some solid references on Baffles and Stops RE: [ap-ug] Baffle question

Richard Crisp

There’s a bit about baffles in Smith’s “Modern Optical Engineering”



also Bely’s “Design and Testing of Large Optical Telescopes” has an entire chapter dedicated to stray light control and baffling (PDF page 205 is the start). Each chapter has an extensive bibliography as well.




From: <> On Behalf Of Len Fulham
Sent: Monday, April 5, 2021 4:44 PM
Subject: Re: [ap-ug] Baffle question



A lot of scopes especially older ones use a single baffle.The baffle is usually dimensioned to fully illuminate a small central area of the field where critical viewing would be done. This generally suits visual scopes and does not cause issues. The single baffle is then very effective in blocking stray light.

Scopes which are optimised for imaging allow for a wider fully illuminated field. To achieve this a single baffle cannot effectively stop stray light so multiple baffles become necessary (unless you use a overly fat tube!). This is evident when one looks into a modern AP scope with numerous quite wide baffles. They are not there for show.

If you wish to revise the scope baffles you should consider the implications. Just making the single baffle opening larger will reduce contrast and may cause you problems. There is simple baffle calculator here:

This will let you play with the parameters and get idea what changes you need to make to have a larger fully illuminated field. Eventually the opening of the focuser tube may become restrictive. It is very helpful to draw (on computer is OK) a proper scaled layout of your scope to examine the possible reflection paths between the objective and the field.

As a practical example I have a Brandon 94mm APO which has a single "mid tube" baffle. It fully illuminates about 3mm of the field. I have designed a set of 4 baffles to allow full illumination of a 40mm field. A lot more work, weight in scope etc. Makes for an interesting project.




Thanks. Just to note that the Bely reference is for cassegrain systems. There is also this approach:

That also seems to imply that the stop in mine is too narrow.