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Difference in contrast and definition for a given apature?


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Hi folks, 

I have some questions for the more scientifically minded people of these forums. 

First a bit of back a story. 

I have two 80mm F15 telescopes. 

I did a comparison of the two telescopes, on the same evening, using the same diagional and eyepieces. Collimation being correct for both telescopes. 

I noticed a difference. In telescope number 1 the definition between high contrast details like black and white was superior, the south polar cap on Mars was alot easier to see, and appeared alot more white than in telescope number 2. Telescope number 2 had superior low contrast performance, being able to resolve the albedo details better on Mars that the other telescope, the albedo details were easier to see, but the polar cap did not appear as such a glowing white, and didn't stand out as much. 

I apologise for not using scientific terms, what would course this performance difference? 

Dave

 

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Assuming the differences were consistent (i.e. remained apparent over time and were not therefore due to seeing or transparency issues) it could be due to differences such as colour cast of the glass (some scopes do impart a slightly yellower or whiter tone to the image, especially on bright objects like planets and moon), optical coatings (as far as I know, less coatings is better for fainter objects and better coatings more effective on bright objects). I am a bit short on time to find reference material but hopefully someone else can confirm

Hope this helps. Interesting question!

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@Moonshane thanks for the reply. The low contrast differences between the two telescopes has been observed in two different observing sessions on planets. The polar cap difference has only been observed in one observation session. So maybe I should do a third session comparing the two telescopes to see if the previous results are repeatable with regards to the southern polar cap.

Dave

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Many things contribute to overall image fidelity, including observer eye-brain system.

Although we might ascribe certain numbers to optical quality of telescope (like aberration level in waves RMS, PV, or Strehl), to understand subtle performance differences, one must look at MTF curve for each scope. This is first factor that plays a part in view quality. Next thing would be, as Moonshane pointed out - quality of coatings and glass used. These don't contribute to wavefront aberrations (well they do, but we addressed that in previous remark), but do modulate frequency response, acting like type of filter (think planetary color filters as being extreme example for this - accentuating certain features while suppressing other). This coupled with amount of light reaching eye create "setup" for third thing - eye-brain system. Eye has non linear response in level of light and perceives different colors as being of different intensity, so any modulation in color or intensity produced by previous two elements will also induce further difference in perception (and this will be dependent on observer, so some might see less difference between two scopes, and some will judge that difference to be greater).

 

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Thanks @vlaiv

I understand the glass type, quality, figure, and polish will impact on the quality of the lens. The Strehl ratio etc. 

I also understand that each observer is different in how they perceive the world. I'm the only observer involved in the above observations. 

If everything being equal. Same glass, same quality of glass, same figure, same polish, same strehl ratio, same observer. Good seeing conditions and transparency. What would cause the above mentioned differences? 

Would light scatter being less well controlled in one telescope over the other course the difference in contrast and definition? 

Dave

 

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Imagine, hypothetically, that you have as you mentioned two exactly the same objective lenses (never going to happen in real life), but each has different type of coating applied, or to different quality standard - it will impact performance as well.

Different type of coating will result in hue shift, which in turn can impact contrast (as our eye/brain sees different colors of being different in intensity, and since it is non linear, once you change perceived intensity it can also change the contrast).

Look at this image of perceived brightness of different colors:

image.png.0f710cd3520bf2a778f76275900a09a8.png

You can easily see that small shift in hue will also result in change of perceived brightness. This will of course impact contrast - as it is "ratio" of brightness, so if some or all "components" of feature change in perceived brightness you will end up in change of perceived contrast.

Other thing that can happen is, depending on coating type, and quality of application - it can create scatter (surface roughness of coating), it can actually alter optical quality also - MTF, because of different thickness of layers in different places. This is probably subtle effect for modern coating techniques, but it can add to considerable effect in dielectric coatings where there are hundreds of small layers applied - this can change RMS/PV values up to significant degree (if you have 1/10 wave surface, and you apply dielectric coating, and you are not careful to achieve uniformity of layers, you can end up with 1/6 surface or worse).

 

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Thanks @vlaiv

Well both telescopes have Magnesium Fluoride coatings on the lenses. 

I was always lead to believe that any unnecessary light in the optical train, stray light etc, like light reflecting off the inside of a focuser tube, robs performance of low contrast planetary details? Is that a definite? 

Dave

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1 hour ago, Dave1 said:

Thanks @vlaiv

Well both telescopes have Magnesium Fluoride coatings on the lenses. 

I was always lead to believe that any unnecessary light in the optical train, stray light etc, like light reflecting off the inside of a focuser tube, robs performance of low contrast planetary details? Is that a definite? 

Dave

Of course, why would we need dark skies to observe faint objects otherwise. A bit of math explains it nicely (I won't include part of non linearity of human eye, but it plays a part too).

Suppose that we have feature consisting of two different brightness components: one having brightness of 1 and other having brightness of 2 (units don't matter in this). Contrast would be 2:1 or x2. Let's have a stray light component of 1 added to the mix. Brightness of parts now becomes 2 and 3 respectively (light simply adds to make image brighter) and contrast is now 3:2, or x1.5. So a bit of light dropped contrast from x2 to x1.5.

Same thing happens when we observe DSOs, background sky light adds into picture and depending on level of LP can be up to x100 times more strong than target light. So you can see variation in contrast of 2:1, but variation in contrast of 52:51 or 102:101 is really hard to spot - we just see uniform sky of same brightness.

Baffling of tubes, blackening of any surfaces all help to get more contrasty view. It is noticeable the most when observing DSOs (perceived reduction of sky brightness) but it helps with planetary detail as well.

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