Metal back distance

Forums Imaging Metal back distance

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    Andrew Read

    Could you help direct me to a technical review or other resources concerning the metal-back distance, by which I mean the fixed distance behind the primary mirror / instrument that cassegrains have their focal plane?  The reason for asking is to be able to work out how much tolerance I should allow in the length of an imaging train and the potential return on effort of minimizing it (e.g. with hand-cut shims).  
    Otherwise if you have your own rule of thumb for allowable metal back tolerance, please do share your thoughts!

    Dr Andrew Smith

    I don’t  think there us a simple answer . There are design equations but it depends on the details of the optical design. With  moving mirror focus where the primary moves it can vary considerably. Can you be more specific?  Do you have a particular scope in mind?

    Regards Andrew 

    Andrew Read

    Thank you Andrew – I’ve started to realize (I also posted on cloudy nights) that the metal back tolerance must depend on each scope’s optical design and that the only way to understand the effect of a sensor being +/- mm off the exact position is probably to look at the manufacturer’s spot diagrams for those positions.  Not easy information to get hold of nor easy to develop a rule of thumb for.

    Martin Lewis


    Why don’t you determine it empirically — as presumably it is the position where aberrations are minimised?

    Star test either side of a know positions further and further out from location of max in-travel. Use a digital video camera to capture the out-of-focus start images and allow you to average them and you’ll have a record of the intra and extra focal images for each mid-location. Best position is where the intra and extra focal images are most similar to each other.


    Andrew Read

    It’s not a bad idea Martin, but I’m dealing with the metal back distance here (focus is on the secondary mirror), so the only way to adjust it is with spacers or custom made adapters between the rear port of the scope and the camera.  At each iteration I would have to refocus and measure the quality of the image (perhaps the FWHM of suitably bright stars). 

    Ray-trace spot diagrams would replicate these tests exactly, and presumably manufacturers must run them in order to specify the optimal metal back distance of the scope.  What doesn’t seem to be specified is any kind of tolerance, or the response of the spot-diagrams to different metal back distances

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