R mirror lenses -thoughts

[dkCambridgeshire]

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On 9/24/2020 at 5:31 PM, jankap said:

There is a large magnification in the pictures, because of the extender and the APS camera. The dog  fills the complete area more or less, almost no unsharp background. There is some busy bokeh, but the light colored dog takes the attention.

The flower picture has the black background, no bokeh donuts there.

The lens had a bad press. One can let the donuts shine by choosing an appropriate scene. There are examples. Those reviewers were termed by somebody as members of the hate brigade.

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I have the APO-extender 2X.

One precaution with mirror lenses to optimise image quality is to allow sufficient time for the primary mirror to acclimate to the ambient temperature. When stored indoors the primary mirror absorbs heat and when moved outside into a cooler environment, the primary mirror acts like a radiator as it cools … creating ‘thermals’ which can distort the image. Astronomers allow at least 30 minutes for a Newtonian astro telescope to acclimate before commencing observing … and for Maksutov Cassegrain models up to 90 minutes. Mirror lens acclimation time is not on many lens testers' / ’reviewers’ radars … it’s seldom mentioned in their reports … hence their reviews, especially comparative reviews, should be read with caution.  

The phenomenon is well documented ... Google 'mirror lens acclimation' for detailed descriptions ... and ways to prevent / minimise convection currents/thermals 

dunk

Edited September 25, 2020 by dkCambridgeshire

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