9 replies to this topic

[orion69]

What can I conclude from this two spot diagrams:
first is from TS 130/780 triplet:
http://www.teleskop-...riplet-APO-1...

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[orion69]

...and second is from APM 130/780 triplet:
http://www.apm-teles...-OTA/Apochro...

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[orion69]

What are left side and upper numbers stand for, and what does numbers for airy disks mean? Also reference...?

Thx

[Eddgie]

The nunbers on the left indicate that the top row of spots represents performance at the center of the field.

The bottom row of spots shows the performance at .5 degrees from the center of the field, so this would represents how sharp stars would be at the very edge of a circle representing 1 degree of true field.

No considering that with a modern chip, the scope may cover much more than this, one has to remember that past the one degree circle of course the performance will slowly get worse the further off axis you move.

The numbers across the top define the wavelenght that the spot was generated for (the color correction for various wavelengths in microns).

I do not know the Airy Diameter reference.

Also, I beleive that the IMA in Zemax refers to the radius of curvature.

Sometimes spots are presented at the best focul surface that is often curved, so using a flat detector (film or CCD) will show a result that is worse than what could be achieved if you curved the detector or film to match the radius of curvature of the best focus of the telescope.

But I believe in Zemax, this refers to the fact that the spots were generated for an infinite focal plane (or perfectly flat, so that you are seeing how they would be spread out on a flat sensor).

If the spots were done at the best curve, then usually the person generating the spots would specify the radius of curvature in millimeters and it would look something like "RC = - 270mm." For scopes with strongly curved focal surfaces, often the spots are drawn for how they would be along a curve with the RC of the scope because to show them as they would be on a flat surface, they can expand off the chart due to defocus.

Here is a link that goes into some detail about Radient Zemax and perhaps you can find specific details here.

Also, and important point. The number of spots used can vary greatly, and if one person used 1000 spots while another used 500 spots for the trace, the trace using more spots might give the appearance of having worse performance simply because more dots will appear outide of the Airy Disk.

Also, the ray trace does not at all factor in diffraction. It is totally geometric, and this means that while the circle often represents the diameter of the Airy Disk, in real life, if you included diffraction effects, there would be spots waaaaaaay outside the Airy Disk circle.

So, not only do you have to know how to read the spots, but for the most meaningful comparisons, it is useful to have the same number of spots traced, and this is not always the case.

Also, knowing the number of rays used helps judge the intensity. If you used 1000 rays, and you found 300 outside of the Airy Disk, you could know that only 70% of the spots went into the Airy Disk circle (but again this does not include diffraction). But if 5000 rays were used and you counted 300 outside, you might not know how dense the spots were in the circle.

Must people try to use a number of spots that don't over-do it, but enough to show a decent depiction of the density.

I do not use Zemax so this is not a definitive answer, but I have worked with other spot diagrams before and this is typical of the kind of information they typicallly display (though the difference wavelengths are usually not present for reflectors of course).

Radient Zemax single lens example

[dan_h]

Eddgie has given good information.

One thing I would add to his response is that you need to be cautious when comparing spots to make sure the wavelengths you are looking at are similar. These are the numbers across the top. The colors used for the plots are simply for graphing. They do not always relate to the actual color of the wavelength plotted.

For example, the TS the dark green spots at 0.54074 need to be compared to the same wavelength on the APM which is 0.54070 and these spots are colored red.

dan

[ValeryD]

Eddgie has given good information.

One thing I would add to his response is that you need to be cautious when comparing spots to make sure the wavelengths you are looking at are similar. These are the numbers across the top. The colors used for the plots are simply for graphing. They do not always relate to the actual color of the wavelength plotted.

For example, the TS the dark green spots at 0.54074 need to be compared to the same wavelength on the APM which is 0.54070 and these spots are colored red.

dan

Most of Zemax users are not professional enough or too lazy. They can't even read the manual and see how they can make all wave lengths corresponds to visible colors on the screen.

In my simulations all wave lengths (usually 9 wave lengths) corresponds to colors they are represent.

5407 - green, 4861 - light blue, 5876 - yellow, 6000 - orange, 4550 - deeeeeeep blue, 4358 - violet, 4047 - deeeeep violet,
6350 - red, 6563 - deeep red.
The background is black. Then on the screen you see the "star" more of less adequately (but much increased in size) to the reality - as much as this possible with such software.

[orion69]

Thanks all!

I think I found what I was looking for here:
http://202.120.223.1...65072942040.pdf

I was puzzled why APM has airy disks size twice smaller than TS scope, I think reason is that APM spot diagram shows radius of airy disk and TS spot diagram shows diameter of airy disks.
Air disk formula:

D = 2.43932 x L x Focal Ratio (6.0)
D = Diameter of Airy Disk in mm
L = Wave Length in mm (e.g. 546nM = 0.000546mm)

Also, important thing is to notice displaying scale...

Well, if I'm seeing this correctly both scopes has similar performance, do you agree?

[Eddgie]

Notice that the second set of spots uses far less density than the first scope does. Using fewer scopes can sometimes mask how the scope will perform.

Also, as ValeryD has mentioned, no great care was used to line up the spots to the colors, so it can make doing an exact comparison difficult.

But I have to ask this question... Even though the bottom trace clearly used far fewer dots (the density is very low) and that this approach can give the impression that the scope does a bit better than it does (but to many dots make any APO look bad), if I look at the traces, I see performance that looks almost identical. Where you can find two similar wavelenght shots and pronounced astigmatism, the patterns would look almost identical if you put in the same density in the bottom picture as the top picture has. Again, the mis-alignment and use of slighly different wavelenghts makes this difficult, but to me the look very similar in the way the rays fan for some specific wavelenghts, but the density makes them look different.

But both are 130mm 780mm f/l triplet, and this seems to be an odd coincidence to me.

Remember, the ray traces say nothing about fabrication or anything else. They just plot the geometric course of different rays through difference glasses with different refraction indexes and spacings.

If the glass types and spacings used are the same, the the ray traces will always look identical if the same wavelengts and ray count are used.

But dude, to me, this looks like the same scope. The traces are so similar that had they been aligned to the same exact wavelenths and used the same dot count, they would look a lot more alike.

And my guess is that it is because they are the same design.

So, to answer your question, yes, they look like the performace would be about the same from what I can see, and my own guess is that the forumla for each scope's optics is the same.

In other words, they are probably ray traces for the same telescope design. Maybe not, but the spots look quite similar if you extrapolate a little due to the mis-match in wavelenghts and densities. I see the patterns just looking far to similar where I can correlate a spot from each diagram.

[dan_h]

Eddgie,

I tend to agree with your thinking on this but I am lacking expertise, and did not say so.

I would also suspect that the two plots have been calculated with different focusing options as indicated by the on axis spots at 0.546070. However, I play with OSLO and don't have any familiarity with the options in Zemax.

dan

[orion69]

Notice that the second set of spots uses far less density than the first scope does. Using fewer scopes can sometimes mask how the scope will perform.

Also, as ValeryD has mentioned, no great care was used to line up the spots to the colors, so it can make doing an exact comparison difficult.

But I have to ask this question... Even though the bottom trace clearly used far fewer dots (the density is very low) and that this approach can give the impression that the scope does a bit better than it does (but to many dots make any APO look bad), if I look at the traces, I see performance that looks almost identical. Where you can find two similar wavelenght shots and pronounced astigmatism, the patterns would look almost identical if you put in the same density in the bottom picture as the top picture has. Again, the mis-alignment and use of slighly different wavelenghts makes this difficult, but to me the look very similar in the way the rays fan for some specific wavelenghts, but the density makes them look different.

But both are 130mm 780mm f/l triplet, and this seems to be an odd coincidence to me.

Remember, the ray traces say nothing about fabrication or anything else. They just plot the geometric course of different rays through difference glasses with different refraction indexes and spacings.

If the glass types and spacings used are the same, the the ray traces will always look identical if the same wavelengts and ray count are used.

But dude, to me, this looks like the same scope. The traces are so similar that had they been aligned to the same exact wavelenths and used the same dot count, they would look a lot more alike.

And my guess is that it is because they are the same design.

So, to answer your question, yes, they look like the performace would be about the same from what I can see, and my own guess is that the forumla for each scope's optics is the same.

In other words, they are probably ray traces for the same telescope design. Maybe not, but the spots look quite similar if you extrapolate a little due to the mis-match in wavelenghts and densities. I see the patterns just looking far to similar where I can correlate a spot from each diagram.

The APM scope is russian LZOS design, TS uses FPL53 glass.
As you mentioned scopes are very close optically but of course not mechanically since APM has far better focuser.
But if it would be possible to install FT focuser (HD version) on TS scope it still would be more then 2000 euros less.
I'm in a market for astrograf and more and more I'm convinced that TS 130/780 would be good move with installed FeatherTouch focuser and Riccardi 0.75 FF/FR which would make it F4.5, fast enough for some nice NB shooting...