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The colorful Tak Sky 90 II - not.

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#1 gezak22

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Posted 24 February 2015 - 01:56 AM

Hi all,

 

This weekend I took delivery of an Alt-Az mount (after 3 months of waiting) which finally enabled me to use my Tak Sky 90 II in its grab-and-go configuration. I was a bit surprised by the outcome and I wanted to share my experience to counter the 'conventional wisdom' that the Tak shows lots of false color.

 

I acquired the Tak three years ago as a narrowband astrograph and it performed those duties very well. But eventually dark nebulas caught my attention and I quickly learned that the doublet was not very happy imaging L data (bloated stars, ...). The solution was simple but expensive: get a TV127is, and sell the Tak to offset the cost. The first part - buying the 127is - was simple, but selling the Tak proved impossible due to a defect in the front element - a coating defect 1x2 mm^2 in size at the extreme outer edge of the front lens. After a couple of days on the market without any interest at all, I decided to do the next best thing - keep the Tak as a visual instrument when the weather isn't good enough for imaging. So I sell the Tak's photo accessories (CAA, FR) and use the money to buy some eyepieces and an Alt-Az mount.

 

Today the weather clears and after letting the scope cool down for an hour I take a look at the moon. At 75x, there was an obvious yellow fringe along the edge of the limb. I swing over to Jupiter and I see a red fringe on one side and a blue fringe on the other. Rotating the eyepiece (ES 6.7mm 82 degree) did nothing to the CA, so I was certain it was coming from the scope. I was not let down a lot. After all, there are numerous accounts of the Tak's poor color correction all over the internet. Returning to the moon, I adjusted the eyepiece position using one of the Tak's adapters and voila, the CA is gone. Jupiter too had lost its red/blue cast. Only when I positioned my eye well off center from the eyepiece did the CA reappear. I guess the CA was due to the diagonal+eyepiece being pushed off center from the light cone, not from the lenses. It made me think how many observers critiqued the Tak's color correction without checking if there wasn't anything crooked with the light path.

 

Of course seeing was poor, so I after 10 minutes of destroying my night vision on the moon I called it a night.

 

This post isn't meant to prop up the Tak doublet as a color-free instrument. I just wanted to share my subjective experience and cast some doubt on the 'common knowledge' that the Tak doublet shows every object as a rainbow. The scope certainly has its shortcomings (price) but when you consider the target audience (travelers) it's hard to fault the little scope.

 

And I am well aware that I only went up to 75x. There is no doubt in my mind that eventually some CA will show up at higher powers especially when compared to scopes with a longer focal length.

 

Bottom line: I'm glad I kept the Tak!



#2 LewisM

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Posted 24 February 2015 - 04:54 AM

Wouldn't a UV/IR cut filter help with minimising bloat in the Lum?

 

One thing I have learned about Taks is if you want ti to work properly, use their stuff. For example, an undercut nosepiece will NOT work well in a Tak EP holder - in fact, the WO ones simply flop around (besides, the Tak prism diagonal is SUPERB and rates in the top 5 diagonals)

 

I admit to never liking the Sky 90, but might look through one with all-Tak gear one day and see if it changes my mind a little. No intention of ever owning one, as it does not fill any niche for me. I am also not a lover of the FS-60 due to the severe colour fringing on bright objects (even using Tak gear)



#3 roadi

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Posted 24 February 2015 - 07:21 AM

Hi all,

 

This weekend I took delivery of an Alt-Az mount (after 3 months of waiting) which finally enabled me to use my Tak Sky 90 II in its grab-and-go configuration. I was a bit surprised by the outcome and I wanted to share my experience to counter the 'conventional wisdom' that the Tak shows lots of false color.

 

I acquired the Tak three years ago as a narrowband astrograph and it performed those duties very well. But eventually dark nebulas caught my attention and I quickly learned that the doublet was not very happy imaging L data (bloated stars, ...). The solution was simple but expensive: get a TV127is, and sell the Tak to offset the cost. The first part - buying the 127is - was simple, but selling the Tak proved impossible due to a defect in the front element - a coating defect 1x2 mm^2 in size at the extreme outer edge of the front lens. After a couple of days on the market without any interest at all, I decided to do the next best thing - keep the Tak as a visual instrument when the weather isn't good enough for imaging. So I sell the Tak's photo accessories (CAA, FR) and use the money to buy some eyepieces and an Alt-Az mount.

 

Today the weather clears and after letting the scope cool down for an hour I take a look at the moon. At 75x, there was an obvious yellow fringe along the edge of the limb. I swing over to Jupiter and I see a red fringe on one side and a blue fringe on the other. Rotating the eyepiece (ES 6.7mm 82 degree) did nothing to the CA, so I was certain it was coming from the scope. I was not let down a lot. After all, there are numerous accounts of the Tak's poor color correction all over the internet. Returning to the moon, I adjusted the eyepiece position using one of the Tak's adapters and voila, the CA is gone. Jupiter too had lost its red/blue cast. Only when I positioned my eye well off center from the eyepiece did the CA reappear. I guess the CA was due to the diagonal+eyepiece being pushed off center from the light cone, not from the lenses. It made me think how many observers critiqued the Tak's color correction without checking if there wasn't anything crooked with the light path.

 

Of course seeing was poor, so I after 10 minutes of destroying my night vision on the moon I called it a night.

 

This post isn't meant to prop up the Tak doublet as a color-free instrument. I just wanted to share my subjective experience and cast some doubt on the 'common knowledge' that the Tak doublet shows every object as a rainbow. The scope certainly has its shortcomings (price) but when you consider the target audience (travelers) it's hard to fault the little scope.

 

And I am well aware that I only went up to 75x. There is no doubt in my mind that eventually some CA will show up at higher powers especially when compared to scopes with a longer focal length.

 

Bottom line: I'm glad I kept the Tak!

There was very slight violet to be seen on jupiter above 50x pr. inch in the sky90 I had. It was in no way distracting the visual details to be seen. This CA was only very slight and only on the bright jupiter. I've had razor sharp and contrasty views of Mars above 200x mags, polar cap, syrtis etc. And one of the amazing things about it is that even at 250x on Mars it didn't seem to go out of light at all.

The sky90 is one scope I realy do regret selling more so than my former FS102. It packs alot of power in such a small package, almost pocket size :)



#4 BillP

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Posted 24 February 2015 - 10:52 AM

At 75x, there was an obvious yellow fringe along the edge of the limb. I swing over to Jupiter and I see a red fringe on one side and a blue fringe on the other. Rotating the eyepiece (ES 6.7mm 82 degree) did nothing to the CA, so I was certain it was coming from the scope. I was not let down a lot. After all, there are numerous accounts of the Tak's poor color correction all over the internet. Returning to the moon, I adjusted the eyepiece position using one of the Tak's adapters and voila, the CA is gone. Jupiter too had lost its red/blue cast. Only when I positioned my eye well off center from the eyepiece did the CA reappear. I guess the CA was due to the diagonal+eyepiece being pushed off center from the light cone, not from the lenses. It made me think how many observers critiqued the Tak's color correction without checking if there wasn't anything crooked with the light path.

 

IMO probably a good number of CA reports from users are due to this issue.  After my diagonal test I was astounded how so many of them were not square.  When I got the diagonal square, then the 2" to 1.25" adapter would also not always sit square.  Basically compression rings and undercuts make the process less precise.  Many times for the testing I had to seat the diagonal out so the compression ring mated outside the undercut to get good alignment.



#5 Erik Bakker

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Posted 24 February 2015 - 12:14 PM

I think to really judge the high power performance of the Sky 90 it needs to be used in conjunction with the Extender Q. They were intended to work as a team for high powers.

 

The size and beauty of the Sky 90 struck me when I first saw it in person 2 years ago. Pictures just don't do it justice. 



#6 gezak22

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Posted 24 February 2015 - 10:51 PM

Wouldn't a UV/IR cut filter help with minimising bloat in the Lum?

Perhaps, but even with the bloating gone, the stars at the edges didn't look perfect. Part of my 30-year-plan was to keep the Tak for <5 years before upgrading to a 127is and finally moving to someting with 1000-1500 mm focal length. Since I'm still young (31) and have no dependents (and no plans for any) I decided to purchase the 127is 2 years ahead of schedule. Since I already had the Mach1, this was an easy upgrade.

 

 

I think to really judge the high power performance of the Sky 90 it needs to be used in conjunction with the Extender Q. They were intended to work as a team for high powers.

 

The size and beauty of the Sky 90 struck me when I first saw it in person 2 years ago. Pictures just don't do it justice. 

Thanks for reminding me. If I'm reading it correctly, this review points to CA even when the ExtenderQ is used.



#7 joseph daukantas

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Posted 24 February 2015 - 11:10 PM

GLAD I HAVE QUESTAR FOR MY GRAB AND GO SCOPE :cool:  

 

Joe D.



#8 plyscope

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Posted 24 February 2015 - 11:49 PM

This review might prove to be more sympathetic.

 

http://www.astrosurf..._sky90_eng.html


Edited by plyscope, 25 February 2015 - 06:04 AM.


#9 Paul G

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Posted 25 February 2015 - 06:29 AM

 

Wouldn't a UV/IR cut filter help with minimising bloat in the Lum?

Perhaps, but even with the bloating gone, the stars at the edges didn't look perfect. Part of my 30-year-plan was to keep the Tak for <5 years before upgrading to a 127is and finally moving to someting with 1000-1500 mm focal length. Since I'm still young (31) and have no dependents (and no plans for any) I decided to purchase the 127is 2 years ahead of schedule. Since I already had the Mach1, this was an easy upgrade.

 

 

I think to really judge the high power performance of the Sky 90 it needs to be used in conjunction with the Extender Q. They were intended to work as a team for high powers.

 

The size and beauty of the Sky 90 struck me when I first saw it in person 2 years ago. Pictures just don't do it justice. 

Thanks for reminding me. If I'm reading it correctly, this review points to CA even when the ExtenderQ is used.

 

 

One of the Japanese astro magazine Temmon Guide's renowned equipment reviews covered the Sky 90. They said the ExtenderQ should be a required accessory and reduces the CA to the level of the FS series. I have a copy of the issue, could scan the pages if someone wants to translate it if it doesn't violate copyright laws. I was told by a Japanese bookstore in New York that the magazine is no longer published, not sure if that is true and, if it is, what effect that has on copyright issues.



#10 Nuphy

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Posted 25 February 2015 - 08:24 AM

Temmon (or Tenmon) Guide is still published.



#11 jrbarnett

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Posted 25 February 2015 - 11:27 AM

Hi all,

 

This weekend I took delivery of an Alt-Az mount (after 3 months of waiting) which finally enabled me to use my Tak Sky 90 II in its grab-and-go configuration. I was a bit surprised by the outcome and I wanted to share my experience to counter the 'conventional wisdom' that the Tak shows lots of false color.

 

I acquired the Tak three years ago as a narrowband astrograph and it performed those duties very well. But eventually dark nebulas caught my attention and I quickly learned that the doublet was not very happy imaging L data (bloated stars, ...). The solution was simple but expensive: get a TV127is, and sell the Tak to offset the cost. The first part - buying the 127is - was simple, but selling the Tak proved impossible due to a defect in the front element - a coating defect 1x2 mm^2 in size at the extreme outer edge of the front lens. After a couple of days on the market without any interest at all, I decided to do the next best thing - keep the Tak as a visual instrument when the weather isn't good enough for imaging. So I sell the Tak's photo accessories (CAA, FR) and use the money to buy some eyepieces and an Alt-Az mount.

 

Today the weather clears and after letting the scope cool down for an hour I take a look at the moon. At 75x, there was an obvious yellow fringe along the edge of the limb. I swing over to Jupiter and I see a red fringe on one side and a blue fringe on the other. Rotating the eyepiece (ES 6.7mm 82 degree) did nothing to the CA, so I was certain it was coming from the scope. I was not let down a lot. After all, there are numerous accounts of the Tak's poor color correction all over the internet. Returning to the moon, I adjusted the eyepiece position using one of the Tak's adapters and voila, the CA is gone. Jupiter too had lost its red/blue cast. Only when I positioned my eye well off center from the eyepiece did the CA reappear. I guess the CA was due to the diagonal+eyepiece being pushed off center from the light cone, not from the lenses. It made me think how many observers critiqued the Tak's color correction without checking if there wasn't anything crooked with the light path.

 

Of course seeing was poor, so I after 10 minutes of destroying my night vision on the moon I called it a night.

 

This post isn't meant to prop up the Tak doublet as a color-free instrument. I just wanted to share my subjective experience and cast some doubt on the 'common knowledge' that the Tak doublet shows every object as a rainbow. The scope certainly has its shortcomings (price) but when you consider the target audience (travelers) it's hard to fault the little scope.

 

And I am well aware that I only went up to 75x. There is no doubt in my mind that eventually some CA will show up at higher powers especially when compared to scopes with a longer focal length.

 

Bottom line: I'm glad I kept the Tak!

I doubt that anyone reporting CA in the Sky 90 was operating at 75x, as you've guessed.  When I owned one, it's color correction under circumstances that challenge color correction (high magnification on bright targets) was quite poor.  I also had (and still have) an FS-78 at the time I owned the Sky 90, and side by side the poor color correction of the Sky 90 was pretty blatant.

 

So while 75x didn't show you false color, 150x certainly would have, but not in the FS-78.  I don't think your experience in any way conflicts with the "common knowledge" of this scope's poor color correction.  You simply didn't test for false color in a manner likely to reveal false color.  By analogy, the old 911s were notorious for oversteer.  That is the rear bias due to their rear engine, a heavy flat-6, would cause the tail to come around quickly in corners unless you were diligent in applying enough opposite steer to reel the tail back in.  Testing for false color at just 75x on the Moon and concluding that there was none would be like testing an old 911 for oversteer by running it in a straight line on a drag strip.

 

The false color isn't all that bad in a 6" f/5.9 achromat at 75x, but believe me it has plenty of false color.  Much more than the Sky 90.  The flaw in your conclusion lies in the inadequacy of your test methods IMO.

 

Regards,

 

Jim



#12 gezak22

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Posted 25 February 2015 - 05:33 PM

I doubt that anyone reporting CA in the Sky 90 was operating at 75x, as you've guessed.  When I owned one, it's color correction under circumstances that challenge color correction (high magnification on bright targets) was quite poor.  I also had (and still have) an FS-78 at the time I owned the Sky 90, and side by side the poor color correction of the Sky 90 was pretty blatant.

 

 

So while 75x didn't show you false color, 150x certainly would have, but not in the FS-78.  I don't think your experience in any way conflicts with the "common knowledge" of this scope's poor color correction.  You simply didn't test for false color in a manner likely to reveal false color.  By analogy, the old 911s were notorious for oversteer.  That is the rear bias due to their rear engine, a heavy flat-6, would cause the tail to come around quickly in corners unless you were diligent in applying enough opposite steer to reel the tail back in.  Testing for false color at just 75x on the Moon and concluding that there was none would be like testing an old 911 for oversteer by running it in a straight line on a drag strip.

 

The false color isn't all that bad in a 6" f/5.9 achromat at 75x, but believe me it has plenty of false color.  Much more than the Sky 90.  The flaw in your conclusion lies in the inadequacy of your test methods IMO.

 

Regards,

 

Jim

 

As I noted in the original post, the goal was not to label the Sky 90 as color free, but to speculate how much of the CA reported by others was actually due to non-square components in the light path.



#13 jrbarnett

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Posted 27 February 2015 - 05:36 PM

 

I doubt that anyone reporting CA in the Sky 90 was operating at 75x, as you've guessed.  When I owned one, it's color correction under circumstances that challenge color correction (high magnification on bright targets) was quite poor.  I also had (and still have) an FS-78 at the time I owned the Sky 90, and side by side the poor color correction of the Sky 90 was pretty blatant.

 

 

So while 75x didn't show you false color, 150x certainly would have, but not in the FS-78.  I don't think your experience in any way conflicts with the "common knowledge" of this scope's poor color correction.  You simply didn't test for false color in a manner likely to reveal false color.  By analogy, the old 911s were notorious for oversteer.  That is the rear bias due to their rear engine, a heavy flat-6, would cause the tail to come around quickly in corners unless you were diligent in applying enough opposite steer to reel the tail back in.  Testing for false color at just 75x on the Moon and concluding that there was none would be like testing an old 911 for oversteer by running it in a straight line on a drag strip.

 

The false color isn't all that bad in a 6" f/5.9 achromat at 75x, but believe me it has plenty of false color.  Much more than the Sky 90.  The flaw in your conclusion lies in the inadequacy of your test methods IMO.

 

Regards,

 

Jim

 

As I noted in the original post, the goal was not to label the Sky 90 as color free, but to speculate how much of the CA reported by others was actually due to non-square components in the light path.

 

But your title is "The Colorful Tak Sky 90 II - not".  

 

It's the "not" that I quibble with.  Even in perfect condition this scope puts up plenty of false color when used in a manner that tests color correction, ergo I disagree with the "not".  There's no doubt that sub-par mechanical condition can affect optical performance.  But this scope has color correction issues wholly independent of such mechanical ones that cannot be repaired or tuned out.

 

I'd probably leave the title as ""The Colorful Tak Sky 90 II".  :grin:

 

- Jim 


Edited by jrbarnett, 27 February 2015 - 05:37 PM.


#14 Astrojensen

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Posted 27 February 2015 - 06:16 PM

 

After my diagonal test I was astounded how so many of them were not square.  When I got the diagonal square, then the 2" to 1.25" adapter would also not always sit square.  Basically compression rings and undercuts make the process less precise.  Many times for the testing I had to seat the diagonal out so the compression ring mated outside the undercut to get good alignment.

There's a very good reason Zeiss was so fond of threaded adapters and accesories and ring-dovetail adapters. It's impossible for it not to sit square and precise every time (unless you really mess them up). Collimation is never an issue. It's also rock solid. 

 

 

Clear skies!

Thomas, Denmark



#15 gezak22

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Posted 27 February 2015 - 11:07 PM

But your title is "The Colorful Tak Sky 90 II - not".  

 

It's the "not" that I quibble with.  Even in perfect condition this scope puts up plenty of false color when used in a manner that tests color correction, ergo I disagree with the "not".  There's no doubt that sub-par mechanical condition can affect optical performance.  But this scope has color correction issues wholly independent of such mechanical ones that cannot be repaired or tuned out.

 

I'd probably leave the title as ""The Colorful Tak Sky 90 II".   :grin:

 

- Jim 

 

You caught me. Yes, I admit to having lured people into the thread with a controversial title. :) I was just trying leave a footprint for other visual Tak Sky users to check that their compoents are square. The level of CA I saw at 75x was quite obvious and I worried that more naiive users might have dumped the scope without a second thought.

 

Once my Tak Sky logs some airplane miles during a trip in May, I might replace it with a bit more aperture in the form of a 4" ES triplet. But I still need to see how well the Tak performs in excellent seeing before I'm willing to pull the trigger.

 

There's a very good reason Zeiss was so fond of threaded adapters and accesories and ring-dovetail adapters. It's impossible for it not to sit square and precise every time (unless you really mess them up). Collimation is never an issue. It's also rock solid. 

 

 

Clear skies!

Thomas, Denmark

 

This is precisely why I bought the Tak Sky for narrowband imaging. The only remaining competition was the TMB92SS and I worried that the TMB's lack of a dedicated flattener would mean that I would have to use at least one non-threaded component. For visual though, I think the 92SS would have been the better choice.



#16 etsleds

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Posted 02 March 2015 - 01:27 PM

I'll take a quibble with this that I'm not convinced that folk who speak so authoritatively about the color of a Sky 90 have ever seen a perfectly aligned one.  After owning one and having seen two others, I had started to wonder whether a Sky 90 ever stays aligned for very long, but my very late model second sample has held up well after a lot of initial tweaking.

 

I'm not aware of another 90mm-ish, wide air-spaced, high quality doublet in this focal ratio range to compare it to empirically with a visual test.  I imagine it must be modeled somewhere?

 

For occasional planetary use, I'm generally using a Pentax 2.58mm XO or a Nagler 3-6mmm, which puts me up into the 85-200x range.  I don't find the false color particularly strong - in a frame of reference I'm familiar with, in terms of false color, it's closer to FS102 than to FS60.

 

I'm always a little curious how it would perform if I had this level of collimation & figure:

 

http://r2.astro-fore...ntrier-sensibel

 

 

 

 

....

 

It's the "not" that I quibble with.  Even in perfect condition this scope puts up plenty of false color when used in a manner that tests color correction, ergo I disagree with the "not".  There's no doubt that sub-par mechanical condition can affect optical performance.  But this scope has color correction issues wholly independent of such mechanical ones that cannot be repaired or tuned out.

 

I'd probably leave the title as ""The Colorful Tak Sky 90 II".   :grin:

 

- Jim 

 


Edited by etsleds, 02 March 2015 - 01:56 PM.


#17 roadi

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Posted 02 March 2015 - 07:14 PM

 

I'll take a quibble with this that I'm not convinced that folk who speak so authoritatively about the color of a Sky 90 have ever seen a perfectly aligned one.  After owning one and having seen two others, I had started to wonder whether a Sky 90 ever stays aligned for very long, but my very late model second sample has held up well after a lot of initial tweaking.

 

I'm not aware of another 90mm-ish, wide air-spaced, high quality doublet in this focal ratio range to compare it to empirically with a visual test.  I imagine it must be modeled somewhere?

 

For occasional planetary use, I'm generally using a Pentax 2.58mm XO or a Nagler 3-6mmm, which puts me up into the 85-200x range.  I don't find the false color particularly strong - in a frame of reference I'm familiar with, in terms of false color, it's closer to FS102 than to FS60.

 

I'm always a little curious how it would perform if I had this level of collimation & figure:

 

http://r2.astro-fore...ntrier-sensibel

 

 

 

 

....

 

It's the "not" that I quibble with.  Even in perfect condition this scope puts up plenty of false color when used in a manner that tests color correction, ergo I disagree with the "not".  There's no doubt that sub-par mechanical condition can affect optical performance.  But this scope has color correction issues wholly independent of such mechanical ones that cannot be repaired or tuned out.

 

I'd probably leave the title as ""The Colorful Tak Sky 90 II".   :grin:

 

- Jim 

 

 

I believe you are pretty much spot on ;)
The trick is when collimating this scope.. "patience, steady fingers and good seeing or an artificial star" while raising magnification as high as posible, preferably higher than one would use in general. The lenses are extremely sensitive to the slightes touch of the alignment screws, but once its done it will perform, mine did.

Too bad that I'm not near the OP as I would more than gladly exchange a new ES 102 triplet for his used "if in mint conditioned Sky90"


Edited by roadi, 02 March 2015 - 07:16 PM.


#18 gezak22

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Posted 04 March 2015 - 01:08 AM

When I bought my Sky 90 II (used) it was completely out of collimation and I believe that it's a crime to use a scope when it's out of collimation. So I built an artificial star using an optical fiber and adjusted the centering of the lens. Since then I haven't had to redo it.

 

I've had more time to evaluate the scope on the moon, Jupiter, and Saturn at 83x (6mm Baader Phantom ortho) and at 120x (4mm UO HD ortho) though seeing has only been mediocre. Jupiter's GRS is a bear to catch. Of the three chances I've had, I've only succeeded once. I'm certain it was not my imagination as in all instances I had no idea if there even was a GRS transit. Shadow transits on the other hand are a real treat - pitch black spots readily visible at 75x.

 

The Cassini division is much easier to see than the GRS, but it's not a clear separation in the ring, just a darkening. I believe Saturn's low elevation at the time contributed in some part to this.

 

Besides collimation and making sure that the elements are square (flush against each other), I've also noticed the following two items that help in minimizing false color.

1. Nailing focus, obviously. But I'm highlighting this because this is a pretty fast instrument, and for me, focus does not snap so it takes a couple of attempts - and I use the Feathertouch focuser! If you switch between EPs a lot, this gets very annoying.

2. Centering the eye. If my eye is off-center from the eyepiece !BOOM! red/blue/yellow edges on bright objects. Also, having the entire lunar disk in the FOV will show color around the full edge of the disk, but once an edge is centered in the FOV the color at that edge disappears.

 

The second point is a bit of a pain as the 6.7mm ES with its 82 deg FOV is great for 'lazy' sweeping while keepig the eye at a comfortable distance from the EP. The problem is that in order to take advantage of the FOV, I have to take my eye off center and this results in some false color.

 

But I'm very happy with the performance at the center of the FOV in the 6mm Baader Phantom ortho and the 4mm UO HD ortho.

 

Too bad that I'm not near the OP as I would more than gladly exchange a new ES 102 triplet for his used "if in mint conditioned Sky90"

My sample is certainly beyond the 'mint'-flavored variety but it's not abused either. See the attached picture of the defect in the front lens. Does it matter? Photographically, I think it does, as shown by the extra diffraction spike in the inset. Disclosing the defect and the effect it had was probably what killed the used market's interest in my scope, so I just kept it, and I'm happy with it.

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#19 Peter Besenbruch

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Posted 04 March 2015 - 01:40 PM

I thought I would throw in a couple of observations with an 80mm ED doublet.

I've had more time to evaluate the scope on the moon, Jupiter, and Saturn at 83x (6mm Baader Phantom ortho) and at 120x (4mm UO HD ortho) though seeing has only been mediocre. Jupiter's GRS is a bear to catch. Of the three chances I've had, I've only succeeded once. I'm certain it was not my imagination as in all instances I had no idea if there even was a GRS transit. Shadow transits on the other hand are a real treat - pitch black spots readily visible at 75x.

 

The Red Spot is easy, as is the white ring framing it. So are gradations within the spot. I tend to use higher magnifications than you. Shadow transits are a different story. Io transits result in a tiny dot, and it appears slightly brownish. Ganymede transits result in a much larger, black dot. I attribute the brownish color of Io's shadow to the 80mm's lack of resolution. Your difficulty with the Red Spot sounds like a contrast issue, brought on  either by the poor seeing, or your optical system.
 

The Cassini division is much easier to see than the GRS, but it's not a clear separation in the ring, just a darkening. I believe Saturn's low elevation at the time contributed in some part to this.

 

Cassini's is pretty easy and blacker, but Saturn's elevation may have been higher from where I viewed.

 

 

2. Centering the eye. If my eye is off-center from the eyepiece !BOOM! red/blue/yellow edges on bright objects. Also, having the entire lunar disk in the FOV will show color around the full edge of the disk, but once an edge is centered in the FOV the color at that edge disappears.

 

That is strange, and I have not encountered such a phenomenon. The closest eyepiece I have to your ES82 6.7mm is my 9mm Nagler Type 1. I also have a bunch of Hyperions. At shorter focal lengths, there is a bit of blue fringing near the field stop on bright, extended objects (think the moon) if you don't position your eye just so. The effect is pretty minor. Your effect sounds more "fun."




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