Evaluation of optics for high resolution imaging

These pages try to bring some light on the theoritical optical performances of various telescope designs for high resolution lunar, solar and planetary imaging. Each design has his pro and cons, and understanding them can help when it goes to the big questions :

- What telescope should I buy / build for HR ?

- What can I do to get the best from the optical performances of my telescope ?

- Should I use only one Barlow lens or two stacked Barlow lens ?

Of course, beyond the theoritical optical performances, there are many other issues involved in the choice of an instrument, to name a few : cost, weight, portability, diameter, thermal issues, ease and stability of collimation, obstruction, mechanical and optical quality.

The analysis is focussed on instruments with diameter larger than 200 mm.

Refractors and Maksutov are not considered. The former because of the cost / weight in larger sizes, the latter because of the weight which makes it unpractical beyond 250 mm.

The key characteristics of a telescope for high resolution lunar, solar and planetary imaging are the following :

Diffraction limited flat field :

- For imaging, we are only concern in flat field. Accordingly, all the simulations presented here are for flat field.

- A 1 arcmin field is required for the planets (ie. maximum diameter of Venus, Jupiter, Saturn rings) from the blue light (or even UV light for Venus) to IR. However a larger field is a benefit in order a have a more robust collimation (specially if the telescope is to be packed in a car when moving to an observing site),

- A larger diffraction limited field is required for solar / lunar imaging when large size sensors are used (eg. CMOSIS 4000 2k x 2k with 5.5 microns pixels).

Backfocus range :

Some type of telescopes are designed for a given back focus (eg. Cassegrain). However, in real life, the back focus may be quite different than the nominal value because of the use of Crayfor focuser, filter wheel, atmospheric dispersion corrector, Barlow lens, field rotator, etc. According, the telescope designs with good optical performances over a relatively large back focus range are a benefit.

Strelh ratio versus wavelength :

Diffraction limited performances are expected over a large range of wavelength. Indeed :

- Planetary imaging is done from UV (350-400 nm for Venus) to IR (650 nm for Mars, Jupter, Saturn, 800 nm for Venus, and even 1000 nm for Venus dark side thermal emission).

- Lunar imaging is usually done in green or red light depending on the seeing conditions and telescope diameter.

- Solar imaging is done in 396 nm (Ca K or K line), 430 nm (G band), 540 nm (continuum), 656 nm (Ha) or red light.

Focus position versus wavelength :

A minimal variation of focus position versus wavelengh is expected. Even if the focus position is set by stepper moter according to the filter used, a large change of focus with wavelenght would make less effective L-RGB imaging technique (eg. Jupiter).

Sensitivity to off-centering, tilt of the primary / secondary mirror :

- Without the use of an optical bench, it might be rather tricky to properly collimate some types of telescopes (eg. Cassegrain, where the axis of the parabolic primary and hyperbolic secondary should be co-axial). All telescopes are not equal to improper collimation ...

When is a telecope diffraction limited ?

- The usual criteria are Rayleigh criterion : wavefront < L/4 P-V, Strehl ratio > 0.8, and about rms > 0.07 L.

What about the quality of manufacturing ?

All of the simulations presented here assume the optics are perfect from the optical and mechanical point of view. This allows comparing and understanding the pro and cons of each design from a theoritical point of view.

However this is not the end of the story... Some designs can turn out perfect when simulated in OSLO or Zeemax, but impossible to make because of too stringent tolerances or surfaces impossible to polish. In real life, a perfect design can be poorly implemented, while an alternate design with more limited theorical performance can give outstanding results because of the choice of quality material and beautifull manufacturing .

In a nutshell ... quality come first for high resolution imaging.

Some good references on telescope optics

Web ressources :

[W1] : Vladimir Sasek : http://www.telescope-optics.net/index.htm

[W2] :Serge Bertorello : http://serge.bertorello.free.fr/

[W3] :Charles Rydel : http://www.astrosurf.com/astroptics/

[W4] :Ken Hutchinson : Clestron SCT Vigneting Analysis, Version 1.5 - C14-Edge HD Forum - (December, 2007)

Books :

[1] : Telescopes, Eyepieces, Astrographs - Gregory Hallock Smith, Roger Ceragioli, Richard Berry - Willmann - Bell (2013)

[2] : Telescope Optics - Harrie Rutten and Martin Van Venrooij - Willmann Bell

[3] : Conception et construction de télescopes et astrographes amateurs - Sous la direction de Charles Rydel

[4] : The design and calibration of popular Schmidt Cassegrain telescopes - Robert Deane (October 1999)

What are the objectives of these web pages ?

The key characteristics of a telescope for high resolution lunar, solar and planetary imaging are the following :

When is a telecope diffraction limited ?

What about the quality of manufacturing ?

Some good references on telescope optics

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