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Equipment Discussions >> Cats & Casses

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Post Laureate

Reged: 10/12/07

Re: I cant believe the size of the LX600........ new [Re: Joe Cepleur]
      #5608072 - 01/06/13 10:59 AM


Personally, I'd love to learn the standard misconceptions and the physics behind them. It would be part and parcel of learning the actual truths, but a more thorough way to learn.

I recommend references to primary sources. Astronomical imaging is complex enough - and when you combine it with human vision in a changing atmosphere - it becomes a even more complex and touches on many disciplines. This makes it unrealistic that you can draw a single graph and make broad claims to compare the expected performance of two very different systems.

Like most threads on this topic, this one refers to Suiter and to various web pages, but it does not refer to textbooks on Fourier optics or journal articles. Suiter is a good intro to aberrations and helps interpret telescope behavior in the context of things like MTF, but it does not go in depth on ways in which MTF is very limited. MTF would be great if the object being viewed were a single, perfect sine wave pattern - but instead even something like the bands of jupiter involve many spatial frequencies that overlap and interfere, and imperfections in both phase and amplitude can generate artifacts that would spoil the view even though the mtf looks "good" in the "frequencies of interest."

Two references on Fourier optics are Gaskill and Goodman, where MTF is treated in more detail and caveats are provided on its usage. I have cited them previously in other threads.

On the topic of large CO in SCT's, I think it's useful to refer to a key early paper on design options for SCT's where some field curvature is allowed. Sigler's 1975 paper in Appl. Optics fits in well here, although it pre-dates more high res. imaging work with ccd's as opposed to film. I can't quote the whole thing, but will provide a brief excerpt:

"It is ... unfortunately true that the flat field designs, due to the Petzval constraint, have large secondary obstruction ratios, small secondary magnfication... These constraints are not too serious for strictly photographic instruments where the secondary obstruction is frequently as large as T=0.5 without unduly affecting performance. However, for photo/visual instruments, the large field of view and low effective focal ratio are sacrificed for higher secondary magnficiation ratios... The attainment of high resolution, especially in the intermediate spatial frequencies of the MTF curve, is facilitated by having the smallest possible secondary obscuration ratio."

So here is a journal article that points out that you can flatten the field by reducing the power of the secondary, but it ends up being bigger - and that will impact high resolution performance in visual work. In summary, a big motivation for the larger secondary in imaging Cassegrains is to reduce Petzval curvature and flatten the field - but doing so is undesirable due to the loss of resolution. Anyway, you can flatten the field and reduce the f/ratio with lenses near the focal plane, while keeping the secondary small.

With regard to comparisons of contrast and sky blackness with a refractor vs. an sct - key differences are the much better baffling that you can have in a refractor, and the potential for less scatter by using only lenses rather than mirror surfaces. If some people feel the overall contrast is better in refractors vs. sct's, it may not be easily explained in terms of mtf - but it may be nonetheless true due to other factors.

People have different feelings for how important the CO is to the view, and my attitude is simply that smaller is probably better. But how much difference it makes depends on many factors that are not captured in an MTF plot or simulations - and also depend on each user's visual system and preferences - and how the optical characteristics combine with the object and visual system to cause the thumb on the human to point up or down in response.


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Just Wondering

Reged: 10/10/05

Loc: Toronto, Ontario
Re: I cant believe the size of the LX600........ new [Re: freestar8n]
      #5608155 - 01/06/13 11:40 AM

My take-away from all of the above posts is this:

If you have a 45% (by diameter) obstruction in your scope and you take a photo of a 1.5 x 1.0 degree field surrounding the Rosette nebula there will be zero contrast loss - as any number of astrophotographers will tell you - the contrast loss is entirely at arc second scales, a level of detail that the pixels on the chip cannot resolve. The telescope is designed with this in mind. Ceravolo and Officina Stallare and others make some very expensive, state-of-the-art astrographs with central obstructions approaching 50%. Visit the websites of these companies to see the astonishing, contrasty images they produce. I bet the farm that if you buy a scope of this configuration you won't even get a diagonal with it - it is not meant for visual use.

If you take those same telescopes and throw on a diagonal and a 6mm eyepiece for a 250x view of the detail in Jupiter's cloud bands, you're going to run into some serious degradation of the contrast.

This seems straightforward to me. To discuss contrast loss in greatly obstructed telescopes without referring to the angular size of the target leads directly to 'myths'.


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