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Mike's 16" Dob Build

ATM DIY dob
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#26 Venator

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Posted 25 February 2020 - 10:52 PM

Love the thread! Great build! I may be jumping out of sequence here, but how do you like the FRP as a material in use in the field? I’m considering it for my build. I’ve read plenty of threads on it and the opinions range from best thing since sliced bread to a little too bumpy. Would like to hear your thoughts on it.

#27 Other_Mike

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Posted 25 February 2020 - 10:52 PM

It was finally time to cut out my rocker box. I'd determine the width based on adequate clearance with the tailgate bolts, the wall height based on the side bearings plus one inch clearance with the mirror box for cables and collimation bolts, and the front-to-back distance just based on the size of the mirror box.

 

Pic 96: the pieces cut out at the club workshop, featuring a cameo by the mirror box and the donated FRP (I also took this opportunity to add a donut sticker to the newly-cleaned mirror, which only had a barely-visible center dot).

 

lrya (96).jpg

 

Pic 97: Back in the garage and lined up, pre-assembly.

 

lrya (97).jpg

 

Pic 98: Arcs for the bearings cut, and sharp corners removed, on the bandsaw. I opted not to bother with the router here, since with only two contact points, the arc didn't need to be perfect. I did, however, clamp the two walls together when cutting on the bandsaw to keep things symmetric.

 

lrya (98).jpg

 

Pic 99: And just for fun, tidying up the front edge with the router.

 

lrya (99).jpg

 

Pic 100: The azimuth bearing and its cemented-on FRP, post-cleanup with a router. I still had trouble with the FRP bonding to plywood, even without edge grain to soak up the cement, and later used a lot of E6000 to tighten down the corners. This also seemed much simpler than cutting out a ring of the correct diameter, and it meant one less surface I'd have to stain or seal.

 

lrya (100).jpg

 

Pic 101: The mirror box fits! There's no gap because I hadn't added the Teflon pads yet; I was saving that for after staining.

 

lrya (101).jpg



#28 Other_Mike

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Posted 25 February 2020 - 10:56 PM

For the ground board, I went with just reusing the old one, so I wouln't need to worry about the center bolt or plastic insert. In pic 102, you can faintly see a bit of the math I did to determine the size of the triangle and location of the feet, which are just drawer pulls from Home Depot.

 

lrya (102).jpg

 

Pic 103: After roughly cutting out the triangle with a jigsaw on the "fast" setting. What a size difference! The original ground board was a 32"-diameter circle that felt like it was four feet across.

 

lrya (103).jpg

 

Pic 104: After sanding and attaching hardware.

 

lrya (104).jpg

 

Pic 105: Installing iron-on edge banding to seal the edges. My wife was convinced going straight polyeurethane would soak in so much it would swell, and she's the more experienced woodworker, so I went with this.

 

lrya (105).jpg

 

I was finally down to staining, sealing (pics 106 and 107), assembling, and accessorizing.

 

lrya (106).jpg lrya (107).jpg

 

Pic 108: Permanent portion of Kydex light baffle installed, using a staple gun. I might eventually use something more durable, as the staples like to pull out a bit.

 

lrya (108).jpg

 

Pic 109: Removable upper light baffle, anchored with Velcro, and pic 110: removable lower light baffle.

 

lrya (109).jpg lrya (110).jpg

 

Pics 111 and 112: Battery and holder, for powering dew heaters and cooling fan.

 

lrya (111).jpg lrya (112).jpg



#29 Other_Mike

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Posted 25 February 2020 - 10:59 PM

Pics 113 and 114: Removable eyepiece holder. Initially only secured with two anchors inset slightly from the edges, but placing too much weight at one end caused it to tip. Adding extra anchors all the way to the edge solved this problem.

 

lrya (113).jpg lrya (114).jpg

 

Pics 115 through 118: Removable filter holder. To be stained eventually.

 

lrya (115).jpg

 

lrya (116).jpg lrya (117).jpg

 

lrya (118).jpg

 

Pics 119 and 120: Initial wiring setup for dew heaters. This has since been improved on a bit, with a single flexible cable guard running along one of the truss poles, more management clips added, and glow-in-the-dark tape highlighting which pieces to disconnect at the end of the night. Each dew heater and the fan have a switch integrated into their power cable so I can run each individually as needed.

 

lrya (119).jpg lrya (120).jpg

 

Pic 121: A peek into the mirror box, just awaiting first light.

 

lrya (121).jpg

 

Pic 122: Lyra, finished!

 

lrya (122).jpg

 

One of the first problems I noticed was that the OTA tends to drift towards the user slightly as the scope is moved in altitude (pic 123). This led to the bolts of the tailgate bumping into the rocker when aiming back up and jarring the scope's movement. I initially thought I didn't get the bearings lined up right, but it doesn't happen when pulling straight down from the center of the upper cage -- so I think it's just due to the slight force imparted toward the user when moving the scope from one side. I used some of my surplus Teflon to act as a spacer / thrust bearings to help keep the OTA centered in the rocker (pic 124). I later added a second one to the other side, as I noticed having a 1/4" gap led to some slop in the azimuth movement.

 

lrya (123).jpg lrya (124).jpg


Edited by Other_Mike, 25 February 2020 - 11:01 PM.


#30 Other_Mike

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Posted 25 February 2020 - 11:04 PM

Pics 125 and 126: First and second light! Second light pic is after the dew heater cables were placed, showing the new management system.

 

lrya (125).jpg lrya (126).jpg

 

Pic 127: the happy astronomer on third light.

 

lrya (127).jpg

 

And that's the end of the build. It's not the perfect telescope, but I'm very happy with out it turned out, and as a first-time ATMer, I don't think I did too badly. It's performed beautifully under dark skies, the movement is smooth, and it's a pleasure to use.

 

Clear skies!


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#31 Other_Mike

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Posted 25 February 2020 - 11:25 PM

Venator, I'm back on mobile and having trouble quoting posts, but so far I love it. I don't have enough experience with Ebony Star to compare, but it feels very smooth. The empty rocker box feels like there's some bumpiness in azimuth, but I don't notice anything with the weight of the OTA. For reference, it's smooth enough that I can easily track a low-flying aircraft in the finderscope.
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#32 C3P0

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Posted 27 February 2020 - 07:10 PM

Congratulations on your first build - as long as a telescope works well and the optics are good, who cares about how good it looks as long as you love it and get to use it often smile.gif. However, you've built a really nice looking scope too, so bonus points there - well done!

 

No telescope is ever perfect. Ever. Even when they are close to perfect, they will probably be too small smile.gif.


Edited by C3P0, 27 February 2020 - 07:11 PM.

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#33 jtsenghas

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Posted 27 February 2020 - 08:43 PM

Your build is coming along nicely! 

 

One way to get cleaner top plies on laminated stock (including plywood) is to use a two flute bit with a down-cutting spiral. This means the flutes have a left handed twist and have some downward thrust while cutting. 

 

Those bits are indispensable for work with thin plastic laminates.  When used on plywood from opposite faces the edges can be virtually flawless. 

 

Up- cutting bits are useful when rapid chip removal from grooves and dadoes is desired,  but those bits are NOT recommended on thin laminates. 

 

I recommend you buy your wife a couple of carbide down-cutting spiral bits. 


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#34 Other_Mike

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Posted 28 February 2020 - 02:16 PM

Yeah, I intentionally did the upper pole seats so the "ugly" side would be against the upper rings and less visible. My wife gave me the tip about using sacrificial scrap so as not to get the breakthrough chipping.

 

I'm sure she wouldn't complain about some new fancy drill bits. ;-)



#35 petert913

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Posted 28 February 2020 - 03:43 PM

Great work, Mike !   Having had the pleasure of viewing through this new scope last night from our local Bortle 4 site,  I can confidently say

that it produces excellent views !


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#36 John Miele

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Posted 28 February 2020 - 07:00 PM

Great looking build Mike and I like the stain color choice a lot.

 

I too rebuilt my 16" Meade Starfinder into a truss scope. And like you, I used the smaller secondary to help get a little better contrast. No regrets on that at all!


Edited by John Miele, 28 February 2020 - 07:00 PM.

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#37 Diego

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Posted 28 February 2020 - 07:14 PM

Hi Mike! Great build! I am in the process of building my own 16" truss dob. I have 90% of the wooden parts cut and dry fitted with screws. I was thinking of different ways to build the truss clamps. You mentioned some issue with your clamps for the UTA. Can you fill me in on the possible issues with that design? I was contemplating something similar as it seems with simple and easy but make...

Thanks!

#38 grzesznypl

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Posted 28 February 2020 - 07:57 PM

Yeah, I intentionally did the upper pole seats so the "ugly" side would be against the upper rings and less visible. My wife gave me the tip about using sacrificial scrap so as not to get the breakthrough chipping.

 

I'm sure she wouldn't complain about some new fancy drill bits. ;-)

There is other way to prevent splintering. You can use masking tape to cover cut from both side but underneath is most important and that will reduce if not eliminate splintering completely.



#39 Other_Mike

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Posted 28 February 2020 - 08:43 PM

Great work, Mike !   Having had the pleasure of viewing through this new scope last night from our local Bortle 4 site,  I can confidently say

that it produces excellent views !

Thanks Pete! I'm glad Lyra's been so well-received within our little group. grin.gif

 

 

Great looking build Mike and I like the stain color choice a lot.

 

I too rebuilt my 16" Meade Starfinder into a truss scope. And like you, I used the smaller secondary to help get a little better contrast. No regrets on that at all!

Thanks John. After much second-guessing, I found another article that gave some back-of-the-envelope math for minimum and maximum recommended secondary sizes, and both came out to 3.1" for my scope -- which is what I thought I was getting when I bought my secondary mirror, but according to the digital calipers, it's the next size down at 2.9". Maybe I'll optimize that aspect down the road.

 

 

Hi Mike! Great build! I am in the process of building my own 16" truss dob. I have 90% of the wooden parts cut and dry fitted with screws. I was thinking of different ways to build the truss clamps. You mentioned some issue with your clamps for the UTA. Can you fill me in on the possible issues with that design? I was contemplating something similar as it seems with simple and easy but make...

Thanks!

The main difficulty is that the poles have to spread out slightly from where they want to be (even with the split blocks and pole seats loosened), and there's a very tight tolerance to get the poles lined up with the openings in the seats. Then I have to go in circles around the scope and nudge each set until everything seats together. When it comes time to tear down, nothing wants to come apart, either.

 

IMG_20200228_172204.jpg

 

The tight tolerance is the red arrow, even though I can loosen things quite a bit in the direction of the blue arrow. Theoretically, the poles should have plenty of clearance to sit in the gap created by opening in the blue arrow direction, but this doesn't pan out in practice.

 

I'm thinking I may open up the spot between the pairs of poles (removing from the yellow zone) so there's a larger opening for them to fit into, while still leaving a bit of material at the top portion to keep the same spread I have now (green zone), as the poles are still about 1/4" from each other when they meet the bottom of the secondary cage.
 


Edited by Other_Mike, 28 February 2020 - 08:45 PM.

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#40 a__l

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Posted 28 February 2020 - 09:54 PM

Replace this wire with a flat one (with adhesive).

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Edited by a__l, 28 February 2020 - 10:43 PM.

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#41 a__l

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Posted 28 February 2020 - 09:57 PM

You will have a mirror deform with such heating. As well as thermal flow on the light path.

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Edited by a__l, 28 February 2020 - 10:03 PM.

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#42 Other_Mike

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Posted 29 February 2020 - 06:46 PM

I like the tape wire! It could be an eventual upgrade. I used what I did because it was easy to source locally and it was cheap. I haven't had conditions bad enough to need the secondary dew heater yet, but I don't expect to need to run it continuously when it gets to that stage - and as far as I can tell, a resistive element on the back of the mirror is how all secondary dew heaters work -- isn't it? This arrangement is only going to put out three Watts, which is what I've found as a recommendation for removing dew.



#43 a__l

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Posted 29 February 2020 - 09:51 PM

Yes, the wires can be painted black (if you wish).
You have an open mirror and you will have to increase the heating power.
I would choose the Astrosystems holder and their controlled heater and adjust the comparator to the minimum possible temperature difference for your mirror.




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