Posts Made By: Vahe Sahakian

"I had my 6"f/12 setup next to the new 6.1" f/9 EDT, and at first glance cast on Saturn, I knew it was all over."

That is what T. Back said when he looked through the 155 F/9 EDT, possibly the finest planetary scope from AP.

Vahe

You are referring to Chromatic Aberration where different wavelengths/colors focus on a different plane in less well corrected achromats compared to apos..
You will have some amount of SA in very fast apos, this shows clearly in defocused star images on both sides of best focus, one big advantage of long focus apos is near elimination of SA from the system.

Vahe

[QUOTEThis is a 1/4 size image of the full 2 panel mosaic taken by Tony Hallas with a 206EDF refractor. [/quote]
Just a question.
Comparing this B&W image to similar images produced by Dave Jurasevich using 160EDF I see that the bright stars are highly bloated in AP206 images, what causes this?

Vahe

[QUOTED & G has had long ratio doublets listed for several years now, at not unreasonable prices. I'm not ready to buy and haven't wasted their time asking about availability yet. [/quote]
I do from time to time visit D&G website, just curiosity, and have always wondered about their F/25 and F/30 objectives, is there really a market for these? Who buys these and in what configuration are these used? A folded refractor IMHO is not easy or cheap to build and introducing two flats will compromise the end result compared to straight through system using the same objective.

That said, I am a big believer in slow F ratio telescopes and just like you I kept dreaming about assembling one but gave up once I factored in the practicality issues.

When it comes to color issues, size and weight, for an apo F/9 is the sweet spot, for an achro F/15 is the most reasonable ratio, making them longer will definitely improve things but the negatives pile up quickly making them not practical.

I never gave up on long focus instruments, currently I have two F/20 TEC Maksutovs, a 10” and a 8”, I use the 8” under less than perfect seeing conditions or when I am not in a shape to do heavy weight lifting, the 10” is reserved for stable seeing, I had it out for Jupiter last night, seeing was near perfect from Houston, the four moons were round discs without any spikes, the detail on Jupiter was amazing @ 260x & 312x and the contrast produced by these two TEC’s are almost as good as my AP refractor., in fact better than the refractor once I go over 240X where the 6” refractor will max out on Jupiter.

To me for anyone considering long focus refractors for planets, Maksutovs offer an attractive alternative, very compact, considerably less color than the finest apos. Cooling is an issue, but most imported Maksutovs do have cooling fans that do control thermal problems reasonably well.

Just something to think about.

Vahe

I would be curious about differences, actual not theoretical, between the new Abbe II and the previous 1.25” ZAO I eyepieces.

For a while I considered getting a pair of Abbe II’s but decided against it since I only needed a pair of 10mm. I do have a pair of ZAO 16mm and the 4mm and 6mm would not be of any use to me since all my scopes are long focus visuals.

Vahe

[I]"Pentax will stop to make telescopes.

The future of apo refractors appears to be moving in the direction of fewer premium brands and a lot more inexpensive “Planet Killer” refractors mostly outsourced to low cost high volume optical shops.

Vahe

Zeiss 150APQ is going to be considerably more expensive than the EDT, if you are really interested and can afford it get the APQ but do not sell the EDT until you had a chance to do a side by side comparison, you can always sell the either scopes once you decide.
The last EDT that I recall listed in Amart (#553850) sold for 10K, a pristine APQ will go for 3 to 4 times that amount.
It is my understanding that APQ’s were corrected to be colorless in visual spectrum only, but ultimately you should compare the two.
I have a 1993 EDT, to me this thing is about as color free as a Newtonian.

Vahe

I have used 18mm Tak Le’s and 18mm Pentax Ortho’s in the Zeiss viewer, the Tak is the most comfortable and is forgiving to eye position, with Pentax 18-O your eye must be at the exact spot with zero tolerance for side movements.
Performance wise Pentax 18-0 is superior, really no contest here. In fact I rate the Pentax 18-O the equal of Zeiss 16 Abbe Ortho in image quality.

Vahe

You are not de-sensitized, what you are witnessing here is gradual loss of hearing when subjected to very high SPL levels, to make up for the loss you need to crank up the volume even higher, that will take care whatever hearing that you still have.

Vahe

Can not really make up my mind as to what my choice would be if I had to decide on only one scope, but I do know for sure what I would not get and that is any scope with eyepiece at the wrong end, that pretty much eliminates Newts, Mak Newts and several other types all with eyepiece position at the wrong end.

Vahe