Using Sharpcap with a Spectroheliograph

[Merlin66]

Using the spectroheliograph involves obtaining a SER video of the solar spectrum while scanning across the solar disk. For optimum results, obtaining circular solar images, it's ideal to obtain an Y/X ratio of 1. This means being able to establish a capture fps based on the solar scan time of the telescope mount. In my case with the NEQ6 at x2 sidereal rate (scan time 9.6sec) a 140 fps capture rate would be ideal.
I can't seem to do this with Sharpcap.
With a small ROI 4000 x 150 and a short exposure 0.05ms I can probably achieve frame rates>250 but the only setting available to try to "fix" the frame rate is the "Frame rate limit" box -BUT it doesn't allow a custom value only 120 or 240 etc.
When 120fps is selected the actual rate drops to around 103fs, and if 240fps is selected I get 204fps......

Is there any way around this? A custom "Frame rate limit" option?
Any help gratefully accepted.
Ken

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[admin]

Hi,

setting the exact frame rate is hard, because when running in high speed more, the camera takes frames at its own pace and sends them to SharpCap. The SharpCap frame rate limiter can drop frames from that stream to keep the rate under the target, but it's impossible to hit an exact rate that way.

The trick is to know that the frame rate that the camera runs at is determined by two main factors:

* Vertical height of the ROI in use
* USB speed setting (Turbo USB, USB speed, etc)
* Bit depth (ie 8 or 16)

A bit of careful tuning of the height of the ROI and/or the USB speed should allow you to find a configuration that naturally gives you the frame rate of 140fps that you are looking for.

cheers,

Robin

[Merlin66]

Robin,
Thanks for the reply.
I've tried all those settings and have yet to find that "sweet spot" which repeatedly gives the optimum frame rate.
Such is life......

[admin]

Hi,

did you use the 'Custom' option when selecting the capture area (at the bottom of the list for most cameras). That lets you set the height to pretty much any value you want (may have to be a multiple of 4 or 8), which should get you close enough I would have thought.

The other approach to try is to set an ROI that can achieve above 140fps, then set an exposure somewhere around 7ms, which should give 140fps. At that point of course you can only use the gain to adjust image brightness (or insert some sort of ND filter if too bright).

cheers,

Robin

[Merlin66]

Robin,
Thanks for the continued interest.
I've tried all those options, the ROI definitely has some impact.
I'll be doing more trials when the weather improves. Winter at the moment -2 overnight!
ken

[Merlin66]

Robin,
Another question...
Can the Frame Rate Limit options be improved to allow a Custom option? I think this could be useful for the SHG.
Ken

[admin]

Hi Ken,

in theory yes, but as I noted above, the limit is a 'do not exceed this rate' limit, and in most circumstances the rate will not exactly match the limit, so even with a custom limit you won't get the rate set on the limiter in 99.9% of cases. Given that, I'm not sure if it adds much.

cheers,

Robin