Orion 120 ED Refractor Telescope Testing and Losmandy GM-8 Mount Analysis

Moonlite CFL 2.5 Inch Large-Format Aftermarket Focuser Upgrade Review

FIRST SECTION:  Orion 120ED Telescope Testing

Background Info:
The Orion 120ED is a doublet objective lens, semi-apochromatic refractor.  It offers superior color correction compared to traditional achromatic designs.  The optics in the telescope were tested as-is, with no changes made to the factory lens cell collimation on the front of the OTA or the optical glass itself.

OTA Alterations:
The factory stock focuser was removed and replaced with a Moonlight "CFL" 2.5 inch, large-format focuser with custom OTA mounting flange machined in-house by Moonlight Telescope Accessories.  This provided an outstanding available back-focus capability of around 220mm, depending on what drawtube adapters are mounted on the end of the drawtube.  This amount of backfocus is very useful for using long, complicated imaging setups utilizing color filter wheels, adaptive optics, etc....  This focuser modification leaves about 25mm more available backfocus than the Orion factory stock focuser does.

Problems With the Focuser Replacement To Watch Out For:
1)  The Orion 120ED OTA uses the standard three screw holes to mount the focuser.  On mine, these holes were not spaced an even 120 degrees apart like the Moonlight flange's holes.
2)  The Orion's OTA inside diameter specs may vary slightly from what they list at the factory.  This combined with the hole spacing problem makes it mandatory that you remove the factory focuser from the OTA and send it in to Moonlight so they can get all this correctly measured prior to machining your mounting flange and drilling the flange holes.
3)  If you wind up having to bore out the OTA screw holes slightly to match the screw holes on the focuser flange, it is imperative that you do this in such a way that the three mounting screws cannot slip backward away from the rear of the OTA, or you will have flexure and collimation problems.  This focuser is much heavier than the factory version, so you must be exact in how you do any OTA screw hole modifications to prevent any slipping or flexing of the focuser inside the OTA.  You can request that Moonlight machine their custom mounting flange .001-.002 inch wider in diameter than the factory flange and this will provide for a tighter fit of the flange and also help prevent any potential for the focuser slipping inside the OTA.  I found that the factory flange was a little "sloppy" and loose-fitting inside the OTA, so an extra thousandth inch of flange diameter really helps in this regard.  Alternately, you could shim the mounting flange surface inside the OTA with some 1mm thick metal shims to get a tighter fit and help prevent any flexure and slipping of the CFL focuser.

Focuser Modifications:
1)  Due to some vignetting with an ST-2000XM CCD camera with focus point at about 1 inch from racked in all the way, the 4.5 inch focuser drawtube was replaced with Moonlight's 3 inch version so the drawtube itself would not protrude into the light path when racked almost all the way in.  This proved to be effective at curing the vignetting.   Since I have not tested this scope at any pixel resolutions greater than 1600 x 1200 pixels or a required image circle greater than 15mm, I do not have firsthand proof of whether there are any vignetting issues beyond this size or not.  However, Orion Tech Support has stated to me that the telescope is designed such that it will fully illuminate a 27mm image circle.  Based on information I obtained from Company Seven, and my own calculations based on this information, I came up with about the same with a 25-26mm fully illuminated image circle.  This size of  image circle will fully illuminate all but the outermost 5-8% of the FOV of a Canon 40D DSLR camera set to its full resolution of 3888 x 2592 pixels for example, according to my own calculations.
2)  With a color filter wheel and adaptive optics also attached (over 6 lbs total imaging payload), the dual-speed micro focuser knob of the CFL focuser would not rack the drawtube inward against the weight of the payload, so a motorized upgrade was installed by Moonlight with 10 lb payload capacity.  It proved to be an outstanding upgrade and made extremely accurate micro-focusing possible by allowing focusing without the image shaking since you are no longer touching the focus knobs by hand.
3)  I ordered Moonlite's standard finderscope bracket for Synta finderscopes.  It fit the Orion 50mm finderscope holder's dovetail base with no problems.

IMAGES AND STAR TEST RESULTS OF THE ORION 120ED:

The Orion 120ED Refractor With Moonlight CFL 2.5 inch Large-Format Focuser Upgrade on a Losmandy GM8 GoTo Mount and Software Bisque Paramount ME Permanent Pier:

Star Test Video (the star Vega was used for the test, very near zenith position):
The webcam of the star test showed nice, evenly spaced and symmetrical diffraction rings both in and out of focus.  The video is a little darker than the visual display while I was imaging it, but visually the star test did very well.  The out-of-focus spherochromaticism is plainly visible in the star test.  But the important point about the SA is that it is well controlled when the scope is *at focus*, and produces a nice, crisp focus.  The chromatic aberration is also very low when at focus.

ORION 120ED STAR TEST VIDEO 

NOTE:  You will need DIVX to be able to see this video.  Download the free DIVX player HERE.

SUMMARY:
Overall, the telescope does very well optically for its price range.  The SA and CA levels are well above what you find in premium apochromatic refractors that use  the most expensive triplet lens cells and premium glass, but are at excellent levels for this type of doublet ED lens design, and also for the price range the scope is offered at  (I used my TMB 152 triplet apo refractor for making comparisons).  But these premium models also cost 3-5 times more than the Orion 120ED.  For the money, the Orion 120ED gives very pleasing views visually, with minimal SA and CA.  For imaging use, mine does a good job and produces nice, crisp focus through the CCD camera. Since I have not evaluated any cameras beyond 1600 x1 1200 pixel resolution or required image circle larger than 15mm on this scope, I do not have first-hand evidence of what the vignetting or flat field issues of the Orion 120ED would be beyond this resolution (if any).

The Moonlite CFL 2.5 inch motorized focuser upgrade is an outstanding addition to the Orion 120ED, and makes the telescope much more useable for imaging purposes.  I highly recommend it for serious imagers, especially due to Moonlite's willingness to custom-fit the mounting flange for precise fit to your specific OTA, and also for their varied line of accessories and adapters to enhance function of the basic focuser.  For serious visual observing by those not interested in imaging, the CFL with dual 8:1 microfocus knob and friction brake is an outstanding upgrade instead of the motorized version, and will make precise micro-focusing of the telescope possible.

USAGE RECOMMENDATIONS:
1)  Wide field imaging using large-format DSLR cameras and large-format CCD cameras: *NOT* evaluated, so any potential vignetting or flat-field issues at higher resolution are unknown by me.  You should first evaluate the size of image circle required to fully illuminate your own camera's CCD chip, then keep in mind the 27mm image circle capability reference provided to me by Orion Tech Support, in order to determine any potential vignetting issues with your camera.  Also, if you plan to use a focal reducer, this could lead to worsening of any vignetting that might be occurring with your camera.
2)  Standard CCD imaging with medium format CCD cameras:  RECOMMENDED.  I saw no vignetting or flat-field problems at the maximum 1600 x 1200 pixel resolution and required 15mm image circle of my SBIG ST-2000XM camera.
3)  Visual use:  HIGHLY RECOMMENDED.  An excellent scope for visual use for the money.  Nice, crisp views and excellent color correction for this price range, with no edge-of-FOV problems noted during my testing, when using Televue eyepieces and star diagonal.

OTHER:
Keep in mind these are mass-produced scopes and there may be some slight variability in optical performance from unit to unit.  You should thoroughly star test your own scope *BEFORE* spending the money on any aftermarket focusers and taking a chance on voiding any possibility of a return or replacement of your scope, just in case yours does prove to have bad optics.

 CCD Inspector Software Analysis:  The Orion 120ED compared favorably on field curvature testing to other brands, according to CCDWare's published test results from various end-users.
CCDWare Test Results of Various Brands of Telescopes  (Note: you may need to join the CCDWare group support forum and be logged in there to be able to review these results (it's free to join).

Note:  The CCD Inspector results are affected by camera flex and collimation.  My Orion 120ED does have some camera flexure present due to using a very heavy camera setup with clamping rings instead of rigid screw-down attachment to the drawtube.  Also, upon testing the scope's collimation with a Takahashi collimator, it was found the objective lens cell is slightly out of collimation.  So fixing these problems would result in even better analysis numbers than what I got for this scope.  The numbers as they are, stack up very favorably as you will see when looking at other types of telescope designs and brands, and the results they gave on these same tests.  Also keep in mind though, that these results were measured based on the inner 50% of the optical path with the type of camera that I used (utilized the inner 60% only of the light path of the scope).  You can most likely expect a higher field curvature results on these tests if performing them on a large-format CCD or DSLR camera that pushes the utilized portion of the scope's light path out to its maximum capacity.  Since I had no way to evaluate this, I do not know how much worse any potential field curvature numbers might be on a large-format CCD chip.

>>>>END OF ORION 120ED AND MOONLITE CFL FOCUSER SECTION<<<<

>>>>SEE BELOW FOR LOSMANDY GM-8 GOTO MOUNT ANALYSIS<<<<

 

 Test Image of M11 using the Orion 120ED and Moonlite CFL Focuser:
M11 open cluster and surrounding star field

This image has some issues due to tracking problems with the GM-8 mount that have since been resolved, but still shows that the Orion 120ED is capable of doing some very nice imaging with a good camera, good processing, and stable mount.

Wide field image of M71, taken after tearing down, cleaning, and re-lubing the GM-8's RA axis.  The improvement in autoguiding shows.  (Note, for both the above test images, adaptive optics were used, which typically cuts the mount's total guide error in half).  You may not get quite as good of results without adaptive optics, although the guide errors should still be at an acceptable level if shooting at 2 arc-seconds per pixel or above.

ADDITIONAL ANALYSIS OF LOSMANDY GM-8 GOTO MOUNT:
The Losmandy GM-8 GoTo mount is a perfect match for the Orion 120ED telescope, and easily handles the OTA weight with imaging accessories.  The GM-8 provides all the hand controller and firmware menu features of their more expensive G-11 and Titan models, and does a good job of tracking and autoguiding accurately.  Below are some mount analysis and graphs using PEMPro V.2 software, that show the Periodic Error and non-periodic spur gear errors of my GM-8, along with a black-and-white test photo of a star field to show imaging results with this mount.  A data collection of 8 PE cycles (approximately 32 minutes of recording time) was used to analyze the graph.

Autoguiding Results:  I routinely get approximately 0.75-1.0 arc-second autoguiding accuracy using an SBIG ST-2000XM camera when atmospheric seeing conditions are moderate-to-good.  Using an SBIG AO-7 adaptive optics unit piggybacked onto this camera, I usually will get 0.3-0.7 arc-second guide accuracy.  This produces nice round stars from this mount when shooting at my typical 1.7 arc-seconds per pixel imaging scale with the Orion 120ED and ST-2000 CCD camera.

Uncorrected Total PE:
 Uncorrected Gear Terms:  There is a whopper random, non-periodic error present of 2 arc-seconds present at 1.73 worm cycles.  This may be random data "noise" though...note the large RMS Error value of 9.990.  This indicates the data was very noisy or the mount gears had problems that made it hard to record a consistent pattern.  I have since torn down, cleaned, and re-lubed the RA axis gears, so hopefully this will improve the PE results.  It is autoguiding much more accurately now, so I believe this helped the RA axis of the mount considerably. Uncorrected Periodic Error Curve:
The basic Periodic Error curve of the mount, uncorrected by PE Playback
Corrected Total Periodic Error:  Note, the PE has been corrected from approx. 23 arc-seconds down to a much more manageable 9.5 arc-seconds.  With a cleaner and longer data collection cycle of 10 or more PE periods, plus utilizing the PEMPro "Refine" feature to further tweak the curve, I think based on past experience that this remaing 9.5 arc-seconds PE error could be further cut down to 4 to 6 arc-seconds PE, especially now that the mount has been torn down, re-lubed, and RA gears adjusted. Corrected Gear Terms:  The largest NON-periodic gear spike is 0.76 arc-seconds in this graph.  Also, the periodic errors are greatly reduced after applying  PEMPro to the mount to correct the errors.  This is much more manageable for autoguiding and will produce nice round stars when using image scales of around 2 arc-sec or greater (typical of many refractor and CCD camera combinations).
 Corrected Periodic Error Curve:  Note the >60% reduction of total Periodic Error after the application of PEMPro to the mount:
LOSMANDY GM-8 MOUNT SUMMARY:  The GM-8 is a a great match with the Orion 120ED and with a little work and fine-tuning of the mechanical worm gear clearance and autoguide software settings, the mount can provide very good tracking and imaging quality when using lighter telescope and accessory payloads of <20 lbs.

 

Wade Van Arsdale
Little Rock, AR., USA
September 2nd, 2007