Effect of Size of Parabolic Mirror in Bad Seeing

[ChaosKnight]

Sometime back there was discussion on whether the size of aperture affects image quality in bad seeing.

Here's something i came up with. The results indicate that a large parabolic mirror degrades image quality more than a small mirror, but whether this degration can be picked out by observations remain to be seen.

http://www.sciforums.com/attachment.php ... entid=3529

[VinSnr]

Sometime back there was discussion on whether the size of aperture affects image quality in bad seeing.

Here's something i came up with. The results indicate that a large parabolic mirror degrades image quality more than a small mirror, but whether this degration can be picked out by observations remain to be seen.

http://www.sciforums.com/attachment.php ... entid=3529

You know I have seen so much of these scientific papers saying big reflectors are a waste of time under bad skies. But I can tell you that the image at the eyepiece will say that these papers are bull.

I dunno if the image did degrade more...but I do know that any objects in a 12" mirror will look nicer than the one in a 6" mirror, iiregardless of the sky condition. In 1992, I think, someone setup a 16" scope in Ngee Ann. Need to climb ladder and all that. But the views were so fantastic, you would think you are under a dark sky. And no...Ngee Ann skies those days were already as bad as now.

So I think all these papers need more work.

[ChaosKnight]

One thing to note: what was found out is that the error at the focal point due to mirror curvature is only about 1% compared to that due to turbulence. And this is for a fast mirror.

Anyway what i did is quite limited in scope. There is no doubt a large mirror degrades image quality, but whether that 1% can be seen with your naked eye is another question.

Also, other overriding factors have to be considered when determining image quality.

[VinSnr]

One thing to note: what was found out is that the error at the focal point due to mirror curvature is only about 1% compared to that due to turbulence. And this is for a fast mirror.

Anyway what i did is quite limited in scope. There is no doubt a large mirror degrades image quality, but whether that 1% can be seen with your naked eye is another question.

Also, other overriding factors have to be considered when determining image quality.

I think what you did was a very good attempt. It might lead to other things if you keep going!

[weixing]

Hi,

Sometime back there was discussion on whether the size of aperture affects image quality in bad seeing.

Here's something i came up with. The results indicate that a large parabolic mirror degrades image quality more than a small mirror, but whether this degration can be picked out by observations remain to be seen.

http://www.sciforums.com/attachment.php ... entid=3529

I do agree that a larger aperture telescope do perform badly compare to a smaller aperture telescope in bad weather. I had done a experiment myself sometime ago... I point my 6" newtonian at the moon on a quite cloudly night. The moon was bright and I can see the background sky was not dark (due to reflection of the light pollution by the particles in the atmostphere) and also the turbulence can be seen easily. I put on my telescope cover and open the off-axis mask on it, so that my 6" Newtonian become a 30mm off-axis Newtonian. I look at the moon again, the contrast of the moon was great... pitch black background, no turbulence can be seen and the moon look very impressive. But all the details on the moon also cannot be seen.

Anyway, you indicate that the main cause of bad image quality of large aperture is due to the incoming light striking the mirror not parallel to the principal axis of the mirror. Yes, this may be also one of the factor that affect the image quality, but not the main one. Because if this is the case, you don't need bad weather... there are always a lots of light striking the mirror not parallel to the principal axis of the mirror. Also, you can minimize this by having a baffle in your telescope. As a result, I have different opinion.

But before I give my opinion, I'll first point out why turbulence cause bad image. Quote from a website: "When light enters the Earth's atmosphere, the different temperature layers and different wind speeds distort and move the image in various ways. The effects of the atmosphere can be modelled as cells of still air about 10 cm in diameter, and this limits the resolution of the telescope." This is also the reason why the stars will twinkle.

IMHO, there are two main reason why a larger aperture telescope will perform badly compare to a smaller aperture telescope in bad weather even the telescope is well baffle:
1) Main Reason: the larger aperture telescope have better resolution power, thus making the effect of the turbulence more obvious than a smaller apeture telescope which has less resolution... the larger aperture telescope able to "resolve the turbulence" better than the smaller aperture telescope.

2) If the atmostphere is hazy or got lots of particles and light pollution is present, the larger aperture telescope will also enchance the brightness of the light reflecting off the particles due to larger light gathering power, thus making the light pollution more obvious than a smaller apeture telescope which has less light gathering power.

Anyway, may be you should also consider what I mention and include the effect of higher resolution of larger aperture telescope in your work.

Have a nice day.

[VinSnr]

Hi,

Sometime back there was discussion on whether the size of aperture affects image quality in bad seeing.

Here's something i came up with. The results indicate that a large parabolic mirror degrades image quality more than a small mirror, but whether this degration can be picked out by observations remain to be seen.

http://www.sciforums.com/attachment.php ... entid=3529

I do agree that a larger aperture telescope do perform badly compare to a smaller aperture telescope in bad weather. I had done a experiment myself sometime ago... I point my 6" newtonian at the moon on a quite cloudly night. The moon was bright and I can see the background sky was not dark (due to reflection of the light pollution by the particles in the atmostphere) and also the turbulence can be seen easily. I put on my telescope cover and open the off-axis mask on it, so that my 6" Newtonian become a 30mm off-axis Newtonian. I look at the moon again, the contrast of the moon was great... pitch black background, no turbulence can be seen and the moon look very impressive. But all the details on the moon also cannot be seen.

Anyway, you indicate that the main cause of bad image quality of large aperture is due to the incoming light striking the mirror not parallel to the principal axis of the mirror. Yes, this may be also one of the factor that affect the image quality, but not the main one. Because if this is the case, you don't need bad weather... there are always a lots of light striking the mirror not parallel to the principal axis of the mirror. Also, you can minimize this by having a baffle in your telescope. As a result, I have different opinion.

But before I give my opinion, I'll first point out why turbulence cause bad image. Quote from a website: "When light enters the Earth's atmosphere, the different temperature layers and different wind speeds distort and move the image in various ways. The effects of the atmosphere can be modelled as cells of still air about 10 cm in diameter, and this limits the resolution of the telescope." This is also the reason why the stars will twinkle.

IMHO, there are two main reason why a larger aperture telescope will perform badly compare to a smaller aperture telescope in bad weather even the telescope is well baffle:
1) Main Reason: the larger aperture telescope have better resolution power, thus making the effect of the turbulence more obvious than a smaller apeture telescope which has less resolution... the larger aperture telescope able to "resolve the turbulence" better than the smaller aperture telescope.

2) If the atmostphere is hazy or got lots of particles and light pollution is present, the larger aperture telescope will also enchance the brightness of the light reflecting off the particles due to larger light gathering power, thus making the light pollution more obvious than a smaller apeture telescope which has less light gathering power.

Anyway, may be you should also consider what I mention and include the effect of higher resolution of larger aperture telescope in your work.

Have a nice day.

Your analysis is spot on..but I think end of the day, the question is "will the degradation of a large mirror due to bad skies OVERIDES the ability to see more details"?

From my experience, the improvements on the object details far exceed the degradation on the large mirror. This is especially true for deep sky.

So there is no doubt that large mirror suffers more due to bad skies, but they also improve the details so much that the degradation became "unnoticable".

[ChaosKnight]

Excellent points, Weixing. Unfortunately including that part about resolving power is difficult, as the derivation of the equations involve wave theory, something i'm not familiar with.

But this is one aspect that i think can be shown experimentally. Lets try something. Tonight i'll take some pictures through my 4" refractor and post them here. Then you tell me if you can see which one is taken through the full 102mm aperture, and which is taken through the masked 53mm aperture.

Because full aperture is about twice that of masked aperture, resolving power is doubled. Lets see if the there is noticeable change in image quality as well.

Since 4" is not considered a big aperture, degration in image quality due to curvature at the lens edges is assumed to be negligible. Also, my scope is f/10, so this should be a fair test on resolving powers only.

As for the part about light pollution, i don't think there's any way we can avoid that. Even between different levels of magnification, the background sky takes on different shades. Practically, maybe some sort of light pollution filter can help.

[weixing]

Hi,
It is best if you use the moon as a target... it is bright enough and do not require a long exposure, so tracking will not be a problem.

By the way, I think it is best you also see the different visually... just in-case the image doesn't show the different clearly or show no different at all.

Have a nice day.

[ChaosKnight]

Cloudy today.....don't think can see the moon tonight.
I'll use some lights in the distance outside my window. And thanks for the reminder, i'll check visually as well.

[ChaosKnight]

Ok here it is. Time for the new game: Guess The Aperture!!

This is a picture of a pair of florescent lamps taken using a webcam and a 4" refractor. The subject is far enough at the horizon so that effects of turbulence is easily apparent. The top four is one group, while the botton four is another.

Can you tell which group is taken using full aperture (102mm) and which is taken using masked aperture (~53mm)? Based on effect of turbulence only.