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Old 04-07-2013, 10:44 AM
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DavidTrap (David)
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Widefield CCD Rig

After Peter Ward posted his widefield rig, I decided I'd better do something about building mine - only been planning it for 6-12 months in my head. Don't look too closely at the camera bracket - all done with hand tools - I'm definitely not a craftsman!

I will be able to try a variety of lenses on the front of the camera and support their weight with the guide rings. I had major problems with a 200mm lens, but I think that was due to flexure of the camera-lens mounting.

Hopefully will get a chance to fiddle with this on Sat night, albeit from Brisbane - hope the weather obliges for those of you heading bush for the new moon weekend!


DT
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Old 04-07-2013, 12:04 PM
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Curious about the focusing there. Is that a stepper motor you've got rigged to the focus ring on the lens? Cool idea. So you can use FMax etc with it?
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Old 04-07-2013, 12:08 PM
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Very impressive, David!

H
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Old 04-07-2013, 12:53 PM
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Thanks H,

Troy, it's a servo motor, not a stepper. I can drive it via the AP mount controller using ASCOM. It is possible to use FocusMax with a servo motor, so I will give that a try in due course.

DT

Last edited by DavidTrap; 04-07-2013 at 07:31 PM. Reason: typo
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Old 05-07-2013, 10:03 AM
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Originally Posted by DavidTrap View Post
Thanks H,

Troy, it's a servo motor, not a stepper. I can drive it via the AP mount controller using ASCOM. It is possible to use FocusMax with a servo motor, so I will give that a try in due course.

DT
That's the tricky part - would be interesting to hear people's thoughts on who to drive a focus motor without the AP functionality

Great rig too BTW David!
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Old 05-07-2013, 02:29 PM
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That's the tricky part - would be interesting to hear people's thoughts on who to drive a focus motor without the AP functionality

Great rig too BTW David!
Thanks Rob,

I think you can get a box from ShoeString (FCUSB??) that will drive a servo motor focuser from your computer.

DT
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Old 05-07-2013, 05:23 PM
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Thanks Rob,

I think you can get a box from ShoeString (FCUSB??) that will drive a servo motor focuser from your computer.

DT
Rob, I've got a FCUSB up here you can have a play with if you would like.

Peter
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Old 05-07-2013, 05:54 PM
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Thanks Peter. Its been so long since I've used the Newt the penny has dropped I've got one of my own here somewhere - used to drive the Orion/Skywatcher cheapy motor on my Crayford.

I don't really want to disassemble the working adaptors etc on the Newt. Does anyone know where you can source a reasonably priced stepper motor? Any need for gearing? (looks like you have a large step down there anyway David going from the small motor cog to the considerably larger lens grip.

More questions - is it really a servomotor, not just a normal motor? I thought servos had encoders etc built into them?
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Old 05-07-2013, 07:52 PM
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I've usually seen the term servo to mean a motor that just runs when current is applied, rather than moving by steps in response to an impulse. The distance a servo motor move is determined by the time current is applied for, so the movements are less repeatable. Most stepper motors don't have positional feedback either, the controllers just count how many steps they have moved, although I believe some do feedback that they have moved in response to the impulse.

I'll let you know if I have any success focusing this setup with focusmax - might have more problems with the 14mm focal length than the movement of the focuser!

DT

Last edited by DavidTrap; 05-07-2013 at 09:38 PM. Reason: Typos. Teach me for posting on an iPhone
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Old 05-07-2013, 08:49 PM
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Even if you end up using a Bhat mask its got to be better than tweaking by hand, then trying to lock it without moving, all while keeping dew heating applied somehow.
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Old 05-07-2013, 09:16 PM
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Not sure how well a Bahtinov mask will work at 14mm. Might have to point at the brightest star I can see and lengthen the exposure to get a diffraction spike! I have had them cut for my 85mm and 50mm lenses, so will see how they go.

DT
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Old 05-07-2013, 09:34 PM
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Looks good to me.

More importantly...what are the images like through the rig?? Looking forward to seeing the results
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Old 05-07-2013, 09:35 PM
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Ta Peter.

Cloudy tonight. Planning to try it out tomorrow night, but there is a rugby game I'd like to watch - decisions!

DT
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Old 06-07-2013, 09:42 AM
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David, I was wondering how you managed to get a filter into that rig - but of course I now realise you're probably using a OSC camera. If not, then how do you do it. The idea of doing some really wide Ha of things like Barnards Loop would be great if it can be done.
Peter
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Old 06-07-2013, 10:22 AM
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Is that a 14-24 lens on that rig? I like the inventiveness you have demonstrated here. Something worth considering using my QSI with if I get a bigger camera for my wide field unit in the future.
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Old 06-07-2013, 10:45 AM
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David, I was wondering how you managed to get a filter into that rig - but of course I now realise you're probably using a OSC camera. If not, then how do you do it. The idea of doing some really wide Ha of things like Barnards Loop would be great if it can be done.
Peter
Those QSIs have a compact filter wheel up against the CCD in the black section I thought Peter. Certainly an advantage for this line of work. Most of us have to fit the filter inside an adaptor. For smaller lenses I've just sat the filter on the top of the lens is another (naughty) way out.
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Old 06-07-2013, 11:00 AM
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Peter, Rob is correct. The QSI has an internal filter wheel - total backfocus including filter wheel and OAG is 53mm - one of the main reasons I chose the QSI. I have to change the front black cover from the one with built in off axis guiding to the plain cover so that the backfocus distance is correct for a Nikon lens - I'll piggyback this rig to another scope for guiding. I can now attach any Nikon lens to this CCD camera.

Yes Paul, it's the 14-24 - you'll need the WS cover and Nikon adapter. Forecast is looking good for testing this out tonight, albeit it from suburbia. Might try a big widefield Ha of the Milkyway.

DT
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Old 06-07-2013, 11:52 AM
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Yeah I have that face plate and adapter already. Was just interested to see you using this lens. Nice lens, I use mine a lot but had not thought of using for this purpose given the dew collecting front lens.
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Old 06-07-2013, 01:22 PM
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I stuck a dew heater strip around the front end when I was using it on my D800 - no problems!

DT
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Old 07-07-2013, 10:14 AM
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Allright - did some experimenting only last night amongst the clouds, but was able to watch a fair bit of the Rugby and Le Tour.

The bracket itself works fine and the focusing motor worked well. I didn't get around to trying a focusmax run, but after watching the size of stars change as I manually focused, I have no reason to think it won't work.

Here is the summary of what I discovered about the 14-24mm lens.

Zoom lenses are incredibly sensitive to backfocus distance. The QSI and it's dedicated adaptor are advertised as providing the correct backfocus distance for a Nikon lens. However, when you put a 3mm astrodon filter into the optical path, it effectively shortens the backfocus by 1mm. This creates major problems. At 14mm the lens would focus, but the distance scale on the side of the lens was reading ~0.35m??? The distortion at the periphery of the field was pretty shocking too - unusable.

So, I tried an empty filter slot and it focused at the infinity marker - much better. I guess the complex optics are designed for a certain back focus and deviating from the distance is not allowed! Next, I'll try to get a 1mm thick piece of aluminium cut (hopefully OzStockman can oblige with his laser) as a spacer - for those with a QSI, I think I should be able to get it to go between the T-mount faceplate and the WS cover.

So how did it go without a filter - at 14mm there is a fair bit of distortion/coma at the periphery, but at 24mm the image is definitely usable. I ran the 24mm image through CCD inspector and it gave ~12% curvature. I only shot a couple of frames amongst the clouds. I'll post some links to them - JPEGS, cloudy, light pollution, but they'll give you the idea.

All in all, I think I will have much more success and less frustration with prime lenses, but was curious to see what the 14-24 could produce.

DT
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