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Exposure time and timings
Posted: 07 Dec 2018, 21:46
by Forum_2015
Hi there,
I have a need to ensure that the time at which I take an image is recorded to the highest accuracy that my controlling PC supports (typically 20ms - and yes I know I need to use a GPS to keep the PC accurate).
I was thinking of using time$ to do this but have found that it returns the seconds component of the system time to integer accuracy.
So I had a look at the JD variable instead and that appears (perhaps I am wrong) to record the current time to milli-day accuracy (84sec) only.
I note that the exposure length can be set to the millisecond so wonder if I am missing something obvious.
Is there any other time related function/value I could use for this purpose? I could call an external routine when the image is taken and have that record the times in a separate log file. But that seems a bit clunky and has uncertain latency. Is there a more elegant solution I have overlooked?
Thanks
Grant
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:47
by Forum_2015
Hello, the JD() function is precise up to 10 ms, for example:
Code: Select all
Print Format(JD(),"0.0000000")
Pause(0.02)
Print Format(JD(),"0.0000000")
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:47
by Forum_2015
Thanks for that. Had not realised the print statement autoformatted floating point numbers. Should have thought of that.
Will have a play with the code you provided.
Why?
Posted: 07 Dec 2018, 21:47
by Forum_2015
I was thinking of the context of getting accurate star and stop positions of image trails for probes in gravity slingshot passbys, satellites and fast NEOs. As the default PC system clock offers 20ms resolution it seemed the thing to shoot for. Greater accuracy (such as provided via GPS) would probably be redundant - especially where a mechanical shutter is involved (50ms not uncommon) and ultimately limited by seeing I imagine (just a guess).
I'm looking at controlling a CCD camera and making it open the shutter repeated times for a single CCD readout - I've done it from within my own VB6 code but wondered if I could do it within Astroart - theres a paper in the BAAJ on this.
Success
Posted: 07 Dec 2018, 21:48
by Forum_2015
Managed to write a script that waits for the time I stipulate, then waits for the seconds part of time$ to change and begins an exposure, it then reads out the image, adjusts the FITS header and save the resulting image to disc. It then waits for the next occasion the second part of time$ changes and repeats for the number of times stipulated.
Seems to work fine. The time value in the header is what I expect, the images are fine. Very simple to use - it reminded me of QBasic.
One thing though, theres a mention in the CCD interface option2 of synchronising two cameras. Have not tried that yet. Can I assume one is named as the main CCD and the other as the guider? How do I save the images for each? How do I define which image is which and name them?
Thoughts anyone?
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:48
by Forum_2015
That syncronising is only a driver options, to support download in background. Using scripts, the secondary camera can be used only for autoguiding, using the Guider.XXXX instructions.
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:48
by Forum_2015
Okay, fair enough. So can I run two instances of AstroArt where one runs (say) a Canon 70D and the other runs (say) a 700D or Canon1100D at the same time? Then I could just set my scripts to save using the image.save routine. Or would it get confused having two Canons at the same time?
Many thanks for the reply BTW.
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:48
by Forum_2015
Two scripts in two sessions would work well, someone also made them communicating, using the clipboard or files. The problem may be the two Canons, they could not work. You will have to try (for quicker tests I suggest to make two separate Astroart folders too, so that .ini file will not overlap).
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:49
by Forum_2015
That's interesting. So, to clarify....
Two Canon cameras cannot work if they use the same .ini file, so separate installations are required. Does that sort the problem or it that just another separate issue.
Sorry to be a nuisance.
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:49
by Forum_2015
It will not solve it. Two different .ini files are useful only to setup quickly the astroart drivers (e.g. to keep two different sets of options).
It can work only if EOS utilities supports two different instances too, but I consider it unlikely.
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:49
by Forum_2015
Okay. Thanks for that. Will try it myself (just in case) but I suspect you are right. Two laptops it is...
Re: Exposure time and timings
Posted: 07 Dec 2018, 21:50
by Forum_2015
Sorry to bother you again.
Have created an AstroArt script that loops round taking 20x unbinned 1 second exposures with a Canon 1100D. It takes about 1 frame ever 5-6 seconds. Given the data writes to internal SD memory card very fast, is the 4 seconds extra delay caused by something else the Canon is doing or is it purely the time it takes the image to pass down the USB cable?