The subject line is a question that comes up every now and then when considering apo refractor performance. I first heard about it in relation to the Zeiss APQ series back in the '90s.
To achieve the very best correction, those scopes require the use of a prism-type star diagonal.
Some apochromatic refractors work best with a mirror diagonal. Others benefit from the refractive properties of a prism. Rarely does the vendor indicate which works best with what.
I've often used a prism with my apo doublet refractors. The dearly departed Tak FS78 was always coupled to a Tak prism 1.25" diagonal and displayed flawless images.
More recently, I tried my Sky-Watcher ED120 with both mirror and prism diagonals and observed that the mirror-type showed slightly more chromatic aberration in the image. Thus, a '70s vintage Celestron 1.25" glass diagonal (original equipment with C8's of the day) has been used with that scope since the testing was done.
This past weekend, I was trying out a used TeleVue 2.5x Barlow with my Megrez 110ED, to see how it compared with a 2.5x Powermate. The Powermate wasn't showing it's best with the Megrez's f/5.95 focal ratio and FPL-51 doublet optics. Sure enough, the old TV 2.5x Barlow (actually 2.8x as measured) worked better than the Powermate. No stopping down to 101mm (f/6.5) was required to sharpen up
the diffraction image.
Just before deciding to bring the scope in for the night, a thought popped into my head ... why not try the
old Celestron 1.25" diagonal with the Megrez, in place of the William Optics 2" dielectric unit? It would be much lighter than the WO diagonal, but how would using a prism work in such a fast ED refractor??
I suspected it wouldn't be pretty.
The Megrez was aimed at Delta Cygni. Using a 12.5mm Ortho and the TV 2.5x Barlow (147x) showed the companion fairly plainly even in the so-so seeing. Not bad. But, SURPRISE-- switching to the Celestron diagonal revealed Delta's faint companion with slightly less difficulty and it looked like there was reduced red defocus in the primary's image. Markedly less!
The view was so promising, it tempted me to up the power to 183x and swing over to Epsilon Bootes (Izar) to compare the two diagonals once again.
With the dielectric, the main star's yellowy color showed it's typical ruddy excess at best visual focus. Swapping
to the Celestron diagonal reduced the unfocused red quite a bit. There was a real improvement.
It was like the FPL51 doublet had been upgraded to FPL52. How best to quantify this?
I'd say the 110ED at f/5.95 using the prism diagonal, showed better correction than the same scope stopped
down to 101mm (f/6.5) using the WO dielectric diagonal. In fact, the Megrez stopped down further to 94mm f/7
displayed about as much c.a. as at full 110mm aperture with the prism. Very surprising.
The upshot of this improvement is the WO 2" dielectric is now packed away in it's box.
It’s stylish shape and carbon fiber trim sure is impressive looking on the back of the Megrez and delivers a wonderfully bright image. A great piece of kit.
BUT, the smaller, lighter Celestron unit results in significantly reduced spurious color at best focus.
All I can suggest from this observation, is that if you currently own a relatively fast ED doublet refractor, it's
high magnification performance (ie. above 150x) could benefit from switching to a prism-type star diagonal.
But, you won't find out until you try.
If you have the chance to pick up a good quality right-angle prism diagonal for a reasonable price, it's fun
to experiment and possibly discover for yourself how to optimize your scope's image quality.
To achieve the very best correction, those scopes require the use of a prism-type star diagonal.
Some apochromatic refractors work best with a mirror diagonal. Others benefit from the refractive properties of a prism. Rarely does the vendor indicate which works best with what.
I've often used a prism with my apo doublet refractors. The dearly departed Tak FS78 was always coupled to a Tak prism 1.25" diagonal and displayed flawless images.
More recently, I tried my Sky-Watcher ED120 with both mirror and prism diagonals and observed that the mirror-type showed slightly more chromatic aberration in the image. Thus, a '70s vintage Celestron 1.25" glass diagonal (original equipment with C8's of the day) has been used with that scope since the testing was done.
This past weekend, I was trying out a used TeleVue 2.5x Barlow with my Megrez 110ED, to see how it compared with a 2.5x Powermate. The Powermate wasn't showing it's best with the Megrez's f/5.95 focal ratio and FPL-51 doublet optics. Sure enough, the old TV 2.5x Barlow (actually 2.8x as measured) worked better than the Powermate. No stopping down to 101mm (f/6.5) was required to sharpen up
the diffraction image.
Just before deciding to bring the scope in for the night, a thought popped into my head ... why not try the
old Celestron 1.25" diagonal with the Megrez, in place of the William Optics 2" dielectric unit? It would be much lighter than the WO diagonal, but how would using a prism work in such a fast ED refractor??
I suspected it wouldn't be pretty.
The Megrez was aimed at Delta Cygni. Using a 12.5mm Ortho and the TV 2.5x Barlow (147x) showed the companion fairly plainly even in the so-so seeing. Not bad. But, SURPRISE-- switching to the Celestron diagonal revealed Delta's faint companion with slightly less difficulty and it looked like there was reduced red defocus in the primary's image. Markedly less!
The view was so promising, it tempted me to up the power to 183x and swing over to Epsilon Bootes (Izar) to compare the two diagonals once again.
With the dielectric, the main star's yellowy color showed it's typical ruddy excess at best visual focus. Swapping
to the Celestron diagonal reduced the unfocused red quite a bit. There was a real improvement.
It was like the FPL51 doublet had been upgraded to FPL52. How best to quantify this?
I'd say the 110ED at f/5.95 using the prism diagonal, showed better correction than the same scope stopped
down to 101mm (f/6.5) using the WO dielectric diagonal. In fact, the Megrez stopped down further to 94mm f/7
displayed about as much c.a. as at full 110mm aperture with the prism. Very surprising.
The upshot of this improvement is the WO 2" dielectric is now packed away in it's box.
It’s stylish shape and carbon fiber trim sure is impressive looking on the back of the Megrez and delivers a wonderfully bright image. A great piece of kit.
BUT, the smaller, lighter Celestron unit results in significantly reduced spurious color at best focus.
All I can suggest from this observation, is that if you currently own a relatively fast ED doublet refractor, it's
high magnification performance (ie. above 150x) could benefit from switching to a prism-type star diagonal.
But, you won't find out until you try.
If you have the chance to pick up a good quality right-angle prism diagonal for a reasonable price, it's fun
to experiment and possibly discover for yourself how to optimize your scope's image quality.
