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8" F/5 Newt palnetary and coma

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#51 Jon Isaacs

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Posted 06 July 2019 - 03:34 AM

I have a 200 BKP which is similar, I don't find it capable for using 300x for the planets or the moon. I had good results once using a 7mm orthoscopic and a 2x barlow giving at 286x and a 0.7mm exit pupil on the moon. Otherwise the images are simply too dark for my taste.

 

I think a bigger Newtonian would be more suited for powers like 300x, like a 12".

 

300x in an 8 inch is a 0.7 mm exit pupil or about 37.5x /inch.  Even my 70 year old eyes can view the planets at magnification levels and more, provided the seeing supports it.

 

I consider 300 x fine for an 8 inch..

 

5372150-jon with the Konus.jpg
 
My 8 inch F/5 Synta/Konus with the Rotating Rings.
 
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#52 JP-Astro

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Posted 06 July 2019 - 05:57 AM

Thanks again for another clarification to everyone. Now I'm confident my choice would be reasonable.



#53 N3p

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Posted 06 July 2019 - 07:14 AM

I agree concerning the Xcel LX eyepieces I think the are pretty good eyepieces with my F5, except for the 25mm. I started with these and I still use them often, the 9mm, 12mm and 18mm, the 5mm was nice too. They give me great satisfaction without breaking the bank.


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#54 CHASLX200

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Posted 06 July 2019 - 08:17 AM

 

300x in an 8 inch is a 0.7 mm exit pupil or about 37.5x /inch.  Even my 70 year old eyes can view the planets at magnification levels and more, provided the seeing supports it.

 

I consider 300 x fine for an 8 inch..

 

 
 
My 8 inch F/5 Synta/Konus with the Rotating Rings.
 
Jon

 

300x is easy in my seeing. I have used 550x with many very good old school 8" Newts.  A top built 8" F/5 will do great on the planets as long as as everything is working right and collimated.


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#55 JP-Astro

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Posted 06 July 2019 - 09:14 AM

 

...

My 8 inch F/5 Synta/Konus with the Rotating Rings.
 
Jon

 

Jon - what brand and design are those rotating rings on your Newt? If they are commercial is there a way to reproduce them via DIY with the help of a machinist?



#56 Jon Isaacs

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Posted 06 July 2019 - 08:28 PM

I believe they were Antares.  They were heavy and pushed the weight of the scope out. Requiring more counterweights.

 

Soon enough I modified the existing rings with Teflon, an added upper ring and some grinding. These were lighter, worked as well 

 

But recently my friend Jack (makeitso) made some rings patterned after some thing he found here on CN. Very simple, a large hose clamp, part of a 5 gallon bucket and some grinding. This works great. And looks good too.

 

Jacks Rotating Rings.jpg
 
Jon

 


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#57 Richard Whalen

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Posted 08 July 2019 - 10:34 PM

How much magnification you can use depends on your optical quality, seeing and your eyesight and aperture. With my 8" scope I am often around 350x to 450x on Jupiter, and 525x on Saturn. Sometimes higher when conditions are perfect.

 

My rule of thumb is 43x the aperture in inches on a very good night with decent optics, higher for very good or excellent optics. Also much depends on which planet you are observing.


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#58 Asbytec

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Posted 10 July 2019 - 04:33 AM

How much magnification you can use depends on your optical quality, seeing and your eyesight and aperture. With my 8" scope I am often around 350x to 450x on Jupiter, and 525x on Saturn. Sometimes higher when conditions are perfect.

 

My rule of thumb is 43x the aperture in inches on a very good night with decent optics, higher for very good or excellent optics. Also much depends on which planet you are observing.

Richard, I am usually between 333x and 400x on Jupiter in my 8", as well, at 0.6mm and 0.5mm exit pupil. I find 333x (~40x per inch) power to be the most productive and my rule of thumb, as well. At 400x, Jupiter is still workable, but it's beginning to dim a little. I was looking at Oval BA the other night, it was easy at 333x. I could see it at 400x, but not as easily.  And I am fairly sure at 500x it would have been even more difficult. I accidentally pulled out the wrong eyepiece and hit 1200x once (0.16mm exit pupil!). Not much to see up that high. I guess my optics are not that good. smile.gif

 

I get that the quality of our optics produce nice sharp and high contrast images at high power, after all it's the same quality image we see at less magnification where (lack of) aberration is apparent in terms of resolution and contrast. But I am always interested in the mechanism of how high quality optics can afford higher magnifications at vanishingly small exit pupils, say a bit smaller than 0.5mm, without excessive image dimming. At some point we begin to lose visual sensitivity and, thus, lose the image itself as the eye is working at a very small relative aperture (less than about 0.5mm f/60).

 

Getting closer to 600x on Jupiter, IME, is unworkable (or at least not as productive as a bit less magnification) in any 8" aperture even in good seeing. I mean, we can still see some detail up that high, I saw some detail at 1200x, too. Just not much detail was perceived by the eye, even though we are viewing the same fine afocal image we observed at 400x and less. At some point, it becomes less about the optics and more about the exit pupil and, I suspect, throughput as well.

 

For example, Jove is fine on both 6" Mak and 8" Newt at 0.6mm exit pupil, (240x and 333x, respectively). But, at 0.5mm exit pupil, the Mak image is unworkable while the Newt image still had some legs. I suspect this has something to do with the throughput of each scope, not so much about their respective quality or difference in aperture. Of course the 8" image is brighter, thus affording higher magnification than the 6". They are pretty close to the same level of quality, not premium but pretty good and roughly the same obstruction. Both were thermally stable and well collimated. Seeing varied from above average to very good in both over time.  (I agree with you in another thread when you talked about stray light control and mechanics, too.) 

 

But, when I hear folks talk about quality optics affording higher magnification, I am always reminded of the small exit pupil involved and how quality might over come the inverse square law and our own personal level of acuity (as a variable). Unless you or they mean magnification higher than say 1mm exit pupil when poor optics start to become visually and visibly soft, while better optics retain their fine imaging properties until the image surface brightness is no longer supported at smaller exit pupils. Sometimes when folks talk about ludicrous magnification in any scope, and especially in premium scopes, I wish they'd elaborate on what they saw up that high. Tight double stars or a bright planetary nebula? 

 

I just do not understand how quality affords higher magnification to smaller than 0.5mm exit pupils (very small relative apertures) and well above the magnification where poor image quality becomes apparent. 


Edited by Asbytec, 10 July 2019 - 04:38 AM.

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#59 CHASLX200

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Posted 10 July 2019 - 05:45 AM

After 500x the image starts to get too dim in a 8". This is where a 14.5" shows it's stuff at 1000x on Jupiter.



#60 Asbytec

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Posted 10 July 2019 - 06:16 AM

After 500x the image starts to get too dim in a 8". This is where a 14.5" shows it's stuff at 1000x on Jupiter.

Chas, I know you have great seeing. My seeing is pretty much the same during our dry season monsoon. So, yea, we're operating at higher magnifications, generally, and on Jupiter, specifically, as well as other objects. I guess that is the crux of my question. Assuming descent optics in both, the 14.5' at 1000x is about the same as an 8" near 550x. In my experience with an 8", the image is less productive starting about 400x and above. Others may vary somewhat, of course.

 

Unless the optics are truly better in the 14.5" in appropriately good seeing. Then my question is why can the higher quality, larger 14.5" aperture show it's stuff at much higher magnification than roughly the equivelent of an 8" showing it's stuff at 400x? The equivelent magnification in the 14.5" would be about 750x, but why does quality allow it to show it's stuff at 1000x (equivelent of 550x in the 8")? I'd love to know what can be seen up that high because, my thoughts are, the 14.5" image is dimming, too, for the same reason the 8" is already dimming at 400x and higher.

 

I've seen the Jovian image at 500x and 600x in the 8", but I would not call it really a great image (on the eye, anyway). There is some detail to be seen, still, and the limb appears to be as sharp. But, a lot of the lower contrast detail is becoming or is already difficult to see. Bright high contrast stuff like double stars are no problem, but Jove is a different animal. It cannot be pushed to ludicrous magnifications, but if it can and optics are the reason, then my question is why and what is seen up that high. A sharp limb, a few belts, the moons, and maybe the GRS? 



#61 CHASLX200

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Posted 10 July 2019 - 06:44 AM

Chas, I know you have great seeing. My seeing is pretty much the same during our dry season monsoon. So, yea, we're operating at higher magnifications, generally, and on Jupiter, specifically, as well as other objects. I guess that is the crux of my question. Assuming descent optics in both, the 14.5' at 1000x is about the same as an 8" near 550x. In my experience with an 8", the image is less productive starting about 400x and above. Others may vary somewhat, of course.

 

Unless the optics are truly better in the 14.5" in appropriately good seeing. Then my question is why can the higher quality, larger 14.5" aperture show it's stuff at much higher magnification than roughly the equivelent of an 8" showing it's stuff at 400x? The equivelent magnification in the 14.5" would be about 750x, but why does quality allow it to show it's stuff at 1000x (equivelent of 550x in the 8")? I'd love to know what can be seen up that high because, my thoughts are, the 14.5" image is dimming, too, for the same reason the 8" is already dimming at 400x and higher.

 

I've seen the Jovian image at 500x and 600x in the 8", but I would not call it really a great image (on the eye, anyway). There is some detail to be seen, still, and the limb appears to be as sharp. But, a lot of the lower contrast detail is becoming or is already difficult to see. Bright high contrast stuff like double stars are no problem, but Jove is a different animal. It cannot be pushed to ludicrous magnifications, but if it can and optics are the reason, then my question is why and what is seen up that high. A sharp limb, a few belts, the moons, and maybe the GRS? 

I can't see see much detail on a small disk.  I find 650x to 800x works best on a 14.5" and a 1000x or more on the best nites.  I use 400x most of the time with the 8" Newt i have.  I also find 400x easy on the SW150ED.  Anything below 250x is just too small for the planets.


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#62 Asbytec

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Posted 10 July 2019 - 06:53 AM

...I also find 400x easy on the SW150ED. 

Yea, I agree. I like a big disc, too, big as I can get it right before it's too dim. I find the comment above interesting and wonder why the 150ED can do 400x like an 8" Newt. Optics? I suspect some high throughput is helping maintain image brightness to survive magnification. This is what I am unsure about. 


Edited by Asbytec, 10 July 2019 - 06:54 AM.

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#63 Starman1

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Posted 10 July 2019 - 09:58 AM

My lifetime-best view of Jupiter in the 12.5" was at 456x (36.5x/inch), and we could see a knotty white swirl in the salmon colored (then, now it's more orange) GRS.

The whole disc looked like the surface of an orb, not a flat disc, and the colors were amazing--ochers, pale ivory, bluish tints, grey-greens, reds, whites, blacks, greys, etc.

It was a technicolor image, and super-sharp--sharp enough we could see the shadows of projections on the cloud banks below.  And an 18 element stack of lenses in the focuser.

Spectacular seeing conditions, obviously.

 

On other nights of superb seeing, I've gone as high as 986x (79x/inch), just to see if it could be done, but I haven't been able to see what I saw that night.

 

The moral of the story is that it is not only optical quality, but seeing that determines how high a magnification we can use.

In absolutely perfect seeing, I've used a superb 7" scope at 160x/inch and the image was OK.  I just couldn't see anything in that scope at 160x/inch

that I couldn't also see at 100x/inch, though the image of Saturn at 1123x was incredibly large.

 

But even after all the crazy high powers, give me 400-500x with spectacular seeing, and I can see details on Ganymede and Neptune.  1000x isn't really necessary.

It's all about the seeing.


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#64 Asbytec

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Posted 10 July 2019 - 10:46 AM

That has to be an amazing view if Jove, Don. My goodness. Cloud shadows? Makes Ganymede look like child's play. Yes, seeing is when the truth comes out.

I was just looking at Saturn at 400x and 500x (8" Newt/Dob) in Antionadi II/III. The rings were sharp at both magnification with Enke min and several dark and bright rings. But I liked the planet better at 400x. A little higher mag than Jove.

Edited by Asbytec, 10 July 2019 - 10:51 AM.


#65 Richard Whalen

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Posted 10 July 2019 - 11:19 AM

Higher quality optics will give a sharper image and greater contrast at higher magnifications. Once you get below .5 exit pupil at some point your individual visual acuity kicks in. Some observers have floaters or other conditions in their eyes that limit them to moderate or large exit pupils. I can go down to just below .25 exit pupil with no floaters or other eye abberations. Also higher quality acts like a buffer zone in larger optics with the seeing keeping small variations of near perfect seeing at bay. 

 

For instance I have never seen encke clearly in any telescope below 600x, best view in my 8" was at 655x and 828x with the higher being easier to see it. Was still a very fine small gap. Never saw it in my 18", though I sure tried enough times. Seeing was just never good enough at my location, and the rings were to edge on when I owned it. Even though it had very good optics for a 18", they were not close to the optical quality of my 8".


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#66 Asbytec

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Posted 10 July 2019 - 06:53 PM

Once you get below .5 exit pupil at some point your individual visual acuity kicks in. Some observers have floaters or other conditions in their eyes that limit them to moderate or large exit pupils. I can go down to just below .25 exit pupil with no floaters or other eye abberations.

 

For instance I have never seen encke clearly in any telescope below 600x, 

Yea, individual acuity plays a role, for sure. Having a sharp and high(er) contrast image as we push higher helps the eye. I guess that combination seems to be the answer.



#67 N3p

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Posted 13 July 2019 - 12:47 PM

Wow you guys are capable of incredible power with a quality 8" telescope, it's amazing but motivating too.

 

De we know the precision of these Synta mirrors such as the 200P or mine the BKP 200? Could their primary be more then 1/4PV ?

 

With my 8" BKP, I really can't see the day where it will give me satisfaction at 285x (7mm with a 2x Barlow) my region is very humid mainly but above that, I am convinced the telescope can't do it at all. For instance, the collimation, I find it limited by the build quality of the telescope, the tube is not perfectly round and because of that, I don't see any way to reach a superior level of collimation.

 

At least not with my current 4 years experience.



#68 Starman1

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Posted 13 July 2019 - 01:30 PM

The roundness of the tube doesn't determine collimation, only the alignment of the mirrors and focuser axis.

Tubes in scopes can be square or even octagonal, after all.

If you have good collimation tools, and use them well, you'll have good collimation, even with a lumpy, bumpy, partially-oval tube.


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#69 N3p

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Posted 13 July 2019 - 06:18 PM

In my case because the tube is not round it's affecting the process of collimation itself. To compensate the tube variations I need to play with the focuser adjusters in order to make everything concentric. Because of that, it's close to 100% sure the right angle between the axis of the focuser and the axis of the telescope is not right or precise, the secondary is most likely not at 45 degrees either.

 

In my opinion, that lack of precision that can't be a good thing for the final results of collimation.

 

Perhaps not highly important but still a limitation, I find this issue serious enough. 

 

I would expect a premium telescope to have a much better precision in the roundness of the tube.


Edited by N3p, 13 July 2019 - 06:19 PM.


#70 jtsenghas

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Posted 13 July 2019 - 07:11 PM

There are some excellent collimation threads in this forum. It matters not at all whether the focuser is exactly perpendicular or secondary is exactly at 45 degrees. All that matters is that the focuser is pointing at the center of the primary via the secondary and the primary is also pointing straight at the focuser via the secondary. That second item is typically the more critical item.

 

If the optical axis is not coincident with the tube axis, or the tube isn't round, or the angles a little different to compensate for any accumulation of such errors it matters not at all for precise collimation. There are so called "low rider" scopes out there with the secondary angled up to another 15 degrees and the focuser angled upwards 30 degrees that are perfectly collimated.

 

If, however, you want excellent GOTO finding and tracking then it helps to have both scope axes precisely perpendicular to each other... 


Edited by jtsenghas, 13 July 2019 - 07:15 PM.

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#71 Vic Menard

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Posted 13 July 2019 - 08:14 PM

...If, however, you want excellent GOTO finding and tracking then it helps to have both scope axes precisely perpendicular to each other... 

Actually, for "excellent GOTO finding and tracking", the scope's axes (specifically, the focuser axis relative to the primary mirror axis) don't need to be perpendicular to each other, but the scope's optical/pointing axis should be perpendicular to the altitude/declination rotational axis, which should also be perpendicular to the azimuth /right ascension rotational axis. Also note that, while perpendicularity is important for orthogonality (which delivers optimal GOTO finding and tracking), it's not necessary for any of the axes to actually intersect each other. 



#72 jtsenghas

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Posted 13 July 2019 - 10:12 PM

Actually, for "excellent GOTO finding and tracking", the scope's axes (specifically, the focuser axis relative to the primary mirror axis) don't need to be perpendicular to each other, but the scope's optical/pointing axis should be perpendicular to the altitude/declination rotational axis, which should also be perpendicular to the azimuth /right ascension rotational axis. Also note that, while perpendicularity is important for orthogonality (which delivers optimal GOTO finding and tracking), it's not necessary for any of the axes to actually intersect each other. 

Yes, I meant the two axes for motion need to be perpendicular to each other. I wasn't referring to focuser and primary axes which, as I said, must merely be pointing at each other via the secondary. For example, the Low Rider dobs with their focusers pointing upward at a higher secondary (in order to keep the observers feet on the ground) could have GOTO finding and tracking provided the scope axes (altitude and azimuth) are perpendicular. 

 

I guess I didn't make myself clear. I was trying to say that for excellent collimation nothing really needs to be square, but for dialing up coordinates the two axes of motion must be perpendicular to each other 



#73 Vic Menard

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Posted 14 July 2019 - 06:44 AM

...I guess I didn't make myself clear. I was trying to say that for excellent collimation nothing really needs to be square, but for dialing up coordinates the two axes of motion must be perpendicular to each other 

The two axes of motion and the optical/pointing axis (three axes total--if you hope to have the dialed up object centered in the field of view).




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