Measuring obstruction % of secondary
Posted 26 January 2004 - 10:28 AM
Posted 26 January 2004 - 11:41 AM
A large secondary will maximize the negative effects of diffraction, but it will also maximize off-axis illumination. It's typical for the percentage of the primary used to drop to 50-75% at the edge of a widest-true-field 2" eyepiece. If you want 100% illumination over the widest possible view (for photography or because you're a DSO kinda guy and don't care about planets), then you would choose a somewhat larger secondary.
The illumination profiles versus secondary size can be computed by "SEC", a DOS program that can downloaded from the Sky and Telescope web site.
Posted 26 January 2004 - 02:00 PM
I figured it would be like everything else with telescopes, everything is a trade-off. If I am mostly into DSO, then I would want a larger secondary. If planets is my thing, then a smaller secondary would help out on contrast. However, if I want a good all around scope that does a respectable job on both, would I be looking for something that would give me about 1/4 degree of unvignetted field or is that getting too small for DSO. Would a 1/2 degree unvignetted field be better and still give good views of planets? For my scope, a 1/2 degree field would be about a 22% obstruction. My DSO targets are usually globulars, open clusters, and nebulae.
Is this the reason that most of you own more than one scope?
Posted 26 January 2004 - 02:23 PM
For photographic use, calculate the 100% illuminated field to the size of the photographic medium you are using (i.e. 35mm for film, or the size of your CCD chip, etc.).
For planetary use, a 100% illuminated field of just 0.1 degree or so is sufficient, but that setup will probably have significant vignetting in low power eyepieces.
I recommend the program Newt 2.0 for playing with the nubmers - very nice program, gives nice visual diagrams of the light rays so you can really see what is happening.
Hope this helps,
Posted 26 January 2004 - 02:26 PM
The way I personally think of it when I am designing a Newt is that for the best general performance I would prefer that the central obstruction be 20% or less with a fully illuminated field that isn't vanishingly small. I'm not tied to numbers on this point, but let's say at least a 1/4 degree 100% illuminated field size. Also, I would prefer that the illumination at the edge of the widest true field ocular that I will use for observing at length be no less than 50%. (Note that the more widely accepted number is 70-75%, but I choose to be more aggressive here because my emphasis is on compact objects rather than on rich field viewing in my Newts.)
These requirements are consistent for my 10" f/5 Teleport, which uses a 1.83" secondary and a low profile focuser. If I find that these requirements are at odds with one another, I choose a slightly larger secondary or consider lowering the profile of the focuser, if the latter is possible and if the problem is the fully-illuminated field size being too small.
Posted 26 January 2004 - 04:46 PM
Also, one thing you will notice if you play with the calculation programs - the faster scopes have a smaller 100% iluminated field, but the dropoff past that is more gradual. Slow scopes (long f-ratio) have larger 100% fields, but the dropoff gets steep past that. Also, bigger scopes can get to smaller % obstruction, because the focuser height and tube diameter become smaller compared to the total light cone (i.e. a 1.6" low profile focuser on an 18" scope is equivalent to a 0.53" focuser on a 6" scope, but you can't get a 0.53" focuser.....).