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How I fixed elongated stars (astigmatism and coma) in my FSQ-106ED

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#1 pathint

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Posted 27 February 2021 - 02:25 PM

A while ago I wrote about my FSQ-106ED having astigmatism, showing as elongated stars across the field whenever focus is not perfect. Tak America deemed it as within spec and refused to further service it. I was able to improve it a bit, but it kept bothering me and I sold it and then got a new FSQ-106ED. 

 

And the new one also has astigmatism... Using the WavefrontEstimate script in Pixinsight, it is confirmed to account for 19.3nm of rms wavefront error to total amount of 23.8nm, bringing the Strehl to "only" 92.4. As you can see from the defocused stars there is a 90 degree rotation going from intra to extra focus. It is also directly mapped in the wavefront error plot.

Screen Shot 2021-02-27 at 1.41.07 PM.png

In a star field, the center of the field is fine, but at some certain corners the elongation is rather noticeable.

 

In addition, there is also coma everywhere, showing as non symmetric elongation of stars. I tried to collimate the scope myself using the three sets of push-pull screws on the front lens G1. It is easy to reduce coma, but astigmatism changed very little. Astigmatism does not occur for rotational symmetric optics, so it means there must be a misalignment in G2. 

 

Being a theoretical physicist I decided to get to the bottom of it. I learned about these wavefront aberrations and did some derivation myself:

Screen Shot 2021-02-27 at 1.49.01 PM.png

Now I understood the physical origin of coma and astigmatism. I came up with a theory what exactly is happening.

 

it turns out the G2 lens is de-centered. This itself introduces both coma and astigmatism. Using the three push-pulls in front, the G1 lens can be tilted in the direction of G2 decentering. This also introduces coma and astigmatism in the same direction. In both lenses, the coma is the main effect. When carefully adjusted, the coma on G1 and on G2 can be exactly canceled.  However, astigmatism is an "even" effect. This means the small amount of astigmatism are of the same sign and always add up. (In principle, one could tilt G1 in a perpendicular direction as the G2 decentering to cancel astigmatism, but this would not have canceled coma.)

 

This is why Astigmatism is so hard to eliminate -- one has to make sure *every* element is perfectly centered and astigmatism free. For a petzval design with four lenses, it is no wonder why so many scopes have some astig. Even if they are precisely aligned in factory, shipping can easily mess things up. It has very tight tolerance for error.

 

(I will write in the next post about what I did to improve.)


Edited by pathint, 27 February 2021 - 07:04 PM.

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#2 pathint

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Posted 27 February 2021 - 03:01 PM

Now it comes to the experimental verification of my theory. I started off with a situation where the dominant aberration is astigmatism. I wanted to reduce it or eliminate it. Here are the guidelines.

 

 

(1) You can screw off the dew shield, and take off the electric tape protecting the gap between G1 and G2. There are six peripheral hex screws on the G2 cell, forming three pairs of push-pull on opposite sides.

IMG_0514.jpeg

Use a narrowband filter with a wider pass range. In my experience a cheap 12nm OIII filter works the best, as usually refractors aim at the best creation for green light. 

 

(2) Now, push and pull the two opposing hex screws that matches the direction of star elongation *intra-focus*, which is the position obtained by moving the sensor *closer* to the objective. This is very important. If you use the 90 degrees rotated direction obtained extra-focus, you will only make things worse. If there are no perfect match of the screws and elongation direction, you need to combine two pairs to make a vector sum.

 

Just by looking at one star, there is no easy way to tell which screw to push and which to pull, and you have to start with a gamble and see if things improve. For this reason, only make a very small move, and always pull before you push so you don't pinch the lens. After you do a push-pull motion, things will appear worse, since coma of G1 and G2 will no longer cancel.

 

(3) Now you can use the push/pull tilt adjustments on G1 to nullify coma. Before you begin, it is *very important* not to over tighten any push/pull set. Always loosen before you tighten, even if you only planned to tighten by a very small amount. Otherwise it will cause strain on the lens. This generates all sorts of unwanted effects, including spherical aberration. You also don't want to move the G1 screws by too much -- if the gap between G1 and G2 cells changes by more than 30%, significant spherical aberration will occur (according to the parameters for TOA-130, but I assume FSQ should be around the same). It is advisable (by Darrin at TNR) to use a high quality caliper to record the initial values of the G1/G2 cell gap and make sure you don't wander off too far. (Mine are 1.3mm +- 5% but yours may vary.)

 

The correct adjustment is to *tighten* the big screw (pull screw) that your G2 push/pull motion is *pointing away from*. For example. if you pushed the bottom G2 screw *up* and pulled top G2 screw *up*, then one needs to *tighten* the *bottom* pull screw on G1. Or if there happens to be push/pull sets on the *top* position, loosen the push screw on that set (and tighten pulls afterwards to lock). A nuisance (or design) on the FSQ is that no G2 screws and G1 screws align -- they always are off by 30 degrees angularly. You may have to adjust another push/pull on G1 by a smaller amount to match the direction. Use the histogram feature on your capture software to highlight the small brightness difference on the diffraction ring. When the brightness of opposite sides on the ring is balanced, you have once again eliminated coma. 

 

(4) Now, examine if astigmatism has improved. One can either check if the diffraction rings are less elliptical, or check if the corner stars have become rounder. If the elongation got worse, it does *not* mean you have chosen the wrong pair of opposite hex screws on G2! Instead you made the wrong gamble in (2), and you need to reverse the direction of push/pull on the *same* pair. Go through (2)-(3) until astigmatism becomes unoticeable to you. 

 

(5) Congrats, you have now eliminated both coma and astigmatism. As long as the gap between G1 and G2 are close to the factory value, and as long as there are no strain on the glass by conflicting push and pulls, there shouldn't be a problem with spherical aberration or chromatic aberration. But it's always worth making sure. If there is a problem adjust the gap. If undercorrected (extra-focal more blurry than intra-focal), reduce the gap by a uniform amount. If overcorrected, increase the gap.  

 

That's it, guys! Here are my improved results. Astigmatism has been reduced from 19nm to 15nm. This may not sound like a dramatic improvement for astigmatism alone, but it is a 20% percent drop. What's more important, with better centering there is less amount of coma to cancel. Since the perfect cancelation of coma in theory only happens at the center, reducing them on each lens in the first place leads to better performance off-axis.

 

The Strehl now reads 96%. I can further improve it if I wanted to, but my 3.8um pixels can no longer detect any elongation.

Screen Shot 2021-02-27 at 3.00.54 PM.png


Edited by pathint, 27 February 2021 - 07:33 PM.

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#3 Midnight Dan

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Posted 27 February 2021 - 03:18 PM

Excellent!  Nice job and nice writeup.  Do you have any before/after images of the stars effect of the problem?

 

-Dan



#4 ntph

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Posted 27 February 2021 - 03:24 PM

Theoretically, I could be a physicist. Theoretically.

 

I have hopefully solved a similar problem, but in a much different way.

 

When I saw that I was getting some very nasty flares on stars on images recently, I investigated the problem very systematically, for me. I took off the camera, put in an ocular and actually looked through the telescope. I know, heresy, but...the stars looked great. So what gives? I racked out and in and saw a problem. Theoretically, it might be solved, but certainly not by me. I got another camera, thought by some here both initiates and veterans to be beyond redemption, a stock DSLR and took the following photos, inside and outside of focus. 

inside focus2.jpg

outside focus2.jpg

 

I solved my problem by picking up a pencil and addressing an overkill-even-for-me padded  package containing my misbehaving objective and shipping it back to Astro-Physics, where it rests awaiting further tender ministrations from Roland and his crew. Hopefully they will deal with it before summer solstice...

 

I marvel that anyone can actually do all this math. It has been so long since I did anything meaningful with math it is a wonder I can count above ten without taking off my socks. 


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#5 pathint

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Posted 27 February 2021 - 03:41 PM

Excellent!  Nice job and nice writeup.  Do you have any before/after images of the stars effect of the problem?

 

-Dan

I took a picture of a star field using Asiair, and confirmed that elongation is gone, but didn't save the fit file.

 

Here is a "before" picture of a corner on an ASI1600MM.

AE061A21-8735-4F01-AB88-1183DAF0DAB2_1_201_a.jpeg
Not that bad, if you are not a perfectionist; consistent with the 92% Strehl. But it was fluctuating, sometimes more astig, sometimes less. that's why couldn't trust my eyes anymore and did a wavefront analysis.

 

BTW, a shoutout to the developer of WavefrontEstimator script in Pixinsight. It's a great tool and I highly recommend it!


Edited by pathint, 27 February 2021 - 04:02 PM.

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#6 pathint

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Posted 27 February 2021 - 03:59 PM

Theoretically, I could be a physicist. Theoretically.

 

I have hopefully solved a similar problem, but in a much different way.

 

When I saw that I was getting some very nasty flares on stars on images recently, I investigated the problem very systematically, for me. I took off the camera, put in an ocular and actually looked through the telescope. I know, heresy, but...the stars looked great. So what gives? I racked out and in and saw a problem. Theoretically, it might be solved, but certainly not by me. I got another camera, thought by some here both initiates and veterans to be beyond redemption, a stock DSLR and took the following photos, inside and outside of focus. 

attachicon.gifinside focus2.jpg

attachicon.gifoutside focus2.jpg

 

I solved my problem by picking up a pencil and addressing an overkill-even-for-me padded  package containing my misbehaving objective and shipping it back to Astro-Physics, where it rests awaiting further tender ministrations from Roland and his crew. Hopefully they will deal with it before summer solstice...

 

I marvel that anyone can actually do all this math. It has been so long since I did anything meaningful with math it is a wonder I can count above ten without taking off my socks. 

That's a great solution!! I wish I could just ask TNR to fix it for me. But they insisted that the astigmatism on my last FSQ was within design parameter and didn't want to have the scope shipped...

 

The math was not super sophisticated -- I didn't calculate precisely by what percentage of a turn each screw needed to be adjusted. I just found a theoretical explanation that helped me identify *which* screws to turn and in what direction.


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#7 calypsob

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Posted 27 February 2021 - 03:59 PM

I have always wondered why optics on refractors do not rest on a custom black molded/die cut/ cryo machined, silicone oring or cylindrical donut in the aluminum cell. Since silicone has no memory you could center the elements and then make micro adjustments with screws. Instead they float in the cell, some I have seen use cork pads. How do they rest in a Tak?


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#8 lucam

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Posted 27 February 2021 - 04:00 PM

Interesting post and I will give a shot at the wavefront estimator tool in PI tool. However, from reading the documentation of the tool in PI it appears that it is really meant to estimate on-axis aberrations while you were concerned with star shapes in the corners. Obviously, if this fixed the problem that's great but isn't one reason for corner problems (other than the obvious sensor tilt, which you may have addressed with the lens plane instead of the sensor plane) Petzval curvature? I haven't worked through the calculation myself but if I were to guess, I would imagine that the field curvature is affected by the spacing between the two doublets, which you didn't touch.

 

Maybe you are pointing to the weak link of the alignment in this telescope that is most easily affected by transportation, for example. So while one may think that the problem is purely off-axis, it is in fact a by-product of the lens shift you described.

 

Thank you for this thought provoking post!

 

Luca (also a physicist)


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#9 pathint

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Posted 27 February 2021 - 04:08 PM

I have always wondered why optics on refractors do not rest on a custom black molded/die cut/ cryo machined, silicone oring or cylindrical donut in the aluminum cell. Since silicone has no memory you could center the elements and then make micro adjustments with screws. Instead they float in the cell, some I have seen use cork pads. How do they rest in a Tak?

I think it has to do with the fact that APO's have many elements and degrees of freedom. For astigmatism to cancel, every single element needs to be perfectly aligned (as I've seen somewhere to a few microns). They cannot cancel each other without producing other more damaging aberrations. This makes micro-adjustments infeasible for end users without interferometry.

 

For an SCT, since all mirrors are circular, you just need to adjust the tilt (which is the same as de-centering for a sphere) of the secondary mirror.


Edited by pathint, 27 February 2021 - 07:01 PM.

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#10 pathint

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Posted 27 February 2021 - 04:32 PM

Interesting post and I will give a shot at the wavefront estimator tool in PI tool. However, from reading the documentation of the tool in PI it appears that it is really meant to estimate on-axis aberrations while you were concerned with star shapes in the corners. Obviously, if this fixed the problem that's great but isn't one reason for corner problems (other than the obvious sensor tilt, which you may have addressed with the lens plane instead of the sensor plane) Petzval curvature? I haven't worked through the calculation myself but if I were to guess, I would imagine that the field curvature is affected by the spacing between the two doublets, which you didn't touch.

 

Maybe you are pointing to the weak link of the alignment in this telescope that is most easily affected by transportation, for example. So while one may think that the problem is purely off-axis, it is in fact a by-product of the lens shift you described.

 

Thank you for this thought provoking post!

 

Luca (also a physicist)

Hi Luca,

 

The thing is, the only true "on-axis" aberrations are spherical aberrations of various orders. So the very fact that "on-axis" wavefront error has astigmatism and coma components means that something is misaligned or mis-shaped. In other words, there is actually no axis in my scope. I'ts hard to imagine Takahashi would fail to use a rotation symmetric lens (they are literally produced by grinding and polishing via rotations), so it must be an alignment issue. 

 

In practice, both the center and the corner has elongation. The elongation direction "flows" across the field (like a liquid crystal if this is familiar to you), sometimes with one or two half-vortices in the imaging field. Other than these half vortices everywhere else in the image has elongation. The astigmatism across the whole field is minimized if the half vortices combine into a full vortex at the image center -- this is when "on-axis" astigmatism is eliminated and the whole pattern becomes rotation symmetric. If you fix the astigmatism "on-axis", the improvement also propagates to elongated stars the corners, since they result from the same misalignment. 

 

The FSQ-106ED design is a Petzval lens design, which minimizes the Petzval curvature, as I understand. In practice I noticed no matter how misaligned an FSQ is, the field curvature generally remains very small. (I don't fully understand the details.)  So I treated as a higher-order error in my adjustments. Plus, mild field curvature alone isn't that bad from a purely aesthetic point of view. It only causes bloated stars off-axis, but it alone does not cause elongation or comets.


Edited by pathint, 27 February 2021 - 07:02 PM.

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#11 thekubiaks

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Posted 27 February 2021 - 06:04 PM

I wonder how Darrin at Takahashi Houston would deal with an FSQ106 that has been returned to them for repair.  I sent my FSQ106 there for repair and the numbers improved somewhat.  I'd love to hear his technique. 



#12 AgilityGuy

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Posted 27 February 2021 - 06:18 PM

My original FSQ-106 purchased about 4 years ago had elongation in all four corners.  It was the previous cream colored model.  I sent it back to Houston and it was replaced with the current model which the optics have looked great by my eyeball evaluation of the images taken through it.  The only time I've seen any strangeness in the corners is using a 0.73 reducer along with my full frame DSLR.  That could be attributed to the reducer as much as anything.  It's so slight I haven't even spent any time trying to work it up.  



#13 pathint

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Posted 27 February 2021 - 07:08 PM

I wonder how Darrin at Takahashi Houston would deal with an FSQ106 that has been returned to them for repair.  I sent my FSQ106 there for repair and the numbers improved somewhat.  I'd love to hear his technique. 

If you take a "half-full" mentality, seeing astigmatism as the main aberration can be a good thing. It means the more severe aberrations (more damaging to PSF and Strehl) -- SA and Astigmatism -- have been carefully distributed on the lens elements to exactly cancel. Astigmatism is one aberration that cannot be canceled this way. Technically it can, but at the cost of severe coma.


Edited by pathint, 27 February 2021 - 07:09 PM.


#14 calypsob

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Posted 27 February 2021 - 07:47 PM

I wonder how Darrin at Takahashi Houston would deal with an FSQ106 that has been returned to them for repair.  I sent my FSQ106 there for repair and the numbers improved somewhat.  I'd love to hear his technique. 

Honestly Im surprised there are not videos on youtube documenting the process by now, a good production on this procedure would be fascinating for even people who dont do astronomy. 


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#15 calypsob

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Posted 27 February 2021 - 07:49 PM

I think it has to do with the fact that APO's have many elements and degrees of freedom. For astigmatism to cancel, every single element needs to be perfectly aligned (as I've seen somewhere to a few microns). They cannot cancel each other without producing other more damaging aberrations. This makes micro-adjustments infeasible for end users without interferometry.

 

For an SCT, since all mirrors are circular, you just need to adjust the tilt (which is the same as de-centering for a sphere) of the secondary mirror.

When you say aligned, are you referring to its spacing or literally the diameters being aligned at their edges? 



#16 pathint

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Posted 27 February 2021 - 08:53 PM

When you say aligned, are you referring to its spacing or literally the diameters being aligned at their edges? 

By aligned I mean the optical axes (at their centers) of all elements needs to be perfectly the same. Aligning lenses of different sizes at their edges isn’t meaningful. In this case there is no astigmatism or coma at the center of the field, no matter how large the spacing errors are. If the spacing is way off, at the center of the image you will have huge amount of spherical aberration, bloating your star, but it won’t be elongated.


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#17 calypsob

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Posted 27 February 2021 - 08:58 PM

By aligned I mean the optical axes (at their centers) of all elements needs to be perfectly the same. Aligning lenses of different sizes at their edges isn’t meaningful. In this case there is no astigmatism or coma at the center of the field, no matter how large the spacing errors are. If the spacing is way off, at the center of the image you will have huge amount of spherical aberration, bloating your star, but it won’t be elongated.

that makes sense, thanks for taking time to explain. 



#18 andysea

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Posted 27 February 2021 - 09:27 PM

Brilliant!

I would have never attempted that.

 

I wonder how Darrin at Takahashi Houston would deal with an FSQ106 that has been returned to them for repair.  I sent my FSQ106 there for repair and the numbers improved somewhat.  I'd love to hear his technique. 

My FSQ had to go back to Japan to get collimated. TNR was not able to remove the elongated stars at the corners. They said that the rear elements needed adjustment.

Now it performs extremely well with the IMX544, at least to my eye.



#19 pathint

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Posted 28 February 2021 - 12:53 PM

Brilliant!

I would have never attempted that.

 

My FSQ had to go back to Japan to get collimated. TNR was not able to remove the elongated stars at the corners. They said that the rear elements needed adjustment.

Now it performs extremely well with the IMX544, at least to my eye.

Yes, elongated stars can be thought of as what's left when other more severe problems of each lens are canceled out. This is the same reason why only the best corrected cassegrain design (Ritchey-Chretian) show astigmatism. Not that Dall-Kirkham and SCT designs don't have astigmatism. It is just swamped by coma.

 

It can only be eliminated if G1, G2, G3, G4 are all aligned. If there is an issue with G4, there is nothing can be done about it without sending it to Japan.


Edited by pathint, 28 February 2021 - 04:40 PM.



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