Telescope Reviews: EON 120mm apo next to Meade 127mm Achromat

My comment was general. Not referring to a specific ED scope in that particular posting.

However, my own exoerience was that f/7 ED DOublet sold in the 80mm Celestron and 80mm ED Orion scopes were not very sharp well off axis.

Since writing a review on my Televue 101, I had a guy with a High End telescope that used ED glasses contact me. I won't say the manufacturer, but it was a 140mm F/7. You can figure it out from there.

He was somewhat concerned because after reading my telescope review on Buffy (my 6" AP), he became much more aware of the off axis abberations in his own premium ED scope. He had owned some premium 4" scopes in the past (A slower premium Japanese made doublet) and to his recolletion, that scope was sharp to the field stop. He was contemplating ordering a larger scope from the same vendor, but started to have doubts.. He was worried that if the 140mm f/7 scope was't razor sharp off-axis, that the larger ED scope wouldn't be either.

Before buying a Televue 101, I called Stellarvue about the f/7 ED Doublet and I talked to the owner. I had asked him if he had tried the 31mm Nagler in his 4" f/7 to see how it performed off-axis.. He said no, he had not.. He then stated that if off-axis performance WAS important, I might consider his much more expensive APO. Hmmmm.

So, my direct experience with the Celestron 80mm f/7.5 (or whatever it was) was that they are not sharp at the edge of a 31mm Nagler field.

Making a fast scope with a flat, astigmatism free field is very difficult. APs arent' cheap, and there are numberous reasons why this is true. But if you are extremely discerning, you start to appreciate that the center 10% of the field is easy to get pretty.. It is the rest of the field that is more challanging.

So, my DIRECT experience was with a Celstron 80mm ED scope. My Email from the 140mm ED owner tells me I was right to pass on the 4" f/7 scope.

But hey, this is me. Buffy was an eye-opener to me. The field in a 31mm Nagler is GLORIOUS. YOu would have to see it to believe it. The stars even at the very very edge of the feild are just about as perfect as they are at the center of the field.

Even the Telveue 101 isn't as sharp at the EXTREME EXTREME edge of field as the AP is.. In fact, no telescope I have ever owned has been as sharp at the edge of the field as Buffy. None... It is remarkable to see. I literally gasped the first time I looked at the double cluster in this scope.

So, f/7 ED doublet in my book is pushing the envelope. To get good color correction at the center of the field, my guess is that they have to live with field curvature and astigmatism at the edge of the field.

My guess though is that a lot of people are delighted with theirs. I admit to being more anal or "Compulsive" than most people. So for me, I had to spend more to be happy... Maybe it is a curse.. Maybe not... I sure don't see it that way... LOL. But I had owned a Genesis years ago, and it was extremely sharp at the edge of the field. The Celestron by comparison was poor at the edge of the field. Heck, starting from 70% out.. If you want to call that the edge of the field.

Now to be fair, unless you are using an eyepiece like the 31mm Nagler, the field may not be wide enough for you to really see this. Someone using their f/7 ED scope with a 24mm Panoptic may never notice it.

Of course now the tradeoff is that they can't match the bigger, slower scope in terms of true field...

Which is how we got were we are when I said that the 127mm Achromat might have given better off axis performance than the 120mm ED scope...

Regards.