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Delmarvascopics 8" f/9 OTA build

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#101 John Jarosz

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Posted 02 December 2012 - 09:36 AM

Here's 3 pics of the focuser. Please excuse the dust on the top surface of the Paracorr. I didn't realize it was that bad. In use (because this is an F4 mirror) the Paracorr stays in it's correct position relative to the main mirror. So this screw focuser doesn't get used anymore, but it keeps the sled in the right position (I built this configuration before I got a Paracorr). There is another helical eyepiece focuser that mounts onto the Paracorr so I can accommodate the different positions of field stops in different eyepieces.

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#102 John Jarosz

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Posted 02 December 2012 - 09:39 AM

Here's a view of the leaf spring and the attachment. In the above photo you can see the curved spider dimly on the right

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#103 John Jarosz

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Posted 02 December 2012 - 09:43 AM

Here is the half nut being disengaged by deflecting the leaf spring. The half nut is made from an aluminum block that is fairly long, tapped to match the threaded rod (3/8 - 24 UNF) and then sliced in half. Some deburring required.

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#104 tim53

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Posted 02 December 2012 - 11:42 AM

Thanks for the pics! They give me some ideas. I like the half-nut solution to going from fine to coarse focus. How much focus travel do you have? I'm trying to decide whether I want to accomodate things like my SBIG ST2000 with CFW10 filter wheel for the occasional DSO imaging - yeah, at f/9 the stars ought to be nice and sharp to the edge of the chip!... but mainly, this is a planetary imaging scope!

-Tim.

#105 Pinbout

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Posted 02 December 2012 - 11:55 AM

Hey Tim,

too bad you can't use the microscope stand, that would be so steampunk'ish. very cool anyway.

#106 John Jarosz

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Posted 02 December 2012 - 11:55 AM

There's about 5" of travel in my arrangement. Excessive, but this is a very old detail from back in the day when eyepiece weren't so standardized. But for photography it will give you all kinds of flexibility; you should be able to accommodate almost any configuration. Making the sled longer than you need won't really affect performance as long as you keep the sled axis parallel to the optical axis.

#107 tim53

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Posted 02 December 2012 - 12:45 PM

yes, and a long sled will be easier to align with the tube

#108 Dick Parker

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Posted 02 December 2012 - 01:14 PM

John -

Very nice sled focuser arrangement. Clever, well executed, clean. Thanks for sharing.

Dick Parker

#109 tim53

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Posted 08 December 2012 - 06:26 PM

So I couldn't stand the look of the rubbed on polyurethane. Sanded it with 320 sandpaper, then rubbed on some more gel stain to even out some bright spots a bit better than I had before. It's nice and smooth now, but I'll need to put something over the stain to protect it from wearing off. Prolly spray a thin coat of poly over it tomorrow or next weekend.

So now, I have to figure out how I'm going to mount the primary and the focuser. Going back to the beginning of this thread, you might notice that the cherry I made the tube from was only 5 1/2 feet long, so the tube is 6 inches shorter than the focal length (exactly 72 inches). I figured I'd make a removable cell for the primary that hangs off the end of the tube, similar to what I did for my 10" f/6, only longer.

Then, for the focuser, if I really do go with a sled, I'm cogitating on making something that could double as a "retractable" section/dewshield/lightshield/focuser that will fit over the outside of the tube and extend beyond the end during use. I've got a pair of full-extension drawer slides that I'd bought a few years back for a similar concept (a moving secondary support to put a camera at the focus of a 12.5" f/3.5 mirror, that i haven't finished). I'd mount the slides to the tube, and mount the extension/focuser to the slides. Then, I'd focus with the threaded rod and half nut, like John's focuser does, only the half nut would be mounted to the tube and the threaded rod to the focuser/tube extension, so it could retract with the extension for stowing the scope.

Since the extension would be a full-circumference gizmo, I could conceivably use a conventional spider and make slots in the tube to allow the spider to retract with the extension. But I think I'll make a curved one instead.

To avoid "ugly", I think I'll start with figuring out how to mount my lazy suzan rotating rings to the tube, so that I can use the same design elements/outer dimensions for both the focuser extension and primary support.

if I were a sketchup sort of person, I'd draft it all up before cutting any wood. But I'm not, really. I tend to make chicken scratch drawings with measurements to keep track of what's in my haid and identify what won't work, then start cutting and fitting as I go.

Whadaya think?

-Tim.

#110 tim53

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Posted 08 December 2012 - 07:09 PM

So here's what I've gots:

The lazy susan rotating rings are very close to the OD of the tube. I'll need to make rings that the inner half of the lazy suzan ring bolts to that bolts (not adjustable) or clamps (better, adjustable) onto the tube. Initially, I was thinking of something along the lines of the forms that I used to clamp the tube during glueup, but I think I'll want something thinner and stronger than that. Thin, in the radial dimension, it can be wide, though. Maybe still set it up so it clamps around the tube, though.

And I remembered something: At one of the Rockler sales about a year ago, I bought a kit to steam bend wood. Will have to make a box for the steaming, depending on the dimensions of the stuff I want to bend, but I could cut thin strips of Maple and bend them around the tube, laminating them up as I go to make a multi-ply ring with the grain concentric rather than cutting something out of plywood and trying to pretty it up later.

Here's the tube with one of the lazy suzan bearings around it. The bearing is aluminum, but I was planning on painting it to look like brass.

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#111 tim53

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Posted 08 December 2012 - 07:11 PM

Another shot showing the tube, the rings, and one of the forms used for glueing up the tube.

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#112 tim53

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Posted 08 December 2012 - 07:13 PM

Tube, full length

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#113 tim53

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Posted 08 December 2012 - 07:23 PM

So, if I make steam bent maple multiply rings that have the same OD as the outer lazy susan ring, and put them on the outside of the rotating ring assembly, I could mount my steambent rings to the inner ls rings, and rig up a braking system that bears against the outer ring to keep the tube from rotating too freely. That way, I could leave the ball bearings in the rings and save a long, involved step of converting the rings into Cave-like rings like I did on my 10" f/6.

-Tim.

#114 tim53

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Posted 08 December 2012 - 07:25 PM

And so, if the steam bending Idea works like it should, I'll make 5 of these rings. Two will go on the outside of the rotating ring assembly, two on either end of the focuser/extension tube, and 3 at the primary end (one to reinforce the tube and two to make the primary cell.

And the primary cell and focuser/extension will be walled with the thin mahogany plywood I bent back when I was experimenting making the tube.

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#115 tim53

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Posted 08 December 2012 - 07:27 PM

Okay, its starting to get dark. Better figure on putting stuff away for the night.

Also, gonna dig up my steam bending kit and try to find those drawer slides and make sure they're tight enough to do what I want them to.

-Tim.

#116 tim53

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Posted 11 December 2012 - 12:14 PM

This past weekend I mostly ran around looking for supplies and ideas, but I did manage to rub on another thin coat of poly (thinned about 60% with mineral spirits). It came out really nice.

I know some will cringe at this, but I've decided to make this puppy focus by moving the primary by mounting a custom cell support on a pair or bearing drawer slides. They don't seem to have any play over a good 6" of travel (more than I'll ever need), and I have 9" of length to use to mount the mirror support to them and them to the cell support, which will bolt to the end of the tube.

If I don't like it, I can replace it with a fixed cell and cut the opening for the eyepieces larger to mount a sled focuser.

Tim

#117 tim53

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Posted 11 December 2012 - 12:21 PM

For the moving mirror focuser, I plan to move it with a threaded rod and nut between the drawer slides.

Since I'll have rotating rings close to the od of the tube, I'll have to run the focusing rod up the inside of the tube. At first, I'll probably put the knob on the sky end of the tube so if I need to change things, I haven't made any extra holes in the tube. But if it works well, I might mount the focus k on on the tube primaryward of the eyepiece and use a pair of bevel gears to turn the focus rod.

Tim

#118 Wes James

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Posted 12 December 2012 - 10:02 AM

Tim- It looked like you pretty well had your focuser design finalized... what made you change your mind and decide to go with the moving primary??? Do you think moving the primary may affect collimation much??
Glad the thinned poly is working good for you... I find it to be an excellent finishing method.
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#119 tim53

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Posted 12 December 2012 - 12:57 PM

Hi Wes:

One of the main reasons for wanting to try moving the primary is that I won't have to make any unnecessary holes in the tube, other than a place to put an eyepiece. If I decide to abandon the moving primary, I can then design a sled focuser and cut the tube to fit it.

I don't expect collimation will change much with motion of the mirror. These linear bearings are nice, smooth, and tight, especially in the range of a few inches. Since they're 9 inches long, I'll also have good control on mounting them parallel to the tube's axis (my corners will serve as straight edges). I may make the mirror cell support that attaches to the slides out of square steel tubing (1/2"), welded together. So I don't expect there to be any flexure there, either. I don't think I'll need springs to take up backlash in the threaded focusing rod either - just let the weight of the mirror take up the backlash.

I would probably not try this with a big mirror, though.

-Tim.

#120 Masvingo

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Posted 12 December 2012 - 05:47 PM

Very impressive workmanship Tim, that tube's a beaut! :bow: :bow: Looking forward to seeing the finished scope and how the moving primary turns out.

The B&L also has a rack under the stage for a light or something, and it's really interesting because it's got a ring there with a set screw that compresses a ring around the light, or whatever was in there.


Re the microscope, my guess would be that a 'condenser lens' fitted in the ring which acts to concentrate light from the illumination source onto or through whatever is being looked at.

James

#121 Dick Parker

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Posted 12 December 2012 - 10:52 PM

Tim -

I have my primaries in a cell that is contained in the OTA on three threaded rods. With this arrangement, I can move the primary. I don't focus it by moving the primary, but I can position the primary so that I can control where within the focuser drawtube travel the focal plane is.

Tme main purpose of the rods is that two of them extend forward and I have knobs on them so I can collimate the primary while looking through the eyepiece at a defocused star.

So, if you have difficulty making a robust moving primary focusing system, There is another alternative that will at least give you control of the location of the focal plane thus allow you to get your focal plane close to the tube and not necessitate making big holes and cut outs at the front end of your beautiful tube.

Dick Parker

#122 tim53

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Posted 13 December 2012 - 10:48 AM

Yep, that's the kind of thing I'm thinking.

I'm betting that it'll be easy to collimate and keep there with such a long focal length. I don't even expect there will be much, if any, image shift focusing this way. But in the end, even if I decide I don't like the moving mirror, I can just rebuild the cell and make a sled focuser at that point.

-Tim.

#123 Norm Folkers

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Posted 13 December 2012 - 11:38 PM

Hi,
I'm a latecomer to reading this post. But just in case no-one has suggested it. I would like to suggest that you consider using laminate flooring to construct a multi sided tube. The laminate comes in different thincknesses. Given it's strength and rigidity, I would use the thinest possible. The laminates click together along their long edge as well as at their ends. So you can make wider panels very easily and the joint can be glued or eopxied to achieve a really good seam. You can even click together sections to make them longer, glued of course. These laminates are already pretty much impervious to water so it would not be difficult to improve on that with paint or varnish, whatever. Once you have glued up sufficient sections for width and lenght, you can mitre cut them in preparation for glueing into a tube. A square tube need only be cut to length, and at 90 deg at that.
The only drawback to this material is that it is heavier than plywood, but I believe it is also a lot more dimensionally stable.
Open stock of laminate flooring is often available at you local harware store of choice. You only need to buy as many sections as you require for making the sections. For a 6" primary (f8 or slightly less) four pieces would likely suffice to make a square tube. Note that this stuff is hard on blades, so use a carbide table saw or buy extra blades.
Hope this suggestion gives some of you ideas to take my suggestion to the next level.
BTW, very nice workmanship on the tube.
Norm

#124 tim53

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Posted 23 December 2012 - 06:18 PM

Hi Norm.

I thought about making a tube out of laminate flooring, but I think I like the uniqueness of wood better. A solid wood tube, made with thin stock and modern glues and polyurethane is amazingly light and strong. I'm pretty sure that this tube is lighter than fiberglass, and stiffer.

Today I put another coat of poly on the tube by rubbing it on. I definitely think that thin ing the poly and wiping it on with a cloth is the way to go from now on. So much neater than brushing and spraying!

Okay, I've *BLEEP*-footed around quite enough! Time to design my rotating ring mounts, end rings, diagonal support, cell, and focus mechanism (sled or moving mirror)!

Tim

#125 tim53

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Posted 23 December 2012 - 06:19 PM

Auto editors crack me up sometimes!






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