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ETX125 (vintage 2000) star test (so how good/bad is it?)

collimation Maksutov optics
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#26 cytan299

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Posted 02 September 2019 - 05:16 PM

Not too bad, not perfect. A small center zone, slight astigmatism and SA. What's interesting is how the DPAC and star test correspond to each other.

Is this a purple tube or blue tube ETX125?

It’s a blue tube.

 

I am really curious about how good my vintage, American made scope is. I don’t think it’s terrible, but is it considered good for 125 mm Maks?

 

 cytan


Edited by cytan299, 02 September 2019 - 05:17 PM.


#27 Boom

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Posted 02 September 2019 - 05:26 PM

It’s a blue tube.

I am really curious about how good my vintage, American made scope is. I don’t think it’s terrible, but is it considered good for 125 mm Maks?

cytan


I have a couple USA ETX125s in my possession and I've been meaning to DPAC them as well. Just haven't found the time to mess with them.

However, from what I've seen on a Russian website that does a bunch of bench testing, they put three old USA ETX125s through the paces and they ranged from OK to Great.
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#28 Boom

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Posted 02 September 2019 - 05:32 PM

https://translate.go...yfuikLzAaBCVZmA

 

https://translate.go...7Fagt_VwAAHT3wQ

 

https://translate.go...L8RYd3nTlpITy8Q

 

Links to the 3 samples.

 

P.S. those are interferograms, not DPACs.  


Edited by Boom, 02 September 2019 - 05:41 PM.

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#29 Asbytec

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Posted 08 September 2019 - 02:27 AM

From the above, I calculate from the ratio of the obstruction:

obs = 1.09/1.46 = 0.75 and thus the spherical wave error is 1-0.75 = 0.25 waves.  (see note below about f number of the etx125)

My guess is that the spherical wave error is between 0.2 to 0.25 waves.

 

Here's the aberrator simulation:

which looks pretty close to what I measured.

 

So, is 0.25 waves of spherical error typical of Maksutovs? Perhaps this is to be expected, see 

 

cytan

First, as mentioned, is astigmatism visible as classic 90 degree ovals on both sides of your defocused images and the plus star image at best focus from your video. You can have about 0.37 waves PV of astigmatism and be diffraction limited at 0.8 Strehl. I have no idea how much is present in your video. Need to look at that harder, but an initial look a Suiter seems to suggest it's not terrible. Not too dissimilar to his sims of 0.37 PV waves. The worse the astigmatism is, the more it will be seen with oval patterns at larger defocus. You might just barely see it in your defocused WinRoddier star test images. 

 

The difficulty with your Aberrator sim is you dialed in 0.25 waves of primary spherical aberration (3rd LSA) which is not going to give good results (although you certainly got close). That would be fine if you were testing the primary alone as the source of lower order spherical, but this scope is a system. You will not see spherical aberration from the primary alone. The reason is because the steeply curved negative meniscus lens generates a lot of higher order (secondary) spherical aberration. This is balanced by adding lower order spherical aberration of opposite sign generated on the primary mirror. Primary and secondary spherical aberrations do not have the same waveform so one cannot fully correct the other, however the correction can be pretty good depending on the residual error present. So, there is spherical aberration present, that much is pretty obvious. It's just difficult to tell what form it takes and how severe it is because it is a mix of aberrations and not the pure form of a single lower order aberration we're all familiar with. 

 

There is going to be some residual higher order spherical aberration (from the meniscus) present in the star test. What we are actually star testing is not a pure 3rd LSA primary spherical wavefront as produced in Aberrator, rather we are testing a hybrid (balanced high/low SA) wavefront that only approximates a spherical one to a varying degree. I recall Vla explaining (in one of your links above) the balanced form is actually represented in Aberrator as 5th HSA input. This residual secondary aberration will affect the star test which is very sensitive close to focus and it may show both characteristics of over and under correction (which your later WinRoddier images show). And the star test images will differ on both sides because of the residual error and that makes it more difficult to evaluate. Since some of the residual error is in the paraxial zones near the secondary obstruction, there is a chance this will affect the shadow breakout test as well. The shadow breakout test seems best suited for the pure lower order spherical aberration with a smooth 3rd LSA curve and a 0.3D obstruction as Eddgie often explains. However, this is not the actual wavefront you are testing, so you're results are likely off. I cannot simulate your patterns in Aberrator. Doing so may be complicated by the apparently under corrected medial zone (shown in Winroddier images) and a mix of additional errors not allowing us to test for the pure balanced high/low pure form of spherical or even the 0.25 LSA results above.

 

ETX 125.jpg

 

But, reading the star test above may be possible without quantifying it. Based on you later images, they do not look like 15 waves defocus to me. At large defocus, you should see more than one ring between the paraxial and marginal rings. I see only 1 ring and part of another (which is a consequence of slight miscollimation). The first thing readily apparent is a bright medial ring inside focus (and the same ring is dimmer outside focus) suggesting an under corrected zone as shown in Winroddier (and somethign Aberrator cannot simulate). Notice in the Winroddier "corrected images", that bright medial ring is missing. I don't know what Winroddier is doing, though, other than maybe showing what it should look like. The next thing is the shadow diameters appear to indicate over correction. As Pinbout often says, the larger shadow indicates the correction meaning larger shadow outside indicates over correction and visa versa. This is likely a result of the meniscus adding over correction of opposite sign (convex wave toward the primary) relative to the normal under correction from the primary. If you are indeed near the same defocus, and I suspect you are, the outside image is also a tad larger indicating some small amount of under correction, too. This is confirmed by the slightly brighter marginal ring outside focus. So, the ETX apparently has both and probably a consequence of its residual higher order spherical aberration. How good is it? I dunno, but I suspect astigmatism is the biggest problem. 

 

So, saying the wavefront error is 0.25 waves of spherical aberration is kind of misleading and not typical of MCTs. An MCT may well have 0.25 waves PV wavefront error or more somewhere, but it will almost certainly not be the same pure form of 3rd LSA seen in star test simulations generated by Aberrator. That amount of error may be present at either the center or the edge or only in a small portion of the wavefront, and even then we have to understand the nature of the aberration causing the PV error. For example, you may have as much as 0.37 PV of astigmatism. A MCT can have as much as 0.4 waves PV of residual 5th HSA and still be diffraction limited. (Scroll down: https://www.telescop..._spherical.htm)


Edited by Asbytec, 08 September 2019 - 02:35 AM.


#30 cytan299

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Posted 08 September 2019 - 12:25 PM

First, as mentioned, is astigmatism visible as classic 90 degree ovals on both sides of your defocused images and the plus star image at best focus from your video. You can have about 0.37 waves PV of astigmatism and be diffraction limited at 0.8 Strehl. I have no idea how much is present in your video. Need to look at that harder, but an initial look a Suiter seems to suggest it's not terrible. Not too dissimilar to his sims of 0.37 PV waves. The worse the astigmatism is, the more it will be seen with oval patterns at larger defocus. You might just barely see it in your defocused WinRoddier star test images. 

 

The difficulty with your Aberrator sim is you dialed in 0.25 waves of primary spherical aberration (3rd LSA) which is not going to give good results (although you certainly got close). That would be fine if you were testing the primary alone as the source of lower order spherical, but this scope is a system. You will not see spherical aberration from the primary alone. The reason is because the steeply curved negative meniscus lens generates a lot of higher order (secondary) spherical aberration. This is balanced by adding lower order spherical aberration of opposite sign generated on the primary mirror. Primary and secondary spherical aberrations do not have the same waveform so one cannot fully correct the other, however the correction can be pretty good depending on the residual error present. So, there is spherical aberration present, that much is pretty obvious. It's just difficult to tell what form it takes and how severe it is because it is a mix of aberrations and not the pure form of a single lower order aberration we're all familiar with. 

 

There is going to be some residual higher order spherical aberration (from the meniscus) present in the star test. What we are actually star testing is not a pure 3rd LSA primary spherical wavefront as produced in Aberrator, rather we are testing a hybrid (balanced high/low SA) wavefront that only approximates a spherical one to a varying degree. I recall Vla explaining (in one of your links above) the balanced form is actually represented in Aberrator as 5th HSA input. This residual secondary aberration will affect the star test which is very sensitive close to focus and it may show both characteristics of over and under correction (which your later WinRoddier images show). And the star test images will differ on both sides because of the residual error and that makes it more difficult to evaluate. Since some of the residual error is in the paraxial zones near the secondary obstruction, there is a chance this will affect the shadow breakout test as well. The shadow breakout test seems best suited for the pure lower order spherical aberration with a smooth 3rd LSA curve and a 0.3D obstruction as Eddgie often explains. However, this is not the actual wavefront you are testing, so you're results are likely off. I cannot simulate your patterns in Aberrator. Doing so may be complicated by the apparently under corrected medial zone (shown in Winroddier images) and a mix of additional errors not allowing us to test for the pure balanced high/low pure form of spherical or even the 0.25 LSA results above.

 

attachicon.gif ETX 125.jpg

 

But, reading the star test above may be possible without quantifying it. Based on you later images, they do not look like 15 waves defocus to me. At large defocus, you should see more than one ring between the paraxial and marginal rings. I see only 1 ring and part of another (which is a consequence of slight miscollimation). The first thing readily apparent is a bright medial ring inside focus (and the same ring is dimmer outside focus) suggesting an under corrected zone as shown in Winroddier (and somethign Aberrator cannot simulate). Notice in the Winroddier "corrected images", that bright medial ring is missing. I don't know what Winroddier is doing, though, other than maybe showing what it should look like. The next thing is the shadow diameters appear to indicate over correction. As Pinbout often says, the larger shadow indicates the correction meaning larger shadow outside indicates over correction and visa versa. This is likely a result of the meniscus adding over correction of opposite sign (convex wave toward the primary) relative to the normal under correction from the primary. If you are indeed near the same defocus, and I suspect you are, the outside image is also a tad larger indicating some small amount of under correction, too. This is confirmed by the slightly brighter marginal ring outside focus. So, the ETX apparently has both and probably a consequence of its residual higher order spherical aberration. How good is it? I dunno, but I suspect astigmatism is the biggest problem. 

 

So, saying the wavefront error is 0.25 waves of spherical aberration is kind of misleading and not typical of MCTs. An MCT may well have 0.25 waves PV wavefront error or more somewhere, but it will almost certainly not be the same pure form of 3rd LSA seen in star test simulations generated by Aberrator. That amount of error may be present at either the center or the edge or only in a small portion of the wavefront, and even then we have to understand the nature of the aberration causing the PV error. For example, you may have as much as 0.37 PV of astigmatism. A MCT can have as much as 0.4 waves PV of residual 5th HSA and still be diffraction limited. (Scroll down: https://www.telescop...ak_spherical.ht

Thanks for the analysis. It's clear that there are aberrations and from what you are saying above, unless I am misinterpreting, is that these aberrations are typical of an MCT, and thus my ETX125. And that astigmatism may be the biggest problem -- however, it seems that astigmatism is also a feature of MCTs. (see http://www.weasner.c...ollimating.html and see Sherrod's comment about "lateral astigmatism") and for the ETX125.

 

I guess there needs to be more data to come to a conclusion about my etx125. My plan is to do a Hartmann analysis. But to do this I need to make a perforated sheet. It's going to take a while because I have other distractions right now.

 

cytan


Edited by cytan299, 08 September 2019 - 12:26 PM.


#31 Asbytec

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Posted 08 September 2019 - 07:07 PM

Yes, the spherical aberration is typical of an MCT using spherical surfaces. It might look bad because it's not perfect, but it may not be all that bad depending on how well the corrected wavefront hugs a perfectly spherical reference wave. Anyway, all of this is balanced by defocus, which is to say, the position of best focus changes. We naturally find this best diffraction (de)focus location where the image is the sharpest and spherical aberration is minimal compared to other defocus locations we see in the star test. So, it's true, in focus matters most. We do not observe or image out of focus.  

 

A glance at figure 37 will help see this. https://www.telescop..._aberration.htm

 

I am not clear on what "lateral axial astigmatism" is, but an MCT can have off axis astigmatism. Simply put, it's caused by oblique off axis rays striking the meniscus in such a way the meniscus has varying thickness along the path. This causes additional refraction along one plane and not the other perpendicular to it. Those planes focus at different points. You seem to show astigmatism on axis. I do not know how to guess at quantifying it. 

 

Anyway, don't forget to enjoy your scope. :)



#32 cytan299

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Posted 08 September 2019 - 09:11 PM

Yes, the spherical aberration is typical of an MCT using spherical surfaces. It might look bad because it's not perfect, but it may not be all that bad depending on how well the corrected wavefront hugs a perfectly spherical reference wave. Anyway, all of this is balanced by defocus, which is to say, the position of best focus changes. We naturally find this best diffraction (de)focus location where the image is the sharpest and spherical aberration is minimal compared to other defocus locations we see in the star test. So, it's true, in focus matters most. We do not observe or image out of focus.  

 

A glance at figure 37 will help see this. https://www.telescop..._aberration.htm

 

I am not clear on what "lateral axial astigmatism" is, but an MCT can have off axis astigmatism. Simply put, it's caused by oblique off axis rays striking the meniscus in such a way the meniscus has varying thickness along the path. This causes additional refraction along one plane and not the other perpendicular to it. Those planes focus at different points. You seem to show astigmatism on axis. I do not know how to guess at quantifying it. 

 

Anyway, don't forget to enjoy your scope. smile.gif

Yes, I do. I use it to take photos of Jupiter and Saturn in the small bore challenge on this site.

 

But I'm just really curious about how good/bad US made vintage ETX125's are because of people's fondness of them. If you do a google search, most posts are about how sharp the stars are with the ETX125. The problem I originally had was that when I bought this scope from ebay, it was way out of collimation. After spending a lot of time collimating it (it's still a little bit off), I'd like to quantify its goodness just to satisfy my curiosity. 

 

Hopefully, collecting data for a Hartmann test will answer my question: how good/bad is my ETX125?

 

cytan

 

cytan


Edited by cytan299, 08 September 2019 - 09:18 PM.


#33 Asbytec

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Posted 08 September 2019 - 10:31 PM

I'm wondering where the astigmatism is coming from. Not sure of the design of the primary mirror cell, but some have mentioned not to over tighten the collimation/set screws for fear of stressing the mirror. Other than that, I think its a good scope. I had one (loan from a friend) that was very sharp. I'd like to see the results of your test.

#34 cytan299

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Posted 09 September 2019 - 11:48 AM

I'm wondering where the astigmatism is coming from. Not sure of the design of the primary mirror cell, but some have mentioned not to over tighten the collimation/set screws for fear of stressing the mirror. Other than that, I think its a good scope. I had one (loan from a friend) that was very sharp. I'd like to see the results of your test.

Hmmm, I didn't think about the screws being too tight. I'll have to play around with it. One thing for sure is that collimating the ETX is not like collimating an SCT. SCT is a piece of cake compared to the ETX. Those push-pull screws just drive me nuts!

 

cytan



#35 Boom

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Posted 09 September 2019 - 02:49 PM

Good suggestion, but there's no way to introduce stress on the mirror through the ETX collimation system. The screws are adjusting the alignment of a steel plate that the baffle tube is attached to. The mirror is mounted on a separate cell that slides on the baffle tube.

Crank the screws all you want. You're only going to deform the steel plate and not affect the mirror.

https://encrypted-tb...hWxcvEHQHSYRfIq

In the photo you'll see the three collimation screws poking out of the steel plate on left side. The silver plate is holding the mirror, and the only thing that is connected to that is the focusing rod.

Edited by Boom, 09 September 2019 - 02:51 PM.

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#36 cytan299

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Posted 09 September 2019 - 04:05 PM

Good suggestion, but there's no way to introduce stress on the mirror through the ETX collimation system. The screws are adjusting the alignment of a steel plate that the baffle tube is attached to. The mirror is mounted on a separate cell that slides on the baffle tube.

Crank the screws all you want. You're only going to deform the steel plate and not affect the mirror.

https://encrypted-tb...hWxcvEHQHSYRfIq

In the photo you'll see the three collimation screws poking out of the steel plate on left side. The silver plate is holding the mirror, and the only thing that is connected to that is the focusing rod.

I'm surprised that having collimation screws done this way can actually work or even hold collimation!

 

Let me get the mechanics straight:

 

The collimation screws tilt the black plate. The black tube which is connected to the black plate tilts. This, in turn, tilts the mirror because it is held on the black tube. That fit between the mirror and the black tube had better be tight so that when you tilt the tube, the mirror also tilts. But the fit cannot be too tight that it will stop the mirror from sliding on the tube.

 

Maybe that's why doing a fake star collimation doesn't work well because I have to set the mirror at a different spot than when focusing at infinity.

 

cytan


Edited by cytan299, 09 September 2019 - 04:07 PM.


#37 Boom

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Posted 09 September 2019 - 05:06 PM

You pretty much have the right idea.

The fit between the mirror holder and the black tube is pretty tight but loose enough to allow it to slide and focus. The thick grease is all that is taking up the slack between the tube and the mirror holder. Some earlier ETX125 had a rather loose fit allowing the mirror to wiggle as you adjust focus. This is what we call focus shift.

You are also correct that ideally we should collimate at infinity focus to put the mirror in the correct spot and also point the OTA about 45 degrees at the sky. This way the mechanics are settled in a manner similar to what they experience during actual observing.

This is why Polaris is one of the best targets for collimation since it doesn't drift too much, it is at infinity focus, and gives you about 40 to 45 degrees of elevation depending on your location.


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