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Pixel scale


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So I have gathered that the ideal pixel scale is 1-2 arcsec per pixel (depending on seeing). But I have seen very nice images significantly more oversampled than this rule.

For example, this image from astrobin https://www.astrobin.com/full/p2adjl/0/ is at 0.658 arcsec/px and looks good to me.  

My setup is at 0.958 and I was a little concerned I was oversampling, but maybe I don't need to worry.

View on the 1-2 arcsec/px rule? When is it ok to break it?

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It is a tradeoff.

If you oversample image - there is nothing going to happen in terms of it looking bad or anything. When you view it in "fit to screen" - it will look as properly sampled image because software will scale it down so it fits inside your window.

Problem with oversampled image comes when you look at it at 100% - as far as visual part goes. Look at the image you linked to at 100% zoom:

image.png.282fdda269f052c36d3b48e7574102f0.png

This is just a crop of it. Stars look large - but that is not the main issue - in above crop that large "blob" star near center - is actually two stars that are not resolved.

Look at that image sampled at 2"/px (which is much closer to actual resolution), when viewed at 100%:

image.png.76e1de25c9731477b602935c0b52d00d.png

Ok, now sampling rate matches level of detail.

If you want to see what image of star field looks like when properly sampled at 0.658"/px - then

image.thumb.png.4157d3409690cc7e96e5bd45ce2a7945.png

I scaled right image to match resolution and that is closer to actual pixel scale. Stars are simply tighter and detail is there that matches resolution. Yes, left image does not go as deep - but it simply does not have detail of right image.

(it has also been deconvolved in order to try to recover some resolution - which caused SNR loss).

In the end - when you oversample you loose SNR in comparison to properly sampling. That is the main problem. In order to get noise free image that goes deep - you need to spend much more time imaging than if you properly sampled.

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2 hours ago, vlaiv said:

It is a tradeoff.

If you oversample image - there is nothing going to happen in terms of it looking bad or anything. When you view it in "fit to screen" - it will look as properly sampled image because software will scale it down so it fits inside your window.

Problem with oversampled image comes when you look at it at 100% - as far as visual part goes. Look at the image you linked to at 100% zoom:

 

This is just a crop of it. Stars look large - but that is not the main issue - in above crop that large "blob" star near center - is actually two stars that are not resolved.

Look at that image sampled at 2"/px (which is much closer to actual resolution), when viewed at 100%:

 

Ok, now sampling rate matches level of detail.

If you want to see what image of star field looks like when properly sampled at 0.658"/px - then

 

I scaled right image to match resolution and that is closer to actual pixel scale. Stars are simply tighter and detail is there that matches resolution. Yes, left image does not go as deep - but it simply does not have detail of right image.

(it has also been deconvolved in order to try to recover some resolution - which caused SNR loss).

In the end - when you oversample you loose SNR in comparison to properly sampling. That is the main problem. In order to get noise free image that goes deep - you need to spend much more time imaging than if you properly sampled.

Thanks vlaiv. I can always rely on you for a detailed answer.

So would you say my current setup is ok (0.958)? Which side of the 1-2 is it best to stray on (oversampled or undersampled)? Because we only have the cameras and scopes we have, so we have limited ability to change pixel scale unless we buy a new expensive piece of equipment.

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15 minutes ago, StuartT said:

So would you say my current setup is ok (0.958)? Which side of the 1-2 is it best to stray on (oversampled or undersampled)?

That really depends on scope in question and sky conditions.

In general - I don't think that 1"/px is feasible resolution for most people. 8"+ of aperture, premium mount and those few nights a year when seeing is great - then yes, 1"/px could be pulled off.

I'd say that for most people - it would be somewhere in the middle - 1.5-1.8"/px. If you use small scope - like 4" or less - just go for 2"/px without worrying too much about it.

Resolution of image to some degree depends on scope aperture as well (it sort of goes into the mix).

If you want to know what sort of resolution your setup makes - just take some of your images - stacked data in linear stage before processing and look at FWHM of stars in that image. Divide value with 1.6 - that is resolution you should be aiming for.

If you get that your FWHM is closer to 1.6" - then go for 1"/px, but if it is 3" or more - then 2"/px makes more sense.

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49 minutes ago, StuartT said:

Thanks vlaiv. I can always rely on you for a detailed answer.

So would you say my current setup is ok (0.958)? Which side of the 1-2 is it best to stray on (oversampled or undersampled)? Because we only have the cameras and scopes we have, so we have limited ability to change pixel scale unless we buy a new expensive piece of equipment.

If you're oversampled (like likely you are), then all you need to do is bin the image - either during capture or post processing.

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11 hours ago, vlaiv said:

If you want to know what sort of resolution your setup makes - just take some of your images - stacked data in linear stage before processing and look at FWHM of stars in that image. Divide value with 1.6 - that is resolution you should be aiming for.

How do I measure FWHM? I assume you mean the FITS image that comes out of Astro Pixel Processor?

10 hours ago, The Lazy Astronomer said:

If you're oversampled (like likely you are), then all you need to do is bin the image - either during capture or post processing.

ok, this is a simple solution! So I can set binning in NINA to 2x2 I think. Does this mean I can increase exposure time and still have round stars?

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9 minutes ago, StuartT said:

How do I measure FWHM? I assume you mean the FITS image that comes out of Astro Pixel Processor?

Yes, FITS from APP is linear data.

Not sure what you use for processing, but I've heard that PixInsight has FWHM measurement. If you don't have any software that will do it - look up AstroImageJ. It has star measurement tool and it will provide you with that info. Just be careful if you are measuring in pixels or in arc seconds (convert between them using your "/px ratio of original file).

image.png.d80b020bdeea52f2625a959fc1fffdf9.png

Shift click will measure star that you click on and will show you FWHM in measurements table.

15 minutes ago, StuartT said:

ok, this is a simple solution! So I can set binning in NINA to 2x2 I think. Does this mean I can increase exposure time and still have round stars?

Round stars are best achieved by mechanically tuning your setup. If your guiding is good - they should be round and tight regardless of exposure length.

In principle, working on smaller resolution will create stars that appear tighter / rounder - due to different pixel scale, so yes, you should be able to expose for longer when binning - but again, that is not solving star shape issue - that is only masking it. Try to solve it properly - do it does not matter if you expose for 2 minutes or 5 minutes or 10 minutes - you should always get round stars (and as tight as possible).

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1 hour ago, StuartT said:

How do I measure FWHM? I assume you mean the FITS image that comes out of Astro Pixel Processor?

ok, this is a simple solution! So I can set binning in NINA to 2x2 I think. Does this mean I can increase exposure time and still have round stars?

DSS can give you a FWHM figure if you load your stacked image into it and analyse (not necessarily sure l particularly trust it to be accurate though - I've seen reports of it always giving a higher figure than PI).

As vlaiv said, good guiding is key to good star shapes. Ideally you'd be looking at wanting to get the RMS as reported by phd2 to be about half (or less!) of your image scale.

I think we have the same mount (EQ6-R?); l typically see guiding RMS of 0.6 - 0.8", which works well for my imaging scale of 1.7"/px. I've gone up to 5 minute subs (with narrowband filters) with no issue (theoretically no need for me to expose for any longer than that).

Your scope's a bit bigger and beefier than mine, but if you see similar RMS, then binning 2x2 should "fix" any star shape issues you experience on longer exposures. 

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7 hours ago, The Lazy Astronomer said:

DSS can give you a FWHM figure if you load your stacked image into it and analyse (not necessarily sure l particularly trust it to be accurate though - I've seen reports of it always giving a higher figure than PI).

As vlaiv said, good guiding is key to good star shapes. Ideally you'd be looking at wanting to get the RMS as reported by phd2 to be about half (or less!) of your image scale.

I think we have the same mount (EQ6-R?); l typically see guiding RMS of 0.6 - 0.8", which works well for my imaging scale of 1.7"/px. I've gone up to 5 minute subs (with narrowband filters) with no issue (theoretically no need for me to expose for any longer than that).

Your scope's a bit bigger and beefier than mine, but if you see similar RMS, then binning 2x2 should "fix" any star shape issues you experience on longer exposures. 

Ok, I should probably point out that

a) I am using a 150mm apo with reducer (focal length 808mm)

b) I am not guiding.

But given that my focal length and camera are both fixed, there is not much I can do about pixel scale anyway, right?

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13 minutes ago, StuartT said:

b) I am not guiding.

I'd put this high on your priority list - get guiding running on your imaging rig.

14 minutes ago, StuartT said:

But given that my focal length and camera are both fixed, there is not much I can do about pixel scale anyway, right?

Pixel binning has the same effect as changing pixel size. You take group of 2x2 or 3x3 (or higher number) of pixels and treat them as single pixel by simply adding the light in each - same would happen if we had larger pixel to begin with - it would capture all light falling on it.

If you change "pixel size" - you effectively change pixel scale although your focal length remains the same.

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I very much like imaging at a pixel scale similar to yours, in my case about 0.9"PP. I find I can present images at full size and be happy with how they look. Ironically, perhaps, I find I use this high res setup to do single panel images which I often then crop to post areas of interest at full size. I also run a widefield rig at much lower resolution (3.5"PP) and I usually make mosaics with that one. So my small field images get smaller and my widefields get wider...  It just seems to work out that way.  Be aware, though, that aiming to post at full size means taking plenty of exposures and you need a guide RMS no more than half your image scale.

0.9"PP gives me results like this:

https://www.astrobin.com/full/miqpyu/0/?mod=&real=

https://www.astrobin.com/full/335042/0/

https://www.astrobin.com/full/393219/0/

https://www.astrobin.com/full/419975/0/

I've imaged at higher resolution but not found that it gave better results so I stick at 0.9 by personal preference.

Olly

 

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I'll use one of Olly's images to demonstrate that what looks perfectly fine at 0.9"/px - in fact has less detail than that and can be happily sampled at say ~1.5"/px instead.

Here is crop of original image:

original.png.4305351e24604e60b6856a39d8059e39.png

I took that crop and resampled it to 2/3 of original size - which is 1.35"/px if original was at 0.9"/px. Here is what that looks like:

reduced.png.6f395963fe898296bf0b99cf19ba802d.png

Now, if original version has some details that can't be recorded at 1.35"/px - then if I enlarge this smaller version - we should be able to tell that it is different than original crop above. So I did exactly that, here is small version enlarged back to 0.9"/px:

restored.png.550201878f02e175352e0f123cf925d3.png

Can we spot any differences between these two (and no, it is not the same image - I really did reduce the size and enlarged back again :D )?

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56 minutes ago, vlaiv said:

I'd put this high on your priority list - get guiding running on your imaging rig.

I avoid guiding by keeping my subs short (35s). This gives me round stars with my setup. I read somewhere that guiding used to be more crucial with CCD cameras because you needed a longer exposure to swamp the read noise. But with newer, low read noise cameras like mine (ASI 2600MC) this is much less significant, so there is little advantage in doing smaller numbers of long exposures compared to larger numbers of short exposures. Plus, it saves all the hassle of setting up guiding! 

55 minutes ago, vlaiv said:

Pixel binning has the same effect as changing pixel size. You take group of 2x2 or 3x3 (or higher number) of pixels and treat them as single pixel by simply adding the light in each - same would happen if we had larger pixel to begin with - it would capture all light falling on it.

If you change "pixel size" - you effectively change pixel scale although your focal length remains the same.

So I'm confused. Earlier in this thread, you seemed to be saying that binning was not a good strategy? Or maybe I am misunderstanding you.

 

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26 minutes ago, vlaiv said:

we should be able to tell that it is different than original crop above. So I did exactly that, here is small version enlarged back

Of course, visual inspection is not a particularly good test.  (In the weekend papers they have a children's puzzle section with "Spot the difference": 6 differences, ... I'm lucky to see four of them!)

So I took both those images, subtracted them (adding a small offset) and multipled the result by 30...

Image13.png.ce05a55d368f5dfb53909368c2140bc1.png

...OK, it is correlated with the image, but I think we'd have to agree that the difference is really "in the noise".

Tony

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2 minutes ago, StuartT said:

I avoid guiding by keeping my subs short (35s). This gives me round stars with my setup. I read somewhere that guiding used to be more crucial with CCD cameras because you needed a longer exposure to swamp the read noise. But with newer, low read noise cameras like mine (ASI 2600MC) this is much less significant, so there is little advantage in doing smaller numbers of long exposures compared to larger numbers of short exposures. Plus, it saves all the hassle of setting up guiding!

Did you actually measure level of read noise compared to other noise sources?

Most dominant noise source is light pollution noise. That however varies with location, but also with sampling rate.

You might be located in heavy LP - but similarly as target signal - LP also is "a signal" - unwanted one that brings its own noise into mix, but it still acts as signal. If you over sample when capturing you are in effect reducing LP signal in the same way you reduce target signal thus not gaining anything like lowering effective light pollution - but you do reduce level of it and hence noise.

This means that LP noise becomes smaller - and you need to spend more time on single frame to swamp read noise.

For example - if you image at 1"/px versus 2"/px - you need four time as long exposure to equally swamp read noise with LP noise. Binning does not help here - as it happens after you read out the pixels and read noise is already added to the mix. It helps with SNR as is after read noise has been added.

7 minutes ago, StuartT said:

So I'm confused. Earlier in this thread, you seemed to be saying that binning was not a good strategy? Or maybe I am misunderstanding you.

Now I'm confused :D - I was under impression that I always advocated for proper sampling rate and in cases where this means binning - to go with binning, unless you are able to actually exploit resolution you are working at.

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3 minutes ago, AKB said:

So I took both those images, subtracted them (adding a small offset) and multipled the result by 30...

This is actually the proper way to asses differences - but I cheated a bit here. I already knew that there won't be perceivable difference as such test needs to be performed on linear scale - and often differences are order of 1/1000 of pixel values. In above image visible differences are more down to 8bit samples being compared than anything else. Data is also stretched which changes some of frequency response - in linear regime difference is even smaller.

In fact - when I asses what is proper sampling rate - I go by SNR - what is level of detail that can be sharpened in image without making noise too visible.

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17 hours ago, vlaiv said:

I'll use one of Olly's images to demonstrate that what looks perfectly fine at 0.9"/px - in fact has less detail than that and can be happily sampled at say ~1.5"/px instead.

Here is crop of original image:

original.png.4305351e24604e60b6856a39d8059e39.png

I took that crop and resampled it to 2/3 of original size - which is 1.35"/px if original was at 0.9"/px. Here is what that looks like:

reduced.png.6f395963fe898296bf0b99cf19ba802d.png

Now, if original version has some details that can't be recorded at 1.35"/px - then if I enlarge this smaller version - we should be able to tell that it is different than original crop above. So I did exactly that, here is small version enlarged back to 0.9"/px:

restored.png.550201878f02e175352e0f123cf925d3.png

Can we spot any differences between these two (and no, it is not the same image - I really did reduce the size and enlarged back again :D )?

Yes, that's a good test! I need bigger pixels. :D

Olly

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Ok, vlaiv. I clearly misunderstood you. I was reading this remark as negative about binning: 

Quote

you should be able to expose for longer when binning - but again, that is not solving star shape issue - that is only masking it. Try to solve it properly 

But I understand now what you are saying (I think). Thanks for taking the time to explain.

 

21 hours ago, vlaiv said:

Did you actually measure level of read noise compared to other noise sources?

I did not. This is bit beyond my knowledge at the moment. I am still very new to all this. I think I was taking this from various other sources about CMOS cameras.

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6 hours ago, StuartT said:

I did not. This is bit beyond my knowledge at the moment. I am still very new to all this. I think I was taking this from various other sources about CMOS cameras.

In principle that is right - CMOS sensors have lower read noise and hence require less exposure length, but one should really measure how much read noise there is compared to other noise sources.

I know that author of SharpCap did YouTube video on how and why regarding all of that - and I believe it is a good material to watch in order to understand.

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