Prism v mirrors

[Visionary]

Is the addition coin that is required to purchase a 2" prism worth the shekels? I have a 8" SCT, I have read that a SCT does not really benefit from a prism and that a good mirror is fine, is this fact or cr$p?
One more question.... The Meade 4000 56mm at around $100 worth it? What function does this style of lens serve outside lunar observation? I am expanding my limited but excellent collection of len's and I would like to know if the Meade 400 2" 56 warrants a place in the lens collection?

[mental4astro]

David, you seem to be misunderstanding the differences between prism and mirror diagonals. A mirror diagonal is a single surface mirror that reflects the maximum amount of light possible. A prism, however, is a three surface item. Light enters the prism, and some light is lost being reflected off the first surface. It then strikes the 'diagonal' surface, and goes out through a third surface, where again some more light is lost by being reflected back. While optical coatings have reduced these reflections, prism diagonals are not as efficient as mirror diagonals. Prism diagonals are now not sought after by people who want the best possible light transmission.

There a few things you need to understand about eyepieces and what the limitations are of the barrel size between 1.25", 2" and 3". Yes, there are eyepieces with a 3" barrel. There are eyepieces with a 4.5" barrel too.

Take two straws, same length, but one is wider in diameter than the other. If you look through one, and then the other, you will notice that you will have a wider field of view with the larger diameter one. This is the same with eyepieces. The maximum true field of view offered by a 1.25" eyepiece will be smaller than a 2". This maximum is seen when one looks at the longest focal length possible for a barrel size and the apparent field of view (AFOV). The longest focal length in a 1.25" barrel size that will give 68deg AFOV is just 24mm. In the 2" format, this is 42mm.

Of course, there are longer focal lengths in these formats. There are 32mm and even 40mm 1.25" eyepieces, but these won't show any more of the sky than a 24mm 68deg eyepiece. With each increase in focal length though, the AFOV gets smaller and smaller. Same happens with 2" eyepieces.

Now, things don't stop here! With a C8, the limiting factor is not a 2" barrel, but the diameter of the bore in the primary mirror. It is actually smaller than the maximum diameter possible with a 2" barrel. This means that the maximum true field of view will be a little smaller in a C8.

That 56mm eyepiece will work well in your C8. The very edge of the field of view may not be very sharp, but it is upto you if this is acceptable or not. It's not like you will be looking through a brick. It will actually throw up a very nice image. But it will only perform at its best from a dark site. It is not an eyepiece for light polluted skies. This is common to very long focal length eyepieces - they concentrate the ambient light pollution too, not just the stars.

[Kunama]

There are other factors to consider than just light transmission, and good reasons (scatter, optical path length, etc) for choosing a prism over a mirror. Whether this applies to you depends on your scope, targets, preferred viewing method (bino v mono).

I still prefer the views of planets through a Zeiss prism over mirrors, just seems to me to show more detail. (I use refractors between F5 and F8)

Here is an interesting read: http://www.baader-planetarium.de/news/mirror-Prism-Dielectric-Diagonal-Comparison_2014-03-06_v2.pdf

[dannat]

Get a mirror diagonal for your CAT rather than prism, a good quality prism ESP. For a refractory can be beneficial but not for your scope

[mental4astro]

Well, looks like I have some humble pie to eat, Matt, :rolleyes: :lol:

Very interesting set of results. Interesting to note that the 'inexpensive' prism diagonals just didn't cut the mustard. I think it is most people's experience with these that has given prism diagonals a poor wrap, including with me. I have had three such diagonals that have come with different scopes I've picked up over the years. All have been poor & all cheap. I don't use these at all.

Most interesting for me is that the better prism ones showed less scatter with bright objects. I am always conscious of the scatter I see with the Moon in my C8 - still very sharp with seeing being good. The Moon is a very different beast to Jupiter. I wonder if a difference is noticeable here. Matt, any experience?

Thanks for the article, Matt. Like you said, these are application dependent. No compulsion for me to change though with my scopes and how I use them - considering the article!

Alex.

[Kunama]

In all honesty I have found the differences to be there but quite minor in most applications. I have used the Tak prism as well as some cheap prisms, Zeiss 34mm prism, some generic 2" mirrors and the Baader 2" dielectric. The cheap prisms are quite obviously lacking when up against the Zeiss.

In the end I have settled on the Zeiss prism for almost all of my viewing as I now only have 1.25" eyepieces and also intend to get the Baader MkV binos when my bank balance recovers. So for me the very short light path is an important factor as well. I can live with a little colour induced by the prism.

[dannat]

BillP on CN did some testing -with the right prism and refractor/ep combination it can slightly help to reduce colour, as opposed to a cheap prism or mirror diagonal

[The Mekon]

Yes prisms can be better than a mirror. I have a University Optics 2" prism. Tested it in an Astro-Physics refractor I previously owned, and it was better than any mirror prism I could find.

[Visionary]

Alex,

Many thanks for wading through my somewhat garbled question/s. I left a huge critical description/question out of the Prism-Mirror question. I am purchasing the prism for daytime use, whale watching. The question I should have forwarded is how good are prisms at astronomical observation within the broader context of day-time terrestrial observation. I shouldn't ask important questions late at night, my bad.
Subsequent to posting my question I have ordered a 2" Williams Optics Prism Diagonal for daytime terrestrial observation eg: Whale watching. Its looks like a beautiful piece of kit, reviews on the diagonal reflect your concerns with erect image prism diagonal terrestrial configured devices and expecting them to work perfectly within the context of dark sky. Erect prism image diagonals cannot work as well as a mirror or even standard prism.
You have answered my question about the Meade 56mm, your explanation is clear and concise, many thanks.
I have profound neurologically induced vision loss that explains in part my absolute fascination for all things optical. I am reading ever more on the subject of optics and I have thousands of questions. Next time I post it will not be late at night with a poorly described/framed question.
Alex, many thanks.