Author Topic: Aperture vs. Size of Secondary Mirror Obstruction  (Read 813 times)

Robert Farley

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Re: Aperture vs. Size of Secondary Mirror Obstruction
« Reply #15 on: January 23, 2018, 06:23:15 PM »
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"Will the bigger aperture of the 8" and the use of higher magnification more than compensate for its larger central obstruction?"

Since the relative obstructions are essentially the same the answer is yes. Brighter images will be more noticeable than a slight increase in resolution. As for weight and ease of handling differences only you can decide if it's worth it.

However, another factor to consider is the effect of different focal ratios on planetary images. Slower scopes tend to produce better images. I would consider it preferable to gain experience with the 6" and scale up later if desired.


I have owned both 6 inch F/8 Dobsonians and 8 inch F/6 Dobsonians. There is no doubt in my mind that an 8 inch provides the better planetary views. Aperture buys you not only greater light gathering but also greater resolution and better contrast transfer.

As others have said, it's the relative size of the central obstruction that is of interest, both these scopes have the same size CO, about 23%. So that is a wash. But the increased aperture is what is really most relevant and that will buy you better planetary views.

Another important factor is that 6 inch Dobs rarely come with 2 inch focusers and none of them I know of come with a Crayford. The standard 6 inch Dob focuser is functional but not of the precision and quality of the Crayfords that come on 8 inch Dobs. The GSO Dobs come with a 2 inch inch 2 speed as do some of the Orion and Skywatchers. Just the cost of the focuser upgrade brings the cost of a 6 inch Dob close to the cost of an 8 inch.

And as Red said, in terms of transport and portability, they're about the same. The base of both a 6 inch and 8 inch are about the same size and weight, around 20 pounds. The 6 inch tube is lighter but not meaningfully shorter so they take up about the same space..

I once summed up my experiences with both 6 and 8 inch Dobs with the following:

The best 6 inch F/8 is an 8 inch F/6...

I later had a chance to see just how true that was. One evening I purchased a used 6 inch F/8 Discovery Dob and it was far and away the best 6 inch F/8 I had ever looked through. Later I discovered it was an 8 inch F/6.

Jon

ardrivunla

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Re: Aperture vs. Size of Secondary Mirror Obstruction
« Reply #16 on: January 25, 2018, 06:01:45 PM »
Here's a different perspective on the generalized subject of pupil obstructions: As the discussions are alluding, profuse concern and analysis has been done on the effects/affects of SM "shadow" upon the PM, spider vanes (straight, thick/thin, curved). I think the topic is SO overblown Because it lends itself to analysis! Ever since the FFT (Fast Fourier Transform) finally became tractable on computers (1950s), optics guys (and then hobbyists) have gleefully written tomes highlighting their computer runs of the impulse responses "point-spreads" of various pupil functions. And, indeed, the central shadow changes the image of a star and MTF... ironically INCREASING the modulation (contrast) of finer detail! (at the expense of reduced modulation of blobbier bigger features). And the spiders... 3 vanes vs 4, curved fat ones vs thin straight ones, wires... bla bla blah... All interesting stuff on the theoretical front. But the GIANT ELEPHANT in the analysis is that nearly all are normalized to a circular entrance pupil, with a radius of (ready for this?) a radius of ONE. So, the first thing you throw out, before running the FFT, is the absolute diameter of your telescope objective. That helps the analysis, but tends to camouflage the dominant characteristic: Bigger lenses form Smaller images of stars. An 8-inch (vs a 6) scope will image the (same) star 33% smaller (tighter), 78% brighter (more intense), and 216% more illuminant (e.g. on the CCD pixels, or on you retina, if the Airyish disk is well-resolved). And THAT improvement is HUGE! Way to think of it is this >> The dominant obstruction in your telescope is the OUTSIDE EDGE, not little things on the inside. And THAT's why a bigger good telescope will always (be capable of) outperform(ing) a smaller good telescope. ~QED~ Tom

ovhercayvic

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Re: Aperture vs. Size of Secondary Mirror Obstruction
« Reply #17 on: January 31, 2018, 02:14:33 AM »
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Thinking about a stable, portable, easy to set up scope that will be used mostly for planets but also brighter DSO. I’m considering a 6” F8 dob (Orion XT6 Plus) and an 8” F5.9 dob (Orion XT8 Plus). The 6” has a secondary mirror obstruction of 34mm while the 8” has a secondary mirror obstruction of 47mm. For both scopes the obstruction to the aperture, by diameter, is 23%. Which scope will give better contrast and detail on planets? Will the bigger aperture of the 8" and the use of higher magnification more than compensate for its larger central obstruction? I'm attracted to the lighter weight and easier handling of the 6". Thanks and best…

If both secondary mirrors have the same obstruction percentage by diameter, then the contrast reduction between the two should theoretically identical. This means by default, the 8" will be the far superior choice for extracting planetary details.

But even if the secondary obstruction of the 6" were say, 20%, and the 8" were say, 30%, the 8" would still be the superior choice. The extra brightness at higher magnifications from the 8" *more* than makes up for the slight loss in contrast. Moreover, the 8" is simply going to extract more details from planetary and lunar features than the 6" is capable of.

For fainter DSOs, you might be able to argue that the marginally higher contrast of the 6" will reveal ever so slightly more faint detail in a few very specific cases, but the 8" in general is just going to be showing you more. At the same exit pupil, you'll have a 1.33x larger image scale, which will be significantly more beneficial than any slight contrast disadvantage of the 8".

Dan Square

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Re: Aperture vs. Size of Secondary Mirror Obstruction
« Reply #18 on: January 31, 2018, 06:25:53 AM »
Thank you everyone for the helpful replies! I learned a lot. I've currently got access to a 6" F5 table top dob (with a secondary that's 31% of the objective diameter) so when I move on it will be to the 8" dob with an upgraded focuser to boot. Best...