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Measuring obstruction % of secondary

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#1 Mark233

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Posted 26 January 2004 - 10:28 AM

How do you calculate the percent obstruction of your secondary mirror? I think I read somewhere that you can take the diameter of the minor axis of the secondary divided by the diameter of the primary. Is this correct? Also, what would be the largest percent obstruction recommended before image quality is degraded? Should you strive to have the largest secondary possible or the smallest?

#2 Mike Hosea

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Posted 26 January 2004 - 11:41 AM

Traditionally, the percentage that we use in discussing the matter is the ratio of diameters, just as you wrote. The conventional wisdom is that below 20% the reduction in contrast is negligible. However, some people like to have the smallest percentage they can manage (given the constraints that the secondary has a job to do) in order, they say, to approach "refractor like" performance. If the secondary is large enough to do its job, the smaller the better, but the largest surface errors in an optical flat tend to be near the edge, so it is somewhat inadvisable (on average) to use the edge of the secondary in the formation of the on-axis image.

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.


#3 Mark233

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Posted 26 January 2004 - 02:00 PM

Thanks Mike!

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?

#4 Jarad

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Posted 26 January 2004 - 02:23 PM

The way I calculate it is that for a general purpose scope you want to have 75% illumination at the edge of the field of your lowest power eyepiece. Visually, 75% illumination will give no noticeable vignetting. So, if you are using say a 35mm Panoptic, with a field stop diameter of 38.7mm, calculate a diagonal size that gives 75% illumination at that diameter (or 19.4mm off axis, depending on how the program is set up).

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,

Jarad

#5 Mike Hosea

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Posted 26 January 2004 - 02:26 PM

I think a 22% obstruction is fine, and a 1/2 degree fully illuminated field is plenty for visual use.

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.




#6 Jarad

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Posted 26 January 2004 - 04:46 PM

Mike's numbers are pretty reasonable - I have a 10" f5.6 scope with a 1.83" secondary too - I can detect some vignetting at the edges of my 35mm Panoptic, but not in my 24mm panoptic. His f5 would probably show a bit more dropoff at the edges of the 35 pan, and maybe a touch in the 24. Still perfectly useable in both.

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.....).

Jarad



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