Telescope Mods & Repair

Nexstar SE Wedge

The final part of my Nexstar project was getting it on a wedge which would make auto-guided long exposure shots possible. Unfortunately, the Nexstar wedge has long since been discontinued and even if it hadn’t — $160 for a weenie little wedge for this scope was a bit much for me. So I decided to buy a cheap used wedge from an older Celestron system and make whatever modifications were needed to get it to work with the Nexstar.

What I didn’t expect was how little modification was needed to get an old 1980’s era Celestron C8 wedge to work with the Nexstar SE. All the holes, both on the tripod and drive base lined up perfectly and the only modification I really had to make to the wedge was to hammer out the center guide peg.

Mounting the Nexstar

In the first photo on the right, the wedge with the center peg removed has been mounted on the Nexstar SE tripod. The center peg on the tripod fits well as it is much smaller than the space left by the removed wedge peg. The tripod bolts hold everything in alignment and there is little to no lateral movement of the wedge in relation to the tripod. The bolts are 5/16″ x 24 x 1.5″ cap screw bolts from the local hardware store. The azimuth adjusters work perfectly with this setup as long as you don’t tighten down the cap screw bolts too much.

One rather major item of caution is the length of the bolts that attach the Nexstar drive base to the wedge. Inside the drive base is a gear (right under the fork arm) that rides very close to the opening for the mounting bolts and is completely unprotected. (Seen in the 2nd photo.) If a bolt is too long and protrudes very far inside the telescope, this gear will bind against the mounting bolt probably causing damage to the drive base. These bolts are 3/8 x 16 — I got lucky and had an extra set from my old Losmandy GM-8 that were the right size but much too long. I cut and sanded them with a Dremel and then added some thick washers just to be extra careful. The final image on the right shows underside of the wedge with the Nexstar mounted.

Another modification I made was to remove two of the rubber feet from the Nexstar base to allow for more latitude range before the Nexstar base impacts with the side of the wedge as the drive base is slightly wider than the wedge. With the rubber feet removed I can get the latitude all the way to the 14 degree mark before the base connects with the wedge — with the rubber feet on, this was more like 30 degrees. The rubber feet peel off without too much difficulty and if I ever what them back on, they’re easy to reattach with some rubber cement glue. (I left one of the rubber feet on to designate the ‘top’ of the mount.)

Adding a Latitude Adjuster

My very first telescope back in 1999 was a Meade LX50 on which one of the first modifications I made to it was upgrading the rather flimsy wedge latitude adjuster that came with that scope. Since then I’ve had the old adjuster kicking around from box to box mainly because I’m a pack-rat but now I’m glad I never threw it away! The length of the bar turned out to be too long for the Celestron wedge by only about 3mm. I cut and sanded the bar down to size with Dremel cutting and grinding disks (at one point I had to wear work gloves because of how hot the bar was getting!) until it was a snug fit and then ‘painted’ any visible scuffs with a black Sharpie. The end result almost looks like it was meant to be there. 😉

Once down to size, I fixed the bar in place to the lowest point on the latitude guide groove on the wedge with a couple of bolts and washers to hold it in place. The long latitude adjustment screw fits well to the underside of the wedge and since this setup is not going to take a bunch of weight (certainly much less than the LX50 it was originally designed for!) it works very nicely. This modification also added a great deal of stability to this wedge. I was lucky to have saved the LX50 latitude adjuster and I think that even if I didn’t have it, I would have eventually wound up trying to make something similar out of wood.

One thing I’d eventually like to add is a pair of knobbed bolts in place of the current cap-screw bolts I have for the latitude adjustment on each side of the wedge so I can loose the allen wrench I currently have to use to loosen them.

Nexstar SE Camera Platform

I recently picked up a used Nexstar 6/8 SE mount and tripod with the idea of using it not only as a grab-and-go for my 72mm f/6 Orion EON (and hopefully a 5 or 6″ SCT OTA down the line) but also in the hopes of using it for some lightweight wide-field long-exposure and time-lapse astrophotography.

I wanted to side-by-side mount a DSLR camera and wide-angle lens combo alongside my unused Celestron 9×50 finderscope from my CPC800 and use an Orion Starshoot autoguider attached to the finder to guide the whole contraption for long exposure photos. That this setup can run on batteries and is lightweight enough to chuck in the back of the car for camping trips made this a really interesting camera platform despite the Nexstar’s well known astrophotography limitations.

Celestron 9×50 Finderscope Autoguider

I found a few pages and forum posts (, and with instructions on making an adapter for Orion’s autoguider using PVC pipe fittings but unfortunately all of them required drilling a hole in the finder which I didn’t want to do. I wanted a solution which would require no modifications to the finder and would be easy to undo.

Luckily, while fooling around with the finderscope one afternoon I realized that the threads on the back of Celestron’s 9×50 Model # 51611 finderscope that the cross hair eyepiece attaches to are common 2″ SCT threads! Given that the 1.25″ adapter on the Starshoot autoguider attaches to a standard T-thread on the autoguider body all I needed to find was a male SCT to male T-thread adapter to fit everything together.

I had no clue if such a beast existed and after a little bit of googling I found what I needed at — their Blue Fireball T / T2 Male Thread to SCT Male & M48 was exactly what was needed to make this work. (Seen in the first photo on the right.) This adapter is very low profile and easily allows the finderscope to achieve infinity focus with the Starshoot autoguider.

As shown in the middle and bottom photos on the right, the whole thing comes together in a nice, compact and sturdy assembly and it fits perfectly on the standard Celestron finderscope mount with no modifications needed.

Side-by-Side Camera/Autoguider Mount

Next up was finding a sturdy way to mount the finderscope and camera on the Nexstar.

From a previous project I had a mounting bar with a center hole tapped for a standard 1/4-20 tripod stud with 4 untapped holes on each side. A couple of bolts and washers from the local hardware store worked to mount the Celestron 9×50 finder mount securely to the bar.

To attach the whole thing to the Nexstar, I used the mounting block from the Orion EON 72mm f/6. (Which normally rides piggyback on my CPC800.)

The second photo shows the system all set up with the finderscope, Starshoot autoguider and an IR modified Canon 300D. One of the lenses I plan to use with this setup is a Zenitar 16mm fisheye but unfortunately, the finderscope is mounted so far forward that it projects into the Zenitar’s 180 degree field of view. So I’ll either need to crop the resulting images or figure out some way of mounting the camera further forward or the finderscope further back.

The last image shows everything attached to the Nexstar SE mount and ready to go. Note that I did have to remove the plastic altitude gear covering for the side-by-side mounting to fit which detracts a bit from the looks but everything still works fine.

I love how compact and multi-functional this setup is and can’t wait to get some time out in the field with it. Running with rechargeable batteries for the mount and with spare batteries for the camera and my netbook I think I can get a solid evening of imaging with this setup without ever needing a power plug.

The next step was to adapt a cheap, old wedge for the Nexstar

Meade LX50 DEC Axis Issues

Hoo boy, did I have a sick DEC axis. l think I only started noticing it when I started guiding long-exposure shots as it was on the most part livable for the visual stuff.

I had a few problems:

  1. Reversing direction quickly back and forth several times would actually “crash” the LX50! The power led would start blinking and all I could do was power off, wait a few seconds and power on to get it to respond. Kinda’ like an operating system or two that I won’t mention here! ;o)
  2. Speed of motion on the DEC axis had nothing to do with the RA axis. Either it was Speedy Gonzalez or the motor would actually come to a grinding (seriously – grinding) halt.
  3. When I reversed direction it would take a long time before whatever was in the FOV actually moved. It was like slack was being picked up somewhere in the tangent arm.

None of this is very good when you’re trying to guide a photo on a $%&# guide star you can barely see and I won’t even go into what an autoguider will try to do…

Problem 1 was the easiest to solve. It was pretty obviously power related but after trying fresh batteries, NiCads, even a blasted 9v the “blinking light” weirdness persisted. After some good advice (always!) from the folks at SCT-User I decided to give it a go with a little 12v, 1A power brick. That did the trick and I’ve never had any problems since. Seems the problem is
related to current and how much current batteries can sustain. Phil Chambers explained what was going on much better than I ever could in a post to the SCT-Users group:

“The issue, as Rod said, is the gearing which loads the motor down. What is probably happening is the motor is loaded, its cold so it moves a little harder so the motor needs more current but as it gets more current, the voltage drops and the spiral is to stall the motor. You must realize that a DC motor uses MAX current at stall. As it goes faster it uses less current. The alkalines simply have to much internal resistance to play this game. As the current starts to rise, the alkalines voltage starts to drop…”

Thanks for the explanation Phil. Since I do all my observing from my back yard this so far hasn’t been an inconvenience. Eventually I’m going to have to deal with powering the scope properly in the field but I’ll do a write up on that bag of beans when I have to face it! :o)

Problem 2 is the #1 gripe about the LX50’s DEC and whenever you mention you have a DEC problem with this scope on most newsgroups about 50 people will step forward and say in one voice: “Scopetronix LX50 DEC Fix Kit” [Link to the Internet Archive – Unfortunatley ScopeTronix went out of business some time in 2008.]

I am now one of those people! 😀 Wow what a difference! I had never noticed how tacky and plasticy those Meade gears were until I had the kit installed on the scope. (I bought the 108 tooth aluminum gear version)

In my opinion if you’re having DEC problems ordering the kit should be the first step. Mechanically the motors no longer “freeze” no matter what you do to them. You can stop the motor turning by grabbing the 108 tooth gear but let go and the motor just keeps on chuggin’ along. I was impressed already.

As far as guiding is concerned the DEC speed is much slower and smoother. At 2x I now have to press and hold down the button on the key pad and just watch the star glide where before it was more like tap… Oh $%&# where’d it go?!

Actually the DEC speed is now slower than the RA speed and as a matter of opinion that suits me just fine! At 32x for me the speed is still quite acceptable as I’ve always gone either straight for the DEC knob or just unlocked the thing to slew more than a few degrees anyway.

A Final note on the motor revision and ROM version. Its stated on a lot of web pages including the Scopetronix page that the best combo is the 16 2/3 motor with the newer ROM (v6.0). My LX50 was bought in late 1999 and I have the newer 33 1/3 motor and the new ROM version and in my case the Fix Kit works great. So far I’ve guided a few shots with the kit installed but have yet to test it with the autoguider.

Problem 3 didn’t take long to locate but I had a heck of a time figuring out how to cure!

Even after installing the DEC Fix Kit I had a lot of back and forth motion along the long DEC screw the tangent arm attaches to.With the DEC locked I could grab the tube, shift it back and forth and watch the DEC knob move slightly in and out.

I won’t begin to go into the amount of little corrections I tried to make or how many times I removed the DEC screw to try to figure out what was going wrong.

Removing the DEC screw is a pretty basic procedure: With an allan wrench remove the DEC knob by loosening the screw that holds it in place. Do the same to the locking nut and push the tangent arm forwards (i.e. the OTA moves north) to disengage the DEC gears. When the gear is loose you can spin it easily until the threads run out on the nut attached to the tangent arm then just pull it out slowly. Do yourself a favor and note where all the little washers go, in which direction, etc. :o)

I tried washers to take up the slack but all they would cause is binding making the motors work harder and make the DEC motion very irregular.

Eventually I noticed the brass ring on the gear side (seen in the top picture with the 108 tooth gear removed) would be pushed out in one direction until it could go no further before the DEC arm would start to move and then be pulled back in when moving in the opposite direction.

AHA! This little ring has a flange on one side that keeps it from sliding through the hole in the fork arm. I had the flange pointing _outwards_ and doing a pretty useless job of staying in one place when the screw would naturally try to push or pull against it as the locking nut is on the inside!

So once again I took the whole thing apart so I could reverse the ring and see what would happen. In the second picture you can see the brass ring in a “corrected” position. There is a washer between the ring and the locking nut to keep the locking nut from binding. After about 2 seconds of slewing in DEC I was positive that this was the gremlin I had been hunting down. DEC motion was smooth in both directions and while the slack was a _lot_ less some was still there and probably could be reduced. With the DEC locked pushing the OTA in the N/S direction would produce minimal movement.

The easiest way I found to reduce the movement that was left was simply to take a cheap pen apart and use the spring to push against the DEC knob. The spring takes up whatever slack is left and so far has done a really good job. There is no binding and the slack between directions is as small as I can make it without replacing the screw and nut with a finer more accurate thread. The spring should be fairly strong. I tried a couple of pens before deciding a spring that I pulled apart slightly so it would have a bit more tension. In the whole process experimentation is important and, as usual, YMMV (your mileage may vary)…

Meade LX50 Hand Controller Fix

This happened to me on my 3rd night out with the scope and from what I’ve read elsewhere its a pretty common problem. Some of the keys didn’t work, the declination axis worked in one direction, etc… Don’t panic if this happens to you! All that’s going on is that a connector inside the hand controller has come loose.

If you open up the controller it should be pretty obvious what’s going wrong.

Remove the 2 screws on the back of the controller and pull off the back plastic panel. Inside you’ll find the printed circuit board connected to the front of the controller (where the keys are) by a blue connector. (Circled in red in the picture) This is what’s come loose. Just push it back in, pop the back on, screw it in place and everything should be fine again.

I used some electrical tape over the connector and the back of the circuit board to keep it in place. The thing’s been fine since.

Meade LX50 RA Axis Moves Slightly When Locked

This one really spooked me. I was finishing up for the night with a quick look at Orion. I aimed, locked the RA and DEC, and noticed that with the RA locked I could move the scope in a left-right motion very slightly. More or less half a centimeter of travel. Yikes!

I took the scope inside and plonked it on the kitchen table with both the RA and DEC axis UNLOCKED. (This bit is very, very important! When you lay the scope on its side, you’re putting pressure on parts that weren’t make to take a lot of weight. If you have an axis locked you could force the scope and really break something.)

The access panel to the RA drive motor is on the bottom of the scope and is held with 4 screws. Once you pop this off you’re looking pretty much at the heart of your LX50, so treat it like such and be very careful! One thing to steer clear away from is the encoder wheel. (In every picture on the right side) This controls the RA drive rate and breaking that means a $200 repair and at least a couple of weeks without your scope. ‘Nuff said.

After fiddling with the motor system for a bit I noticed the problem was with a weird little screw. (Circled in red in the first picture) This thing basically keeps the spring mounted motor drive from traveling more than a certain distance. Through vibration, (my scope travels a bit), or general use, this thing was going off in its own direction: up!

The space is a bit cramped for my fingers so using a flathead screwdriver and pushing it in a counterclockwise motion I managed to rotate the thing back down into place. (Second image)

I didn’t like the idea of pushing it all the way back so I left a bit of space. (Last picture) I suppose this is the way the telescope is supplied.

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