I think balance is as important. A lot of scopes intended for AP seem to be rear heavy and have limited back-focus.



My SW 80 ED DS Pro is really well balanced even with the aftermarket focuser. Shown here with a Takahashi diagonal and Takahashi 'before the diagonal' Barlow. I'm using a 3x SvBony Barlow housing (minus the element) as an ersatz extension tube. This way, the balance is still excellent and vibration is at a minimum. The Barlow needs an extension to achieve focus.



I think the perfect Strehl number is 1.0, although apparently it can never be achieved. So, the nearest to 1.0 is the best number. According to Wiki:

"The Strehl ratio S is frequently defined as the ratio of the peak aberrated image intensity from a point source compared to the maximum attainable intensity using an ideal optical system limited only by diffraction over the system's aperture."



So, your guess is as good as mine lol.



I want to get the 102mm Altair Starwave out soon as well. This balances well with 1.25" accessories on a Porta II/TL130. I'm pretty sure the Starwave and the EDF are made by Kunming United Optics. They both have excellent FPL 53/lanthanum glass.

 

The Tak' Barlow is a bit sui generis in the fact that it was deliberately designed to be used in front of a 1.25" diagonal for 2x and to be used conventionally in a 2" diagonal (in an adapter) for 1.6x. It can't be used conventionally in a 1.25" diagonal.



The field lens is a diameter that would miss most safety shoulders and contact the prism/mirror. It was also designed to be used with an extension, ostensibly for a Tak' f/7.5 refractor. The 80ED is f/7.5.



It can dampen vibration this way, if the eyepieces are small. The diagonal prism is very high quality, the only thing better is the Baader BBHS. Unfortunately the housing is like it came out of a Christmas cracker. Although I've been reliably been informed it's strong enough to hold a bino. The twistlock is fussy with undercuts. I discussed it here:

https://astronomyconnect.com/forums/threads/1-25-prism-comparison.13091/

The diagonals are about a hundred quid (£) here. Tak' are basically giving them away. I believe they're more expensive Stateside.

 

I think the Tak' prism housing is light gauge pressed aluminium. It's stronger than it looks but the twistlock is a bit fussy with undercuts. The prism itself is excellent though. The Ohi ortho's are 42° whereas the Tak' ortho's are 44° as they have a bigger field stop. They are still only really 42° though, the extra 2° is probably to aid target acquisition. The Tak's have better contrast but less overall transmission; so basically a case of 'swings and roundabouts'.



Also, the 4mm is ergonomically easier to use than the Ohi equivalent and there is a unique 32mm Takahashi version. FLO are selling Ohi ortho's now:

https://www.firstlightoptics.com/ja...anese-genuine-abbe-orthoscopic-eyepieces.html

They are sold under different names but they're all made at the Ohi factory in Tokyo.