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how much power do you get out of your meade 6-SN?

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#1 erik

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Posted 04 July 2005 - 06:22 PM

just curious what kind of magnification you get out of this scope. i've been happy with mine so far, except that it doesn't seem to handle power that well. last night, the seeing was pretty good, and i was still only getting about 100x on jupiter, despite the scope being cooled down, collimated, etc. what are your experiences with high power in this scope? :)

#2 jmcdonald

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Posted 04 July 2005 - 07:13 PM

FWIW, my 10" f/4, seems to like power. I was using a 6mm Radian yielding 160x and resolving the heck out of some summer globulars last night. M22 was particularly spectacular despite being right over my roof. I estimated At least a hundred pinpoint, very steady stars.
At this time of day, Jupiter is too low in the Tucson sky glow by the time the 100 degree heat dissipates enough to let the atmosphere stabilize.
My very inexperienced estimat is that cooldown (well, heatup) time is at least one hour -- of course I haven't punched out the back like you have :p

#3 Tim2723

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Posted 04 July 2005 - 08:33 PM

Hi Erik,

Well, you're certainly not one who needs the usual dissertation on cool down and collimation, but 100x seems a remarkably low limit.

Can you describe what you were seeing in more detail?

#4 erik

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Posted 05 July 2005 - 01:19 AM

just a soft image. the seeing was pretty good too...

#5 Clive Gibbons

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Posted 05 July 2005 - 01:43 PM

just a soft image. the seeing was pretty good too...


Hi Erik.
What do star images look like?
Assuming good collimation, you should see a decent Airy disk and a few diffraction rings looking at moderately bright stars in the 150x to 200x range. If not, perhaps the scope has some residual spherical aberration? :question:


Clive.

#6 erik

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Posted 05 July 2005 - 02:58 PM

the star test and ronchi test look perfect. these scopes have a spherical mirror and corrector plate, right? i'm not that familiar with how that type of optical system works. would spherical abberation be a result of the mirror or corrector plate? i was expecting at least 150x out of this scope...

#7 Patrick

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Posted 05 July 2005 - 03:05 PM

I was using a 6mm Radian yielding 160x



My 6" f/8 Newt can go up to 300x on good nights, 220x or so on average nights.

My 80mm refractor performs decently at 100x on good nights, so 100x on a 6" SN (and even 160x on a 10") sounds pretty low.

Patrick

#8 erik

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Posted 05 July 2005 - 03:13 PM

yeah, that's what i thought. i've used 6" newts that were capable of over 200x. is the design of the SN less condusive for high power use?

#9 tcraig

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Posted 05 July 2005 - 03:28 PM

Erik

I have a sn10 that about 100x starts getting real soft. I figured it was bad eyepieces and/or barlow. Now I wonder a little bit. I am currently waiting on a better 3x barlow to show up in the mail.

Let us know what you discover.

#10 jmcdonald

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Posted 05 July 2005 - 04:08 PM

Well, I was at 160x out of necessity -- lack of short focal length eyepieces - the 6mm was borrowed and I didn't barlow it. I now have a 10mm plossl and 3x barlow. I'll try 300x next time I take the scope out and report back.

#11 jrcrilly

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Posted 05 July 2005 - 04:12 PM

yeah, that's what i thought. i've used 6" newts that were capable of over 200x. is the design of the SN less condusive for high power use?


Hi, Erik.

I've not owned a 6", but the 8" and 10" SNT's I've had went to 200X with no problems. My skies usually stop me there no matter what telescope I use.

#12 Tim2723

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Posted 05 July 2005 - 06:19 PM

Erik,

Before you loose any sleep over this, you should simply repeat the test. The collimation could have slipped, you may have overestimated the collimation or cooling, a smudged or dewed surface may have gone unnoticed, or you may simply have misjudged the seeing. Even the most experienced observers are not immune to these problems. While the SN design is not heralded as the premier planetary scope, 100x is unexpectedly low if you are correct about everything else.

Confirm and repeat is my advice.

#13 erik

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Posted 05 July 2005 - 08:18 PM

thanks, i'll do that. i've had it out 3 times now, and it's been that way every time. i got it mainly for deep sky and a wide fov in a portable package. but i would like it to perform decently on the planets as well. can someone explain to me the technical aspects of how this optical system works, or does someone have a link on SN optics? is it possible that the mirror or corrector plate is flawed?

#14 jrcrilly

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Posted 05 July 2005 - 08:27 PM

can someone explain to me the technical aspects of how this optical system works, or does someone have a link on SN optics? is it possible that the mirror or corrector plate is flawed?


Hi, Erik.

An optical defect is possible, though I haven't seen any reported. Those optics were figured & tested in California and their track record is pretty good.

Optically, it's nearly identical to a conventional Newt. The main difference is that the primary is spherical rather than a paraboloid. This provides a view with much less coma than would an equally fast paraboloid but introduces spherical aberration, for which the corrector compensates.

Was it new? Has it been apart? I believe the corrector and primary are rotationally optimized at the factory.

#15 Tim2723

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Posted 05 July 2005 - 10:15 PM

Sorry Erik, you didn't mention that you'd tried it three times. I was just trying to be helpful and encouraging.

#16 erik

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Posted 06 July 2005 - 10:21 AM

can someone explain to me the technical aspects of how this optical system works, or does someone have a link on SN optics? is it possible that the mirror or corrector plate is flawed?


Hi, Erik.

An optical defect is possible, though I haven't seen any reported. Those optics were figured & tested in California and their track record is pretty good.

Optically, it's nearly identical to a conventional Newt. The main difference is that the primary is spherical rather than a paraboloid. This provides a view with much less coma than would an equally fast paraboloid but introduces spherical aberration, for which the corrector compensates.

Was it new? Has it been apart? I believe the corrector and primary are rotationally optimized at the factory.

thanks john! i have had it apart, but i didn't change the vrotation of the corrector or anything. the secondary tilt was slightly off when i got it so i had to adjust that slightly. i bought it new. the spherical mirror doesn't normally cause a problem at high power? i only have experience with spherical mirror newts, which are often useless at high power, at least if they're fast like an f/5. but i assume the SN design is supposed to compensate for that?

#17 jrcrilly

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Posted 06 July 2005 - 10:53 AM

the spherical mirror doesn't normally cause a problem at high power? i only have experience with spherical mirror newts, which are often useless at high power, at least if they're fast like an f/5. but i assume the SN design is supposed to compensate for that?


Hi, Erik.

Yes, the spherical Newts fall down at higher mags because of the spherical aberration and their generally poor quality. The SNT's work about as well as a somewhat slower Newtonian of similar quality.

#18 Clive Gibbons

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Posted 06 July 2005 - 12:07 PM

thanks, i'll do that. i've had it out 3 times now, and it's been that way every time. i got it mainly for deep sky and a wide fov in a portable package. but i would like it to perform decently on the planets as well. can someone explain to me the technical aspects of how this optical system works, or does someone have a link on SN optics? is it possible that the mirror or corrector plate is flawed?


Hi Erik.
The scope has a spherical primary mirror and an aspheric corrector plate, which corrects the s.a. of the primary mirror.
Two possible optical causes for what you're seeing:
1) there might be residual s.a., caused by either the primary mirror or corrector being improperly figured.
When the system was assembled, the technician might not have matched the primary mirror with an optimally figured corrector.
2) the corrector might have a "rough" figure, due to the high-speed method these optics are polished with. This has been seen in SCT's. The overall correction can be good, but low-level "ripple" in the optical figure will spray light over the image, making it seem a bit soft or fuzzy.

In any event, if you can't get a sharp image over 100x, it's probably worth seeking a replacement scope. Try another one in the dealer's stock.

One more thing... stop the scope down a bit. Try it with a 5" or 5.5" aperture mask. If that sharpens the image up appreciably, the mirror probably has an edge defect.

Clear skies,


Clive.

#19 erik

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Posted 06 July 2005 - 08:32 PM

thanks john and clive! things to consider... one thing that might be a cause, is the mirror had a "ring" that meade used as a mirror clip. it covered over 1/4" of the primary mirror, so when i had the scope apart, i removed it. maybe meade installed this to hide a TDE or to improve the performance by stopping the scope down and making the f/ratio slightly slower. i thought i was buying a 6" scope, not a 5 3/4". guess i'll put it back and see if that improves things...

#20 jgraham

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Posted 07 July 2005 - 10:23 AM

I've had mine up to about 190x before and the image looked very clean. I've not had my SN6 apart, but I have worked on my Meade N6. In the N6 the clip ring just barely clears the edge of the mirror. When you re-install it be sure not to over-tighten the screws as this may stress the mirror. I leave mine on the verge of loose; just enough to pull the rubber pads down to the mirror.

Collimation on a short focal length telescope is everything. I found I have to use a centering eyepiece (the one that came with my StarBlast) to get it right. Be aware the secondary may look a little off-center when it's lined up right; again a consequence of the short f/ratio. The manual mentions this in the alignment section. I thought mine was off-center until I used my centering eyepiece, once I used that everything lined up okay. If we get a break in the weather I'll see how far I can push it.

-John

#21 erik

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Posted 07 July 2005 - 08:56 PM

thanks john. i don't believe my collimation is a problem. i'll try to get it outside again soon if the weather ever cooperates...

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Posted 08 July 2005 - 01:26 AM

I have an SN8, and I get 203X with a 4mm Omni Plossl on any night of average seeing. I get great views of the moon with crisp detail. No significant ghosting or flare. I just got a new Borg 4317 helical focuser and a TeleVue 2.5X Powermate, so that magnification figure will probably raise a bit.

FWIW, I haven't touched my collimation since I got the scope. I get good views of M57 Ring Nebula with a 6mm Omni Plossl at 135X. M13 looks quite spectacular as well at that power. I have a fairly dark site near Lake Texoma away from all homes for a couple of miles. I just have to put up with a chugging oil rig on the same plot of ground.

My problem is shake in my tripod and mount. They give out before my scope does on a night of good seeing. Putting this SN8 on a Losmandy G11 or Celestron CGE might not be a waste after all.

#23 jgraham

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Posted 08 July 2005 - 12:42 PM

I waited too late last night to catch Jupiter, but I did take a close look at the Double-Double in Lyra at 190x (LXD75-SN6 with a 4mm Plossl). The stars were cleanly split with beautifully sharp Airy disks and diffraction rings. I haven’t touched the secondary on this telescope, but I have aligned the primary. So the optics in mine look pretty good.

-John

#24 erik

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Posted 08 July 2005 - 08:17 PM

it was cloudy once again last night, so i didn't get a chance to test it again. i put the "stop down ring" back on the primary mirror though, maybe that'll help things a bit. i borrowed my friend's laser collimator to verify my collimation was "on' using my chesire and coll. cap, and it was dead on. so there must be another issue...

#25 erik

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Posted 12 July 2005 - 10:02 AM

got the 6" out last night for a couple of hours. for a small scope, it sure takes a LONG time to fully cool down! it seemed to take about as long as my 16" newtonian for the "boiling" to go away on jupiter. as jupiter is sinking lower these days, it was hard to tell how well the scope was taking power. once again, about 100x was the maximum before the image began breaking up. however, the seeing was such, that my 16" was only allowing about 150x, so i think poor conditions and jupiter's low placement in the sky may have been contributing. the 6" SN did do well on the moon. 200x was no problem there, despite the moon being low in the sky. although the moon was "swimming" a bit in the ep, nice detail was visible. jupiter is always more fussy about magnification, so i'm really looking forward to testing the 6" on mars and saturn, which should give me a better idea on how well it takes power...


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