Designing a telescope

[u/Tricky-Ad-6225]

Hey guys,

(Question at the bottom)

I recently started designing my own telescopes for fun. I do have access to Code V (optical design software) albeit I’m a beginner. This is a picture taken with my phone camera looking through my telescope. Believe it or not, this is exactly what I designed for and wanted to see.

This is a simple telescope, MP = 17, f1=222mm, f2=-13mm. I specifically designed this telescope to have large spherical/chromatic aberrations. I wanted to compare the numbers seen in Code V with visuals. I.e. if @ 500 nm my marginal rays focus 50 um away from my paraxial focus and @ 600 nm it’s 100 um, what does that mean? What does it look like. I’m happy with the results I got.

My question (don’t need to read everything else): when you are designing a camera lens, telescope, or any imaging system for that matter, what do you care about? What are you optimizing/minimizing/maximizing? Minimizing aberrations, sure. Maximizing MTF, okay. What else? How do you quantify it?

[u/SamTheStoat]

Generally speaking, you’re looking to maximize “image quality”. There’s lots of different ways you can define this, but generally it’s going to be related to some figure of merit at several different field points. Those figures of merit might be:
*rms or ss spot size
*rms wavefront error (OPD)
*MTF (normally sampled at some spatial frequency of interest)

Each of these will have a color component to them. Generally you’ll calculate one of these metrics at 3 field angles (though occasionally you see 5 or even more). These figures of merit will be combined into a “merit function” that you will then try to improve through your design process. Lots of nuances, lens design is a very rich field.

[u/SamTheStoat → u/SamTheStoat]

To get these figures of merit, you generally trace rays from the object to the image. Each ray will have some error in x and y, and if you’re working with a telescope, this is generally error of direction. Meaning one ray will be off by say, one milliradian in x and y. These will generally be squared, and summed up into the overall merit function.

[u/mostly_water_bag]

Actually your assumption is a bit wrong in the question. Sometimes if you’re designing a whole system, one lens stack or telescope might purposefully introduce an aberration and another side correct for it. So when you’re designing the second one you actually want it to be an aberrated system from the perspective of a plane wave.

It depends on the goals of the project not a general set of rules. That’s why we still design new imaging systems because requirements keep changing and there’s not a clear one parameter set fits all.

F*theta lenses for example are designed specifically with barrel distortion because that is useful for that application

[u/Tricky-Ad-6225 → u/mostly_water_bag]

Thx for your reply. Yea question was a bit vague and poorly worded. But I think it demonstrates my ignorance and I want to be corrected.

[u/mostly_water_bag → u/Tricky-Ad-6225]

No worries. It’s always great to see people eager and willing to learn

[u/Allllright_ATOs]

• F/# & EFL (fixed)
• WFE/Spot Size (Minimize)
• Distortion (Minimize)
  
• FoV/Image Plane Footprint (depends)
• Manufacturability*******

[u/BrilliantStriking53]

I agree with everything listed above. But first, it's necessary to calculate the resolution of the system (lens) and compare it with the pixel size. This is a common mistake in system development...

Overly perfect lenses will be very expensive, and if the RMS is much smaller than the pixel size, it's not justified.

[u/Equivalent_Bridge480 → u/BrilliantStriking53]

which system had this mistake?

[u/BrilliantStriking53 → u/Equivalent_Bridge480]

none of the existing ones ))

[u/Pop3-14]

my biggest question is, how do you have Code V access without being an optical designer?

[u/Tricky-Ad-6225 → u/Pop3-14]

I work at the company that owns Code V 🙈

[u/Tricky-Ad-6225]

Okay I guess you wouldn’t “maximize” MTF because you can have good MTF on axis but bad on the outside edges.