55 replies to this topic

[Scott in NC]

A few months ago I purchased an Astro-Tech AT125EDL. This is an f/7.8 doublet apochromatic refractor which is sold by Cloudy Nights's host Astronomics, and manufactured by Kunming United Optics in China. Its optics utilize a Hoya FCD100 ED element and a lanthanum mating element, and is suspected to have been figured and interferometrically tested in green light (based upon anecdotal and circumstantial evidence). It's turned out to be a fantastic scope which I believe represents an excellent value for its price, and I've had a great time observing with it in the short time that I've owned it.  As some of you may recall, I owned another AT125EDL last year which compared very favorably to my prized Takahashi FS-128 in a side-by-side observational session, which I've previously reported here.  

 

I've presented some preliminary DPAC test results from this AT125EDL before, but I've learned a lot since I originally posted that information, especially with regards to photographic technique. I thought it would be interesting to write this up in a more formal fashion, similar to the way I just did with a very similar scope, the Stellarvue SVX127D, which is an f/8 doublet apochromatic refractor which is manufactured in Stellarvue's facility in Auburn, CA, with hand-figured optics using a Hoya FCD100 ED element and a lanthanum mating element, as documented on the Stellarvue.com website.

 

The build quality of this Astro-Tech scope is very nice, and the powder coating is evenly done in a beautiful matte finish. Some people may prefer refractors with a glossy white finish, but matte white reminds me of the finish on another of my favorite scopes, the TEC 140.

Here’s the scope set up on my optical testing bench (a.k.a., dining room table).

 

[Scott in NC]

As previously stated, I have a strong suspicion that these optics were figured and tested using interferometry using a green laser.  I base this belief upon the fact that the optics appear to be best corrected in green, as well as due to the fact that other 125mm f/7.8 doublet refractors with similar optics produced by KUO have also been determined to have been tested in green.  I can't imagine that KUO would produce variations of the same scope for multiple vendors (Astronomics, TS-Optics, Altair Astro) and test some of them in green and others in red.

 

First I'm going to present the Ronchigrams as tested in green light.

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

 

[Scott in NC]

In green, spherical control appears to be very good, with only a very minor hint of overcorrection, which I can see more clearly in the outside-focus image.  It's subtle, yet there.  The lens appears to be quite smooth, with only a small, shallow central zone.  There may be a very small edge abnormality as well (turned down), but I've seen this in many scopes and when it's this small I have a difficult time deciding whether it's truly there or is merely a diffraction artifact.  Regardless to say, this is not something that I can see while observing through the scope.

[Scott in NC]

Next are the Ronchigrams in red:

 

Inside focus:

 

 

 

Outside focus:

 

[Scott in NC]

To my eyes, spherical correction looks just as good (or very nearly as good) in red as in green. In this case it's in the opposite sign as in green (undercorrection here, vs. overcorrection in green).  So far so good!

[Scott in NC]

Next the blue Ronchigrams. In several of the scopes that we've seen DPAC tested so far, blue is really where the spherical correction starts to fall off, especially if the scope was figured all the way on the other side of the visible light spectrum, i.e., in red.

 

Outside focus:

 

 

 

Inside focus:

 

[Scott in NC]

As you can see, the Ronchi bands are more bowed in blue than in any other color, indicating less well corrected (in this case overcorrected) spherical aberration than in green or red.  It's still not too bad IMO, and the fact that SA is slightly undercorrected in red, slightly overcorrected in green, and overcorrected (but a little more so) in blue, suggests to me that this lens was indeed figured in green.  And that's how it should be IMO.

[Scott in NC]

And finally, here are the white light images, which are useful for judging control of chromatic aberration.  As expected with an FCD100 and lanthanum doublet at a focal ratio of f/7.8, this scope does pretty well.

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

Overall, I think the Astro-Tech AT125EDL is a very nice scope, and I still don't know how they're able to manufacture and sell this for the price that they do and make any profit.  Astronomics originally sold this scope for $1849, but has had it on sale for the past few months for $1695.  At that price I consider it a steal!

 

As I mentioned, I've had the opportunity to test this scope (although not this particular sample) outdoors for visual observation side-by-side against my Takahashi FS-128.  When time and weather allow, I'd like to test it outdoors against my recently acquired Stellarvue SVX127D.  I think that that would be a more relevant comparison, as both of these are currently-produced scopes with similar apertures (125mm vs. 127mmm), similar focal ratios (f/7.8 vs. f/8), and the same glass (Hoya FCD100 for the ED element, and lanthanum for the mating element).  I'll try to post more when I've had the opportunity to do that testing, and will likely put my findings in a thread of their own.

 

This has been a really enjoyable project, and I thank everyone who's stuck with me and managed to read this far!  

[Regulus 1.36]

Wow! That's a lot of scope, Congrats. 

[PhotogTom]

That is really nice. I wouldn't know how that translates to a strehl value at each color, but I see straightness of those lines as a good thing.

 

I wonder how well this quality correlates to my 102EDL. Also, I'm going to try to compare to the 130 EDT, keeping in mind that we're seeing a sample of one here. 

 

Still learning this stuff, and probably will never have a master-level command of the subject. 

[Scott in NC]

Tom, so far I’ve DPAC tested an AT80EDL and an AT125EDL, and have visually observed through those two scopes as well as a second AT125EDL. No, that’s still not enough to make a generalization, but from what I’ve seen so far, the ATxxxEDL line appears to be pretty consistent.  I would expect your AT102EDL to test just as well.  I wish I could get my hands on one, but unless one shows up on the used market for such an incredibly low price that I can’t resist, I don’t have any plans to purchase one.

[Alan S]

Right...I have a feeling you will be testing a 102EDL soon enough!

I bought my 102EDL for a very good price used, and the scope appeared as new.

So looking forward to your testing of the 102EDL 🤣

[PhotogTom]

Tom, so far I’ve DPAC tested an AT80EDL and an AT125EDL, and have visually observed through those two scopes as well as a second AT125EDL.  No, that’s still not enough to make a generalization, but from what I’ve seen so far, the ATxxxEDL line appears to be pretty consistent. I would expect your AT102EDL to test just as well.  I wish I could get my hands on one, but unless one shows up on the used market for such an incredibly low price that I can’t resist, I don’t have any plans to purchase one.

The new one was an incredibly low price at $999.

 

The interesting thing is that I have this 102EDL as well as the 130EDT, and while the light gathering of the 130 is superior, the 102's visual image quality seems to be on par in most situations.  Love them both, though that 130, all dressed out, is a little bit heavy. 

 

I know that there's other testing that can/should be used to ascertain the full quality of the optics, but this tells a lot the way I see it.

 

In the photography world, we have it easier in a way - we take pictures and then pixel-peep them to death to determine sharpness, distortion, and contrast, as well as making subjective judgements on other aspects of the image produced - bokeh, color cast, lens flare, etc. And, of course, in the extra long lens category, the atmosphere plays an increasingly large role in IQ as the subject distance increases. Shooting images across a hot desert suffers from the same heat distortion that we might see from a telescope in the same situation, but less magnified.

[Scott in NC]

You’re probably right about that Alan, lol.  Unfortunately, there is a very real cost associated with this testing, and so unless I’m lucky enough to be able to borrow a scope, I need to make sure that anything that I purchase purely for the purpose of testing has to be purchased inexpensively enough that I’ll be able to resell it without taking a huge loss.  

[Joe G]

Scott,

 

Your photographic technique has really improved.  Great images.

 

I am struggling with the at focus images.  Our camera setups are mostly the same: DSLR with 100mm macro lens.

 

Can you elaborate on your technique for the in-focus images?

 

Thanks.

 

Great scope.

[Scott in NC]

Thanks, Joe.  It’s taken a lot of practice, but using the 100mm f/2.8 macro lens has really helped.  Another good (or bad, depending upon your point of view, lol) thing about this lens is that it gives a large enough image scale that small lens imperfections that weren’t easily seen with my prior setup easily show up now.

 

For the at-focus images I adjust the scope’s focuser so that I’m focusing in on a single Ronchi band and allowing it to “blur” off to one side so that it becomes nearly invisible.  I may have to take several photos in order to get one that really shows the wavefront well.  It also helps that I have a potentiometer hooked up to my power source so that I can adjust the LED’s brightness up or down.

 

That’s really all that I do, and can’t explain it much better than that.  The more you fiddle with it and the more practice images you take, the easier it will get.

[Joe G]

Thanks.  Pretty much what I have been doing.  I have a potetiometer too.

 

Is the focus point right after the band disappears as opposed to in the middle of where the band on the other side starts appearing?

[PhotogTom]

Scott, out of curiousity, do you use a tripod for the camera? I know that companies like Manfrotto make a pretty sturdy, small tabletop tripod that might be useful if you can't get things quite aligned here. Could be helpful to allow slower shutter speeds and avoid higher-ISO/Gain settings on the camera.

[Jeff B]

Scott, I really do have to  compliment you on those white light images.  A very nice at focus image.

 

Well done sir. 

 

Jeff

[Scott in NC]

Thanks.  Pretty much what I have been doing.  I have a potetiometer too.

 

Is the focus point right after the band disappears as opposed to in the middle of where the band on the other side starts appearing?

Good question, the answer to which I don’t really remember off the top of my head. I’ll say that it’s probably right after the band disappears, but I’ll often take multiple images right at and around what I think is likely the best focus point and then use the best of those images.

[Scott in NC]

Scott, out of curiousity, do you use a tripod for the camera? I know that companies like Manfrotto make a pretty sturdy, small tabletop tripod that might be useful if you can't get things quite aligned here. Could be helpful to allow slower shutter speeds and avoid higher-ISO/Gain settings on the camera.

So far I’ve taken all of my images handheld, but at 1/20 second exposure I know that a tripod would likely help to reduce blur and sharpen the images up. I don’t think a tabletop tripod would work, as the way I set my rig up, the focuser end of the scope is hanging off the back end of the table.  But I do have a Manfrotto camera tripod with a pan head, and may give that a try one of these days.  I’ve resisted so far, as my DPAC rig is pretty simple, and this will add yet one more piece of gear to have to set up.  But I really do need to experiment with it one of these days, and if it leads to improved images then I’ll likely stick with it.  Thanks for the suggestion!

[Scott in NC]

Scott, I really do have to  compliment you on those white light images.  A very nice at focus image.

 

Well done sir. 

 

Jeff

Thank you, Jeff!  I like how the at-focus image can really give you a good idea of the amount of chromatism present.  And I think I’ve mentioned this before, but in addition to the usual three bands in the Ronchigram, I usually like to focus in or out just enough so that two more bands are just barely creeping in from the sides. This allows you to see the color fringes a little better.  Notice here how they’re green inside focus and purple outside focus.

[Joe G]

So far I’ve taken all of my images handheld, but at 1/20 second exposure I know that a tripod would likely help to reduce blur and sharpen the images up. I don’t think a tabletop tripod would work, as the way I set my rig up, the focuser end of the scope is hanging off the back end of the table.  But I do have a Manfrotto camera tripod with a pan head, and may give that a try one of these days.  I’ve resisted so far, as my DPAC rig is pretty simple, and this will add yet one more piece of gear to have to set up.  But I really do need to experiment with it one of these days, and if it leads to improved images then I’ll likely stick with it.  Thanks for the suggestion!

I don't know how you do that, hand held.  I assume you are using live view and then you press down on the shutter button?

[Scott in NC]

Yes, exactly. I guess I’ve got pretty steady hands.