Book Mania

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.

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