Flip Mirror vs 2" diagona

[konstantinos75]

Hello

I have the following flip mirror which accepts 1.25" eyepieces
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p242_Vixen-flip-mirror-for-2--systems---adapts-to-2x-T2---1-25-.html

Now I am thinking to upgrade to this 2" dielectric diagonal
https://www.firstlightoptics.com/diagonals/william-optics-2-dielectric-diagonal.html

If I place a 1.25" eyepiece to this 2" dielectric diagonal will I have

- wider field of view
- better image quality
- sharpher and brighter image

???

Thank you
Konstantinos

[michael.h.f.wilkinson]

I have both a flip mirror (1.25" from Vixen) and a couple of 2" diagonals. I only use the flip mirror for imaging, as the 2" diagonals offer much wider views by allowing the use of 2" EPs. My diagonals are also a touch better in quality than the Vixen flip mirror

[konstantinos75]

ok so you advise me to go ahead buying the 2" diagonal.

Many thanks

[michael.h.f.wilkinson]

Certainly. I had the WO you are considering, and liked it a lot. I only sold it when I got a Denkmeier (with WO-built diagonal by the look) with filter switch, allowing easy change of filters. It works like a charm in the C8. It later turned out the Filter-Switch diagonal does not come to focus with my APM 80mm F/6. I do have a 2" Amici prism, which does come to focus in the APM, but an Amici prism (which is perfect for wide field views because of the correct image) does not do well at high magnification. Therefore I got a 2" dielectric diagonal again.

[Ruud]

4 hours ago, konstantinos75 said:

If I place a 1.25" eyepiece to this 2" dielectric diagonal will I have

- wider field of view 
- better image quality
- sharpher and brighter image

No, no, no, but do get a 2" diagonal and buy at least one long focal length eyepiece with a wide field of view for it.

You can get wider views of the sky from a 2" diagonal, but 1.25" eyepieces will show the same field in both 2" and 1.25" diagonals.

An eyepiece is like a loupe. You use it to study the image that the objective forms at its focal plane. An eyepieces has a field stop to prevent you from seeing those parts of the focal plane that it can't show sharply. The larger the field stop, the more sky you see. The field stop cannot be bigger than the barrel of the eyepiece. It has to fit. The size of the field stop is proportional to the apparent field of view of the eyepiece and to the focal length of the eyepiece.

You can calculate an eyepiece's field stop: 
Field Stop (mm)  =  0.0175 * apparent field of view of eyepiece (degrees) * focal length of eyepiece (mm)
When you get values larger than 29mm for the field stop, the eyepiece can't be made in a 1.25" version.

The true field of view (tfov) is the bit of sky that you see in the eyepiece, it increases with increasing field stop. The apparent field of view (afov) is how wide the view appears to the eye. Manufacturers advertise the afov in degrees, for instance 68°.

The widest tfov you get from a 2" barrel is obtained (for example) by a 40mm 68° eyepiece, whereas the widest tvof from a 1.25" barrel is reached with a 24mm 68° eyepiece. The 40mm will show a 1.67 times wider view than the 24mm. The field stop of a 24mm 68° eyepiece is about as wide as a 1.25" barrel has room for. You can not fit a 40mm 68° in a 1.25" barrel because its field stop is too wide for that.

The same goes for a 32mm 50° eyepiece. This too has a field stop that nearly uses all the space of a 1.25" barrel. For focal lengths longer than 32mm, 50° eyepieces will need a 2" barrel to accommodate the field stop.

Unless your current 1.25" diagonal is bad, you won't get better image quality from a 2" diagonal. 

The reflectivity of diagonals varies from something like 95% to 99%. This influences the brightness of the view, but the gain from higher reflectivity is not enough to be noticeable. When you go from 95% to 99% you gain 2.5*log(99/95) = 0.045 magnitudes in brightness. You won't notice the difference, although it can be measured.

The most important reason to have a 2" diagonal is to be able to use eyepieces with wider fields tops than will fit in a 1.25" diagonal. Those wider field stops will show you more sky.

You may find these calculations useful:
--> magnification = focal length telescope / focal length eyepiece
--> tfov = afov / magnification
--> exit pupil = objective diameter / magnification

A few examples on how to use them:

I have two eyepieces that need a 2" diagonal, one is of 34mm and the other of 28mm focal length, both have a 68° apparent field of view. I use them in my f/5 refractor, which has a focal length of 500mm. The objective diameter is 101mm.

The 34mm eyepiece gives a magnification of 500/34 = 14.7x and a tfov of 68°/14.7 = 4.6°. That is as wide as I can go without getting an exit pupil wider than my own pupil. The exit pupil is 101mm/14.7 = 6.9mm.

The 28mm gives a magnification of 500/28 = 17.9x. The tfov is 68°/17.9 = 3.8° and exit pupil 101/17.9 = 4.6mm.

A 40mm 68° eyepiece would give a 500/40 = 12.5x magnification and a tfov of 68°/12.5 = 5.4°. That is a wonderful wide vista, but the exit pupil would be 100/12.5 = 8mm. That is wider than my eyes can handle. I would be wasting light. 

I do want wider fields than 4.6°, but for those I use binoculars.

Many observers, btw, prefer to keep the exit pupil under 5 mm, for reason that he sky brightness gets too high with bigger exit pupils. There is also an age related reason: when people get older, their maximum pupil size gets smaller, and by the time you're really old it is about 5 mm.