MTF of a telescope

[jetstream]

2 minutes ago, Deadlake said:

I don't see any discussion of optical quality, hence the question.

Vlaiv did- he assumed perfect optical quality for the scopes used for the examples. Optical aberrations will cause sags in the MTF graph. It would be interesting and very involved to discuss less than perfect optics.

[jetstream]

10 minutes ago, JeremyS said:

Ok, so this applies to standard optics and the everyday world. But what about Takahashi?

 

 

🤣
 

Depends on your diagonal 😉😃

[vlaiv]

1 minute ago, JeremyS said:

Ok, so this applies to standard optics and the everyday world. But what about Takahashi?
 

Everything that has been said so far applies to Strehl 1 optics - so absolutely 100% perfect wavefront optics. We are comparing exceptional specimens of any particular design.

In reality - things are going to be only "worse".

2 minutes ago, Deadlake said:

@vlaiv @alex_stars

How would the quality of the scope effect the MTF functions?
How would poor seeing (say atmospheric turbulence) in combination with optics quality effect the MTF, presume poor optics would propagate errors introduced by atmospheric turbulence?
Thanks

 

 

Quality of the telescope just "lowers" MTF in some way. It will not impact cut off frequency - that is solely dictated by aperture size.

Everything else - like central obstruction or optical aberrations just makes MTF with lower values. Something like this:

Actual telescope performance can be obtained by examining either wavefront or PSF.

General relationship is:

PSF is power spectrum of FT of wavefront aberrations

MTF is frequency spectrum of FT of PSF

In order to have MTF we need either wavefront or PSF.

Seeing is exceptionally tough topic to tackle - since it is very random. Best that we can do is run some sort of simulations by having for example range of wavefront parameters - average aberration and standard deviation from that value. We also need how fast it is changing and then we need to integrate over period of time - usually 30ms or so - time that our brain / eye system perceives as single image.

Established way to handle wavefront aberrations is via Zernike polynomials. That is orthonormal basis set for deformation of unitary circle. This means that any sort of wavefront aberration can be decomposed into Zernike polynomials - some of which we know under different name like - lower / higher order spherical, defocus, piston, tilt, coma, astigmatism and so on ...

Seeing is approximated by mean value for each of these polynomials and standard deviation - so we can then use random process to "bend" wavefront that is otherwise produced by telescope and that will give us integral deformation of PSF which will in turn give us MTF

 

[jetstream]

8 minutes ago, vlaiv said:

Quality of the telescope just "lowers" MTF in some way. It will not impact cut off frequency - that is solely dictated by aperture size.

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis  (no coma)and perfect seeing.

[mikeDnight]

21 minutes ago, JeremyS said:

Ok, so this applies to standard optics and the everyday world. But what about Takahashi?

 

 

🤣
 

Oh you are a trouble maker Jeremy!  😈😂😂

[Deadlake]

13 minutes ago, jetstream said:

Depends on your diagonal 😉😃

I've been on this forums for 7 months and I'm sure @JeremyS has been threatening to buy a Baader diagonal for that time. 😄

[jetstream]

Just now, Deadlake said:

I've been on this forums for 7 months and I'm sure @JeremyS has been threatening to buy a Baader diagonal for that time. 😄

Hes gonna need it after he sees the next graph!

[jetstream]

Am I correct in reading this that the 4" refractor has more than 10% difference in the frequencies ranging from about 20 pixels to 100?

[Deadlake]

9 minutes ago, jetstream said:

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis  (no coma)and perfect seeing.

I suspect up to 30% obstruction will impact the answer. If possible interesting to see what a 11" SCT (like a C11) would do. 

[jetstream]

Just now, Deadlake said:

I suspect up to 30% obstruction will impact the answer. If possible interesting to see what a 11" SCT (like a C11) would do. 

Yes but now were comparing a near perfect refractor (Tak etc), optimised shall we say to a mass produced, highly obstructed SCT. Even so, if the optical quality of the SCT is the same as the frac, results might be surprising.

What I'm interested in as well is how the graph shape might impact image processing.

[vlaiv]

6 minutes ago, jetstream said:

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis  (no coma)and perfect seeing.

Just saying Strehl 0.9 - does not give me enough information to actually plot MTF.

Infinite number of different wavefronts will produce Strehl 0.9.

If we want to compare actual scope to other actual scope or perfect scope for that matter - we need either:

wavefront diagram or Zernike polynomials.

Say one went and did optics test of a telescope. Likely result will come in form of report that looks like this:

Above diagram is what is needed to produce PSF and hence MTF of actual telescope. This is in fact for primary mirror - but in reality one would do such report for whole system.

Alternatively, we could assume one particular type of aberration - say spherical and then find which term corresponds to Strehl of 0.9 (say 1/5.6th of a wave or something like that) - then I could generate such wavefront and based on that do comparison.

[andrew s]

14 minutes ago, jetstream said:

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis  (no coma)and perfect seeing.

Sounds like rigging the comparison to me. Regards Andrew 

[jetstream]

1 minute ago, andrew s said:

Sounds like rigging the comparison to me. Regards Andrew 

Maybe, but will it not show the effect that aperture has on things vs "perfect" smaller aperture APOs?

[vlaiv]

Ok, so it shell be 12" 20% obstructed newtonian with spherical term that is equivalent to Strehl of 0.9 versus perfect 4" or 5" APO? Which one shall we go for? Maybe both to see the difference?

I'll do both MTF and simulated views of Jupiter in perfect seeing later - got to take one of my dogs to a vet now.

[John]

17 minutes ago, jetstream said:

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis  (no coma)and perfect seeing.

If we get a clear night I can compare my Tak FC100-DL with my OO 12" F/5.3 dob which has a 21% obstruction and, according the paperwork, a primary of Strehl .987

What targets would be the most suitable for a fair comparison I wonder

[jetstream]

4 minutes ago, vlaiv said:

This is in fact for primary mirror -

Sure use this one, my 15" report is similar I think, but not sure where it is.

[jetstream]

1 minute ago, John said:

If we get a clear night I can compare my Tak FC100-DL with my OO 12" F/5.3 dob which has a 21% obstruction and, according the paperwork, a primary of Strehl .987

What targets would be the most suitable for a fair comparison I wonder

You know John 😃 and many of your reports indicate which ones show more?

[Deadlake]

10 minutes ago, vlaiv said:

Just saying Strehl 0.9 - does not give me enough information to actually plot MTF.

Infinite number of different wavefronts will produce Strehl 0.9.

If we want to compare actual scope to other actual scope or perfect scope for that matter - we need either:

wavefront diagram or Zernike polynomials.

Say one went and did optics test of a telescope. Likely result will come in form of report that looks like this:

Above diagram is what is needed to produce PSF and hence MTF of actual telescope. This is in fact for primary mirror - but in reality one would do such report for whole system.

Alternatively, we could assume one particular type of aberration - say spherical and then find which term corresponds to Strehl of 0.9 (say 1/5.6th of a wave or something like that) - then I could generate such wavefront and based on that do comparison.

I have something similar but for a 152mm/F8 scope so not what we want. Shame as very good figures...

Edited February 3, 2021 by Deadlake

[jetstream]

4 minutes ago, vlaiv said:

k, so it shell be 12" 20% obstructed newtonian with spherical term that is equivalent to Strehl of 0.9 versus perfect 4" or 5" APO? Which one shall we go for? Maybe both to see the difference?

I'll do both MTF and simulated views of Jupiter in perfect seeing later - got to take one of my dogs to a vet now.

Thank you Vlaiv!

Yes my GSD wants to go out now, back soon.

[vlaiv]

3 minutes ago, John said:

If we get a clear night I can compare my Tak FC100-DL with my OO 12" F/5.3 dob which has a 21% obstruction and, according the paperwork, a primary of Strehl .987

What targets would be the most suitable for a fair comparison I wonder

Since we are talking about pure telescope performance without influence of atmosphere - I would say either marbles or printed image of Jupiter stapled about 150 meters away .

In fact, given that you'll be using 12" scope - a target at least 100 meters away to minimize spherical aberration from close focusing.

 

[vlaiv]

1 minute ago, Deadlake said:

I have something similar but for a 152mm/F8 scope so not what we want. Shame as very good figures...

 

Yes, one of the problems that we face when doing these type of comparison is that refractors have different strehl based on wavelength.

That is something that is very hard to simulate as far as rendering a target / simulated view of the planet. We both need to take defocus into account but also need to have target in much more wavelengths than standard 3 - red green and blue.

Ideally we would do something like at least 10 different wavelengths and then combine results.

[Deadlake]

34 minutes ago, vlaiv said:

Yes, one of the problems that we face when doing these type of comparison is that refractors have different strehl based on wavelength.

That is something that is very hard to simulate as far as rendering a target / simulated view of the planet. We both need to take defocus into account but also need to have target in much more wavelengths than standard 3 - red green and blue.

Ideally we would do something like at least 10 different wavelengths and then combine results.

Would this help, on MTF tab and called modulation?



At the same time this scope is near perfect, need to find a lesser scope which is not in the premium class.....

 

Edited February 3, 2021 by Deadlake

[Carl Au]

15 hours ago, John said:

MTF ?

Modulation Transfer Function

I think that is all that I can contribute to this

 

 

You are not alone...

[alex_stars]

Now its getting interesting as we leave the perfect optics world and close in on reality.

Just a reminder, also consider how good you can focus you scope, and possible tube currents ..... and .... and .....a yes, Baader prism or not 😄

I'll get the 🍿 and enjoy.

Edited February 3, 2021 by alex_stars

[jetstream]

26 minutes ago, alex_stars said:

a yes, Baader prism or not

and how the length of the optical path in the prism affects the refractor 😃

I test my fracs "straight through" just for its and giggles