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First Impressions of the Lunt 152mm f/8 ED-APO



First Impressions of the Lunt Engineering 152mm ED-APO

                                                                                                                                      By: William Paolini, 4/10/2014 v2

 

Image by the Author

The recently released, Lunt 152mm f/7.9 ED-APO is an air spaced doublet refractor that is the result of a unique collaboration between APM Telescopes in Germany and amateur astronomers.  This collaboration, a portion of which can be seen in a thread titled “Nobody is making a 6" doublet ED scope” on www.cloudynights.com, helped determine, among other things, what would be acceptable levels optical and color performance from a design that uses less than the most premium and expensive glass types, so an affordable 6” doublet scope could be made with APO-like performance.  The result of this collaboration is an optical design by G.Düring in Germany that utilizes Chengdu CDGM FK61 and Lanthanum H-LAF53 glasses. This new APM Telescopes lens design optimizes performance over the human eye’s Photopic color vision range, enabling this 6” f/7.9 ED objective to achieve similar color performance that a conventional 6” Achromat would have at f/30!  Since this performance level is close, but not quite fully apochromatic (which would more typically require a triplet design for this short focal ratio), the Lunt-152 is designated as an “ED-APO” which is the popularly used nomenclature in the marketplace to indicate semi-apochromatic performance.                      

                       

Image by the Author

 

Basic specifications for the Lunt 152 ED-APO are as follows:


°  Aperture: 152 mm

°  Focal ratio: f/7.9

°  Focal length: 1200 mm

°  Resolution: 0.76"

°  Limit value: 12.7 mag

°  Back focus: 180 mm

°  Tube diameter: 156 mm

°  Observing length: 1220 mm

°  Transportation length: 1020 mm

°  Dew cap diameter: 186 mm

°  Tube weight: 23.5 lbs

°  Case weight: 26.4 lbs

°  Total weight: 55.7 lbs


°  Lens optimized for operation at +5° to +104° Fahrenheit

°  Tube end uses M109x1 and M95x1 threads to accept  3" or 2.5" focusers


First handling impressions of the Lunt-152 ED-APO are that it is a very well built telescope, having excellent fit and finish with many well appointed features.  The telescope sports an all-metal tube, a retractable and lockable dew shield, all-metal lens cover, a beautifully finished heavy gauge fully rotatable 2.5” Rack & Pinion focuser with a 10:1 reduction gear, a lens cell that is fully collimatable (including adjustments for lens centering), a finder accessory dovetail shoe on the focuser, mounting rings, losmandy mounting plate, a 2” dielectric diagonal with 1.25” eyepiece adapter, and a padded hard-shell transport case.  On the mechanicals, the biggest surprise was the quality of the standard focuser.  Lunt offers the telescope with an optional Starlight Instruments premium focuser which I was very tempted to order.  However, upon inspection of the standard focuser I am glad I stayed with the more economical decision as this focuser has the same robust precision look and feel; and after several weeks of use in the field has proven to be thoroughly excellent for visual observing with no play in the mechanism, providing very precise and positive control.

 

Image by the Author

 

I was also pleased to see the use of standard threading so the focuser can easily be unscrewed from the back plate for servicing or retrofitted with a Starlight focuser.  The rear plate can also take larger 3” focusers.  The rear plate that the focuser attaches to easily unscrews from the main tube, making access to the inside of the telescope very easy.  In my case, I chose to flock the inside of this rear plate for a final bit of internal light suppression.  Finally, since the finder scope shoe is mounted on the focuser housing itself, this leads to two unusual situations: first it means that the finder’s eyepiece is fairly close to the eyepiece you use for the telescope (which can be a good thing or problematic depending on the size of the eyepiece), and secondly it restricts some finder scope brackets from attaching well to the shoe.  Case-in-point is a red dot finder (RDF) I was going to use had a dovetail stop located on the forward facing portion of the dovetail.  This meant that this particular RDF had to be slipped onto the shoe from the front of the shoe.  Unfortunately, with the shoe mounted on the focuser the main tube of the telescope did not allow enough clearance for the dovetail to engage the mounting shoe.  The easy fix of course was removing the dovetail stop from the RDF, which I did with some common work shop hand tools.  Luckily though, when I took the Orion 8x50 right angle finder from my Dob, the dovetail stop on this was located at the rear of the shoe so there was no issue using this finder as-is and it easily mounted to the Lunt with no problems. 

 

Overall, all the materials, fit, finish, and mechanicals on the Lunt-152 ED-APO are high quality and convey the impression of a study and capable telescope.  The Dew shield easily extends and retracts, and has a locking screw to keep it at any position desired.  The supplied tube rings have large locking screws making them easy to hand tighten, and the lens cell collimation screws have locking screws on either side of the push-pull screw to ensure a solid locking of the lens so it hold collimation.

 

As with any refractor, a primary concern is always its level of color correction.  This is of course important more for lunar and planetary observing so fine details are not lost in a mix of out of focus colors.  On stars and deep sky objects color correction tends to be less of an issue, although it can impact overall contrast and many observers dislike seeing blue or purple halos surrounding very bright stars.  With the Lunt 152 classified as an “ED-APO”, this means its color correction may not be as complete over as wide of a spectrum as with a full Apochromat.  The best way of determining what, if any visual impact this may have is of course the field test.  So to determine how precise the visual color correction was I put the Lunt-152 through its paces on a full range of celestial targets in a range of magnifications from 100x to 300x and more to determine color correction behavior.  Planetary observation was also heavily conducted as this is typically the “acid test” for a telescope.  Finally, to determine if the Lunt-152 ED-APO was showing any subtle color casts on targets, a Takahashi TSA-102 Super Apochromat telescope was used as a reference telescope, and targets were dual observed at both equal magnifications and equal exit pupils to see if any visual color differences were present.  What follows is the visual assessment after several weeks of field testing.

 

Brighter Stars – Magnitude 0.1 Capella was observed at 100x, 150x, and 200x to determine if any chromatic halo, fringing, or cast was present.  Examining this star in the Lunt-152 and comparing it to the TSA-102 revealed virtually the same rendition of the star, which to my eye was a creamy-orange color. Characteristic of a full APO the TSA-102 held this color regardless if the star was in focus or out of focus.  In the Lunt-152, when the star was out of focus a fringe of color would appear on the perimeter of the out of focus image of either a reddish or yellow-greenish color, but in focus there were no visual chromatic artifacts or fringing.  Turning to magnitude 0.5 Betelgeuse, it also showed similar coloration in both scopes at the same range of magnifications.  However, its color to my eye was orange with a slight creamy under tone in the TSA-102, whereas it had as a slightly stronger orange tone in the Lunt-152.  Finally, moving to the beautiful Mizar-A and Mizar-B pair, these also showed very similar colorations in both scopes at 100x, 150x, and 200x.  Mizar-A was white to my eye, and Mizar-B appeared off-white with a cream under tone. This cream under tone of Mizar-B showed slightly stronger in the Lunt-152 than in the TSA-102.  So overall, only a few subtle nuance differences in tone were observed in the Lunt-152 compared to the color pure reference TSA-102 telescope.  Additionally, if the atmospheric seeing was poor and the star was strongly scintillating, sometimes a momentary flash of color would be observed on the perimeter of the star’s image as it was defocussed by the seeing.  Other than these rare occurrences, the Lunt-152 provided excellent chromatic performance on even the brightest stars.

 

Star Clusters – During all observations of open clusters, regardless of magnifications, all stars were presented with no apparent chromatic artifacts when using either scope.  Open clusters were also very wonderfully presented in the Lunt-152, showing many delicate star points on richly dark backgrounds.  Clusters I am well familiar with also presented with many more stars in the field of view of the Lunt-152 than I am used to with my TSA-102.  The open clusters M35 and M37, in particular, were incredibly rich in the Lunt-152 compared to the TSA-102.  Finally, with open clusters being beautifully rendered at low to moderate magnifications, I decided to take advantage of the extra aperture of the Lunt-152 and observe M37 at 400x.  At this magnification, the view was still rich and beautiful, only now the entire field of view of my 82º AFOV eyepiece was densely filled with stars.  Going deep into rich open clusters is one of my favorite observations and the view in the Lunt-152 was a memorable observation making me feel like I was up close and inside the cluster itself! 

 

Nebula – The Great Orion Nebula, M42, was chosen as a showcase object to test for performance relative to this class of celestial objects.  As always, M42 was beautiful to observe, being both spectacular and mesmerizing in in the Lunt-152. The mottled structure and extent of the nebula was outstanding, being much more detailed that what can be seen using the TSA-102. I particularly enjoyed the view using the 30mm XW (40x with a 3.75mm exit pupil).  The extent of the nebula, particularly the “wings” was much greater in the Lunt-152 than in the TSA-102, as expected due to the increased aperture.  However, the diamond like appearance of the various stars within and behind the nebula was visually striking, something I do not experience in the smaller 4” APO.  And unlike the smaller 4” APO, in the 6” Lunt the Trapezium-F component was never a challenge observation and it displayed itself prominently as a direct vision star when viewed through the Lunt-152.  M42 gave a spectacular show as it always does, and no unexpected chromatic artifacts of any kind were noticed, displaying a bright and vivid nebula with beautiful blue-white and white stars shining intensely within the nebula. 

 

Moon – The first observations of the Moon with the Lunt-152 also permitted me the opportunity to conduct a cool-down evaluation and assess how quickly the Lunt-152’s doublet objective took to reach sufficient equilibrium for high magnification lunar observing.  Indoors, the house and telescope were at 72ºF.  Outdoors for the evening of the test, the temperature was 52ºF.  This 20º temperature difference took only 20 minutes before the Lunt-152’s 6” doublet objective was able to provide a sharp and steady view at 240x -- so excellent cool down characteristics.  On successive evenings, I found similar fast cool-down performance, and the 6” doublet was always thermally adjusted about the same time as the 4” triplet of the TSA-102.  Nothing special was done to cool the scopes down other than placing them outside with the tubes horizontal, dew shields retracted to expose the lens cell the most, and the diagonal removed so internal warm air could escape.

 

With the Lunt-152 cooled, lunar observation revealed that the limb of the Moon virtually colorless visually when the image was in-focus.  With a 1/4 turn of the fine focus knob inward a razor thin magenta-red fringe would appear on the defocused lunar limb, and going out of focus in the other direction by the same amount produced a slightly larger but less color-intense yellow-green fringe instead. But as long as the image was focused, either no visual fringing could be detected, or just the very slightest hint of a magenta hue would sometime be at the edge of perception when working at magnifications from 170x to  240x.      

Lunt-152 Afocal Lunar Photo @ 170x (color not-altered)

Image by the Author

 

Moving to observation of the terminator features, bright crater wall tops and mountain peak tops that were just off the terminator surrounded by black showed no chromatic fringing whatsoever at 170x.  However, when the magnification was pushed to 240x, then one or two of the very brightest features just off the terminator into the dark side showed a light magenta fringe that stayed even when in focus. This chromatic artifact appeared no place else other than on those few brightest features that were off the terminator and on the dark side. All similar features within the illuminated portion of the terminator and the rest of the Moon remained of proper coloration when in focus and showed no false chromatic artifacts.  Finally, pushing the Lunt-152 to 342x, no additional chromatic artifacts surfaced, and all details were sharp and etched.  In summary, on the Moon’s entire terminator only two of the very brightest peaks and/or crater wall tops illuminated in the dark side off the terminator showed the slightest magenta fringe, and only when the magnification was in the range of 240x-342x. Except for these two features, all other lunar features at the full range of magnifications from 40x to 342x remained or proper color and hue, and were sharply rendered with excellent contrast.

 

Mars  – Always an interesting target, Mars is generally a challenge typically being very small angularly and requiring stable seeing to see its very delicate features.  Luckily, during the testing timeframe Mars was near Opposition making productive observations much easier.  The first thing of note when I viewed Mars in the Lunt-152, compared to the TSA-102 which I am most familiar, was how bright Mars appeared.  It was very much brighter than I am used to using my 4” APO and was actually unexpectedly brighter even at smaller exit pupils and 300x.  I decided to take advantage of this brightness and added the Baader Contrast Booster filter to accentuate my observations. Observing Mars in the Lunt-152 showed the north polar cap as distinct, bright, and very white. Mars itself appeared a pale creamy orange color.  Utopia was nicely contrasted compared to the white of the polar cap and distinctly rendered.  Limb haze was also showing nicely.  Mare Sirenum and Mare Cimmerium were a much lighter shade than Utopia, but still an easy catch during the stable moments.  Magnifications between 200x to 300x were used for observing these details, which the Lunt-152 easily accomplished showing a bright orb even with the added filtration. 

 

Comparing the Lunt-152’s performance to the TSA-102, initial observations were conducted with the magnifications being leveled between the two telescopes.  At 171x, Mars coloration was identical in both scopes, although as expected brighter in the Lunt-152.  Polar caps were clearly visible in both scopes, with a dark line bordering close to the northern cap making it appear quite distinct. Limb haze, Mare Sirenum and the east most portions of Mare Erythaeum and Mare Acidalium visible were visible through both scopes, but only the Lunt-152 revealed a portion of Utopia peeking from around the right limb of the planet.  At equal magnifications the various Martian dark albedo features appeared richer, more contrasted, and more detailed in the Lunt-152 than they did in the TSA-102.  This was expected given its larger aperture, and the larger brighter exit pupil the Lunt-152 was operating at when the magnifications were leveled.

 

Moving to observing Mars with the two telescopes operating at the same exit pupil (170x in the TSA-102 and 240x in the Lunt-152), things changed.  At a leveled exit pupil of 0.6mm, the contrast of the Martian Maria showed a slightly richer black-brown in the TSA-102.  Detail levels were about the same though as the atmosphere was not allowing much over 170x, and the Lunt-152 was at a much higher 240x to attain the same 0.6mm exit pupil.  The colors and hues on Mars were the same between both scopes, and the stark white of the polar caps was also equivalent when operating at the same exit pupil in each scope.  Of course, the TSA-102 showed no fringing or artificial color artifacts at any time, and the Lunt-152 showed this same chromatic performance when the image was focused, except for the slight occasional quick color fringe when the seeing defocused the image sufficiently.  Overall, observing Mars was excellent in both scopes, with the Lunt-152 showing more details than the TSA-102 when the seeing was stable, and with the Lunt-152 able to go to higher magnifications easily due to its larger aperture.  Chromatic performance in both scopes was also equivalent when the image was focused and the seeing stable or somewhat unstable.

 

Jupiter – A constant favorite to observe with its wealth of details and prominent features, Jupiter was well placed during the observations being at or near the zenith.  First observations of Jupiter in the Lunt-152 were done at 200x and showed highly detailed views with no color fringing on the planet when the view was in focus.  A defocus by racking the focuser out or in from best focus with the TSA-102 showed no chromatic artifacts, and in the Lunt-152 showed the often typical semi-APO characteristic of a slight color fringe.  When Jupiter was defocused inward a thin red-purple would appear on the perimeter of the planet, and when the defocus was outward a slightly larger and less intense yellow-green fringe would appear.  However, when in focus there were no false chromatic artifacts.  Increasing the magnification to 240x, the Lunt-152’s view of Jupiter remained clean and sharp.  The colorations on Jupiter between the TSA-102 and the Lunt-152 were identical.  When the magnifications of the two telescopes were such that the exit pupils were the same, the TSA-102 showed a slightly higher contrast view, with NEB and SEB showing a richer dark hue, but the details were far inferior to what the Lunt-152 was showing.  As example, in a typical observation the TSA-102 would show the NEB and SEB with detailed irregular boundaries and some internal albedo differences or possible eddies, polar regions with shadings but no structure, a fully defined NTB line, and only a hint of a partially defined STB line.  Moving to the Lunt-152, and the same observations showed all the details that the TSA-102 was showing, but in addition it resolved the hinted SEB swirl into a fully defined GRS just exiting around the planet, complex trailing eddies behind the GRS, a NEB barge and several white ovals as well.  So a wealth of more details were plainly visible in the Lunt-152 that were invisible in the TSA-102.  Overall, there was so much more detail to see it was like looking at a different planet in the 6" APO vs. the 4" APO.  Colorations on Jupiter remained the same with no in focus fringing at any of the magnifications used (170x-240x). 

 

Saturn – During the observational tests, Saturn was an early morning object and was observed during the end of some of the testing evenings.  Not near as bright as Mars is now at opposition, Saturn was however still able to put on a beautiful show at 200x using a 6mm ZAO-II eyepiece.  Like in the other tests, no false chromatic artifacts were visible as long as the image was in focus and all colors were consistent with what the reference TSA-102 was showing.  However, the Lunt-152 showed Saturn with a greater level of details than the TSA-102 could attain.  With the Lunt-152, Saturn’s polar region was not only showing nice gradations of grey to steel blue hues, but also significant internal structure and definition as well.  The ever popular Cassini Division was stark and an easy catch clear around the rings, excepting just the smallest sliver directly in front of the planet. The A, B, and C rings were also all clearly portrayed and the entire planet had a very three-dimensional look to it.  Of all the features though, the polar region was the most spectacular for me through the Lunt-152 as the color and hue banding was highly detailed and exceedingly beautiful.  

 

My initial impressions of the Lunt 152mm f/7.9 ED-APO are that it is a apochromatic refractor that is fully capable, excellently performing, well built, and substantially chromatic aberration free in focus visually; and all at a price point that is truly exceptional.  False color only appeared when a bright target was out of focus, and vanished when in focus.  Its doublet design allows very fast cool down and exceptionally reduced weight, being easier to carry and move around fully mounted than is my 10” Dob.  Compared to my TSA-102 Super APO, the Lunt-152 ED-APO represents a meaningful increase in visual performance, showing my familiar observing targets with more details and visually at the same level of color purity.  Excellent value with excellent performance, the Lunt-152 ED-APO with its APM designed lens makes the 6” APO experience both affordable and enjoyable.

 

 

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Disclaimer - Given difference between equipment, seeing conditions, observer physiology, and observer psychology (i.e., likes, dislikes, and expectations), your outcomes can be different when compared to those in this article.  All advice and information in this article is given in good faith as an amateur astronomer and hobbyist and is based on sources believed to be reliable and accurate at the time of release. The author does not accept legal liability or responsibility for the content of the advice or information or any consequences arising from its use, and should not be considered a substitute for professional advice.  It is the responsibility of the user to make their own decisions about the relevance, accuracy, currency and reliability of information found in this article.  The advice and information in this article does not necessarily reflect the views of the organizations hosting the article.

 

This article is placed in the public domain with no restrictions when made available in its entirety.

All images Copyright 2014 by William Paolini and require written permission if used apart from this article.

For a formatted PDF version of this article, please contact the author at wapaolini@hotmail.com.


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