Dedicated Video Scope
I used a 6" F/4 with a surveillance camera a prime focus (really, where the diagonal would usually be) for imaging Hale-Bopp and really felt there was a niche for a dedicated Video Astrograph in my arsenal. So I'm making a 14" F/5 dedicated to video. I put together a wish-list and came up with this scope.
What My Video Astrograph Needs:
- Light gathering power.
- Non-rotating Electronic Focuser.
- Equatorial Drive.
- Rotatable Tube.
- Wind Resistance.
- Portable.
How I addressed those needs:
- Light gathering power: A 14" F/5 primary gathers a pretty good amount of light. By placing the camera at prime focus I can avoid light loss and aberations caused by a second reflecting surface.
- Non-rotating Electronic Focuser: My current scope uses a low profile helical focuser that is just fine for visual work, but awful for trying to focus a video camera. I swore that electric focus would be a priority. When Kurt Sidor of the ATM list made some precision slides available, I purchased an extra one for the camera carriage. I have an old disk drive that doesn't know it yet, but it is going to be donating its stepper motor to this higher cause.
- Equatorial Drive: I pondered all the usual suspects: Alt-Az-FR, a split ring equatorial, an english yoke. Alt-Az-FR was very tempting, particularly considering I could leave out the Field Rotator most of the time, as you don't get a lot of rotation in 1/30 of a second video frame. But the inherent simplicity of an axis pointed at Polaris kept me coming back to true equatorials. I looked at and dismissed the Sphere a dozen times. I just about settled on a split ring mount, mostly because every time I tried to reduce the Sphere to something easier to make, I ended up with something that looked a lot like a split ring on drugs. Then it finally hit me one night (all ATMs dream about their next scope, don't they?), the sphere was actually no more difficult than any other design that incorporated all of its benefits.
- Rotatable Tube: See above.
- Wind Resistance: See above.
- Portable: OK, I missed the boat on this one. A 26" sphere is BIG. Fortunately, it scales well, the 18" should fit in a 28" sphere.
Mirror
The mirror is a plate glass 14" diameter and a hair under 1" thick. It was from a bunch that Tim Walters had water jet cut and made available very reasonably priced to the ATM List. They are gone now but an 18" piece is waiting under my workbench for my next project. I cast a tool out of UGL dental plaster and went at it. Grinding went quick but polishing is taking forever. I will go into detail and post some images later.
The UGL PoP needs a lot more water for this application than it says on the box. I found that 4 cups of water per box gave a nice pourable consistancy, and it set up fine, though more powdery than the box ratio. A couple coats of Shellac sealed it up nicely.
I found out the hard way that you just don't multiply everything by 8/14ths to go from an 8" to a 14" my tiles and pitch squares were way too big. I suffered through the tile tool (tended to go right from slip-slidey to stuck, bypassing heavy slow drag most of the time), but I re-channeled the pitch lap, quartering all my 2" squares into 1" squares.
Mirror Mount
I used PLOP to design the simplest mirror cell that wouldn't compromise performance. I arrived at a nice 12 point design that was easy to make and light in weight. It will need to be collimated from the front of the tube so the collimation screws are just beyond the edge of the mirror. Here is the cell in the framework of the sphere:
The cell actually is larger than the opening when laid flat, but turned on its side the cell and mirror will just slide through the opening. Note the adjustment knobs (2.5" disks of 3/4" plywood, cut with a hole saw) around the opening of the sphere. Here is the cell at its first full assembly:
Here is a close-up of the adjustment rod, spring, and cell. The rod is 5/16" 16TPI. They fit into T-nuts in the mirror support board.
Here is the rocker arm sub-assembly:
Here is a close-up of the rocker arm support, the piece in the middle is cross drilled. With a 1/4" clearance hole in the short direction and 1/4-20 threaded hole through the long dimension. The individual rocker arms are supported by 1/4" shoulder bolts, I measured the shoulder length and cut the threaded section back to about 5/16" long.
Here is the complete mess. The white squares with 1/4" clearance holes are 1/8" teflon, they still need to be trimmed to final size. They are spacers so the outside dimensions aren't critical, but they do have to allow clearance for adjacent moving parts. The arms are 1/2" solid square bar stock, the supports and frame is 1/16" wall 1" square tubing. The disks were cut from 1/16" stock with a hole saw, I couldn't find fender washers with right size hole and OD so I cut my own.
The framework is very stiff. Each arm pushes against a block on the next arm. The whole thing was drilled on a simple jig made of scrap plywood. After a final going over with a file all the non-moving joints will be joined with JB-Weld, I don't trust screws in thin Aluminum enough to leave it at just that.
Mount/OTA
I have wanted a sphere mount ever since I first saw the Norman James version in Texereau. I built my 8" F/6 using techniques that I could leverage in the design I planned for this. The advantages of a sphere mount are:
- you get a rotating tube for free
- your tube is almost the whole mount; the baseboard is simple
- it is equally easy to use in altazimuth or equatorial modes
- aesthetically it is just cool
The negatives are:
- somewhat greater difficulty to design
- tighter construction tolerances (but not any worse than a rotating tube)
- setting circles will need 3 encoders and custom SW (ditto goto if I ever try)
The sphere has a 26" diameter, the end ring and mirror platform are 18" diameter, the clear opening is 15". Here is the parts layout before assembly:
Here are the pieces dry fit. It still needs a little filing here and there before I get out the screws and glue.
The entire frame required only 1 and 1/3 sheets of 1/2" plywood because the rings nest. I debated whether to add two additional rings between the "equator" and the end ring and mirror cell platform, but once everything was screwed and glued it was plenty rigid, so I'm glad I didn't.
The next step is to cover it with 1/4" steel mesh (AKA hardware cloth), then carbon fiber over the large empty areas, then fiberglas over the whole thing. Then a nice smooth finish with resin and glass spheres (AKA body filler). I am toying with the the idea of painting it to resemble a pool ball and use a cue for the camera boom :-).
Looks like a hamster cage for the rodent space program :-)
Video Captures
Jupiter 10/15/99 c. 3:19 AM EDT
This one looks larger because it was captured at 1150x1500 resolution, the ones below were 640x480 captures. I am still trying to decide if the higher resolution actually reveals more detail. The actual video feed doesn't have that many lines, but when co-adding several images I *think* it allows for more precise registration. Comments welcome.
Jupiter and Saturn 10/13/99 c. 3:30 AM EDT
Taken with a P23C Video camera, captured directly to disk with a Snappy. The connection between the camera and Snappy was a Video Transmitter and Receiver. No cable into the house. (www.x10.com, no affiliation) Taken with a 2x Barlow at the newtonian focus of the 8" F/6 shown below.
MAIL ADDRESSES:
WORK: omalleyj@dialogic.com
PLAY: omalleyj@eclipse.net
