Is focussing using 2x bin accurate or not?

[Martin-Devon]

I'd like to know if focussing using 2x2 bin mode gives identical results to when 1x1 bin mode is used. Most of my targets are narrowband, and as you know the stars are much dimmer in H-alpha than they are in Lum. So far, I've been focussing & optimizing my FWMH figures in the Artemis capture software on the laptop screen by choosing a star with the H-alpha filter in place & using the 2x2 bin mode. The FWMH signal is stronger and the screen downloads are quicker in 2x2 bin mode. Once I've got my FWMH number down best I can to below 1.0, and it's steady, I'm assume I'm in good focus.

However - it's crossed my mind whether I'd get identical focus if I used 1x1 bin mode instead, ie., exactly the same binning as will be used for the rest of the night to capture the data over several hours. I tried to hop between 2x and 1x binning mode on Monday night to check, but the weather was useless, intermittent clouds/haze so I had no confidence that any comparisons were real.

I'd be interested to hear feedback from others though on this - is it best to use 1x1 bin mode for optimal focus, or will 2x2 bin mode give identical results?

Thanks,

Martin

[martin_h]

I use Nebulosity for capture and its focus routine and it runs at the maximum bin mode for the CCD in use, with mine thats 4x4 bin. I assume that focusing on a binned star would be more accurate because you are focusing on a slightly bloated star and when you get that to focus, going back to 1x1 should make it even tighter.

Just my thoughts, I dont have a clue on the maths of it.

[kookoo_gr]

i use a bahtinov mask and bin 2x2 when i focus and when i check again at bin 1x1 the focus is identical

[davew]

I've never been happy to check focus in 2x2 but the more I read other's accounts, the more it seems I'm wrong !

In my defence I do use Camera lenses more often than not and sometimes find a very slight difference

I will happily use 2x2 to get close and then switch over to 1x1. I won't be changing my routine.

Dave.

[Martin-Devon]

Very helpful feedback - thanks everybody. I'll stick to 2x bin then for focussing - it really does make it easier & faster than 1x bin.

[ollypenrice]

I focus with a Bahtinov in Bin 2 to get a good signal sometimes. But I never focus using FWHM in Bin 2, only Bin1. I have no theory behind this but when I have accidentally noticed that I've been binning for focus and then go into Bin 1, I find I still have a little refining to do. This tells me that Bin 1 should be better.

I use FWHM in Artemis Capture and don't get on at all well with Nebulosity's variant, though that is in Yves' ODK rather than our own refractors. I find Nebulosity often gives incoherent figures so for now I focus using the diff spikes and find that the best method to date. Some of the problem maybe that the Starlight Xpress H36 does not like being binned and creates triangular artefacts beside stars. This may be mucking up the focus routine in Nebulosity. I don't know that, though.

Olly

[ian_bird]

Hi Olly

"so for now I focus using the diff spikes and find that the best method to date."

Are you talking about the diff spikes from the secondary mirror support vanes? If so - how do you do that?

Cheers

Ian

PS - Can you tell Monique that Beverley has fallen in love with her salad! Which I make at least three times a week!