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

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#1 Borodog

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Posted 03 April 2021 - 01:51 PM

I have read some conflicting information about the C90 in regards to aberrations. I have read that the Mak-Cas design produces a flat field, but I have read elsewhere that later C90s have internal field-flatteners implying that perhaps earlier designs did not in fact produce a flat field. So, does the original orange tube C90 produce a flat field? What about coma? Being a reflector I presume it produces no chromatic aberration, yes?

 


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#2 photomagica

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Posted 03 April 2021 - 02:50 PM

I have an original C90, however I have not used it for night sky imaging. I have used it visually and as a 1000mm telephoto lens on an SLR. Others may be able to speak to specific aberrations, however  these points may be of use:

  1. There is sample variation among the original orange tube C90s.  I've seen very good samples and ones I consider unacceptable.
  2. The design works well over the field of a typical mid-power 1.25 inch eyepiece - a field diameter of perhaps up to 20mm. Visually the sharpness over that field is good and I have not noticed other aberrations or field curvature - however I was not looking for field curvature.
  3. Flare is serious and a long, internally flocked lens shade is needed to reduce this to acceptable levels. Flare is not distributed uniformly across the field.
  4. Vignetting outside the central sharp field is significant and non uniform.
  5. Sharpness deteriorates outside the central field - this is acceptable in many daytime photographic circumstances even across a full 35mm frame, for example where there is a bird in the sharp central field. I'm estimating the star images would be pretty bad outside this field. In other words - yes - there are very significant aberrations outside the small central sharp field 
  6. Don't take the 20mm sharp field as a given. For critical astrophotography uses it is likely less.
  7. I have not observed any chromatic aberration in the sharp central field.

Whether a C90 will serve your purposes depends upon the use you have in mind and the field you want to cover. Even with all of these limitations, I've enjoyed mine as a travel telescope and with a custom made bracket on top as a platform for other cameras to provide tracking - I have the original one-arm equatorial mount.

Bill


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#3 davidmcgo

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Posted 03 April 2021 - 03:02 PM

I have owned around 5 orange tubes over the years and a black original style 500mm f5.6.  All suffered from some degree of chromatic aberration, spherical aberration, and collimation errors.  It was a cute package but no where close to the hype.  I think the tolerances were too loose on the inner and outer radii of the corrector and the lack of precise centering limit them to at most around 100x, some didn’t work well at 80x, and the 500 f5.6 was bad over 20x.

 

For performance I would take a current 90mm Mak from Orion or the current C90 any day.  But the originals were really cute and fun.  Hence why I have owned so many, just hoping for a good one.  But my standards are too high.

 

Dave


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#4 Borodog

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Posted 03 April 2021 - 03:20 PM

Thank you. My main intended use for the scope is lunar and solar photography, mainly with a ZWO ASI183MC camera at prime focus. The camera pixel size is well matched to the scope for critical sampling. The sensor is 13.2x8.8 mm, 15.9 mm diagonal. So, not small, but smaller than APS-C sized. I am not too concerned about vignetting, which I can handle with flats, so much as aberrations like coma, field curvature, CA, or the dreaded poor collimation, since there is no way to collimate the scope. It's no great loss if I can't use the scope for lunar, since I am planning to mainly use other scopes for that (C8 and 10" Dob), but it would be nice to use the C90 for solar.

 

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#5 GreyDay

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Posted 03 April 2021 - 03:44 PM

I've owned one orange tube(f11), one black tube(f11) and borrowed a second orange tube. Of the three the black tube was best and one of the orange tubes had around 10% curvature at the edge of field but as Dave says above these vintage C90's aren't the telescope their popularity would have you believe. The black one i owned gave a much more contrasty view than either of the orange ones. Focusing with these scopes can be frustrating if the focusing collar isn't buttery smooth with little resistance. I stripped and relubed both C90's i owned and neither were bad but still not as smooth as the modern Synta rear focusing 90mm models. As for aberrations.. little noticable CA on bright targets, though eyepieces used may introduce some abberations, particularly widefield ep's which suffered vignetting. They were a really good scope for their time and i can see where their legendary status was earned, but.... the latest Synta models offer better contrast, native 1.25" fittings, better coatings, easier collimation and the deal breaker... easier focusing.

 

If you're getting one cheap then maybe if you get a good one or don't mind tinkering, they're not a bad portable scope for quick views and general stargazing. The modern Synta's are preferable in my experience for the reasons stated but i found the 102mm Mak a much more versatile scope for a little extra cash. For a comparison of old vs new C90's watch the video below but watch to the end where they're star tested.

 

https://www.youtube....h?v=AXwC1YH0UHg


Edited by GreyDay, 03 April 2021 - 03:46 PM.

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#6 Borodog

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Posted 03 April 2021 - 04:59 PM

Thanks. The only possible candidate is the orange tube C90 I already own. It's the third oldest listed astro variant on the CN C90 registry. It was given to me by my father-in-law and will not be going anywhere. :O)

 

I agree with you on the kludgy focus mechanism, which is why I put a big honking SCT dual speed Crayford focuser on it (to be shared with my C8). The Crayford attaches via the external threads (via a Mak-SCT adapter ring), so it allows full use of the rear cell opening, and puts the focal length at exactly the advertised 1000 mm f/11.1.

 

A related question then is, if the Mak-Cas C90 is subject to coma and curvature like an SCT, given that it is a similar design and similar focal ratio, would it benefit from an SCT f/6.3 reducer/corrector? Or is there no way to know that until I try it? It would move me farther from critical sampling, but I would probably be willing to accept that trade for increased image quality.

 

For what it's worth, here is a mosaic shot with the scope at 805 mm (no Crayford installed at the time). The focus is a bit out and there appears to be some color fringing, possibly related. It looks sort of like CA to me, but I'm not sure. I probably would be willing to accept this quality for solar imaging, but not lunar, except in a pinch.

 

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Edited by Borodog, 03 April 2021 - 05:24 PM.

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#7 GreyDay

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Posted 03 April 2021 - 05:18 PM

A related question then is, if the Mak-Cas C90 is subject to coma and curvature like an SCT, given that it is a similar design and similar focal ratio, would it benefit from an SCT f/6.3 reducer/corrector? Or is there no way to know that until I try it? It would move me farther from critical sampling, but I would probably be willing to accept that trade for increased image quality.

A good question, the best place for an answer would be the AP forum, maybe someone there has already tried a corrector.


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#8 Borodog

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Posted 03 April 2021 - 05:32 PM

A good question, the best place for an answer would be the AP forum, maybe someone there has already tried a corrector.

Thanks; I went with the Cats & Cas forum. Seemed like probably the closest fit maybe, although any of these three forums might have someone prowling around who'd know.



#9 RichA

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Posted 03 April 2021 - 05:57 PM

I have read some conflicting information about the C90 in regards to aberrations. I have read that the Mak-Cas design produces a flat field, but I have read elsewhere that later C90s have internal field-flatteners implying that perhaps earlier designs did not in fact produce a flat field. So, does the original orange tube C90 produce a flat field? What about coma? Being a reflector I presume it produces no chromatic aberration, yes?

They are not and never have been very good scopes.  Modern Maks from Orion, Meade, Celestron, Sky-Watcher, etc., are superior. 

No point in letting nostalgia get in the way.  I do wish some company other than Questar and Celestron made a decent metal fork mount like the C90 Astro had though.


Edited by RichA, 03 April 2021 - 10:48 PM.

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#10 wfj

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Posted 03 April 2021 - 06:00 PM

The original C90 is a short focal length Gregory design, with an f/2 primary and a f/11 focal length. These aren't optimal for it's Maksutov Cassegrain design.

 

Here's a link that details such optical aberrations: https://www.telescop...CT_off_axis.htm

 

Modern C90's are physically longer -   f/3 primary and f/14 focal length. They are more uniform and narrower in field, better baffled.

 

There are some excellent original C90's. They are modified by having an aspheric corrector front surface. Because this complicates for an relatively inexpensive scope, it's often the source of quality control issues.

 

Also, for f/11 you really need an a two element corrector for balanced chromatic aberration. As it is, you'll notice slightly different color effects in/out of focus due to the thin corrector. A well made C90 will have no color at focus, and its one way to hint at such - if the scope isn't well-made it won't be sharp and always show color. But even this is a difficult comparison because of the difficult focuser and the glued primary mirror permanent collimation, which may not be either well collimated nor permanent.

 

There are other fixable, annoying defects that rob from its optics, When remedied it performs excellently.


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#11 Borodog

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Posted 03 April 2021 - 06:32 PM

The original C90 is a short focal length Gregory design, with an f/2 primary and a f/11 focal length. These aren't optimal for it's Maksutov Cassegrain design.

 

Here's a link that details such optical aberrations: https://www.telescop...CT_off_axis.htm

 

Modern C90's are physically longer -   f/3 primary and f/14 focal length. They are more uniform and narrower in field, better baffled.

 

There are some excellent original C90's. They are modified by having an aspheric corrector front surface. Because this complicates for an relatively inexpensive scope, it's often the source of quality control issues.

 

Also, for f/11 you really need an a two element corrector for balanced chromatic aberration. As it is, you'll notice slightly different color effects in/out of focus due to the thin corrector. A well made C90 will have no color at focus, and its one way to hint at such - if the scope isn't well-made it won't be sharp and always show color. But even this is a difficult comparison because of the difficult focuser and the glued primary mirror permanent collimation, which may not be either well collimated nor permanent.

 

There are other fixable, annoying defects that rob from its optics, When remedied it performs excellently.

Thank you, wfj. That is very insightful. If I am reading you and your link correctly, it seems to imply that the original C90 design at f/11 is actually quite low in coma and astigmatism but suffers from significant field curvature. Would you agree with that? Would the Antares f/6.3 SCT reducer (but not corrector) be a good fit for this scope or no?

 

In the image above you can see some color fringing, but as I said the focus was a bit out and I have since fitted a Crayford dual speed focuser to the rear cell so that I don't have to touch the original mechanism. Also, the color you see is exaggerated because the saturation is pushed quite hard. In the "natural color" version (whatever that may mean) the fringing is less noticeable (see below).

 

I guess what it comes down to is this. I've only been doing AP for a few months so I have little experience to refer to. Would you say this is about as good as I could hope for, other than better focus, for the 3rd oldest known astro variant C90? Or would some sort of corrective optics in the train significantly improve the result? Again, I am willing to accept reduction if it results in higher image quality. But if this is as good as it's going to get, then I will stop worrying about it, as I am in fact pretty happy with it as-is.

 

gallery_346195_16100_10513430.png


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#12 photomagica

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Posted 03 April 2021 - 08:56 PM

Thank you. My main intended use for the scope is lunar and solar photography, mainly with a ZWO ASI183MC camera at prime focus. The camera pixel size is well matched to the scope for critical sampling. The sensor is 13.2x8.8 mm, 15.9 mm diagonal. So, not small, but smaller than APS-C sized. I am not too concerned about vignetting, which I can handle with flats, so much as aberrations like coma, field curvature, CA, or the dreaded poor collimation, since there is no way to collimate the scope. It's no great loss if I can't use the scope for lunar, since I am planning to mainly use other scopes for that (C8 and 10" Dob), but it would be nice to use the C90 for solar.

Over the sensor size you are using the C90 should be OK for solar if you have a good sample. It is hard to tell from a Moon photo but if the softness is indeed a focus issue, you may indeed have a good one. I photographed several total solar eclipses with mine and the results were pleasing, thought not as good as the two eclipses I photographed with a Zeiss Jena 100/1000 Semi-apo refractor. The relatively high flare was an issue. Flare was also a problem when I used a 3.5 inch Questar for one eclipse.

 

I agree with several previous posters that the original C90 suffered from quality issues. I was very fortunate to be able to select mine from a set of six that were available to test. Mine showed fairly good, though not perfect diffraction images at high power. Several years after I bought it, I actually took it in to Celestron in Los Angeles because the collimation had shifted. The technicians invited me to observe while they tested it on their huge laser collimator. At first they said it met spec. but I urged them to look harder and they then agreed it could benefit from adjustment. I received it back as good as new after they adjusted it. I still have it but have not critically checked it for several years.

Bill


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#13 photomagica

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Posted 03 April 2021 - 09:02 PM

Have a look at this thread: https://www.cloudyni.../?hl=+c90 +test

The test described here is on a black f11 C90, however I expect an orange tube one would perform similarly. The field covered is a Canon HPS-C frame -so larger than you are intending.

Bill


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#14 wfj

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Posted 03 April 2021 - 09:04 PM

Yes field curvature. But blindly correcting might not help. Correctors are specific to telescope type and its specifics. Unfortunately generic combination of parts seldom works.

 

Some items to help. I use a focus mask on the C90 to focus on a nearby star to the object I wish to photograph, viewing it through the camera (magnifying when possible). Made one for 3D printing: https://www.thingive...m/thing:4590007 Bahtinov masks create an aberration that you balance to determine exact focus.

 

After focusing the camera/scope combo, take a picture of a high contrast feature like the partial moon phase terminator, near the center of the field of view. Then move the same feature to the edge of the field of view and take another picture.

 

With field curvature, you should see a difference with the feature. That’s how you can prove that the issue is field curvature.

 

If it is and you do the same with a corrector in place, then you can see how the corrector helps.


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#15 Borodog

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Posted 03 April 2021 - 09:45 PM

Thanks. I already have a recently acquired Bahtinov mask for the scope, acquired at about the same time as the Crayford focuser, so after the Moon photo taken above. I have not had a chance to put either to the test.

 

My plan is to focus on a star at center field, image it, move it to the edge, image it again, and finally refocus at the edge and take one last image. This should let me know to what extent I am dealing with any poor collimation, coma, or curvature. If the worst of the lot is curvature, I may be able to get away with simply focusing to the midpoint. 


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#16 wfj

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Posted 03 April 2021 - 10:15 PM

Not a bad plan. However, star images may depend on the right longer exposure to catch the details you’re after. The images might be affected by movement or lack of tracking. Also, out of focus star images show collimation better, and star testing inside/outside focus can quantify spherical aberration (which is when the C90’s corrector hasn’t had the aspheric done on it).

 

And ... let’s say everything checked out right and you still have a muddy moonshot. The secondary baffle has a flat front which for the moon will cause veiling glare that won’t show up on other tests.

 

Oh, and you should be able to do better with your picture. You should be able to see collapse on crater walls not present in your photo as well as better contrast.

 

add:

When I wrote this, I could only see it on a cellphone screen. Using my laptop screen I do see more. It is one of the best pictures of a near full moon through a C90 I've seen.

 

The focus seems spot on - no "yellow" around the rim. I can make out details in the crater rims. I'd like to try next moon phase to replicate your experience myself, to see how they compare, health/weather permitting. What resolution and magnification were you using? Mirror or prism diagonal, or straight through?


Edited by wfj, 04 April 2021 - 11:55 AM.

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#17 Terra Nova

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Posted 04 April 2021 - 08:39 AM

I found my original orange C90 to be on a par with an average (definitely not an excellent) 60mm achromat, say for example a middle of the road Towa 60 x 800. My Mayflower 814 bested it. The best mine would ever do on a good night was to split both doubles of the double-double, (just barely, but split them) using my Meade 7mm research grade ortho with a very good hybrid prism star diagonal. It would show all four stars of the Trapezium with my 12.5mm and 9mm circle T, volcano-top orthos. It was compact, I will say that for it. If your expectations aren’t real high, it can be a fun little scope. Also, definitely craft a dew shield for it. It needs one!

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#18 beanerds

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Posted 04 April 2021 - 11:11 AM

I have one of the Black C90's and as already said I use this sweet telescope more as a Terrestial long distance Microscope  , focus at 6 feet ,massive to watch a BEE clean its self after collecting pollen   . of Ant's doing their stuff ,, ??
 

Astronomy wise ? ,, I have a great 90mm f10 refractor and truthfully it is a better astro scope than the C90 in every way ,, but !!

 

The C90's close focus is awesome ! ,, and without the 90mm Frack I would be happy nwith the C90 as a small astro scope .

 

Enjoy mate , these are cool telescopes 

 

Beanerds.


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#19 wfj

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Posted 04 April 2021 - 11:37 AM

I found my original orange C90 to be on a par with an average (definitely not an excellent) 60mm achromat, say for example a middle of the road Towa 60 x 800. My Mayflower 814 bested it.

That's a fair description of what I've seen with many of them. Did you star test it? I've found that many didn't seem to have the aspheric to the front of the corrector.

 

Mine splits Delta Cygni fine. Use it with tight doubles with any modern 4mm. Does better on the Dumbbell Nebula then my NK65. About the same FL, easy comparo.

 

 

Also, definitely craft a dew shield for it. It needs one!

Redoing the baffle design eliminated that for mine. Don't use the dew shield except when humidity necessitates, and then it needs to be more than the length again of the C90's stubby tube, like when in Hawaii. Made it easier to focus.
 


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#20 Borodog

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Posted 04 April 2021 - 01:38 PM

Not a bad plan. However, star images may depend on the right longer exposure to catch the details you’re after. The images might be affected by movement or lack of tracking. Also, out of focus star images show collimation better, and star testing inside/outside focus can quantify spherical aberration (which is when the C90’s corrector hasn’t had the aspheric done on it).

 

And ... let’s say everything checked out right and you still have a muddy moonshot. The secondary baffle has a flat front which for the moon will cause veiling glare that won’t show up on other tests.

 

Oh, and you should be able to do better with your picture. You should be able to see collapse on crater walls not present in your photo as well as better contrast.

 

add:

When I wrote this, I could only see it on a cellphone screen. Using my laptop screen I do see more. It is one of the best pictures of a near full moon through a C90 I've seen.

 

The focus seems spot on - no "yellow" around the rim. I can make out details in the crater rims. I'd like to try next moon phase to replicate your experience myself, to see how they compare, health/weather permitting. What resolution and magnification were you using? Mirror or prism diagonal, or straight through?

 

Ok, that is comforting to hear then. In that case I will most likely leave it as is. Thank you very much; that is exactly the feedback I was looking for.

 

Also, a correction. That image *was* taken with the Crayford in, because the focal length (measured by the image scale) was exactly 1000 mm. Without the Crayford in the focal length is exactly 805 mm. It was focused without a Bahtinov, however. In the originally posted version, if you look at the upper limb, you can see color fringing. Interestingly, you can see on the dark side of the bright upper limb that it is blue on one side of a stitch in the mosaic and red on the other side of the stitch. So it was clearly blue when shooting one panel and red when shooting the other. I am not sure what could cause that; perhaps a slight focus shift between panels? This would be even more apparent at the original resolution, before I downsampled the final image by ~50%. In any event you can also see some color in the craters at the terminator. For all I know some of this is atmospheric dispersion, though. 


Edited by Borodog, 04 April 2021 - 01:40 PM.

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#21 GreyDay

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Posted 04 April 2021 - 01:53 PM

I found my original orange C90 to be on a par with an average (definitely not an excellent) 60mm achromat, say for example a middle of the road Towa 60 x 800. My Mayflower 814 bested it. The best mine would ever do on a good night was to split both doubles of the double-double, (just barely, but split them)

 

It was compact, I will say that for it. If your expectations aren’t real high, it can be a fun little scope. Also, definitely craft a dew shield for it. It needs one!

I found the same with the Orange C90's, At the time i compared them to a Synta 80x900 refractor that i'd bought for £15. i was expecting a little more for the cash i'd paid for the C90.

 

If it had been the only scope i'd owned then maybe i'd have been happy with it, as Terra says "it can be a fun little scope". There are several other "popular" scopes i've been lucky enough to try out over the years that are also held in high esteem that didn't meet my expectations. The C90 isn't on it's own in that regard.



#22 wfj

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Posted 04 April 2021 - 02:23 PM

Ok, that is comforting to hear then. In that case I will most likely leave it as is. Thank you very much; that is exactly the feedback I was looking for.

 

Also, a correction. That image *was* taken with the Crayford in, because the focal length (measured by the image scale) was exactly 1000 mm. Without the Crayford in the focal length is exactly 805 mm. It was focused without a Bahtinov, however.

Good, am pleased too. Glad to be of help.

 

So you are shooting direct to imager - to get the 805mm FL you have to have the image plane about an inch past the rear flange of the C90. I've a scale on the helical focuser to tell the infinity focus focal length at those positions, since it changes considerably.

 

You may wish to take another image with the Bahtinov and see if you have the same color cast.
 

In the originally posted version, if you look at the upper limb, you can see color fringing. Interestingly, you can see on the dark side of the bright upper limb that it is blue on one side of a stitch in the mosaic and red on the other side of the stitch. So it was clearly blue when shooting one panel and red when shooting the other. I am not sure what could cause that; perhaps a slight focus shift between panels? This would be even more apparent at the original resolution, before I downsampled the final image by ~50%. In any event you can also see some color in the craters at the terminator. 

I saved the picture and maxed out the saturation. Don't see red at all. See a suffuse yellow cast on the outer limb of three quadrants, with the remaining first quadrant having a blue suffuse cast.

 

The C90's corrector is not achromatic, so inside/outside focus does yellow/blue a bit, a side effect of the thin corrector design choice. At a guess, could a slight miscollimation, or corrector wedge, or perhaps dirt in the corrector's front mating surface to the tube, "tilt" it so that focus is slightly skew?

 

The C90 wasn't always assembled with due care, as I've found foibles. My own needs to be stored for long times vertically, as horizontally causes it to be "off". So I store it corrector face down and it's just fine for any length of time. I think the adhesive in the base that holds the mirror in collimation (I use nylon thumbscrews in place of the cap screws to touch up collimation) isn't fully cured and sags otherwise.

 

Oh, and the moon does have subtle brown hues I can see with my C90 and refractors - in part why I did the baffle improvements.

 

For all I know some of this is atmospheric dispersion, though. 

Depends on the altitude of the moon in the sky for this shot. Usually the gradient spans the picture, and it forms a line. Don't see that here.

 

Nice to here from someone else with a good experience with the C90, instead of the usual bashing. I'd never in several decades had a good one until recent, and in my pursuit of seeing how good the good one could be made, came away with an entirely different view of their flaws and virtues than prior experience.

 

Best of luck with your AP.
 


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#23 PETER DREW

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Posted 04 April 2021 - 05:12 PM

I found that on the test bench that a well collimated early orange C90 would give a good star image.  However, if the focus collar was turned to suit an added accessory, the rotation caused all sorts of distortion to the star image.



#24 Terra Nova

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Posted 04 April 2021 - 10:53 PM

That's a fair description of what I've seen with many of them. Did you star test it? I've found that many didn't seem to have the aspheric to the front of the corrector.

 

Mine splits Delta Cygni fine. Use it with tight doubles with any modern 4mm. Does better on the Dumbbell Nebula then my NK65. About the same FL, easy comparo.

 

 

Redoing the baffle design eliminated that for mine. Don't use the dew shield except when humidity necessitates, and then it needs to be more than the length again of the C90's stubby tube, like when in Hawaii. Made it easier to focus.
 

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.


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#25 Terra Nova

Terra Nova

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Posted 04 April 2021 - 11:01 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.


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