Aperture obstructions and light transmission. - Forums

Topic starter

Frequent contributor

Jan 31, 2025

Sorry if this is a bit long and naive, but here is my dilemma. I have reflector and refractor telescopes of various focal ratios. My question concerns actual light transmission between scopes with central obstructions and those without(refractors. My reflectors typically have a significant central obstruction, such as 36% by area central obstruction in my edge 9.25", and I wonder how much light that is costing. For example, video lenses are rated in "T stops," which refers to their actual light transmission(i.e. the f-stop may be f 2.8, but the T-stop could be 3.5). So, if I had a 150mm refractor at f7 and a 150mm reflector at f4, would the T stop or light transmittance be similar due to the loss of light gathering by the central obstruction?
Thank you
Scott

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Tobiasz

Jan 31, 2025

Don't forget that th primary of your edge telescope has a hole in it, so the effective obstruction by area is MUCH smaller.

Tony Gondola

Frequent contributor

Jan 31, 2025

No, not at all. I don't know how large the secondary is in your post but if it's around 30% then the reflector would have a t-stop of about F/4.4. The refractor, if it has 3 main elements and a flattener with 2 elements would be approximately F7.1

Wei-Hao Wang

Jan 31, 2025

For a 150mm F4 reflector to become T7, the central obstruction needs to have a diameter of about 12.3 cm. I don't think I have ever seem such a scope. Even if you assume each of the two reflective surface has a reflectivity of only 85%, the central obstruction still needs to have D = 11.1 cm.

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Dan H. M.

Jan 31, 2025

You can calculate the effect of a CO yourself very easily. John Hayes recently made a spreadsheet for it and there is also an online tool.

https://lambermont.dyndns.org/astro/code/compare-telescopes.html

https://www.astrobin.com/forum/c/astrophotography/other/astro-imaging-channel-tonight-7724-the-quest-for-aperture-why-are-big-telescopes-better/

As was said above the f/7 is vastly slower than the reflector unless you resample the data like crazy, which would make little sense.

Tobiasz

Jan 31, 2025

Dan H. M.:
You can calculate the effect of a CO yourself very easily. John Hayes recently made a spreadsheet for it and there is also an online tool.

https://lambermont.dyndns.org/astro/code/compare-telescopes.html

https://www.astrobin.com/forum/c/astrophotography/other/astro-imaging-channel-tonight-7724-the-quest-for-aperture-why-are-big-telescopes-better/

As was said above the f/7 is vastly slower than the reflector unless you resample the data like crazy, which would make little sense.

The tool from the first link is very useful and I use it frequently. Can recommend.

ScottF

Topic starter

Frequent contributor

Jan 31, 2025

Ok, so the bottom line is the CO makes little difference and the faster scope is always better for a given aperture. That's what I wanted to know, thank you.

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Tobiasz

Jan 31, 2025

Ok, so the bottom line is the CO makes little difference and the faster scope is always better for a given aperture. That's what I wanted to know, thank you.

Depending on your desired FOV, always decide for the scope with the bigger aperture - faster and more resolution. CO has little effect in astrophotography the way we do it.

andrea tasselli

Frequent contributor

Jan 31, 2025

Except for fast scopes with very large COs and tight NB filters. That is where the things get interesting…

Dan H. M.

Jan 31, 2025

Ok, so the bottom line is the CO makes little difference and the faster scope is always better for a given aperture. That's what I wanted to know, thank you.

Well, CO does have an impact when comparing similar systems. For example, a fast refractor/lens will be more efficient than a reflector with a similar focal ratio because the frac doesn't have a CO. As with everything, though, this doesn't tell the full story. The reflector might have better optics and be overall mechanically superior and thus preferable despite its CO.

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Tony Gondola

Frequent contributor

Jan 31, 2025

Where the central obstruction really matters is in it's effect on strehl ratio of the system. The amount of light actually lost isn't nearly as important as the loss of system contrast. This is nicely shown if you compare the MFT of both systems. We compensate for this in our processing but non-obstructed systems will always have the edge, at least up to a diameters that are available. After that, the power of large aperture wins.

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