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Determining best backfocus distance through star testing

Equipment Refractors Optics
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Eyecon avatar
Eyecon avatar
I'm trying to confirm a practical observation of how star shapes changes from being radially stretched to tangentially stretched on various optical systems and the relation ship between star shapes and back focus. I know everyone keep referring to the picture below as a guide for setting back focus distance. But with two different setups I regularly use for AP, I noticed that the extra focal and intra focal star shapes are the opposite between these two different systems: on my refractor, intra focal corner star shapes are radially stretched while extra focal edge stars are tangentially stretched. On my Newtonian with the 1x coma corrector it's the opposite: extra focal corner star shapes are radially stretched, while intra focal are tangentially stretched.




Based on this observation , instead of relying on the above guide image, shouldn't the rule for adjusting/tuning backfocus be as follows:
First run autofocus, observe edge star shapes and nudge focus in or out by a few steps: if edge star shapes improve but nudging the focus point outwards it would mean the sensor is too far from the optical elements and if they improve by nudging the focus inwards it would mean the sensor is too close? Image below illustrates what I mean, the lens shaped elements represents the optical elements that have a specific back focus requirement for optimal performance. 





Very curious to know what you all think.
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Wim van Berlo avatar
Wim van Berlo avatar
If the star shape changes as you describe, isn’t that a sign of astigmatism?

cs,

Wim
ManuManu avatar
ManuManu avatar
Hello,
This default is said "coma". The classic astigmatism (due to overstress on optical elements) appears also in the center. this not the case here. I am very surprised by the opposite behavior of the refractor versus the reflector. For instance on my Epsilon (reflector), I tune the tilt adapter while checking the figures like Eyecon: If I am too close, the stars appears like figure 1.
My experience is that you have to find the correct backfocus by experimenting with small spacers because you are not fully sure of the backfocus of all of your elements. For instance what is the backfocus introduced by a tilt adapter (like the built-in in the ZWO cameras)? Maybe 0.2, maybe 0.3?
By the way some people propose to focus not excatly to the center, to have a lower dispersion between center and corners.
Not sure it helps but this my experience on that…
CS
Eyecon avatar
Eyecon avatar
Hello,
This default is said "coma". The classic astigmatism (due to overstress on optical elements) appears also in the center. this not the case here. I am very surprised by the opposite behavior of the refractor versus the reflector. For instance on my Epsilon (reflector), I tune the tilt adapter while checking the figures like Eyecon: If I am too close, the stars appears like figure 1.
My experience is that you have to find the correct backfocus by experimenting with small spacers because you are not fully sure of the backfocus of all of your elements. For instance what is the backfocus introduced by a tilt adapter (like the built-in in the ZWO cameras)? Maybe 0.2, maybe 0.3?
By the way some people propose to focus not excatly to the center, to have a lower dispersion between center and corners.
Not sure it helps but this my experience on that...
CS

yeah I was equally surprised because my refractor and my SCT behave the same, but my Newtonian does the opposite. If my logic around BF adjustment is correct, the radially  stretched edge stars means the back focus distance is too big in case of my refractor and my SCT, and needs to be reduced which is the opposite of what the commonly used BF diagram suggests. I have the Askar BF adjuster which allows me to adjust backfocus with everything installed in increments of .05mm.  I just want to confirm that the logic from my diagram is correct and that the common BF diagram is not applicable to all optical designs
Chris H avatar
Chris H avatar
I have found your technique to be much better than the images of star shapes also. I saw this article by Roland Christen of Astro-Physics a few years ago which convinced me to use that technique. 

https://www.astro-physics.info/tech_support/accessories/photo_acc/optimizing-your-field-flattener.pdf
John Hayes avatar
John Hayes avatar
…The classic astigmatism (due to overstress on optical elements) appears also in the center. this not the case here.

I want to clarify that “classic astigmatism” is a 3rd order Seidel aberration that varies as the square of the field height.  It’s a common misperception that anamorphic errors in the optics cause astigmatism.  Anamorphic errors in the optics (as well as misalignment errors in two mirror systems cause “astigmatic errors”, which look like astigmatism in an image but don’t vary with the field.  It may seem like a fine point to distinguish between “astigmatic errors” and astigmatism but it’s important to understand that these are two different things.  The diagrams shown by the OP show astigmatism in the field, which is what you’ll get from a anastigmat.  A Newtonian is NOT an anastigmat and it will show primarily coma, which is linear in the field.

Having said all that, I like the method that the OP proposes.  The only thing that I might recommend is that when you focus, you restrict the algorithm to stars in the central zone of the image.

John
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Eyecon avatar
Eyecon avatar
Chris H:
I have found your technique to be much better than the images of star shapes also. I saw this article by Roland Christen of Astro-Physics a few years ago which convinced me to use that technique. 

https://www.astro-physics.info/tech_support/accessories/photo_acc/optimizing-your-field-flattener.pdf

Thanks for sharing this article, I'm glad it's a known procedure, I find it surprising that every time I see a post about edge star shapes that people refer to the diagram from OPT where in fact this diagram is only true for certain optical designs but not others.
Eyecon avatar
Eyecon avatar
John Hayes:
…The classic astigmatism (due to overstress on optical elements) appears also in the center. this not the case here.

I want to clarify that “classic astigmatism” is a 3rd order Seidel aberration that varies as the square of the field height.  It’s a common misperception that anamorphic errors in the optics cause astigmatism.  Anamorphic errors in the optics (as well as misalignment errors in two mirror systems cause “astigmatic errors”, which look like astigmatism in an image but don’t vary with the field.  It may seem like a fine point to distinguish between “astigmatic errors” and astigmatism but it’s important to understand that these are two different things.  The diagrams shown by the OP show astigmatism in the field, which is what you’ll get from a anastigmat.  A Newtonian is NOT an anastigmat and it will show primarily coma, which is linear in the field.

Having said all that, I like the method that the OP proposes.  The only thing that I might recommend is that when you focus, you restrict the algorithm to stars in the central zone of the image.

John

Thanks for the clarification about astigmatism John, very helpful to understand how these effects are classified and named.

Great suggestion on restricting autofocus to near on axis stars, I'm currently verifying the procedure indoors using my 80PHQ with .76x reducer and a 9um fiber optic artificial star. I'm using NINA to verify the results of the procedure (if the star shapes get better if I follow the procedure suggested vs. OPT's diagram) and I only have one star to worry about at the moment. Each time I change the spacing, I slew the telescope so that the artificial star is in the center, then I run autofocus in NINA, and then I slew the telescope so the star moves to the edges to see the results of the adjustement. So far, the results of the suggested procedure, tell me to reduce the back focus distance when my edge stars point to the middle of the sensor (radial stretching) which is the exact opposite of the commonly used diagram!
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Ashraf AbuSara avatar
Ashraf AbuSara avatar
That's how the Aberration Inspector in Hocus focus works inside NINA to determine Tilt and Backfocus. It splits the image into different squares, and then calculates the focus curve for each corner and center, and uses that information to make further determination about those factors.

If you calculate the actual distance your focuser moves with each step and feed it into the application, it will even tell you exactly how much closer or farther you need to move your backfocus and tilt in microns and tells you whether you need to do it inward or outward..
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Eyecon avatar
Eyecon avatar
Posting a result of my indoor testing of the procedure suggested above. According to the commonly used OPT diagram, increasing the spacing  between sensor and reducer should change the star shapes around the edges but as you can see from the test below, reducing the distance was the way to go. I'm not sure if  I can make definitive conclusion on tilt based on the below but it seems the left side of the image is not as consistent as the right, so it could be an indicate of left to right tilt. 


17.5mm spacing


16.5mm spacing

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Eyecon avatar
Eyecon avatar
Ashraf AbuSara:
That's how the Aberration Inspector in Hocus focus works inside NINA to determine Tilt and Backfocus. It splits the image into different squares, and then calculates the focus curve for each corner and center, and uses that information to make further determination about those factors.

If you calculate the actual distance your focuser moves with each step and feed it into the application, it will even tell you exactly how much closer or farther you need to move your backfocus and tilt in microns and tells you whether you need to do it inward or outward..

Yes understood, when you are very close to ideal BF I find that the results of aberration inspector are not very accurate especially with less than ideal optics. This is understandable because for example as you can see above, the 80PHQ .76x reducer is just not capable of producing perfectly round stars near the edges. It's my understanding that aberration inspector just looks at FWHM values and not at eccentricity when assessing the focus curves of the different sections.

In any case, I'm just happy to have some solid experimental data to support the procedure and show that following the common diagram for BF adjustment is not the best way of addressing back focus.
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James Peirce avatar
James Peirce avatar
It should not be different. What is *usually* happening when things seem reversed is that coma, which can be confused against a chart like this as stars radiating out from center, prompts a desire to move the sensor out, and likely away from ideal backfocus.

And it can even *look* like things are improving because other aberrations (typically resulting in collective bloating/defocusing/spectral separation) of stars start to hide the coma, making the stars more “round” even as light is being scattered.

Not that I’m confident that’s what’s happening here, because there’s other room for misinterpretation and I’ve not seen star sample images. But it’s a pretty easy trap to fall for when chasing a chart like this and not being quite sure how to tell those aberrations apart from those caused by incorrect backfocus.

If a spot diagram is available for your telescope+corrective optics that can provide a good hint.
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Eyecon avatar
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James Peirce:
It should not be different. What is *usually* happening when things seem reversed is that coma, which can be confused against a chart like this as stars radiating out from center, prompts a desire to move the sensor out, and likely away from ideal backfocus.

And it can even *look* like things are improving because other aberrations (typically resulting in collective bloating/defocusing/spectral separation) of stars start to hide the coma, making the stars more “round” even as light is being scattered.

Not that I’m confident that’s what’s happening here, because there’s other room for misinterpretation and I’ve not seen star sample images. But it’s a pretty easy trap to fall for when chasing a chart like this and not being quite sure how to tell those aberrations apart from those caused by incorrect backfocus.

If a spot diagram is available for your telescope+corrective optics that can provide a good hint.

understood, in the above case with my 80PHQ refactor there was clear change (not necessarily improvement) in the shape of the edge stars that was only possible by doing the opposite of what the generic star shape chart showed. Below is the spot diagram for the corrector I'm using. The star shapes certain match although the scale is off but I'm also testing using a relatively large source vs an actual star. At the edge of my APS-C (~12.5mm from center left to right), my edges stars look like the 13.914mm spot below. So I guess that's just the best that this reducer can do. Am I reading the spot diagram correctly?

James Peirce avatar
James Peirce avatar
understood, in the above case with my 80PHQ refactor there was clear change (not necessarily improvement) in the shape of the edge stars that was only possible by doing the opposite of what the generic star shape chart showed. Below is the spot diagram for the corrector I'm using. The star shapes certain match although the scale is off but I'm also testing using a relatively large source vs an actual star. At the edge of my APS-C (~12.5mm from center left to right), my edges stars look like the 13.914mm spot below. So I guess that's just the best that this reducer can do. Am I reading the spot diagram correctly?

What you’ve got there is rendering from a sample as measured from center (it sounds from your reply like you’re reading it correctly). What you get in an image is going to vary depending on tilt, pixel size, etc., and it’s going to shift a bit depending on whether you are inside or outside of ideal focus. But it can give you a good idea of what you might see in practice.

In practice, with copy variation and small pixel sizes, as well as other imperfections in the optical train like tilt, the aberrations you see tend to look a bit worse unless dialed in with particular care, recording to large pixels, etc.

But if what you are seeing corresponds as you say, you may actually be quite close to correct backfocus (which I imagine needs to be met when using the reducer, even if not needed for the telescope). I don’t know about this specific telescope and reducer, but in many cases it is close to what is recommended. Especially when the focal ratio is pretty aggressive as there’s a small margin of error.

You may be at or close to what the telescope and reducer can reasonably achieve. I would expect there to be some fairly noticeable aberrations with that telescope and reducer on APS-C.

Looking at the two charts you posted, it sure looks to me like you are balancing some aberrations, as you’ve got light scattering both away from center and radially relative to center. Your best result might be in just trying to balance those aberrations, but maybe not. It is nice to use a color sensor for this, if you have one, as you can see how spectra is separating. For example, with my SVX080T-35V, and the rebranded reducer StellarVue sells for it, there is some noticeable coma on the outside using an APS-C sensor at the more ideal backfocus. If I move outside that backfocus it can *look* like the stars at the edges star to clean up a bit, but light is starting to misalign across spectra more, and you start to get more noticeably varied spot sizes across spectra at the center which begins to scatter detail and affect star color rendering. This negative adjustment also shows up as larger numbers when measuring star sizes.
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