Guide and image with the same camera

[higginsdj]

Is it possible to guide and image with the same camera?

My use case - I image Occultations and therefore image sub second integrations. Guiding would also use these same frame. Obviously I wouldn't want to guide at this sub second rate but only send guide pulses when the scope moves a predefined distance from the 'guide' point. This way I could keep my target in the central zone of a small camera FOV with a poorly tracking Alt/Az mounted scope. Occultation setups are usually mobile and therefore have less than ideal setup and alignment.

Cheers

David

[admin]

Hi David,

SharpCap's feature tracking mode almost covers this ground, but it is designed for surface (lunar/solar) or planet tracking - it will not track on a starfield (at least not by design).

The real thing would be to somehow control the camera in SharpCap for capture but also allow access from a full guiding application (like PHD2) - that way you get all the tried and tested guiding algorithms in PHD2 available. As far as I know this is probably only possible with QHY cameras, which have a weird option to allow the images captured in SharpCap to be also made available as a fake Webcam for other applications to use (see https://www.facebook.com/QHYCCD/posts/q ... 900277875/). Note that this not something I have tried myself, so I cannot guarantee it will work for a particular QHY camera or with PHD2. Worth trying if you already have a QHY camera, probably not a good reason to rush out and buy one!

cheers,

Robin

[Borodog]

It does seem like it would be relatively simple to use the alignment infrastructure from live stacking to calculate corrections for the mount, no?

[admin]

Hi,

the live stack alignment info solves part of the problem, but what you don't get is

* What direction does the mount need to use to move the image up/down/left/right (ie how does image direction relate to mount axis movement directions)
* How much backlash is there in the mount movement in each direction

Both of these things are calculated by the sort of calibration that PHD2 does. The first one can also be worked out by plate solving.

Having tried to make the same sort of calibration work in SharpCap in the feature tracking, I've found that it is actually pretty hard to make this sort of thing work reliably for different brands of mount. At some point I want to come back and look at that again, but it is a significant project

cheers,

Robin

[Borodog]

Got it. I was assuming that yes, you would be using the feature tracking infrastructure and the star alignment infrastructure together to make it work. But I don't have to write the code, so it's really easy for me to claim things should be "simple." ;O)

[Jean-Francois]

Hello Robin,

I'm interesting to know more about this fake Webcam.
I have several QHY cameras ... so I can try it.

For long time ago, I had the idea for this kind of guiding. But it was not possible on "normal" CCD camera.
For the CCD type camera, the full detector is read line by line at the same time.
It is possible to imagine some custom CCD detector with some separate area with a separate readout.

But now ... the most of the new cameras are CMOS type ... that is different.
May be in the future it will be possible to read a small sub region (50 x 50 pixel somewhere in a corner) at a different frequency.
Ideally ... this sub region position should be selectable ... so around a guide star.

The idea is: the sub region is exposed between 0.1 s to some seconds for the guiding and the other full detector is exposed for the light image up to several minutes.

Regards,
Jean-Francois

[admin]

Hi Jean-Francois,

I have had the QHY fake webcam thing working when I first implemented support for it. Of course you are limited to the exposures being the same for both applications, since the application that is talking to the real QHY camera controls all the settings.

I'm not sure if the sort of sub-area thing you are suggesting is likely to happen - almost all the current developments in CMOS sensors are being driven by non-astro requirements (industrial imaging, car cameras, DSLR/Mirrorless cameras, mobile phones), so unless they want that feature it is unlikely to happen.

cheers,

Robin

[SteveInNZ]

I have a different but real application. It's a bit too niche to make you drop everything but I may as well describe it.

I have a CEM25EC mount and want to do some astrophotography on film. The EC mounts don't require guiding over typical sub-exposure times of a few minutes but any mount is going to struggle with a single, one hour image. So I was going to put my cooled astro camera next to the film camera and image the same thing on both mediums. If I could use the digital sub to keep the mount on track, I'd being getting the most out of everything. The EC mounts also don't like being guided at a fast rate anyway, so it all sort of falls nicely into place.

Of course, I can use an additional guide camera and even my lowly CGEM since it's now being guided, so I'm not stuck. I just prefer the neatness of the EC solution.

So if it was there, I'd use it.

Steve.