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Central Obstruction, A Doable Test

Started by jonisaacs, 11/17/2003 07:15AM
Posted 11/17/2003 07:15AM Opening Post
Again and again the effects of a central obstruction on planetary viewing are debated with much information, analysis, experience, opinion, vague recollections etc. When comparing two scopes it is really not possible to determine the cause of any observed differences, subjective or objective, there are just two many variables.

The simplest way to eliminate these variables is to just use one unobstructed scope or two identical obstructed scopes.

If one wants to compare an 6 inch scope with 33% obstruction to a 4 inch unobstructed scope then simple masks allow one to do this easily. A 2 inch disk in the center of the objective and an ring with a 4 inch hole will make a 6 inch unobstructed scope into either of these. This eliminates all the other variables and reduces the problem to how a central obstruction affects the view. One can also investigate how adding a central obstruction affects the image for a given aperture.

Some further comments:

1. The best way to do this is a "blind test." When looking to see the effect of central obstruction for a constant aperture, this is quite doable and quite important. An assistant should change the size of the central obstruction and the observer should make every effort not to cheat. A quick defocus would suffice to see the size of the CO. Doing a comparison between apertures, the first test, the brightness difference should be obvious so a blind test would be difficult.

2. The outer ring used to reduce the aperture is easy to do, it just fits inside the dew shield. Attaching the inner ring is a bit of a challenge. One suggestion is to simply glue the ring to the objective. (NOT!!)

Actually fine wires could be used or possibly sticking a piece of plastic to the objective using a bit of water. Not quite sure how this is best done.

4. Unfortunately, this requires an APO or off axis Newt because color correction of an Achromat is affected by a mask. Adding a central obstruction will increase the false color and adding a ring that masks the aperture will reduce the false color.

5. Maybe someone else has already suggested these, I have not read all the posts on this topic, they are overwhelming.

Think about it.

Damon Rinard, well known for testing various aspects of bicycles had this statement on his website:

"One measurement is worth 50 expert opinions."

jon isaacs

Posted 11/17/2003 11:41AM #1
There is the (in)famous Dickinson/Dyer experiment in which an APO was fitted with paper obstructions of differing sizes. The aperture was held fixed in that case. Is this in the Backyard Astronomer's Guide? I read about it while browsing at a bookstore years ago--haven't ever owned a copy. IIRC, they came up with the 20% figure that gets tossed around nowadays as being the threshold for noticeable CO effects (but needless to say, some people think the threshold is lower).

I don't know that anybody has done a similar experiment allowing the obstructed configuration to have greater aperture.