LX-200 Review

I bought a used Meade 10" LX-200 F/10 telescope in April 1998 hoping that it would improve my efficiency and the bottom line answer is: it has. My telescope lives up to the Meade advertising claims to near perfection.

I use the telescope in a permanent setup so its extra weight and bulk (compared to an 8" say) is no problem. I also bought the super wedge and was able to get an excellent polar alignment using the star drift method. (See below.) I trained the PEC using the ST-7 in tracking mode feeding corrections to the LX-200 every two seconds or so for the 8-minute period of the worm. I haven't measured the residual periodic error, but it is quite small and entirely acceptable.

As the telescope has an accurate polar alignment, only one star fix is needed to tell the computer where in the sky the telescope is pointing. I put a bright reference star on the cross hairs of the 8x50 finder and check by taking a quick (0.1 sec) focus image with the camera. It is, afterall, the camera field of view that I want to match up to the sky. A few adjustments with the very slow motion mode and the telescope is ready to point whereever I desire.

(As a parenthetical statement, I did set up the telescope on a tripod in alt-az mode when I first got it. I used the two star alignment procedure spelled out in the manual and found that the scope could locate objects all across the sky with ease. Alt-az mode, of course, is no good for long exposure photography due to field rotation - unless you get the derotator with which I have no experience.)

I only tried a few of the built-in library of objects, but the telescope went directly to the ones I tried. Instead, I fabricated the serial cable described in the manual to control the telescope from my computer. The cable end that plugs into the telescope is a standard 6-wire telephone plug although only three wires are used which are, fortunately in the 'middle' of the plug so I was able to use an ordinary telephone extension with four wires. I cut off the other end and mated it to a 25-pin connector (pins 2,3, and 7) from Radio Shack (for my computer, although a 9-pin might be needed for other computers).

I use Guide 6 to control the telescope from the computer. It is simplicity itself. Guide 6 provides an extensive array of means to find objects. I am usually looking for asteroids and Guide 6 includes a huge database of asteroids which can be accessed. In addition, one can add new asteroids or comets from their published (or calculated) orbital elements. Once the desired object is centered on the computer screen, a click on 'Slew Telescope' sends the LX-200 right to the field. I have been able to completely eleminate printing paper finder charts.

Optically, the telescope is also superb. There was a slight mis-collimation after all the shipping, but that was easily adjusted out following the procedure in the manual. One tip I learned from Thane Bopp is to buy three Allen wrenches (5/64) and to put one in each of the adjusting screws. Use only the smallest of movements of the wrench for each adjustment (if you can feel it move, that is enough).

I don't do very much visual observing anymore, but I did take a quick peek at the first quarter moon and was very pleased with the view. Castor split cleanly at 100x, 200x and 300x.

My CCD images are also better, at least subjectively. I haven't taken the time to make any quantitative assessments of the images compared to the 8". On the aperture gain alone, I should have gained about a half magnitude for the same exposure, but newer coatings and better tracking can also lower the limiting magnitude. Looking at my recent images makes me believe I have probably gained a full magnitude for my asteroid work. The longer effective focal length means smaller pixel sizes, 2.2 arcsec vs 2.74 for the 8". I have taken a few 'pretty pictures' and I am thrilled with the results. I am starting to look for a way to print out some of these images to decorate my office.

As a by-product of an Eclipsing Binary run, I obtained data relating to the polar alignment and tracking efficiency of the LX-200. For EB work, I point the telescope to the field of the EB and set the ST-7 to the AutoGrab mode wherein the camera takes a prescribed exposure every prescribed period of time. In this case, I was pointed at V836 Cyg and took exposures every 4 minutes for a three hour span. It is not possible to make the ST-7 self guide in this mode, so we are dependent upon the telescope's alignment and sidereal tracking. The resulting 45 images were analyzed for the changes in the EB's brightness, but I could also record the pixel position of each image. Transforming pixel sizes to arcseconds and plotting the data gives the results shown below.

The steadily decreasing line representing the DEC drift indicates that the polar alignment is still off a bit. The polar axis is too far West, but the rate of change, some 70 arcsecs per hour is negligible for my purposes. The slightly rising, then decreasing nature of the RA drift indicates a slight imbalance in the telescope. Note the inflection point aligns with the point in time (shown by a diamond symbol on the x-axis) where V836 Cyg crossed the meridian. To date, I have made no attempt to balance the ST-7 on the telescope. I just hung it on and started to work.

Another measure of the pointing ability of the LX-200 came as a byproduct of an evening when I measured two eclipsing binaries whose predicted minima were separated by only 30 minutes. I set up the ST-7 to take 15-sec exposures every 3 minutes, and moved the telescope back and forth between the two variable stars such that the odd-numbered exposures were of the first and the even-numbered exposures were of the second. Thus, each star was sampled on a 6-minute interval over a 2 1/2-hour period. Altogether, there were 42 'jumps' of some 53 degrees from one star to the other and back. I measured the position of a convenient field star in each such frame and calculated the 'miss distance' for each jump. The data are plotted below. The average miss distance was 23 arcsecs and the worst case was just over 80 arcsecs.