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How to focus in Ha?

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Replied by Radek Kaczorek on topic How to focus in Ha?

Mathematically speaking...
Size of a star in any optical system can be calculated as an angular size of Airy disk e.g. for a 8" scope angular size of a star is 0.345 arc sec.
The above is limited by:
- Resolution of a scope calculated according to Dawes' Limit or Rayleigh Limit e.g. for a 8" scope respectively 0.57 or 0.68
- Resolution of a scope + CCD (aka scale) which depends on focal length and pixel size e.g. for 800 mm FL and 4.54 um pixel size it is 1.17 arc sec / pixel
- Seeing, which can vary from below 1 arc sec (exceptional) to 4 arc sec (poor) with average something around 2.5 arc sec.

All these means that for a 8" scope with 800 focal length (f/4) and CCD with pixels of 4.54 um a real star angular size is around 2.5 arc sec, and is spread over 4 pixels (2x2). It will not get better than this for such a setup.
BIN2 will fit a star in a single "virtual pixel" (2x2) at the best. This is perfectly fine because you do not register any subpixel values anyway. BIN3 would lose required resolution (undersampling) and BIN1 would gain additional resolution (oversampling).

In other words if your scope + CCD resolution is above seeing (i.e. > 2.5) you should not use binning, because you lose focusing precision. If it is below seeing (i.e. < 2.5 or even < 1.25) binning will not harm focusing.
For simplicity reasons I skip the fact that focusing is based on FWHM/HFD/HFR ratio, which makes the system more sensitive to resolution. In any case binning is fine as far as a star fits to at least 1 pixel (binned or unbinned).

This is how I see it. Any other views are welcomed.
The following user(s) said Thank You: Eric
5 years 6 days ago #36840

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Replied by Magnus Larsson on topic How to focus in Ha?

Hi!

My idea with binning has been precisely that I get a better S/N ratio, and therefor the focusing should be improved. However, I've also done it to find suitable stars, something that in some parts of the sky is not that easy with my f/9 scope.

I found this article about binning, where they also argue that it can be useful for focusing:

hamamatsu.magnet.fsu.edu/articles/binning.html

Regarding multiple stars - the full frame focusing in Ekos - anyone got good experiences with that?

Magnus
The following user(s) said Thank You: Eric
5 years 4 days ago #36924

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Replied by Eric on topic How to focus in Ha?

Thanks for all these insights, very interesting indeed.

Solving the field to guarantee the procedure uses a real star is a sure method, but that's a chicken and egg problem in the general case as a frame that is out of focus may not solve at all. I believe that's the reason the Ekos Scheduler first processes focus then processes alignment. Ekos could however use the star catalog of KStars to pick proper stars once alignment is done. And if alignment cannot be done because the frame is out of focus, Ekos could just attempt to focus before reattempting alignment.

The maths behind choosing which binning to use could help automate the focus settings. Ekos knows the properties of the telescope and of the sensor, thus can deduce the theoretical resolution and give an advice. But I would expect setup owners to already know about under- and oversampling when assembling the optical chain: there, binning is more about capturing in general than it is to focusing, isn't it?

If binning is used to increase signal-to-noise, perhaps stacking could be used too. Ekos is able to stack multiple frames before estimating HFR. The process needs improvement as it is not visually explicit: the last frame captured is displayed, instead of the stacked frame. The other problem is that it would require sub-pixel guiding to be effective, as we do not want the target star to move while we are exposing multiple times, potentially for a long time each in narrowband. Currently Ekos runs the first focus procedure before guiding, but again that could be updated. Also it doesn't cover the case of off-axis guiding which will need a preliminary focus anyway, and will probably dislike a focus procedure while running. I suppose Mr Baudat should enter now with his On Axis Guider (www.innovationsforesight.com/wp-content/..._Imaging_Channel.pdf)...

I use full-frame focusing very often on my setup. It works relatively well in luminance as it smooths the samples and can eliminate hot pixels, galaxy cores and double stars, but has some drawbacks. As an example, the number of stars detected depends on the focus quality for the same exposure, and this changes the convergence of the procedure. Also, for some reason probably inherent to the algorithm, the computed mean HFR ends up equal to 1 (no decimals, just 1) too often for my taste. And finally, its processing is heavy on my low-end setup and could cause cpu issues with other processes such as guiding. As a side note, I recently added the ring-field option to that feature, to exclude stars too far (hence possibly elongated) and too close (hence possibly a galaxy core) from the center of the frame.

-Eric
4 years 11 months ago #37380

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