Meade ETX-90 : a rusty rescue with potential for restoration

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An ETX-90 scope popped up on Facebook Marketplace at a price that was WAAY too good to be true. We figured it was one of three things: the scope’s mirror was damaged, it was stolen, or someone just wanted to get rid of it (i.e., the kids have grown up, the person didn’t take to telescope-ing like they though they would, etc.).

From the photos that the seller posted, the scope’s main mirror looked fine, so we decided it was worth the 30-minute drive to go see what was what.

FirstLook

The ETX-90 turned out to be in varying degrees of decrepitude. Most of its pieces—screws, eye pieces, etc.—were rusty, dusty, and/or in need of a cleaning. In spite of that, we decided it was worth the purchase because the main mirror was clean, unlike the outside of the scope. We couldn’t tell if the mirror would focus, but the focus knob turned and it felt like something was moving inside…which we took as a good sign 🙂

The field tripod was functionally sturdy, but in considerable rough shape. Fixing it up could be a good winter project and something we might take on, or we might pass it to someone else to take on. That is TBD.

If we do take on the project, it would require: a complete strip down; repainting; and re-lubrication. We are confident it can be done, because all the work on the tripod is mechanical, not electronic.

Disassembly

We knew that the mount of the ETX-90, while good for parts, would probably be too much work to return to functionality. We decided that the best option was to extract the main tube from the mount, clean what we could of the optical pieces, and determine exactly which components can be salvaged.

Shaking and tapping resulted in a lot of loosely contained debris to fall out of the scope. Pointing a light into mount showed some were even packed into the arm. The only way to get it out would be to take it apart. It was time to break out the tools 🙂

Removing the optical tube assembly (OTA) from the mount appeared simple enough as four screws on each side held everything in place. The problem was that many of the screws were very rusty. One head actually disintegrated when we tried to turn it! Drilling it out was the only option. That slowed things down, but we were eventually successful in separating the OTA from the mount without damage to either.

In the process of taking everything apart, a surprising amount of pine needles came out of the unit…something not typically expected 😉

Cleaning

We will not be disassembling the OTA unless we find a problem with the focusing mechanism.

With the ETX-90 in pieces, the next task was cleaning the glass so we could evaluate the optical quality of the components. 

Ethanol, cotton swabs, and microfiber cloth allowed for gentle and residue-free cleaning of both glass and metal. The key is to not to force-clean anything, go slowly, and when in doubt…stop 🙂

After the initial cleaning, some optical components were still not as clear as we wanted, but most appeared to be usable. The one disappointment was the 8x21mm erect viewfinder: its internal prism was damaged beyond repair.

The good news: after cleaning front of the OTA, we could clearly see (geddit?) that the mirror was in good shape. Next will be to see if we can get it to focus.

FirstView

To assess the quality of the ETX-90’s mirror, we set the OTA up to look across the field to a brightly lit target.

With fingers crossed, we turned the knob and things came into focus…but a little foggy. To see if the problem was with the eyepiece which came with the scope, we used one of our higher quality ones and the difference was dramatic….YAY!!!

Below is an image of what we saw with the better eyepiece.  Considering we took it handheld using an iPhone, through the eyepiece, while looking through a double-pane window…it’s a pretty good image. More  important is that we have now determined that the OTA will focus 🙂

Next up is to see is to see what is the BEST image we can get from this ETX-90. The result will determine whether we will use it as a spotting scope, a quick grab-n-go astronomy field scope, or something else.

We’ll post some pics of our tests once we figure out which adapters we will need to mount a DSLR to the axial port of this OTA….yes, this is our idea of fun 🙂