"Mushiness" of SCT Views

[beka]

1 hour ago, vlaiv said:

There is no hard limit to what we can detect - no red line which represents "above" and "below".

https://en.wikipedia.org/wiki/Just-noticeable_difference

With visual astronomy, you will often read reports of target appearing and fading out of view. It will become stably visible after certain amount of time - as we build up mental image of it, sort of train our brain to see it. This is combination of two effect - JND and the fact that we can "burn" certain image in our brain.

Further more, besides JND - there is issue of sampling, particle nature of light and the way our brain interprets things.

We don't see continuous signal although it might seem so to us. We see in pixels - or rather special neurons that are evolved to sense light. These are finite in size and are arranged in irregular grid.

Particle nature of light means that light is not continuous signal, but rather arrives in photons and thus have associated Poisson noise.

We can detect very faint signals - just 7-9 photons strong. At that photon rate - noise will be very big, however we never seem to see such noise. This is because of how our brain works in order for us to see something faint. Several criteria must be fulfilled for sensation to happen.

Few adjacent photo receptors must be triggered, and then brain decides on some threshold to produce actual signal - but denoised.

Then there is matter of magnification. You need to increase magnification in order to start resolving Airy disk of source star. To a certain point - star is just point source, and two point sources look the same to our eye - or rather have same shape on our retina that depends on "optics" of our eye. Eye lens distorts point of light just enough so it covers few receptor cells in order to be detectable - and we see it as point because our brain filters things.

When we start increasing magnification - we reach place where airy disk is no longer point source and is resolved. This spreads the light over more surface and reduces photon count. If star is already at threshold visibility - further increase of magnification will push it below that threshold (here threshold again is not clean line but about +/- 7% of intensity and depends on how long you stare at the thing).

All of this shows that you can't explain things with such a rather simple model as constant cutoff point.

 

Hi Viaiv, I understand that our vision is extremely complex. But surely we can speak of broad thresholds, like the 6th magnitude naked eye limit or that specified for their instruments by the likes of Celestron. In my plot the red line could be like the limiting magnitude for the Green scope.

But let me ask a related question. If we had two APO scopes (perfect optics) differing only in aperture and ignoring other factors like seeing, sky brightness etc. Would we see the airy disk of the same star to be the same size in each scope - or will it be "bloated" in the scope of larger aperture? I think the latter 🙂.

Best

[vlaiv]

1 minute ago, beka said:

Would we see the airy disk of the same star to be the same size in each scope - or will it be "bloated" in the scope of larger aperture? I think the latter 🙂.

That depends on magnification used in each scope.

If you match magnification to aperture - then it will look the same.

[Mr Spock]

The airy disc size is determined by the formula ϑ = 1.22 λ/d where  ϑ is in radians, λ is the wavelength of the light in meters, and d is the diameter of the aperture in meters. 

From that you will see the airy disc in a 200mm scope is half the size of that in a 100mm scope (for the same magnification).

Scopes with central obstructions will put more light into the first (and subsequent) diffraction ring(s) than a scope with no obstructions. The same applies to the scope's wavefront with a ⅛λ scope putting more light into the airy disc than a ¼λ scope.

[michael.h.f.wilkinson]

11 hours ago, WJC said:

Having been in military and civilian optics for over 50 years, and owned telescope of all sizes and configurations, you will find no dispute from me. But I was amazed that is all the words so far, no one even mentioned the size of the secondary obstruction.

I mentioned the percentage of CO both for my planetary Newtonian (23%) and my SCT (34%) in the discussion so far. I have experience in military optics, but studied astronomy at the Kapteyn Astronomical Institute in Groningen, specialising in optical astronomy. 

[vlaiv]

11 hours ago, WJC said:

But I was amazed that is all the words so far, no one even mentioned the size of the secondary obstruction.

 

On 09/09/2022 at 16:13, vlaiv said:

SCTs have spherical primary mirror and have rather large corrector plate in the front. If that corrector plate is not figured properly - there will be residual spherical aberration. Add to that substantial central obstruction and the fact that it is F/10 scope with long focal length that makes high powers very accessible - and mushiness best shows at high powers.

 

[beka]

2 hours ago, Mr Spock said:

The airy disc size is determined by the formula ϑ = 1.22 λ/d where  ϑ is in radians, λ is the wavelength of the light in meters, and d is the diameter of the aperture in meters. 

From that you will see the airy disc in a 200mm scope is half the size of that in a 100mm scope (for the same magnification).

Scopes with central obstructions will put more light into the first (and subsequent) diffraction ring(s) than a scope with no obstructions. The same applies to the scope's wavefront with a ⅛λ scope putting more light into the airy disc than a ¼λ scope.

Well, the FWHM will be smaller for a larger aperture regardless of magnification . But for the theoretical PSF intensity profile,  there seem to be intensities (like at the level of the red line) where the airy disk is narrower for the smaller aperture. What i would like to know is how that translates to what we see at the eyepiece.

Cheers.