Self Guiding

[festoon]

I was wondering if with any software it might be possible to self guide during my image acquisition.

I have thought about this before and had good feedback here on SGL that some natural image drift is good and would naturally dither the image....so maybe if the software would every say 20 frames re-align the chosen guide star - wouldn’t that be a useful feature?

The set up I’m using is a Samyang 135mm f/2 lens with an Atik 414ex CCD on a HEQ5 - so quite wide field. At the moment I’m running unguided.

I understand normally I could auto guide with a separate guide scope and camera.
However, as my imaging set up is widefield I should be able to have a decent guide star within my FOV. So my question is why wouldn’t I be able to choose a star from my live main image FOV and guide from that? If so what software would be able to do this? Maybe I’m missing something, but just wondering why a separate guidescope is needed.

 

Edited January 9, 2021 by festoon

[festoon]

I think I’ve answered my own question - the purpose of the guide cam is to keep the guide star not moving during the exposure, not to keep the star at the same position between frames.

So the guide cam needs to be exposing for short times and updating the position to enable longer exposures. Which it can’t do if it’s doing long exposures. 

Forget my comment earlier - I was being thick  

[discardedastro]

So it comes down to what you're trying to correct for.

Widefield, you won't see - generally - fine mount errors, which is generally why guiding isn't as necessary as it is with narrow FOVs. This is mostly what guide systems work to correct. In this case you need to respond to errors more rapidly than once every 30, 60, 120, 300, 600, 900s as you'd normally use for imaging.

Consequently guide systems work on the basis of a separate, short-time-domain exposure because this gives you enough information quickly enough to adjust the mount motion to correct your error before it affects the image - because at a narrow FOV we see those errors more rapidly. Widefield, depending on exposure time, is much less sensitive because stars won't appear to move visibly in response to minor mount errors. Guiding also corrects for periodical and systematic errors like polar alignment errors, which don't need faster correction periods but benefit from it.

You also want non-saturated, small stars for guiding as these give you the best accuracy of the star's centroid on your sensor - imaging data will normally have quite large/saturated stars by comparison.

However, those errors won't be completely invisible in wide field - it may just be hard to see or minor in your images depending on exposure time and accuracy of polar alignment. You do want some drift but doing this between exposures rather than during is much better.

So guide systems won't take your imaging data and correct your positioning based on it. You can use plate-solving to re-align your mount with your target between frames, of course, but that's about all 99% of systems will do for you.

Essentially, you can either add a guidescope and camera, or an off-axis guider (if your system will support it - I'm guessing with the Samyang probably not) to produce those short-duration guide images and feed your guider (e.g. using PHD2) but the image from your main sensor isn't frequently updated enough to give a guide system enough to work on!

Edited January 9, 2021 by discardedastro

[festoon]

Thank you @discardedastro. This makes really good sense.

Is there any software that will automate dithering without guiding?

[discardedastro]

Just now, festoon said:

Thank you @discardedastro. This makes really good sense.

Is there any software that will automate dithering without guiding?

Quite a few bits of imaging/guiding software will do dithering, but most will only do that through guiding - e.g. applying a small guide "correction" to intentionally dither. The reason being that some (many) mounts don't do small corrections well if instructed to slew to new coordinates, and software support for sending correction pulses directly has been a bit variable historically (so ST4 was used along with special ST4 ports provided on guide cameras or in separate adapters).

[Davey-T]

You can dither manually by moving the mount up, right, down and left a few pixels between frames then crop off the overlapping images in processing after aligning them

Dave