Atmosfringe report for new scope

[GavStar]

For the first time I’ve received a scope that came with a test report. Now I know that I should probably just ignore it since the key test is how it performs under the sky.

But I’d be interested if someone can give me an easy to understand explanation of the various figures. I know Strehl ratio is the most often quoted as an indication of lens quality but what about all the other numbers - what relevance do they have? I’ve had a bit of a search but the descriptions I’ve found were quite technical.

 

[vlaiv]

All quoted numbers relate to each other, but it is not an easy calculation to make - hence usage of software.

P-V or Peak to Valley is measure of how much wavefront reaching focal point is deformed. Let me explain this a bit better. When light reaches objective from a star it is in perfect sync, think waves, and all the light reaches objective with crest of wave at the "same place". Light after that reflect/refracts (depending on scope), and surface that does bending of the light introduces slight delay in waves if not perfect. Light refracted on one side is a bit ahead, while on the other side, trails a bit. Diagram in top right corner shows this. Higher on diagram (red color) - there is shift to one side, lower in diagram (blue color) - shift to other side. For perfect star image and in general image of telescope it is important that light reaches focal plane "at the same time" - or in sync. Back to P-V number - it shows how much "last" part of light trails behind "first" part of light. It is highest point in diagram minus lowest point in diagram. In general this number does not tell you much about optics.

RMS is another measure that describes diagram in the top right. It is root mean square - or deviation of wavefront from perfect. It is similar to P-V but it is calculated on many points over objective, not only maximum and minimum point. This gives better idea how much wavefront deviates from perfect.

Strehl ratio is probably most important measure, because it includes effects of interference of those waves that might be out of sync. It tells you how much power is concentrated in airy disk vs rings in comparison to perfect optics. You want this number to be as close as possible to 1. 0.956 is very good value.

One thing that must be noted is that this report is for light of 532nm. Since this is refracting telescope, for different wavelength of light one might get different numbers in analysis. This analysis is geared towards visual usage because human eye is most sensitive in green (532nm is green light).

Diagram in central lower region - MTF is another important measure of performance of telescope. It shows deviation of contrast for respective frequencies. On bottom axis labeled spatial frequency left (lower numbers) - means bigger detail, while right (larger number) - means smaller detail. As you see from the graph at some point line effectively reaches 0 at very small detail - this is maximum resolution of telescope - it cannot show smaller detail past this point due to physics. Graph on this report shows that you will be able to view tiny detail very much as perfect telescope would show it. There is small drop in contrast for a bit larger features compared to perfect telescope. But it is rather small.

Astigmatism, Coma and Spherical are another way to characterize how the wavefront is deformed from perfect (again, it is approximation to actual deformation), but it tells a bit of how it will impact final image. Those represent Zernike terms / polynomials. It is a bit like having a set of data, and then fitting - linear function to it, or quadratic function to it, or cubic function to it. Each will only approximate data (with some error) but you will be able in general to see how data behaves. So these terms also approximate wavefront surface in similar manner. Again there is some error to this, but it helps characterize wavefront to those who regularly deal with optics.

Hope this explains numbers a bit. In general 0.956 is very good Strehl ratio, and MTF is almost as that of perfect scope, so you should be able to get excellent view (at this wavelength of light).

[GavStar]

That’s a very clear and helpful explanation - thank you.

[Ben the Ignorant]

For green light (532nm):

 

1/4 wave peak-to-valley: good

1/6 wave peak-to-valley: very good

1/8 or better wave peak-to-valley: splendid but rare and usually expensive

 

1/20 wave rms: good

1/30 wave rms: very good

1/40 wave or better: excellent

 

Congrats, your little scope is a very good one!

[vlaiv]

16 minutes ago, Ben the Ignorant said:

For green light (532nm):

 

1/4 wave peak-to-valley: good

1/6 wave peak-to-valley: very good

1/8 or better wave peak-to-valley: splendid but rare and usually expensive

 

1/20 wave rms: good

1/30 wave rms: very good

1/40 wave or better: excellent

 

Congrats, your little scope is a very good one!

Almost, but from P-V error alone one is not really able to judge quality of optics.

Maybe this page will help a bit in further understanding P-V, RMS and Strehl:

http://www.rfroyce.com/standards.htm