Guiding for DSI (Deep Sky Imaging)

[bunburystargazer]

Looking at how the centre of mass guiding works, is there any reason that the centre of mass could not be a star?

My idea is this--
If we use a guide camera, which would need to have a long focal length-f5+, could we actually zoom in on that star, and use it as the centre of mass to guide on with an alt/az mount.

First, open up 1 copy of sharpcap for your main imaging scope, and carry out your alignment.
For me, my mount automatically connects to sharpcap when I select my camera.
Once aligned, disconnect the mount from sharpcap, and open a second copy of sharpcap, and connect your guiding camera that is on your guiding scope, and your mount to that second copy of sharpcap. Once this is done, slew to your target by whatever means you use to goto your target-eg, stellarium. Then centre the target within your main imaging scope, and let it track while you set up your tracking via your guiding scope and camera.

By this time, your guide scope and camera should be close to your selected target.
You then pick out the brightest star that is within view of your guiding scope, and zoom in on that star, so as it is of good size within the frame.
Then using the guiding features, we use centre of mass to guide the mount. If this worked, then your main imaging scope would stay on target when live stacking, for as long as required. I am not sure on how much of the sensor the object has to take up before centre of mass guiding can be used.
I have not been able to try out this idea, as I have had nothing but full cloud cover from the day I thought about trying this idea out, so thought I would just ask here in the forum until I get a chance to try it.

I know it would be trial and error to get the correct framing times, exposure, etc, but could it work?
Good seeing would be required, so as we could get the star as stable as possible, and some playing around with the settings in the guiding feature, but it might work. If not, then how hard would it be to add to sharpcap--guide on a bright star, where you can select the star in the crosshairs or such, and have sharpcap guide on that star? Or would it work using-guide on image features, where the stars them self would be selected by sharpcap as the features to guide on?

If this could be done, that would be a game changer for alt/az mount owners for DSI.

Thanks
Brad

[admin]

Hi Brad,

it might track the star if you got all the settings right, but I don't think it would be useful for deep sky imaging.

The reason behind this is that the feature tracking in SharpCap (whether tracking surface features, or center of mass, etc) is not designed to keep the image absolutely steady, it's designed to stop the target from drifting out of view over the long term. It will allow a bit of drift to happen, then decide to act on the drift and make a movement to try to get the image back to roughly where it works before - this will then repeat as needed.

The approach above is fine for keeping the sun/moon or a planet in view, since you use short exposures when imaging those sort of targets, so the gradual slight drift followed by a more rapid correction doesn't cause any problems with smeared images. On the other hand, for deep sky, you are taking long exposures and both of these would lead to smearing of the captured images.

To be honest, your best bet is to use a dedicated deep sky guiding application like PHD2 with your guide scope for this sort of thing (if possible - may not be depending on details of alt/az mount control). PHD2 is designed to try to keep the target perfectly steady with a series of very small corrections rather than less frequent but bigger ones. If your AltAz mount has an ST4 guide port then that may be operational in alt-az mode even if the ASCOM driver for the mount does not allow for pulse guiding. PHD2 can send guiding commands via the ST4 port of the guide camera (again, presuming it has one), which may let it guide in alt az mode.

cheers,

Robin

[bunburystargazer]

Hi Robin.

Thanks for the reply.
I and many others have tried PHD2 guiding with alt/az synscan mounts, with very limited success.
Seems to work well for about 5-10 minutes, then the mount just starts doing it's own thing, like they do when using a wifi connection.
At this point, the mount by itself would track better alone, without guiding. With alt/az mounts, we have to use the single star option, due to multiple stars rotating, and messing the tracking up even sooner.

So, I was just hoping that there was some other method of keeping the scope on target for a longer period of time using the centre of mass on a single star option in sharpcap.
I have been using the solve and sync option while live stacking, and that works well most of the time, but sometimes it just fails, and says-cannot sync to target.

Anyway, I will keep experimenting and see what I come up with that works.


Thanks.

[admin]

Hi,

did you try the recenter option in the 'Guiding' tab of live stacking? That's fully integrated with live stacking and is designed to use plate solving to recenter the target once the total drift exceeds an adjustable threshold. It obviously can't do anything about the rotation you get with an Alt-Az mount, but it should keep the drift under control. See the documentation at https://docs.sharpcap.co.uk/4.1/#Automa ... ecentering for more info.

cheers,

Robin

[turfpit]

Brad

Sadly, for your wallet, the solution for deep sky is an equatorial mount. I started off with an alt/az mount but after some struggles I bit the bullet and purchased an equatorial mount.

With a second hand Celestron AVX, cost £500, this has opened the door to long exposures. This is 7x1200s Ha frames from a single 3h 20s session (the Aurora affected the last 3 frames), guiding with PHD2.

Dave

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