The first thing most users add to the Odyssey 8 is a finder as they did not come standard. Coulter sold Telrads for $50 and many buyers went with this option. I received a Telrad with my scope and haven't needed anything since. I understand the newer Telrads have a finer dimmer control but the older ones are entirely useable.

The Focuser
The focuser is usually the first part of a Coulter to be replaced. Coulter had really taken to heart the original concept of John Dobson. The Dobsonian telescopes sold commercially today are slick metal tubed affairs with custom designed bearings, rack and pinion focusers and even electronic setting circles. Originally, dobsonian telescopes were made as cheaply as possible, often with off-the-shelf parts modified to telescope use. Dobson's ground his original mirrors out of porthole glass.A modest man, he prefers the term "sidewalk telescope" rather than dobsonian. In a way, the Coulter line of scopes was in the "sidewalk telescope" tradition whereas the newer dobs on the scene, like the Orion SkyQuests, are better thought of as standard newtonian reflecting telescopes that happen to use a dobsonian type mount.

The dobsonian/sidewalk telescope design was supposed to bring telescopes to the masses at very affordable prices. The Coulter was exactly such a beast.

The focuser was made out of PVC plumbing parts and a steel draw tube. I believe they technically called this a helical focuser but that would be a real stretch of the truth. In reality it was a friction fit focuser. You would move the draw tube in and out until the image was in focus and then turn the PVC ring so it would lock the draw tube in place. You might recognize the pieces involved if you've ever worked under your kitchen sink. I have scanned the original instructions Coulter sent out with the focuser for reference. It is a very clever design and a model of economy. I have built my own version for a quick refractor I threw together one afternoon and it cost me around $3.00. In practice however, it was very hard to achieve a crisp focus with this original focuser. [Note for ATMs: in the photo to the right you can see that the inside of the pvc fitting was sanded down by Coulter so the steel draw tube would fit smoothly inside.]

The solution I chose was ascrew-on low profile helical focuser from Lumicon International. For $70 they make a steel helical focuser that screws right onto the PVC base plate of the old Coulter focuser. With the Lumicon focuser solution you do not have to remove the entire old focuser and worry about patching up holes in your tube or aligning the new focuser correctly. The draw tube is replaced by an actual threaded tube making it a true helical focuser. The brass knob [pictured] holds the eyepiece in place while a white plastic knob applies pressure to the draw tube. Like many helical focusers, when racked far-out, it is a bit wobbly. Tightening the plastic knob eliminates the wobble.

One problem is that tightening the bolt on the focuser draw tube changes the collimation of the scope! With my laser collimator I can see that tightening the bolt and changing the focus of the focuser can make the laser beam miss the center of the primary mirror by as much an an inch. This is unacceptable in any scope but especially bad in an f/4.5.

Another nice option many people opt for is the reverse crayford from JMI. It is more expensive but I now wish I had gone this route. I decided not to because couldn't bring myself to spend as much on a focuser as I spent on the entire scope back in 1987. The crayford also requires making the hole in the tube larger which would have taken a bit more work. One nice thing about the JMI focuser is that if you decide to opt for a newer scope it can come with you. The Lumicon focuser is for the odyssey only.

One final note about the screw on focuser: Due what I believe is poor thread quality on either the the pvc piece or the focuser itself, it is impossible to screw the focuser all the way onto the base. This is frustrating and can be an issue when trying to focus with my highest power eyepieces. I tried it on a spare PVC piece and it had the same problem. Lumicon does have a work around for this. You take the pvc piece off the outside of the tube, stick it through the hole from the inside of the tube then hold it in place by screwing the focuser on the threads sticking through the hole.

This seems like one more reason to go with another brand. Overall, while it is an improvement over my old focuser, I am still less than satisfied by the Lumicon focuser.

Eyepiece Holder

At the same time I added the new focuser, I also added a metal eyepiece holder from Orion. I found it very convenient and eventually added a second one. An eyepiece holder is one of the cheapest upgrades you can make to a Coulter Odyssey but what a difference!

Bearings

Up next for replacement were the bearings. The original azimuth bearings consisted of two parts. On the bottom of the rocker box was what looked like a thin sheet of opaque white plastic. It was attached with staples. Yikes! On the base, the plastic sheet rode on several round self-adhesive clear plastic pads that were probably designed to stick on the bottom of chair legs.

I replaced the plastic with a square of Ebony Star laminate countertop material from Wilson Art. I attached it with contact cement. The bottom pads were replaced with Teflon chair sliders. The sliders ride a little high and cause some trouble since it adds wobble. I hope to replace them with lower profile teflon pieces soon. As you think about the size of your teflon pieces remember you want your load to be about 15 lbs per square inch. The Odyssey 8 is 36 lbs so 36/3 = 12 lbs. Once inch square pads should do the trick!

Spider
I next replaced the secondary mirror holder and spider.

When people looked down the tube of the scope they were often puzzled by the odd looking spider. It is technically known as a two vane spider(in this case the "two" vanes are actually one long vane that spans the diameter of the tube.)

Again, the design is a model of economy. It consists of two pieces of bent metal and three bolts. The cross beam is bolted into the side of the and can rotate about the axis of the mounting nuts. Attached in the center of the cross-beam is another piece of metal with a 45° bend in it. One side of the bend attaches to the cross beam while the other side has the secondary mirror glued to it. Essentially, the user can adjust the pitch and yaw of the secondary with this system. It cannot be moved up and down inside the OTA so it was critical that Coulter align this correctly at the factory. Also, when moving it side to side you changed the angle of the mirror but you also moved where the center of the mirror was which makes it pretty much impossible to collimate.

Well, they did a less than perfect job at the factory. It turns out that it is was easy to obtain optical collimation but more difficult to obtain mechanical collimation. It seemed impossible to obtain both at the same time. In a way I liked the system because it rarely got much out of alignment even after bumping over a gravel road in my car. This is the advantage of the two vain system. It is very stable. Of course, they are not as adjustable and that is a hugely critical issue.

Some people have found that their secondary needs to be moved closer or farther from the primary mirror (the direction not allowed by this system). One solution I am familiar with is to drill out the hole on the 45° piece into a long slot. The mirror can then be raised or lowered as needed and held in place by tightening the nut.

I opted to replace the whole assembly.

The system I chose was the four vane spider from the company that bought out Coulter, Murnaghan Industries. They can be reached at e-scopes.cc. I purchased the 9.5-10.5 inch spider with 1.8 inch hub. As the old Coulter mirror was epoxied to the two vane spider so I purchased a new secondary mirror from University Optics.

The Murnaghan spider is well constructed though some may balk at the use of plastic adjustment screws and a plastic secondary hub. The metal pieces seem well machined. Special note if you purchase this spider, the directions sent by Murnaghan are slightly incorrect for this scope. As this spider is actually a few millimeters larger than the inside diameter of the scope you have to drill the four holes with a 3/16th inch bit not a 1/8 inch bit. This way the holes are the same size as the end of the vanes. The tubes on the end of the vanes actually stick into the holes in the side of the tube. You then attach the bolts directly though the outside and into the spider. Use the included nuts on the outside of tube as spacers to prevent the bolts from being tightened too far in. Because this is the non-standard installation method, without the nuts as spacers you could damage the spider by screwing the bolts in too far.

You can see the bolts and white patches where the old spider was attached.

The Murnaghan secondary uses a mirror hub without clips so the mirror is actually glued to the hub with GE silicone II. This way, there is no tension on the mirror from clips.

The new spider is much more adjustable and I have been able to improve the collimation of the scope.

You can check out my full review of the Murnahgan spider as well.

The Primary Mirror Cell
The mirror cell takes a really long time to cool down. It isn't a problem when at a star party because I'll come early and socialize and by the time it is dark I'm ready. It is a real pain if I want to do some sidewalk observing. I live in south Minneapolis and leaving a telescope on your front step or the sidewalk is probably asking for trouble.

One idea is to add a cooling fan to the scope. The sono tube used for the OTA could easily be cut to accommodate a "muffin fan" but then I would have to either make it removable or re-balance the scope. I'm not sure it is worth the slight advantage it would give me. I also like that I'm not dependent on a power supply to use my scope.

The real problem with cool down is the Coulter cell. Again, it is simple, functional and economical but not a model of fast cool down technology. The mirror rests on a thin circle of masonite which in turn sits on the collimation bolts which stick through a chip board base. The mirror and particle board circle are held onto the base with duct tape and a hose clamp. I know, it sounds frightening but actually works. The base is exterior grade chip board too so it stands up to the elements well. The real problem is that the back of the mirror is completely un-ventilated and you have to wait for the mirror and a few inches of wood to cool as one unit. I will try to post a picture of the cell here soon.

As the night progresses, the air cools faster than this huge mass of wood/glass and problems occur.

I'm intimidated by the idea of replacing the cell because if it puts the mirror at a different height I'll have issues with focal lengths and I'l perhaps have to move the secondary, etc. If done right, a new cell would be great. If done wrong, I'll be worse off than I am now. It would also significantly change the center of balance of the scope and I would probably have to move the bearings to keep it balanced.

I understand some people have cut holes in the center of the coulter cell for better ventilation and added springs to the collimation bolts so they can get rid of the lock bolts. These are two fixes short of replacing the entire cell and may be worth looking into. Of course, removing mass from the rear of the scope will change the balance so be ready for that.

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