MTF of a telescope

[vlaiv]

57 minutes ago, Deadlake said:

Would this help, on MTF tab and called modulation?

That is actually derived data.

These three contain actual data needed to generate MTF graph (any of the three - they are just different representations)

This is also important point:

That data is produced for/with 546nm wavelength.

Reflectors produce same wavefront regardless of wavelength used, but refractors do not.

If we want to perfectly simulate refractor - we need wavefront per each wavelength (close ones are very similar - but say each 30nm is good sampling - about 10 different wavelengths used for 400-700nm range).

[Deadlake]

10 minutes ago, vlaiv said:

That is actually derived data.

These three contain actual data needed to generate MTF graph (any of the three - they are just different representations)

This is also important point:

That data is produced for/with 546nm wavelength.

Reflectors produce same wavefront regardless of wavelength used, but refractors do not.

If we want to perfectly simulate refractor - we need wavefront per each wavelength (close ones are very similar - but say each 30nm is good sampling - about 10 different wavelengths used for 400-700nm range).

I don't have those.

[mikeDnight]

All I need is an Ant' 1 night with good transparency to tell which is the better scope. Not a graph or Strehl claim in sight!

[paulastro]

18 hours ago, John said:

MTF ?

Modulation Transfer Function

I think that is all that I can contribute to this

 

 

Well, you contributed more than I could have done John.  I thought MTF was a type of hardboard 🤔.

[vlaiv]

2 minutes ago, paulastro said:

Well, you contributed more than I could have done John.  I thought MTF was a type of hardboard 🤔.

Isn't that MDF?

 

[paulastro]

Just now, vlaiv said:

Isn't that MDF?

 

Well spotted vlaiv 🤣.

[Carl Au]

So to clarify, a bigger scope is better than a small one regardless of type? 

[vlaiv]

Just now, Carl Au said:

So to clarify, a bigger scope is better than a small one regardless of type? 

Not really.

If they are close in aperture size - like 4" and 6"  - then optical quality and central obstruction plays a part. But once aperture is large enough and both scopes are diffraction limited - aperture wins in cases where there is no atmospheric influence of it is minimized.

Atmosphere often reduces aperture to equivalent of 2-3" anyway in poor seeing and in such cases effect it has on smaller aperture is not as detrimental as on larger aperture. It is often said that smaller aperture "cuts thru seeing" better.

[jetstream]

35 minutes ago, mikeDnight said:

All I need is an Ant' 1 night with good transparency to tell which is the better scope. Not a graph or Strehl claim in sight!

Agreed!

and I invite you 3700 miles to compare your 100mm to my 15" under those conditions!😃

[Deadlake]

19 minutes ago, jetstream said:

Agreed!

and I invite you 3700 miles to compare your 100mm to my 15" under those conditions!😃

@mikeDnight isn't there only one answer from someone who started the thread "Is the FC100DZ the best DOUBLET ever made?" 😀

Edited February 3, 2021 by Deadlake

[vlaiv]

Here are results of 4" and 5" ideal clear aperture telescope vs 12" 1/5.6 PV with 20% CO and diffraction limited 12" 1/4 PV with 20% (I threw in diffraction limited scope in 12 for comparison)

MTF diagram - black is 4", blue is 5", red is 12" 1/4PV and orange is 12" 1/5.6PV

Top row: 4", 5", bottom row: 12" 1/4PV, 12" 1/5.6PV

[mikeDnight]

33 minutes ago, jetstream said:

Agreed!

and I invite you 3700 miles to compare your 100mm to my 15" under those conditions!😃

I'm on my way  Gerry. Swimming was the only sport I was ever any good at. Plus it's a win win with a 15" and a TSA120  to play with. ☺

[jetstream]

Just now, mikeDnight said:

I'm on my way  Gerry. Swimming was the only sport I was ever any good at. Plus it's a win win with a 15" and a TSA120  to play with. ☺

Always welcome Mike!

Lots of scopes here to play with, I'm going out with a marble pretty quick lol!

[mikeDnight]

4 minutes ago, mikeDnight said:

 

 

Edited February 3, 2021 by mikeDnight

[jetstream]

9 minutes ago, vlaiv said:

Here are results of 4" and 5" ideal clear aperture telescope vs 12" 1/5.6 PV with 20% CO and diffraction limited 12" 1/4 PV with 20% (I threw in diffraction limited scope in 12 for comparison)

Excellent Vlaiv, in my limited experience around 20% CO doesn'r really hinder things too much and espc when used with good optics. Makes me wonder why I have a weakness for good refractors  lol!

Another interesting thing is to compare the fracs with real world test results, I think a lot go 1/8pv-1/10 and with high strehls. Is it possible to include the effect of miscollimation in those graphs?

Your work is truly appreciated Vlaiv.

[Joe G]

4 hours 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

I’m interested in this topic but it’s mostly beyond me. However, I would love to see a mini comparison review between your Tak & the Dob on some sample targets. It would be a great addition to any final conclusions. Someone once said that “ In theory, theory & practice is the same thing, but in practice it’s not”

[vlaiv]

14 minutes ago, jetstream said:

Is it possible to include the effect of miscollimation in those graphs?

In principle yes - as long as we have wavefront of miscollimated scope as is - we can create simulation and add MTF to comparison.

I think there is very small difference between 1/8 and 1/10 PV to be honest. Look at difference between 1/4PV and 1/5.6PV - I was expecting more, but it actual image - it's not that much at all.

It is hard to see side by side - best way to notice differences is to have images blinking and they are minimal.

 

[Captain Scarlet]

All utterly fascinating.  I think one of the unquantifiable factors influencing our in-the-field-perceptions of frac vs bigger reflector is that fracs are generally supplied having been assembled collimated and calibrated on an optical bench by professionals. Whereas the reflectors, especially Newtonians but even the Cassegrains, have been supplied as barely more than a bag of bits with a useless set of instructions essentially saying "you put it together".

M

Edited February 3, 2021 by Captain Magenta
Typo

[vlaiv]

I really want to try this comparison in real life.

Although image on computer screen can be quite telling - looking at the eyepiece is different experience. Closest approximation to seeing free view that I could think of is looking at a terrestrial object that has appropriate size of features and possibly contrast levels. Either printed image or maybe physical object like marble or even image on phone screen.

I don't own excellent optics, but I do want to see difference between 4" Mak and 8" Newtonian first hand. I do have experience with what 8" can deliver - but there is always that residual - "how much was it down to seeing and how much down to quality of optics?".

If I can get similar results in actual testing to these theoretical values - that will give me confidence that comparison between 0.8 Strehl and 0.98 Strehl scope is also relevant.

Taking images during tests is also interesting proposition - that way we can document actual test.

[Captain Scarlet]

In London I have a bog-standard SW 200p f/5 8" newt, and a superb Leica 60mm spotting scope. On one occasion I put them side by side on my Skytee2, and pointed at Polaris. Polaris B was trivially easy to see through the 8", a lovely obvious little blue pinpoint. I simply could not see it through the Leica, and I tried really hard!

M

[jetstream]

I just came in from a marble test and might start another thread unless its ok to post pictures etc here. Very very interesting... the little 90mm SV outperformed my expectations. Phone got cold and went dead, images after it revives.

[vlaiv]

Just now, jetstream said:

I just came in from a marble test and might start another thread unless its ok to post pictures etc here. Very very interesting... the little 90mm SV outperformed my expectations. Phone got cold and went dead, images after it revives.

I certainly don't mind seeing pictures here.

[andrew s]

We will have to club together to buy one of these Shack-Hartman wave front sensor to remove any subjective bias in real world tests! 🤩

Regards Andrew

[jetstream]

The marble and set up. I used a Vixen 2.4mm and the VIP barlow. Not sure if the fs is at the shoulder but if it is the mag is around 393x. Scope is a 630mm 90mm f7, APO triplet that star tests well. Distance is 54 yards on the Zeiss rangefinder. I was shocked at how well this scope did, I could see the 2 separations of colors- yellow and blue very well. The Nagler 3-6 was first tried but it was too easy.

The scope is always cooled and I test straight through.

 

[mikeDnight]

Well, after looking at the four images of Jupiter posted above, I have to say that if my 4" refractor gave an image like the supposed 4" refractor above, I'd give up on planetary observing. The views of Jupiter when its high in the sky are much more like the bottom two images, but with less vivid colours.

Edited February 4, 2021 by mikeDnight