Imaging Performance at f/6 with Flattener, 51mm Image Circle

Polychromatic Strehl Ratio for Imaging Wavelengths

Strehl Ratio measures the lens correction on-axis to show how well the colors of the visual spectrum are focused across the wavelength range. Best correction occurs at 555nm to match the peak visual acuity of the human eye. Eye response falls off to 2% in the deep red and violet ends of the spectrum. The correction level of the 155 lens design exceeds 1/10 wave for all colors in the visual wavelengths.

RMS Spot Radius Over a 51mm Diameter Field

Theoretical Ray Trace Performance over 51mm Image Circle, 435-656nm

Actual Diffraction Performance over 3.1 Degree 51mm Image Circle, 

Although the spot sizes are usually given for performance comparisons, the actual smallest spot diameter will be limited by diffraction. The Airy Disc size for an F6 lens is approximately 8 microns as shown below.

This diagram shows the expected star shapes over the field as well as for 0.1 and 0.2mm defocus.

Summary of Imaging Performance at f/6 with Flattener, 3.8 Degree 64mm Image Circle (i.e., Medium Format)

Visual Use Performance at f/6

Polychromatic Strehl Ratio for Visual Wavelengths

Strehl Ratio measures the lens correction on-axis to show how well the colors of the visual spectrum are focused across the wavelength range. Best correction occurs at 555nm to match the peak visual acuity of the human eye. Eye response falls off to 2% in the deep red and violet ends of the spectrum. The correction level of the 155 lens design exceeds 1/10 wave for all colors in the visual wavelengths.

Polychromatic RMS Spot Size Over a 0.9 Degree Field at f/6

The 155 f/6 lens is a 3 element coma-free design which produces diffraction limited images in any wide-field eyepiece up to 1/2 degree. This means that high power views up to full Moon size are sharp and coma-free. Although field curvature is present due to the fast focal ratio, the actual star sizes are well within the Airy Disc up to 1/2 degree. Eyepieces that cover larger field sizes up to 100 deg apparent field will still show small sharp stars because the magnification of those eyepieces is not large enough to resolve the Airy Disc.

The two diagrams below show the RMS Spot sizes over the 1/2 degree visual field. The actual smallest star size of a 155F6 optic has an Airy Disc radius of 4 microns. The polychromatic spots all fit well within the Airy Disc.

Longitudinal Spherical Correction

This graph shows the color correction as well as the spherical correction of all the wavelengths in the visual range. The closer together the color curves are the lower will be the chromatic aberration of the lens. The straighter the lines are from the on-axis point (0% Pupil Radius) to the outer diameter of the lens (100% Pupil Radius) the higher will be the spherical correction for each color.

Normal ED and Fluorite Triplets with close spaced elements will only be corrected for spherical aberration in the middle green part of the spectrum. The red colors will be under corrected and the blue will be overcorrected. It is known as Sphero-chromatism. This does not cause color errors to appear in most of the images. The stars, Moon and planets will still look color-free but the overall detail will be softer and lower in contrast.

The 155F6 uses a Gauss derived optical design where each color from deep red to deep violet has less than 1/40 wave P-V of spherical error. To get that kind of correction in a normal triplet lens, even with a Fluorite element, requires greater than F14 focal ratio in a 155mm aperture. Wide airspaced triplets can be made to have very high polychromatic spherical correction at faster focal ratios, but the large air spaces can cause thermal cool-down issues. The 155F6 has relatively small air spaces and comes to thermal equilibrium in a normal manner.

Specifications

Specifications subject to change without notice as details are finalized.  The pictured scope is the prototype.

What's Included?

• Tube Assembly with Astro-Physics 3.5" Focuser, Dew Shield and Dustcap
• Flat Field Corrector
• 2.7", 2" and 1.25" Adapters
• Carrying Case

Items That You Will Need For Your Scope

• 6.5" Mounting Rings (65RING)

Accessories to Consider

• 10 x 60 Vario-Finder/Guidescope with Quick Release Guider Bracket (1060GUIDE)
• Handle with Dovetail Slot for Guidescope and Finder Brackets, PICQRM2 & STARFMD (QRHANDLE)
• MaxBright 2" Mirror Diagonal (PMDMAX)
• 2" Advanced Convertible Barlow (BARADV)
• Baader Mark V Binocular Viewer (BPMARKVS)
• Baader Cool-Ceramic Safety Herschel Prism (BPHERP)
• Kendrick Dew Remover Premier Heater (KDR006)
• Solar Filter options, all feature Baader Planetarium AstroSolar SafetyFilm, Density 5-Visual
• Baader AstroSolar Filters (ASTF-160), Diameter of filter material: 160mm
• Baader AstroSolar Film (AS50A4). 200mm x 290mm (7.9"x11.4") sheet to make your own filter. Instructions included.
• Baader AstroSolar Film (AS50HM). 500mm x 1000mm (19.7" x 39.4") sheet to make multiple filters, Instructions included.

Availability

Spring 2025 Update
The 155mm CS is not available for ordering and there is currently no available notification list. 

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