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Addendum: In the year or so that has passed since the information here was prepared, it has become apparent that carrying out the practice of using a separate guide scope is not at all easy and some have reported difficulties. Therefore when attempting to do this, be sure you have read all of the guidescope related articles on this website and all of caveats. This is not a trivial mechanical system to get right.
The ETX can make an good guider scope for use on a larger SCT. I have mounted an ETX on both a 10" and a 12" LX200 for use as a guider scope. A separate guide tube is, in my opinion, much easier to use than an off axis guider. And, it can be just as accurate when mounted and adjusted carefully. I later I switched to a Celestron C-5 for the 12" telescope. This was principally to gain the added light gathering capacity of the C-5. However, this switch required the addition of an external flip mirror at considerable extra cost. The ETX is not only suitable but better sized, for the 10" telescope because of its lesser weight its lesser cost and the very convenient build in flip mirror. I believe that there are two or three limitations and slight problems with the ETX that do need to be addressed. One is that the ETX is optically a bit slow (f14) so that brighter stars need to be selected for good guiding. I believe that one can live with this limitation by using appropriate eyepieces and suitably light adapting one's eyes. Optically, the ETX is excellent. In fact, I had a chance to compare three ETXs directly against two Questars. The ETXs were in every case superior. The mechanical structure of the ETX is not quite as elegant as the Questar but it is very good. The ETX is also about one fifth the cost of the Questar. There are two mechanical issues in the ETX design that need attention if it is to be used as a guider telescope.
The guider which has been used with the ETX is a 216XT which is used in stand alone mode or on one of the COM ports on a computer. The guider can be placed in operation after the telescope has been pointed to the correct location. The guider takes over automatically when it is engaged and guides the LX200. The ETX is a very good guider telescope. It has a focal length of 1250 mm which is long enough to guide a 10" LX200 for imaging at prime focus (2400 mm) and especially suitable then telescope is used with a Meade 0.63 reducer or a MAXfield 0.33 reducer. Additionally, the ETX has a built in flip mirror which gives full view of the part of the sky to be guided upon and makes it easy to choose the desired guide star and place it in the field of the guider CCD. The two ports on the ETX are easily made parfocal if the 9 mm eyepiece is used with the 216XT. Thus, with the ETX,one gets a full view imager and a built in flip mirror. The savings on the latter item alone is significant. It would be hard to find a better combination.
It is very important to have the ETX firmly mounted on the main telescope tube. Piggy back mounts are never adequate for precision mounting of accessories. (piggy back mounts will not hold a camera with al ens longer than about 100 mm rigid enough for any use in my opinion.) Thus I have installed a Losmandy rail on the 10" and a pair of Losmandy rings to hold the ETX. Remember that to maintain 1 arc second alignment of the main tube and any attachment to it, the ends of the auxiliary tube may move no more than 0.05 thousandths of an inch in a distance of ten inches. Flexing of the mount as the telescope moves across the sky must be minimized. The Losmandy rail and rings is one combination I have found that does the job well.
The ETX is shown below mounted in 125 mm Losmandy rings, which fit perfectly.
Notice that the ETX is nicely proportioned compared to the 10" LX200. It is
light in weight and of suitable focal length. The ETX is shown without
the 216XT guider or 9 mm eyepiece which are normally used with it for guiding
purposes. All in all, the ETX is a good and even handsome guider telescope.
It looks like it belongs on the LX200.
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Checking and Fixing the Mirror Flop.
One of the main problems with using a second SCT as a guider telescope is that
you now have to worry about two mirrors flopping about. Thus I carefully checked
the main mirror flop in the ETX and modified/adjusted it to eliminate the mirror
flop. Fortunately the guider telescope can have the mirror fixed essentially
at infinity focus and every thing, including the guider imager firmly locked
in place. First, the optical tube must be opened and the focus mechanism
inspected. The open tube is shown in the photograph below. Several
photos also show close ups of the focus mechanism as well.
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As can be seen, the focus mechanism is not very robust. The end of the focus screw is loosely retained in a plate mounted to the rear of the mirror. Considerable slack exists at this point. Additionally, the point at which the focus rod penetrates the back plate has a relatively loose bearing. When the mirror is near infinity focus the mirror is quite a way toward the back plate. and thus the spring under considerable tension. Fortunately the movable slide is quite tight on the center mounting tube. I have not completely solved the issue of ensuring that the mirror is absolutely tight on the center mounting tube. Removing the existing lubrication and re-lubricating it with beeswax seems to hold the mirror securely enough for most applications. Since the guider telescope is used only at infinity focus, it would be possible to fix it firmly with small springs or set screws. I have not done that at this point but will certainly do so if there appears to be any residual mirror flop in the guider telescope.
Fixing the Flip Mirror in the ETX
I found that the flip mirror, when in its 90 degree position, was not as stable as I would have liked. Each time the mirror was flipped, the centering of the guide star in the eyepiece changed considerably even though the star was usually on the guider chip. But in some cases the flip mirror was badly enough set that a star in the center of the eyepiece did not fall on the chip at all. This problem needed to be fixed before the ETX could be used with the tiny 216XT chip.
The reason for this problem is that the up position of the mirror is established by a spongy pad on the back of the mirror which does not firmly establish a stopped position. The fix allows for precision adjustment of the angle of the mirror in its up position. (90 degree position) The angular accuracy required for the mirror is quite high. A good fix can be effected in the following way.
Remove the black case from the body of the telescope. Done by removing
three screws. You will see the mirror mechanism shown below in two views. One
with the mirror out of the path and the other with the mirror in the 90 degree
position. There is a small rubber pad on the paddle like extension sticking
out on one side of the mirror mount. Scrape the rubber pad off and scrape the
paddle clean. Drill a No. 3 tap drill size hole through the back of the
casing immediately next to the attachment ring and in line with the paddle. This
location is shown in one of the lower photographs immediately next to the thread
on the back adapter tube. As can be seen, there is just enough room to
do this. Be careful to not ruin the thread on the rear mount ring. Tap
the hole and insert a long No. 3 set screw. Turn the screw inward until
it engages the paddle. This screw can now be used to set the mirror angle
very precisely. The mirror will now come up and into contact with the set
screw with a nice firm click. A dab of cement can be applied to hold the
screw firm.
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If the screw is too short it will fall through the threaded hole and if too long it will interfere with attachments to the rear thread. If the screw is too short, use a longer one and grind or file it to the correct length. It needs to engage the paddle but not stick out the back of the hole. To set the mirror to the correct angle first move the mirror to the down position (so it is out of the way of the light beam) and center a target (star) on the chip. Then, without moving the ETX remove the camera and put the mirror in the up position (so it intercept the light beam and directs it at 90 degrees). Insert the screw and turn it until the target is centered in the eyepiece. The mirror is now aligned so that it directs the target into the center of the eyepiece. Glue the screw in place from the inside. This operation can be seen in the photographs.
In the final photograph, the telescope is shown with a standard adapter the converts the small thread on the back of the ETX to standard Schmidt thread. This adapter also covers the set screw hole. I prefer to have the Schmidt thread rather than the small thread so that standard accessories fit easily.
The chip and the eyepiece will now have concentric images which will repeat reliably when the mirror is flipped. If there is a very slight lateral offset of the image it will be stable so it can be corrected with the moveable cross hairs in the 9 mm eyepiece. If the lateral offset is too great, the mirror will have to be shimmed. I did not have to do this. You will find that the mirror mechanism has a detonate spring that is supposed to hold the mirror against the set screw when it is in the 90 degree position. It might be necessary to adjust this spring to insure that it pushes the mirror paddle against the set screw firmly. I had to adjust the spring on the ETX shown.
I have found that with the two adjustments described above made, the ETX makes a very suitable guider telescope.