Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Intes Scopes?

This topic has been archived. This means that you cannot reply to this topic.
27 replies to this topic

#26 Guest_**DONOTDELETE**_*

Guest_**DONOTDELETE**_*
  • -----

Posted 21 June 2004 - 09:34 AM

But here again, these are limited. Would an SCT and MCT or even your Mewlon have the same plots?

Yes, if it is designed and fabricated with enough precision so that aberrations are negligible compared to diffraction. It's the theoretical limit; no matter how perfectly you fabricate the optics, that's all you'll ever get.

How does focal ratio change this? (MTF is associated with resolving power which changes with focal ratio.)

It varies linearly with focal ratio. That is to say, if you plot the MTF curve as a function of angular resolution, it will be independent of focal ratio. (Which I should have done, but it involves exporting data and plotting with another software.)

What would the off-axis plots look like - most of the image is off-axis.

I already gave the plots for 0, 0.02 and 0.1 degrees off-axis. I'm not sure if it's useful to go further off axis; 0.1 degrees corresponds to the edge of the field at 300x with a 60-degree AFOV eyepiece.

And how do the radial and tangental off-axis data compare?

What does "tangential off-axis" mean?

And last but not least, how does this affect the image you see; an MTF curve can show differences that are not perceptable.

I admit I can't say. What would you propose is a good way to approach this question? I suppose I could convolve a Hubble image with the calculated MTF, but considering that released Hubble images are not linear I'm not sure how useful that would be.

Incidentally, the Hubble has only 12.5% central obstruction...

#27 Rhadamantys

Rhadamantys

    Mariner 2

  • -----
  • Posts: 214
  • Joined: 25 Apr 2003

Posted 21 June 2004 - 12:47 PM

Hi Ken. According to Françon, a telescope with more than L/16 of SA is questionnable (you can begin to see the difference with something perfect), at least for visual planetary work where low contrast details are seeked. Usually, this translates into a strehl ratio of 0.99 or less. In France, such scopes (usually Newtonian or very well made Cassegrains) are said to respect Françon's criterion. I suggest you draw the MTF of a perfect Newtonian on axis and of one which has L/16 PtV of spherical aberration: the difference, as displayed by the MTF will be minimum, but this difference will be enhanced for very low intrinsic contrasts if one takes the human's eye response into account (see Texereau for a better explanation of the Françon's criterion and Telescope Optics for the MTF/human eye response: most enlightening).

Practically, it will be quite difficult to see a difference between a telescope respecting Françon's criterion and something with, say L/8 PtV of SA, and that's assuming that both telescopes are as well collimated.

PS: for the Clavius 166, my latest infos point to a weight of 2.8kg for the OTA... That's near the weight of a 85mm refractor. You can guess that I am eagerly waiting for mine.

#28 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5746
  • Joined: 30 Sep 2003

Posted 21 June 2004 - 07:03 PM

But here again, these are limited. Would an SCT and MCT or even your Mewlon have the same plots?

Yes, if it is designed and fabricated with enough precision so that aberrations are negligible compared to diffraction. It's the theoretical limit; no matter how perfectly you fabricate the optics, that's all you'll ever get.

How does focal ratio change this? (MTF is associated with resolving power which changes with focal ratio.)

It varies linearly with focal ratio. That is to say, if you plot the MTF curve as a function of angular resolution, it will be independent of focal ratio. (Which I should have done, but it involves exporting data and plotting with another software.)

What would the off-axis plots look like - most of the image is off-axis.

I already gave the plots for 0, 0.02 and 0.1 degrees off-axis. I'm not sure if it's useful to go further off axis; 0.1 degrees corresponds to the edge of the field at 300x with a 60-degree AFOV eyepiece.

And how do the radial and tangental off-axis data compare?

What does "tangential off-axis" mean?

And last but not least, how does this affect the image you see; an MTF curve can show differences that are not perceptable.

I admit I can't say. What would you propose is a good way to approach this question? I suppose I could convolve a Hubble image with the calculated MTF, but considering that released Hubble images are not linear I'm not sure how useful that would be.

Incidentally, the Hubble has only 12.5% central obstruction...


Hi Ken:

They were more rhetorical questions. but thanks for your answers. I was trying to illustrate the the complexity of the problem and the factors that can influence an MTF plot. I think it is important how a central obstruction can influence optical performance, but it is only one ingredient in a complex recipe.

But let me sneak in a few comments on the issues you raised.

I understand your plots are theorical limits, but I'm wondering if other parameters should be added to model "standard" telescopes. It may be more useful as a comparison. I think the design (SCT, MCT, etc.) can affect an MTF. For example, how flat the field is varies with the scope design. This curvature of field will affect off-axis resolution. Spot size will also limit resolution and MCTs are said to have a better spot size than SCTs.

I'm curious why you chose 300x. I rarely get to use those powers. I think calculating for the rquired image circle for an eyepiece at 100x with a 60 degree FOV would be more revealing. While this would not be an issue with the planets, the moon and sun are 1/2 degree in extent.

I don't think radial and tangental plots are usually done with telescopes, but it is usual for photographic optics. Radial and tangental MTF plots are rarely the same off axis. The radial plot would be the resolution along directions emanating from the optical axis. The tangental plot is the resolution at the tangent to the radial axes. Near the optical axis, these plots are very similar, but toward the edge of the field the tangental resolution can suddenly become worse.

There is a way to know if differences are in the MTF plot is perceptable, use the scope. MTF is just an indicator of the optical performance. The engineers still have to use the scope or lens to see if the performance is acceptable. Optical testing always ends with a subjective test. The lens needs to be used with the eye, film, or CCD to know if it is performing correctly.

I didn't realize the Hubble CO was so small. Maybe it is the wrong one. They have made mistakes before. ;)


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics