Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

In and out of love with a Dob?


Recommended Posts

Last fall my wife, Monica, and I volunteered to bring a couple of telescopes to a star party organized by our local Land Trust. My wife had never operated a telescope other than to focus one of our small refractors on a target I had located, so thinking it would be easiest to start small and wide I put her in charge of our 50mm achromat mounted on a photo tripod, while I watched over our 102mm refractor.

Fifteen minutes into observing and without a word Monica abandoned her post, disappeared, and left me to run both scopes on my own. I never even saw her leave. It wasn’t until the end of the party two hours later that I found Monica working an old 12” Orion Dob owned by the Land Trust, but without anyone at the Trust who knew how to run it. Adjustment knobs were missing, the finder was hopelessly out of alignment, and the filthy 10mm Plössl eyepiece mounted in the focuser made for a dim and tiny field of view, but Monica somehow worked through it all and was happily showing Jupiter and Saturn to the party guests.

A little over a month ago we again volunteered to help at the next Land Trust star party and I thought it might be worthwhile to go through the dilapidated Dob and bring it up to snuff so Monica could run it again. We borrowed a larger vehicle, retrieved and brought the Dob home, and I went to work. Here are the two scopes compared.  I suppose I can see the attraction…maybe…

IMG_1874.jpeg.064628264f6009b7afa9c187644a16bf.jpeg

The Dob mirror was reasonably clean considering it resided at a dusty ranch, but since I hadn’t ever worked on a Dob I decided to try my hand at cleaning the mirrors. Here’s the before photo showing the not so great secondary mirror alignment and some curious markings on the primary mirror:

IMG_1832.jpeg.a2336b2170c5effc166d3f8ef91b3b19.jpeg

I used a few tomato cans to lift the mirror from the cell…

IMG_1849.jpeg.5bed0bf079ceed11214c2db70a00c3b3.jpeg

and took a closer look at the mirror which showed some curious marks, some of which were scratches:

IMG_1845.jpeg.c672ba451d931a6fcefd3c4c74654428.jpeg
 

IMG_1843.jpeg.dd9d57ad86f9b820a75df77410eda000.jpeg

IMG_1846.jpeg.b3ffc32d81af8613b3fbdffe9d41c876.jpeg

Perhaps this little fellow I found wandering within the tube had something to do with the marks:

IMG_1855.jpeg.3694259f555b330c77d51497bc311caa.jpeg

It appears the Dob was jostled a bit as well, as shown by the witness marks showing the mirror rotated a bit on it’s cell:

IMG_1841.jpeg.0b08b0c215f7461fc1f4c21546834d5c.jpeg
 

And here’s part of the mirror after cleaning.  Shame about the scratches, but I’m pretty sure they won’t effect viewing:

IMG_1851.thumb.jpeg.f6be347ec963a5cc6f1a19e1cb5322dc.jpeg

Long story short: the mirrors cleaned up nicely, I centered the secondary holder and focuser, reset the main mirror cell, collimated the secondary mirror and Barlow laser collimated the primary mirror, assembled the various remaining bits and bobs, and all just in time for several weeks of strong gale winds.

To pass the time while waiting for a break in the weather to star test the scope, Monica and I were watching Ed Ting’s video comparing various Dobs and completely out of the blue Monica declared, “I’m in love with Dobs!”…. “WHAT DID YOU JUST SAY!?!”………“I’M IN LOVE WITH DOBS!”

I’m a committed refractor fan, perhaps an overcommitted fan, and it’s possible I may have overreacted a bit, but I shot her a glance that possibly looked a little like this:

image011.jpeg.78a2f33b6020a5795b9c5028b647075b.jpeg

I eventually regained my composure and dismissed Monica’s declaration of love as simple infatuation, but I freely admit I was a bit taken aback and perhaps a little hurt.

A couple of days ago we had a brief pause in the storm and we wrestled the Dob through the kitchen and into the back yard. The seeing was atrocious and so I wasn’t able to evaluate collimation or optic quality, but we were able to wrestle the big Dob in the general direction of M13. When I say wrestle I’m not kidding.  It took tapping, shoving, rapping and banging to move the Dob onto M13, and more tapping, shoving, rapping and banging to keep it there. The Teflon azimuth bearings were by no means up to the task, the stiction was horrendous, and Monica and I were both frustrated and disappointed by how poorly the mount performed. I asked Monica whether she was still in love with the Dob and she sadly confessed that much of the shine was lost.

I felt bad for Monica, and I want her to have a great experience at the star party, but nothing I did after our short observing session seemed to help make the mount livable. That is until I remembered a trick one of the regulars at CloudyNights mentioned: Ivory Soap. I removed the scope tube, flipped over the mount, and rubbed the Ivory bar soap liberally over the bearing surface. Initial indications are that the soap’s made a huge difference in reducing stiction, but the final test will be Friday night at the star party. Fingers are crossed for favorable weather and a successful night with the Dob. We’ll see.

 

 

 

Edited by Jim L
  • Like 18
Link to comment
Share on other sites

3 hours ago, Jim L said:

A couple of days ago we had a brief pause in the storm and we wrestled the Dob through the kitchen and into the back yard. The seeing was atrocious and so I wasn’t able to evaluate collimation or optic quality, but we were able to wrestle the big Dob in the general direction of M13. When I say wrestle I’m not kidding.  It took tapping, shoving, rapping and banging to move the Dob onto M13, and more tapping, shoving, rapping and banging to keep it there. The Teflon azimuth bearings were by no means up to the task, the stiction was horrendous, and Monica and I were both frustrated and disappointed by how poorly the mount performed. I asked Monica whether she was still in love with the Dob and she sadly confessed that much of the shine was lost.

I felt bad for Monica, and I want her to have a great experience at the star party, but nothing I did after our short observing session seemed to help make the mount livable. That is until I remembered a trick one of the regulars at CloudyNights mentioned: Ivory Soap. I removed the scope tube, flipped over the mount, and rubbed the Ivory bar soap liberally over the bearing surface. Initial indications are that the soap’s made a huge difference in reducing stiction, but the final test will be Friday night at the star party. Fingers are crossed for favorable weather and a successful night with the Dob. We’ll see.

Try surfacing the bearing surface with Fiberglass Reinforced Wall Panel in a nubby texture.  It should be glossy to have low friction, but fairly coarse to reduce stiction.  You can run a piece of Teflon across it to get an idea of the feel of it.  I won't lie, it can sound a bit like rumble strips on a highway when rotating the scope, but it works great.

  • Like 1
Link to comment
Share on other sites

4 hours ago, Sunshine said:

Enjoyable read, I'm sure you'll get the mount up to snuff and Monica will be a crack shot with dob's soon enough, that mirror certainly did clean up nicely.

There are a lot of mirror cleaning videos and some of them are real horror shows. I cleaned the mirror using the technique presented in the Optical Wave Lab video. It seemed to me their technique presented the least risk to sleeking or scratching the mirror without resorting to polymer film.

 

  • Like 1
Link to comment
Share on other sites

Michael and Louis, thank you very much for the tips. Louis, thank you for the link to the fiberglass reinforced wall panel; I’ve heard very positive comments about its efficacy as a Dob bearing surface.  Michael, where might one source that large diameter bearing you’re using?

Link to comment
Share on other sites

41 minutes ago, Jim L said:

Michael and Louis, thank you very much for the tips. Louis, thank you for the link to the fiberglass reinforced wall panel; I’ve heard very positive comments about its efficacy as a Dob bearing surface.  Michael, where might one source that large diameter bearing you’re using?

I bought the same one as Michael from the same supplier for my 12” dobsonian. The supplier is called “Simply Bearings”. If you type in Lazy Susan bearing you get a variety of different sizes. Measure the base of your dobsonian and buy accordingly. HTH.

  • Like 1
Link to comment
Share on other sites

Great story @Jim L. Love the pic of the little refractor beside the big dob! I'm an out and out refractor man but people's descriptions of DSOs through big dobs is very compelling. I'd love to try and make one, including grinding my own mirror. 

Malcolm 

  • Like 1
Link to comment
Share on other sites

7 hours ago, Louis D said:

Try surfacing the bearing surface with Fiberglass Reinforced Wall Panel in a nubby texture.  It should be glossy to have low friction, but fairly coarse to reduce stiction.  You can run a piece of Teflon across it to get an idea of the feel of it.  I won't lie, it can sound a bit like rumble strips on a highway when rotating the scope, but it works great.

I’m not a fan of the lazy Susan bearing type things. And I tried to find the above wall panel stuff in the UK without luck. But this -  stucco embossed aluminium - worked really well for me. Cheap too. I paid £13 for a 50cm (20 inch) square from a local sheet metal supplier. Lager sections are available. And easy to cut with tin snips.

Works well for altitude bearing plus Teflon too. 

IMG_1654.thumb.jpeg.3b5049fc5a3f44a01b4646c556d3667c.jpeg

Edited by PeterStudz
  • Like 1
  • Thanks 1
Link to comment
Share on other sites

Be careful.... One always leads to another and they just keep getting bigger.  The real fun with dobs is decking them out.  Lasers, finders, setting circles oh my. 

IMG_20221125_111907454.jpg

IMG_20220607_194155507.jpg

IMG_20221201_184255941.jpg

Edited by Mike Q
Link to comment
Share on other sites

3 hours ago, Jim L said:

Michael and Louis, thank you very much for the tips. Louis, thank you for the link to the fiberglass reinforced wall panel; I’ve heard very positive comments about its efficacy as a Dob bearing surface.  Michael, where might one source that large diameter bearing you’re using?

Just get a bearing that is roughly the same diameter as the ground boards.  It wont have to be exact. Close counts.  When i get around to doing my 10 inch I will do a inner and outer bearing with teflon on the edges for additional support, overkill for sure, but that is what i do.  Plug lazy susan bearings into amazon and that will get you headed in the right direction 

Edited by Mike Q
Link to comment
Share on other sites

7 hours ago, PeterStudz said:

I’m not a fan of the lazy Susan bearing type things. And I tried to find the above wall panel stuff in the UK without luck. But this -  stucco embossed aluminium - worked really well for me. Cheap too. I paid £13 for a 50cm (20 inch) square from a local sheet metal supplier. Lager sections are available. And easy to cut with tin snips.

Works well for altitude bearing plus Teflon too. 

IMG_1654.thumb.jpeg.3b5049fc5a3f44a01b4646c556d3667c.jpeg

Good find.  Basically, you just need to find a hard, nubby surface that is glossy.  The finer the nubbiness, the higher the sticktion (more contact points to overcome static friction) and the smoother the motion (like driving on a finely rutted road vs. a potholed road).  The bearing surface area of the Teflon blocks also plays into the whole motion equation as well.  Larger blocks will play better with larger nubs by spanning multiple at once.

There's definitely an art to it based on materials availability and matching them up properly.  For instance, nylon furniture glides may work better than Teflon blocks on certain surfaces.  You just have to experiment to find out what works best.

  • Like 2
Link to comment
Share on other sites

Welcome to the world of playing with dobs! That looks like a great find. Reading the other comments is testimony that most dob owners have different views and preferences on what works best with their scope. My weapon of choice is a 14" truss by Orion USA, but it took me a while to get the azimuth as I wanted it. I tried teflon and furniture sliders, but still had too much stiction, and a lazy Susan on its own had too little: the scope wouldn't stay where I put it. However, I then cut up a bathroom mat with foam backing and mounted it inside the bearing- and that added the perfect amount of friction. 

It's not an elegant or pretty machine, and at 80kg you wouldn't call it grab and go. But the performance is terrific 😃

  • Like 1
Link to comment
Share on other sites

3 hours ago, Whistlin Bob said:

It's not an elegant or pretty machine, and at 80kg you wouldn't call it grab and go. But the performance is terrific 😃

And here I thought my 15" Tectron truss Dob was heavy at a little over 100 pounds.

  • Like 1
Link to comment
Share on other sites

40 minutes ago, Louis D said:

And here I thought my 15" Tectron truss Dob was heavy at a little over 100 pounds.

My 16 inch dwarfs them both at 195 pounds lol

Link to comment
Share on other sites

I don't understand the problem with Lazy Susan Bearings. My StellaLyra 8" had one but crucially, with a knob to vary the stiction. For normal scanning of the heavens, the knob was loose or slightly tightened (ensuring easy movement) and while changing EPs on a target, I'd tighten it. Simple job and for me, it worked perfectly.

Link to comment
Share on other sites

6 hours ago, bosun21 said:

That’s only because it’s a go to.

While that is definitely a factor i think there is some substantial weight in the steel tube that protects the mirror, then the weight of the mirror cell itself.  Then there is the base itself.  Inch thick boards.  If i made a non go to base for it, it would still weight 140 to 150 pounds. 

Link to comment
Share on other sites

32 minutes ago, cajen2 said:

I don't understand the problem with Lazy Susan Bearings. My StellaLyra 8" had one but crucially, with a knob to vary the stiction. For normal scanning of the heavens, the knob was loose or slightly tightened (ensuring easy movement) and while changing EPs on a target, I'd tighten it. Simple job and for me, it worked perfectly.

It’s personal, so one isn’t better or worse than the other. But for me (when I tried a lazy Suzan) your description of having to use a knob or similar to vary stiction was one of the things that annoyed me. With the setup I have now there’s no need to fiddle with any knob! 

Link to comment
Share on other sites

11 minutes ago, Mike Q said:

While that is definitely a factor i think there is some substantial weight in the steel tube that protects the mirror, then the weight of the mirror cell itself.  Then there is the base itself.  Inch thick boards.  If i made a non go to base for it, it would still weight 140 to 150 pounds. 

It’s the go to motors that account for a good bit of the weight of the base. The boards are thicker to accommodate them. I used to own the 12” go to and the base was a pain to move manually so I sold it.

Link to comment
Share on other sites

25 minutes ago, PeterStudz said:

It’s personal, so one isn’t better or worse than the other. But for me (when I tried a lazy Suzan) your description of having to use a knob or similar to vary stiction was one of the things that annoyed me. With the setup I have now there’s no need to fiddle with any knob! 

Well, as you say, each to his own. But I like the idea that I can alter stiction to suit me, not rely on an average.

Link to comment
Share on other sites

6 minutes ago, cajen2 said:

Well, as you say, each to his own. But I like the idea that I can alter stiction to suit me, not rely on an average.

But for me it’s not average - it’s perfect, that’s what I want! I also think that there’s some confusion or misunderstanding of “stiction”. 

Edited by PeterStudz
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.