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Baffling Cass 2ndary question

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#1 AlphaGJohn

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Posted 27 June 2013 - 12:17 PM

Hi Folks,

For those with experience with Cass scopes:
Is baffling of the 2ndary necessary/useful with a closed tube? (If it is, can you explain where the light comes from that the 2ndary baffle would be blocking?)

How much extra sky-end beyond the 2ndary do you recommend having (with or w/o a 2ndary baffle)?

Current setup:
~12" ID tube
10" Primary/4" 2ndary
Primary baffle
No 2ndary baffle
About 3" of tube beyond the back of the 2ndary

Thanks for whatever you can tell me.

John

#2 GlennLeDrew

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Posted 27 June 2013 - 02:50 PM

The minimal secondary baffle length should block direct sky light from being seen from the edge of the largest anticipated FOV. A longer baffle will block such light from reaching (and reflecting off of) eyepiece barrel and diagonal walls. A longer secondary baffle, in order to not clip light from the edge of the primary, must have a larger diameter, thus presenting a larger secondary obstruction. It's a balancing game.

#3 AlphaGJohn

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Posted 27 June 2013 - 03:49 PM

Thanks, Glenn. At the moment, the primary baffle is so long I can't see the edges of the secondary w/ the focuser racked all the way in (which may well be due to incorrect separation). But just speaking off the cuff here, if, say, the primary baffle is long enough that you can just see the edges of the secondary, I'm having a hard time envisioning a light path for "direct sky light" to reach something shinny at the eye end (the end of the focuser tube or eyepiece barrel or whatever) that would be blocked by a 2ndary baffle; maybe I need to draw myself a picture....

#4 MitchAlsup

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Posted 27 June 2013 - 03:53 PM

Is baffling of the 2ndary necessary/useful with a closed tube? (If it is, can you explain where the light comes from that the 2ndary baffle would be blocking?)

How much extra sky-end beyond the 2ndary do you recommend having (with or w/o a 2ndary baffle)?


If the tube extends to RoC of the primary then you can get away with essentially no baffling around the secondary. This is a lot longer (almost 2X what the typical closed tub length ends up being.)

#5 GlennLeDrew

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Posted 27 June 2013 - 04:03 PM

You don't want the primary baffle to be too long, for it will clip light from the central portion of the primary which is just outside the secondary shadow, thus effectively increasing the central obstruction.

And on top of that, a too-long primary baffle can clip light from the primary's edge, thus reducing aperture. Talk about a one-two punch!

Again, it's a delicate balancing act, settling on the appropriate baffle lengths and diameters for the desired field illumination characteristics. Have you explored the concepts with even a crude ray trace sketch? This is an instructive exercise.

#6 Dick Parker

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Posted 27 June 2013 - 04:27 PM

John

http://mirrorworksho...affleStory.html

Dick Parker

#7 Mike I. Jones

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Posted 27 June 2013 - 05:24 PM

If you subscribe to Astronomy Technology Today, log on and read the article I submitted in November 2008, Volume 2 Issue 11, on optimization of Cassegrain baffles. I took Cassegrain baffling a little farther, by deriving an algorithm that solves for THE optimum baffling, defined as the baffle geometry that minimizes the central obstruction of the pupil due to the baffles while providing 100% stray light shielding of a field of specified diameter.

I implemented that algorithm in the CassDesign program I wrote. When I get home I'll post it up.
Mike

#8 AlphaGJohn

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Posted 27 June 2013 - 05:30 PM

Good advice Gents,
Dick, that article is excellent.
Mike, thanks for being ready to give me optimal baffle setups as soon as I've nailed down the mirror spacing. I don't subscribe to the journal in question, but I'll check out what that would cost.

I have not gotten the spacing figured out, as I essentially have to finish off a lot more of the tube than I'd expected before I can put the scope on a mount and check the separation using a star/Ronchi test.

The diagrams in Dick's article helped me visualize what the 2ndary baffle is for.

Thanks!

John

#9 Mike I. Jones

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Posted 27 June 2013 - 06:41 PM

Yep - if you don't baffle the secondary, the primary baffle has to stick forward so far that its end starts to shadow the primary center with an obstruction larger than the secondary. There is an optimal ratio of the two baffles that minimizes that obscuration.

Cone-shaped baffles are better than cylinders. Cones can dump graze-angle stray light away from the axis and out of the image plane better. Also, some people add knife-edge rings on the primary baffle exterior to further tamp down stray light. Might be overkill for you though since your tube is closed.

Mike

#10 Mirzam

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Posted 27 June 2013 - 08:05 PM

One thing that I always thought was cool is the way the Questars use a spiral baffle inside the primary baffle tube to reduce grazing reflections--the baffle is basically like a long spring.

JimC

#11 Mike I. Jones

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Posted 27 June 2013 - 10:31 PM

John,
I did a few updates to CassDesign 2.0 tonight. Download and unzip, have fun.
Mike

Attached Files



#12 MitchAlsup

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Posted 27 June 2013 - 11:32 PM

Yep - if you don't baffle the secondary, the primary baffle has to stick forward so far that its end starts to shadow the primary center with an obstruction larger than the secondary. There is an optimal ratio of the two baffles that minimizes that obscuration.


No doubt that there is an optimal ratio, however see attached image. I don't see any primary vignetting.

Attached Files



#13 Dick Parker

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Posted 28 June 2013 - 07:13 AM

Mitch -

Not sure what your point is, but I see a need here for a larger than optimum secondary baffle needed to block light from coming around the secondary off axis. The effect of this would be a brightening of the sky progressively as you view toward the perimeter of your field of view. Consequently, low brightness objects (nebula and galaxies) would fade from recognition anywhere other than the center of the FOV, The optimizing comes from blocking the light from around the secondary on axis,(primary baffle) and light from around the secondary due to parallax off axis (secondary baffle) with minimum obstruction and/or vigneting from both baffles.

Dick Parker

#14 AlphaGJohn

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Posted 05 July 2013 - 10:11 AM

Well, although the question of primary/secondary spacing is still undetermined, I'm coming up with some really interesting values for baffle lengths by playing around with Novak's formulae in Cassegrain Notes (I don't know what people think of these nowadays)--I can play around with the parameters within a wide range and I get negative length values for the primary baffle (his formulae 4 & 5)--very odd.

It's fairly clear that the current baffle is too long--at least at its current diameter. Using its actual ID with Mike's CassDesign 2.0 is not possible (it chooses the diameter for you based on other settings). But, CassDesign 2.0 does recommend a substantially larger diameter primary baffle. That's something I hadn't really considered. But given the diameter of the current baffle (sized to match the mirror's perforation), it's definitely got to be shorter--with a longer, larger diameter baffle over it, apparently--unless my values for other things are off.

So, it still comes down to getting the primery/secondary spacing correctly determined so I can nail down these other issues.

Thanks again,

John

#15 GlennLeDrew

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Posted 05 July 2013 - 04:35 PM

Have you verified things via a ray trace? Once the mirror surfaces are located with respect to each other, the focus is located, and the size of the circle to be baffled at the focus set, the key rays are automatically defined and the optimized baffles practically define themselves.






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