Some report improvements when using a prism, but that is mostly due to just a change in the way the light is distributed and does not mean the contrast was improved. The goal is to preserve as much contrast as possible.
For example, say that you noted a slight red fringe on the ring of a very bright star. Now, suppose you make the spherical aberration of the color red worse, rather than better. The more of the energy in that red ring gets pulled out of that ring and the Airy Disk is re-distrusted across a wider area. The perception could be that the system is working better because the red ring is dimmer, but the reality would be that the contrast is being lowered. The red fringe is simply dimmer because it does not have as much energy concentration due to the SA pushing it further out into the pattern.
A prism adds spherical aberration and the three variables are the length of the path through the prism, the color of the light , and the focal ratio of the system.
In most refractor design, the best correction is for green. By this, we mean that the green has no, or a very small amount, of spherical aberration. This means that the other colors will have different amounts of spherical aberration. Typically the red is overcorrected and the blue is under-corrected but this is not always the case.
Remember, the prism adds spherical aberration. Now, suppose you see that there is a blue fringe, but when you add a prism, it gets a bit better. Some would say that it has improved the performance of the system, but the reality is that to get the improvement in blue, you added spherical aberration to what was perfect green, and made the red worse. Have you really made anything better or have you just shifted the error to a different colors? The contrast transfer is probably slightly affected. The blue is more focused, and the red is more spread out so harder to see, but more energy is taken out of the Airy Disk to get a better cosmetic view.
A refractor is typically designed to have the best contrast transfer possible when a mirror is used, and if you use a prism, you shift everything. At best it is neutral, but in reality, it will almost always lower contrast unless you use a glass path corrector.
On the other hand, these issues are far more important when the scope is fast and at f/7.8, it probably does not make enough difference in the overall performance to be easily seen, but unless the scope was designed for a prism, the contrast will usually be lowered by using a prism diagonal. Might you see a change in fringing? Sure, but does that mean the contrast transfer is improved? Unlikely. The designer optimized the performance for one wavelength, and when you use a prism, you change the correction of that wavelength, and usually not for the better.
I am attaching a chart that shows the Sperhochroamtism in green. Blue might be slightly improved, but green will be lowered and red will be even more affected, and this is why the contrast will be lowered, but at f/7.8, probably not enough to see.
Last time I will say it though. Clap now... Prisms introduce spherical aberration and if the scope has the best correction at green, the green and red SA will be lowered by the prism, unless it is a scope that was purposefully designed to under-correction in green, which the prism would correct.
This is unescapable. If you correct for one color perfectly by the design, and you use a prism, that color will be over-corrected. If you use a mirror, that line stays in correction.
My advice it to not take anecdotal evidence of seeing a change in the color fringing as meaning things are improved. You can't improve one color without affecting all of the other colors. Again, at f/7.8, it is not so important, but with faster highly corrected refractors, I recommend a mirror unless the manufacturer has specified a prism. A prism may change the color fringing of one color, but it comes a the expense of changing all of the other colors as well. It is far to complex to easily predict how all of the colors will behave, but they will be modified from the intent of the optical designer, who strove to make a design that would deliver the best performance when a mirror was used. If the maker does not specify a prism, you are almost assuredly getting the best performance possible with a mirror.
I know this is a wordy answer, but giving an answer without a deep explanation doesn't really do anything to dispel anecdotal evidence. If the system is perfectly corrected in green, and you put in a prism, it is no longer perfectly corrected in green. The only question now is does it matter to the user.
Edited by Eddgie, 12 February 2025 - 01:30 PM.
