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More Focussing..


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You can get good focus with it, i mainly use fwhm to help with focusing.

Apt has another measure tool HFD wich is said to be more precise than fwhm( correct me if 'm wrong please)

Go to a bright star and try  the lowest value you can get.

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Bahtinov mask has one advantage. It provides are fixed point where the focus is perfect. But to reach it there is need to use a computer and software. Bahtinov Grabber (and APT which licensed the technology) has a method to analyze the spykes on sub-pixel level and providing much better analysis of the metrics than by naked eye. 

On other hand FWHM and HFD are industry standards and are widely used with good success :)

So if you focus manually by eye on steady atmosphere, I would suggest to use FWHM or HFD. If there is much turbulence then Bahtinov can give better results. If you use automated focusing then the application takes the care :)

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Thanks Yodhha. So you are saying Bahtinov is not affected by turbulence in the atmoshphere? I'm using FWHM in Backyardnikon. I wear glasses and the adjustment on the camera viewfinder isn't enough to make it possible to see a focus.

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Bahtinov is also affected by turbulence in the atmosphere. When I see the values in APT to oscillate around the zero point, I take it as good focus :) For example if I see values -0.10 then 0.10 and this is repeating for several seconds I know that the atmosphere will not give me better reading...

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When using FWHM there are two important things to consider;

1) The star must not be saturated, meaning it must not reach the highest brightness values your camera can record. So a bright star may easily be too bright.

2) Use exposures that are long enough to set the seeing (turbulence) average out a bit. I use three seconds. Less than this and the FWHM readings bounce around too much. 

Olly

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I'm getting the impression, when watching FWHM in live view that it can be used as a gauge of turbulence. I focus to the lowest value, although that may be around 2, on a turbulent night it may run up and down and go as high as 40+.

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Bahtinov mask has one advantage. It provides are fixed point where the focus is perfect. But to reach it there is need to use a computer and software. Bahtinov Grabber (and APT which licensed the technology) has a method to analyze the spykes on sub-pixel level and providing much better analysis of the metrics than by naked eye. 

had a play with Bahtinov Grabber last night, looks very good - straight into my toolkit thanks !

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Sorry to crash this thread - is "FWHM" a specific software package? I know what the acronym stands for, and I can see a few software packages talk about it but I can't seem to find any software actually called FWHM.

Thanks.

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Sorry to crash this thread - is "FWHM" a specific software package? I know what the acronym stands for, and I can see a few software packages talk about it but I can't seem to find any software actually called FWHM.

Thanks.

Hi Bob,

 Some capture software has the option to use fwhm to help with focusing. The one I use is backyard eos.

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FWHM stands for 'Full Width Half Maximum'. A bit detailed information can be found here - http://en.wikipedia.org/wiki/Full_width_at_half_maximum

In short it is a metrics that gives possibility to measure the focus changes. It has lowest value when the focus point is reached :)

EDIT: With other words it measures the star diameter...

That's helpful, thanks.

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You are welcome :) Take a look on APT too, it also has FWHM as well as HFD (Half-Flux Diameter - which is other way to measure the star diameter) ;)

Just had a quick look at APT, that looks like it has a lot of features and very cheap too!

Think I'll have to go and have a play with them both.

Thanks.

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Just had a quick look at APT, that looks like it has a lot of features and very cheap too!

Think I'll have to go and have a play with them both.

Thanks.

Yes, Yoddha is right to point out apt. Ive not used it but from what I've heard, it is as good as byeos. I think they both have free trials so give them both a go :)

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Apt is a great software, i highly recommend it.

you can do pretty much everything with the free version, but i just bought the license key for the full version.

can't wait to test the auto focusing as soon as the clouds break :)

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