphaned telescope collection and donated a telescope to me. It was a 60 mm Monolux of Japanese design, likely decades old, that needed some tender loving care. I reviewed my experiences bringing this orphan back to life here.

Once I'd waded through the problems, like discovering that one objective lens was in backwards, and solving the astigmatism issue, I found this telescope to perform far better than I'd ever expected. From my first views through my college's 10 inch to my more recent views through a quality 60 mm, I was impressed. Having a long focus refractor of quality seemed to serve up very pleasing images almost regardless of size. More details in bigger telescopes, of course. But pleasing views, always.

Tycho Through 60mm Refractor
So here in my old age I find that the 2.4 inch (60 mm) Unitron that I'd almost bought many times in my teens would have thrilled me. I'd have probably worn it out and been on my 3rd one by now. Since refurbishing the Monolux 60 mm, I've constructed (yes, I finally built a refractor) a longer focus Carton 60mm Telescope. At left you see an example of what it can do.

So at this point, having exposed my sparse but varied past with refactor telescopes, I strongly suggest that you don't let the telescope snobbery keep you out of the game. If you're just getting started, or know someone who is, don't be afraid of getting a 60 mm to 90 mm refractor, something like the Orion 09882 Observer 70mm Equatorial Refractor Telescope. What you'll have is a telescope that can deliver high contrast steady views, is small enough to be easily portable, and requires very little maintenance for all that. Just get one with solid construction and a steady mount.

It's hard to go wrong with these long focus types. The long focal ratios reduce color fringing, a nagging characteristic of refractors, to a very tolerable level. Just don't count on that being true if you buy an inexpensive short focus refractor. If you want to go inexpensive -- go long.

So what if you're more into wide field observing? Wouldn't it be handy to have a telescope about 18 inches long, perhaps some 60mm to 80mm in diameter, for star observing? Light weight, portable, wide field, low maintenance -- is it possible?

Yes, it is possible. But for precise, color corrected images, the price goes up a bit. To get good color correction and sharp images out of a short focus refractor, you can't get away with the classic crown and flint glass achromats. You have to go to the ED optics design, which use more exotic glass compounds that minimize color dispersion (translate to color fringing).

If the short focus, wide field, portable telescope is what you want, a perfectionist version would be like the ED Triplet shown at left. Short, wide field, 80mm star pulling aperture, and yes -- higher price. You don't need to pay that much if you get an ED doublet, like the BARSKA Magnus ED 560x80 Refractor Telescope at half the triplet price.

If you're satisfied with low power and a bit of color distortion, you can even try a classic achromat short focus instrument like the Orion 9946 ShortTube 80-T Refractor Telescope, which comes it at less than $200. You'll still get pleasing wide field images, but may see a bit of color fringing on the brightest objects, and may find less clarity at high magnification.

Monday, May 12, 2014

Petavius, A Lunar Crater Of Wonder

Crescent Moon
I was finally blessed with a clear, relatively calm night recently, so I took advantage and went out to observe Jupiter and Mars.  Mars was still available in this 2014 opposition, and I wanted to get in as many observing sessions as I could while it was still of sufficient angular size.

While my telescopes were acclimating, I could't help but admire the crescent Moon that was still up. I decide to start by viewing some lunar craters, since Mars was still a bit low in the East.

The Moon was in about the phase shown in the image at left. Mare Crisium  was the most prominent feature, surrounded north and south by typical rugged terrain. Since the moon in this early phase is to a direction where I tend not to get great views, I've not often observed the features highlighted in this lunar phase.
60mm x 1000mm

I started my observations with my trusty 60mm x 1000mm Refractor. My pipe tripod mounted 60mm is much like the one in the Orion 24445 Lunar Explorer Telescope Bundle. It's an ideal instrument for this type of viewing, big enough to produce some great images, and small enough to be generally little affected by atmospheric turbulence. Plus, it is ready to go in a hurry, being very portable and needing very little cool-down time.

So I and my 60mm began our investigation of the eastern limb of the Moon. Since the Mare Crisium feature was so prominent, I started my lunar examination with it. I enjoyed perusing the periphery of the mare, looking for craters and mountains that lead into the interior. I also enjoyed the view of the frozen "waves" that stretch across portions of the mare floor. Here and there were small pockmarks that I could make out.

But then, as I explored the more rugged regions along the lunar terminator, I was totally taken aback by a large crater some distance south from Mare Crisium. The crater looked normal enough for the most part, like many of the larger craters in the 60 to 100 mile diameter range. And like many such expansive craters, this crater had a small jumble of hills at its center. But running straight out from the central peaks to the rim of the crater was a thin, straight line, like a spoke. It ran for miles, and was easily noticeable in my 60mm telescope.

I couldn't imagine what it was. It was far too long to be a shadow cast by a central peak, yet so straight it seemed that it must be a shadow. So I changed to my other scope for the evening, my planet busting 6 inch Dobsonian. I could, as I expected, make out a bit more detail, but not enough to tell me precisely what I was seeing. The only explanation seemed to be a fault line, like the Straight Wall, or a deep rille. But it was so straight, it still amazed me.

After viewing that puzzle for awhile, I decided to get to my main objectives for the evening, Jupiter and Mars. Jupiter was interesting as a comparison object on that night, showing mostly the major cloud bands, with a bit of mottling in the NEB. The mottling was just visible in the 60mm, and of course more so in the 6 inch f/10. But there weren't, on that evening, any moon transits or Great Red Spot to examine, and the atmosphere wasn't giving up any more tantalizing details, so I moved on.

My next stop was Mars. On that evening, Mars was only at about an apparent size of 14 arc-seconds, not a very big target. It looked approximately like the image at left through my 60mm, though smaller. That is, what's shown is approximately the level of detail that I saw. Not a lot, but I could easily see the dark feature at the right of the image, which was Syrtis Major. I didn't feel too bad about being able to see features like Syrtis Major on a 14 arc-second Mars with my 60mm telescope.

I was using a 17mm Plossl eyepiece and my 3x Barlow, yielding a magnification of about 180x. I've found that on Mars and Jupiter, being that they are so bright, I can often push the 60mm telescope up to around 200 times magnification.

After viewing Mars for about a half hour, I put up my equipment, and retired for the evening (not enough of a trooper to stay up for Saturn). I'd seen the targets I'd set out to see, and while most anticipating the view or Mars, it was the mysterious crater on the Moon that had provided the most excitement.

Crescent Moon
Since that evening, I've had a chance to go back and research the enigmatic crater with the single linear spoke. Using the moon map included with the Xephem Planetarium planetarium program, I was able to locate the crater I'd seen.

At left is an image showing about the phase of the moon on the night I was observing. I've marked the feature most observers are familiar with, Mare Crisium, the large, dark circular patch near the bottom of the image. The crater I'd ran across that had the strange linear feature was the crater Petavius, which I've also marked.

You may well be familiar with Petavius, but I have to tell you, I was not. And I was definitely not familiar with the odd feature that I saw that evening. It turns out, according to the Wikipedia Petavius Crater article, that the odd, radial spoke feature is a large rille. Apparently, lunar orbiter pictures show the floor of Petavius crater to be full of odd cracks and unusual scars.
NASA/GFSC/Arizona State University Photo

At right is a clear image of the spoke feature I saw. This is a satellite photo, taken from a near overhead position. Viewed through a telescope from Earth, the crater appears more elongated, given that it's seen from a more peripheral angle.

This photo, and others, are featured at Lunar Networks. You can see other strange features in this crater, like the grove that rings the inside left rim of the crater. Another, more sinuous rille is seen moving upward from the center peaks.

So for me it's a mystery solved, but also a reminder that many exciting lunar features lurk near the moons periphery. Features that I've tended to overlook.

The cause of the many odd features on the floor of Petavius remain a mystery for scientists who study the moon. Usually, impacts like that which likely caused Petavius, break the lunar surface and allow lava to flow and cover the basin floor. For some reason, with Petavius this lava coating was incomplete, leaving a major break in the crater floor exposed.

If you're already familiar with Petavius, you're probably laughing at my naivete. But if you're not familiar with Petavius, try looking at that region of the Moon shortly after the lunar phase that exposes it. See if the linear feature of the rille jumps out at you like it did me. I'll certainly be looking for it again. But note that like the Straight Wall, the Petavius rille is a feature that's only easily visible when the Sun angle throws a shadow on the bottom of the rille. If you observe Petavius much after the Moon's crescent phase, that area will be too evenly illuminated to reveal the rille.

And one final point. I remind you that I was able to pick out the illusive rille with my modest 60mm refractor. So if you have a telescope, even a small one, you can likely see the Petavius rille -- when the time is right.

Sunday, May 4, 2014

Mars Observing Hints

If your astronomical juices get flowing every time Mars comes near Earth, then perhaps it's time you learned a few Mars observing tips. After all, you wait over 2 years between Mars oppositions, and Mars only stays near max apparent size for a scant few months each time. So get ready.

Go Long

First suggestion: "Go Long!" 

Say what?

I mean pick an instrument out of your telescope arsenal that has a long effective focal length. The longer, the better. Why?

For one, it's to get the most magnification out of your eyepiece selection. Mars isn't a gas giant after all. At best Mars gets as close as around 35 million miles to Earth, and that's not often. Mars is only a tad over 4000 miles in diameter, about half the size of Earth. And when at closest approach, it doesn't stay around long. Earth laps Mars in their respective orbits and rapidly pulls ahead.
Mars, when on the opposite side of the Sun from Earth, is only about 4 arc-seconds in apparent size. That's about the angular size of a 4 mile diameter crater on the moon. That's small. Don't expect to see much detail when Mars appears that small.

At best, on the infrequent very close approaches, Mars can get to about 25 arc-seconds in size, a size that though small, can yield some tantalizing details to the determined observer. Usually, Mars ranges from around 12 arc-seconds to perhaps 20 arc-seconds at opposition. At the time of this writing, 2014, Mars presented a disc of only about 15 arc-seconds at opposition. So, you'll need magnification, at least 150 times. 200 to 350 times even better.  

Another thing about long focus telescope is that optics of longer focal lengths tend to have less optical anomalies, mostly because long focus optics are easier to make accurately. Longer focus optics in refractors have less color distortion, and in both reflectors and refractors long focus optics have less curvature of field. The depth of field of long focal length telescopes makes it easier to achieve optimal focus since focusing isn't as sensitive. And as it happens, longer focal length optics, because of the ease of construction, tend to be cheaper than short focal length optics made well enough to perform at high magnification.

One thing in your favor is that Mars is quite bright. It's not that much further from the Sun than Earth, so it gets lit up pretty well. So you can push even a small telescope to at least 200 times and still see a nice image. I recently spent an evening looking at a 15 arc-second sized Mars image with a 60x1000 mm refractor, and was able to push it to 200 times without difficulty. The image was amazingly good. I was able to easily spot Syrtis Major, and some hints of other features near the opposite Mars pole.

What are some good telescope options?  A long focus refractor will do nicely. It can be as small as 60mm, like my Long Focus Carton Refractor that I constructed from parts. Most of today's 60mm scopes, while good in optics, come with pretty shaky mounts, so shop carefully or consider making your own mount. You may prefer something a bit bigger than a 60 mm telescope,  say the Orion 9024 AstroView 90mm Equatorial Refractor. On it's equatorial mount, it might be a more capable long term choice.

Long focus Newtonians make excellent and inexpensive planetary telescopes. A six inch f/8 DOB, like the Orion 8944
SkyQuest XT6 Classic Dobsonian Telescope, makes a great all around performer, and performs on the Moon and Planets very well. Even a 4.25 inch DOB, like the Star Gazer Steve 4 1/4 Inch Planetary DOB will do a respectable job on planets.

Maksutov Cassegrains like my ETX 90 work well on Mars and other planets as well. A bit bigger Maksutov that's attractively priced is the Celestron NexStar 127SLT Mak Computerized Telescope. I know, I said go long, and the NexStar 127 or its cousin the Schmidt Cassegrain NexStar 125 appear to be very short. But because of their Cassegrain designs, these telescopes have very long effective focal lengths, great for high magnification. It's only the optical tubes of the Cassegrain designs that are short.

Go High

If possible, observe Mars when it's significantly higher than the horizon, at least 30 degrees. The higher Mars is in elevation, the less atmosphere you are trying to observe through, and the better chance the image will be stable. Take a look at the Seeing And Transparency tutorial to see an animation that shows what poor seeing looks like. Transparency, or how dim of objects you can see, is not particularly important in Mars observing. Sometimes good images occur even when transparency is poor. But you definitely need the good seeing of a stable atmosphere.

Go Late

Mars: ETX 90
If you can, observe late. Of course, you have to observe when Mars is up. But early in an opposition when Mars is rising late, you'll likely get better views. After midnight, the atmosphere often settles down from the churning heat of the day. I obtained my best Mars photo, shown at left, by taking it past midnight at 3 in the morning.

In truth, I started much earlier in the morning than that, perhaps around 1 in the morning. But I was using Barlow projection into a Modified Webcam Astro-Camera, and the field of view of the combination was very small. It took me the two hours to finally get Mars visible on my computer screen so that I could take the photo. But it was worth it. Not a bad image for only a 90mm telescope, yes?

The photo does illustrate that you can definitely get decent results with modest optics, if they are of good quality and seeing is good.  On that 2003 opposition, Mars was near it's max possible 25 arc-seconds in apparent size. Thus the amazing detail for the small telescope.

Be Patient

To wring out the most planetary detail, be patient. For one, your telescope may take awhile to acclimate to the outside ambient temperature. For another, the atmosphere, sometimes even on an unsteady night, may have periods of clarity. So keep looking for those windows of opportunity. Also, once you've noticed the most obvious details, start looking for more subtle ones. You'll be amazed at what you eventually see that you didn't previously notice.

Use A Color Filter

This is not a necessity, but color filters, like those in the Celestron 94119-10 1.25 " Eyepiece Filter Set, can be a real help. Red is the color often recommended for Mars, as the red of the planet will go through the filter, but the darker areas, which can have a blueish tint, tend to get blocked. Thus the non-red areas look darker. On the subject of color filters, yellow is often recommended for observing Saturn and Jupiter.

Try Sketching 

I'm not a particularly good sketcher, though I've had my moments. The images above show one of my best efforts. On the left is a sketch I made of Mars during the very favorable 2003 opposition. I used my 6 Inch Planetary DOB to make the the observations for the sketch. The image on the right is the same as the previous Mars photo. It's the image taken with my ETX 90, and it was taken about 2 days after I did the sketch. You can see that I managed to sketch the major features fairly accurately, as confirmed by the ETX 90 photograph.

You might check out this Astro-Sketching tutorial for some hints about that particular skill. The sketcher in the tutorial took on Moon craters, a much more difficult topic for sketching.

So why sketch? For a couple of reasons. One, you document your session, and some years later you'll enjoy reviewing your sketches, as well as showing them off. Two, it will make you a more attentive observer, able to eke out those elusive details.

Try Photography

Planetary photography is not particularly easy, other than the occasional times you might be able to hold a digital camera up to the eyepiece and grab an image. Even that is hard enough, and doesn't give the best results. My best images have been obtained by using converted Webcams, both a homemade one and a commercial one like the Orion 52175 StarShoot Solar System Color Imaging Camera IV . I have an earlier design, the Celestron NexImage. With it I've managed to get some very nice Moon and Planetary pictures.

But planetary photography is meticulous and time consuming work. The main difficulty is getting the planetary image to show up on the small CCD of the camera. If you don't have the system in relatively good focus, you can have a planet go right through the field of view and not even see it. Once you get a planet into the field of view, obtaining images isn't so bad. My procedure is to take movies (avi files) of perhaps 20 seconds long. The camera takes 10 pictures per second, so that gives me about 200 images to work with from each avi file.

I use a Yorick Language program I created to process frames from my avi files. The program lets me find the highest correlated frames of the movie, then align them and average them. The final output can have contrast adjustments made to bring out the most detail. More time is spent at the computer working with the images than is spent taking them. A program that's available online for processing digital images is RegiStax. It is similar to my Yorick program. It has more features, but I enjoyed the challenge of writing the Yorick program.

The tedious aspect of taking and processing planetary images is another reason you may want to consider sketching. With sketching you get to spend your time observing, and not twiddling with equipment. Even if your sketches come out worse than you'd hoped, you'll still have the enjoyment of having had some great observing sessions.

Hopefully some of the suggestions will help you get the most out of your next Mars opposition. Many of the hints work for Saturn and Jupiter as well.  And the biggest suggestion I can make is to observe as often as you can. You never know when an exceptional night offers up marvelous observing, and it would be a shame to miss those rare opportunities.

Thursday, May 1, 2014

A Telescope Dynamic Duo

Tycho Through 6 Inch Newtonian
At left you see a Tycho photograph I'm pretty proud of. I took it with my 6 inch f/5 Newtonian and an inexpensive Webcam Astro-Camera. I processed about 50 images with a Yorick Programming Language program, averaging the best of the images.

Now that I have whetted your astronomical appetite, I'd like to tell you a tale of two telescopes, one of them being the one used for this photograph.

In The Beginning

Two score and 10 years ago, I got my first big boost in my pursuit of astronomy. I obtained, with the help of my high school principal, parts for a 6 inch f/12 Newtonian telescope. The parts included the mirrors, a mirror cell, an aluminum tube, a secondary spider, and a focuser.

The tube, as it turned out, was barely long enough, so I had to add a bit of an extension. But with the help of a high school friend, I was able to figure out how to assemble the parts and complete the telescope.

I constructed a somewhat crude but functional equatorial pipe mount for the instrument. The mount used smoothed black pipe for shafts, and bronze sleeve bearings.  It was a bulky and heavy mount, just big enough to hold the lengthy telescope.I needed a small step ladder to view through the telescope when viewing high elevation targets.

Then I purchased a copy of Making Your Own Telescope by Allyn J. Thompson. Initially I bought it to get ideas for a better telescope mount, but got excited about the idea of grinding my own mirror. Allyn's book is addictive that way.  He mentioned in the book that a six inch f/4.5 Newtonian would make a great partner for a long focus Newtonian. With this complimentary pair, one would be well equipped to view anything from wide star vistas to minute planetary detail, without compromise.

So I purchased a mirror kit and proceeded to use a summer to make a short focus Rich Field Newtonian. At the time, I didn't know enough ATM to realize that I needed a larger secondary than the one that came with the kit, which was more appropriate for an f/8 mirror telescope. But the objective mirror I ground came out well, probably about 1/4 wave.

Then, I got sick. Yup, I caught a serious case of aperture fever, as indicated by the Aperture Fever t-shirt design at left. Aperture fever is an insidious illness that drives amateur astronomers to aperture excess.

I bought a 10 inch mirror that seemed to satisfy my compulsion. That started me on a new project, even though I'd never realized the benefits of my six inch telescope pair.

I had to adapt my f/12 tripod to accommodate my 10 inch project, which more or less removed the long six inch from contention.

Then, after a couple of post-college house moves, I grew tired of hauling numerous telescopes around. It seemed that the 10 inch was the keeper at the time.

So, somewhere back there I sold my complimentary pair of six inch telescopes -- both of them. Yeah, it was a dumb thing to do.

Forward To The Past

Now, some 45 years later, I'm back to near the optimal telescope pair that Thompson suggested. I have a six inch f/10 DOB (shown on the left), and a six inch equatorial mounted Rich Field Newtonian (shown on the right).  The f/10 DOB I purchased as a custom kit from Stargazer Steve Dodson. The f/5 Rich Field was obtained as a unit from Discovery Telescopes. Alas, Discovery doesn't sell the model I have anymore, but the Celestron 31057 Omni XLT - 150 is very similar.

The Long And Short Of Newtonian Telescopes

There are ample articles about the differences in design and characteristics of long and short focus Newtonian reflectors. I'll only cover the design and operational differences briefly here. After that I thought it might be interesting to talk about differences between my two telescopes in a different dimension. The other dimension concentrating on the fact that one telescope (the Rich Field) is an example of a mass produced product with concentration on popular appeal and convenience, and that the other (the f/10 DOB) is an example of a telescope produced by hand by a master in ATM.

As you likely already know, as to basics these two telescopes are fundamentally similar in both being of Newtonian design. But because of their vastly different focal lengths, the two have significantly different characteristics.

The f/5 Newtonian Optical Characteristics

The f/5 Rich Field is much more typical of today's Newtonian market. Longer focus Newtonian telescopes, even the venerable f/8, are hardly sold anymore except as Dobsonians, like the Orion 8944 SkyQuest XT6 Classic Dobsonian Telescope. Even most Dobsonian telescopes, especially in larger than 6 inch size, tend to be made in relatively short focal ratios. This again is to make them as portable as possible.

The f/5, as is necessary, has a larger secondary than does the f/10. The f/5's secondary is about 30% the diameter of the main mirror. In operation, this means that while it gives twice the field of view of the longer telescope, its larger secondary reduces the contrast it can deliver. The f/5 also shows noticeable coma on the outer 30% or so of the field of view.

It's positives are it's portability and the marvelous wide star fields with which to wow you (and it does -- wow you I mean). Objects like the Beehive cluster, the Andromeda galaxy, the Pleiades, and the Double Cluster in Perseus are the types of items that only wide field telescopes like this can give in all their glory. Thus the commonly used name: Rich Field Telescope, or RFT.

The f/5 requires meticulous collimation to provide sharp images of high resolution targets. But when properly aligned and cooled down, the f/5 can perform admirably, as the initial Tycho photo demonstrates. To get consistent high resolution performance, I resorted to purchasing a Celestron Collimation Eyepiece, which has helped immensely.

The f/10 Dobsonian Optical Characteristics

The f/10 DOB is strong where the f/5 Rich Field is weak. It has a very small secondary, one only 16% of the diameter of the main mirror. This helps it deliver high contrast images competitive with like-sized refractor telescopes. It is less picky about alignment, and offers a flat field -- no coma. The moon, planets, and double stars are targets where it excels. In fact, other than field of view and portability, it performs better than the f/5 in about every optical category.

This quick summary is fairly common knowledge. But that other dimension I mentioned, I think is interesting. Now I'm not out to bash makers of mass produced telescopes. I love my f/5 Rich Field, and in fact it's my most used telescope. But there are some things of which to take notice.

Commercial Design VS ATM Design

A detailed examination of these two telescopes, fairly typical of their respective types, shows what differences in design occur between mass produced telescopes and hand crafted ones. The differences, at least for my two telescopes, don't relate to how sturdily they are built -- both are fine. The differences don't relate to how reliable they are or how long they'll likely last. No real difference there.

The differences have to do with what I'd call optical efficiency. That is, getting the most performance out of the optics. The following descriptions will reveal what I mean.

A Typical Commercial Short Focus Newtonian

I'll start with the short focus Rich Field Telescope. It is a Chinese import, but with American made optics from Discovery Telescopes. The optics are very good. It was my decision to purchase this telescope with a soda lime glass mirror. For a somewhat larger price I could have had a Pyrex mirror. I assumed at the time that in a six inch size, mirror cooling time would not be much of a factor. I think I was wrong.

The f/5 has a rolled steel tube. It's only 7 inches in diameter, which isn't that great for air flow. It also complicates focusing. When I received the telescope, I found that when using short focal length eyepieces or my Barlow, the focuser would extend into the optical path. Not good. Had the tube been a larger diameter, that likely would not have been the case.

6 Inch f/5 Eyepiece End
In the image at left you can see the focuser. It's racked all the way down for storage, but with short focal length eyepieces, it still extended nearly an inch into the optical path. I rather assumed that this played havoc with the diffraction pattern.

The tube is also just long enough to house the optics. The eyepiece end of the tube barely extends past the secondary mirror. This can allow stray light to enter the eyepiece. You can see by the image at left that the spider is just within the lip of the main tube.

The secondary mount (spider) was the three-strut mount shown in the image at left. The struts on this spider were nearly a quarter inch thick. This literally blocked about 4% of the incoming light, and caused 6 horrible broad spikes radiating from all bright objects. This diffraction tutorial gives more details about the effects of different type secondary mounts on diffraction.

The large three-strut spider made a hideous image of the bright planet Mars, making it difficult to see features near the planet's limb. Double stars with bright primaries could easily have dimmer companions hidden in one of the spikes.

6 Inch f/5 Mirror Cell
The objective mirror cell, a sturdy plastic mount that uses the push/pull type of alignment adjustment, was designed to seal off the bottom end of the telescope tube. I think this served the dual purpose of acting like a lens cap, and making the plastic mirror cell sturdy enough to do its job. But this hinders air flow, and thus significantly extends cool down time -- especially with the soda lime glass mirror.

So while the tube and tripod are plenty sturdy and functional, the optical efficiency is impacted by the minimal sized tube, the closed nature of the tube, the extension of the focuser into the optical path, and the thick-strut spider. The emphasis on making the smallest, most portable six inch reflector possible came at some cost.

A Typical ATM Master Telescope Design

So now consider the Stargazer Steve DOB design. The f/10 planetary DOB has an over-sized sonotube. Sonotubes are renowned for their thermal characteristics, and the tube for this particular telescope is over 8 inches in diameter -- what ATM experts suggest. The air currents that crawl along the inner wall of the tube are out of the optical path.

The mirror cell does not block off the tube, but allows ample air flow through the tube so the mirror can stabilize more quickly. The objective mirror is Pyrex on this telescope, and the cool down time difference between it and the f/5 mirror is significant, being only about 1/2 hour for the f/10, and well over an hour for the f/5.

6 inch f/10 Eyepiece End
The eyepiece end of the tube extends at least 8 inches past the secondary, which helps block out unwanted stray light.

The diagonal mirror is minimum size for the telescope length, reducing diffraction issues to a negligible amount.

The secondary mount (spider) is of a thin metal curved design. Minimal light is blocked, and there are NO diffraction spikes. This helps deliver refractor-like performance.

The Difference Is In Optical Efficiency

So when you compare the aspects of the two telescopes relating to the optical path and optical environment, you can see that what an ATM master delivers gives about the best performance out of a set of optics that you can possibly expect. The mass produced telescope delivers a solid instrument with fine optics in a desirable and popular package -- but not one with optimal optical efficiency.

The Silver Lining 

The good news?  Since the f/5 is of the simple Newtonian design, I was able to tune out most of the foibles. I did that on my f/5 telescope, leaving it still a very portable and easy to use telescope, but with significantly better optical efficiency.

I started with the focuser issue. I moved the objective mirror forward about 3/4 of an inch, which was the minimum necessary to let the focuser stay out of the optical path when using even my shortest focal length eyepieces.

I put black flock paper on the inside of the telescope tube, which with it's total non-reflectivity, helps mitigate any problems from stray light. One could also add an extension to the eyepiece end of the tube, but this would impact the portability and ease of transporting the telescope.

6 inch f/5 Curved Spider
I removed the thick three-strut spider and replaced it with a thin, curved metal spider. Now the amount of light being blocked is only about 1%, and there are no spikes when looking through the telescope, even on the brightest targets. You can see the curved secondary mount in the image at left. Now isn't that better?

I was going to replace the tube with a larger diameter and longer sonotube, which would have cut down on mirror thermal instability, but would have impacted portability. I was initially willing to make that trade off, but I didn't want to risk damaging the objective mirror. The mirror is glued to its current mirror cell, so I decided not to risk trying to remove it. Since the current cell would not adapt well to a larger sonotube, I scrapped that upgrade.

So I still have the rolled steel tube, and the mirror cell still seals off the bottom end of the tube. It mainly impacts cool down time, which is usually of little consequence since the telescope is primarily used for viewing wide star fields at low power.  If I want to view a mix of stars and planets on any given evening, putting the planets last on the list usually provides the required hour or so of cooling time necessary for reasonably good viewing.

Can You Improve A Well Built DOB?

6 inch f/10 DOB Wheels
As to the f/10 DOB, there wasn't much I could do to improve it, optically at least. I did add small brackets along the bottom of the base so that I could slip an axle with wheels in place. This way I don't have to lift the telescope off of the base and move the instrument in two trips. I just roll the telescope into place and slightly tip the base and remove the wheels.

Even with the wheels, however, my DOB wasn't any easier to find star targets with. Not nearly as convenient as the clever push-to telescopes like the Orion 10018 SkyQuest XT8i IntelliScope Dobsonian Telescope. The Orion IntelliScopes use angle encoders and a small computer to tell the user where to point in order to find objects. Thousands of objects are available in the computer. For those of us who are not effective star-hoppers, this is a magnificent convenience.

Failing that level of convenience, I was able to add setting circles to my f/10 DOB. It took me awhile to figure out how to do that, and in the end I didn't figure it out as much as copy the techniques of another person at the Manual DOB Setting Circles webpage. I had to alter the designs a bit to fit my mount, but found the basic ideas to be of great help.

6 inch DOB AZ circle
At left is an image of my DOB azimuth setting circle. I printed out a precise circle with a Perl language program, making the circle as large as would fit my mount. I lifted off the rocker box and glued the setting circle on the base. Then I cut a slot in the rocker box of the mount to reveal a section of the setting circle.

I added a pointer (just visible in the image), and by peaking into back of the rocker box I can set the telescope azimuth. I get the azimuth and elevation from either a calculator program I use, or a laptop program. One could use a computer planetarium program on a laptop.

As long as the base is setting reasonably level, either of my programs will let me align to north, and enter an azimuth bias if the mount isn't perfectly aligned with Polaris. The azimuth bias feature helps in that I don't have to take inordinate time to perfectly align the telescope base with north. I just get within a few degrees and inform the respective program of my alignment error.

Then either program will let me choose an object out of one of a few target lists, like the Messier list. Once a target is selected, either program will provide me with the correct azimuth and elevation of the target -- accounting for the azimuth bias.

6 inch DOB Elevation Circle
 Elevation is simplicity in itself. I'd have never thought of it without the article I found. I use a dial level that happens to have a magnetic base. It was easy to affix a small metal plate to a side of my hexagonal telescope cradle because of its flat surfaces. I can plop the level onto the plate and be off and running.

So, yes I was able to improve, slightly, my well designed DOB. But not pertaining to it's optical efficiency, which was already optimized. However I have  made the instrument easier to use, and decreased significantly the challenge of finding star objects. With my version of computer assist, inspired by the commercial push-to design, I can use the telescope to easily locate and observe any of hundreds of targets.

Hopefully you've found the comparison of long and short Newtonian telescopes informative. And if you're a tinkerer, perhaps you've been inspired to add some features to either your commercial or hand-made telescope. For me, like many of you, I find tinkering to be part of the entertainment of the astronomy hobby.