| article: Transmission of Light in different Telescope Designs |
Different optical designs for telescope have different advantages and disadvantages.
Some of them like contrast, definition and even resolution are hard to quantify and topic of *philosphical* discussions even though mathematical methods are known to estimate that.
In this article I just want to deal with one fact: the transmission of light through the OTA and loss of light coming from unperfect mirror or lens reflection and obstruction.
Mirrors
The better to standard mirror with aluminum coating reaches reflectivity of 90% in wavelength ranges of visible light.
There are special coatings going up to 98% but they are very expensive.
90% will be used in further calculations.
Lenses
Uncoated lenses are reflecting up to 20% of the light.
Unlike with mirrors, this reflectivity means a loss of light.
Average multi-coated lenses will reflect only 5%, hence transmit 95%.
Very expensive coatings can transmit up to 98% (by now).
95% will be used in further calculations.
Obstruction
Obstruction is called the fact that any secondary mirror of a reflecting telescope will cause a shade onto the primary mirror.
This light cannot be reflected through the OTA and is lost.
The Schiefspiegler is an exception, of course and obstruction can be made to zero.
The different designs of reflectors like Newtonian, Schmidt-Cassegrain, Maksutov and others cause a different degree of obstruction.
With faster focal ratios of the same telescope type the obstruction is increased also.
Usually the obstruction is expressed in percent of the primary mirror, i.e. 20% for a slow Newtonian and 40% for a SC.
To calculate the transmission it is necessary to calculate the area of the shade and therefore the square-diameters are used:
D1 is diameter of the primary and D2 of secondary.
In further calculations we will use the following values:
| design | focal ratio | transmission by
obstruction |
| Refractor | any | 1.00 |
| Newtonian | 10" f8 | 0.94 |
| Newtonian | 10" f4 | 0.91 |
| Schmidt-Cassegrain | 10" f10 | 0.85 |
Total Transmission
The total transmission is calculated from the multiplication of all single transmission.
The table assumes the following components:
Achromatic Refractor: 2 air spaced lenses = 4 air/glass surfaces
Apochromatic Refractor: 3 coupled lenses = 2 air/glass surfaces
Newtonian Reflector: 2 mirrors = 2 mirror surfaces + obstruction
Schmidt-Cassegrain: 2 mirrors + 1 correction plate = 2 air/glass surfaces + 2 mirror surfaces + obstruction
Fixed FL Telephoto Lens: 3 air spaced lenses = 6 air/glass surfaces
Zoom Telephoto Lens: 4 air spaced lenses = 8 air/glass surfaces
The design can vary, especially of the telephoto objectives tending to have even more lenses.
| design | focal ratio | total
transmission |
| Apochromatic Refractor | any | 0.90 |
| Achromatic Refractor | any | 0.81 |
| Newtonian | 10" f8 | 0.76 |
| Fixed FL Telephoto | any | 0.74 |
| Newtonian | 10" f4 | 0.73 |
| Zoom Telephoto | any | 0.66 |
| Schmidt-Cassegrain | 10" f10 | 0.62 |
Conclusion
For imaging the faintest details of galaxies or nebula the loss of light is an important factor.
But transmission is only a part of the truth, for sure.
A Newtonian at f/4 is almost 4 times faster than an f/8 of same size because of his focal ratio.
Because of the bigger secondary mirror of the f/4 system this is not exactly 4 times better.
And all Newtonians being 10 inch will outperform any 6" Apochromatic Refractor at f/6 which costs much more.
But only if you can live with the off-axis coma of the Newtonian design.
If you need to include a coma corrector to the newtonians then of course you add lenses and hence decrease the transmission.
Schmidt-Cassegrains on the other hand can more easily be moved to very dark sites and so gain back a bit of their bad transmission by better sky conditions.
Used Telephoto Lenses (without auto-focus and zoom) have a different advantage: they are *cheap*!
In fact I have used all the discussed designs with success.
In other words: you can make great photos with each of the different telescope desings!
Have a look at the top photos of the astroscopic labs and compare the different designs.
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