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What aberrations is the original orange tube C90 subject to, if any?

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#26 wfj

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Posted 05 April 2021 - 11:52 AM

I did star test it. My comments here are based on memory of events several years ago. To the best of my recollection, my C90 produced fairly round and concentric rings inside and outside of focus, The Airy disk, as I remember was more or less overcome by the brightness of the first defraction ring which was also much brighter than those further out. Also, as I remember there weren’t a lot of rings visible, I suppose because so much energy seemed to be concentrated in the first. Overall, they were prettymuch, just outright unspectacular-passible but blah. Certainly nothing like the clean Airy disk and delicate rings of diminishing brightness falling away from center as seen in a good refractor. As I said in my prior comment, it did split εand ε2 but barely; ie. each pair were hardly distinct stars with just the most minute bit of black separation, tho the difference in orientation of the two oblong pairs was easily seen. I think that it was the way that the energy was distributed in and around the Airy disk that hampered the clean separation of the stars. So there was certainly no wow factor; a passing mark at best.

Thank you for responding, I didn't know.

Your description suggests significant SA - did you do the "breakout test"? Where you measure the focuser change between the in focus to secondary shadow appearance(breakout), and compare to the the out focus breakout focuser change? If they differ, there's a measure of SA. If a 20%+ difference, it's likely not 1/4 wave.

 

I think you had one with an improper aspherization. It was there, but not done/tested correctly.

 

Also, there's the appearance of zones if as you go in/out and the brightness/gap of a ring shifts. Many CAT's have zones, usually because of lazy figuring or someone tried to "fix" a zone with a subdiameter tool (often a finger) and just split the bad zone by hitting the peak. I've seen this in refractors too, especially your favorite Towa's.

 

Hanging around John Dobson for a few hours decades back was worthwhile, even enduring his crazy talk about physics ...


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#27 wfj

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Posted 05 April 2021 - 12:26 PM

PS- I found the dew shield (I will call it that really out of convention rather than utility) to be important more in blocking stray light rather than stray moisture. The corrector plate being exposed at the very front of the telescope seemed susceptible to picking up contrast robbing stray light, and given that my neighbors often have their unshielded porch lights on, I would get reflections at times as well. The dew shield that I fashioned was about 6” in depth from its outer edge to the surface of the corrector plate and eliminated the problem entirely. It seemed to make a noticeable improvement in contrast and overall sky darkness.

Had same before baffle mod. The secondary baffle is made of shiny styrene. It's intended for terrestrial light from the side of the OTA tube to be blocked, and with the truncated "cap stop" of the primary baffle is an ineffectual compromise for both, as they can reflect oblique light. Microbaffles in the tube top near corrector, a narrow stop black band in the secondary (only one inch of the 1.375" spot is used, that's the ID of the removed baffle), a carefully calculated longer cone baffle with stops ... means no reflections and better contrast.

 

When people flock a C90, they get some of this benefit, but they don't cure the reflections. Doesn't get the internal reflections of the exposed corrector are incident on the flat of the original primary baffle/stop, and glance off the inside of the secondary baffle. To paraphrase Elon Musk, the best secondary baffle is no baffle.

 

Can test this during full moon or a light source, at different angles from the source. Look for the glow/reflections. Proof the baffle design is correctly calculated.

 

This is just another aspect of a nice design hobbled by poor QC as Celestron did high volume of them.


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#28 Borodog

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Posted 05 April 2021 - 05:07 PM

It seems possible that since this is one of the earliest C90s made that perhaps QC hadn't fallen behind yet.


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#29 wfj

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Posted 05 April 2021 - 07:25 PM

It seems possible that since this is one of the earliest C90s made that perhaps QC hadn't fallen behind yet.

Correct. They probably didn't know how many they could sell if any.

 

They'd handcraft early production, ones that didn't come out well would become the spotter line, and then they'd seed early sales to aficionados for word of mouth as "astro" models that passed muster. Then repeat batches as needed to supply warehouse (didn't do on demand back then).

 

Perhaps the lousy glued mirror and focuser elements were meant to probe market, and when nobody complained, they kept with them rather than be replaced.



#30 Terra Nova

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Posted 05 April 2021 - 07:36 PM

I only did the basic star test no other tests. I did do a star test on several occasions and also looked at Epsilon Lyra more than once and the results never changed. I pretty much lost interest after I got my Questar and sold it not long afterward. I just thought, those times when I decide to observe with a 3.5” Mak, I’m going to use my Questar. 


Edited by Terra Nova, 05 April 2021 - 09:10 PM.

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#31 Terra Nova

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Posted 05 April 2021 - 07:39 PM

Correct. They probably didn't know how many they could sell if any.

 

They'd handcraft early production, ones that didn't come out well would become the spotter line, and then they'd seed early sales to aficionados for word of mouth as "astro" models that passed muster. Then repeat batches as needed to supply warehouse (didn't do on demand back then).

 

Perhaps the lousy glued mirror and focuser elements were meant to probe market, and when nobody complained, they kept with them rather than be replaced.

Mine was an early one. Number 9429. But it was also labeled ‘Mirror Lens’ and was obviously sold as a photographic scope and terrestrial spotter and there was no provision for a finder.


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#32 wfj

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Posted 05 April 2021 - 08:43 PM

I only did the basic star test no other tests. I did do a,star,test on several occasions and also looked at Epsilon Lyra more than once and the results never changed. I pretty much lost interest after I got my Questar and sold it not long afterward. I just thought, those times when I decide to observe with a 3.5” Mak, I’m going to use my Questar. 

Much as I've done with every C90/Q/ETX I've been disappointed by - many.  Have two Maks I find hard to part with, both star test and DPAC 1/8 wave. And both are earliest production. Like most, find it hard to give them up, although my daughter reminds of too many/much scopes/duplication, now under 10 she wants it under 5.

 

The reason I conducted many tests and fixed a bunch of stupid things was that I had a 6mm in it on Jupiter before going to turn it over. Expected a belt or two, got much more. I had heard a long time ago when they first came out fantastic stories, and I'd used a Q back then. Then came 4+ years at Berkeley and govt roles, and peeked through them at star parties as I reentered the field, and they were definitely not Q's. Got to be a habit to "debug" **** the C90 story was about: great scope concept that was cruelly, cynically exploited by a desperate scope vendor trying to stay in business.

 

Mine was an early one. Number 9429. But it was also labeled ‘Mirror Lens’ and was obviously sold as a photographic scope and terrestrial spotter and there was no provision for a finder.

Probably a botched aspherization. Likely many tries to work out production processes and procedures. Unethically sold their mistakes. Cash flow.
 


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#33 RichA

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Posted 05 April 2021 - 09:41 PM

I did star test it. My comments here are based on memory of events several years ago. To the best of my recollection, my C90 produced fairly round and concentric rings inside and outside of focus, The Airy disk, as I remember was more or less overcome by the brightness of the first defraction ring which was also much brighter than those further out. Also, as I remember there weren’t a lot of rings visible, I suppose because so much energy seemed to be concentrated in the first. Overall, they were prettymuch, just outright unspectacular-passible but blah. Certainly nothing like the clean Airy disk and delicate rings of diminishing brightness falling away from center as seen in a good refractor. As I said in my prior comment, it did split εand ε2 but barely; ie. each pair were hardly distinct stars with just the most minute bit of black separation, tho the difference in orientation of the two oblong pairs was easily seen. I think that it was the way that the energy was distributed in and around the Airy disk that hampered the clean separation of the stars. So there was certainly no wow factor; a passing mark at best.

What I never liked about the C90 was the off-axis vignetting, which if you defocus a star and shift it off-axis is readily apparent.


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#34 Terra Nova

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Posted 06 April 2021 - 08:04 AM

I’ve been perhaps too critical on the little orange C90. It did have sharp optics and provided nice bright, crisp views when used as a terrestrial spotter. It also did well on the sun and moon and even Jupiter and Saturn if It wasn’t pushed. The best images were with 25mm, 18mm, and 12.5mm circle T orthos so that was keeping it between 40X and 80X. Don’t get me wrong and I don’t mean this in a snotty way, I did have a lot of fun with it the first year or so, but truly, the Questar was in such a different class that it really did ruin the C90 for me. Had I been a true collector, I would have kept it, but I’m really an observer and I just don’t see any reason to keep scopes that I’m not going to use. You have to think of it kind of like an ST80. It’s small, compact, and can be a lot of fun to use as long as you realize it’s limitations and stay within them. The comparison really is not a bad one, both are small, compact, and very portable. Both have around the same operating limit on magnification (at least my C90 did). The offsets are the ST80 gives you much, much wider fields of view, but the C90 has less false color (I never found CA to be an issue with my C90 although others have). Also, the C90 allows you to use longer focal length eyepieces to achieve the same magnification and snaps to focus easily (I never had a problem with the C90 focus, crude as it was, I liked the big focus ring in spite of the plumbing fixture pipe threads- it worked well). From what I understand tho, (there is a huge, long running threat in the CATs forum), the new C90s are in a whole other league optically from the old ones, and since they cost about the same amount of money, I would encourage someone to think of that as a viable alternative unless they just want the C90 for collectability. Also, if you want small, collectable orange, I would strongly encourage a C5 instead, especially if you intend to use it for astronomy observations. It’s much better than a C90 in every way.


Edited by Terra Nova, 06 April 2021 - 08:11 AM.

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#35 Borodog

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Posted 08 April 2021 - 05:13 PM

What I never liked about the C90 was the off-axis vignetting, which if you defocus a star and shift it off-axis is readily apparent.

This is exactly why I got an SCT adapter, which uses the exterior threads of the rear cell rather than relying on a 0.965" adapter. Preserves the entire rear opening diameter. I won't say it doesn't vignette, it does noticeably on an APS-C sized sensor, but it is night and day better even on small sensor 1.25" cameras and rock solid to boot. I wouldn't want to hang my mirrorless camera from the set screw and the 0.965" opening, that's for sure.



#36 Borodog

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Posted 08 April 2021 - 05:15 PM

I’ve been perhaps too critical on the little orange C90. It did have sharp optics and provided nice bright, crisp views when used as a terrestrial spotter. It also did well on the sun and moon and even Jupiter and Saturn if It wasn’t pushed. The best images were with 25mm, 18mm, and 12.5mm circle T orthos so that was keeping it between 40X and 80X. Don’t get me wrong and I don’t mean this in a snotty way, I did have a lot of fun with it the first year or so, but truly, the Questar was in such a different class that it really did ruin the C90 for me. Had I been a true collector, I would have kept it, but I’m really an observer and I just don’t see any reason to keep scopes that I’m not going to use. You have to think of it kind of like an ST80. It’s small, compact, and can be a lot of fun to use as long as you realize it’s limitations and stay within them. The comparison really is not a bad one, both are small, compact, and very portable. Both have around the same operating limit on magnification (at least my C90 did). The offsets are the ST80 gives you much, much wider fields of view, but the C90 has less false color (I never found CA to be an issue with my C90 although others have). Also, the C90 allows you to use longer focal length eyepieces to achieve the same magnification and snaps to focus easily (I never had a problem with the C90 focus, crude as it was, I liked the big focus ring in spite of the plumbing fixture pipe threads- it worked well). From what I understand tho, (there is a huge, long running threat in the CATs forum), the new C90s are in a whole other league optically from the old ones, and since they cost about the same amount of money, I would encourage someone to think of that as a viable alternative unless they just want the C90 for collectability. Also, if you want small, collectable orange, I would strongly encourage a C5 instead, especially if you intend to use it for astronomy observations. It’s much better than a C90 in every way.

I'm sure this is excellent advice for someone thinking about getting one of these type of scopes, but as I said, this one was given to me by my wife's father who bought it new in 1977, and is the third oldest known astro variant C90, and it is not going anywhere. :O) I'm just trying to get the most out of it that I can.


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#37 Terra Nova

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Posted 08 April 2021 - 08:42 PM

WFJ: Oh well, it was my attempt to be conciliatory. That’s all I got. I guess it wasn’t enough.
The opinions I expressed were admittedly based on a sample of one.

To sum up:

  1. Mine’s gone.
  2. It wasn’t very good.
  3. I don’t miss it.
  4. I have no desire for another.
  5. I tried my best to answer all your questions.
  6. To each, his or her own.

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#38 RichA

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Posted 09 April 2021 - 01:38 AM

This is exactly why I got an SCT adapter, which uses the exterior threads of the rear cell rather than relying on a 0.965" adapter. Preserves the entire rear opening diameter. I won't say it doesn't vignette, it does noticeably on an APS-C sized sensor, but it is night and day better even on small sensor 1.25" cameras and rock solid to boot. I wouldn't want to hang my mirrorless camera from the set screw and the 0.965" opening, that's for sure.

There is a limit on mods to these things.  Like putting a 2" adapter on an ETX125.  All of a sudden, you can't use the forks because there is no more swing-through.  I cobbled a 2 inch diagonal and a 20mm Nagler on a C90 once, just for the heck of it.  It worked ok.



#39 GreyDay

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Posted 09 April 2021 - 04:18 AM

 

WFJ: Oh well, it was my attempt to be conciliatory. That’s all I got. I guess it wasn’t enough.
The opinions I expressed were admittedly based on a sample of one.

To sum up:

  1. Mine’s gone.
  2. It wasn’t very good.
  3. I don’t miss it.
  4. I have no desire for another.
  5. I tried my best to answer all your questions.
  6. To each, his or her own.

 

My opinions were based on a sample of three, (2 orange 1 black). The opinions i expressed were toned down for a larger audience but I agree with Terra, The "Vintage" C90 was good for it's time, but times have changed and what was considered good then has been surpassed. Some people have good examples, some poor, but the overall impression i have of the "vintage" C90 is of poor QA, Average mechanical quality, average performance. To sum up:

 

1.Mine are gone.
2.they weren' very good.
3.I don’t miss them.
4.I have no desire for another.


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