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How does the SC1 mod work?


Glider
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I've read lots of descriptions of how to make the SC1 long exposure modification to various webcams.

I can appreciate what is happening technically by following the circuit schematics that people have drawn and downloading the datasheets for the chips which are used.

However, I'm just a little unsure about how the modified camera is then used.

It seems that you have to hook it up using a serial or parallel connection so that a piece of image capture software can control the exposure. Something like K3CCD.

My Neximage came with something called Amcap. You can select frame rate and exposure.

I can see the logic that if you select 5f/s then the exposure is 200ms or shorter. If you select 10f/s then it is 100ms or shorter, etc, etc.

However, this software can't interface to the SC1 mod.

My question is, if I manually drive the SC1 mod at 1Hz whilst the camera was downloading at say 5f/s would each frame have a 1s exposure, or would I see groups of 5 frames with exposures of nominally 200, 400 600, 800 and 1000ms.

Otherwise, how does something like K3CCD sync capturing a frame to the start of an exposure?

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As I understand it, the SC mod intercepts the frame transfer pulses, even though the webcam chip is sending pulses to get 5 f/sec the connection is broken for as long as you want the exposure this allows the more time for the charge on the ccd to accumulate and then the software makes the connection and then the next frame pulse in the webcam is allowed through to end the exposure.

I've been fiddling around with a few of these webcams to get a guidecam sorted.

One thing I have found if you operate an SC modded webcam remotely ie: 10mtrs away using a serial to usb converter, there are bad timing issues which result in black or non exposed frame intermittantly.

I'm now a proud owner of an old laptop and use this to remotely operate.

Oh, I've just found that a laptop with 2 usb ports can't run 2 webcams not enough bandwidth so I'm going to try a cardbus adapter.

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Thanks Astromerlin.

I was thinking of making myself an interface board using something like a Microchip PIC18 (I do this sort of thing for a living). My thought is to make this fit in the webcam enclosure or rehouse the entire innards in a simple plastic box. On the outside would be a simple on/off switch to enable/disable long exposure mode.

What I thought was;

..The PIC18 intercepts the frame transfer pulses from the webcam controller going to the imaging chip.

..Depending on how the PIC is configured by the exposure switch it stops the pulses for a user programmed exposure time.

..After the exposure time has expired the PIC lets a frame transfer pulse through.

I was thinking of a simple multiply by 10 factor based on the frame transfer rate. So, if the PIC detects frame transfer pulses at 5/s, it counts 10 and lets one through, effectively a 2s expsoure. If it detects pulses at 10/s it would give an exposure of 1s, etc.

I'd keep things short to avoid the problems that people quote with the gain amplifier causing heating and image blooming. Infact, once you've got a PIC on board it could handle turning this on/off as necessary depending on a long exposure length limit.

Am I correct in thinking that any webcam driver could then be used to capture long exposure images without needing a special serial/parallel interface?

Does anyone have any comments?

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Am I correct in thinking that any webcam driver could then be used to capture long exposure images without needing a special serial/parallel interface?

Not all webcams, only ones that have the same processing chips in them (I forget the actual number) you can get more info at Steve Chambers site

SC1

Having had troubles with the LX mod and my wish to image remotely I have had a similar idea but havn't the time to experiment.

I am dumping the USB to serial as that doesn't work at distance and I'm trying a long cable with line drivers on the parallel port idea.

It did occur to me that I could try cobbling together an 8051 based micro as I can programme in 8051 assembler.

My thoughts are that you could sync the micro to the frame pulses and start an exposure for n number of frame pulses and

you can also provide amp-off timing to give a trouble free exposure all the time, well needed for a guidecam.

I'm not sure you could have the exposure working free of the capture software, you'll need to experiment here. I would plan to use the LX pulse provided by the software to start the exposure and then count the number of pulses for the amount of exposures required.

I would be interested in your experiments.

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My basic problem is that, like you, I'm not sure you could have the exposure working free of the capture software. I would plan to use the LX pulse provided by the standard Windows software to start the exposure and then count the number of pulses in the PIC18 for the amount of exposures required.

However, what would the capture software do when it sent the intermediate number of pulses and didn't get anything back until the last one.

Would it show n-1 dark frames and then one long exposure frame?

This would make the captured video look jumpy.

Does anyone have any ideas?

Anyone got an SC1 modified webcam that they could hook up to a signal generator at say 0.5Hz and give it a go?

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Ok, I've tried this for you as I've done with work today.

I connected to the webcam LX a 700ms pulse 50% duty cycle and used WxAstrocapture as the capture program.

I pressed the record on the capture program without LX mode and observed 3 types of image that occured regularly. A blank image, no recording. A normal exposure image, recording at 5f/sec then a long exposure and the capture rate was reduced considerably.

So it seems that the capture, recording rate is done in the webcam driver.

I think the reason for the 3 types of image was the duty cycle, it really needs to be a short pulse to start and end capture.

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Thanks very much for that.

It sounds like the capture program initiates the exposure and expects to control the timing of reading of the resulting long exposure frame.

It sounds like your 50/50 exposure start signal and the capture frame read rate where different and over time kept coming into alignment and then drifting out again.

Sometimes this gave the desired result and sometimes not.

It doesn't sound like my idea will work. Would you agree?

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The webcam driver expects a video stream and arranges capture accordingly. This is why other LE capture software was developed, to step around the driver system and retain the good frames.

You might consider holding the exposure pin open whilst controlling the snapshot button maybe though as that bypasses the driver's attempts to create an avi.

Arthur

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Hi Milamber.

Yes, I can see that the many programs that are available are one way to go, using a serial or parallel interface for control.

I'm just trying to see if a 'simpler' hardware approach would also work. To me 'simple' means basic Windows capture utility and no serial/parallel leads.

Do you mean that I forget about capturing video but instead manually capture multiple still snapshots to load into something like Registax.

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I realise what you are doing, and my point was that without firmware control at that level along with the hardware mods there is little you can do to interupt the avi save sequence, meaning that the video stream is going to be difficult to use the way you are proposing.

So yes, your only way past that is via the snapshot routines, which themselves already save frame by frame. One would still need to interrupt the exposure time sequence though, and it's for that reason I think you are not going to make a new guiding system within your initial requirements... believe me, *every* way of controlling the exposure and save routines were looked at a long time ago before Steve published the initial mods.

Arthur

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Ah, now this is starting to sound like a nice little project again.

I've bought a Logitech Quickcam 3000 and aim to make the SC1 mod on it and get it working with something like K3CCD in the conventional way. The aim after all is to try some long exposure work.

Then I'll have more of a play with signal generators and scopes and see if a PIC18 approach is possible. I'm not saying it will have any benefit over the conventional approach but it'll keep me busy.

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Had a plan a while back to use a 556 instead of a laptop to initiate and control the LE, thereby allowing (via a preset for exposure length) a standalone guidecam - the idea being to then build it into a small scope with preset focus... Just a thought, never took it further ;)

Arthur

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  • 2 weeks later...

I finally managed to pick up an SPC900NC on eBay.

Bearing in mind my job is hardware and software engineering I was keep to crack it open. Bearing this in mind, and considering what I saw I basically didn't fancy bodging around inside with a soldering iron, seemed like a disaster in waiting and all credit to anyone that has made the mod on this sort of surface mount components.

However, what I have come up with for my own peace of mind is attached. This 'carrier board' basically does a long exposure mod without any 'bodging'. It connects directly to a serial port or via USB-Serial converter and works with wxAstrocapture.

It might seem over the top compared to a 'dead bug' but in my case it is now a platform to experiment with amp-off, peltier cooling and a fan.

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And this is how it looks when it is all together.

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The AC414 nose piece can be screwed so far into the old lens holder that it touches the CCD housing with thread to spare, so its held off using washers.

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9pin D type for connection to USB to RS232 dongle, running with wxAstroCapture.

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Using the original Logitech USB lead saved a lot of messing about.

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5s LX across the room with the lights off, I was going to post a pic without the LX on for comparison but take my word for it, it's just a black screen.

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Now I'm turning a 50mm to 1.25" adaptor to use the 9x50 finder as a DIY guide scope.

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  • 1 year later...
I finally managed to pick up an SPC900NC on eBay.

Bearing in mind my job is hardware and software engineering I was keep to crack it open. Bearing this in mind, and considering what I saw I basically didn't fancy bodging around inside with a soldering iron, seemed like a disaster in waiting and all credit to anyone that has made the mod on this sort of surface mount components.

However, what I have come up with for my own peace of mind is attached. This 'carrier board' basically does a long exposure mod without any 'bodging'. It connects directly to a serial port or via USB-Serial converter and works with wxAstrocapture.

It might seem over the top compared to a 'dead bug' but in my case it is now a platform to experiment with amp-off, peltier cooling and a fan.

Glider, Has your carrier board design for the SPC900nc ever been published to allow one-off builds? If so, could post details of the site? Thank You. Bog Trotter

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  • 2 weeks later...

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