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What is the typical FWHM that I can get in the UK?


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Mike

FWIW, I ran your image through CCD Inspector.  Results attached.  Not knowing your imaging scale, the results are in PIXELS. 

I'm impressed with the flatness. 

CCD_Inspector_result.thumb.jpg.e056d2a481df14d90bf8df239f883313.jpg

 

Although I don't consider myself qualified to comment critically on others' images, so you may discount my thoughts, I'd say that it has a bit of a green tinge and the background level is slightly high.  To remediate this, I've run it through an SCNR process and also applied a little tweak of the low-end curve (following the practice of @ollypenrice) which is a simple but effective noise-reduction technique.

M15-scnr-onr.thumb.jpg.8f9c7cfc7550556eb81bba5e2e2173f4.jpg

 

HTH,  Tony

 

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The main issue is nothing very theoretical at all. FWHM is a measure that has units of degrees, preferably in arc-seconds or similar, but most of the time it gets reported in pixels since that is what

I see that there has been quite a few responses to my original question about the typical FWHM values that I should be getting in my system since I last wrote anything here. I have been away sinc

It's very simple, embarrasingly so, and can't be used on images in which dark, obsucuring, dust structures pull the background down below that of the unobscured background sky. All I do is stretch the

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34 minutes ago, AKB said:

Mike

Although I don't consider myself qualified to comment critically on others' images, so you may discount my thoughts, I'd say that it has a bit of a green tinge and the background level is slightly high.  To remediate this, I've run it through an SCNR process and also applied a little tweak of the low-end curve (following the practice of @ollypenrice) which is a simple but effective noise-reduction technique.

HTH,  Tony

 

Hi Tony ...

Thanks for your thoughts .... I am glad to hear that the "flatness" of my image is apparently good.

Can I ask a couple of questions about what you have said?

(1) I did think that the colour of my image was not quite right (your modified version look much better) ... but I was amazed to get even this far!!! Can you please tell what the "SCNR" process is that you refer to?

(2) How can I find out about " Olly Penrice's " technique that you refer to?

(3) I have no knowledge or experience of CCD Inspector (apart from what I have just read briefly on their web site).... I am using a refractor so is it any use to me ?

Mike

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4 minutes ago, SlimPaling said:

(1) I did think that the colour of my image was not quite right (your modified version look much better) ... but I was amazed to get even this far!!! Can you please tell what the "SCNR" process is that you refer to?

You can read about SCNR here: https://www.pixinsight.com/doc/legacy/LE/21_noise_reduction/scnr/scnr.html

...although I don't actually use PixInsight, it's very easy to implement in other packages.  It just limits the maximum value of green as a function of the red and blue intensities.

 

4 minutes ago, SlimPaling said:

(2) How can I find out about " Olly Penrice's " technique that you refer to?

I'm searching his 28,000+ posts to find the right reference, but in the meantime, perhaps he will drop by and let you know!

 

21 minutes ago, SlimPaling said:

(3) I have no knowledge or experience of CCD Inspector (apart from what I have just read briefly on their web site).... I am using a refractor so is it any use to me ?

It depends.  I play around with 'fast' systems (like and F4 Newtonian, or an F2.3 Hyperstar) and things like spacing and tilt are critical.  For this, CCD Inspector is a good tool.  I'd say you don't need it at the moment, but I've heard others say that they don't know how anyone can image without having used it.  Each to their own!

 

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2 hours ago, AKB said:

 

I'm searching his 28,000+ posts to find the right reference, but in the meantime, perhaps he will drop by and let you know!

It's very simple, embarrasingly so, and can't be used on images in which dark, obsucuring, dust structures pull the background down below that of the unobscured background sky. All I do is stretch the image till the brightest background pixels are, say, at 23 in Photoshop Curves. I put a fixing point on the curve at 23 and put a few similar fixing points above that to pin the straight 'curve.' I then lift the lowest part of the curve, below 23, to compress all the values below 23 towards 23...

The advantage of this as a noise reducer is that it does not work by getting adjacent pixels to exchange values, so it does not introduce the oily blur look. The background sky pixels remain entirely independent of one another, so the noise in the image is preserved as a relationship but reduced in its values.

Olly

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Returning to the original question, I have some interesting figures!

My guidescope uses a 50mm by 183mm fl coated achro doublet from Astromedia that cost about £5. I'm using it with an ASI120MC, 1 second exposures.

While doing the Deer Lick/Stefan's quintet it was reporting the guide star as FWHM of 3.5-4.5.

Owing to tripod/trees I have just slewed to M33, and it is reporting FWHMs of 2.5 to 3 and a bit, occasionally less than 2, but rarely so perhaps unreliable.

So it looks like I have bingo seeing tonight and underlines that in practical terms seeing is more important than aperture - note that tonight the upper air is dry and windy, ideal conditions as the wind mixes up all the air cells.

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On 2017-10-06 at 21:26, ollypenrice said:

I rather hope that someone comes up with a proper theoretical explanation of the FWHM values because I don't really understand them. From experience, the coarser the pixel scale of the system the lower the FWHM. At 3.5"PP in our Tak 106/Atik 11000 it is not unusual to get down to 0.85. The seeing-related range is also low, with a likely 'worst' value of about 1.2. However, at higher resolution (0.9"PP in the TEC 140/Atik 460) the FWHM is both higher and more variable with the seeing. On a glorious night we might see 1.2 but it can be far worse, as bad as 2.5 or more. In this case I would only shoot colour and I'd wait for better seeing for luminance.

The main issue is nothing very theoretical at all. FWHM is a measure that has units of degrees, preferably in arc-seconds or similar, but most of the time it gets reported in pixels since that is what the software defaults to unless it has the proper data (pixel size and focal length used). If it doesn't offer the option to input this I would consider it to be faulty software. Since the FWHM value in pixels is proportional to the actual seeing and in the same range as proper arc-second values this looks resonable at first glance but is worthless for comparisons.  When measurements using arc-seconds are compared everything tends to make more sense.

Edited by glappkaeft
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On 15/10/2017 at 00:32, Stub Mandrel said:

Returning to the original question, I have some interesting figures!

My guidescope uses a 50mm by 183mm fl coated achro doublet from Astromedia that cost about £5. I'm using it with an ASI120MC, 1 second exposures.

While doing the Deer Lick/Stefan's quintet it was reporting the guide star as FWHM of 3.5-4.5.

Owing to tripod/trees I have just slewed to M33, and it is reporting FWHMs of 2.5 to 3 and a bit, occasionally less than 2, but rarely so perhaps unreliable.

So it looks like I have bingo seeing tonight and underlines that in practical terms seeing is more important than aperture - note that tonight the upper air is dry and windy, ideal conditions as the wind mixes up all the air cells.

Just realsied these are pixel values, so actual figures are about twice, so a bit more than 5" as the best FWHM.

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59 minutes ago, Stub Mandrel said:

Just realsied these are pixel values, so actual figures are about twice, so a bit more than 5" as the best FWHM.

That don't really mean much since your guide camera is limited by the pixel size and focal length to a pixel scale just above 4". Using the Nyquist theorem (which isn't 100% accurate in the case of digital sensors - pixels have an measurable area) any measurement below 8" would be questionable due to sampling errors. Fortunately sub-pixel guiding comes to the rescue when it comes to actual guide errors.

The best I seen on a night of truly excellent atmospheric seeing in southern Sweden (should be reasonably similar to the UK) is 1.1-1.2" for 20 minute exposures using a well collimated 41cm F/9 RC with very good dome/tube seeing, a very stable mount, working auto focus, excellent tracking/guiding and a camera (Atik 11000) that gives ~0.5" pixel scale (at Uppsala Amateur Astronomers dark site - Sandvreten Observatory). The "average" seeing with the same system was around 2.5", anything much below 2.0" was pretty rare and numbers above 3" was pretty common.

Edited by glappkaeft
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 The values jumped around all over the place and told us nothing whatever. We had to use a Bahtinov mask on that setup.

This happens to me all the time.  The only time I have managed to use FWHM was when I was at your place Olly.  

Carole 

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