84 replies to this topic

[peleuba]

It's been a hot minute (slightly over a year) since I last posted a bench test of a refractor.  Its gratifying to see some other members picking up the slack.

 

The AT90CF caught my eye the moment I received the email from Mike Bieler introducing it.  This was, perhaps 6 months ago (or more).  What intrigued me was the price point.  In this aperture class I already had an A-P Stowaway with exquisite optics which I keep in our West Palm Beach condo but wanted another small APO - like a Stow -  here at my fulltime home in the Baltimore area.  The chances of me actually having an opportunity to procure a second Stowaway from A-P were slim, so I started to formulate a Plan B hopefully in time for the eclipse.   

 

As I started looking around, I noticed several vendors selling the same scope - a 90mm F/6 triplet with FCD100 advertised as the ED element - StellarVue, Orion, Astronomics here in the U.S. You may hear that these are not the same.  I assure you they are.  For good performance at a given price point, there are few mating elements that work with FCD100.  Moreover, I examined photographs of the different OTA's and the hardware appears to be quite similar - including the flanges that attach the dew shield to the OTA and allows it to slide fore and aft.  See the photo below.  My sense is that these OTA's and many others up through 152mm aperture sold by many retailers world-wide are sourced from Kunming United Optics.

 

I ultimately went with the Astro Tech AT90CF after Vic Maris (StellarVue) told me that:  "We sold more than we anticipated, so we ran out of a precision part. So, we cannot make them right now".  All things being equal, I would have preferred the aluminum tubed StellarVue scope but with the eclipse getting close, I decided to go with the next best option - a carbon fiber OTA from AstroTech.  Astronomics has become my favorite online vendor - I've purchased 4 refractors from them in the last 5 years.  Super nice to speak with and excellent pricing especially with the CN member discount.

 

To sum it up, the scope has exceeded my expectations.  I find it remarkable that a $1595 scope can be this good.  The focuser is terrific.  Its equal to the FeatherTouch rack and pinions on both my Stowaway and TSA120.  If there are any differences to the visual astronomer, they are not material to the feel or performance.  I hope I grow to like the carbon fiber tube!

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Edited by peleuba, 04 April 2024 - 03:06 PM.

[peleuba]

White  (click image to make larger)

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Edited by peleuba, 04 April 2024 - 01:20 PM.

[peleuba]

Green (click image to make larger)

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Edited by peleuba, 04 April 2024 - 01:20 PM.

[peleuba]

Blue (click image to make larger)

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Edited by peleuba, 04 April 2024 - 01:20 PM.

[peleuba]

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Edited by peleuba, 04 April 2024 - 01:20 PM.

[peleuba]

Star Test  (click image to make larger)

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Edited by peleuba, 04 April 2024 - 01:20 PM.

[peleuba]

Here are some photos of the scope during testing...

 

 

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Edited by peleuba, 04 April 2024 - 12:29 PM.

[peleuba]

Another

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[peleuba]

Overall, a very nice lens.  The scope has excellent spherical correction with low-ish spherochromatism and only subtle chromatic effects visible in the star test.  There is some very slight astigmatism that seems to come and go.  Even so, its at a very low level.

 

The Ronchigram shows green to have nearly neutral correction - somewhere between ⅛ to about ⅒ wave.

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Edited by peleuba, 04 April 2024 - 12:54 PM.

[photomagica]

I've checked my AT90CFT with DPAC visually and on a star with a 180 lpi ronchi eyepiece and overall the results are the same as you are seeing. It is a truly outstanding telescope. Thanks for sharing a very well conducted test.

Bill

[RichA]

White

Just a quick question, what are you using for the grating?  Is it a precision chrome-on-glass one or is it a plastic sheet type?  Reason I ask is, I personally have not seen bands as smooth as some of the tests I've seen here and I think it's because the plastic printed ronchi gratings I've used just have very course lines. 

Edited by RichA, 04 April 2024 - 01:27 PM.

[Polyphemos]

Looks like a terrific scope. Congratulations!

[jmillsbss]

Well, that does it....I'm gonna get one...

 

No. Wait.  I already HAVE one!  It really is an amazing telescope.

 

I'm glad to hear of others' reports, technical or not, that reinforces what I'm seeing with my eyeballs.

 

It was the most expensive scope I'd ever bought, but seems to be a very good bargain for the comparison to other more expensive scopes.  I don't know how much better an additional $4000 would perform.

[peleuba]

Just a quick question, what are you using for the grating?  Is it a precision chrome-on-glass one or is it a plastic sheet type?  Reason I ask is, I personally have not seen bands as smooth as some of the tests I've seen here and I think it's because the plastic printed ronchi gratings just have very course lines. 

 

Hi Rich - I am using, for this test, 133LPI gratings that, I think were printed on some type of hard plastic substrate (not a cellophane sheet).  I bought several of these from ronchiscreens.com when they were still in business.  Each of the colors is using a separate grating.

 

I agree, its smooth.  The lens is fairly smooth, too.  Only very slight micro-ripple and a subtle edge zone.

 

The smoothest lens I have ever tested (using the same Ronchi gratings) is here   

Edited by peleuba, 04 April 2024 - 08:34 PM.

[Heywood]

Overall, a very nice lens.  The scope has excellent spherical correction with low-ish spherochromatism and only subtle chromatic effects visible in the star test.  There is some very slight astigmatism that seems to come and go.  Even so, its at a very low level.

 

The Ronchigram shows green to have nearly neutral correction - somewhere between ⅛ to about ⅒ wave.

 

This is very good news, indeed.  Thank you for your efforts.  

 

My understanding is that the AT90CFT is the first scope in series of carbon fiber FCD-100 triplets with a guaranteed minimum Strehl, with the next one to arrive this coming summer.

Edited by Heywood, 04 April 2024 - 01:37 PM.

[peleuba]

This is very good news, indeed.  Thank you for your efforts.  

 

Glad to do this.  Its a terrific, high Strehl scope.  

[saemark30]

Paul, nice work. Can you divulge which mating elements work well with FCD100?

[saemark30]

To separate good diffraction gratings from the bad, take a look at it under a good microscope at 40-100x and see if the lines are truly uniformly dark and straight, and the spacing to line thickness is 50/50.

[peleuba]

Paul, nice work. Can you divulge which mating elements work well with FCD100?

 

While I do not know this "for sure".  My sense is that its Hoya BSC7 (the equivalent of Schott BK7 and Ohara BSL7).  Its relatively inexpensive and very high quality glass that will provide good performance with FCD100.   I saw no marketing info that "Lanthanum" glass was used, and trust me, if it was, it would be marketed as such.

 

Anyway, an FCD100 triplet doesn't need Lanthanum mates for high performance at F/6.  From what I've learned from from experienced lens makers is that Lanathanums do not have the highest internal quality and are more expensive then plain vanilla BK7 type of glass.   

 

So, that's why I think its BSC7 or some equivalent.

[photomagica]

Printer encoder strip provides a source of quality grating. It seems to come in only 180 lpi and higher counts. It is available very inexpensively on the auction site.  I got a strip 5m long by 20mm wide for under $15. For visual DPAC I was using a 133 lpi screen with reasonably clean lines that appears to have been printed with a film recorder, not an inkjet.

 

To provide some detail, I could see a very, very faint central circular zone and something at the edge that may have been diffraction. Visually, I find it difficult to distinguish an edge condition from diffraction unless the condition is significant. Neither of these things were significant at all. At very high powers under good seeing the diffraction pattern of a star is textbook. While DPAC or the Ronchi test is not quantitively, the good results give confidence that the lens does meet or exceed the promised 0.95 strehl.

Bill

[peleuba]

Visually, I find it difficult to distinguish an edge condition from diffraction unless the condition is significant. 

 

This.

 

I am glad I am not the only one who sometimes has trouble differentiating diffraction artifacts from true rolled/turned edges!

[Mike Sandy]

Printer encoder strip provides a source of quality grating. It seems to come in only 180 lpi and higher counts. It is available very inexpensively on the auction site.  I got a strip 5m long by 20mm wide for under $15. For visual DPAC I was using a 133 lpi screen with reasonably clean lines that appears to have been printed with a film recorder, not an inkjet.

 

Thanks for that tip!  I just went on and ordered a 5m strip and there were some available under $10 including shipping.  I have the 133 lpi that I had printed from a pdf file - this looks like a better, and cheaper, option.  Thanks again!

[Scott in NC]

Very nice, Paul. Thanks for sharing this. I’ll add it to the archives.

[MortonH]

How is the balance of the scope for visual use?  I remember reading that the SharpStar 90mm f6.6 was rear heavy.  For visual I like my scopes to be balanced without using a dovetail that's as long as the tube!

[kwyjibo]

How is the balance of the scope for visual use?  I remember reading that the SharpStar 90mm f6.6 was rear heavy.  For visual I like my scopes to be balanced without using a dovetail that's as long as the tube!

With a 2" diagonal and a Pentax 30xw eyepiece and the focuser 40mm out the center of balance on mine is right where the rear mounting ring is.