46 replies to this topic

[peleuba]

I had some time over Easter holiday weekend to bench test my Takahashi Sky 90-II, an F/5.6 Fluorite doublet utilizing a very wide air space between R2 and R3.

 

I have owned this telescope on/off (mostly "on") for the last 5 years or so.  I sold it to a friend and subsequently bought it back.  I really like the size of Sky90 and have always been a fan of its performance, but, perhaps I am in the minority.  Ed Ting (scopereviews.com) does not like it and years ago the Japanese Tenmon Guide panned its performance visually, without the Extender Q attachment.  And, when Takahashi introduced the FSQ series it made the Sky 90 obsolete as an astropgraph.

 

In any event, here is my Sky90-II in double pass autocollimation (DPAC) at three wavelengths. 

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[peleuba]

As you might expect, best overall correction is in Green.  Red and Blue have opposite correction which is to be expected as they are on opposite side of the spectrum from a wave length perspective. - A little under-corrected in Red and a little over corrected in Blue.  

 

Overall, the telescope is sharp to look through with a little bit of spherochromatism noted on the star test.  The first time I looked through it a few years back, I was pleasantly suprised.  I had read all of the negative reviews, but I like what it shows me.    

 

Here is photo of my test LED eyepieces.  They were 3D printed by CloudyNights member Moshen.  Having tested optics in DPAC for years, these eyepices make is easier to get acceptable photos to publish.    

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Edited by peleuba, 22 April 2019 - 10:58 AM.

[Paul G]

Nice scope! I have the Tenmon Guide article around here somewhere.

[Traveler]

Here another sky fan!

The Sky is an excellent astrophotography scope with its flattener/reducer (F4.5). Portability is very good. Traveling by airplane is no  problem (at all): it carries fine in a backpack.

[peleuba]

Nice scope! I have the Tenmon Guide article around here somewhere.

 

Thanks!  Yes, I am afraid to read the whole thing...  You sent be bits and pieces over the years.  I do have the Extender Q but I never use it.  Part of the allure of the Sky90 is the size.  The Extender Q, well, extends the size of the telescope!

 

Its a good lens, perhaps not in the same league as 92mm A-P triplets, but its good. 

[Tyson M]

Thanks!  Yes, I am afraid to read the whole thing...  You sent be bits and pieces over the years.  I do have the Extender Q but I never use it.  Part of the allure of the Sky90 is the size.  The Extender Q, well, extends the size of the telescope!

 

Its a good lens, perhaps not in the same league as 92mm A-P triplets, but its good. 

Now you got an AT92 to throw in the mix....interesting to see your thoughts on the scope. 

[t.r.]

Can you put some numbers to it? Roddier test or maybe star test? I too loved my Sky 90II...really a fine performer for its day and size. Better than the TEC 110 you tested or not quite?

[hoes]

I don't think Ed TIng ever tested the version ii  - I wonder if that would have made a difference?

 

Steve

[peleuba]

Can you put some numbers to it? Roddier test or maybe star test? I too loved my Sky 90II...really a fine performer for its day and size. Better than the TEC 110 you tested or not quite?

 

Sharper then my TEC110, for sure; bench images tell the story there.  I love numbers as much as anyone but these images and the star test really will tell you all you need to know.  I will get some bench star test images as time permits then see if any of those can be run through Roddier.  In green, this Sky90 is very, very slightly undercorrected.  The scope was in thermal equilibrium having sat in the V-blocks for more then 24 hours.

 

I've always liked the Sky90.  The CFF92 was better in sheer performance, but its a 35% premium in cost and it was a lot heavier.  There is just something about the TAK that I really like.  Overall, the problem I see with small APO's (up through 90mm) is not so much the overall quality of the lens as there are several good ones out there, but you begin to run out of light when you approach magnifications of 150X or so on the planets.

[StarryHill]

What's the difference between the original and the ii versions? How can you tell?

[peleuba]

I don't think Ed TIng ever tested the version ii  - I wonder if that would have made a difference?

 

Steve

 

Its the same lens/design/prescription.  The lens cell is what has changed.  There are screws installed radially around the lens elements to adjust tilt/collimation.  Its a fast Fluorite doublet that is limited by its aperture, it for sure, does not appeal to everyone.  But I will say, from bench tests and using it on/off over the last 5 years, its better then the reputation that precedes it.

 

All IMO, of course.  

[t.r.]

Curious how the Borg 90 FL would do on the bench...it is the Sky 90 successor or perhaps better stated as a sibling with Canon Optics? A couple who have had the Borg were not all that excited about the performance.

[starman876]

I have an earlier version of the Sky 90. The performance of the lens in such a small package is really outstanding.  My DPAC results are very similar to the ones you show.    It is my favorite small scope to use.   Truly outstanding performance.

[peleuba]

Curious how the Borg 90 FL would do on the bench...it is the Sky 90 successor or perhaps better stated as a sibling with Canon Optics? A couple who have had the Borg were not all that excited about the performance.

 

I have never seen a Borg telescope in person.  I do know that Canon-Optron made the lens for Borg.

[jay.i]

Curious how the Borg 90 FL would do on the bench...it is the Sky 90 successor or perhaps better stated as a sibling with Canon Optics? A couple who have had the Borg were not all that excited about the performance.

Moshen ran his 90FL through DPAC: https://www.cloudyni...-7#entry8682322

[StarryHill]

Really enjoy my Sky90 for visual use. However, I once tried it for astrophotography without a flattener and the coma was terrible away from the center.  See one sub below. This was with a fullframe Canon6d but I've done astrophotography with other OTAs and have never seen anything like this in them. Having read Ed Ting's reviews I always thought it was just a Sky90 thing and the reason for the mixed reviews. Maybe my Sky90 has bad optics or needs collimation (don't know if I have the ii). Or maybe a flattener can correct this.  In any case, it has been my most-used visual scope over the years and a ton of fun.

 

 

 

 

 

 

Edited by StarryHill, 22 April 2019 - 01:58 PM.

[chemisted]

I have SN 0018 which will have been in my possession 20 years this coming November.  I've stated before the views, the size and the FOVs I get with this scope led me to keep it and sell the AP Traveler that I owned (and pretty much just sat on the shelf).  Recently, with the encouragement of another CN member, I got the extender Q.  With this add on, I can say unequivocally that I have never seen a better star test.  Anyone interested in the scope and its history should read Todd Gross' review:

 http://www.newxspott...an/scope.htm#68

[peleuba]

 

Really enjoy my Sky90 for visual use. However, I once tried it for astrophotography without a flattener and the coma was terrible away from the center.  See below. This was with a fullframe Canon6d but I've done astrophotography with lots of other OTAs and have never seen anything like this in them. Having read Ed Ting's reviews I always thought it was just was a Sky90 thing and the reason for the mixed reviews. Maybe my Sky90 has bad optics or needs collimation (don't know if I have the ii). Or maybe a flattener can correct this. Any insights on this? In any case, it has been my most-used visual scope over the years and a ton of fun.

 

I do see what you mean.  The way to check collimation is to view a bright star in the center of the field and defocus it.  Then see if the image is round.  Here is an example from mine...  Its round and in (near) perfect collimation.

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Edited by peleuba, 22 April 2019 - 02:01 PM.

[25585]

I don't think Ed TIng ever tested the version ii  - I wonder if that would have made a difference?

 

Steve

How do versions 1 and 2 differ?

[bobhen]

I wish Takahashi would offer a TSA 90 at maybe F6 or F6.5. The TSA optics are just absolutely superb and Tak doesn’t have anything between 76 and 100mm. And making a fast triplet is less troublesome than making a really fast apo doublet.

 

There must be a market as AP sold 200 and CFF sold out a couple runs of their 92mm offering as well.

 

Bob

[StarryHill]

How do versions 1 and 2 differ?

If I'm understanding the info above, the only difference is that 2 has collimation screws available within the front lens cell and 1 does not. So, if properly collimated, both versions should be identical optically... again, if I'm understanding correctly. I'm quite comfortable collimating a reflector but pause at the thought of trying to collimate an apo. I imagine this requires a fair amount of skill and perhaps specialized equipment.

Edited by StarryHill, 22 April 2019 - 11:02 PM.

[etsleds]

There are several sets of screws in the lens cell, only one of which we're supposed to use to collimate the lens cell, the others I'm not sure exactly what they do.

 

I like the Sky 90 II a lot, it's fun for rich field sweeping up to about 150x for me, holds up past that just fine but then I'd rather be using a larger scope anyway.  It's generally a well-executed scope with sharp, contrasty, smooth optics whose various aberrations (field curvature and chromatic aberrations) are entirely expected from the physics of the design, once it's well-collimated.  I wouldn't be surprised if most reports of coma weren't just field curvature, miscollimation, or optics pinching.

 

 

If I'm understanding the info above, the only difference is that 2 has collimation screws available within the front lens cell and 1 does not. So, if properly collimated, both versions should be identical optically... again, if I'm understanding correctly. I'm quite comfortable collimating a reflector but pause at the thought of trying to collimate an apo. I imagine this requires a fair amount of skill and perhaps specialized equipment.

[gts055]

My understanding of the difference between the Sky 90 version 1 and version 11 is as follows : Version 1 has just three screws radially set  at 120 degrees to adjust the tilt of the front (fluorite) element. Version 11 has nine screws in total with six of these seen as three pairs adjacent to each other and a further three single screws.

The rear screw in the adjacent pairs adjust the centering of the flint and the front screw adjusts the centering of the fluorite. These six screws are in pairs set at 120 degrees apart. The remaining three screws adjust the tilt of the fluorite. Attached is an image of the cell showing one of the pairs and one of the single screws.  Hope this makes some sense, sounds very complex to me   Have fun,  Mark 

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Edited by gts055, 23 April 2019 - 07:05 AM.

[CHASLX200]

Always wanted a 90 but the collimation stories scared me away.

[Fomalhaut]

Always wanted a 90 but the collimation stories scared me away.

...plus "pinched optics" issues in low temperatures...