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From the desert to the swamp...

dob ATM reflector
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#1 lphilpot

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Posted 03 September 2018 - 08:00 PM

...or something like that, at least!

 

A couple of months ago I finally sold my 14.5" f/4.5 TeleKit and ordered a 12" f/5 DobSTUFF scope from Dennis Steele in Palm Desert, CA to replace it, since I needed something lighter and more manageable. The Albert Highe inspired design of the DobSTUFF (and Plettstone, etc.) scopes appeared to be a reasonable / workable compromise between other too-heavy / too-big or too-fragile / too-dew-prone designs. It's a straight-strut design, so there's some flexure compared to triangular truss designs (as I expected) but it doesn't seem to be excessive (?). Actually, other than a couple of brief test-runs on my patio, I've had it out for only one observing session since it arrived in early July, and even that had less-that-clear skies. Such is summer in the Deep South, alas... So hopefully I can put it through its paces before too long and validate what I've done to it.

 

Anyway, I can read others' reports of scope mods for hours on end, so hopefully my process of adapting a fundamentally dry-climate design to survive in humidity will be entertaining and / or useful to others (particularly if they're considering the same task). FYI, this isn't really a review of my DobSTUFF scope as much as what I done to it. However, that said, Dennis is very easy to work with and prompt with communication. Also, the things I've done aren't really a reflection on the DobSTUFF design nor Dennis' execution as much as they are a reflection of my wannabe perfectionist tendencies (that is, I want perfection but I almost never achieve it in any sense!). I also had a self-imposed budget and I'm aware that some of what I've done is already available commercially.

 

Being an open-air design, I knew I'd have to enclose it a bit to combat dew. I opted for the primary dew shield from Dennis, which is a cylindrical piece of plastic that encloses the mirror "box" (mirror cylinder? wink.gif). I contacted Heather Teeter about a custom shroud but was turned down. I came up with a shroud design and between my wife and I we made it from some $18 / yard spandex we found at Jo-Ann Fabrics. I also installed a DewBuster controller, four dew heaters, primary fan and a Kendrick secondary heater.

 

 

Photos and other commentary

 

The completed (yeah, right!!) scope in dew-prevent configuration (i.e., I won't install the dew control stuff at TSP, etc.):

 

sideview.jpg

 

I put a 3/8" strip of hook-side Velcro around the end ring and top ring of the mirror unit to secure both ends of the shroud:

 

Velcro.jpg

 

Although spandex will inherently stick to hook-side Velcro, we put 2" long strips of hook-side sew-on Velcro every 4" to make sure it's secure. I also noticed that even after only a couple of tests, the non-Velcro spandex was showing signs of "picking", so who knows how long it would last directly on Velcro.

 

Using Velcro on the scope complicates the process of installing / attaching the shroud, since it's difficult to pull the shroud over the end ring without it hanging on the Velcro. As a result, I slide the shroud down over the struts before attaching the end ring. Of course that complicates other parts of the assembly routine, but that is what it is. I just need to get the proper sequence resolved in my mind. The first full assembly took me thirty minutes... I think it'll get faster with practice. smile.gif

 

There are alternatives to Velcro: drawstrings / bungee, snaps, hooks, etc. but this seems to work well. I wish there was stretchable Velcro - That would've made the ends a little less wrinkled, but whatever. I have mixed feelings about Velcro - It's incredibly useful but annoyingly imprecise for us analytical types. grin.gif  I get a slight twitch when I see Velcro stuck all over scopes and various bits of what-not attached to it a slightly imprecise angles.  lol.gif

 

How do I keep the spandex from getting in the light path? I initially thought about using longitudinal bungee cords, but actually a stretchy cord is a installation convenience and a liability, not an asset, against the "squeezing' force of the spandex. Instead, I installed a few open screw hooks (see above) as well as utilizing other attachment points already on the scope (knobs and two hooks Dennis added for securing the metal "strings" he included) to hold black 550 parachute cords with aluminum tensioners like this:

 

tensioner.jpg

 

After a bit of field-use I may find I can eliminate one of the cords, but at the moment I have three. They lace around and in conjunction with Dennis' "strings", when tight they form a nice framework to hold the spandex out:

 

cords.jpg

 

(sorry for the backlit photo - It was about to rain outside and my carport is a little dark... New scope curse and all that)

 

(continued next post)


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#2 lphilpot

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Posted 03 September 2018 - 08:03 PM

I was kinda hoping the cords would contribute to OTA stiffness a bit, but the jury's still out on that. It may look like a lot of trouble, but I can get them all in place in about a minute. You can see a couple of the cords in faint relief against the shroud here:

 

rightrear.jpg

 

I cut a slit on each side near the bottom to clear the strut, then added a pair of small hooks and eyes to hold the slits more or less closed (sewing those tiny black hooks / eyes on black cloth with black thread was an insanity-inducing experience...). They may get replaced with zippers, not sure yet...  I rounded over the corners, ends, edges, etc., of various wooden clamps and other parts to minimize wear on the shroud. I also moved the position of the counterweight setscrew to avoid it rubbing the shroud too much.

 

Around the focuser and the finderscope mount, we just cut rectangular openings and hemmed them the best we could:

 

finders.jpg

 

All the dew heater cables are Velcro-wrapped to the struts. I use the same heaters (mostly) as I use on my 4" refractor and since I had shortened the cables to fit the refractor, I made extensions from the cut-off cable pieces (which I had fortunately kept). I also had to make extensions for the secondary heater cable and the DewBuster temperature probe. I don't want any loose cables dangling from the scope components when disassembled, so there are jacks installed on the end ring instead. (starting to see why the order of assembly is critical?? lol.gif)

 

To power it all, I use a 12 VDC battery. I didn't buy a powered ground board, so I made a removable shelf / tray to carry the battery:

 

battery.jpg

 

I added a Powerpole connector to drive the DewBuster so the two cigarette lighter plugs remain available for a blow dryer, etc. That dual jack was one of the only benefits I got from the purchase of a Power Tank years ago... Despite appearances the jacks aren't bolted to the battery. They're screwed to an aluminum plate that's double-stick-taped to the battery (the bottom side of the jack mount is ribbed and sticks poorly if attached directly). I put a 7 lb piece of flat Inconel in the opposite side of the rocker box bottom to balance things out.

 

I made a little platform / shelf to mount the DewBuster controller on the near-side altitude bearing (using a hole already there). It can be removed with a thumbscrew as well:

 

DewBuster.jpg

 

The right-most plug is on a 12V full-power jack and runs the fan. There's a little rolling switch on the wire to turn the fan off / on.

 

(continued next post)


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#3 lphilpot

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Posted 03 September 2018 - 08:06 PM

Instead of using wood, I bought a cutting board and made a fan mount from it. It's kind of a prototype and I may replace it with something more visually appealing, but it works fine:

 

fan.jpg

 

That's a 120 mm 83 CFM fan I found at Parts Express for virtually nothing ($3 maybe). Although I wanted to have the mount fully 'floating' in rubber grommets, it didn't work out that way. But there are still neoprene washers and other vibration-absorbing pieces at most contact points. The fan is virtually vibration-free so we'll see how it goes. I tend to use fans mostly for cool down before observing, anyway. I had an extra 12V power supply laying around and I put a plug on it so I can also run the fan from shore power when appropriate.

 

 

Other mods / additions

 

Dennis provides a mirror cover:

 

cover.jpg

 

But I also wanted a lid over the top of the entire unit. That gives me options when closing up for the evening and I'm not thinking / acting carefully due to lack of sleep. Putting a cover right down on a mirror isn't usually an ideal 3 AM task... Plus, with a lid, I can transport soft / lightweight items in the mirror unit since space is usually at a premium on star party trips. So I made a lid:

 

lid.jpg

 

I also made a little bungee to ;lace through the handle and hold it in place when the OTA is horizontal:

 

bungee.jpg

 

Those are two zinc-plated hardware store hooks stretched open a bit and covered in Dip-n-Grip. A proper latch would be preferable, but the plastic dew shield provided would slightly get in the way without further modifications (future project maybe?).

 

(continued next post)


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#4 lphilpot

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Posted 03 September 2018 - 08:07 PM

The default feet on the scope are little metal furniture slides like on the bottom of chair legs. They work, but I like larger and slightly taller feet on soft and / or wet ground so I replaced them with hockey pucks:

 

feet.jpg

 

The end ring is a kinda weird shape, particularly when it's in "travel mode", that is, not on the scope. The secondary seems awfully open to damage so I made a cover to protect it at least a little bit:

can.jpg

 

That's a regular old green beans can with a rubberized wrist strap attached to hold it in place over the spider vanes. The top edge is slightly soft to help minimize rattle during transport.

 

But even so, the end ring is still a real strange item to transport, so I made three "transport struts" that allow me to reassemble a mini version of the scope, for travel-time only:

 

travel.jpg

 

They're wood because by that point in time I was too cheap to order aluminum tubing, but I may end up replacing them. I put a threaded insert recessed into each end of the "struts":

 

strut.jpg

 

By the way, with only hand tools (and hand-held power tools), that was a booger to do!! Ever tried to accurately center something in the middle of a round wood dowel, with a hand-held drill? It can't be done, but I got close enough, more or less. At any rate, once in travel mode the end ring is considerably more protected and I can sit the entire "OTA" (sans rocker box) on a car seat, with the seat belt around it.

 

(continued next post)


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#5 lphilpot

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Posted 03 September 2018 - 08:09 PM

To minimize the chance of the "strings" (i.e., cables Dennis provided) tearing the shroud, I covered the ends in shrink-tubing:

 

shrink.jpg

 

Likewise to minimize wear on the custom-sized Telegizmos cover I bought, I rounded over the back "corner" of the altitude bearing:

 

roundover.jpg

 

And to avoid the OTA tipping down too far when stored horizontally (as will be the case at star parties), I made a "stop" from aluminum to prevent downward travel past horizontal:

 

horizontal2.jpg

 

Dennis provides a light baffle that mounts opposite the focuser, but since it's attached with only a single screw it can 'swing' a bit at times. I added a slot and second screw that stabilizes it. The screw is intentionally a bit above the wood, so the baffle just slips under it and is held in place once the thumbscrew is tight.

 

baffle.jpg

 

However, I don't use the baffle when the shroud is on the scope.

 

(continued next post)


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#6 lphilpot

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Posted 03 September 2018 - 08:11 PM

I had a bit of a problem with the finderscope mount clamp rocking instead of tightening down securely and repeatably. Apparently the inside of the hole (i.e., the hole "walls") were ever-so-slightly smaller in the middle than on the edges. Or at least not totally flat. That (and the criticality of finder alignment) meant that no matter how tight I clamped it down on the strut, it still rocked. That would mean my finderscope would never be accurately aligned. I came up with this solution:

clamp.jpg

 

There are two identical screws on the other side; all of their heads extend over the hole edge by about 1/32". That means the clamp actually contacts the strut at five points comprising two triangles with a common apex.

 

If I hadn't been able to make this work, I probably would've requested Dennis make me a second clamp and a 'platform' between the two to mount the finderscope. But that would've been heavier and as it was the scope was marginally nose-heavy. I was able to raise the altitude bearings (i.e., lower the mirror) unit by 1" and that was enough to give me plenty of 'balance latitude'. Now I can once again use the counterweight on the nose end to trim balance properly.

 

 

Some other things I did

  • Smoothed up the altitude bearing flanges to fix motion - There was dried contact cement on the altitude bearings flanges, but once removed the motion is very good.
     
  • Added a felt pad inside the rocker box for the mirror box to rest against - Just a minor thing but it helps prevent scarring the inside of the rocker box where the mirror unit touches at zenith.
     
  • Painted the front of the mirror board - The front side of the mirror board was varnished and somehow looked kinda bright down there, so I coated it with flat black paint. I may (or may not?) flock it at some point.

Possible future mods

  • Make a end ring extension (dew / light shield) from plastic to extend the OTA farther beyond the secondary
     
  • Replace the secondary holder and spider - The Destiny holder / spider is ....OK.... but I'd rather have a conventional straight vane spider with a "can-style" secondary holder. The Destiny holder uses bolts that IMO are undersized (#10). I'd like to have a minimum 5/16" center bolt to minimize secondary vibration. I'm also not a fan of glued-on mirrors, primary nor secondary.
     
  • Convert the primary cell to a wiffle-tree design - See my comment above about glued-on mirrors. As it is now, the mirror hangs from its back surface via RTV. I'd like to convert the primary cell to (probably) a simple wiffle-tree design with clips to prevent front-side fallout. I wonder how much collimation shift is a result of flexure in the RTV itself...?

ANYWAY... If you're still reading, thanks!!!  :)  Obviously I had a new scope on my hands and no weather to use it!  :)

 

(not continued next post lol.gif)


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#7 Garyth64

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Posted 03 September 2018 - 08:24 PM

Did you say all that in one breath?  lol.gif

 

I like every post, the scope, and all the little things here and there.  Very nice.

 

I wanted to ask a couple of things, but now I have to go back and take notes. smile.gif


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#8 ToxMan

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Posted 03 September 2018 - 08:25 PM

Nice build, Len. I hope it gives you hours of pleasure. 



#9 Dlhudgens

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Posted 03 September 2018 - 11:14 PM

Great job!  The pictures, the tweaks, the explanations—all work for me.  I’m not sure whether its a work of art or of technical skill.  Let’s just say I bet it works great in the field.  Keep us posted on further updates / tweaks and your experiences in the field.



#10 lakland5

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Posted 04 September 2018 - 04:37 AM

Great posts, very informative--thanks for all the descriptions and pictures.   I really like your setup.



#11 lphilpot

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Posted 04 September 2018 - 06:39 PM

Thanks for all the comments and compliments. As I said (implied) if it had been clear I probably would've been out using it instead of sitting inside thinking, "hmmm... wonder how I can adjust that?"  :)



#12 lphilpot

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Posted 21 September 2018 - 09:41 PM

Final (?) item - Strut pouch.

 

Made from canvas, has a bungee draw cord with a spring-loaded cord stop. The flap is longer than necessary, but that was the full length of the fabric, so we left it there. I can cut off and re-hem if necessary. There's a buckle-style Velcro strap around it as well. Once again, I designed, marked and cut; my wife sewed.

 

truss_pouch_all.jpg

 

truss_pouch_cord.jpg

 

truss_pouch_open.jpg


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