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Old Japanese SCT corrector plate misalignment?

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#1 FedericoB

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Posted 10 August 2020 - 02:51 AM

Hi everyone, long time lurker here.

 

I recently managed to get my hands on a pretty good condition JSO Space 10. 

For those of you that don't know it is an old 100mm f/10 SCT  from the mid 80s by a company that no longer exists (Japan Special Optics). Mine seems to have been purchased in 86' right around the time when comet Halley was swinging last!

 

I cleaned off some mold from the corrector plate and mirror (it was a small amount), afterwards I decided to conduct and indoor star test and collimate (really lousy weather these days...). 

 

After successful collimation I decided to check that optics quality (indoor star test). The results were intriguing to say the least:

 

1. Perfect in focus image shows a bright center with 4 or 5 disks surrounding the center spot (spherical aberration??).

2. Being slightly, very slightly, out of focus both in and out appear to show these spikes surrounding the bright spot (astigmatism?).

3. When in and out of focus the spot is no longer a circle and it is slightly elongated along one axis, the elongation is orthogonal between the in and out of focus images.

4. The in and out of focus are not exactly identical (even when not counting the elongation).

 

It is the third result that makes me ask, is the corrector plate not properly aligned on the telescope?

I searched through CN for people with similar questions and it appears that if the corrector plate where to be misaligned it would present itself as astigmatism.

 

I then carefully opened the scope and unfortunately there are no alignment marks on the telescope at all... So I closed it back up.

 

So if I want to align an SCT corrector plate with no markings is there anything I can do?

 

Also, I must admit I am a novice at optics quality assessment, so these are the only major issues I noticed (thanks to Suiter's star testing book). I am not even sure what a perfect SCT's star test looks like...

 

The reason I would like to get it as close to perfect alignment as I can is because I'd like to use this telescope for planetary imaging, and having the best resolution and contrast requires a perfect alignment of the optics. It's such a light scope I might even get away with using it on my Sky Adventurer mount!

 

If any of you want more pictures let me know! This telescope is beautiful! But I don't just want a pretty scope, I want a scope that blows me away with its optics too!

 

Telescope Image:

UJ9cpIT.jpg


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#2 luxo II

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Posted 10 August 2020 - 03:04 AM

#3 is astigmatism. You can't fix that and its in nice condition cosmetically, so I'd say leave well enough alone, and enjoy it. 

But if you want a scope that blows you away with perfect optics, this isn't it and it's going be pretty ordinary for planetary imaging. 

 

For that in a small CAT you really want something like the deluxe version of an Intes MK66, MK67 or M615...


Edited by luxo II, 10 August 2020 - 03:12 AM.


#3 FedericoB

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Posted 10 August 2020 - 03:29 AM

#3 is astigmatism. You can't fix that and its in nice condition cosmetically, so I'd say leave well enough alone, and enjoy it. 

But if you want a scope that blows you away with perfect optics, this isn't it and it's going be pretty ordinary for planetary imaging. 

 

For that in a small CAT you really want something like the deluxe version of an Intes MK66, MK67 or M615...

 

I see! I guess I gave JSO too much credit, since I had read around and they seem to have been regarded quite highly when they used to exist due to their superb optics.

 

Funnily... I do have an MK66 (not sure if it's the deluxe version) but it's back home in Canada and I need to upgrade its focuser, get a mount for it, etc... which is why I have started going the small scope route and wanted to find a nice compact cass that I can use to planetary viewing while still being portable enough for my Japan life.

z6EcZSB.jpg


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#4 yellobeard

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Posted 10 August 2020 - 04:10 AM

With Meade and Celestron putting them on the world by the millions, it might be hard to accept, that an SCT still is a difficult type of scope to manufacture.

Where millions of people successfully build newtonians, only a handful of people actually managed to build great performing (small) SCT's

So I take my hat off, for companies like Meade and Celestron, for the fact that they saw an oppertunity, to make so many SCT's, with a very decent price/quality rate!

There were some other brands that tried the same, and did not acheve the same.

As for your challenges: The spherical aberration is a power mismatch between the two mirrors and the Schmidt Corrector Plate, and cannot be solved other than polish either the primary or secundary, and give them a (slight?) aspherical shape.
For the astigmatism however, there is a slight chance it might improve with turning the Schmidt Corrector in small bits, during star testing.
You only (if possible) need to loosen the mounting ring just a bit.

This only is a guess of mine, a few pictures of a sligtly defocussed, bright star would hopefully give us more info.


..

Edited by yellobeard, 10 August 2020 - 04:13 AM.


#5 FedericoB

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Posted 10 August 2020 - 04:27 AM

With Meade and Celestron putting them on the world by the millions, it might be hard to accept, that an SCT still is a difficult type of scope to manufacture.

Where millions of people successfully build newtonians, only a handful of people actually managed to build great performing (small) SCT's

So I take my hat off, for companies like Meade and Celestron, for the fact that they saw an oppertunity, to make so many SCT's, with a very decent price/quality rate!

There were some other brands that tried the same, and did not acheve the same.

As for your challenges: The spherical aberration is a power mismatch between the two mirrors and the Schmidt Corrector Plate, and cannot be solved other than polish either the primary or secundary, and give them a (slight?) aspherical shape.
For the astigmatism however, there is a slight chance it might improve with turning the Schmidt Corrector in small bits, during star testing.
You only (if possible) need to loosen the mounting ring just a bit.

This only is a guess of mine, a few pictures of a sligtly defocussed, bright star would hopefully give us more info.


..

 

 

Thank you for your insightful comment yellobeard!

 

I am happy to hear that it may be possible to improve the star test output.

 

I will happily provide slightly defocused star images, the thing is the weather is horrible in my city these days, extremely smoggy. Do you think indoor star images would also help?

Or perhaps even if it's not a perfectly clear night a star image is still useful?

 

How careful should I be with corrector plate centering?



#6 yellobeard

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Posted 10 August 2020 - 04:47 AM

Hi Frederico,
Are you willing to tell me how you do the indoor star test?
The fact is, that when you focus an optically perfect SCT at an object nearby, you definitely will get spherical aberrations!
They (SCT's) mostly have the oppertunity to focus on an object very close (by moving the primary mirror), but they are not optically corrected for short distances!

So an indoor star test can only be done right, by creating an artificial star inside an optically perfect (bigger) scope, and put the two scopes facing each other, looking at the star inside the SCT at high magnification.

For inside (and outside!) testing, your artificial star needs to be at least some 200 yards away!
With another scope, you create a parallel beam of light, as ling as you have the artificial star exactly in primary focus of that scope.

..

Edited by yellobeard, 10 August 2020 - 04:48 AM.


#7 Andrew Brown

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Posted 10 August 2020 - 04:49 AM

From detailed research on this very forum re C8 corrector plates..

 

two pathways available ~~ First, optics are not hand finished/tuned whatever then centering does become the bigger issue

Second they have been hand finished/tuned whatever then as yellobeard says it becomes a case of trying to find the sweet spot by rotating the corrector.

 

However I would suggest that you start at THE BACK and see how the optical path aligns itself.

 

PM incoming..

 

Edit ~~ looking at my rigel picostar for a 100mm aperture the minimum operating distance for the artificial star is 8m.


Edited by Andrew Brown, 10 August 2020 - 04:53 AM.


#8 FedericoB

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Posted 10 August 2020 - 06:21 AM

Hi Frederico,
Are you willing to tell me how you do the indoor star test?
The fact is, that when you focus an optically perfect SCT at an object nearby, you definitely will get spherical aberrations!
They (SCT's) mostly have the oppertunity to focus on an object very close (by moving the primary mirror), but they are not optically corrected for short distances!

So an indoor star test can only be done right, by creating an artificial star inside an optically perfect (bigger) scope, and put the two scopes facing each other, looking at the star inside the SCT at high magnification.

For inside (and outside!) testing, your artificial star needs to be at least some 200 yards away!
With another scope, you create a parallel beam of light, as ling as you have the artificial star exactly in primary focus of that scope.

..

 

Okay well I definitely did not have my artificial star far enough...! The spherical aberrations I am seeing might be because of that!

 

My star test currently is as follows:

 

  1. Using an artificial star with a pinhole of less than 50um, created with a flashlight and a 26 gauge needle.
  2. Place artificial star some 8 or so meters away and bring it to focus.
  3. Use extension tubes to bring it to focus as it is quite close.

However after reading your comment I think it is not appropriate... I guess I must wait for a clear night. Is it ok if it's smoggy?

 

 

 

From detailed research on this very forum re C8 corrector plates..

 

two pathways available ~~ First, optics are not hand finished/tuned whatever then centering does become the bigger issue

Second they have been hand finished/tuned whatever then as yellobeard says it becomes a case of trying to find the sweet spot by rotating the corrector.

 

However I would suggest that you start at THE BACK and see how the optical path aligns itself.

 

PM incoming..

 

Edit ~~ looking at my rigel picostar for a 100mm aperture the minimum operating distance for the artificial star is 8m.

 

I wonder... I believe I read somewhere that JSO handmade their scopes and each was made to be diffraction limited, that would mean that the corrector plate is more important.

Interestingly, I do not seem to have much play in terms of center alignment for the scope and seems to fit on the tube quite snugly.

 

I need to get a copy of Robert's book first!

 

Though I am curious, what does it mean to start at the back?



#9 luxo II

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Posted 10 August 2020 - 06:27 AM

Think he means start at the primary mirror not the corrector, if you’re going to rotate something in the hope of curing the astigmatism.

Another possibility is that one of the mirrors is being crimped in whatever cell it sits in, optics should not be fixed with any pressure on the glass.

If your going to try rotating the corrector, set up one evening with it pointed at a bright star near the zenith, loosen the parts retaining the corrector and rotate that in situ, maybe 30 degrees at a time, and looking through it to see if the star test is any better.

Edited by luxo II, 10 August 2020 - 06:32 AM.

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#10 FedericoB

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Posted 10 August 2020 - 06:38 AM

That makes sense luxo!

 

I do realize that there is no way to rotate the primary easily as there is only one possible direction to hold it down due to asymmetrical screws at the bottom.

 

I can definitely try to loosen the corrector and do slow rotations that seems to be quite easy just loosening the front 3 screws.

 

Could it be that centering isn't so critical? It seems there is no side to side play of the corrector and it fits in a very tight slot at the front of the tube.



#11 Bowlerhat

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Posted 10 August 2020 - 06:52 AM

Crikey I just lost an auction of this like weeks ago. Beautiful scope!


Edited by Bowlerhat, 10 August 2020 - 06:53 AM.


#12 FedericoB

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Posted 10 August 2020 - 07:45 AM

Seems to be built like a tank! 

It is definitely gorgeous!

 

It seems the 100mm version (space 10) came in three colors, black, yellow and white.

 

They also seemed to have a non coated version aimed at spotters but for around 60USD from the 1980s you could get the multi-coated version, aimed at astronomers.

 

The scope I got thankfully is the multi-coated version.

 

If anyone is interested here's the catalog from 1986, it's in Japanese, but there's nostalgia written all over it.

 

http://yumarin7.saku...kan/JSO1986.pdf


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#13 davidc135

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Posted 10 August 2020 - 10:05 AM

If you are able to set up an artificial star at, say 10 to 20 metres it can still be useful in spite of the induced SA. First, you can check the SA by viewing the 'star' at 5mm inside and outside the focus by comparing the size of secondary shadow. If the outside of focus shadow is larger than the inside that would be OK and from the expected over-correction. A torch shining on a ball bearing makes a good star.

 

Regardless of the SA you can still test for astigmatism and any beneficial effects of rotating the corrector if the weather is poor.

 

I'd go ahead and experiment with rotation of the corrector. It may be that time has cemented it to the rubber gaskets either side of the plate margin(?) and will need easing loose or you could be lucky. But reading post #1 again I see you have opened it up.

 

Best to mark the start position before any clocking. You will probably have to recollimate the scope with each rotation.

 

Hopefully astigmatism in both the primary mirror and the corrector plate can be made to partially or fully cancel out but it's a long shot. It's not likely that any lateral shift in either the corrector plate or the secondary within the plate is responsible for, or can cure astigmatism if that is what you have.

 

It looks like the whole cell is held in by the screws through the tube but how is the corrector plate retained in the cell? is it collimated in a similar way to the scope in #3

 

David


Edited by davidc135, 10 August 2020 - 10:30 AM.


#14 Bowlerhat

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Posted 10 August 2020 - 01:20 PM

It seems the 100mm version (space 10) came in three colors, black, yellow and white.

 

They also seemed to have a non coated version aimed at spotters but for around 60USD from the 1980s you could get the multi-coated version, aimed at astronomers.

 

The scope I got thankfully is the multi-coated version.

 

I always like that colorful tone of vintage scopes-there's an appeal in simple plain colors rather than modern anodized metal tubes

 

 

If you are able to set up an artificial star at, say 10 to 20 metres it can still be useful in spite of the induced SA. 

I would get a minimum of 30m


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#15 Nippon

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Posted 10 August 2020 - 01:37 PM

I have an old orange C8 that had astigmatism so bad that at focus stars were a cross. I experimented by rotating the corrector and got rid of the astigmatism and the optics now are excellent with near identical and perfectly round intra and extra focus circles and a clean airy disc on steady nights.


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#16 FedericoB

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Posted 10 August 2020 - 06:42 PM

If you are able to set up an artificial star at, say 10 to 20 metres it can still be useful in spite of the induced SA. First, you can check the SA by viewing the 'star' at 5mm inside and outside the focus by comparing the size of secondary shadow. If the outside of focus shadow is larger than the inside that would be OK and from the expected over-correction. A torch shining on a ball bearing makes a good star.

 

Regardless of the SA you can still test for astigmatism and any beneficial effects of rotating the corrector if the weather is poor.

 

I'd go ahead and experiment with rotation of the corrector. It may be that time has cemented it to the rubber gaskets either side of the plate margin(?) and will need easing loose or you could be lucky. But reading post #1 again I see you have opened it up.

 

Best to mark the start position before any clocking. You will probably have to recollimate the scope with each rotation.

 

Hopefully astigmatism in both the primary mirror and the corrector plate can be made to partially or fully cancel out but it's a long shot. It's not likely that any lateral shift in either the corrector plate or the secondary within the plate is responsible for, or can cure astigmatism if that is what you have.

 

It looks like the whole cell is held in by the screws through the tube but how is the corrector plate retained in the cell? is it collimated in a similar way to the scope in #3

 

David

 

Hello David,

 

Thanks for the suggestions!

 

How far should do torch be from the ball bearing?

 

 

As for how I adjust the corrector plate, here is an image of the front with a mark I put on to remember its location.

 

By loosening those 3 screws I can rotate the plate, interestingly there is very little sideways movement if at all, so not much of a centering marging, I'd say it move less than 1-1.5mm.

 

As an aside, I was reading Robert Piekiel's book and he talked about this type of collimation mechanism, since I have a central screw it probably means everytime I will collimate the secondary I must loosen one screw and tighten the other two.

 

UJnzYGq.jpg

 

@Nippon

I should really get some defocused images, but they might be hard to take... I'll give it a shot with a camera with a small sensor.

 

For this scope the focused image seems to have very little astigmatism, but when I defocus them the star looks a bit elongated and if I am near focus the PSF looks a bit flared, don't think it looks like a cross.

 

I really hope I can get similar results!!


Edited by FedericoB, 10 August 2020 - 06:55 PM.

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#17 Nippon

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Posted 10 August 2020 - 07:25 PM

I rotated the corrector on my C8 90 degrees and it fixed it. I did out under the stars with the scope on a star about 60 degrees up so I could see the results. Just remember to put the screws back in the retainer ring before you lower the scope.


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#18 stevew

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Posted 10 August 2020 - 10:03 PM

 

The reason I would like to get it as close to perfect alignment as I can is because I'd like to use this telescope for planetary imaging, and having the best resolution and contrast requires a perfect alignment of the optics. It's such a light scope I might even get away with using it on my Sky Adventurer mount!

 

 

 

While I'm sure it's a nice portable telescope I'm not sure a 100mm SCT with a 33% central obstruction would be a good planetary imaging scope.



#19 FedericoB

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Posted 10 August 2020 - 11:42 PM

While I'm sure it's a nice portable telescope I'm not sure a 100mm SCT with a 33% central obstruction would be a good planetary imaging scope.

 

Due to the theoretical max resolution such a small scope would have?

 

My other scope is a 72SED doublet, so I was hoping there could be targets I can image with the 100mm SCT but not with the 72mm refractor.



#20 davidc135

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Posted 11 August 2020 - 02:30 AM

Frederico

I have a torch 3 metres from a 4mm ball bearing at 25m. I imagine it's better if it makes a narrow angle with the returning beam as otherwise the 'star' pattern may be distorted.

Astigmatic inside and outside of focus star patterns will have their long axes mutually perpendicular. Just checking that that is the case when you mentioned orthogonal.

 

It looks like the three collimating screws engage in a plate which is free to tilt about the end of the centre screw? It's possible that it's the other way around.

 

Depending on expectations it may be a decent planetary scope if the optics are good. Hasn't the Questar 31/2 a 31% co? My B&L4000 which is reasonably free of optical defects but has a 38% obstruction will visually show some detail within the bands of Jupiter and sub 3km craterlets on the moon. A good 75mm refractor would just have the edge though. That's visual. Maybe positions would be reversed in imaging.

 

David


Edited by davidc135, 11 August 2020 - 02:32 AM.


#21 yellobeard

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Posted 11 August 2020 - 04:42 AM

That scope looks quite neat!! And as far as I can see, not much wrong with the collimation as well.
To be sure, you need more pictures, from different angles and orientations.

As for the minimum distance to the artificial star: A minimum mostly is calculated towards the minimum distance where pherical aberration starts. Which means that with that distance, you exactly are at one end of the spherical aberration tolerance.
Always better to keep more distance if possible, as you will be more safe within the tolerance.

So indeed a minimum of some 30 yards would be advisable


..

Edited by yellobeard, 11 August 2020 - 04:44 AM.


#22 FedericoB

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Posted 22 August 2020 - 04:26 AM

Hi everyone,

Just wanted to quickly update on the situation.

So I finally received some adapters that allowed me to connect a 1.25 inch visualback to the telescope.
I was able to collimate the telescope, except that due to horrible seeing and light pollution I ended up using Jupiter... In any case it was slightly out of collimation because as soon as fixed it, Jupiter's contrast and resolution got much better! I guess it really is true what they say about SCTs with regards to collimation.

I don't have a proper cooled camera but I do have a camera that is usually used for fast moving objects due to its global shutter (not that it helps imaging...), I also had a Raspberry Pi and an external SSD to save the data. So I used my phone to connect to the Pi and I used Qt V4L2 test utility to capture the raw data.

So off I went to a nearby park with my Star Adventurer, the telescope, tripod and my imaging equipment. After polar aligning as best as I could I decided to shoot video of Jupiter and Saturn. This was my first time doing any sort of semi-serious planetary imaging, so I wasn't expecting much, but honestly the results were better than I anticipated in spite of the heavy smog during the night. I live really close to Tokyo so at night it's hard to see anything but the brightest objects. Honestly I even had trouble finding Polaris...

I wish I had taken pictures of defocused stars but I decided against it due to the bad light pollution, since it would not give me an adequate true test of the telescope, but perhaps I shouldn't worry too much since I was surprised by the quality of the results (Saturn in particular) despite the light pollution condition and my lack of a proper planetary camera.

The most difficult thing was to focus the planets, in fact I am pretty sure my Jupiter image is defocused... but the Saturn one is acceptable. Does anyone have tricks and tips for focusing them? I should probably start with a sturdier tripod because the one I have shakes so much!

Here is a gallery with the results:
https://imgur.com/a/jSiMdEr

Saturn tho:
DgN5LuK.png

Here's a picture of my setup: Everything that you see on the image is my imaging setup, plus my phone to connect to the Raspberry Pi for the imaging. You can see the power bank and raspberry pi wrapped around the tripod. I will build a tray for them soon!

peQmEOR.png

One thing I am curious is which factor would affect the quality of the imaging the most. In my case, would it have been the light pollution, the smog, the non astronomy camera, the telescope optics or perhaps an outside temperature of 34C (93.2F)... Does anyone have any insight here?
I know the pictures are not perfect so I am trying to pinpoint the places I can improve. Like that halo around Saturn.

I am trying to learn to get as good as I can with the current setup I have because I want to try and get a nice picture of mars when it is at its closest this October.

Edited by FedericoB, 22 August 2020 - 05:08 AM.

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