Otto Piechowski's Review of the Intes Micro and Orion Shorty Barlow Lenses
Barlow Physical Descriptions
Performance Comparison
Practical Usefulness
Decision and Conclusion
Credits
Appendix I: Magnification Factor Algorithm Formulae:
Appendix II: Jeff Barbours Comments
This report contains a side by side comparison of the 1.25 inch Orion Shorty 2X Barlow and the 1.25 inch Intes-Micro 2X Barlow. Both are inexpensive. Both are good quality. Either would be a welcome addition to an assortment of good quality eyepieces for a good quality telescope being used by an amateur stargazer-scopist.
Barlow Physical Descriptions
The Intes-Micro is 82 millimeters long. The clear aperture of the negative lens element is 20 millimeters (wide). A deep blue lens coating is apparent. Eyepieces and accessories are held very snuggly by a compression ring, tightened and loosened by a single exterior screw. It is a single solid unit, the lens held in place by a thin threaded retraining ring. The lens consists of two elements glued together. The edges of the lens are well darkened. This Barlow was purchased used for $30. It was in pristine shape. It had no markings indicating the make or the magnification factor. The seller stated that it was a 2X Intes-Micro. Dozens of attempts to find product information about an Intes-Micro 2X Barlow on the web revealed nothing. Though sold by a couple telescope dealers, it does not seem to have had an extensive production run.
The Orion Shorty 2X Barlow is 78 millimeters long. The clear aperture of the negative lens element is 22 millimeters (wide). A light blue lens coating can be seen. Purchased new, the edges of the lens were not darkened. Eyepieces and accessories are held in place by a single exterior screw. The Barlow consists of two parts; the tube holding the negative lens appears to be a standard eyepiece drawtube. The negative lens, also a glued doublet, is contained in a small cell that is screwed onto the back of this drawtube. This drawtube screws into the front part of the Barlow that looks quite similar to the eyepiece holder tube on many Meade, Celestron or Orion diagonals. In short, it appears that Orion has decided, wisely, to make the parts of the Shorty 2X uniform and interchangeable with normal eyepiece and accessory stock items. Originally selling for around $50, with shipping, this Barlow is now obtainable for around $35. Product information about this Barlow is available on the web. The unit is clearly marked as an Orion Shorty 2X Barlow.
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Performance Comparison:
The telescope used to compare the optical performance of both Barlows was an MK-67; the 150 millimeter maksutov-cassegrain made by Intes of Moscow. A good quality performer in itself, exhaustive comparative studies by Jeff Barbour and Cor Berrevoets have shown the MK-67 (and other similar telescopes) to be equivalent to apo-chromatic refractors of between 102 millimeters (4 inches) and 178 millimeters (7 inches), depending on the application. Details of these comparisons can be found at:
astro.geekjoy.com
The Intes-Micro provides an actual magnification factor of 2.67X (see Algorithm Formulae below for how this is determined). Compared to the Shorty 2X, the images it gives of Jupiter, Saturn and the moon are decidedly yellowish. In high contrast areas such as at the terminator of a quarter moon, there is significantly less glare visible than in the un-darkened Shorty 2X. Optically, this Barlow is well corrected and has a smooth surface. At a magnification of 1,300X (220X per inch or 8.7X per millimeter of aperture), though washed out, lunar detail is still easily visible, bright stars in a stable transparent sky reveal a well delineated airy disk and a clear first diffraction ring. However, there is a glow around bright stars that covers most of the field of view (FOV) in the eyepiece.
The Shorty 2X performs as advertised; providing a magnification factor of 2X (see Algorithm Formulae below for how this is confirmed). The images are noticeably whiter and bluer than in the Intes-Micro, especially on the moon, Jupiter and Saturn. When the negative lens is removed and the edges are darkened with a felt tip marker, the glare is significantly reduced when viewing high contrast areas such as the terminator of a quarter moon. However, there is still a bit more glare visible than through the Intes-Micro. At a magnification of 1000X (166X per inch or 6.6X per millimeter) detail is still easily visible on the moon and bright stars reveal a clear airy disk and first diffraction ring.
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Practical Usefulness
The decision to obtain a Barlow depends on its optical quality and the use to which it will be put. This use will depend on the desires of the individual scopist-stargazer. In my case, I wanted to be able to provide high magnification views using large focal length eyepieces for guests looking through the telescope, who might have some difficulty observing through, say, a 7 millimeter orthoscopic eyepiece. For example, the Barlow would allow basically the same view of Jupiter through an 11.4 millimeter eyepiece as through the 7 millimeter eyepiece without the Barlow.
Also, though the applications of high magnifications (over 60X per inch or 2X per millimeter of aperture) are few, there are some. In the MK-67 magnifications of 125X per inch (5X per millimeter) have been necessary and used effectively to observe the ring-hugging faint moons of Saturn or to elongate tight double stars; in both cases unobserved at lower magnifications. In this MK-67, this magnification required a 5 millimeter orthoscopic eyepiece and a 2X Barlow.
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Decision and Conclusion
Both are good. Both will serve the amateur well. Nevertheless, I have decided to keep and use the Shorty 2X.
The reasons for this choice include (1) I do not like the yellowish tone given to bright objects, (2) the magnification factor of 2.67X is excessive for my needs, and (3) the glow which covers a significant part of the FOV at very high magnifications.
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Credits
I wish to thank Joe Donahue, Pete Rasmussen, Jeff Barbour, Cor Berrevoets , Bill of Fort Worth and "T" of Ohio for their feedback and guidance in making this report available.
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Appendix I: Magnification Factor Algorithm Formulae:
How does one determine the comparative magnification factors of various Barlow lenses?
Take a fairly bright star. Though it is not necessary to use a star near the celestial equator, doing so will reduce the amount of time you will need to do this experiment. You will view the star using an eyepiece alone, the same eyepiece in one Barlow and the same eyepiece in the second Barlow. Though any eyepiece may be used, the shorter the focal length of the eyepiece, the better, it terms of the least amount of time needed to do this experiment.
Turn off the drive on your telescope if you have a drive. Starting just outside of the field of view (FOV), let the star drift across the FOV. Make certain that the star drifts across the middle of the FOV; that its line of transit crosses the exact center of the FOV.
Do this with the eyepiece alone. Do this three times recording the number of seconds it takes each time. Average the amount of time the transit takes. For example, in my telescope using a 7 millimeter eyepiece, the star transit took 40 seconds (timings of 39 and 40 and 41, added together equaling 120, divided by the number of timings (3), giving a result of 40).
Then, inserting the eyepiece into one of the Barlows, again time the stellar transit across the FOV. Do this three times and average. For example, using the Shorty 2X in my telescope with the same 7 millimeter eyepiece, gave a result of 20 seconds.
Since this is, what is called, a simple scalar comparison, one has now determined that the Shorty 2X actually gives a magnification factor of 2X; 40 divided by 20 equals 2.
Inserting the same eyepiece into the Intes-Micro Barlow, and conducting the experiment in the same way with the same star, a timing was derived of 15 seconds; resulting in a magnification factor of 2.67X (40 divided by 15 equals 2.67).
[Another way to look at this is; if one knows the magnification factor of one Barlow, then a comparison of star transit times between the two Barlows will also reveal the magnification factor of the second Barlow. For example, if the Shorty is known to actually have a magnification factor of 2X; one can multiply this magnification factor by the ratio of the transit time of a star in the Shorty by the transit time of the same star seen in the Intes-Micro. For example, 20 divided by 15 is 1.33. 2X multiplied by 1.33 gives a magnification factor of 2.67X for the Intes-Micro Barlow.]
How does one determine the actual magnification provided by a given Barlow used with a given eyepiece?
There are two ways to do this; one simple but based on more assumptions. The second a bit more difficult but based on fewer assumptions.
The simpler but less certain way.
One obtains the magnification provided by a given eyepiece by dividing the focal length of the telescope by the focal length of the eyepiece. Similar units of measurement must be used for both. What this means is that if I am measuring the focal length of the telescope in millimeters, I need to use a measurement of the eyepiece’s focal length, given in millimeters. Usually, these focal lengths are stated on the telescope or eyepiece or in accompanying literature. For example, my telescope is listed as having a focal length of 1,800 millimeters. If I use a 7 millimeter eyepiece, I divide 1,800 by 7 and obtain a magnification of 257X for that eyepiece. [Please note, at times, the actual focal length of the telescope is not given (e.g. 1,800 millimeters) but an F-number is given. The F-number is the focal length of the scope stated in a different way; i.e. the focal length of the telescope divided by the aperture (width of the objective lens or meniscus or objective mirror) of the same scope. For example, 1,800 millimeters focal length, divided by 150 millimeters of aperture equals an F-number of F12. Again, always be sure to use the same units of measurement; in this case, millimeters.]
Once one knows the magnification given by the eyepiece, all one then needs to do is multiply the magnification of the eyepiece by the magnification factor of the Barlow being used. If the Barlow has a magnification factor of 2X, then the magnification given by the 7 millimeter eyepiece with that Barlow is 514X.
The more difficult but more certain way.
One obtains the magnification given by the eyepiece by using the transit time of a star situated on or very near the celestial equator. A good example of such a star is delta orionis (one of the stars in the belt of the constellation Orion). One uses the stars transit time to determine the True-Field-Of-View (TFOV) of the eyepiece being used (or of the eyepiece/Barlow combination being used).
With the clock drive turned off, this star transits the FOV of a 7 millimeter focal length eyepiece in my MK-67 in 36 seconds. One should, of course, do at least three timings and average the result, in this case 35, 36 and 37, resulting in an average of 36. One then divides this 36 by 4 to obtain the TFOV of that eyepiece measured in arc-minutes; in this case, 9 arc-minutes is the TFOV (36 divided by 4 equals 9). One then divides the 9 by the number of arc-minutes (60) in a degree of arc and gets a result of .15 degrees. (4 divided by 60 equals .15). This .15 represents the TFOV of this 7 millimeter eyepiece measured, not in arc-minutes, but in degrees of an arc.
To obtain the magnification of this eyepiece, one then takes the "claimed" Apparent-Field-Of-View (AFOV) of the eyepiece (usually found as part of the product-information listed in a catalog or in supplied literature or on the website) and divides this by TFOV. The AFOV is almost always given in degrees of an arc. For example, the "claimed" AFOV of the 7 millimeter eyepiece is 42 degrees. To complete this calculation, the TFOV must be measured in degrees of an arc as well. In the case of the 7 millimeter eyepiece, the magnification is revealed to by 280X. (AFOV divided by TFOV: 42 divided by .15 equals 280X).
One can measure the exact magnification of the eyepiece/Barlow in the exact same manner. The transit time of this star is measured to be 18 seconds using the 7 millimeter eyepiece with the Shorty 2X Barlow. 18 divided by 4 is 4.5. 4.5 divided by 60 is .075. 42 divided by .075 equals 560X. [One caution is required here. On occasion the construction of a given Barlow "vignets" the eyepiece; that is, reduces the Apparent Field of View. Such a reduction, indicating Barlow vignetting, would change the magnification because the AFOV would now be less than the stated amount for the eyepiece alone in the product information. Thus, so that one is comparing apples to apples, one needs to determine that there is no vignetting; no AFOV reduction. "Eyeballing" can ascertain this well enough. All one does, is look through the telescope, using just the eyepiece and then using the eyepiece and the Barlow in combination. If the "size" of the circle (FOV) looks pretty much the same, there is likely, no vignetting.]
One benefit of this measurement of magnification using celestial equator star transits is that it possibly gives you a more accurate measurement of the true focal length of your telescope. For example, having determined that the magnification given by the 7 millimeter eyepiece in this telescope is 280X, one can then multiple the magnification by the stated focal length of the eyepiece. In the case of my telescope, 280 multiplied by 7 gives 1,966. That is significantly longer than the advertised system focal length of 1,800 millimeters. Another way of saying the same thing is that the actual F-number (focal length divided by aperture) is F13.1 rather than the advertised F12 (1966 divided by 150 equals 13.1). The difference between the claim in the product information and the reality is 8%. Though not desireable, this is not unusual. In the case of the MK-67, Bill Burnett and Mike Palermiti of Internet Telescope Exchange have noticed a product variation all the way from F11 to this F13. The benefit of a longer F-number is that it might create a slightly crisper/flatter view in wide field of view eyepieces. The drawback is that it reduces the true field of view. For example, if this system were F12 the TFOV would be .96 degrees of an arc. At F13, it has an TFOV of .88 degrees of an arc, a loss of over 8%.
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Appendix II: Jeff Barbour's Comments
Fellow observer Otto Piechowski and I share much in common. Otto and I both have an interest in observing all aspects of the night sky. We both use Intes Micro MK67 family scopes. (Mine is the Orion Argonaut.) And finally we both possess Orion Shorty Barlows. (Mine may be a slightly earlier version.)
In testing my Shorty, I've found that it provides a gain of 1.8x in magnification. In fact it may well be that all older vintage shorties (mine was purchased in early 2001), come in at about this value. (General knowledge of this probably resulted in Orion's releasing the newer, truer, 2x Shorty version that Otto owns...)
In comparing the Shorty to my only other barlow (a more expensive 3 element "Longy" apochromatic barlow also sourced from Orion), I've found perceptible differences in performance and utility. For one, the 3 element "Longy" gives a visibly flatter field of view. Due to its superior nulti-element coatings, and incorporation of an internal baffle, the "Longy" provides darker-background, higher contrast views as well.
Despite this, I've found that owning both barlows is essential to getting a full spread of magnifications through the F12 (1800mm FL) Argonaut, and fast F5 (400mm FL) ST80 used from Backyard, Boulder Creek. For instance, the "Longy" only accommodates focus through 150mm MCT Argo in the 3x "pre-diagonal" configuration. While the Shorty, may only be used in the normal "post-diagonal" 2x mode. Through the fast achromat (the "Pup"), both barlows must be used in tandem (the "longy" before the diagonal, and Shorty after). This is essential to achieve magnifications needed for lunar, planetary, and close double star observation.
One final virtue may be ascribed to the Shorty barlow. As Otto noted above, the negative doublet pair may be unscrewed from barrel. This allows it to be threaded into the filter region of any inch and a quarter eyepiece. In terms of my own Orion Ultrascopic series eyepieces, the resulting gain is roughly 1.3x. So for those with a "magnification coverage gap" in their eyepiece series this becomes an optional way to close it.
There is a final "vice" to the Shorty as well. The doublet lens obtrudes well outward from its thread on barrel making it susceptible to damage if improperly handled...
Overall, I am very pleased to include several barlow lenses in my own personal observation kit. Negative lenses are now de rigeur among eyepiece manufacturers. "Built in" barlows allow for improved eye relief in high-end, short focus, multi-element eyepieces. Impact on light transmission is less than one might expect. (Using averted vision, I frequently detect the 13.0 magnitude star adjacent to the Ring Nebula through the 80mm achromat with both barlows installed.) In fact, every amateur should consider a high quality barlow as an alternative to a wide range of what often amounts to more expensive and fixed magnification eyepieces.