Summary Remarks
Introductory Note
ST80: A Scope in Progress
Doublet Pair Orientation Procedure
Point Source Performance of Lens Rotation Corrected ST80
Field Performance of Lens Rotation Corrected ST80
In Sum:
Additional Discussion by the Editor
About the Author
About the Editor
About Astro.Geekjoy

Summary Remarks

Mechanically sound, optically fair, easy to use and extremely portable - a fine value at less than 200 USD. Good wide low power views. At 43mm (F9.3) a fine near focus and terrestrial scope - otherwise (at F5) moderately afflicted by chromatic aberration. Requires upgraded diagonal upon purchase and deserves improved eyepieces post-purchase. Lunar-planetary, double-star, and flat field performance may be improved significantly by the owner - despite a complete dearth of user collimable adjustments. Aperture-limited on deep sky (Messier Catalogue primarily). Makes an excellent starter scope, yet due to its extreme portability and wide-field capabilities remains the "quick-look" scope of choice for all but the most discriminating and well-heeled of optophiles.

Comes with a 6*25mm erect image finder scope, dovetail mounting bracket and two inexpensive 3 element eyepieces. May ride on just about any mount. A full description of the as-shipped version of the ST80 plus appropriate mounting options can be obtained at the Orion Binocular and Telescope website.

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Introductory Note

Jeff Barbour has thoroughly reviewed the Orion Short Tube 80mm F5 achromat. His articles, describing in detail what can be seen through the Short Tube 80 (ST80) and how to improve the performance of the ST80, can be found at: Sky Training the Pup and Argo, the Pup, and the World Jeff’s two articles are the best reviews of the ST80 I have found on the web. What follows is a consolidation of my work on and observations through the ST80 along with some of Jeff’s points previously made.

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ST80: A Scope in Progress

The ST80 is a substantial little scope at a very modest price. The value of the scope lies in the good quality figuring and polishing of the two elements of the air spaced achromat objective, a decent job of collimation and the better than reasonably good mechanical functionality. But, as Jeff has pointed out, one should not expect at this price that every effort has been made to ensure perfect collimation, planarity of the objective elements, proper orientation of the objective elements and perpendicularity of the optical elements to the optical tube. Probably, one should be satisfied if the ST80 arrives from the supplier providing ½ wave performance or better. An experienced observer is going to be able to tell that an ST80 is not visually optimized at ½ wave. Even with a very stable and clear sky, at ½ wave front error one will not be able to see much more on Jupiter than two cloud bands, Saturn’s Cassini division will be invisible and Mars' maria - even at opposition will be faint and indistinct. One will probably notice that a wall is hit at any magnification above 100X, after which views become mushy and are no longer pleasing.

Editor's Note: Through ultra-stable skies and a superb small-aperture scope of the highest optical figure and alignment image quality delivers very satisfying views on low contrast extended studies to .4mm exit pupil - some 200x in an 80mm scope. An 80mm scope that is just "diffraction limited" (1/4 wavefront error in yellow-green light) will hang in there to 160x (.5mm exit pupil). It is Astro.Geekjoy's contention that we, as amateur astronomers, have a responsibility to work with all scope suppliers and manufacturers to encourage at minimum "diffraction-limited" capability in all scopes sold to the general public. The reasoning for this lies in the fact that a good, clear look at the starry realm is perhaps the only promotion required of our High Art and Science needed!

At ¼ wave things are quite different. Under steady skies, an observer with normal vision will be able to see the Cassini division and a noticeable improvement of detail on Jupiter and the moon. Under good seeing conditions, most of us will not be able to tell the difference between views of celestial objects through a ¼ wave ST80 and a better corrected model. However, a scopist adept at star testing or double star observing is easily able to tell that the ST80 performing at ¼ wave is far from perfect. As such, intra and extra focal images of bright stars will betray the imperfection as will attempting to see any faint star near a brighter primary (such as Polaris). Also, she will notice that as the sky becomes more and more subtley unstable, views through a ¼ wave scope are more detrimentally affected than those through, say, a 1/6th wave front error scope.

There is a street light less than fifty yards from my front porch. Perhaps because of some micro-fractures or perhaps because of some etched fissures in the light’s cover, the street light presents a dozen or so nice bright point-sources of light, each giving a nice airy disk and diffraction ring(s) through a well corrected and collimated optic. There is also a telephone pole nearby with a dried sap excretion that, in sunlight, provides a number of nice point-source images. During the day I use these sap-sources to test the correction and collimation of an optical system. As night falls I will then use the street light. Upon receiving the ST80, I could tell that the in focus image of bright point sources was not as it should be. The central airy disk was oblong; not circular as it should be. Also, there was no diffraction ring visible on one side of the airy disk. On the other side were a number of arcs. Also, I noticed curious chromatic aberrations. On bright point sources and bright extended objects I could see a violet hue on one side and a greenish hue on the other. These aberrations led to the conclusion that the scope was not adequately collimated, that the elements of the objective were not properly oriented to one another and that there was a lack of planarity between the elements. All in all I would hazard a guess of a wave front error of between ½ and ¼ wave.

Not satisfied with these results I proceeded to attempt some improvement of the collimation, orientation of elements and planarity of the objective. I removed the objective lens cap. I then noticed that the objective lens cell is a very simple arrangement of lipped-cup that holds the air-spaced objective. The two elements are separated by three tiny paper thin spacers. The entire objective is fastened into the lens cell by a plastic ring that is screwed into the lens cell.

The trick with optimizing the performance of a telescope is to approach the work methodically. One needs to make a small change in the orientation or planarity or perpendicularity of the system and then reassemble it and check how a point source of light looks. Then one takes the system apart, makes the next tiny adjustment, reassembles the system and checks its performance on a point source of light.

I had no luck at all attempting to methodically rework the planarity of the elements. Jeff, in his articles, has outlined his successful attempt to improve the planarity of the objective elements of the ST80. Fortunately, I discovered that I did not need to adjust the perpendicularity of the system. What I was able to correct was much of the orientation deficiency of the elements and, by chance, some of the planarity problem. What follows is the methodical approach I took to improve the visual performance of my ST80 from its received condition of ½ to ¼ wave front error to a condition of about 1/5th wave front error.

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Doublet Pair Orientation Procedure:

The following procedure may be used by the owner of the telescope to significantly improve ST-80 scope performance. It will correct for any negatively reinforcing aberrations in the doublet pair. It will not result in perfect alignment of the scope vis-a-vis the mechanical components of the scope (what most amateurs consider to be "collimating" the objective to the eyepiece). Owner's using this technique must do so at their own risk. The rule being "If it ain't worth breaking, it ain't worth fixing."

1. Make or find a bright point source of light. This will be used to determine the appearance of the airy disk and diffraction ring(s).
2. Remove the objective lens cap.
3. There are three screws that hold the objective lens cell to the telescope tube. Pointing the telescope tube upward, remove the three screws and gently lift the objective lens cell out of the tube.
4. Holding the base of the lens cell in one hand (the cup side), gently unscrew the plastic retaining ring (which should be on the top side as it is unscrewed) and set aside.
5. Cover your free hand with a clean unscented Kleenex. Place your Kleenex covered fingers snuggly against the objective on the retaining ring side, with the fingertips touching the objective around the perimeter of the objective. Tip the cell upside down and let the objective slide out of the cell and onto your fingers. Place the cell down. The front objective element (the one usually against the retaining ring – a positive convex shape) is now resting against your fingertips separated by a Kleenex. Gently rotate the back objective (a negative concave lens) clockwise a quarter (90 degree) rotation relative to the front element. (The spacers should not come lose. If they do you will need to reposition them.) Now it is time to reassemble the objective cell and retest the visual performance of the optical system.
6. Gently lower the cup portion of the objective cell over the two elements till the cell sits snuggly on the back element. Using your fingertips (covered with the Kleenex) to hold the objective elements snuggly to the cell, tip the cell over so that the cup is down and the retaining ring screw-slot is pointing up.
7. Hold the objective cell in one hand. Remove the Kleenex from the other hand, pick up the retaining ring and screw it back onto the retaining cell.

It is especially wise to rotate the retaining ring counterclockwise till it “clicks” into the proper orientation to the retaining screw slot, and then to tighten the retaining ring by turning it clockwise. It is important to never “force” the ring tight. This may strip the retaining ring. This is especially so since both ring and slot are made of plastic. Screw the retaining ring on until you feel a slight resistance/snugness and then loosen the ring ever so slightly. You have obtained the proper tightness if you have loosened the snugness but, when you shake the cell, you hear or feel no rattle. EDITORS NOTE: If you over-tighten the retaining ring you will see evidence of pinch when observing stars. This will be seen as a “tri-partitite shield” shape in the eyepiece.

8. With the telescope tube pointing upward, lower the cell onto the tube, insert the three screws and tighten them until a slight resistance is felt.
9. Proceed to test the collimation and orientation of the optic by looking at the bright point source, observing the appearance of the airy disk and diffraction ring. A magnification of 100X or more is needed. This can be obtained with a good 4mm eyepiece or some combination of eyepieces and Barlow lens. What you are looking for is a circular Airy Disk surrounded by one or more concentric circles around the Airy Disk. This/these Diffraction Ring(s) ideally will appear equally distant from the Airy Disk all the way around. They will also display uniform illumination.
10. Continue repeating steps 2 through 9 until the appearance of the Airy Disk/Diffraction Ring(s) begins to worsen, at which point, repeat steps 2 through 9 with the exception that now you want to rotate the rear element counter clockwise each attempt by 1/8th of a turn. Continue by this amount and in this direction until the appearance worsens again. The ideal orientation of the elements now lies somewhere between the two points where the appearance worsened. Through trial and error, by repeating steps 2 through 9, going clockwise again, attempt to find the ideal orientation of the two elements relative to each other, existing somewhere between the two points where the appearance worsened.
11. Step #10 should, in all likelihood, provide the ideal orientation for your objective elements. There is a possibility, however remote, that a somewhat better orientation can be obtained at an orientation around 180 degrees (half way around) from the point of present orientation. If you wish to pursue this possibility, repeat steps 2 through 9 by rotating the rear element 180 degrees, testing the appearance, and then repeating steps 2 through 9 with ¼ (90 degree) turns. Once the appearance starts to worsen, then follow the directions iterated in step #10.

Editor's Note: Scope's using the ST80's doublet pair (sourced from China) may be purchased from other distributors. They may be of a price comparable to Orion's, somewhat less, or much more. More expensive models are likely to have already had the doublet pair pre-rotated to give optimal lens alignment - as described above by Otto.

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Point Source Performance of Lens Rotation Corrected ST80:

Having performed the steps listed above on my own ST80, the appearance of the Airy Disk/Diffraction Ring(s) now is closely approximated by inputting Telescope parameters of 80mm of aperture, a focal ratio of 5.0 and a coma of .1 to .15 into Cor Berrevoet’s Aberrator Program and creating a Star image at 367X with a +1 magnitude enhancement. The result is a circular Airy Disk with the Diffraction Ring(s) more pronounced on one side than another.

The actual appearance of a bright point source of light through the eyepiece of my ST80 is very similar to the above described Aberrator results with the exception that at about 55X of magnification per inch (160X) the Airy Disk appears oblong. The interpolation of this obliquity with the wave front results indicated by Cor’s program indicates a total wave front error of between 1/4th and 1/5th wave.

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Field Performance of Lens Rotation Corrected ST80:

Once a reasonable well-figured achromatic doublet element-pair is properly lens rotated:

• The Cassini Division is easily seen appearing as a slightly worn graphite pencil line at the ansae of Saturn’s rings.
• Jupiter shows the two main belts, darkening in the polar areas, some indication of variations in the main belts and the presence of the Great Red Spot.
• At 32X and above all four elements of the Trapezium are easily visible.
• A 30mm Rini Modified Plossl provides a sharp and mostly flat four degree field of view. Through this eyepiece, the crescent to first quarter moon is stunning and creates a feeling of dis-equilibrium. One sees a three dimensional globe, perfectly immobile, hovering unsupported in space. This view of the crescent moon provides me an indescribable awareness of the reality of gravity in the immensity of space.
• The entire Pleiades can be seen through the 30mm eyepiece. In fact, two entire Pleiades could be seen. The entire belt of Orion can be seen in one and the same field of view. Almost all of the Hyades are seen at once.
• Saturn’s moons Titan, Rhea and Tethys are seen.
• M81 is easy. M82 reveals its spindle shape. NGC 3077 is glimpsed with averted vision. NGC 7789 is faint but visible. Galaxy IC10 at magnitude 11.8 is just visible with averted vision.
• Polaris B is visible at 80X and easily so at 114X. Rigel B is always seen. The 3.1 arc second separation of the 4.9 and 7.2 magnitude stars of ADS1140 is easily visible as a gray dot on the first Diffraction Ring. The same is true of 1 Aries (5.8 and 6.6 magnitude stars with a separation of 2.9 arc seconds). Gamma Aries is easily resolved at 32X (3.9 and 3.7 magnitudes with a separation of 8 arc seconds) and elongated at 13X. Albireo’s components are gold and blue, not straw-yellow and pastel-blue. Epsilon Lyrae are two pairs of headlights. Castor is a nice bright pair of stars displaying unequal sized airy disks. Pi Aquilae (magnitudes 6.4 & 6.8 at 1.4 arc seconds) is just resolvable under the finest of conditions and highest magnifications.
• The crater Ptolemaeus reveals its major indentations. The Domes of Hortensius and two craterlets inside Plato are detectable as unresolved brightenings during optimal solar illumination.
• The views of Saturn are sharp at 80X, 114X and 160X (UO orthoscopic eyepieces). Terminator craters on the moon continue to present a fine appearance even at 80X per inch of aperture (250X). The moon shows no chromaticism at 13X. However, a violet fringe on one side and a green fringe on the other is visible at 32X, 57X and 80X. Interestingly, when a Barlow is used to up the magnification to 114X and 160X, no chromatic aberration is seen on the moon. This may be because the Barlow effectively turns the scope into an F10 system.

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In Sum:

The ST80 will not see the faint fuzzies that a light bucket can see. The ST80 will not show the level of detail on the planets visible through an optically excellent maksutov. The ST80 will not be able to present the icy-cold sharp contrast of the edge of the white moon against the black sky presented by an apochromatic refractor. The ST80, as Jeff has stated in one of his articles, can frame some celestial objects better than many other larger apertured scopes, catadioptric scopes, optically exquisite maksutov and apochromatic refractors. Further, the casual stargazer will see more celestial wonders and terrestial offerings through the ST80 than through larger and better scopes because the ST80 is so very easy to assemble, move and store. It was Alan MacRobert who wrote that a 3 inch scope will show more than a 12 inch scope because it will be used more.

• In the right low power eyepiece, the moon will hang suspended and motionless in the sky, creating a visceral awareness of the perfect celestial balance of gravity and centrifugal force found in space.
• With one hand one can pick up the whole scope, mount and all. The other hand is free to hold the screen door open. Because of this ease of use, one often takes advantage of spontaneous urges to observe.
• One can see a perfectly focused miniature ringed Saturn hovering amid the embracing silhouette of tree branches and leaf buds.
• As twilight envelops the earth, one can easily follow gold and red jet exhaust contrails, or the daytime aspect of hot air balloons floating through the sky.
• One does not feel a need to run out whenever the sky is clear. When one has a large aperture scope or a scope of near optical perfection one often feels the obligation to take advantage of any clear sky so as not to pass up the rare opportunity of a sky steady enough to optimize such a scope. As Jeff has pointed out elsewhere, the number of optimized evenings for the ST80 are significantly more than through larger and better scopes.
• One is perfectly pleased to look at the moon’s craters, or the moons of Jupiter, or sunspots…through the glass of a living room window. The lack of optical perfection allows one to use the conditions presented rather than to only use perfect conditions.
• Birds on the telephone wire, a spider on its web, a chain link fence seen from the edge covered in snow and, under the right conditions, microbes in pond water are thoroughly enjoyable.
• Between tasks and chores one can take a quick peak at the sparkly details of ornaments hidden within the mysterious confines of a Christmas tree twenty feet across the room.
• A large dobsonian or a moderate sized catadioptric might take an hour to reach a thermal equilibrium that maximizes the potential of that scope when the temperature differential is 60 degrees. The ST80 will take about fifteen minutes. One needs no unheated storage shed, no cooling fan or sital substrate.
• There are apochromatic refractors in the 70mm range that can provide everything that the ST80 can provide as well as better resolution and crisper contrast on many objects. However, this type of performance will cost five to ten times as much as the price of the ST80. For a fifth of the cost, one receives 70-80% of the performance.
• 80mm short tube refractors have joined the six inch Dobsonian mounted Newtonian reflector and 7X50 binoculars as the most recommended observing instrument for the beginning stargazer or budding scopist.

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Additional Discussion by the Editor:

There are many things 200 dollars American can buy new – a fine apochromatic refractor of 80mm’s in aperture is not one of them. However 200 dollars will buy a dobson mounted 4.5 inch F8 Newtonian. Such a scope is probably the best competitor to an ST-80 as a beginner scope on the new astro market. If one is to purchase a “once and only in a lifetime” scope and must choose between the two designs, the Newtonian – due to its greater magnitudinal reach may make the better choice.

But few astrophiles tend to keep a “one and only” scope in perpetuum. Where the 4.5 dobsonian might languish in the closet once a larger model is obtained, a fast achromat ( such as the ST-80) is likely to continue to see service throughout any amateur’s observing career.

The main virtue of such an achromat (vis-à-vis the 4.5 dob) is not its portability – but its superior flat field performance. In fact such a scope is capable of framing the entire Cygnus Loop in a single 3 degree field… This is not possible – even in a relatively slow Newtonian – given significant off-axis coma outside one degree fields.

One might argue that a smaller Maksutov-Cassegrain would be a superior choice to the fast achro. And this would be true - should such a scope exceed 4 inches in aperture. However, scopes of the MCT architecture have longish focal ratios – typically F12 and beyond. This limits wide field viewing. Meanwhile such a scope is likely to be usurped once its purpose (typically lunar-planetary observation) is eclipsed by larger models costing significantly more than what most beginning observers are willing to invest.

It is for this reason that Astro.Geekjoy enthusiastically endorses the acquisition of the Orion ST-80 and Celestron WA-80 as examples of a “first and lasting” scope architecture for all amateurs – if not for all times…

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