A framerate setting question

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mconsidine
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Joined: Fri Jun 17, 2022 2:18 pm

A framerate setting question

#1

Post by mconsidine »

Hi,
I've searched the forum/manual and believe the following is not possible. But I'd like to confirm it.

Specifically, I'd like to be able to set a specific exposure and framerate combo, recognizing that they would need to be "legit" to begin with (e.g. you can't have a 1ms exposure and 2000 fps, because the math doesn't work. But technically, 5ms at 171 fps could, it seems).

I cannot find a way to do this, but am hoping I have overlooked something. Is it possible this could be added as an option in the drop down?

The motivation for it comes from a method of building up solar disc images via slit images from a spectrograph. In order to get the aspect ratio correct, the resolution in the scanned (say, horizontal, direction) needs to match the resolution in the perpendicular direction on the chip. The latter is a function of the chip - the former is a function of quickly one is scanning the solar disc, which in turn is a function of the framerate used to acquire the slit images.

So it's helpful to be able to specify both an exposure and framerate that "work", in order to be able to get the aspect rate right as well as for repeatablity.

Hope that makes sense and I look forward to any feedback.
Thanks in advance,
mconsidine
Jean-Francois
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Location: Germany

Re: A framerate setting question

#2

Post by Jean-Francois »

Hello Matt and Robin,

The "Frame Rate Limit" allows to set a fix value ... but it does not really work fine. I never have the correct value.
Try with the Test Camera (high speed) ... SharpCap can not stabilise on the wanted value.
Other problem ... the final rate depends on the data transfer to the disc and other Windows application(s) load.

Concerning the "math" ... some camera can produce for example 1 second integration at 25 Hz (?) ... yes, my old Mintron analogue camera can this.
The solution of the logical problem is "intern buffer" :-)

For my sun imaging with my Sol'Ex, I used all the time the maximum possible fps.
For integration time below 3 ms, SharpCap can read the ROI at ~340 fps (+/- 3 fps).

I control the sun aspect ratio with the telescope rate (I used the ASCOM driver from my SharpCap script and it is possible to set all the rates, not only 4, 8, 16, ...). And at the end, the INTI script correct the aspect ratio automatically (OK, it is for that necessary to have both side of the sun on the image. Alternative is to set manually the ratio). But, if I remember correctly, you use an old (historical) telescope and you can not change the scanning rate.

So ... yes, it could be interesting to have a more precise (and stable) camera readout rate ... (it could be a script function ... no, it could be at least a script function :-) ).

Regards,
Jean-Francois
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Re: A framerate setting question

#3

Post by admin »

Hi folks,

I don't think that this is easily achievable sadly :(

Astro cameras tend to run in one of 2 modes - video and still.

In video mode, the camera takes images continuously and sends them to the PC. Frame rates are limited by the exposure and potentially the data bandwidth available to transfer images to the PC. In this mode there isn't a way to say to the camera 'take 5ms exposures, but start them 6.72ms apart'. The way that the SharpCap frame rate limit works in this mode is that SharpCap accepts all frames that come from the camera, but discards frames if they are arriving too quickly to get a frame rate that is close to (but not exactly) the one requested.

In still mode, SharpCap has to ask the camera to run each frame individually. There is obviously more control this way, but also more latency around the capture of each frame (sending message to the camera, camera starting, exposure, readout, transfer to PC, transfer to SharpCap, repeat). This means that high frame rates tend to be hard to achieve (depends on make/model), and exact timing is still not possible.

You may be able to get closest to what you need in video mode by choosing an exposure length that divides into the 1/(max frame rate) time, or is close to that. By some careful adjustment of exposure, you may be able to get to the point where (for instance) the max frame rate code is skipping every 2nd or 3rd frame fairly regularly.

Actually, would that work - if there was an option to skip every 2nd, 2 out of 3, 3 out of 4 frames, etc?

cheers,

Robin
Jean-Francois
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Location: Germany

Re: A framerate setting question

#4

Post by Jean-Francois »

Hello,

May be the only camera (without going to exotic camera) that allow a precise timing is the QHY-174-GPS. In the "slave" mode, it should be possible to synchronise the capture with the GPS signal. For long time ago, I tried to set the camera in the slave mode with a script ... but without success.

Regards,
Jean-Francois
mconsidine
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Joined: Fri Jun 17, 2022 2:18 pm

Re: A framerate setting question

#5

Post by mconsidine »

Hi,

Thank you both for your replies. I understand now better the problem. It almost sounds like a post processing step that takes the resulting video and does a binning only in the scan direction (ie horizontally) to get the desired frame rate would be what I need.

Another alternative may be to do a limb to limb sweep and derive the aspect ratio from that, hoping that it does change during a study of an active region.

JF - yes, this is an old spectrohelioscope, which was never designed for photography, much less high cadence digital work! We cannot rotate the slots, so we are left with trying to get smooth repeatable E-W motion out of old coelostat hardware. Introducing a slow rotating Andersen prism may accomplish that for imaging small regions.

Perhaps I should try to get more frames than needed and let the INTI code figure it out, but I'm concerned I'm not going to get repeatable results I can stack.

Matt
mconsidine
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Re: A framerate setting question

#6

Post by mconsidine »

Actually, I don't think my binning idea would give me what I want. I think it would essentially increase the slit width as well ... Skip that idea.
Matt
rnschrantz
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Re: A framerate setting question

#7

Post by rnschrantz »

Hello,
I think I have a similar question about Sharpcap using a spectrohelioscope. A spreadsheet I use indicates a framerate of 349 frames per second is exactly correct for my 16X mount slew rate (to produce a round raw image). My computer can handle that framerate. This framerate corresponds to an exposure of 2.87 milliseconds (1/349). The camera can do all that fine with Sharpcap. BUT.....a 2.87 millisecond exposure overexposes the image even at zero gain. I need to drop down to, let's say, 1.5 milliseconds for a correct exposure. But then, the framerate automatically increases to near 400 fps (maxing out there)! There are framerate limit choices, but only a few.

Just as there is a custom ROI box size choice, is there way I can custom-specify framerate at 349 fps? I want to be able to image at 1.5 milliseconds exposure and 349 fps. My computer could handle this, but Is there a way to do it in Sharpcap?

Rick
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Re: A framerate setting question

#8

Post by admin »

Hi Rick,

unfortunately there's no option to choose a custom frame rate, and even if there was, I don't think it would do what you wanted...

The way the frame rate limit works is to let the camera run at full rate but to discard frames as required to get approximately the rate selected sent to the screen and saved to file. I'm not aware of any cameras in mainstream Astro use that have the ability to adjust the inter-frame interval separately from the exposure for high speed work. It might be possible on some of the industrial cameras, but I've never really experimented enough with them to be sure.

Can you put a neutral density filter onto the camera to drop the brightness a bit and then you will be able to set the right exposure?

cheers,

Robin
rnschrantz
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Re: A framerate setting question

#9

Post by rnschrantz »

Hi Robin,

Thanks for responding so quickly. Yes, I can do the neutral density filter, and that is certainly "plan-B" (or maybe "plan-A" now!). However, I have experimented and found that if I slowly decrease the Turbo USB slider, I can slowly reduce the framerate.
For example, if I set my exposure to 1.5 milliseconds on a 3076 x 100 pixel narrow ROI box (for SHG imaging).....the framerate maxes out at 460 fps. That is as fast as my computer can download frames at that ROI size regardless of exposure. But, the spreadsheet calculation shows that I need a 346 fps framerate for my slew speed to produce a round solar image. I found that reducing the Turbo USB setting in manual mode gradually reduces the framerate. If I reduce Turbo USB down to around 70......the frame rate hovers right around 346 fps (bouncing around the 345 - 347 range). This seems like exactly what I need.
I keep my 1.5 milliseconds exposure and get my 346 fps framerate. I'll still record 346 fps, but my exposure for each frame will be 1.5 milliseconds to not overexpose. Will this weird way to seemingly achieve what I need actually work? Or is there some sort of problem doing it this way that I don't know about?

Thanks!

Rick
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Re: A framerate setting question

#10

Post by admin »

Hi Rick,

yes, that should largely work (and it's a creative solution to the issue). Basically you are making the camera delay taking the next frame because it needs to send the data from the last one to the computer, so you are inserting a gap between one frame and the next.

The only thing to watch out for is if this is stable enough for your requirements - the only way to tell is to experiment.

cheers,

Robin
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