AT 92mm f5.5 - looks interesting

[Stu]

I've often had my eye out for one of these but they seem pretty rare and are out of production. A 92mm triplet at f5.5 appeals from a balance of scope size, aperture, lovely widefield views and also the portability to get it under the dark skies it would need.

I'm pleased to see that Astro Tech are re-releasing a revised/improved version of this product in early 2018.

Sourcing AstroTech products in Europe seems a little tricky these days, but it is certainly a scope that would appeal to me when selling turns back to buying!! I have fond memories of the 106mm f6.5 AstroTech triplet I owned some time ago.

https://www.astronomics.com/astro-tech-at92-f55-triplet-apo-refractor-ota_p20545.aspx

A potential issue with fast fracs like these is field curvature so I would want to be sure that widefield views were not too badly affected by this, will be interesting to see first reviews.

[John]

It does look a nice package

Astro Tech have had some nice items under their branding over the years. The original Paradigm eyepieces (AKA BST Starguider now) were AT's.

The AT 92 can see to the "furthest reaches of infinity" as well. My scopes only get to the nearside of infinity

 

[Stu]

5 minutes ago, John said:

The AT 92 can see to the "furthest reaches of infinity" as well. My scopes only get to the nearside of infinity

Only looked at the scope, didn't bother with all the flannel  

[MarsG76]

I got the email advertising the re release of this scope and it definitely peaked my interest... perhaps its time to upgrade my frac for wide field astrophotography.

 

[DirkSteele]

Sounds very promising. I heard Astrotech end up with some of the late Tom Back's designs including the TMB 92mm Signature series. I recall a very positive review for that scope in Sky & Telescope many years ago. If the design has been refined to take advantage of technology advances since the original hit, and it is executed well, the scope could be rather special!

[Stu]

8 minutes ago, DirkSteele said:

Sounds very promising. I heard Astrotech end up with some of the late Tom Back's designs including the TMB 92mm Signature series. I recall a very positive review for that scope in Sky & Telescope many years ago. If the design has been refined to take advantage of technology advances since the original hit, and it is executed well, the scope could be rather special!

Yes, I agree! One to keep an eye out for for sure.

[DirkSteele]

Here is the review by Alan Dyer of the original TMB 92 in Sky and Telescope from a fair few years back.  It was a great little performer.  Though I think the light weight model which came with a 2" focuser was the better choice for travelling grab and go.  If the new model has improved further, and it is built to tight specification....

 

http://www.skyandtelescope.com/wp-content/uploads/documents/TMB-92.pdf

[Stu]

Thanks Matthew  

[Piero]

Very interesting Stu!

There are other portable refractors with similar specs by Baader and CFF, both sold in Europe, but the price is double.. it seems they have excellent performance though!

[Stu]

1 hour ago, Piero said:

Very interesting Stu!

There are other portable refractors with similar specs by Baader and CFF, both sold in Europe, but the price is double.. it seems they have excellent performance though!

Interesting, do you have links, just for information really, I can't afford one!

[Piero]

1 hour ago, Stu said:

Interesting, do you have links, just for information really, I can't afford one!

Sure! Here you go: 

Baader: http://www.baader-planetarium.com/de/teleskope/baader-apo-95560-caf2-travel-companion.html  

CFF: http://cfftelescopes.eu/refractors/   (and in particular this:  http://cfftelescopes.eu/refractor-92mm-f6/  ) 

Expensive but seem top-notch! 

 

p.s. I'm also interested in a portable 95-100mm refractor.. It's rather funny that a 100mm F7.5 which can be split in two parts would fit the purpose and cost half of these fast telescopes. It would also be lighter..

[michael.h.f.wilkinson]

At double the price I would have to wonder how much real advantage I would get over the AT offering. THat sounds like a potential upgrade for my APM 80mm F/6 triplet

[Stu]

19 minutes ago, Piero said:

Sure! Here you go: 

Baader: http://www.baader-planetarium.com/de/teleskope/baader-apo-95560-caf2-travel-companion.html  

CFF: http://cfftelescopes.eu/refractors/   (and in particular this:  http://cfftelescopes.eu/refractor-92mm-f6/  ) 

Expensive but seem top-notch! 

 

p.s. I'm also interested in a portable 95-100mm refractor.. It's rather funny that a 100mm F7.5 which can be split in two parts would fit the purpose and cost half of these fast telescopes. It would also be lighter..

Regarding your PS, the FC100DC doesn't do a bad job in this respect. I have thought a few times in the past about getting a custom tube made for the optics with a sliding dewshield and a splittable tube. The standard product will go on a plane though and is very light.

My main interest in the 92 is for the faster optics/shorter focal length to get that wider field. These things are always a compromise, I loved my Genesis for the wide (5 degrees roughly) flat field, but didn't love the CA. The AT92 may give the fov but not have a flat field!

So, my ideal scope might be a lightweight 100mm fluorite doublet with a petzval reducer/flattener taking it to f5, splittable tube and sliding dewshield . I guess that is not so far from an NP101, but they are too big and heavy really.

[Piero]

As far as I can see many of these new telescopes address the needs of imagers rather than visual observers. 

For instance, many of these fast telescopes have 2.5" or even 3" focusers which are essentially suitable for large cameras or CCD.. These add a lot of weight too..

Is TS (Europe) the equivalent of AT (US)? They seem very similar..

[Piero]

1 hour ago, Stu said:

Regarding your PS, the FC100DC doesn't do a bad job in this respect. I have thought a few times in the past about getting a custom tube made for the optics with a sliding dewshield and a splittable tube. The standard product will go on a plane though and is very light.

My main interest in the 92 is for the faster optics/shorter focal length to get that wider field. These things are always a compromise, I loved my Genesis for the wide (5 degrees roughly) flat field, but didn't love the CA. The AT92 may give the fov but not have a flat field!

So, my ideal scope might be a lightweight 100mm fluorite doublet with a petzval reducer/flattener taking it to f5, splittable tube and sliding dewshield . I guess that is not so far from an NP101, but they are too big and heavy really.

The FC100DC is a lovely scope and judging from your images, it does indeed a good job as far as transportability concerns.  

You are right that it is all a compromise. Some time ago, I noticed this: http://www.apm-telescopes.de/en/telescopes/refracting-telescopes-ota/apochromates/ts-photoline-90-mm-f-6.6-fpl-53-triplet-superapo-telescope.html . 

[John]

I guess the Tak Sky 90 is very compact at F/5. I believe though that even the use of fluorite can't produce a CA free doublet at that focal ratio ?

My Vixen ED F/6.5 does pretty well for it's focal ratio and that it uses FPL-51 (we think) in it's ED doublet. The dew shield just slides off leaving a pretty compact OTA.

[Stu]

1 hour ago, John said:

I guess the Tak Sky 90 is very compact at F/5. I believe though that even the use of fluorite can't produce a CA free doublet at that focal ratio ?

My Vixen ED F/6.5 does pretty well for it's focal ratio and that it uses FPL-51 (we think) in it's ED doublet. The dew shield just slides off leaving a pretty compact OTA.

I tried a Sky 90 once John. Wasn't impressed to be honest. It might be good on axis but I didn't enjoy it for widefield which is what I was after. Maybe I had a duff one as I know they can be sensitive but it was also quite big and heavy. In that sense I much preferred my FC76DC with splittable tube.

I have always envied your Vixen because of the slightly faster Optics. The 106mm f6.5 triplet I had was similar focal length and very nice but significantly heavier with longer cooling time.

I guess, going back to my desire for a Petzval, f5 is perhaps just too fast without some form of correction/flattening.

I sometimes used a x0.8 reducer with the Tak to take in the whole of the Veil or NAN more comfortably, worked well.

If only I was designing these things, I would come up with something perfect (providing I was allowed to bend or ignore a few minor details like the laws of physics  )

[YKSE]

25 minutes ago, Stu said:

I tried a Sky 90 once John. Wasn't impressed to be honest. It might be good on axis but I didn't enjoy it for widefield which is what I was after. Maybe I had a duff one as I know they can be sensitive but it was also quite big and heavy. In that sense I much preferred my FC76DC with splittable tube.

I have always envied your Vixen because of the slightly faster Optics. The 106mm f6.5 triplet I had was similar focal length and very nice but significantly heavier with longer cooling time.

I guess, going back to my desire for a Petzval, f5 is perhaps just too fast without some form of correction/flattening.

I sometimes used a x0.8 reducer with the Tak to take in the whole of the Veil or NAN more comfortably, worked well.

If only I was designing these things, I would come up with something perfect (providing I was allowed to bend or ignore a few minor details like the laws of physics  )

I'd think this TS has more or less all the feathers you want, the picture in TS site explain it well.

http://www.teleskop-express.de/shop/product_info.php/info/p5964_TS-Optics-Photoline-90mm-f-6-6-FPL-53-Triplet-SuperApo---2-5--focuser---3-focus-positions.html

e.g. you can can have fast f4.9 and flat field, and you can use it with binoviewer in low power mode, it's fairly compact and the price with a flattner is in par with 92mm AT.

[Stu]

1 hour ago, Piero said:

The FC100DC is a lovely scope and judging from your images, it does indeed a good job as far as transportability concerns.  

You are right that it is all a compromise. Some time ago, I noticed this: http://www.apm-telescopes.de/en/telescopes/refracting-telescopes-ota/apochromates/ts-photoline-90-mm-f-6.6-fpl-53-triplet-superapo-telescope.html . 

Looks interesting too. I wonder if the reducer or flattened can be used visually? 

I think too often these scopes are just designed for imaging and mlss a trick. Is it the WO 71mm which is an f4.9 quad but you can only use a 1.25" diagonal with it so cannot fully exploit its short focal length and flat field visually which is a shame.

[Chris]

2 minutes ago, Stu said:

Is it the WO 71mm which is an f4.9 quad but you can only use a 1.25" diagonal with it so cannot fully exploit its short focal length and flat field visually which is a shame.

Interesting point, Stu. Still, I bet it would be stunning with a Pan/ES 24mm 68* in the 1.25" diagonal when compared to the field curvature of most other short ED's. I'm giving thought to the new WO61 ED, but I do need to factor in field curvature which can be substantial judging by memories of my Starwave 70ED plus ES24mm.

9 minutes ago, Stu said:

I wonder if the reducer or flattened can be used visually? 

I second this question!    

[Piero]

Another with similar specs (+ FT focuser) is the Borg 90fl f5.6: 

https://www.firstlightoptics.com/borg-astrograph-telescopes/borg-90fl-f5-6-basic-ota-with-feather-touch-focuser-set.html  

 

[Stu]

9 hours ago, Piero said:

Another with similar specs (+ FT focuser) is the Borg 90fl f5.6: 

https://www.firstlightoptics.com/borg-astrograph-telescopes/borg-90fl-f5-6-basic-ota-with-feather-touch-focuser-set.html  

 

I had no idea there were so many lovely options out there! That Borg looks fabulous with the FeatherTouch doesn't it?

All have been filed away for future dreaming  Thanks Piero  

[Piero]

15 minutes ago, Stu said:

I had no idea there were so many lovely options out there! That Borg looks fabulous with the FeatherTouch doesn't it?

All have been filed away for future dreaming  Thanks Piero  

The specs are very interesting indeed.

I suspect it will show some lateral colour because of the fast focal ratio and not being a triplet, though. 

On the other hand it fits in one's pockets when unmount! LOL!