Homemade Hyperbolic Astrograph - 1, Optical Layout

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Dear PTS,

I agree totally with you that Mr. Chan's brave endeavor and skills should be admired. While the current design of a 102mm mirror may not beat a traditional ED/APO camera lens, when the mirror size increases, the percentage of central blocking will drop and this design, if prove workable, will soon beat the camera lens.

With due respect, I do not agree that a camera lens has similar light reduction. Let’s consider, for argument sake, a camera lens has eight elements. It has four elements more than the Baadar MPCC. If these are uncoated lenses, each surface will lose about 4% of light, there are eight surfaces more than the MPCC, so light reduction is 0.96^8 = 0.72 or 72%, still better than the combined effect of central obstruction and simple metallic reflecting surfaces (64%). However, even camera lenses 50 year ago are at least coated, i.e. the loss of light per surface is significantly reduced to 1.3% per surface, so such a camera lens with 8 additional surfaces has a throughput of 0.987^8 = 0.901 or 90%. Modern camera lens are multicoated, light loss per surface can be as low as 0.3%. So the additional 8 surfaces of a camera lens over MPCC have a light throughput of 0.997^8 = 0.976 or 98%. So 98%, as compared with 64% is a significant different.

However, when the size of the mirror gets bigger and if dielectric coated mirrors (~98% reflecting) are used, the story is different. For example, a 8” astrograph Newtonian by Orion Optical (AG8), 200mm primary mirror with 90mm secondary, 45% obstruction, 1-0.45^2 = 79.7% light. Hilux reflective coating @97% reflectivity. Total light reaching the corrector = 79.9% X 97%^2 = 75% through put. It has a focal ratio of 3.8 which equals to an effective f-stop of SQRT(3.8^2/.75) = 4.4. So, it is a 635mm f/4.4 lens. It would be extremely expensive to buy a 600mm f/4 to f/5 ED/APO camera lens. Even the factory made AG8 costs only about 50% of a similar lens made by Canon or Nikon.

Regards,
Tang

[PTS]

Dear PTS,

I agree totally with you that Mr. Chan's brave endeavor and skills should be admired. While the current design of a 102mm mirror may not beat a traditional ED/APO camera lens, when the mirror size increases, the percentage of central blocking will drop and this design, if prove workable, will soon beat the camera lens.

With due respect, I do not agree that a camera lens has similar light reduction. Let’s consider, for argument sake, a camera lens has eight elements. It has four elements more than the Baadar MPCC. If these are uncoated lenses, each surface will lose about 4% of light, there are eight surfaces more than the MPCC, so light reduction is 0.96^8 = 0.72 or 72%, still better than the combined effect of central obstruction and simple metallic reflecting surfaces (64%). However, even camera lenses 50 year ago are at least coated, i.e. the loss of light per surface is significantly reduced to 1.3% per surface, so such a camera lens with 8 additional surfaces has a throughput of 0.987^8 = 0.901 or 90%. Modern camera lens are multicoated, light loss per surface can be as low as 0.3%. So the additional 8 surfaces of a camera lens over MPCC have a light throughput of 0.997^8 = 0.976 or 98%. So 98%, as compared with 64% is a significant different.

However, when the size of the mirror gets bigger and if dielectric coated mirrors (~98% reflecting) are used, the story is different. For example, a 8” astrograph Newtonian by Orion Optical (AG8), 200mm primary mirror with 90mm secondary, 45% obstruction, 1-0.45^2 = 79.7% light. Hilux reflective coating @97% reflectivity. Total light reaching the corrector = 79.9% X 97%^2 = 75% through put. It has a focal ratio of 3.8 which equals to an effective f-stop of SQRT(3.8^2/.75) = 4.4. So, it is a 635mm f/4.4 lens. It would be extremely expensive to buy a 600mm f/4 to f/5 ED/APO camera lens. Even the factory made AG8 costs only about 50% of a similar lens made by Canon or Nikon.

Regards,
Tang

One more point I missed.
I strongly believe this is just Chan's first "trial" into this complicated system. I believe he will continue with larger aperture once his experiment is successful.

pts