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Hi, anyone measure the typical FWHM of the images they take?

I typically see fwhm 3.5+ usually less than 4 with my 530 mm f/5 FSQ and qsi583 and a 20min exposure with good guiding.

i had hoped that this would be lower around 2.5 fwhm but i do not see this at all.

Any one better than mine?

Ray 

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Not quite comparable since we run long focal length scopes (2 and 3.6 meter) and we are located in Sweden (should have the same, i.e. poor, seeing as the UK though). Best long exposure 1.2-1.5", good seeing about 1.8-2", typical 2.5-3.5". We have found that to be able benefit from the good days means that everything else must perform up to spec, focus, tracking, guiding, local seeing, collimation, etc.

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My problem is that most software gives you FWHM as a dimensionless number, which means you can not compare your value with anyone else's. For instance BackyardEOS tells me the FWHM in live view, but as it does not know the focal length of my scope it can only be giving it to me in pixels. This means that if I look at the same star at the same point in time using a different camera or a different scope I will get a different FWHM value.

Ideally all of the different pieces of software would always record FWHM in arc seconds, so that you could compare them. However that would require the software to understand the focal length of the scope and the size of the pixels. In the mean time I just use it to help focusing by getting the lowest value I can

Edited by frugal
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In Maxim it will tell me the average figure in an image. My 30 minute narrowband subs at 1.6m generally give me a figure of between 6-7. I know that's well focused,so I don't bother even looking at it.

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For short focusing exposures in broadband I sometimes get down to 0.85 at 3.5 arcsecs per pixel but sometimes have to settle for 1.5. At 1.8 arcescs per pixel I never go below 1. More like 1.5 on a good night.

Olly

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Hi, anyone measure the typical FWHM of the images they take?

I typically see fwhm 3.5+ usually less than 4 with my 530 mm f/5 FSQ and qsi583 and a 20min exposure with good guiding.

i had hoped that this would be lower around 2.5 fwhm but i do not see this at all.

Any one better than mine?

Ray 

It is all seeing dependant since your scope has very cause for concern. A couple of nights ago I got 3.7 on average for 900s subs with WO star 71 and an Atik 383L+ Mono but the seeing was not brilliant, it never is in the city centre I also have a feeling that the focus had shift a bit as the temp dropped quite a bit past 10pm and I did not adjust it.

A.G

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Thanks for all the replies...I started to trial Sequence Generator Pro a couple of days ago, and it measures focus using H F R (half flux Radius) I'm not sure what the difference is but it reported HFR of 1.65"

I agree with the point about seeing effects.... and this was at the Zenith or thereabouts..the guiding was good, however if i guide in the west and over the roof of my home; the guiding can become atrocious, i assume that heat from the roof plays a part in this and consequently raises the star profile.

Ray 

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