I have been doing some comparisons with my 150mm x 1200mm refractor regarding light scatter and colour
correction of various diagonals.
What I have found in some older Zeiss documents mention that to get maximum performance from their
refractors they suggest a 90deg prism diagonal.
This got me thinking so a few trials were in order.
A 1.25 older Al coated diagonal, a cheaper 1.25 BK7 diag and a newer 2" dielectric diagonal.A wide angle 25mm
and a UWA 6.7mm eye piece was used.
On Jupiter the Al 1.25 appears clean and bright with quite an extended violet halo. Next the
2" dialectric showed a very slightly brighter image with a slightly extended violet halo however
showing some light scatter that reduced visual contrast.The BK7 prism diagonal provided quite a bit
more control of the violet light and gave a noticeable increase in contrast.Also noticeable was
a greater violet control on the moon giving a much sharper limb.Further
A noticeable increase in contrast and detail on several DSO's in particular various gob clusters.
A prism diagonal (which adds overcorrection, depending on the f/ratio of the objective lens--
the faster the lens the more overcorrection results) can help null the optical system.
Slightly under corrected objective lenses are fairly common in the marketplace.
While my Achromat is no APO it may(in some instances)try a BK7 Diagonal
and see what you find.I would suggest that a BK4 diagonal may not work as well for colour control due to it's denser properties and different refractive index.
My $49 spent on a BK7 diagonal was well worth it.
