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Mounting scopes and camera on a beam

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#1 kw6562

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Posted 04 June 2012 - 11:37 PM

For the past month I have been working on an alternative strategy for attaching my scope to its mount. There are a couple of reasons for this. First, I bought a large camera last year (a Roper Scientific Quantix with a KAF6303 chip - those of us who bought them have a thread going in the "Vendor Announcements" forum). Although my focuser was able to handle its weight I was always nervous about the fact that the thin metal case of the filter wheel and a few threads were supporting it, so I wanted to come up with a safer alternative. Second, I have always thought that an AP setup should be built on an optical rail or bench - a solid, massive base from which one can mount the scope, camera, OAG or guidescope, etc. Obviously weight restrictions place a practical limit on what is possible, but for years I wondered if there might be a way to make something like that.

I decided to make a stage to focus the camera using an old lead screw stage and a stepper motor that I had in my junk pile. To drive the stepper I'm using a Rigel Systems nStep controller. Fortunately the driver transistors are able to handle the motor, which is rated at 5V 1A, but I had planned to modify the circuit if necessary after helpful consultation with the folks at Rigel. To increase the torque I removed a 20:1 gear head from an old DC motor that I found in surplus store and adapted it to the stepper. The resolution is 0.25um/step.

So the stage had to sit on something which lead to the idea of using 80/20 aluminum extrusion as the "rail". It is 1.5" x 1.5" in section and is very stiff. I bought a 3 ft. piece from the same surplus store for $5. The scope, guider, focus stage, and dovetail are connected to it with 1/4-20 bolts. Tonight I finished the assembly and everything seems to be working correctly. The whole thing weighs 35 lbs. - a little high, but I'm fairly certain that I won't have any issues with it on my mount, which is an Atlas with ceramic bearings mounted on a reinforced cement pier. Of course its raining in New England for the next few weeks...

I would appreciate feedback, particularly if anyone else tried anything like this and what their experiences were. Once I get it back on the mount and everything is balanced and tuned I plan to take lots of data to determine flexure and guiding accuracy. Please ask questions - I am leaving out a lot of detail for brevity. Thanks and clear skies --Keith

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#2 kw6562

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Posted 04 June 2012 - 11:41 PM

Back end view...

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#3 Roy McCoy

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Posted 04 June 2012 - 11:58 PM

I think you will like what you have done. I will be easy to find and adjust the center of gravity. Your Atlas will certainly be able to carry the weight. Whether or not it will work as an AP platform, well I guess you will find out soon enough.

Here is a picture of a dual mount I used with my CG5 (no longer have the MN). It was slightly overloaded, but for visual I thought it was just fine.


Best Regards,

Roy

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#4 kw6562

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Posted 05 June 2012 - 11:07 PM

Thanks Roy - That's quite a setup! I had a brief break in the weather tonight (missed the transit, unfortunately) so I installed it on the mount and balanced it very easily, as you said. I was able to refocus the scopes and try out the focus stage - so far everything is working great. Actually the CWs are up higher than they were previously when I had the scopes mounted side-by-side - a little more weight but less of a moment arm, I would guess. No time to try to guide but I didn't noticed any oscillations in the stars from frame to frame, so I don't think its struggling. Now if the weather would just cooperate...--Keith

#5 freestar8n

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Posted 06 June 2012 - 04:05 AM

Hi-

I'm interested in any flexure measurements you get with that setup. It looks good and rigid, but I'm afraid I'm not optimistic it will help a lot because of inherent flexure within the SCT OTA - which is why so many of us use OAG. Typical flexure values may be perhaps 0.1 to 0.5"/minute - which may be fine if the stars aren't too small in the images and the exposure isn't too long - but otherwise can be a problem.

Also - if you are interested, MetaGuide, which I wrote and is free, can measure the flexure in realtime very sensitively using two video cameras. You can use it to measure the flexure over time as you track a star, or abruptly with change in altitude as you go from a low star to a high one.

Frank

#6 rwiederrich

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Posted 06 June 2012 - 03:34 PM

Kieth..I did something similar with my 4" schmidt. I machined a mount that ran on a rail that I bolted to the existing rings of my homemeade 6"f/15. Camera and filter could also be supported in like manner as you described. Here is one image.

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#7 rwiederrich

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Posted 06 June 2012 - 03:39 PM

A bit closer view so you can see the locking screws and the extension for the camera. The whole set-up can be slid up and down the rail for balance of the system.

Rob

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#8 rwiederrich

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Posted 06 June 2012 - 03:45 PM

And one final one of the entire scope with rail mounted schmidt camera.

Rob

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#9 kw6562

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Posted 06 June 2012 - 11:05 PM

Thanks for the comments - Frank, I will definitely post data when I get everything running and we have some clear skies. The idea was to support all of the optical components with one rigid structure. The SCT primary support and the guider objective cell (which is plastic) are probably going to be the worst sources of movement. I'm under no illusions that this will be as good as an OAG but I went with a guider for other reasons. When I had mounted side-by-side with the SCT on a 3/4" thick aluminum plate it worked reasonably well. We'll see...

Rob that is a very impressive setup! I take it that the channel worked well for the SCT. I like the idea to be able to slide the scope along the channel for balance.

--Keith

#10 macona

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Posted 08 June 2012 - 04:15 AM

You might check out my thread where I extensively modified the rigel usn-nstep. I was not happy with the way it drives the motors. In this day and age with cheap integrated stepper control ICs there is no reason to be building a circuit like that. I now can handle up to 2 amps, micro stepping, and bipolar motors. Their circuit was limited to unipolar motors.

http://www.cloudynig...&vc=&PHPSESSID=

#11 kw6562

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Posted 16 June 2012 - 08:57 PM

Macona - I agree, a stepper motor driver IC is a better way to go - nice job (and definitely NOT a hack). I lucked out with my motor and I didn't need to replace the output FETs so I was able to use their circuit as-is. Are you also building your own focuser for the Quantix? --Keith

#12 kw6562

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Posted 16 June 2012 - 09:15 PM

So I obtained some flexure data last night and am pretty pleased with the results. Conditions were:

Target: M27
Scope: EdgeHD 8
Guidescope: ST-80
Main camera: Quantix KAF-6303
Guide camera: DSI Pro
Mount: Atlas EQ-G
Filter: Luminance
Exposures:100 x 10s (total duration about 26 mins. with downloads)
Main control SW: Maxim DL
Guide SW: PHD

I used DSS to obtain the offsets and plotted them in Excel with correction for resolution; also added in quadrature to find the total distance. As can be seen in the chart below the total was about 0.1"/min., probably as good as I can get with a guidescope. --Keith

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#13 kw6562

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Posted 16 June 2012 - 09:16 PM

Also this is the stack unaligned.

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#14 freestar8n

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Posted 17 June 2012 - 04:12 AM

Hi - and thanks for posting the numbers. That looks about right. The flexure may vary a bit depending on where you are in the sky, but my guess is that is about as good as you can do.

Did you have the mirror locks enabled during this? If not they may help reduce it a bit more.

If you end up with numbers that are pretty consistent and let you take 3-5m exposures reliably, it looks like a good setup.

Frank

#15 kw6562

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Posted 17 June 2012 - 10:41 AM

Thank you Frank - Yes, I did have the mirror locks enabled. Typically I take 5 min. subs for most DSOs so I expect this will be okay. Clear skies --Keith

#16 macona

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Posted 19 June 2012 - 02:46 AM

I ended up getting a moonlight focuser for my setup. I adapted it to a small stepper motor to drive it. I am hoping it does not make the light path too long or I will be going back to the drawing board. I really need to get the scope out. Weather has been *BLEEP* all year.

I do worry about the friction drive being enough to move the camera. I might end up going a similar route as you. I have some square rail linear guides that would work as well as some nice ball screws that would make up a nice slide. I also have some of the Bosch version of 80/20 lying around as well.

#17 kw6562

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Posted 19 June 2012 - 09:26 PM

Hi Macona - I also used a Moonlite focuser before this and it worked okay - it had a DC motor driven by a Shoestring FCUSB - but I had to tighten the adjustments screws to make it stable with the weight of the Quantix and the FCUSB wouldn't drive it. However, the motor is 12V and the FCUSB only puts out 5V (runs directly off of the USB 5V) so I used a power opamp that I had in my junk and a 12V supply to drive the motor. That worked great, but I wanted to be able to autofocus using a stepper and I didn't like the fact that the weight was hanging off of a compression ring and the thin metal of the filter wheel housing. So rather than upgrade the motor (~$400, IIRC) I decided to take a more radical approach that included the rail mounting. If you want more details for your camera PM me.

And the weather here in New England has been lousy too, I feel your pain...Clear skies --Keith

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#18 macona

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Posted 20 June 2012 - 12:59 AM

This is what mine ended up like. The motor I got from all electronics. Pretty low backlash, really cheap too. Everything is pretty sturdy. The color wheel is well built and I made the off-axis guider so that it fits between that and the scope with as little extension of the path as I could get away with. I do need to figure out something to balance things out. Maybe a constant tension spring balancer.

Posted Image
Stepper motor on Moonlite focuser by macona, on Flickr

The guider I built:

Posted Image
DIY Off Axis Guider by macona, on Flickr






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