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 Secondary Mirror Computations
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ejbragg
 Monday, August 06 2007 @ 10:32 pm EDT (Read 2670 times)  
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Still waiting on the primary mirror to arrive......

By the way, I have NOT ordered the secondary. I have decided on where I will buy the secondary, as per the suggestion of the maker of the primary, Mr. Royer; however, the size of the mirror apparently needs revisited. There is apparently no clear answer, since at least ONE of the variables is a subjective value.

I have juggled this back and forth with charts and formulas from a plethora of sites. I started with Krieg & Berry's book, "The Dobsonian Telescope", which is a thick, comprehensive book on the subject. According to their charts (which don't even go down to 12.5" aperture on the primary!), the estimation seems to be about 2.55" on the minor axis for a 20mm fully illuminated field of view, f/5 dob.

Now a lot of people claim that THAT much light is not required. On the other hand, I do like to see distant objects other than planets, so my first thought is that I like this size.

I have also used a (freebie) telescope CAD program (Newt) which verifies a calculation for about the same.

However, I have read other materials that suggest that such a highly illuminated field is unnecessary and even wasteful.

The following dimensions are those I used for my design:

D (primary): 12.5"
F (Focal Length, f/5): 62.5"
H (eyepiece height): 3"
T (UTA inside diameter): 14"
L (folding distance): 10"
d (diameter of 100 illm): 0.79" (or 0.5"?)
a (secondary minor axis): 2.55" (or 2.2?)

And we've seen the formula:

a = d+ [(D-d)L] / F

There are other suggestions that the fully illuminated field need be only 0.5" (in the same textbook) and another source suggests a "realistic" 0.4"! This would bring the secondary diameter down close to 2" even! That's a pretty big difference.

The two actual sizes made by the manufacturer I'm interested in are: 2.14" & 2.60".

It seems I'm up against several factors here:
1) Difference in diffraction caused by the secondary
2) Amount of light blocked by secondary
3) Overall reflected light provided by secondary

From the CAD program, I discovered that the amount of light blocked by the larger secondary is far outweighed by the amount of reflected light it provides. So I guess it's down to the diffraction pattern caused by the larger secondary. So... how bad is this larger secondary going to distort the image?!

Although I've been leaning toward the larger 2.60" secondary (as this is, after all, a faster scope than average), I don't want to be stuck with a serious (and expensive) flaw. What might be the consensus on this topic?

Thanks, guys.

P.S. One other important note is that I do, in fact, intend to get some 2" eyepieces for this scope - perhaps another factor in looking toward a 20 mm fully illuminated view field. Does that change anything?


"Thus the heavens and the earth were completed in all their vast array." - Genesis 2:1, NIV
 
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nspace01
 Wednesday, August 08 2007 @ 01:48 pm EDT  
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Sounds like you have done a lot of research as is required to get everything "Right".

A simple fomula that I have played with, when trying to design my "One Day" project, is as follows:

minimum secondary = DC / F

where C = the distance from the secondary to the focal plane, D = the primary diameter, and F = the focal length.

In you design,
C= 10 inches
D= 12.5 inches
F= 62.5 inches

So:

(10 x 12.5)/ 62.5 = 2 inches would be the Minimum diameter of the secondary mirror for 100% illumination.

Like I say, this is a very basic formula and doesn't take a lot of other factors into account.

BTW, I really injoyed our joint observation of a few deep sky objects on Bob's 12" dob Saturday night...........(The ones we could find, anyway...) Considering we had a 9mm eyepiece, observing at 200x (I think) and a telrad that was way off, we did pretty good........................


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bobmoody
 Friday, August 10 2007 @ 01:45 am EDT  
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About 2" eyepieces......

I almost feel like the hype about 2" eyepieces is overblown. I've bought, and used, and sold 2" eyepieces several times. In some instances, actually a limited number of instances, a 2" eyepiece was really best for the object and the optical system I used. As a pure experiment one evening last year, I used a 2" 40mm Celestron E-Lux that came with our 14" CGE1400 IN the same 14" scope, but I also had a Meade 40mm 1.25", AND, a .965 40mm eyepiece just to see what differences I could see between them. I expected BIG differences, but I had a hard time seeing ANY differences in actuality.

Seeing how closely a $29.00 .965 40mm performed compared with the $60.00 1.25" 40mm, and the $89.00 2" 40mm, I couldn't believe my eyes. Others that were observing with me that night were also amazed. For any system of f/6 or certainly f/7 and higher, there is NO need for a 2" eyepiece. The longer focal ratio gives a diameter of the light-cone at the entry to the eyepiece skirt or focuser skirt small enough that you don't usually need that extra diameter or the higher prices associated with it.

However, for optical systems of f/6 or below, especially from f/5 down to f/4.5 or even f/4, a 2" eyepiece IS usually required. You may be cutting off some of the light-cone by trying to use 1.25" oculars with those systems. That super-slow focal ratio that you wanted to couple the dimmest of light to your eye for better details can be undercut by the mirrors inability to bring every photon to focus.

The other gremlin associated with short focal systems is coma around the edge of the field. This is when every star out at the edge of the field has a short little "tail", or comet-like coma always pointing away from the center of the field. That deep curve of these super-short focal systems frequently induces this aberration which is best recovered by eyepieces specifically designed for this purpose. Its said that the mirror can be parabolized to a point that this effect is minimized, but that's very hard to do. Most of these "coma-correcting" eyepieces are 2" in size and this is also where the HIGH, HIGH prices also come in. To get all you can from a 10" or 12" f/4 or f/4.5, that's just a given that you need these special eyepieces for the best view.

Bottom line....consider carefully whether you think you NEED a 2" eyepiece or not. Choose instead eyepieces with good eye relief and wide apparent fields-of-view. You'll nearly always get great views and you can be confident that you won't lose any photons in the process. The extra money you save can make up what you need to get another 1.25" quality eyepieces in the medium-to-high magnification range. Only buy the 2" eyepieces if your system really needs them.

Sorry for the protracted explanation, but its just my 2-or-3 cents worth.....
Bob


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ejbragg
 Sunday, August 12 2007 @ 02:05 pm EDT  
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Thank you guys for your inputs. Bob, I will certainly take my time in buying any 2" eye pieces. I appreciate your comments. Hopefully, I can "test drive" any such size against a 1.25" version before actually buying it. And thanks, Leonard for the formula. I will certainly stick to that as a minimum diameter.

I fired up the "Newt" program again, and here's what the program estimates:
With a 2.14" secondary (the closest available to the 2" minimum), the 100% fully illuminated field comes out to be about 0.07 inch in diameter - on that scope..... basically, zilch. Only the 75% (or less) field is present, out to about a half inch diameter. If I stick a 2" eyepiece in there ... no change, of course, except that I've completely wasted my money on a larger eyepiece that will gather more of [very little]!

If I upgrade to the 2.6" secondary, the program says that my 100% fully illuminated field changes to about 5/8" diameter, and my 75% illuminated field grows to about 1-5/8". More light. However, the 75% field suffers vignetting with a 1.25" eyepiece. No big deal, I suppose, since it only means I won't capture all the light available. I will still get [ALL] the 100% field illumination. If I THEN decide to own a 2" eyepiece for any reason, I will then gather the full 75% ring, as well. And then some.

What I'm saying is that according to the program, i.e. THEORETICALLY, a 2" eye piece adds no advantage whatsoever over the 1.25" eye piece if my secondary is only 2.14" on the minor axis. And THEORETICALLY, a 2" eyepiece DOES add more light-amplifying power than a 1.25" eye piece if my secondary is 2.6" on the minor axis.

You are saying that the experiment suggests that the real world is not the same as the theoretical world! (Oh, I can't believe that!) Mr. Green Well, good information, for sure. I have some more questions on this below....

Thanks for revealing that the eyepiece diameter should not be the primary driving factor in secondary size, once you've reached the ballpark figure. So tossing that aside, I still suppose that the difference in light gathering ability still points me toward the larger secondary, if I want to still see more distant objects (since they will appear brighter). But the question still remains: If I use the larger secondary, will the resultant diffraction overshadow the scope's ability to light up those distant objects? In other words, will there be sufficient blur added to the picture which makes the added brightness - and therefore magnitude ability, a moot gain? Again, please let me know if I'm wrong, here.

Bob, I have another question about your experiment. Undoubtedly, nothing tests a theory any better than looking at targets for yourself. But I'm wondering if you've done this test with say, a pair of higher dollar eyepieces. For example, would a TeleVue Nagler 2" & 1.25" be just as similar? (I'm supposing the Naglers will be cleaner than the Meades or Celestron, unless the guys on the forums are off base in their claims. - again, please correct me!) But let's say I'm looking at a nebula. We know how faint they are, even to a telescope. Would there be any difference between the barrel sizes in this case? To more exactly state my question, what exactly were you testing when you did your comparison?

Thanks.


"Thus the heavens and the earth were completed in all their vast array." - Genesis 2:1, NIV
 
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nspace01
 Thursday, August 23 2007 @ 01:36 am EDT  
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Your dad and I breifly discussed some possibilities of materials other that wood for telescope construction, at the last star party.

Check out the following website for ideas for an alternate construction material....

www.ultralightscopes.com/MarkIV.htm

This material sounds to be fairly strong but would reduce the wieght by 4/5ths over plywood.


nSpace01
 
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ejbragg
 Friday, August 31 2007 @ 04:49 pm EDT  
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Leonard,

That's an interesting site, with interesting material. It would likely save someone in weight; however, I bought my wood about a week before you posted the message. My dad and I even looked into another material, Lexan, in the form of construction foam. Because of ups and downs in the material properties, and the trade-offs between the known and the unknown, we decided to stick with wood, as it is tried and proven able to survive if treated properly, over hundreds of years.

Anyway, I already have a couple big sheets of 1/2" Russian Baltic Birch, from Plunkett's Distributing, here in town: high quality, 9 ply.


"Thus the heavens and the earth were completed in all their vast array." - Genesis 2:1, NIV
 
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