Restoration of Tasco-Vixen 8V Jones Bird Telescope

Before doing any alignment, I needed the OTA to be in its final geometric configuration. This meant getting the dings out, which might change the shape or orientation of the optics at the ends. (In retrospect removing the dings probably made no difference to the optics, since alignment was within range of the adjustments, but removing the dings did make the OTA look nicer.)

Figure 8. OTA primary end thumbnail

The photo above shows the cell holder from the outside. It is two pieces of metal. The inner part, the mirror holder, has 3 pairs of screws. One screw of each pair screws into the outer ring, which itself screws into the OTA with 3 screws. I first removed both pieces together, by removing the 3 screws holding the outer ring to the OTA (again marking the orientation of the outer ring and the mirror holder). I then removed the mounting rings for the equatorial mount, which slid off the OTA. I didn't remove the focuser or shoe; there were no dings in this region.

The primary surface was OK to a cursory inspection and I decided initially not to mess with it.

I beat out the dings from the inside using a brass hammer and a block of wood. The face of the hammer was slightly curved, approximately matching the curvature of the OTA. (A flat hammer may not have worked so well. Presumably a soft faced ball peen hammer would have been OK, but I didn't have one.) The brass was probably not necessary. It was the smallest hammer I had and the small size of the head was needed, to get inside the OTA. To see the bumps inside the black OTA, the OTA needed to be illuminated from the other end, to cast shadows. Both a regular indoor light and outdoor light, through a window on a cloudy day, worked well. I'd put the hammer on the bump, then raise the hammer about 1cm and give the bump a tap. Usually first or second tap was enough.

Figure 9. beating out the OTA thumbnail

The photo shows the two mounting rings for the equatorial mount, after being lined with cork. The far ring is vertical, the near ring is flat on the table. Next is the block of wood, the brass hammer and the OTA.

The sides of the OTA were now flat (well flatter). I left the circular paint scars from the ends of the screws. There was no way I could paint to match the nice enamelled finish on the OTA. I decided that the scars added a touch of well travelled experience to the scope.

Inspection of the mounting rings and the screws showed no obvious way to fix the problem to alternately allow rotation of the OTA, then to lock it down. First I tried lining the rings with cork. It didn't work. Here's my cork method ^[15] .

Later I got a better fix from Milt Hays Jr., who I found through Cloudy Nights (he sold me my 2nd Tasco 8V), and who sent me this photo. Rather than having the screw push directly on the OTA, the screw pushes wooden blocks, and the larger area of the wooden blocks pushes on the OTA. It works perfectly. (I glued fender washers to the top of the blocks, so the screw pushes onto the fender washer and not the wood.)

Figure 10. Method for locking the rings from Milt Hays Jr. Zodiacwks (at) aol (dot) com thumbnail

There are two sets of knobs on the mounting rings. One set of knobs stops the OTA rotating (and inflicts the damage on the OTA described elsewere here). The screws on the other set of knobs clamp onto the equatorial mount. I had missmatched sets of screws. Presumably Max had lost the originals and picked up non-matching replacements with the right thread. I wanted a matched set. The scope comes from Japan, is metric (the label on the OTA has focal length and aperture in metric), the screws on the OTA are metric (M4x0.70, M3x0.5), the screws on the tripod are metric (M8x1.25, M5x0.8), so the screws on the rings are going to be metric, right? The nearest I could get was M6x1.25 but these didn't fit. Wrong. All four of these screws are 1/4"-20. Mars Climate Orbiter anyone ^[16] ? (I can't believe how long it took to figure this out. It never occured to me in a million years. I didn't even pull out my Imperial taps and dies to measure them against.)

So off to get some 1/4"-20 knobs. My hardware store doesn't have any knobs. (My told me people are asking for them all the time. Well, why don't you sell them then?)

I found knobs at Lee Valley Tools ^[17] .

These nuts (M4x0.7) are inside, are not captive and don't have washers. They have survived 30yrs held with paint and luck, but some were loose. I don't like hardware on the inside of the OTA that can escape and crash into the optics. To remove the focuser, you first have to remove the plate, so you can hold the focuser nuts on the inside of the OTA as you unscrew the focuser. It would be nice to be able to remove the focuser without first removing the plate. This would require captive nuts and would be a bit of bother. I decided I wouldn't be removing the focuser a whole lot. Instead I replaced the inside nuts (both shoe and focuser) with aero/nylocks.

The Synta shoe on the scope is slightly trapezoidal (the current batch of Synta shoe hardware has parallel sides) and narrower than current Synta feet (the feet wouldn't fit) (maybe the new ones are metric :-( ). In my first attempt to fix the problem, I widened the shoe with a file. My 2nd Tasco 8V had the same problem. I replaced both shoes with current wider Synta shoes, which have a large gap between the shoe and the foot. The head on the screws holding down the shoe were countersunk oval. The oval head allowed the original finderscope shoe to fit, but not the foot on the red dot finder or the green laser holder. I replaced the screws on the shoe with countersunk flat.

While trying to align the scope on an artificial star in my living room one day, the image of the artificial star, instead of coming to a point, had spikes coming out radially from the point. Jim diagnosed it over the phone as a reflection inside the tube. Sure enough, the chromed draw tube was projecting into the body of the scope. Either arrange the geometry such that the draw tube is outside the OTA at focus, or only look at stars at night.

These were new to me. I used the Hubble Artificial Star ^[18] . This device isn't particularly well made. As well, after inserting batteries, you have to iteratively align the LEDs inside with the holes on the outside, to get any light out. The mask is separate from the device, and can get lost, when it could easily be an integral part of the device.

However the device provides satisfactory artificial stars. Now to see stars, you no longer have to wait for a clear night, go outside to fumble and drop expensive hardware, in the dark and freezing cold. Instead you can do it in your well lit living room, at a comfortable temperature, anytime, day or night. You can scratch your head and think about things, without having to pull everything down and bring it back inside. Amateur astronomy has come a long way.

Being in the living room, the artificial star is close (about 15' in my case) rather than at infinity, so you have to raise up the eyepiece to focus on the artificial star. I bought 2 extender tubes 2" long. This put the eyepiece as high off the OTA as the OTA is long. I assume the close distance of the target adds spherical abberation to the image. (Since the equatorial mount for my scope is in Australia, I'm using my Dobs mount to rest this scope.)

Figure 13. artificial star testing thumbnail

About this time I found out about Vixen 70001 extenders through a posting by Jae to Cloudy Nights ^[19]. Here's the photo ^[20] . These 70001 extenders are what you need to get height. They screw together (so don't wobble like the standard sleeve extenders) and having a large diameter, don't produce vignetting as do the sleeve extenders. Jae was kind enough to send me a spare 70001. Thanks Jae! It's the black cylinder (extender) between the chromed draw tube of the focuser and the Vixen 3720 36.4mm to 1.25" eyepiece adapter, in the two photos of the completed scope in the intro.

With Max's scope, I had the devil of a time trying to adjust the orientation of the primary using the adjusting screws. The primary moved erratically or not at all, and certainly not in response to my attempts to adjust the mirror.

There were 3 pairs of screws. Jim suggested I pull the cell apart to find out what was going on. The mirror was solidly glued to the back plate, as you would expect. He thought that I'd find that one of the pair was a pusher and one a puller, and I'd have to figure out which was which. It would be a good idea to put a spring on one of the screws, converting it to an adjuster, when the other would become the lock screw. Sure enough, that's what it was. The scope was designed so that to adjust the mirror, I had to simulateously adjust two screws; the pusher and the puller, in each of the 3 pairs. This was insanity.

I converted one screw to a spring tensioned adjuster screw. The other screw would be the lock screw, to hold the mirror in place. I expected I'd have to go on-line and wait for the springs, but my local hardware store had just what I wanted; springs that just fit over the screws. They were longer than needed, so I cut them down to length. To stop the cut end from bighting into anything, I folded it down with pliers and bent it inwards so that the spring gripped the screw and stopped the screw from falling out, on disassembly.

Figure 14. primary adjusting springs thumbnail

The photo shows the primary in the mirror holder. On the near side of the mirror holder is the site of a pair of screws. The left side is the locking screw. The adjusting screw has been removed and separated from its spring, in front of the mirror. An orginal length (uncut) spring from the hardware store is on the right.

Although I already had marked the mirror holder and mounting ring with marker pen, I needed further help to line up the screws with their holes, which became hidden as you joined the two pieces. Between messing around and aligning the scope, I wound up removing and putting back the mirror holder several times., I marked the locations of the screw holes on the outside of the mounting ring, so I could line up the screws with the marks and then push the two pieces together. Then the screws would just screw into their holes.

After inserting the springs, I found that one of the screws no longer bit the thread. Inspection of the cell showed that the first 4 turns of the thread were stripped. This is poor design - you can't hold the weight of a 125mm mirror in 4 turns of a 4mm thread in aluminium. The threads should have been designed longer or wider (see also tripod tray). I changed the screws to the longest that would fit the whole length of the thread. (They were about 30% longer than needed. I took up the slack with 6 extra washers on the outside of the back cover.)

I had read (Cloudy Nights?) that one fellow's primary was not adjustable for tilt. He had only 3 screws (not 6 as in Max's scope). He found no rubber gasket to fill the role of pushing the mirror against the adjusting screws. Milt's scope had only 3 screws. Adjustment of these did not affect the tilt either. Max's primary had been glued to the back plate. When I unscrewed the cell in Milt's scope, the mirror fell face down onto my work bench, covered in tools and screws. This was like opening up the scope to find a 30yr old turd left in there by Tasco. I was very lucky - the mirror missed everything. This is negligence. The mirror was not attached to the back plate or to the cell - it was just rattling in there. There was no rubber gasket pushing against the screws, and because the primary was not attached to anything, adjustment of the screws just moved the back plate, but not the primary. It was not possible to adjust the primary and it had never been possible. Tasco did not intend it to be adjusted.

I glued the mirror to the back plate.

Inspection of the mirror cell showed that one of the drillings, for the 3 screws from the back, was through the wall of the mirror cell. This would have been visible at the time of manufacture. I replaced the screws with long one and added springs on the screws sitting between the back plate and the mirror cell.

The first thing you do when collimating/aligning a newt is to adjust the roll and tilt of the secondary, to put the center of the primary under the cross-hairs of the cheshire. The center of the primary needs some distinguishing mark to identify its location. If the primary is even approximately adjusted, then behind the image of this identifying mark will be the image of the inside of the focuser, a uniformly black disk (if you have a cheshire in the focuser, you can illuminate it with a flashlight/torch and you will see a white circle in the middle of the focuser). The mark at the center of the primary must _not_ be a dark blob or you'll not be able to distinguish it from the inside of the focuser. I suggest the white self adhesive reinforcing rings, available at office supply stores. Even better, cut the outside of the ring to a non-circular shape, e.g. a square. If you use a barlowed laser (blug) for alignment, the image of this square ring will be projected back into the barrel of the blug.

To find the center of the primary, The usual procedure is to cut out a paper circle the size of the mirror, and cut out a hole in the center and lay the paper on the mirror and mark the center of the mirror with a marker pen. Somehow, I always wind up hitting the surface of mirror with the paper. Since paper isn't soft (it contains silica), I've stopped doing this. Instead I lay a soft (wood, plastic) ruler across a diameter of the mirror (you can find this by looking for the maximum size of the mirror) and marking a short line normal to the ruler. Rotate the mirror 90° and repeat. The center of the mirror is the intersection of the two marker pen lines. (The soft ruler contacts the mirror at at infinitely thin line. Laying a ruler across the mirror like this, won't affect the image as much as a paper scratch to the surface of the mirror.)

Because of some later confusion, when for a while I thought that the mirror wasn't centered in the OTA, I checked that the dot was in the center of the mirror, and at the center of the cell. It was.

Here's how I apply the square hole to the center of the primary.

	Warning
	This is not a particularly safe thing to do. The square ring landed correctly the first time. Repeating the exercise on the 2nd Tasco 8V, the ring flipped off the pencil, onto the wrong spot where it stuck. Fortunately there was a slight air gap in one spot. I had to do some careful surgery to move it with a box cutter, without scratching the primary.

I center the eraser over the central marker lines and allow the tilted corner of the square to touch the mirror. Then I move the square into place with the (blunted) point of the pencil, before tacking it down, by pressing lightly on the 4 corners of the square with the pencile tip.

Figure 15. applying square center marker thumbnail

Initial alignment with the laser, found that the dot in the middle of the primary was off by about 1/4". This is normally fixed by tilting the secondary or the focuser, neither of which I thought were adjustable on this scope. (As I found out afterwards, from a posting by Steve_M_M, see plate, the secondary has the usual adjustments.) I found by giving a judicious heave to the focuser that the laser dot now lined up reasonably well with the dot at the center of the primary (I wouldn't do this again, now knowing that the secondary is adjustable). Inspection of the outside surface of the OTA using reflection of daylight along the cylinder, showed no bumps or irregularities. Presumably the focuser was sitting on an undeformed OTA. Further heaving on a similar scale, didn't change the alignment any further.

I adjusted the primary to reflect the laser beam back to the laser. The alignment as viewed in the Cheshire, was sort of OK, but wasn't spot on and the more sensitive autocollimator showed that collimation was out of range. A defocused artificial star test with a 25mm eyepiece looked OK, but with a 10mm eyepiece, showed the scope not to be collimated. I was unlikely to be using a 10mm eyepiece on this scope. Did I care? Well yes I did. I'd come this far, I wasn't going to stop at this last step.

Jim suggested I use the star test, to iteratively adjust the primary (my only adjustment) to minimise the collimation error. I expected I'd have record the results and then shim the corners of the focuser (with pieces of manilla folder) and minimise again, till I found the minimum. Presumably it wouldn't be the exact minimum, but close enough.

Before I did this, with a near alignment, I wanted to check the vignetting. I did a few sweeps of the artificial star with my longest focal length 1.25" eyepiece (25mm). At focus I couldn't detect any vignetting. However even the dimmest of the 5 stars was bright, so this wasn't a good test. To look for vignetting, you want something that's just visible in the center of the FoV, and you watch for its disappearance as you move it to the edge of the FoV. In an attempt to dim the stars further, I defocused till one of the rings was quite dim. I swept the artifical star from side to side across the FoV to find to my surprise that the defocused rings changed shape. At the center of the FoV, the rings were circular and centered on the airy disk. As I moved the star to one side, the rings became elliptical, with the long axis pointing to the center of the FoV. The airy disk is at the focus of the ellipse nearest to the center of alignment. By adjusting the primary, I could circularise any elliptical pattern, offset from the center of the FoV, moving the center of alignment to this spot. Presumably I'm seeing uncorrected spherical abberation. I realised that until then, I'd made no attempt to bring the image under test, to the center of the FoV, this not being needed in a conventional Newtonian scope, which has little spherical abberation. Presumably the astigmatism I had seen was really spherical abberation, which I missed because I didn't undestand how the scope works.

Three parts of the secondary mount require a pin-type face spanner ^[22] to disassemble/unscrew/remove. You'll be using the face spanner near the glass plate. If you think you might slip, cut out some thick plastic (e.g. a ziplock bag) and tape it to the plate. You may have to clean your plate later. This is easier than removing a scratch.

My face spanner does not have enough inside clearance, to fit into the pin holes and clear the threads on the secondary mount (into which the secondary stalk is screwed). I filed down the inside of the jaws of the pin-type face spanner to fit. This photo shows the plate with the corrector lens removed.

Figure 17. plate with pin-type face spanner thumbnail

I used plastic gloves to avoid leaving fingerprints on the optical surfaces. As you can see in the photo below (secondary, cap removed), when I was finished, there are white patches on the OTA. I'd forgotten that the gloves were dusted with talc, which helps when removing sweaty hands from gloves. Talc is fine for working on cars. I just grabbed the gloves I had in the garage. For optical work, it would be better to use talc-free gloves, or at least first wipe off the talc.

The grey coloured intermediate sized annulus has three grub screws (M3x0.5, 1.5mm hex key) in the plane of the plate, which adjust pitch and yaw on the secondary.

One of the threads was tight enough to make alignment difficult. The opening for the thread was blocked so that I couldn't remove the grub screw. A drop of oil didn't help. Presumably the outer part of the thread in the aluminium body had problems. For the alignment, I didn't need to remove the grub screw, but I wanted alignment to be easier. With the secondary stalk out of the plate, I screwed the grub screw all the way in, allowing the outer part of the thread to be chased with a M3x0.5 die. I then removed the grub screw and chased the whole thread with the die. I cleaned the swarf with greenhouse gas blaster.

I was not happy to be using a hex key in close proximity to glass. Reinsertion of a standard (straight ended) hex key required some jiggling, a slow and accident prone process. Here are my solutions to the hex key problem (best to worst)

Here is the plate at furthest disassembly.

Figure 21. partially disassembled plate thumbnail

Left to right the pieces are

In front: pin-type face spanner for locking rings and for inner annulus on underside of stalk.

This photo show the plate after unscrewing the secondary mirror stalk.

Figure 22. secondary partly disassembled thumbnail

From top left, going clockwise

Here's the secondary stalk

Figure 23. secondary mirror stalk from bottom thumbnail

The perspective foreshortens the secondary stalk. The hex key is to stop the stalk rolling over, which would put the mirror face down. The photo shows

The three grub screws tilt the secondary. It works this way

Here's the secondary stalk disassembled. This was the last thing I figured out how to do. By the time I got here, I'd already figured out how to remove the secondary mirror.

Figure 24. Secondary Stalk disassembled thumbnail

While working on my first Tasco 8V (Max's), I couldn't find any way of further disassembling the secondary stalk. Doing so, I'd hoped, might show me a simple path to fixing the longitudinal offset problem of the secondary mirror (see longitudinal adjustment). (As it turns out, further disassembly didn't help. I reglued the secondary mirror instead.)

The inner annulus/slotted blade were frozen together. (Liquid Wrench didn't help, but then was not expected to, since the frozen surfaces weren't rusted iron.) Presumably one screwed into the other. Depending which screwed into which, you would unscrew the slot one way or the other, while holding the annulus with the pin-type face spanner (I was wrong; they were glued together). I tried both ways. A major problem with this approach was that the slot was narrow enough for only smallest screwdrivers. The small screwdrivers aren't wide enough to exert any significant torque. The screwdrivers that match the length of the slot are too wide for the slot and tear out the slot walls. If you're going to have a screwdriver slot, at least have one that fits a screwdriver, or better have a hex head which won't tear out.

The mystery, of unfreezing the joint, was solved by accident. My 2nd Tasco 8V (Milt's) was perfectly aligned, but I changed the inside nuts to aeros (nylocks) on the Synta shoe, and the focuser. While I had the scope open, I put a square ring in the center of the primary. Naturally the scope was now misaligned. On attempting to align it, I found the secondary stalk was stiff, sticky and difficult to adjust. It felt like it had been glued in. The outside surfaces of the secondary mount were covered in a varnish, which felt like glue. I soaked the base of the secondary stalk in a (glass) cup of acetone for 3hrs, to find that all pieces were now finger loose, including the inner annulus. In the meantime, before I discovered this, my efforts resulted in a badly torn up slot on Max's scope.

From this discovery, I saw that the slot is only to rotate the secondary mirror on alignment. It is not to be used to separate the inner annulus from the secondary stalk. Tasco had glued them together and didn't expect anyone to attempt to separate them. In this case the slot should have been small enough to indicate that only a small screwdriver was to be used. Poking a blade type screwdriver into the front of the OTA in proximity to the plate is not a good idea. Better would be a Phillips head or a hex head.

After successfully unfreezing Milts secondary stalk, I repeated the process with Max's secondary stalk. It wouldn't budge after 1 day in acetone, but after 2 days, could be screwed off, with the help of the pin-type face spanner. The threads were covered in sticky glue, which dried and hardened as the acetone evaporated. I removed the dried glue by repeated threading and unthreading of the annulus onto the secondary stalk.

Note the curved surfaces on the base of the secondary stalk, the top of the intermediate annulus, and the top of the inner annulus. (There is a curved surface on the outside of the intermediate annulus which fits the inner annulus, but it's hidden in this photo.)

There was no obvious way to affect the longitudinal offset of the mirror by shimming the base of the stalk.

On reassembly, there was nothing to hold the inner annulus (the circular nut) onto the thread at the end of the secondary stalk. It rotated freely and moved with the adjustments necessary for alignment, when you wanted it not to move. My first attempt to solve this was using a 7mm wavy washer on the thread. This provided too much tension. On tightening the inner annulus enough to stop it spontaneously falling off the thread, it became difficult to move the inner annulus with grub screws. The wavy washer worked, but not well. I tried a 5/16" ID neoprene washer instead. Being rubber, it provided friction to stop the stalk from moving around. The extra friction meant that less tension was required to stop the inner annulus from spontaneously falling off the thread, so less force was needed on the grub screws when moving the inner annulus. The inner annulus now projects out of the scope (is no longer flush with the end of the secondary mount), but it's better than gluing it in position.

Figure 25. Secondary stalk with neoprene washer, before reassembly thumbnail

This is a complicated device. I'd be OK with a complicated device, if they'd spent the extra dollar to make a brake for the OTA, which didn't crumple the OTA. Clearly there were separate budgets and design teams for the OTA mounts and the secondary mounts. The secondary design has the following problems

In my scope, as expected, the aperture (on the corrector cylinder) is centred in the cheshire. The cheshire looks through this aperture, to the secondary. The projection of the secondary should also be in the center of the cheshire's field. However, in my scope, the secondary is offset 3mm out of the OTA. The edge of the secondary is occluded by the aperture. This could be fixed by screwing the secondary stalk into the OTA, but it's already at its innermost limit, up against the plate. If you wanted to, you could move the secondary out of the OTA by unscrewing the secondary stalk and relocking it with the ring, but this doesn't help with the current problem.

	Note
	My second Tasco 8V has the secondary offset longitudinally by about 1.5mm. It only just fits into the aperture facing the focuser.

This occlusion of the secondary mirror causes vignetting, but doesn't prevent alignment. When the scope is aligned, not all of the primary is seen by the secondary, and part of the secondary is looking at the inside of the OTA.

You cannot adjust the secondary longitudinally into the OTA, requiring modification of the scope to fix the problem. You would expect that removing the locking ring entirely would allow you to screw the secondary stalk into the secondary mount, by the thickness of the locking ring. However there is only a couple of extra turns of thread on the secondary mount. This doesn't get you very far into the 3mm offset and was abandonned as a solution.

	Note
	When screwing in the stalk, without the outside ring, do this with the plate out, so you can watch the end of the secondary mirror as you screw it in, as if moves towards the corrector lens. You don't want to accidently screw the secondary mirror into the corrector lens.

This uncorrectable 3mm offset of the secondary shows abysmal QC at Tasco/Vixen.

A possible fix would be to remove the secondary mirror (presumably destructively, since it's glued down), and put a pad under the new secondary mirror to move it further into the OTA. Inspection of the secondary mirror holder, showed that it was recessed; the glued surface is not accessible and you can't even get a razor blade under it. This contravenes on of the big rules of making something - it must be able to be serviced, particularly if you aren't going to make it right the first time.

Warning

Using Si glue is not a good solution. The solution here was my first attempt at the problem. The Si glue is sticky and when applying it I wound up getting it on the front surface of the secondary mirror. I couldn't get it off without risking scratching the mirror. The Si glue is not reversible; if you don't like the job, you're can't remove all of the Si glue from the back (or sides or front) of the mirror and you're stuck with it. I would suggest (but haven't tested) the clear double sided sticky pads from office supply stores, that are used to stick posters onto walls. These are easy to peel off, but I don't know how well they'd be for a decade inside an OTA. Would the secondary mirror fall off one day?

To solve the longitudinal offset problem (see longitudinal adjustment), I had to move the secondary mirror into the OTA. This required removing the secondary mirror and repositioning it further into the OTA on top of a pad of Si glue II. But first I had to remove the secondary mirror. It appeared to be glued in (rather than being held reversibly by clips). Having already tried everything else I could think of, I found that soaking the secondary mirror in acetone for a day dissolved the glue (secondary sitting end down into a glass cup of acetone, covered with a saucepan lid, sitting in the garage). This separated the secondary mirror from the stalk, giving me the secondary mirror in pristine condition. This discovery, later allowed me to separate the inner annulus from the secondary stalk (they had been glued together, see glue in inner annulus).

Warning

There is a clip on the end of the secondary stalk. When I dissolved the glue on my first Tasco 8V, it did not cause any problems. The clip on the 2nd Tasco 8V, was folded down onto the secondary mirror. Not expecting any problems with the 2nd mirror, I undid the screw on the back of the clip, whereupon the clip rotated, crashing into and scratching the secondary. This clip has no function that I can see; it doesn't hold the secondary and if the glue came loose, it would not stop the secondary from falling off the stalk. Its only function is to scratch the secondary when you unscrew it. Another disaster from Tasco. I covered the scratched area with black marker from a felt tipped pen.

Figure 26. Ungluing the secondary: L-R secondary stalk, clip and screw, secondary mirror thumbnail

	Note
	Ungluing the secondary mirror exposed a shaft in the stalk, down to its base, but didn't give me any clue how to unscrew the central annulus on the outside end of the secondary stalk. At this stage I was hoping that doing so would allow me to move the secondary mirror longitudinally in the OTA. However once I found that I could unglue the secondary, I had another way of handling it.

I reglued the secondary onto the stalk with silicon glue II (the one that releases water on curing, not the one that releases acetic acid - smells like vinegar - which will pit the aluminium stalk). I put 4 blobs of glue onto the stalk large enough that the base of the secondary mirror is above the plane of the end of the stalk. The next person inspecting the mirror, will see that it's glued on by 4 blobs, and won't take long to figure out that the mirror can be removed from the stalk with a razor blade. The secondary mirror is now 3mm too far into the OTA. This is compensated for by unscrewing the secondary stalk in the secondary mount.

Here's the secondary mirror sitting on the tilted stalk, while the glue cures.

Figure 27. Gluing the secondary thumbnail

The clip (to left of secondary) is rotated 90°. In this position it scrapes against the inside of the cylinder. I rotated it a further 90° bringing it parallel to the axis of the OTA. (Presumably I should throw it away. It never had a purpose. I covered the screw with a blob of Si glue. It's unlikely to come loose.)

Longitudinal adjustment occurs by looking down the focuser and screwing in the secondary stalk till the secondary mirror is in the middle of the field of view.

I was lucky: This worked the first time (on Max's scope). If I ever wanted to remove the secondary mirror, I just had to cut with a razor blade, parallel to the top of the secondary mount. The bottom and sides of the secondary mirror would have Si glue on them, but this didn't seem to be a major problem. When I repeated the process on the secondary from Milt's scope, The mirror, instead of sitting on its bed of uncured Si glue, sunk into the cavity at the end of the stalk. (Perhaps I was trying to use less glue, so that if I ever wanted to remove the mirror, I would have less glue to cut through.) I had to hurredly fish the mirror out, with the blade of a box cutter. This resulted in removing the mirror with festoons of uncured Si glue hanging off it and on the end of the box cutter blade. On separating the blade from the mirror, I accidentally put a blob of wet Si glue on the mirror. Maybe I should have removed the wet glue, but thinking I would more likely wind up smearing it everywhere, I decided I would try to remove it when it was dry (I now think this was a mistake). I couldn't get it off with cotton balls at least and left the blob there. In the meantime I has a secondary covered in dripping Si glue. I rested it on the box cutter, to let it dry while tried to think of a better solution. When dry, I used the box cutter to cut off the excess Si from the secondary mirror, when I saw clearly that you just can't remove all of it.

I was now committed to the Si glue method. So that the secondary mirror would not sink into the cavity at the end of the secondary stalk again, I did the gluing in two stages. I put 4 blobs of Si glue into the end of the secondary, let them dry and cut them flush with the surface of the end of the secondary stalk.

Here's the four pads (note small blob of Si glue on surface of secondary mirror, closest to viewer)

Figure 28. Four pads of Si glue thumbnail

Here's the secondary curing

Figure 29. Secondary curing thumbnail

	Note
	You only need to do this if the aperture in the corrector assembly is wildly out of aligment. The aperture only has to show the whole of the secondary. The aperture is much larger that the secondary and almost any small misalignment can be ignored. The aperature on my scope was adequately aligned and I only removed the corrector cylinder to find out how it worked.

The corrector lens assembly is a cylinder with

The plate now only has the secondary mount, into which are screwed the secondary stalk (from the outside) and the corrector assembly (from the inside). This secondary mount is held in place by the inner locking ring.

Removal of the inner locking ring reveals a thin inside washer. On Max's scope, the inside washer did not separate from the secondary mount, nor did the secondary mount budge with pushing or attempts to rotate it. To remove the washer, I would have had to stick my fingernail between the glass and the washer. I didn't want to touch the glass. I didn't know whether the washer was hard rubber that, after 30yrs of being compressed in place, would shatter on attempts to remove it. On Milt's scope, the inside washer was loose and the secondary mount rotated freely.

On removal of the secondary mount, the only thing you could do with it is rotate it. However the piece is radially symmetrical and rotating it doesn't get you anywhere. I stopped the disassembly here.

With the corrector cylinder on the plate, reattach the plate (secondary stalk is optional). The axis of the corrector cylinder should be co-linear with the axis of the OTA. Through the cheshire, note the placement of the corrector cylinder in the direction transverse to the axis of the OTA. The profile of the cylinder will make chords with the field of view of the cheshire (you may have to raise the focuser to see the whole width of the cylinder). The chords should be the same length. (This can be seen if you illuminate the inside of the OTA, opposite the focuser, with a flashlight/torch. The cylinder will be black. The only light will be from the two segments bounded by the two chords and the circumference.) In Max's scope, one chord is about 50% longer than the other. Either the focuser is tilted transversally or the secondary assembly isn't in the middle of the plate. This is poor manufacturing, but can be compensated for during collimation.

Next you are going to align aperture under the cheshire. The aperture only needs to allow the secondary to be visible to the focuser (i.e. assuming the secondary is centered under the cheshire, which it isn't on my scope). If too far offset, it will only vignette the image in the eyepiece, but will not affect alignment/collimation. If the aperture is near enough to centered, you're done.

If adjustment is needed, the cylinder may have to be adjusted longitudinally and tranversally. This has to be done iteratively, removing the plate from the scope each time. It should be possible to do all adjustments from the outside. It's bad design when adjustments are done from the inside. However the procedure is simple in principle and should only take a few iterations.

Longitudinal adjustment probably won't be neccessary, but is done by successively rotating the cylinder 360° about its axis, moving it along the thread of the secondary mount. Once the cylinder is in the correct longitudinal position, transverse adjustment is done by small rotational adjustments.

I found a flashlight/torch useful to illuminate and identify various parts of the secondary, when I was looking through the cheshire. Identifying the secondary is not easy; the black insides of the OTA don't reflect much light from the flashlight/torch into the aperture.

You do the alignment like a conventional newtonian. Here are some tutorials ^[26].

Although almost any route you choose will get you there, by the time I'd finished restoring two scopes, I must have done the alignment dozens of times, the alignment changing a bit as the mechanics changed. Here's the way that matches my final setup.

When you attempt to tighten the wing nut on the tripod leg height adjustment, the bolt rotates with it. (This is a solved problem outside Tasco.) The size of the wing nut on the top of the leg, at the equatorial mount, and at the bottom, at the height adjustment, are different. Presumably Tasco is trying to maximize the number of parts used.

When removed from the tripod, the wing nut rotates freely on the bolt. The wing nut doesn't rotate freely on its washer when tight in the tripod. Some of the torque tightening the nut, instead is going to rotate the washer on the wooden tripod leg. Inspection of the bottom surface of the wing nut shows it isn't flat; it has a circular ridge around the perimeter left over from the stamping.

The wing nut is small and to get enough torque on it to tighten a tripod leg, your fingers feel the bight. This is not a tripod for little old ladies or retired men.

Miraculously both problems were solved by replacing the wing nut with the largest knob I could find for a M8x1.25 thread ^[28]. It has a flat base and rotates on the washer, rather than binding to it.

Table 3. Comparison of Tasco 8V original wing nut, ETX-80 tripod tightening knob and Tasco 8V replacement knob

Figure 37. Tasco 8V wing nut This is not a knob.

Figure 38. ETX-80 tightening knob This is a knob

Figure 39. Tasco 8V with M8x1.25 knob This is also a knob.

Note plywood protecting carpet.

After you tear up your fingers tightening the wing nut (or later a knob), and you put the tripod leg on the ground, the leg collapses under the weight of the scope.

What happened? Tripod wooden legs usually have flat faces and a pair of bolts to keep the centre element aligned as it moves. When you tighten the bolts, the center element having flat faces, is clamped down. The center element of the Tasco tripod leg however has an octagonal cross section and only one bolt (the bolt that rotates with the wing nut). The orientation of the faces is such that when the leg is in the air (and no weight on the point), the center element gets clamped a little askew (a little like cross threading). When the weight of the scope goes onto the point, the centre element is pushed out of its askew position, into an air gap where it's not clamped and the leg collapses. To get the leg clamped you have to rattle it as you're clamping it, to make sure that, at the final tightening, the center element really is centered. The construction of tripods is a solved problem. Tasco should use one of the known solutions.

Disassembly of a leg shows that the concave inside surface of the outer two leg members is rough cut. Presumably this unfinished surface is left over from the router, which cut out the concave surface. The rough surface doesn't allow the tripod leg members to slide freely. Instead they jam whereever they are when you first start clamping. I could have sandpapered the inner surfaces of the outer tripod leg members. Instead I thought it would be faster to line the rough cut surfaces with felt left over from making the OTA brake blocks. The outside convex surface of the inner leg member is lightly (1 molecule thick) varnished. I assumed it was smooth enough, but on sliding the inside leg member back into the felt lined leg, it caught on the felt as it slid in and out. The partially varnished surface of the inner member was rough to the hand. It hadn't been sandpapered before applying the one molecule thick layer of varnish. I removed the roughness with fine sandpaper (150 mesh, don't let the grit get anywhere near the optics; sand outside on the lawn then blow all the dust off the leg, before bringing it back in the house).

The pictures here show a leg of the tripod with the middle element removed and resting on top of the two outer elements.

	Note
	This worked OK, but not particularly well. One piece of felt rolled over a little bit and the sticky side caught on the sliding leg element and peeled off. I don't expect the felt solution will work for a long period (e.g. 10yrs) without having to reline the tripod legs all the time.

Table 4. Tripod leg before and after lining with green felt

Figure 40. Tripod leg before modification tripod leg before modification.

Figure 41. Tripod leg after modification Tripod leg after modification

I sanded the inside of the outer elements on the leg where the felt peeled off. Over a couple of night's observing, this worked just as well as the felt lining. Since sanding will last longer than the felt, I removed the felt and sanded all three legs.

The tripod has pointed feet. The scope and tripod weigh 11kg. The points puncture carpet and do nasty things to hardwood floors. The scope and tripod are not house trained. It's hard to imagine anywhere you'd need these points - ice? Do people sledgehammer the legs into the ground? The scope isn't going anywhere in even a stiff breeze even without the points. Even if you cut off the points, the remaining foot is still too small to be house safe and sinks into soil wet after rain making it hard to keep the polar alignment.

I know many tripods have points. Is it because everyone else's tripods have them and people have come to expect them? (Hey, this tripod wouldn't puncture cardboard!) I have one photographic tripod where you can at least retract the points.

The pointed feet are held in by two small Phillips head wood screws.

Figure 42. Tripod leg point

tripod leg point.

In principle, you can thread the wooden end ^[29] and attach a glide ^[30] but the glides are limited to a tilt of 15°. A tripod needs at least 22.5° (30° would be nice). I've run into this problem before, when trying to make feet for tripods for indoor use.

Amateur astronomers operate where the stars are visible. The ground underfoot can be anything (including wet grass in winter that becomes mud after a couple of hours of walking on it). You put down a tarp or mat so that in the morning, you can find what you dropped in the darkness (lens caps...) and to save eyepieces falling in the mud. Tarps and mats are incompatible with pointed tripods. Here's a collection of tripod legs and tarps at the Spring 2013 Staunton River Star Party (one person had the tripod legs sitting on the large plastic lids from pain buckets).

Table 5. Tripod Legs at the Staunton River Star Party

Figure 43. homemade equatorial	Figure 44. unknown	Figure 45. Meade
Figure 46. modified commercial	Figure 47. wooden

It's clear that few regard their tripod legs as suitable for amateur astronomy. Don't amateur astronomy tripod suppliers go to star parties to see how their products are used?

I took my Tasco 8V to this star party and wasn't prepared to rip up my astroturf carpet. As it turns out both lens caps came off my polaris scope during the night (even though I didn't use the polaris scope; it must have happened when I was taking the cover off). I noticed one was missing before I left the party. The other I didn't notice till I got home. I returned and found it (whew!). All this because Tasco can't supply a tripod suitable for amateur astronomy.

Follow the above examples, I drilled some wooden blocks at an angle to take the points.

Figure 48. Tasco 8V tripod leg pad; scope is now observing safe and house trained

Note carpet. The larger part of the foot (about 1" diam) is sitting on top of the wooden block. Only the point is in the wood. This setup seemed like a good idea, till I forgot to take the wooden blocks with me to an observing session. Here's the cure. The rubber feet are glued on with Si glue II.

Figure 49. Tasco 8V tripod rubber feet

The tripod tray is attached by screws (M5x0.8, 6mm long) on the tray to a flange on the tripod leg. The length of the thread on the screw is short (3 turns), requiring exact alignment of the flange before you can attach the knob. In the field, you'd like to be able to attach the tray without looking underneath to line up the screw exactly. A longer screw thread would have solved this problem. The tray is designed to be attached on a bench at home with the equatorial mount not attached, not in the field with the mount and scope already attached. Tasco seems to like short screws (see also primary cell threads stripped). I changed the screws to 10mm long screws. Unfortunately the knobs aren't threaded the whole way through, so you can't use all of the extra length. I offset some of the extra length with 2 washers under the screw head on the top side of the tray.

I glued the rubber washers between the flanges and the tray, to the tray.

The tripod has only limited extension of the legs (19cm, giving extra height of 19cos(22.5°)=17cm). You have to do your observing from a chair, rather than standing up.

If observing overhead, you need to use the finder scope (which I replaced with a red dot finder) to initially locate your object. To get under the finder, you need to get your head down really low. You can either lie on your back in the mud, kneel with your knee in mud, assume the position of a suckling calf or have a neck that swivels like a goose. (You could have put a tarp on the ground, but the tripod is not compatible with a tarp.) This is painful. This would have been solved if Tasco has used a 2' longer center element for the tripod. When used on sloping ground, you loose the height necessary to compensate for the slope. Clearly no-one who reviewed this scope ever used it.

The tripod height is fine if you're looking at the horizon or testing on an artificial star inside the house.

Here's the tripod in the lowered (unextended) position.

Figure 50. Finished tripod

where otherwise not noted