Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

ZWO Camera comparison


Recommended Posts

1 hour ago, ollypenrice said:

The reducer took the system beyond 3.5"PP and the stars were certainly blocky.

 

5 minutes ago, david_taurus83 said:

Tbf vlaiv, there's a few square stars in there if you peep!

I still don't get it.

How can you tell a star is blocky if it for example covers only one pixel?

@david_taurus83

Which ones?

Above image was resampled RC image that has diffraction spikes on some stars - which could give impression of blockiness but that sort of thing won't happen on refractor

image.png.cc025d95073d8c92ffcd17ae2d1c5b2a.png

After all, we might be talking about different thing?

When people say they see blocky stars - I imagine this happens:

image.png.478b274cb6d511715e58c4a1c8cf9a52.png

but that happens when someone tries to zoom in past 100% and uses nearest neighbor interpolation algorithm. This is by the way that 5"/px image zoomed in to say 300% or so.

Look what happens when I choose different interpolation algorithm:

image.png.62ec96c64f94c9dcfc7c6fd244306275.png

Stars don't look blocky anymore (they do have small ringing as interpolation artifact here)

That is the sort blockiness that I think people are talking about, but maybe it is something entirely different?

Link to comment
Share on other sites

31 minutes ago, david_taurus83 said:

A few of those look square to me. Most image viewers don't give the option to change interpolation algorithms. What you see is what you get!

Screenshot_20210727-220653_Chrome.jpg

I'll have to see if I can find the original data but it was taken a long time ago. However, David's post above gives a very good impression of the 'look' I disliked in my image and why I gave the stars a slight blur.

Olly

Link to comment
Share on other sites

1 hour ago, david_taurus83 said:

A few of those look square to me. Most image viewers don't give the option to change interpolation algorithms. What you see is what you get!

I was right then to think it has to do with zooming in?

In that case - stars don't look blocky because they are under sampled - they look blocky because of interpolation method used when zooming beyond 100%.

Link to comment
Share on other sites

Just now, david_taurus83 said:

How are you changing the method when you zoom in?

Depends on software you are zooming in with. If your software does not have this option - you can always change software.

I also avoid zooming in past 100% because I know that there won't be any detail to be seen.

In any case, my point was - under sampling does not cause blocky stars - software interpolation does. If you have two pieces of software and one shows blocky stars and other does not - will you conclude that data contain blocky stars or that one software makes them look blocky while other one does not?

Link to comment
Share on other sites

On 26/07/2021 at 19:38, Davey-T said:

You need something with decent sized pixels to achieve a reasonable image scale, images can be binned although it works better with mono cameras.

I use an Atik 314L that has 6.4um pixels and bin those with 10"SCT.

At the top of this page under resources there is a calculator where you can enter your scope and camera to see what image scale it will give, to see if it's under / over sampled.

Dave

There is also a a calculator for “Seeing”, i.e. optimum camera/telescope combination for your skies

https://astronomy.tools/calculators/ccd_suitability

Link to comment
Share on other sites

2 hours ago, iapa said:

There is also a a calculator for “Seeing”, i.e. optimum camera/telescope combination for your skies

https://astronomy.tools/calculators/ccd_suitability

Except that "theory" behind that is flawed in many ways.

Quote

In the 1920s Harold Nyquist developed a theorem for digital sampling of analog signals. Nyquist’s formula suggests the sampling rate should be double the frequency of the analog signal. So, if OK seeing is between 2-4” FWHM then the sampling rate, according to Nyquist, should be 1-2”.

Wrong. Nquist sampling theorem clearly states that if you want perfect representation of band limited signal you need to sample at twice highest frequency component. There is no rationale to equate FWHM with highest frequency component of the signal. In fact, if you approximate star profile with Gaussian distribution - you don't have band limited signal at all.

Explanation goes on about pixel being square and what not - all completely irrelevant to this discussion and ends with flawed conclusion that:

Quote

Our calculator, at typical seeing of 2-4”, uses the Nyquist formula of 1/2 and the 1/3 to stop stars becoming square so the optimal range is between 0.67” and 2”. (0.67 = 2 / 3, 2 = 4 / 2).

 

Link to comment
Share on other sites

Hi,

I'm just thinking that it might not be worth spending loads of money on a dedicated cooled camera straight the way if you're only going to occasionally dabble with deep sky imaging?

A second hand full frame DLSR or Mirrorless camera would match well in terms of sensor size and pixel scale, e.g.  The Sony A7s and Canon 5D mk1 both have massive pixels and sensors for not much money :)

If you find yourself doing more and more DSO imaging jump up to a dedicated cooled camera at that point. 

Admittedly the above will be better for galaxies, globular clusters and small bright planetary nebulae, rather than extended faint nebulae, but then again galaxies, globs etc are what the C9.25 will be best at when it comes to DSO's :)

 

 

Edited by Chris
Link to comment
Share on other sites

On 30/07/2021 at 00:21, vlaiv said:

Except that "theory" behind that is flawed in many ways.

Wrong. Nquist sampling theorem clearly states that if you want perfect representation of band limited signal you need to sample at twice highest frequency component. There is no rationale to equate FWHM with highest frequency component of the signal. In fact, if you approximate star profile with Gaussian distribution - you don't have band limited signal at all.

Explanation goes on about pixel being square and what not - all completely irrelevant to this discussion and ends with flawed conclusion that:

 

I wish I had time to delve into the maths of all the apps that are available, rather that taking, at face value, the rationale given for may common applications.

At 60+, it must be time to brush of the dust and cobwebs of the last few decades.

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.