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DPAC Test - C5 SCT and TEC 7 Mak

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#1 Jeff B

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Posted 12 February 2020 - 07:32 PM

I thought I'd post these even though my DPAC set up does not lend itself well to covering the full apertures of these type of scopes with their tiny fully illuminated fields of view.  But there is enough information to be useful.  Hover your cursor over the images to read their titles.

 

Both scopes were placed face down on my flat and carefully shimmed with thin paper to get the largest FOV I could manage.  Then I placed the Ronchi screen/LED holder at the "correct" back focus position for each scope and focused the scopes to get the results.

 

The C5 is a garden variety sample, about 10-12 years old.  Not too bad spherical correction really but the optics are rough as you can see with the ragged lines.  Visually the scope is rather sharp but planetary/lunar images get softer than my 5" APO's past about 120x or so.

 

The TEC, on the other hand, is corrected a little better but is much smoother.  Visually, this is an intensely sharp scope which goes toe-to-toe with my 6" APO's.

 

Enjoy!

 

Jeff

Attached Thumbnails

  • C5 in DPAC.jpg
  • C5, DPAC, Inside of Focus.jpg
  • C5, DPAC, Outside of Focus.jpg
  • TEC7 A.jpg
  • TEC 7, DPAC, Inside of Focus.jpg
  • TEC 7, DPAC, Outside of Focus.jpg

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#2 Richard O'Neill

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Posted 12 February 2020 - 10:51 PM

It appears the TEC has a turned edge. I'm a bit surprised and disappointed.



#3 Jeff B

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Posted 13 February 2020 - 12:02 AM

It appears the TEC has a turned edge. I'm a bit surprised and disappointed.

I noticed that too but I'm not so sure it's real as I can see a similar pattern in the C5 images as well.  The top and bottom edges of the primaries are chopped off by the baffle tube ends as the LED source and view through the grating are both slightly off axis and the fully illuminated FOV is very small.  I suspect there is something going on with that like maybe a little thermal and/or diffraction thing at work.  The intra and extra focal patterns during star test of the TEC 7 shows no hint of a turned edge either, plus the views are very sharp up to very high powers.  In my experience, a turned edge shows up as a softening in sharpness and loss of contrast on planetary surface structure.  I don't see that with the TEC 7.  

 

But it's easy enough to set it up again and index the holder and screen around and have a closer look.

 

Jeff



#4 luxo II

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Posted 13 February 2020 - 05:34 AM

Both have a hint of TDE, but it’s not objectionable.



#5 Eddgie

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Posted 13 February 2020 - 09:27 AM

It appears the TEC has a turned edge. I'm a bit surprised and disappointed.

I noticed that too, but did not want to say anything.

 

The C5 has either a turned edge or an S zone that falls right at the edge.  Hard to say.  I see S zones a lot in Celestron scopes.  I had been told that this was probably due to the use of sub aperture polishers, where the polisher does the center of the mirror first (or last) and the outside is polished in a seperate opration with the mirror rotated under the polisher.  Now I don't know that to to be true, but I have seen a number of Celestron SCTs with a zone and this zone always appears to be 8" out from the center. That would suggest that the C8 polisher is used to polish the center, then the mirror is rotated for polishing the outer area, and that leaves a zone at 8".   On the C9, the zone, when it appears, is kind of like what I see here.  On the C11, it is just inside the edge, and on the C14, it is aboujt 3" in from the edge.

 

I had a C5 a decade ago and had reported seeing a turned edge using even a single pass.

 

The C5 also appears to have some zonal stuff going on near the center.

 

This is a great case of death by a thousand cuts.  The SA correction is good, but not great, there is some kind of turned edge or zonal error at the outside, and one at the inside.   While there is no one serious error, there would be some contrast loss due to the accumulation of small errors.  

 

Elsewhere in another forum, I was clear with my lack of confidence in interpreting DPAC.  I mean I can identify errors, but I don't know how to quantify severity.  Still, I have seen enough DPAC test or wavefront simulations with interferometer that I feel confident in saying the C5 is typical of the finish on many small mass produced MCTs and SCTs.

 

The TEC looks great. Again, I don't know how to estimate severity of zones when shown in the DPAC, but this is a single small error in an otherwise high quality instrument.  No one dies from a single tiny paper cut.  I am sure this is a superb instrument.



#6 Eddgie

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Posted 13 February 2020 - 09:38 AM

 In my experience, a turned edge shows up as a softening in sharpness and loss of contrast on planetary surface structure.  I don't see that with the TEC 7.  

 

But it's easy enough to set it up again and index the holder and screen around and have a closer look.

 

Jeff

Yes, a turned edge will affect contrast on subjects closer to the center of the field, so would lower contrast on planets.  An S zone that is more towards the center tends to broadcast a faint veiling glow across the entire field (a slightly lowering of contrast across the frequency range the way a spider vane does).

 

Now the only way you could really know if it was enough to lower contrast would be to have a perfect sample side by side and look through them both with a high quality MFT chart as a target.   Without a reference standard, one cannot really depend on a quality judgement based on just looking through.

 

That all being said, if you don't see it in the star test, then even if there is a turned edge, it is of no consequence.

 

If a turned edge is bad enough to be seen in a star test, or in a single pass Ronchi test, then is is almost for sure doing enough damage that it would be possible to see when compared to an excellent example.

 

To bring this around to my previous post, I said I had a C5 that had a turned edge, but I also had one that was free of this.  The turned edge showed even in a single pass Ronchi test (which is far less sensitive than double pass) and while the scope was otherwise good, the reference C5 was free of any meaningful error and it did put up a tiny bit crisper planetary performance.   

 

In the TEC though, I am guessing that the TE is too small to make a meningful difference becuase the DPAC is very sensitive and the error looks tiny (vs the C5) and if it does not show in the star test, my guess is that it is just too small to matter.



#7 davidc135

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Posted 13 February 2020 - 10:17 AM

Maybe they are only diffraction effects in the Tec images?

 

David


Edited by davidc135, 13 February 2020 - 10:29 AM.

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#8 Eddgie

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Posted 13 February 2020 - 10:33 AM

And for those that do not know how to judge the severity of a turned edge, here is the way it works.

 

Many think it is the severity of the angle that is the major attribute of a turned edge that affects the image but in reality, it is the width of the turned edge that is more significant.

 

The physics here is just based on simple geometry.  

 

Suppose you had a 1mm wide band running around the diameter of the mirror or lens 50% from the center of the mirror that had a little ridge ( called an S zone).   Because the diameter of that band is smaller than it would be at the edge of the mirror, there is less surface area (light reflecting area) that is exposed to the incoming light, and that means that this small amount of energy and due to the shallow angle, that energy is broadcast over the entire field does little damage.

 

If though, you made that small band 3mm wide, you are tripling the amount of energy involved (and this energy is also taking away from the total energy at the center of the field.)

 

Now because the diameter of a 1mm band out by the edge of a mirror would be far larger, the total surface area affected would be many times greater than the 1mm band closer to the center of the mirror so this is why a zonal error is more serious if it is further from the center of the mirror.  It is the simple geometry that the further from the center of the mirror the band is, for a give width, there is more surface area involved and this means more energy that is going somewhere rather than where it is supposed to go.

 

Now looking at the TEC vs the C5, the total amount of area that is involved in each is considerably different. In the TEC, it is this tiny tiny little sliver right at the very edge of the mirror and while there is more area encompassed in this band than if the error was near the center of the mirror, it is still a very very tiny amount of surface.

 

In the C5 though, it is clear that the turned edge (though I believe it to be a S zone, but a turned edge is essentially an S zone that has the outer half of the ring sliced off) extends far deeper into the diameter of the mirror. The C5 S zone or turned edge is covering far more total area than the zone or turned edge in the TEC does, and appears far sharper.  But look at the total area of the mirror involved in the C5. If the mirror was unobstructed, a 5mm wide zone at the outside of the mirror would cover 1.3 square inches of the mirrors total area, but in the C5, we only get to use 90% of the mirrors area, so of the total area avialable (lets say 15 square inches) the zonal area would constitute almost 10% of the total energy that was falling on to its surface.  (I am not trying to estimate the area involved in the C5 above, only using a figure of convenience.)

 

I see a lot of tests with a very narrow but sharp turned edge and because of the surface area involved, it is hard to believe that this small amount of area would be of any consequence (I mean it all has a consequence, but I am speaking of a consequence that would have a meaningful affect on performance).   I do see tests though where there is either an S zone or turned edge that is wide enough to involve a significant amount of the mirror's or lenses total area, and these are the ones that are going to have an effect on the image.

 

Sometimes we see zones in premium refractors (sub aperture polishing?) but these zones are often closer to the center (less area) and mild, so once again, since there is less area, the zone would do far less damage than if a zone of that same width and height was moved out to the edge of the mirror, were far more surface was affected.  

 

An S zone near the center of the optic will have the tendency to broadcast the energy over the entire field as a veiling glare, but since the total area is low and the energy is diffused over the entire field, the zone would have to be quite severe to have much consequence.  The consequence would be that the background glow across the whole field would rise very very very slightly.  The global conrtrast transfer (the contrast of all size features) would be lowered perhaps a fraction of a percent or in other words, far below the amount that even the most skillful observer in the world would be able to see. 

 

When it is at the edge of the field, the energy tends to be directed and concentrated towards the center of the field, and because more area is involved, and that energy is now being concentrated, a strong zone or wide turned edge will have a meaningful affect on planetary observing

 

I am sure many readers already have read Suiter's book on star testing and know how each kind of error manifests itself in terms of contrast transfer function, but for those that don't this post was intended to explain what is going on with turned edges (a kind of zonal error) and how to know when and when not to panic.  A razor thin and mild zone at the edge of the lens or mirror is not fun to see, but if it does not show in the star test, it is safe to ignore it.   If on the other hand, the zone is wide and steep, and can easily be seen in the star test or DPAC or even single pass Ronchi test, the effect can be so serious that for best planetary work, one needs to make the decision whether it is better to mask off the zonal error and reduce the aperture, or live with the zone.  This can be calculated and modeled, but the easiest thing to do would be to make a mask that covered the zone and compare with and without the mask in place.  

 

With the C5, I see some zonal errors that I take to be meaningful. With the TEC, while I do see a slight mild turned  edge, I doubt that it has any meaningful affect on Strehl or contrast transfer.  These are super super super sensitive tests. That is why people that want to assess optical quality use them. They disclose even the tiniest and most inconsequential error. 

 

One of these instruments is near perfect, one is further away.  How far away each is, I can only guess. My guess is that the C5 is perhaps a bit below average in terms of mass produced quality standards, but as I have said twice before, I do not know how to quantify DPAC tests and am just correlating these images to tests I have seen elsewhere that also included interferometer tests.  Since I don't consider even an excellent C5 to be a good choice for serious planetary observing, it gets a pass from me because it is an inexpensive, mass produced instrument, and I hold no great expectation that such an instrument will have high quality optics.  For the money, I would have been shocked if it looked close to the TEC.  Not that this never happens, but I just would not expect it to happen and people buying MCTs and SCTs at this price point would be well served to adopt the same expectation. 


Edited by Eddgie, 13 February 2020 - 10:42 AM.

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#9 davidc135

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Posted 13 February 2020 - 10:47 AM

I wouldn't like to say definitely if the Tec has tde or not but in testing some optics once I saw a similar edge to the Tec. When I reduced the aperture by 10mm with a circular mask the edge effect still remained which made me feel it was illusory. Might be worthwhile doing a check.  David


Edited by davidc135, 13 February 2020 - 10:48 AM.


#10 Mike Spooner

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Posted 13 February 2020 - 10:54 AM

Nice write up, Ed. Looking at the TEC I notice the edge effects on both inside and outside appear to turn outward which is indictitive of diffraction effects in my experience. Suiter mentions the width of TE vs noise which is a really good examination and an example of just how sensitive the out of focus star test is. 

 

Mike Spooner



#11 coinboy1

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Posted 13 February 2020 - 10:58 AM

I would say a possibility of the TDE is because you are resting the telescope directly on the optical flat. How about setup a test bench where the weight of the scope is not resting on the optical surface. You may be distorting the optical surface of the flat when you put a heavy scope on top of the flat like that. Cant be good for the coating also. 


Edited by coinboy1, 13 February 2020 - 10:59 AM.


#12 davidc135

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Posted 13 February 2020 - 11:12 AM

If the Ronchi grating is substituted by a knife edge, errors can be quantified. A ruler is OK for F/10. 

 

Also a good follow up test for tde.   David


Edited by davidc135, 13 February 2020 - 04:37 PM.


#13 Jeff B

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Posted 13 February 2020 - 04:16 PM

I love these discussions and thanks Eddgie for the postings.

 

And I learned by doing.

 

My DPAC holder does not let me see the entire aperture of most SCTs and Maks as I'm looking a tad off axis and the end of the baffle tube chronically blocks a portion of the aperture.  You can see that in well in the TEC 7 photos.  The top, small sharper edges are the edges of the primary while the lower, edge is really the out of focus edge of the baffle tube.

 

So I carefully re-did a couple of images but for one I flipped the Ronchi screen holder over so I could see the other edge of the primary.  For that one, the sharpest edge is on the bottom of the image.  I think this clearly shows there is really no turned edge but there is a diffraction thing going on which is in full display when the out of focus baffle tube end clips the aperture.  

 

Put you cursor over the image to read its title.

 

I'm not sure which side of focus they were taken on but the differences you see in the overall line shape are due to subtle focus and holder/screen alignment differences that occurred when I flipped the holder over.

 

I'm sure there will be some comments...which is great and keep them coming.

 

Jeff

Attached Thumbnails

  • TEC 7, DPAC, Screen Rightside Up.jpg
  • TEC 7, DPAC, Rightside Up.jpg
  • TEC 7, DPAC, Screen Upside Down.jpg
  • TEC 7, DPAC, Upside Down.jpg

Edited by Jeff B, 13 February 2020 - 04:20 PM.

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#14 TG

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Posted 18 February 2020 - 10:36 AM

I have seen similar images with my TEC-6. The diffraction is worse with a 200lpi grating which is what I used. Also, remember that the TEC has twice the focal ratio of the MN that Jeff did earlier. The ronchigrams will look worse on the longer focal ratio scopes for the same errors.

Tanveer

#15 Astrojedi

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Posted 19 February 2020 - 10:18 AM

I noticed that too, but did not want to say anything.

 

The C5 has either a turned edge or an S zone that falls right at the edge.  Hard to say.  I see S zones a lot in Celestron scopes.  I had been told that this was probably due to the use of sub aperture polishers, where the polisher does the center of the mirror first (or last) and the outside is polished in a seperate opration with the mirror rotated under the polisher.  Now I don't know that to to be true, but I have seen a number of Celestron SCTs with a zone and this zone always appears to be 8" out from the center. That would suggest that the C8 polisher is used to polish the center, then the mirror is rotated for polishing the outer area, and that leaves a zone at 8".   On the C9, the zone, when it appears, is kind of like what I see here.  On the C11, it is just inside the edge, and on the C14, it is aboujt 3" in from the edge.

 

I had a C5 a decade ago and had reported seeing a turned edge using even a single pass.

 

The C5 also appears to have some zonal stuff going on near the center.

 

This is a great case of death by a thousand cuts.  The SA correction is good, but not great, there is some kind of turned edge or zonal error at the outside, and one at the inside.   While there is no one serious error, there would be some contrast loss due to the accumulation of small errors.  

 

Elsewhere in another forum, I was clear with my lack of confidence in interpreting DPAC.  I mean I can identify errors, but I don't know how to quantify severity.  Still, I have seen enough DPAC test or wavefront simulations with interferometer that I feel confident in saying the C5 is typical of the finish on many small mass produced MCTs and SCTs.

 

The TEC looks great. Again, I don't know how to estimate severity of zones when shown in the DPAC, but this is a single small error in an otherwise high quality instrument.  No one dies from a single tiny paper cut.  I am sure this is a superb instrument.

A couple of comments/questions on your point about the SCT mirror making...

 

- Since these are compound scopes how do you know what optical element is introducing the error? Corrector? Secondary? Primary?

 

- In my ATM days I made a few mirrors and spherical mirrors were ridiculously easy to make to a high degree of accuracy even in an automated fashion but had crazy coma. Parabolic were much harder. I just find it hard to believe that so many errors exist in SCT primary mirrors. In my experience 99% of the errors in SCTs are related to optical alignment. Although not saying that is the case here.

 

- Also just a side comment the OP’s C5 is at least 15+ years old - Pre 2005. One of the poorest vintages of the C5.


Edited by Astrojedi, 19 February 2020 - 10:21 AM.

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#16 davidc135

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Posted 19 February 2020 - 01:14 PM

IMO, surely correctors are the most likely culprits behind rough and zoney Ronchi tests, at least in SCTs. But having said that, I don't know of many tests, apart from on a couple of primaries, that have shown the two mirrors to be fault free. But a fair assumption, I think. David


Edited by davidc135, 19 February 2020 - 01:21 PM.



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