determining aperture reduction from shorter truss tubes experimentally

[Stonius (Markus)]

Hi there,

I've got a 16" truss dob. The trusses are adjustable in four increments of 10cm. Currently it's on the second to lowest one. I'd quite like to get my Baader Mk V binoviewer going on it. However, even with Baader's 1.7 correcting lens for newts, it still won't quite achieve focus. I can put my truss tubes on their lowest setting, but I'm concerned I maybe inside the light cone from the mirror and trimming my aperture as a result.

Is there a way to experimentally figure out if this is the case and if so, how by how much? I feel like there should be a simple way but I just can't think of it.


Markus

[JA]

Hi Markus,

Your 16 is a Hubble 16" f/5 according to your signature. Their website lists the f/5 as having a 70mm secondary and 2032mm focal length, the same as would it be computed simply just based on the 16'' f/5. One might therefore question the accuracy of the focal length, based solely on the 16" f/5, but the same principles apply when you have the exact measured focal length.

To understand how much you are using of your light cone it is far simpler to think of the light cone from the primary mirror to the focal point as a simple triangle without the 90degree bend in the light path following reflection form the secondary. Then using the geometrical concept of similar triangles one can predict the width of the light cone at the secondary mirror position and compare this with the width of the actual secondary mirror (a 70mm central circular obstruction, as seen by the primary mirror.). Then simply adjust the distance between the secondary mirror position in 10cm steps and you could answer your questions for all the rod positions. If the light cone width is say 100mm wide at the secondary mirror position then you will loose about one stop of light, and your f/5 scope is more like f/7.
Twice or half the Area represents a change of one f/stop.
1.414.... times the diameter or its reciprocal, 0.7071 represents a change of one f/stop.

I could post a diagram if that helps.

Best
JA

[Stonius (Markus)]

So basically, I need to measure the distance from the center dot to the center of the secondary somehow? I'm not sure how to do that to greater than +/-5mm accuracy