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Message: <blockquote data-quote="elkcub" data-source="post: 1347457" data-attributes="member: 14473">I guess the best time to ask for a discussion is when the opportunity presents itself.  I emphasize discussion since I'm really conflicted about this star-testing with booster procedure that you guys recommend.  I also admit to limited knowledge of the subject.  Here's my concern.  When two telescopes are used in cascade, as described, they are coherently coupled.  The real images inside each instrument are not projected onto a screen, for example; hence, the transfer function of the complete system is not the simple product of the component transfer functions.  Under such circumstances, P. Mouroulis ("Visual Instrumentation," pg. 28-29) says that "... For coherently coupled systems, we may not multiply the transfer functions even approximately."  After describing the correct relationship by means of wavefront aberration, which is a phase term, he then exemplifies how systems in cascade can each suffer serious aberrations, with the combined system being diffraction limited.  As I understand it, therefore, an unknown number of interaction profiles could occur with a booster, depending on the particulars, the results of which are not necessarily attributable to flaws in the instrument being examined, or flaws in the booster, but rather to the combined instrument plus booster.  This means there is the possibility that the combined effect masks or minimizes an underlying problem with the instrument, or that the combined effect makes the instrument look bad when it really isn't.  As if this concern were not enough, those who recommend star-testing with a booster seem to feel universally that almost any low power optic can be used for the purpose.  This requires somewhat more faith than a confirmed skeptic like myself can deal with.  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />  I'd love to be talked out of this concern, preferably on a theory basis.  But, while we're at it I guess something else that's been gnawing away is the absence of references to this procedure in textbooks — i.e., the use of a booster.Ed</blockquote>

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