19 replies to this topic

[Scott in NC]

And finally we arrive at part 3 of yesterday's telescope testing marathon at my friend's house. This is a Takahashi TSA-120 that he's had for a while, but which I finally had the opportunity to test out.  I've had a TSA-120 of my own for over a year now, so already knew what to expect, but was eager to test another sample. Unlike the other two scopes that were tested yesterday (TOA-130 and Agema SD 130), the TSA-120 uses a conventional air-spaced triplet arrangement, without the wide air gaps that the other two utilize. It has a single FPL-53 element surrounded by two glass mating elements, and is only slightly faster than the other two at f/7.5. Compared to the other two scopes, it's a relative lightweight, weighing 6.7 kg (14.7 lbs.) according to the spec on Takahashi's website.  The one I tested may weigh slightly more though, as instead of the stock Tak 2.7" focuser, this one had the upgraded Starlight Instruments FeatherTouch FTF3035 3" R&P focuser available from the manufacturer as an option.

 

Here's the scope set up for testing.

 

[Scott in NC]

[Scott in NC]

This scope looks pretty nice, I think. There's some slight over correction in green, balanced out by slight under correction in red. Blue is a little more over corrected, but not excessively so, and is comparable in degree to what I've seen in many other high-end apochromatic refractors.  Chromatic aberration is very well controlled, although not to the same level of the TOA-130 (but then, that's a really high bar to set, as pretty much nothing else is). Polish looks pretty smooth, and I only see a shallow central zone near 35-40% of the radius, which to me doesn't look like anything to be concerned about.  I only see this on the "at focus" image, and don't see any evidence of it on the Ronchigram bars (i.e., notching of those bars) on the intra focal and extra focal images. The bottom line is that is a scope that I would be pleased to own.

[Erik Bakker]

That looks like a scope with very nice optics. And it is good to see it tested in the same way as the TOA-130 and SD 130 you just published recently. It puts the relative performance of these 3 scopes into perspective. 

[scoale]

So this is a scope I have had more experience with.  The scope initially had some coma that required a trip back to Japan, but I've had it under the stars multiple times since getting it back and it is very satisfying in use.

 

A few star test images below.  No filter on top images; green filter on bottom. You can see a bit of spherical error, consistent with DPAC result.  Tonight was way too turbulent to get any in focus images.  I also need to grab a few images at greater defocus to pickup the zones.  I can also confirm that there is a bit of color outside of focus, but none in focus.

 

Edited by scoale, 09 November 2024 - 09:12 PM.

[Jeff B]

Smoooth with a very nice edge. 

 

Regarding the star test images I find them really nice and that center zone is trivial to me and maybe only shows up as a center brightening on one side of focus with perhaps a dimming on the other side.

 

To me, there is no real secondary spectrum and the only remarkable bit of SA is in the blue, "classically" overcorrected.

 

Scott, I took the white light images and chopped out the blue, giving a blended yellow image.  Doing that, yellow looks really very "neutral". 

 

Scott's sample is right in there too.  I can see why these scopes are so nice visually.

 

Once again, great work gentlemen!

 

Jeff

[Scott in NC]

Thanks, Jeff! Please feel free to add your yellow images to this thread if you have the time.

[RichA]

TSA-120 (SC).png

Stick with the TOA?

[ris242]

The scope initially had some coma that required a trip back to Japan,

wait............a bad TSA 120?

[Scott in NC]

Stick with the TOA?

The TOA is technically a better scope, but there’s a cost (price, complexity, size and weight). The TSA is very good for what it is. The TOA is just “over the top” good. But it’s not for everyone. I’m happy to have both, but I use my TSA more than my TOA, mainly because it’s smaller and lighter, and I can use it with a lighter weight tripod and mount (DM4 and Uni-18, rather than DM6 and Planet).

[Scott in NC]

wait............a bad TSA 120?

I’ll let Stephen explain, but I believe it had to do with focuser misalignment, as the adapter that attached the upgraded FTF3035 wasn’t quite orthogonal with the rest of the tube. Or something like that. I don’t believe it had anything to do with the lens cell.

[scoale]

wait............a bad TSA 120?

 

I’ll let Stephen explain, but I believe it had to do with focuser misalignment, as the adapter that attached the upgraded FTF3035 wasn’t quite orthogonal with the rest of the tube. Or something like that. I don’t believe it had anything to do with the lens cell.

 

First, when people tell you there are no perfect telescopes....they are probably speaking from experience.  Every single TEC, TAK, Televue, Stellarvue, and Agema I have owned was/is imperfect - but very nice.  A quest for a perfect scope is sure to be painful (at least in my experience).

 

That said, my understanding of the root cause of the coma in my first TSA is imperfect, so I'm not going to say anything that might mislead or misrepresent. I have a cheshire to check lens alignment and a Glatter to check focuser alignment, and a camera - so I was able to provide info to TAK. Suffice to say, it was confirmed by TAK NA and deemed necessary to send to Japan for remedy.

 

My second TSA had obvious astigmatism and zones.

 

The first row of star test images are from the first TSA with coma.  This scope is now back in my possession and is the scope Scott DPAC tested for this thread.

 

The second row of star test images are from the second TSA with astigmatism and zones.  BTW, the spherical correction on the 2nd scope was fabulous.

 

Both of these scopes performed quite well under the stars, but I am OCD about coma and astigmatism.  Once I see them, I can't unsee them.

 

Edited by scoale, 11 November 2024 - 08:16 AM.

[Scott in NC]

Sorry, didn’t mean to mislead anyone about the cause of the issue. That’s what I get when I try to rely upon memory about things I know little about. Thanks for clarifying, Stephen!

 

But on another note, I think it’s great that we have the ability to test our scopes, whether it be via star testing, or DPAC, or better yet, both. No man made items, including our precious scopes, are going to have a 100% track record of perfection. But if we can identify issues, we have a much better chance of getting them rectified.

[scoale]

Scott, your memory is correct, there's just a bit more to it.

 

Completely agree that star testing and dpac can help you understand what you have, and help you resolve issues if need be.

[25585]

The TOA is technically a better scope, but there’s a cost (price, complexity, size and weight). The TSA is very good for what it is. The TOA is just “over the top” good. But it’s not for everyone. I’m happy to have both, but I use my TSA more than my TOA, mainly because it’s smaller and lighter, and I can use it with a lighter weight tripod and mount (DM4 and Uni-18, rather than DM6 and Planet).

Same here. I still use my AZ100, but it can go on an EQ5 rather than EQ6, if I wanted. My 130s go on the '6 or a Planet.

[scoale]

Just adding some better star test pictures for anyone who may reference this thread in the future. 

 

Images are all unstacked single frames (I really need to learn to stack) of an artificial star at 180ft with a 58 green filter.  Scope cooling for 2 hours.  Ambient temperature of 34 degrees.

 

Star test result is consistent with DPAC.  Both show the central zone, a very slight outer zone, and some slight spherical error in green (star test and dpac wavelengths are probably off a bit).  There is also a small amount of astigmatism and coma visible in the star test, and one of the in-focus images shows some pinching.

 

IMHO, this is a superlative lens.

 

Outside focus on left, inside on right

 

Edited by scoale, 15 December 2024 - 10:11 AM.

[CHASLX200]

Just adding some better star test pictures for anyone who may reference this thread in the future. 

 

Images are all unstacked single frames (I really need to learn to stack) of an artificial star at 180ft with a 58 green filter.  Scope cooling for 2 hours.  Ambient temperature of 34 degrees.

 

Star test result is consistent with DPAC.  Both show the central zone, a very slight outer zone, and some slight spherical error in green (star test and dpac wavelengths are probably off a bit).  There is also a small amount of astigmatism and coma visible in the star test, and one of the in-focus images shows some pinching.

 

IMHO, this is a superlative lens.

 

Outside focus on left, inside on right

 

ABOUT AS GOOD AS IT GETS.

[25585]

There you go Chas, time to try out a TSA. 

[RichA]

I’ll let Stephen explain, but I believe it had to do with focuser misalignment, as the adapter that attached the upgraded FTF3035 wasn’t quite orthogonal with the rest of the tube. Or something like that. I don’t believe it had anything to do with the lens cell.

Focuser misalignment (tilt) would make ronchi bands appear to converge at one edge of the circle.  Lateral misalignment (if that is even possible) would have almost no effect on the image. 

[RichA]

Just adding some better star test pictures for anyone who may reference this thread in the future. 

 

Images are all unstacked single frames (I really need to learn to stack) of an artificial star at 180ft with a 58 green filter.  Scope cooling for 2 hours.  Ambient temperature of 34 degrees.

 

Star test result is consistent with DPAC.  Both show the central zone, a very slight outer zone, and some slight spherical error in green (star test and dpac wavelengths are probably off a bit).  There is also a small amount of astigmatism and coma visible in the star test, and one of the in-focus images shows some pinching.

 

IMHO, this is a superlative lens.

 

Outside focus on left, inside on right

 

Looking at it on this test, it looks quite good.  In-truth, the only time I ever saw a true bullseye diffraction pattern with intact rings was done using an environmentally-controlled test, otherwise, ring break-up seems just to happen, and has little to do with the optics themselves.  "Seeing" if you will. Though of course rings not appearing on one side is usually collimation.