yesterday evening I could finally observe and compare my 152/900mm achromat (slightly undercorrected) and the new 152/1200 ED (Strehl 96.2%). On the planet there is no need for comparison. The difference would be obvious. For us the question remained whether differences can be seen on deepsky objects as well.
In general it can be said that the ED shows natural, more intense star colors and the contrast is noticeably higher. But does it also show more details?
Before I get into details, one important note: I am comparing two concrete instruments here, whose optical quality alone is quite different. So one should not consider this in a generalized way. But for me personally, this is enough, because I only have these instruments. However, today it might be easier to get an optically & mechanically decent ED than an achromat.
The achromat is equipped with a Baader Maxbright T2 mirror, the ED with a William Optics Zenit mirror. The conditions were pretty good in Leipzig (Bortle 6, SQM-L 19.2, NELM 5m2+). Both mirrors are of comparable quality.
First I used the well-known Egg Nebula (PK80-6.1) in Cygnus. In the ED at 171x (7mm DeLite) the nebula was clearly elongated, the northern "knot" appeared much brighter & rather round. The southern extension was slightly separated and slightly elongated. The image was very contrasty and the stars appeared pinpoint. Even at 300x the image was really nice and the structure was even more clearly visible. The achromat at about 165x (Nagler zoom ~5.5mm) showed the nebula also elongated, but the whole image looked more blurred and less contrasty. Fainter stars were less obvious. The separation of the two "knots" was hardly visible. At 300x this impression became stronger. But the stars themselves were also sharp in the achromat with defined focus. The nebula seemed to be a bit larger and more diffuse.
Next I observed the Cheeseburger Nebula (NGC 7026). At about 171x the ED showed clearly the more beautiful image. The nebula could also be magnified very well. At 300x the achromat showed a nearly round nebula with a very diffuse edge, getting little bit brighter towards the middle. It was and remained a diffuse nebula. Later I added the Baader long pass filter (light yellow), which increased the contrast. The nebula appeared a bit smaller and fainter. The dark band was not visible. When I first set the nebula in the ED at 300x, I was overwhelmed by how defined and clear the nebula appeared. The dark band was immediately visible! Regardless of this comparison I would not have expected it under these conditions. I tried to see the dark band in the achromat, but without success.
In between I observed the small Dumbbell Nebula, where the ED also revealed internal structures more easily. But I only left it at a short observation.
Just the observation of NGC 7026 under urban conditions showed me that my ED is clearly superior to my achromat. Under dark skies the differences could be even greater.
Edited by Robert Zebahl, 08 September 2020 - 03:28 AM.