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Is There Any Hope for this old C90 500mm f5.6

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#1 jstrandberg

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Posted 23 August 2019 - 10:05 AM

I bought this lens in a thrift mall hoping to use it for some lunar and semi-wide field astro shots. It is the camera "mirror lens" version black tube. After much playing around with it, I cannot seem to get good results. Here is the best I could get on the moon. 

 

I have a set of extension tubes of varying lengths and have tried all sorts of back-focus distances but the results all seem to be the same. First photo was taken with the 65mm tube and the 40mm tube. Other than my exposure differences, as you can see, both pics have soft focus and lots of CA.

 

Any ideas out there on how to correct this, or did I just get one that was poorly made?

 

 

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#2 Taosmath

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Posted 23 August 2019 - 10:15 AM

Have you checked the collimation?


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#3 bbqediguana

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Posted 23 August 2019 - 10:41 AM

I was going to say the same thing - collimation. I'm not sure how it can be adjusted - is this a Maksutov or a Schmidt? If it is a Mak, then maybe there's some adjustment screws on the primary... if it is a Schmidt, then there should be 3 adjustment screws on the secondary.



#4 jstrandberg

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Posted 23 August 2019 - 10:47 AM

Have you checked the collimation?

I did a check using an artificial star and it looked good. Perhaps I should verify that again.



#5 jstrandberg

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Posted 23 August 2019 - 10:49 AM

I was going to say the same thing - collimation. I'm not sure how it can be adjusted - is this a Maksutov or a Schmidt? If it is a Mak, then maybe there's some adjustment screws on the primary... if it is a Schmidt, then there should be 3 adjustment screws on the secondary.

It's a Mak and there are three adjustment screws on the back.


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#6 Vla

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Posted 23 August 2019 - 10:55 AM

Focused and out of focus star at high magnification would be much more helpful.



#7 nicoledoula

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Posted 23 August 2019 - 10:56 AM

No hope. It's a Mak pretending to be a short refractor. Better off with a Meade Adventurescope 80mm for under $100, I don't think collimation will remove the CA. Something wrong with your AT 72 ED? That will take fantastic photos



#8 jstrandberg

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Posted 23 August 2019 - 11:25 AM

No hope. It's a Mak pretending to be a short refractor. Better off with a Meade Adventurescope 80mm for under $100, I don't think collimation will remove the CA. Something wrong with your AT 72 ED? That will take fantastic photos

Just bought this on a lark because of its small size. The AT72 does take amazing pictures but overall it is nearly twice as long.


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#9 jstrandberg

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Posted 23 August 2019 - 11:29 AM

Here is a shot using the artificial star. I think the collimation is ok, right?

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#10 Boom

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Posted 23 August 2019 - 11:53 AM

Have you practiced taking pictures of static objects during the day time?  Preferably on something indoors from one side of the house to the other, using a solid tripod at base ISO.  Or early during the day on something outside before it heats up.  This way you can rule out poor focus, shaky mounts, moving targets, and air currents as sources of poor images.  

 

These mirror lenses are very sensitive to focus and air currents, and in an effort to prevent blurring, users crank up the shutter speed resulting in high ISO and loss of detail.  It may be f/5.6 but the t-stop is way slower due to the CO and old coatings.  

 

I was surprised the razor sharp images I can get out of mirror lenses when I shot indoors, on a solid tripod, using base ISO, with careful focus.  Once I pointed the lens outside, the air currents alone softened the image.

 

 

As for the collimation, that's too much defocus to tell how well it's collimated.  You need to check using a high powered eyepiece.



#11 jstrandberg

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Posted 23 August 2019 - 12:21 PM

Have you practiced taking pictures of static objects during the day time?  Preferably on something indoors from one side of the house to the other, using a solid tripod at base ISO.  Or early during the day on something outside before it heats up.  This way you can rule out poor focus, shaky mounts, moving targets, and air currents as sources of poor images.  

 

These mirror lenses are very sensitive to focus and air currents, and in an effort to prevent blurring, users crank up the shutter speed resulting in high ISO and loss of detail.  It may be f/5.6 but the t-stop is way slower due to the CO and old coatings.  

 

I was surprised the razor sharp images I can get out of mirror lenses when I shot indoors, on a solid tripod, using base ISO, with careful focus.  Once I pointed the lens outside, the air currents alone softened the image.

 

 

As for the collimation, that's too much defocus to tell how well it's collimated.  You need to check using a high powered eyepiece.

Thanks, for the input on focus for collimation, I will attempt another go at it after work today. Unfortunately, this was marketed as a telephoto lens when originally sold in the 1980s, and I don't own any of the necessary eyepieces or adapters etc. to use it as a telescope, so my collimation test is using the camera on live view.


Edited by jstrandberg, 23 August 2019 - 12:22 PM.

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#12 Boom

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Posted 23 August 2019 - 01:04 PM

There probably isn't much you can do about the CA.  Didn't someone say this lens has refractive elements in the baffle tube?  I think the scope is capable of a sharper image, though.  It's likely soft due to movement, focus, and low light. 

 

Were you using a tracking mount?  Care to share the EXIF data?

 

Edit:  I see it was 1/200th & 1/50th ISO-400.  One seems underexposed and both probably have blurring from shutter speed.  That CA sure is annoying though.


Edited by Boom, 23 August 2019 - 01:20 PM.


#13 Taosmath

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Posted 23 August 2019 - 01:49 PM

Thanks, for the input on focus for collimation, I will attempt another go at it after work today. Unfortunately, this was marketed as a telephoto lens when originally sold in the 1980s, and I don't own any of the necessary eyepieces or adapters etc. to use it as a telescope, so my collimation test is using the camera on live view.

If you use an artifical star, place it as far away from your camera as you can (maybe in your yard if you have one ?) and get the best focus on the live screen that you can. Then defocus very slowly in one direction until you see a central point surrounded by 2 or 3 rings.  Take a picture.  Then focus again and this time defocus very slowly in the opposite direction until again you see 2 or 3 rings.  If you have a collimation problem it will look like the images  in the second row (taken from Suiter's book).  For comparison I also attach images from the book showing perfect optics.

 

Screen Shot 2019-08-23 at 12.41.51.png

 

Screen Shot 2019-08-23 at 12.41.11.png

 

Let us know how you get on.

 

Colin


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#14 Bowlerhat

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Posted 23 August 2019 - 04:48 PM

 C90 500mm f5.6

What scope is it? I've never seen this configuration before.



#15 bbqediguana

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Posted 23 August 2019 - 04:57 PM

What scope is it? I've never seen this configuration before.

I believe it was made by Synta and their model is called the MC90. I had one many, many years ago as the Canadian version (Vista MC90). As I recall, the views through my Orion Short Tube 80 were way better.

 

https://www.cloudyni...cassegrain-r731

 

Cheers!

Rick



#16 Boom

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Posted 23 August 2019 - 05:31 PM

What scope is it? I've never seen this configuration before.

 

 

I believe it was made by Synta and their model is called the MC90. I had one many, many years ago as the Canadian version (Vista MC90). As I recall, the views through my Orion Short Tube 80 were way better.

 

https://www.cloudyni...cassegrain-r731

 

Cheers!

Rick

 

 

No.  While Synta did produce the MC90, the lens discussed in this thread is the old USA C90 500mm Mak.  It's the short FL cousin to the ubiquitous 1000mm C90

 

https://www.picclick...-500mm-f-56.jpg

 

https://www.keh.com/...nual-focus.html


Edited by Boom, 23 August 2019 - 05:33 PM.

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#17 bbqediguana

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Posted 23 August 2019 - 05:54 PM

No.  While Synta did produce the MC90, the lens discussed in this thread is the old USA C90 500mm Mak.  It's the short FL cousin to the ubiquitous 1000mm C90

 

https://www.picclick...-500mm-f-56.jpg

 

https://www.keh.com/...nual-focus.html

Oops! Sorry about that! Thank you for posting the correct information.



#18 Bowlerhat

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Posted 23 August 2019 - 07:37 PM

Oh what? that one has 500mm focal length? I thought it is just a version of a C90 with longer focals.

Interesting.

 

Ah  that MC90 reminds me of the Meade97.



#19 jstrandberg

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Posted 23 August 2019 - 11:43 PM

If you use an artifical star, place it as far away from your camera as you can (maybe in your yard if you have one ?) and get the best focus on the live screen that you can. Then defocus very slowly in one direction until you see a central point surrounded by 2 or 3 rings.  Take a picture.  Then focus again and this time defocus very slowly in the opposite direction until again you see 2 or 3 rings.  If you have a collimation problem it will look like the images  in the second row (taken from Suiter's book).  For comparison I also attach images from the book showing perfect optics.

 

attachicon.gif Screen Shot 2019-08-23 at 12.41.51.png

 

attachicon.gif Screen Shot 2019-08-23 at 12.41.11.png

 

Let us know how you get on.

 

Colin

This is my test with the artificial star 24 feet away. I never did see the nice concentric circles that are in your pictures, as the image got more out of focus the image just began to look like a tie-died shirt pattern with all fuzzy bands of different colors.

I don't know what inside and outside focus are so I used counter-clockwise and clockwise rotation of the focusing nob for labeling the pictures.

Although I hope I am wrong, I get the feeling by the amount of CA in these pictures, that this lens just might be a POS.

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  • fOCUS.jpg

Edited by jstrandberg, 23 August 2019 - 11:46 PM.


#20 Taosmath

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Posted 24 August 2019 - 01:43 AM

This is my test with the artificial star 24 feet away. I never did see the nice concentric circles that are in your pictures, as the image got more out of focus the image just began to look like a tie-died shirt pattern with all fuzzy bands of different colors.

I don't know what inside and outside focus are so I used counter-clockwise and clockwise rotation of the focusing nob for labeling the pictures.

Although I hope I am wrong, I get the feeling by the amount of CA in these pictures, that this lens just might be a POS.

Well, I don't consider myself an expert on star testing refractors, and your artificial star is too close to be a stringent test, but I see two things.

 

1. The fact that the central dot in the  3rd column of pictures is in the center of the image both inside and outside of focus suggests that collimation is good.

2) The fringing does seem to suggest significant chromatic aberration.  While I am not knowledgeable enough to say for sure that the CA is the cause of the softness of your images, it seems plausible.

 

I would wait for the input of experts before making that conclusion.



#21 Boom

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Posted 24 August 2019 - 01:54 AM

Have you tried taking pictures of static, bright terrestrial objects?

At least you'll find out whether the lens is sharp, before moving on to more challenging subjects.

#22 Jon Isaacs

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Posted 24 August 2019 - 03:03 AM

Just a comment:

 

If the Collimation procedure is the same as with the 1000 mm FL version, the primary mirror must be removed to adjust the Collimation.  This is not for the faint hearted.

 

Jon



#23 jstrandberg

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Posted 24 August 2019 - 09:23 AM

Have you tried taking pictures of static, bright terrestrial objects?

At least you'll find out whether the lens is sharp, before moving on to more challenging subjects.

Yes, after my moon shots and the dismal results, I took some pics of a real estate sign about 50-75 feet away -- same soft focus and CA.



#24 Vla

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Posted 24 August 2019 - 10:31 AM

This is my test with the artificial star 24 feet away. I never did see the nice concentric circles that are in your pictures, as the image got more out of focus the image just began to look like a tie-died shirt pattern with all fuzzy bands of different colors.

I don't know what inside and outside focus are so I used counter-clockwise and clockwise rotation of the focusing nob for labeling the pictures.

Although I hope I am wrong, I get the feeling by the amount of CA in these pictures, that this lens just might be a POS.

For monochromatic correction, green filter is necessary (and less of overexposure). I put such design through raytrace, and, unless it has a subaperture corrector of some kind, it has to be heavily compromised optically.To get such a fast focal ratio in a Gregory-Maksutov, corrector radii have to be much weaker than what the optimal configuration requires. That causes excessive chromatism, which in addition requires compromising the central line correction (it could be somewhat improved by weakening rear radius, but it would further worsen chromatism). Better performance could be achieved with aspherizing the primary (about -0.18 conic would eliminate spherical aberration, or allow for less chromatism with still acceptable spherical), but they nearly certainly didn't go this route here. The central line correction is little worse than 1/2 wave p-v, red F-line is at about 2.5 p-v level, and blue F-line somewhat more (comparable to a bit faster than f/5 100mm achromat, but actually worse than that because it is primary spectrum). Field is practically flat, with negligible astigmatism/coma, some lateral color (c.obstruction is 48mm). Simulation of edge image, and 3-bar pattern (right) for shows it leaves much to be desired (note that there is no way to know how close it is to the actual design, but should be fairly close).

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  • 56.png

Edited by Vla, 24 August 2019 - 12:39 PM.

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#25 jstrandberg

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Posted 25 August 2019 - 12:04 AM

For monochromatic correction, green filter is necessary (and less of overexposure). I put such design through raytrace, and, unless it has a subaperture corrector of some kind, it has to be heavily compromised optically.To get such a fast focal ratio in a Gregory-Maksutov, corrector radii have to be much weaker than what the optimal configuration requires. That causes excessive chromatism, which in addition requires compromising the central line correction (it could be somewhat improved by weakening rear radius, but it would further worsen chromatism). Better performance could be achieved with aspherizing the primary (about -0.18 conic would eliminate spherical aberration, or allow for less chromatism with still acceptable spherical), but they nearly certainly didn't go this route here. The central line correction is little worse than 1/2 wave p-v, red F-line is at about 2.5 p-v level, and blue F-line somewhat more (comparable to a bit faster than f/5 100mm achromat, but actually worse than that because it is primary spectrum). Field is practically flat, with negligible astigmatism/coma, some lateral color (c.obstruction is 48mm). Simulation of edge image, and 3-bar pattern (right) for shows it leaves much to be desired (note that there is no way to know how close it is to the actual design, but should be fairly close).

Thanks for taking the time to produce such a detailed report. Although I understand very little of the technical analysis, I think you were saying that this is a flawed design that cannot be adequately corrected and that I should toss this lens in the trash.




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