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Old 22-07-2018, 07:18 AM
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gregbradley
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I don't think that is correct. Here is a quote from the Astrodon site:

Please note that camera manufactures often specify the MECHANICAL back focus of their products. 3 mm thick filters will ADD 1 mm (0.04") to that mechanical backfocus as measured from the telescope or from the MonsterMOAG prism. The camera window, often 3 mm thick will add another 1 mm (0.04"). So, You may have to add 2 mm (0.09") to the mechanical backfocus.
There may be some confusion as camera manufacturers measure backfocus from the focal plane of the CCD to the outer surface of the camera. When they account for the thickness of the filters, the SUBTRACT the 0.04", which is correct as measured from the CCD. However, most people measure backfocus from the back of their scope of from a corrector, and then add/subtract spacers to arrive at the correct backfocus. In this case, as measured from the scope, the 0.04" must be ADDED. A subtle point, but does get people in trouble from time to time.


There also is a thread on Cloudynights that discussed this QSI note.

Clearly the definition of optical backfocus has been misunderstood by QSI. They must be measuring mechanical backfocus.

I would research this point more before you order to satisfy yourself what is the correct spacing.

There is a thread that shows a ray diagram where the filters cause the light to bend less in extending the focus point of the light a bit further out.


Greg.
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