The Ubiquitous Meade 8" Schmidt-Cassegrain Telescope

The Ubiquitous Meade 8" Schmidt-Cassegrain Telescope
(A review and comparison of the LX-90 and LXD-75)
By Mark Muccio

I.  Introduction

A lot has been written about the ubiquitous Schmidt-Cassegrain telescope.  They have been mass produced for decades, and, it would seem that everyone has owned one at some point in time or another.  Two most frequently asked questions are “What are the differences?” and “Which telescope setup should I get?”

In an effort to answer those questions myself I owned two Schmidt-Cassegrain telescopes (SCT) at the same time.  One was German equatorial mounted and the other altitude / azimuth mounted.  Not only were they both SCTs, they were both Meade 8” telescopes.  My basis for this review is a comparison of the popular Meade LX-90 and LXD-75 telescope packages; and, I hope to assist others that may ask “How do they compare and what would you get?”

II. General Overview

Webster defines catadioptric as an “adjective: belonging to, produced by, or involving both the reflection and the refraction of light.”

Therefore, a catadioptric telescope uses a combination of mirrors and lenses in its configuration.  One common catadioptric design is the Schmidt-Cassegrain Telescope (SCT) where light first enters a Schmidt correcting lens - refraction.  The Cassegrain portion is comprised of a larger spherical primary mirror where light is reflected back up the tube to a smaller convex secondary mirror which reflects the light back out an opening in primary at the rear of the instrument.  The light path is folded by the optics thus enabling for a very short, compact telescope in relationship to aperture.  This makes SCTs portable and a popular type of telescope that can be found in a range of sizes.

Schmidt-Cassegrain Advantages

• General-purpose telescope design.
• Excellent optics from a mass-produced OTA.
• Very good for lunar, planetary, deep sky and binary star observing.
• Closed tube design with low maintenance.
• Compact and portable.
• Less expensive than equivalent aperture refractors.
• More standard accessories available than with other types of telescopes.

Schmidt-Cassegrain Disadvantages

• More expensive than equivalent aperture Newtonians.
• Slight light loss due to secondary mirror obstruction compared to refractors.
• Longer cool down times.

Meade first announced the Model 2080 8" Schmidt-Cassegrain telescope in September 1980.  The Meade OTA has changed little in the way of the optical design, although, the optical coatings have been enhanced over the years.  Also, various upgrades and electronics have been added to the mounts.  The two models under review are the Meade LX-90 (non-LNT / non-GPS) and the Meade LXD-75.  Both OTAs are 8” SCTs with Ultra-High Transmission Coatings (UHTC).  The clear aperture is 203mm and the focal length is 2000mm making for a forgiving f/10 system.  The UHTC maximize light transmission and image brightness.  It is applied at the factory and often increases the resell value on the secondary market.

Both scope packages included: tripod, Series 4000 Super Plossl 26mm eyepiece, 497 AutoStar controller, 8x50 finder with quick release mounting bracket, 1.25” SCT visual back, 1.25” prism diagonal, manual, and software.  The LX-90 also included the Lunar Planetary Imager (LPI) and AutoStar Suite bundle.  Both packages were double boxed for shipping.  The LXD-75 was fully foam lined.  The foam could easily be reused within a durable case for transportation and storage.  The LX-90 utilized just enough foam to support shipping and is not reusable except in the original shipping box.  This necessitated the purchase of a fitted foam case (from JMI) for taking the LX-90 scope on the road since I didn‘t want to use a cardboard box.

III. A More Detailed Comparison - Optical Tube Assembly

Our comparison begins with the optical tube assembly (OTA).  Both OTAs were of very good fit and finish.  They were nearly identical with the exception of the baked enamel paint - one in Meade Blue and the other in a classic white.  Again, the SCT design is one of folded optics consisting of a short closed tube.

LXD-75 8” f/10 OTA Schmidt Corrector

At the front of the tube is a Schmidt corrector lens made of water white glass for higher light transmissions.  It has multicoatings, and, the lens is designed and figured to correct for spherical aberrations.  The correctors on both scopes seem to disappear at times and I interpret this to be a good thing.

At the other end of the tube is the spherical primary mirror.  It is slightly larger than the corrector lens at 8.25” and is multicoated.  A word on Ultra-High Transmission Coatings (UHTC), the manufacturer claims a 15% increase in light throughput when compared with standard coatings.  I have read a number of debates and at least one test report in regard to the benefits of UHTC.  The common thought is that the coatings do offer a slight advantage.  These scopes were new and it was not a surprise that the optics on both OTAs were in factory new condition out of the box.  Looking inside the OTA we find a baffle tube to block off-axis light.  The inside of the tube is painted a flat-dark-gray, not exactly flat-black.

At the rear of the scope we find a standard a 1.25” SCT adapter and a 1.25” prism diagonal.  The diagonals appear to be identical; however, the one included with the LX-90 had poor coatings.  Visual tests at higher magnification (200x) on bright targets (Luna and Saturn) display a lot of light scatter.  This was confirmed by switching scopes.  I also validated that the other diagonal did not display this issue.  I have since taken the bad diagonal apart.  Streaks in the coatings are easily seen on one side of the prism and I suspect this was one that got past QC.

The Meade SCTs are focused by moving the primary mirror cell.  There is a focuser knob protruding from the rear of the scope.  The mirror cell is moved forward and backward via a threaded shaft driven by the focuser knob.  There was a great difference in how each of the focusers performed.  The LX-90 was very smooth with next to no play.  It wasn’t stiff with only slight resistance.  The focuser on the LXD-75 was very stiff in comparison.  There was slightly more play in the LXD-75 than the LX-90 and this resulted in more noticeable image shift in the LXD-75 OTA when focusing at higher powers.

LXD-75 OTA with 8x50 Finder
and William Optics Crayford Focuser

Both telescopes come with an 8x50 finder and 6 point mounting bracket.  The finders are interchangeable with the exception of painted color.  I found both to be functional and served the purpose of telescope alignment just fine.  The cross-hair eyepiece is not removable and is likewise not illuminated.  Focus is adjusted by loosening the primary objective, rotating the cell to move forward/back until desired focus is achieved, and then tightening the primary objective to secure it into place.  The mounting bracket is quickly released by two nylon set screws in the mounting base attached to the scope.

Also included was a 26mm Series 4000 Super Plossl eyepiece with both telescopes.  Some final differences in regard to the OTAs are:  LX-90 had a plastic front corrector cover and the LXD-75 was metal, the LXD-75 also included a full-length mounting rail and standard dovetail (not needed for the LX-90), and the LXD-75 had a small handle at the rear of the scope to aid in moving and positioning the OTA.

IV. Setting Visual Expectations

Both telescopes are capable of providing years of viewing pleasure for the average amateur observer.  The overall performance visually was nearly identical.  Collimation is identical; however, I found that the LXD-75 needed fine tuning more often.  Fine tuning collimation on an SCT is best done with a star test on an exceptional night, and, I often felt that the LX-90 had moments of better seeing when the conditions did not permit fine tuning the LXD-75 OTA.  I was very pleased with the performance of both OTAs.  The SCT may be considered a compromise in design, however, Meade’s fine execution of this design and 8” of aperture may be all that some will ever need to be happy.  Here are a few observational highlights…

Planets: Mars, Jupiter and Saturn
I had the opportunity to use both telescope to view Mars in 2005.  Neither scope disappointed as I used both on almost every occasion to view the Red Planet.  Using various Televue Plossl eyepieces provided the highest contrast.  When seeing conditions permitted, subtle details and changes on the surface were worth the effort and wait.  Jupiter and the four Galilean satellites that are visible put on an extraordinary display.  The various belts and zones can be readily seen.  Special treats are the Great Red Spot and transits of the moons.  Finally, Saturn (viewed with this telescope) is the reason that a couple of my close friends are now observing.  Saturn is wonderful at 150-200x in this telescope, and, I have had a few cold, still nights at 300-400x.  The images are everlasting impressions.  As with Jupiter, Saturn’s satellites offer a changing view while the rings and the planet’s shadow produce an almost three-dimensional effect.

M13: Globular Cluster in Hercules
Beyond just a fuzzy ball with a bright center, M13 is a sight to behold.  This scope easily resolves this cluster into the countless points of dazzling lights against a dark background.

M42: Orion Nebulae
With this scope the complex structure can be clearly seen, and, the sight is greatly enhanced when using the UHC filter.  At 100x the four stars (A, B, C, D) of the Trapezium are identified.  Doubling the magnification to 200x will usually reveal Theta-1E when seeing permits, and, I have seen Theta-1F on exceptional nights with the scope sufficiently cooled.

M45: Pleiades
This is one cluster that actually looks better in scopes with a wider field of view.  I include it here to demonstrate a limitation of an f/10 system.  Although the stars are very nice points of light, M45 does not have the same impact because the full cluster can not be framed against a dark background.  The finder scope actually is a better view…

M57: Ring Nebula
At 150-175x the Ring can be clearly identified with a dark center.  On clear, dark nights variations in light intensity within the Ring can be discerned after some study.  Again, the UHC filter does help when viewing this DSO.  While you are in the neighborhood Epsilon Lyrae (the Double-Double) should not be overlooked.

The Moon:
Whole books have been written on observing the moon.  I own a couple and still have yet to explore all that there is to see.  Perhaps it will take the rest of my lifetime to complete my journey; however, I can be assured that this journey will not be limited by what an 8” SCT can reveal.  The details on the moon seem to amaze and delight even the most uninterested of my neighbors when on occasion they stop by during their late evening strolls.  At times the moon is very bright in this telescope and I do use a variable polarizing filter when needed.

V.  A More Detailed Comparison - Mounts

Thus far the telescopes compare very similarly, however, the mounts are very different.  As expected the LXD-75 German equatorial mount (GEM) and the LX-90 altitude / azimuth mounts (Alt/Az) make for the most contrast in this review.  What follows is a brief description of each. 

The Meade LX-90 OTA is supported by an aluminum double tine fork mount.  The mount is very solid and stable.  It is painted black and has a nice fit and finish.  The LX-90 mount sits upon a very sturdy tripod that is shared by much larger bothers in the Meade SCT lines of telescope.  The range of operation will allow for leveling on uneven surfaces - a must for good alignment - while accommodating varied OTA heights of approximately 30“ - 44“.  The mount along with the OTA is attached via a single bolt that threads through the spreader, tripod and into the base of the fork mount.  I find setup and assembly to be very easy.

Meade LX-90 8” f/10

At the risk of stating the obvious it should be mentioned that for all intent and purposes the LX-90 OTA is permanently attached to the mount.  Although it can be removed, this setup is intended to be “integrated” into a single package.  This could be viewed as a limitation and might not appeal to everyone.

Slew speeds on the LX-90 are selectable from 1x sidereal to 6.5°/sec in 9 increments using the AutoStar controller.  Pointing precision is specified at approximately 5-arc minutes and I found this to be good enough to place objects consistently centered at 200x magnifications.  The worm gears measure 4.9" for both axes and allow for approximately 3-arc minutes if “High Precision” is enabled on the AutoStar controller.  This is accomplished by the telescope first slewing to a bright star near the selected object and then synchronizing the telescope.  At that point the telescope has a higher precision alignment and will continue to slew to the object that was originally selected.  This function was verified to work very well at magnification of 300-400x; however, I usually leave the LX-90 telescope in the normal mode and view at magnifications less than 200x.

The Meade LXD-75 German Equatorial Mount (GEM) is built solid of die-cast aluminum and sits upon a variable-height field tripod.  The mount itself is off-white with black hardware, motor covers, controller, cables, etc.  The LXD-75 mount incorporates steel ball bearings on both the right ascension (RA) and declination (Dec) axes and is an upgrade from the LXD-55.

LXD-75 Mount

The movement is very smooth with a balanced load.  The worm gears on the LXD-75 are 2.83” and the offers somewhat less precision in GO TO accuracy when compared to the LX-90.  Meade states approximately 15-arc minutes as compared to 5-arc minutes with the LX-90.  In the field, the LXD-75 was not as consistent in GO TO operation as the LX-90.  High-precision mode on the AutoStar controller did make a slight improvement.  The slew speeds on the LXD-75 are selectable from 1x sidereal to 4.5°/sec in 9 increments.  The mount is loud when slewing; however, this seems to be the norm with other GO TO mounts costing under $1000 that I have owned.

Included with the LXD-75 mount is an illuminated polar alignment finder.  The on/off-dimmer switch is nice and I like it better than some other polar finders I have owned.  I don’t have an issue of the polar finder cover falling off either.  I have owned a couple other GEMs and have experienced this with prior setups.  I find I can get a good alignment for visual tracking with just the finder and a single star alignment.  When aligning with the AutoStar two and three star alignment modes, the LXD-75 mount tracks very well. 

VI. The Meade #497 AutoStar Controller

The interface and brains of both mounts is the Meade #497 AutoStar controller.   With over 30,000 objects in the controller’s library one can “GO TO” both popular and more obscure targets in the night sky.  The menu driven controller is easy to use.  Input date, time and location followed by telescope star alignment to get things started.  Selection of planets, stars, galaxies, clusters, and nebulae is straight forward.  The popular targets can be accessed by name and all by various catalogue designations.

Meade AutoStar Controller #497

I found the more advanced feature of creating guided tours very useful.  I create lists (constellation tours, double star tours, open cluster tours, Messier Tour, etc.) of seasonal objects I want to view and store these in the AutoStar controller.  The tours are ready for use on those clear nights when I want to be sure to view objects that I might otherwise miss because of limited time.  Updating and programming the AutoStar controller has a small learning curve and requires a computer connection.  Learning to use the AutoStar features is a very worth while effort to get the most from these mounts.

VII. AutoStar Suite Software and Computer Control

I had my doubts when I first loaded the AutoStar suite software included with the Meade telescopes.  “Desktop Observatory“ is how Meade describes this product and it does come very close.  I was surprised at the functions built into this application.  The planetarium functions enable views of the sky from your location with scaled representations of planets, the moon, asteroids, comets, 13,000 deep sky objects and over 45,000 stars in color.  Furthermore, you can slew the telescope from the display.

The remote control functions are completely integrated and can be accessed with a personal computer using a direct connection or from a remote network.  Everything you can do from the AutoStar hand-held controller can be performed within the software version.  Other basic functions include new object uploads, creating tours, updates to the controller and saving profiles (settings).  The imaging functions range from guiding, camera control, image capture, processing and analysis.  Included with the LX-90 was the Lunar Planetary Imager (LPI) that was easy to use on the first night right out of the box.  More advanced functions for the observer that has everything include dome controls and weather sensors, etc.

Installation of the software was straight forward and the updates were downloaded from Meade’s website.  Everything needed to connect the telescope to my laptop was included.  I mainly use the software with my LPI and to update r update the controller.  Until recently I limited my use of the telescopes to mostly visual, however, I plan to use the LPI as an auto-guider for my new DSLR camera and short APO refractor on the LXD-75 mount.

VIII. Setup and Operation

There is a learning curve to most anything new in life; moreover, for a first-timer or novice of GO TO telescope operation, both of theses setups is no different in that regard.  After some experience (both daytime and night time) it becomes much easier to assemble the components and operate the telescopes.

Because SCTs usually require “cool down” of the optics to get the best views I usually setup the mounts prior to sunset.  I find the overall setup of the LX-90 a little more simple.  The tripod is easily leveled and the OTA and mount - a single unit - placed on top.  It is secured with a single bolt threaded through the spreader.  This is tightened by hand and is a very secure setup.  The OTA is pointed north and finally leveled.  The LXD-75 also sits upon a level tripod.  It will next need to be pointed north and the latitude adjusted.  After this is complete the OTA is attached and everything should be balanced.  I can setup either mount in just a few minutes now with some experience.  The LXD-75 will take a couple more to balance the OTA.

The alignment for the AutoStar controller is usually performed after sunset when a few brighter stars can be seen.  The alignment for visual tracking and GO TO will take a few minutes on the LX-90 and a couple more minutes are added to polar align the LXD-75 prior to star alignment.  I should note that the LXD-75 can be used with a manual controller.  You can slew the scope and track without AutoStar and GO TO.  This does require a more precise polar alignment, but that is easy enough.  You will still need batteries or power source.  There is not a completely manual operation except to loosen the right ascension and declination locks.

I found everything performed as described.  Overall, the operation of both setups does not disappoint.

IX. Optional Accessories: Suggested “Nice to Have“ Items

Denkmeier 2" SCT Power Switch Diagonal

(Suggested for both LXD-75 and LX-90)

This 2” diagonal has both reducer and Barlow lenses.  Three magnifications can be obtained without removing the eyepiece of bin viewer by moving either of the switches.  I use mine on my LX-90 with a William Optics 16mm UWAN on most nights the telescope is out.  Included are caps and a 2” to 1.25” adapter.

Bob’s Knobs
(Suggested for both LXD-75 and LX-90)

Why aren’t replacement thumb screws for the three collimation hex screws standard?  You know you are going to collimate the OTA.  You know it will be fine tuned with a star test at night.  Why not make it easier on yourself?  Need I say more?

William Optics 2” SCT Crayford Focuser

(Suggested for LXD-75)

A rotating focuser is a must have on any GEM mounted telescope because of the change in viewing positions.  On the LXD-75 I get a slight image shift without the focuser and zero with the focuser.  The WO focuser’s travel is long enough that once the course focus is completed by adjusting the mirror I find that I can use just the WO focuser for the rest of my viewing session.  Balance was not an issue on the LXD-75, however, when using a 2” diagonal and larger 2” EP the balance center-point moved significantly enough to reposition the OTA.  This was not possible with the LX-90 unless a counter weight and rail was installed.  Also, the length of the focuser when extended limits the scope travel on the fork-mounted LX-90.  Worst case is that it will stop at approximately 63-64 degrees and will not reach zenith.

Peterson Engineering Corporation EZ Clutch™ Kit

(Suggested for LX-90)

This upgrade modifies the declination clutch system and reduces the need and/or potential of over-tightening the clutch knobs.  The procedure is simple and converts the factory lubricated "wet clutch" system to a "dry clutch" system.  After reading a lot about how things get lubricated during assembly I was expecting the worst in the form of excess grease, etc. It really wasn't bad at all. Maybe a little excess on the worm drive but not anything to complain about.  The cleaning took the most time. I wanted to insure that everything was degreased properly. It was a snap to put everything back into place.  It really now only takes two fingers to tighten and lock the OTA.

8x50 Right Angle Correct Image (RACI) Finder

(Suggested for both LXD-75 and LX-90)

I really had a hard time with the upside-down and inverted images of the standard finder telescopes.  This replacement was a personal choice that I felt was a needed change.

Flexible Dew Shield

(Suggested for both LXD-75 and LX-90)

The front corrector lens of an SCT will collect dew.  Passive dew prevention with a flexible dew shield is a must have in my honest opinion.  The shield also helps to prevent stray light from hitting the front corrector and I use one on every observing session.

Peterson Engineering Corporation EZ Focus™ Kit
(Suggested for both LXD-75 and LX-90)

When I decided to not use the WO Crayford focuser on my LX-90 (due to not being able to reach zenith) I modified the stock focuser by replacing the nylon washers with a precision thrust bearing.  I really had no complaints with my LX-90's factory focuser, but could it be better?  Upon completion and with minor adjustments, I was more than pleased. This modification really exceeded my expectations. It is just that smooth…

Pedestal Replacement for Tripod

(Suggested for LXD-75)

Replacing the LXD-75 tripod with a more rigid 4” steel tube pedestal was a good upgrade.  It measures 40” tall, the legs have screw type levelers and it came with an adapter specific to the LXD-75 mount.  I did notice a difference in improvement in vibrations and I can now use some of my longer OTAs (Newtonians and refractors) with worry of hitting the tripod.

A.C. Adapters and/or Portable Power Tank
(Suggested for both LXD-75 and LX-90)

Both the LXD-75 and LX-90 require power to operate.  Stuck with low power (or no power) I have tried to use both of these mounts manually and it is not worth the effort. 

X.  Summary

Below is a table of expected sale prices base on my research.

* Note that all models include optional UHTC.

Some “pre-owned” averages include values that were derived from package deals by removing the “fair market value” of extras to get a larger sample of data.  The LXD-55 and LXD-75 averages were assembled by summing the separate mount sale prices and OTA sale prices.

Both the LXD-75 8” Schmidt-Cassegrain telescope and the LX-90 8” Schmidt Cassegrain telescope are fine instruments.  With 8” of aperture both telescopes can go deep with a good range of observable objects.  As expected the optics of both setups compared very similarly.  The LX-90 is slightly easier to setup for visual use.  The LX-90 is a slightly more stable setup and has more precise GO TO.  The LXD-75 mount is much more versatile considering the range of other OTAs it could carry when compared to the 1-to-1 arrangement of the LX-90 mount and OTA.  Observing sessions can be taken to new levels if desired with the AutoStar controller and function rich AutoStar Suite software. The following is a quick comparison chart of some of the items discussed.

I have bought and sold many telescopes and mounts during the few short years that I have enjoyed this hobby.  Both of these setups are capable of providing years of viewing pleasure for the average amateur observer.

The initial review was inspired by asking the question “How do they compare and what would you get?”   The answer is “It will depend.”  The answer for my situation was to keep the LX-90 for visual use and sell the LXD-75 setup.

I hope the information shared in this review and comparison will help others in finding their answer.

Footnote: I recently purchased another LXD-75 (mount only this time) to learn the basics of astrophotography with a small refractor.  But, I still use the LX-90 every chance I get!