prism vs mirror diagonal performance at different focal ratio

[Piero]

if I remember correctly I read somewhere that prisms are better for F7+ refractors, whereas mirror diagonals are generally advised for shorter refractors.

Now I don't know if this is true and I don't have any experience with this, so, any idea? 

Do prism diagonal increase FC or add other aberrations when used in sub F7 refractors?

[johninderby]

This is worth a read.

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

           John

[mikeDnight]

I've read the same information somewhere, probably in a review about diagonals. However a couple of years ago, a group of friends and i had our scopes set up for a side by side compare. One of the scopes was a Takahashi Sky 90. We spent the evening looking at the moon, Venus, Jupiter and numerous star images to try and define any differences. The Sky 90 was the weakest of the scopes as it just seemed to lack the punch of the other refractors. Part way through the session, I suggested to my friend who owned the Sky 90 that he should try a different diagonal. Without giving any consideration to the short F ratio of his scope I handed him my Tak prism to try. The planetary performance of the Sky 90 with regard to the level of detail on Jupiter was instantly, and very noticeably enhanced. He chucked his mirror in the bin and bought a Tak prism.

Now I'm not suggesting that the same result would occur on other short refractors. It may be that the Tak Sky 90 is a special case, as its lens configuration is different from the similar looking but longer FS series. On the Sky 90 the two elements have a greater seperation than the Fraunhofer FS. This was done i believe to allow better correction of various aberrations that would normally occur in such a short focus scope. The Fraunhofer lens design of the FS series of fluorite refractors reach their lower limit at F8, as any shorter and the level of CA would be unacceptable. By altering the lens design slightly and widening the seperation between the elements, the Sky 90 still maintains excellent control of CA. The alternative would have been for Tak to have used a Steinheil design for the lens, which allows excellent colour correction at shorter focal ratios, as in the modern FC series.

I have seen some truly awful diagonals in my time, most of which were prisms made by or for Celestron, so i suppose prism quality could play a very important roll in short refractors. 

Mike

[Merlin66]

In fast f ratio systems say ,f8 a prism diagonal will introduce chromatic aberrations.

They were "originally" invented for use on long focus f15 achromat refractors....

[Piero]

Thanks for your very interesting comments. Appreciated! 

[Solar B]

I commented (below) on another topic but I think it relevant 

 

In the HIFI world the correct chain of importance is 

1. Source .... CD player or Turntable 

2. Amplifier 

3. Speakers 

So it stands that in the Astronomy world the correct chain of importance would be

1. Telescope / Optics

2. Diagonal

3. Eyepiece 

Now many will happily spend hundreds on an EP but only around £100 on a diagonal 

I suggest that this may be an error as the diagonal in the chain is actually more important than the EP

Brian 

[Piero]

1 hour ago, Solar B said:

So it stands that in the Astronomy world the correct chain of importance would be

1. Telescope / Optics

2. Diagonal

3. Eyepiece 

Now many will happily spend hundreds on an EP but only around £100 on a diagonal 

I suggest that this may be an error as the diagonal in the chain is actually more important than the EP

Brian 

Hi Brian, 

what is your basis on the fact that a diagonal is more important than the eyepiece?

Also, for about £100 you can get a 1.25" tak prism or tv everbrite which are nearly as good as straight through observing.

Cheers

[Solar B]

Hi Piero 

 

If using a diagonal then as I suggest like the Hi Fi chain 

1...Telescope / optics are the most inportant component as this gives you the image or signal , once the signal has been accessed

it cannot be improved upon.

2... Diagonal , the image / signal will then pass to the diagonal , here it will be degredated or tarnished a little.

3... EP , the quality of the image or signal reaching the EP is totally dependant upon the quality of the previous 2 components thus

the EP is the least important within the chain.

Agreed operating a Telescope straight through with an EP would be preferable and I'm a Big fan of the Tak 1.25 prisms which offers

great value for money.

 

Brian 

[Astrobits]

From a purely theoretical ( and not exhaustive ) point of view:-

A mirror doubles the surface error so a 1/10th wave error becomes 2/10ths wave on the wavefront

A prism also has a reflective surface so it also has the same effect as the mirror. However, the prism also has two plane transmission surfaces which will, individually, affect the wavefront only half as much as the surface error, so x2 1/20th is 1/10 error. Thus a prism could have a 3/10ths error on the wavefront if all surfaces are 1/10th wave. In addition, the two plane surfaces of the prism act as a plane parallel window and to a converging wavefront this will add some spherical aberration and the angled light will also undergo some dispersion in relation to the wavelengths of light,  i.e. chromatic aberration. These last two effects will have very little effect on the final image as the distance over which they can operate is very small.

Of course, not every diagonal will live up ( or down ) to these expectations.

Nigel

 

[YKSE]

10 hours ago, Astrobits said:

From a purely theoretical ( and not exhaustive ) point of view:-

A mirror doubles the surface error so a 1/10th wave error becomes 2/10ths wave on the wavefront

A prism also has a reflective surface so it also has the same effect as the mirror. However, the prism also has two plane transmission surfaces which will, individually, affect the wavefront only half as much as the surface error, so x2 1/20th is 1/10 error. Thus a prism could have a 3/10ths error on the wavefront if all surfaces are 1/10th wave. In addition, the two plane surfaces of the prism act as a plane parallel window and to a converging wavefront this will add some spherical aberration and the angled light will also undergo some dispersion in relation to the wavelengths of light,  i.e. chromatic aberration. These last two effects will have very little effect on the final image as the distance over which they can operate is very small.

Of course, not every diagonal will live up ( or down ) to these expectations.

Nigel

 

Very itneresting, Nigel  the added spherical aberration with a prism diagonal, will it be under- or over-correction?

[timwetherell]

On my F7 refractor, my 1.25" prism is just a hair better than my 2" dielectric diagonal on planets and other contrast critical targets. maybe 1% better. Enough that I would use it for serious observation of Jupiter but not enough that I would bother changing it most of the time. Can't say I've noticed any detrimental effects of CA or SA introduced by the prism though I'm generally only using very narrow field eyepieces which may mostly see  "on axis" rays