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Lets talk about the Meade SN-6 schmidt newt..

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#226 jgraham

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Posted 05 November 2011 - 01:50 PM

If I recall right the center screw pulls the secondary support up against the 3 adjustment screws, it dies not tighten the holder in the window. It's tricky, but what you need to do is to grip one of the ribs inside secondary support to keep it from rotating while you snug the lock ring that holds the support in the window. I'm pretty sure that I carefully held one of the ribs with a pair of needle-nose pliers in one hand and snugged the ring with the other, and I've never touched it again!

#227 pgatrick

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Posted 05 November 2011 - 03:41 PM

Once I build up the courage to attack this again, I'll try that out. I have a pair of mini locking pliers that will probably fit perfectly, don't know why using them to hold it still didn't occur to me.

#228 Jb32828

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Posted 17 January 2012 - 10:34 AM

I wanted to revive this thread and see How you all a coming along with your SN-6s.

Now that I am used to how to get it collimated I am really, really happy with this scope. I can get it collimated in 10 minutes to where if I focus on Jupiter's moons I can defocus and refocus and the airy disk all the way down to the point of light is a perfect concentric circle. I actually used this scope for my eyeballs last weekend and got a very nice view of Jupiter with a 2x barlow.

So, I am a happy camper, I hope everyone else is too...

#229 seryddwr

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Posted 28 January 2012 - 12:14 AM

I'm possibly looking to get one of these. Since I already have an imaging scope, I plan on using it visual-only. Has anyone thought of putting the optics in a longer tube, to put the focal plane closer to the secondary? I'd like to cut down on the vignetting.

#230 Karl Fabian

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Posted 30 January 2012 - 01:16 PM

Used visually my SN6 performs very well as is both in low power wide field and high power lunar planetary. A good helical focus adapter is handy for high power. As designed you should not encounter vignetting especially in a visual mode with any normal eyepiece. However,if you still want to put the focal plane closer to the secondary and install a low profile focuser, it would be much easier to extend the length of the existing tube and move the main mirror back the required distance. Be advised however that image contrast could suffer because the longer original focus tube acts like a baffle in suppressing off axis stray light bouncing off the inside walls of the main tube. That could be remedied with flocking. Building an entire longer tube would be a lot of unnecessary work and not easy especially when you consider properly mounting the corrector plate and lining things up. It could end up a disaster. :(

#231 seryddwr

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Posted 30 January 2012 - 10:58 PM

especially when you consider properly mounting the corrector plate and lining things up. It could end up a disaster.

I wondered about that. So, if I change anything, it will be to extend the tube, like you said. Thanks for the suggestion.

#232 jgraham

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Posted 30 January 2012 - 11:23 PM

I lengthened the tube of my original N6 to bring the focal plane in a bit, but I don't think that you can do this with the SN6 as the spacing between the mirror and the corrector is fixed. However, I don't think that vignetting is a problem anyway and it works pretty well as-is.

#233 seryddwr

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Posted 30 January 2012 - 11:49 PM

OK, I just worry about the 2.14" secondary being too strongly vignetted, when using the 31mm Nag. (The vignetting calculator on Mel Bartel's web site seems to indicate that as being the case.) But if the spacing is fixed... :mad: Well, I haven't bought one yet, anyway. :shrug:

#234 Karl Fabian

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Posted 02 February 2012 - 11:43 PM

It is irrelevant that the position of the secondary is fixed. Moving the main mirror back accomplishes the same thing. It brings the focal point to a lower position with respect to the tube surface. Then with a lower height focuser the lower position of focus allows the secondary to field the light cone more effectively. That is why low profile focusers are used on planetary Newtonians. It allows a smaller secondary to field the light cone...or put another way it simply optimizes field illumination no matter what size the secondary is.

#235 davebuechler

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Posted 03 February 2012 - 03:35 PM

Hi Guys

I have a SN8 and while overall the performance is acceptable, I can never seem to get sharp focus. Collimation to the best of my abilities is right on. Do you get a good snap at focus with your SN?

#236 jgraham

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Posted 03 February 2012 - 05:13 PM

The SNs are fast scopes so the focus can be a tad finicky. I use a Bahtinov mask to get the best possible focus even when I use the scope visually. This keeps me from endlessly fishing for the sweet spot. The focus should be sharp. Look at the diffraction patter as a check for good overall collimation. I then use a Ronchi screen in place of an eyepiece to look at the overall system figure (being an ol' ATM helps). A knife-edge works just as well. I've not seen a bad SN yet, but I did have an N6 with an under-corrected primary once (which Meade promptly replaced). One thing I have seen with SNs is either the secondary being out of alignment causing some odd diffraction patterns from the structure behind the diagonal or the tube extension being installed when it should have been removed. The later caused the draw tube to extend into the light path, again causing odd diffraction patterns.

#237 Karl Fabian

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Posted 03 February 2012 - 07:40 PM

I get a nice sharp focus on my SN6 and have pushed it up to over 200x with sharp images. Collimation is important on this scope. I used a laser in the focus tube to verify the beam hits the donut in the center of the mirror. If it doesn't adjust the 3 secondary screws until it does. I then adjusted the main mirror till the return beam passes through the target hole on the laser collimator. Then the final tweek is to adjust the main mirror using the blue ghost method which assures that the corrector plate is perpendicular to the optical path. This may throw off the laser beam slightly but it is more important that the corrector plate be perpendicular. Also if using a laser collimator verify that it also is collimated. A simple method is to use a chshire and get the colimation as close as possible and then do the blue ghost. A helical focus adapter is very handy for achieving critcal focus easily. I have the Orion version and put a little straight STP oil treatment (the oil thickener stuff for worn engines) on the threads to make it silky smooth with to slop.
Clear skies,
Karl

Hi Guys

I have a SN8 and while overall the performance is acceptable, I can never seem to get sharp focus. Collimation to the best of my abilities is right on. Do you get a good snap at focus with your SN?



#238 Karl Fabian

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Posted 04 February 2012 - 12:20 AM

I lengthened the tube of my original N6 to bring the focal plane in a bit, but I don't think that you can do this with the SN6 as the spacing between the mirror and the corrector is fixed. However, I don't think that vignetting is a problem anyway and it works pretty well as-is.



John, sorry I may have misunderstood you. Did you mean the mirror on the SN has to be a certain distance from the Schmidt plate for proper correction? That is a good question, and if that is the case moving the mirror back might not be a good idea.

Karl


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