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Mike's 16" Dob Build

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#1 Other_Mike

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Posted 25 February 2020 - 12:24 AM

In May of 2019, someone was selling their old 16" Meade Starfinder for a very reasonable price (pic 1). I didn't take a look at the mirror, or even offer more than a passing glance, because I was worried about spending money that I shouldn't, and I knew there was no way I could fit that thing anywhere convenient. After much internal back-and-forth, and thinking about the possibility of a truss tube design, I ended up buying it a few months later when it hadn't sold yet. Here are the chronicles of my build -- things might seem a bit out of order in places, because I kind of jumped around during the process.

 

lrya (1).jpg

 

I had to shrink all of these pictures considerably so I could share more than one or two per post. The full-sized versions are on Imgur here: https://imgur.com/a/...nTy/layout/grid

 

My initial, and overly-optimistic idea, was: cut off the top portion of the OTA, build a mirror box around the existing mirror cell, repurpose the old bearings into a new rocker box, and connect both ends with some trusses. Discussions with folks in my club and other ATM-ers thankfully steered me towards Kreige and Berry's book on the matter, and I decided to start from scratch. This decision was cemented when I took the scope apart and found the flotation cell and tailgate it came with to be woefully inadequate. The floating cell was nine points, with extremely thin corner pads, and the "tailgate" was just a thick piece of particle board with holes drilled for the flotation cell and collimation bolts.

 

I skipped K&B's measurements for the floating cell and tailgate, and instead used PLOP for the cell and designed the tailgate to accomidate the collimation bolts. PLOP told me I'd be best with an 18-point cell. I used 1/4" aluminum (pic 2) because between that and 1/8" steel, the aluminum would get to my house in time to use the metal-cutting bandsaw at my club's workshop.

 

lrya (2).jpg

 

As it turned out, the bandsaw didn't have a metal-cutting blade installed, so my wife's tool collection got the new jigsaw she'd always wanted, and I spent a few days in the garage.

 

Pic 3: floating cell pieces cut out and pilot holes drilled, lined up on the old particle board to make sure things looked symmetric.

 

lrya (3).jpg

 

Pics 4 and 5: assembled floating cell components, using K&B's guidance on fasteners. The springs are the old collimation springs from the original design. I opted to reuse them instead of tapping threaded holes in the tailgate.

 

lrya (4).jpg

 

lrya (5).jpg


Edited by Other_Mike, 25 February 2020 - 12:38 AM.

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#2 Other_Mike

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Posted 25 February 2020 - 12:28 AM

Pics 6 and 7: cutting one of the split bolts for the sling support.  This was pre-tailgate, so I had to get creative with a vice to hold the bolt steady.  I wish I'd been more patient; the second one cut much easier after having installed it in the tailgate.

 

lrya (6).jpg lrya (7).jpg

 

Regarding the tailgate, this was another place I deviated from K&B. I was looking up build videos on YouTube and found one person who suggested using square aluminum tubing and U-channels instead of flat stainless steel, because the only tools required are a drill press and jigsaw, versus hiring someone to weld it. I found all the aluminum I needed in the scraps bin at a local metal supplier and paid less than $30 for them.

 

Pic 8: Tailgate roughly assembled without fastening, but with the floating cell components attached.

 

lrya (8).jpg

 

Pic 9: Same, with mirror resting on floating cell. It fits!

 

lrya (9).jpg

 

Pic 10: Tailgate and floating cell assembled, with side pins installed, but prior to split bolts being added.

 

lrya (10).jpg

 

Pics 11 and 12: Mirror in the finished tailgate, front and back.

 

lrya (11).jpg lrya (12).jpg

 

Pic 13: Ronchi test; someone in the club really wanted to test my mirror. I'm not trained in interpreting the results, but the concensus I got was "there's a bit of a bump in the middle that won't really matter since it'll be directly below the secondary."

 

lrya (13).jpg


Edited by Other_Mike, 25 February 2020 - 12:38 AM.

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#3 Other_Mike

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Posted 25 February 2020 - 12:31 AM

Pics 14-16: Painting the tailgate in appliance epoxy, as suggested by some of the ATMers in the club. I only did the upper and side surfaces, as the back shouldn't contribute to scattered light in the mirror box. I may do it eventually for looks.

 

lrya (14).jpg

 

lrya (15).jpg

 

lrya (16).jpg

 

Pic 17: a support for my cooling fan, before being stained and anchored via one of the collimation bolts.

 

lrya (17).jpg


Edited by Other_Mike, 25 February 2020 - 12:39 AM.

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#4 Other_Mike

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Posted 25 February 2020 - 12:34 AM

Up next was the secondary mirror.  The mirror that comes with a 16" Starfinder has a 4" minor axis, which seems to be oversized from what I could turn up. Just looking in the focuser before disassembly, I could see a lot more of the inside of the OTA than I'm used to with Newtonians.

 

I picked up a never-opened 80mm mirror on the classifieds here (which turned out to be 75mm, but it's close enough and has since been field-proven, so I don't really care). After removing the original 4" mirror, I cleand up the holder and glued the new secondary in place. I accounted for a 4mm offset, used nickles as spacers, and used three big globs of extra-viscous E6000 for adhesive.

 

I did a bit of research in this area -- lots of mixed responses from various ATM forum posts, and a heavy divide on the consensus with E6000. The biggest complaint seemed to be low viscosity, but the stuff my wife works with is super thick, and it ended up working great.

 

Pics 18-20: prepping and gluing the secondary to the holder.

 

lrya (18).jpg lrya (19).jpg

 

lrya (20).jpg

 

Pics 21 and 22: after 48 hours cure time, plus reassembling the secondary holder. As a test, I gripped the mirror and holder and gave a bit of a twist -- there was some give, but it held strong.

 

lrya (21).jpg lrya (22).jpg

 

Pics 23 and 24: before and after flocking the sides of the mirror with Black 3.0.

 

lrya (23).jpg lrya (24).jpg


Edited by Other_Mike, 25 February 2020 - 12:40 AM.

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#5 Other_Mike

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Posted 25 February 2020 - 12:37 AM

This is where things stalled for a month. I found out that A) my upper cage inner and outer diameters should be 18" and 21" respectively, and B) the plywood I already had in the garage was 20" wide. I spent the month waffling over options without really deciding anything, before eventually settling on cutting out the rings and leaving a flat spot where the 21" circle intersected the 20" board. This ended up working out very well for me, as it A) gave me a flat surface to rest the cage vertically on in the garage, and B) gave me a secure flat surface to attach the focuser board to.

 

Pics 25-31: the process of cutting out, drilling pilot holes and seats, and staining the upper cage rings.

 

lrya (25).jpg lrya (26).jpg

 

lrya (27).jpg lrya (28).jpg

 

lrya (29).jpg lrya (30).jpg

 

lrya (31).jpg

 

I used as steady a hand as possible and a jigsaw to cut out the upper rings. They weren't quite identical, so I clamped them together and used the jigsaw again to clean them up. I get kind of anxious trying new tools for the first time, which is why I didn't use my wife's router for this step (also, there wasn't enough clean workbench surfaces for that yet). As a result the upper rings are a little messy, but have worked well enough.


Edited by Other_Mike, 25 February 2020 - 12:45 AM.


#6 Other_Mike

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Posted 25 February 2020 - 12:44 AM

I ordered a set of four 12" long aluminum tubes from David Kriege. They needed to be cut down slightly (done by rotating them against a stationary jigsaw), then have holes drilled to hold the secondary spider.

 

Pic 32: my hole-drilling jig to get every spider hole in the same spot.

 

lrya (32).jpg

 

Pic 33: secondary cage assembled with tubes.

 

lrya (33).jpg

 

Pic 34: focuser board test run.

 

lrya (34).jpg

 

Pic 35: focuser board stained, attached, and adding braces to the back when it was found to flex too much.

 

lrya (35).jpg

 

Pic 36: upper cage now complete with focuser board, Telrad base, and finder dovetail.

 

lrya (36).jpg

 

I also wanted to build this scope with as much dew-removal capacity as possible (thank you, Pacific Northwest weather). I added three 12-ohm resistors to the back of the secondary mirror and ran a power supply cable along one of the spider vanes, with a quick release on one of the support tubes.

 

Pics 37 and 38: secondary mirror dew heater in progress (later added more tape and painted cable black).

 

lrya (37).jpg lrya (38).jpg


Edited by Other_Mike, 25 February 2020 - 12:45 AM.

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#7 Other_Mike

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Posted 25 February 2020 - 12:47 AM

I think that's all for tonight. I need to find food and get some sleep -- I'll post more tomorrow night.


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#8 grzesznypl

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Posted 25 February 2020 - 02:01 AM

I love those "build threads" because from every single one I can learn something if not from OP then from members commenting. I like your telescope except 

"floating cell pieces cut out and pilot holes drilled, lined up on the old particle board to make sure things looked symmetric" .... you did used PLOP, right? They do not only have to look symmetric but have to be in exact, optimal spots to reduce stress on mirror. 

Secondly, from what I have learned, kevlar belt is know to be probably one of the worst choice of edge support and is notoriously known to create stress at the edge of the mirror. Howie Glatter's Cable Sling or whiffletree is much better choice. 

Otherwise looks nice ... good job.



#9 Other_Mike

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Posted 25 February 2020 - 02:45 AM

Yes, I did use PLOP, and I measured everything as precisely as I was able for drilling the holes and positioning the supports. The layout on the particle board was just a reality check to make sure there weren't any glaring errors up to that point.

 

Regarding the edge support, I've seen discussions about fabric sling vs cable vs whiffletree. Fabric sling vs cable seemed to be splitting hairs in terms of optical benefits (based on discussion threads here on CN), and I wasn't convinced that the extra effort of precisely aligning a cable to the center of gravity of the mirror was worth it. The fabric I got is as wide as the mirror is thick, so it should be distributing the weight sufficiently,and I haven't seen any detectable astigmatism or other aberrations on the nights I've been able to take the scope out.

 

Whiffletrees look like a great option, but that was something I wasn't ready to redesign the tailgate to accommodate at this point. It also would have likely been one more thing I'd have to buy instead of building myself, and I was trying to keep the project on a minimal budget where possible.

 


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#10 grzesznypl

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Posted 25 February 2020 - 11:52 AM

Original telescope was f/4.5, right? From imgur.com looks project is finished. What is total weight of your new telescope?  How did the first light go??



#11 Other_Mike

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Posted 25 February 2020 - 12:06 PM

The original scope was listed as f/4.5, but when we measured the FL during the Ronchi test, we got 71", so it's more like f/4.4375, which I've been rounding to f/4.4.

I weighed most of the individual components when calculating the balance, but haven't put it all on a scale. The OTA should be approximately 66 pounds.

First light went great! Every eyepiece came to focus, which was great because my biggest concern was that I'd measured something wrong. I confirmed at low power that I got an appropriately-sized secondary mirror, as I could detect no vignetting at the edge of the field, and bright stars just blinked out as they left the FOV without any dimming.

Second and third light went better, because the moon wasn't up. I could see loads of detail in M42 that I haven't seen with my 12", and the shape of the Horsehead Nebula became apparent after looking for enough time - previously, I'd only seen a bit of a dark gap in IC 434.  Also, all of my dew heaters worked great, and my 12V battery kept things running without a struggle.

I'm really waiting for galaxy season to put it through its paces.



#12 Other_Mike

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Posted 25 February 2020 - 12:09 PM

Back to the build . . . I'll post a few more updates before work.

 

K&B's book left the section on pole seats for the upper cage pretty open-ended, so I came up with something of my own design that let me A) set the cage directly on top of the poles without needing three arms, and B) didn't risk me dropping anything on the mirror (Yes, I have a cover for the mirror box, but that doesn't mean I'll remember to do everything in the right order each night). The design I came up with has some flaws -- the shape makes it a struggle to get the poles seated, and even removing the cage at the end of the night can be difficult. I'll probably modify the seats at some point, with a design that has a central plug that presses the poles into the upper cage when tightened. But, in the meantime, this has proven itself adequate.

 

Pics 39 and 40: a brick of two 3/4" pieces of plywood glued together, after drilling a series of 1.25" holes at an approximately 10-degree angle (based on theoretical calculations of what angle the truss poles should eventually sit at).

 

lrya (39).jpg

 

lrya (40).jpg

 

Pics 41 and 42: cutting the pole seats on the bandsaw.

 

lrya (41).jpg

 

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Pic 43: pilot holes drilled, bolts for tightening in place temporarily.

 

lrya (43).jpg

 

Pic 44: staining.

 

lrya (44).jpg



#13 Other_Mike

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Posted 25 February 2020 - 12:16 PM

For the clamping blocks on the mirror box, I'd heard a few folks say there were some design flaws with K&B's split block design, and I frankly couldn't make sense of their instructions. I went with the simplified design. Pics 45-47 document this. The blocks themselves were 1/4"-3/4"-3/4"-1/4" plywood sandwiched together and glued. The holes were drilled with a large forstner bit, later widened a bit with a spindle sander, and the splits cut with a table saw. Each block has a bolt glued into a recessed opening in the back of the split section, and is tightened with a wing nut.

 

lrya (45).jpg lrya (46).jpg

 

lrya (47).jpg

 

Pic 48 shows one after staining and attaching to the mirror box -- I used a popsicle stick to shim them inward slightly, a whole lot of wood glue, two screws going all the way through from the front, and two shorter screws coming up from underneath so as not to bridge the gap of the split. Final screw and pilot hole positions were determined AFTER figuring out the truss pole positions much later on. Each also had a small bracket installed as a stop for the truss poles.

 

lrya (48).jpg

 

I initially hadn't planned to shim the blocks inward, but noticed how much tension was in the poles when trying to bend them inward to fit in the cage after they arrived. If I find myself building a mirror box in the future, I'll integrate the shim angle into the blocks themselves, probably with a jig on a table saw (now that we have one at home).



#14 Other_Mike

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Posted 25 February 2020 - 12:20 PM

I waited until I could use my club's table saw to begin cutting the mirror box out. My balance calculations said it should be 18" high, and the inside dimensions would be 18.25" square to account for the tailgate U-channels. Since I didn't initially account for the wall thickness of the U-channels, the top rung would eventually need to be spaced from the mirror box walls using 1/8" thick nuts.

 

Because sometimes engineers are bad at math, I got to the telescope workshop and realized I didn't have enough plywood to cut out all four walls (remember my "should I buy more plywood?" dillema from cutting out the upper cage rings). I redid my math and figured I could cut out two of the side walls and just cut the front much shorter than K&B's design where a slight arch connects the sides to the bottom of the front.

 

Pic 49 shows the three walls I managed to cut out, post-assembly.

 

lrya (49).jpg

 

In the meantime, I could at least work on the top of the mirror box (and check for tailgate fit). Pics 50 and 51 show the top cover being marked, and fitting on the box after cutting.

 

lrya (50).jpg

 

lrya (51).jpg

 

Pics 52 and 53 show that the tailgate does indeed fit (horray for correct measurements!) with a piece of scrap plywood standing in for the missing wall.

 

lrya (52).jpg

 

lrya (53).jpg

 

Pic 54: the fourth wall cut out and installed (my circular saw worked great for this; not so much when it came time to cut out the rocker components later on).

 

lrya (54).jpg



#15 Other_Mike

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Posted 25 February 2020 - 12:23 PM

Pic 55: clamping the tailgate in place to mark and drill pilot holes.

 

lrya (55).jpg

 

Pics 56-58: mirror box fully assembled, tailgate in place, and mirror in tailgate.

 

lrya (56).jpg lrya (57).jpg lrya (58).jpg

 

Pics 59 and 60: corner braces pre- and post-installation.

 

lrya (59).jpg

 

lrya (60).jpg

 

Pic 61: Now with corner edges cut! The arc that K&B suggest is pretty minimal, so I just went with a decent curve that ended at the shortened wall I cut out previously.

 

lrya (61).jpg

 

Pic 62: Split blocks roughly installed at what should be close to their final position, partly so I can begin staining, and partly so I don't misplace them in the garage.

 

lrya (62).jpg

 

Pic 63: a look inside the mirror box at the tailgate, after staining the interior.

 

lrya (63).jpg



#16 Other_Mike

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Posted 25 February 2020 - 12:26 PM

Last update today before I leave for work.

 

I stained the mirror box in stages. If I'd had a bigger shop, I would have done it all at once at the very end, but I wanted to get the tailgate permanently in place so I'd have somewhere safe to store the mirror, and I wanted to add handles so the thing was easier to move around with a heavy mirror inside. Since I wanted stain and polyurethane under all the hardware, I took care of those areas first, then installed the hardware.

 

Pic 64: mirror box top, and areas to be under hardware, stained and sealed.

 

lrya (64).jpg

 

Pic 65: split blocks now stained, more areas of mirror box stained. Sides were left unstained so as to have a bare wood surface where the side bearings would later mount.

 

lrya (65).jpg

 

Pics 66 and 67: a look inside the nearly-complete mirror box, now with the interior stained black and sealed.

 

lrya (66).jpg lrya (67).jpg



#17 Garyth64

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Posted 25 February 2020 - 04:47 PM

Very nice looking build.

 

Can I interrupt and ask how far inside the focus is the secondary?

 

And do you know how far inside the focus the secondary was in the original tube that used the 4" secondary?



#18 Other_Mike

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Posted 25 February 2020 - 05:27 PM

It's 14" from the center of the secondary to the focal plane, with that being 1/2" above the fully racked-in focuser. I'm pretty sure I took measurements of the original tube but I don't have them on me - I'm pretty sure the distance was very similar. I'll report back if I find them.

Edited by Other_Mike, 25 February 2020 - 05:28 PM.


#19 Garyth64

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Posted 25 February 2020 - 07:17 PM

I thought it would be something like that, so with that information, I'm not sure if a 80mm (3.14") secondary is adequate. 

 

with a fully illuminated field of .64" . . . according to an old formula"

 

(16" - .64") x 14"       +     .64"       =   3.66" secondary

         71"

 

The cone of light alone is 3.2", and the 80mm doesn't even work for that.

 

I would have used the 4" secondary.   smile.gif



#20 Other_Mike

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Posted 25 February 2020 - 07:39 PM

There was a lot of back and forth about this, and the consensus I found was that the 4" was oversized - both from people who have used the 16" Starfinder, and from discussions with my local ATM group. The spider also vibrated badly with that mirror on it, but is much more steady with the smaller mirror.

I used the percent-illuminated-field-simulator after being suggested it several times, and found that with my lowest-power eyepiece, the 4" would fully illuminate to the edge of the field. I also learned that this is only necessary or practical for imaging - that the amount of vignetting with a smaller mirror wouldn't be detectable to the human eye.

After some field tests, I can safely say that I can't discern whatever level of vignetting is present, so I'm happy with the smaller central obstruction.

#21 Garyth64

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Posted 25 February 2020 - 08:00 PM

Ok, but the math is still there.  I, myself, only trust the math.

 

The 4" only blocks about 6% if the incoming light.  That is still pretty good.

 

The 3.1" blocks about 4%.  I don't know if I could tell the difference of 2%.

 

Back to the build!

 

-clear skies!


Edited by Garyth64, 25 February 2020 - 08:03 PM.


#22 Other_Mike

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Posted 25 February 2020 - 10:39 PM

When it came time to figure out bearing surfaces, like many ATMers, I found that what had been considered the gold standard was discontinued and impossible to find. I even got a free sample of Ebony Star, and found it to be totally smooth when it arrived. Thankfully, the folks in my club pointed me to a thread here on CN that referenced fiber-reinforeced plastic as a suitable alternative, with some people even swearing it was better than Ebony Star. My local Home Depot sold 4x8 sheets of the textured stuff for $34, so I picked one up and somehow managed to cram it into my car (pic 68). This stuff was so floppy I had to use tie downs just to stiffen it enough to get it in the car.

 

lrya (68).jpg

 

This stuff is also a pain to cut, leaves dust everywhere, and smells awful. I highly recommend safety glasses, gloves, and a respirator when dealing with it (and keep your shop vac handy). I managed to get through it with a combination of tin snips, and scoring / folding / tearing. Pic 69 shows my cut pieces. Of the big sheet that I started with, I sold a 4x4 section to another ATMer in my club, and donated the remainder to my club's workshop.

 

lrya (69).jpg

 

The bearings themselves are 1" plywood, made by sandwiching two sections of 1/2" plywood with copious amounts of wood glue and a pin nailer, then left for 24 hours with weight applied (pic 70).  I went with a 22" diameter disk.

 

lrya (70).jpg

 

I had initially planned to make a circle-cutting jig for my jigsaw (pics 71 and 72), but as pic 73 shows, it refused to cut straight.

 

lrya (71).jpg lrya (72).jpg

 

lrya (73).jpg

 

The blade would both wander inward, and tilt. I finally got over my own hesitation and built a jig to use with my wife's router (pic 74).

 

lrya (74).jpg

 

The initial pass (pic 75) was definitely on the right track compared to the jigsaw.

 

lrya (75).jpg


Edited by Other_Mike, 25 February 2020 - 10:41 PM.


#23 Other_Mike

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Posted 25 February 2020 - 10:43 PM

The router could only plunge just under 1", and we didn't have any longer bits, so the final cut was made by jigsaw, the bearings were flipped over, and the remaining fraction of an inch cut by the router from the other side. Unfortunately, the weight of the scrap when making the final cut tried to delaminate the top ply, so I glued it back on (pics 76 and 77) and cleaned up the remaining edge on another day (pic 78).

 

lrya (76).jpg

 

lrya (77).jpg

 

lrya (78).jpg

 

Pic 79: bearing disk cut in half.

 

lrya (79).jpg

 

Pic 80: outlines for split blocks marked.

 

lrya (80).jpg

 

Pic 81: outlines for split blocks cut out.

 

lrya (81).jpg

 

Pic 82: bearings resting passively on the mirror box after opening up my initial estimates a bit with the bandsaw and jigsaw.

 

lrya (82).jpg


Edited by Other_Mike, 25 February 2020 - 10:44 PM.


#24 Other_Mike

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Posted 25 February 2020 - 10:47 PM

I glued the bearing surfaces on using contact cement as K&B suggested, but it didn't work out so great. On the first try, the cement mostly got absorbed by the grain of the wood, so I had to repeat the process. I added a lot more cement the second time around and clamped the FRP in place while the cement cured. Pic 83 shows the bearings after trimming excess with the router, a very smelly and messy process.

 

lrya (83).jpg

 

After evaluating the bearings' placement on the mirror box, I opted for five anchor points in each one. In an effort to make the bearings line up with each other as closely as possible, I clamped them together and drilled all 10 pilot holes on the drill press before unclamping them (pic 84). I used a single cardboard template to mark where the pilot holes should be on each side of the mirror box (not pictured), using the top and back of the mirror box as reference points independent from the split blocks.

 

lrya (84).jpg

 

It turned out that my split blocks had enough variation on them that I had to further trim the left side bearing to make all the pilot holes line up, but I eventually got it to work (pic 85). Finally, I glued some "pizza slice" wedges to bridge the gap formed by the front truss poles (pic 86).

 

lrya (85).jpg lrya (86).jpg

 

Note: I made the bearings earlier on, but only installed them once the truss poles arrived -- thus the poles being present in pic 85, even though we haven't gotten there yet.

 

I figured now was as good a time as any to clean the primary mirror (pics 87-90). I knew from the previous owner that there were a few places the coating was gone thanks to "rodent activity" while in storage, plus some scratches I found that were due to the original mirror clips rubbing on the edges. Even so, the mirror looked worlds better after a quick bath and rinse with distilled water. I'd still like to get it recoated in a year or two -- some people in my club have experience with spray-on silvering -- but so far it's performed beautifully.

 

lrya (87).jpg lrya (88).jpg

 

lrya (89).jpg lrya (90).jpg


Edited by Other_Mike, 25 February 2020 - 10:49 PM.


#25 Other_Mike

Other_Mike

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Posted 25 February 2020 - 10:50 PM

It was at about this time my truss poles arrived. David Kriege no longer carried the 1.25" by-the-foot thin wall aluminum tubing (not sure why it's listed on his site), but I happily found a source on Amazon with only slightly thicker walls (0.050, I think) and a length that would leave minimal scrap.

 

Pic 91: first run with poles installed. Looks like I had the split blocks at too great of an angle!

 

lrya (91).jpg

 

Pic 92: one screw removed from each split block. That's better!

 

lrya (92).jpg

 

Pic 93: With secondary cage attached for the first time, and it looks like a telescope.

 

lrya (93).jpg

 

I spent a lot of time fiddling with pole positions and taking measurements awkwardly until I finally convinced myself that I knew where to make my cuts. Pics 94 and 95 show the before-and-after, which went very quickly thanks to a pipe cutter a friend lent me.

 

lrya (94).jpg lrya (95).jpg




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