36 replies to this topic

[scoale]

Are the results of the test something I should be concerned about? Someone here earlier posted a nearly perfect star test of a 60mm Takahashi scope. It seems like that is the expected standard for Takahashi telescopes. How does my star test compare?
 

 

I haven't heard of any places locally where someone with the equipment and experience in optics testing could take a look at it, so I'd like to hear your thoughts on what might be visible in the star test that I was able to do on my own.

Piotr

 

In terms of perfect star tests, I see much better than I hear. Unless presented with proof, I immediately discount assertions of a perfect star test.  None of the scopes I have tested, including four Takahashi's, were perfect. And, BTW, I don't think that the other 60mm Tak star test is perfect.

 

That said, I would encourage you to reach out to Takahashi with your concerns.

[kgb]

Nice work with the images Piotr. The extrafocal image, as Lagrange had pointed out, does appear soft which would point to the lens possibly being under corrected. Am I correct to assume that you are still seeing haloing on the in-focus image when the extender is removed?

What I find peculiar is that when the extender is in, there appears to be a distinct “rind” effect around the outer radius of the lens. Adding the haloing and possible SA, I wonder if there might be a zonal defect that is being amplified by the extender. Rather than speculate, either, contacting Tak and forwarding your results or doing further testing with the Ronchi is the way to go.

In the event you experience problems with the availability from TS, it looks like it is the Gerd Neumann available at FLO. https://www.firstlig...i-eyepiece.html. Hopefully you will get better weather to test this out.

[Psion]

I've had a look through my FS-60Q to see what the star test looks like. Unfortunately I don't have the means to photograph it and the seeing here is pretty bad at the moment but I was able to get a reasonable look at the diffraction patterns.

 

Inside and outside focus were fairly similar with clearly visible diffraction rings, but inside focus seemed to be more defined as the rings were very sharp and clearly visible.

 

There certainly wasn't the softness that I can see in your images, especially the outside focus patterns which look very different to the ones I saw.

 

I don't know enough about this stuff to give you any definitive advice, but it looks like there might be something odd about your scope. Hopefully other FS-60Q owners can do their own tests to give us some more data.

 

In focus, the Airy disc and first couple of diffraction rings were very clearly defined from what I could tell through the less than ideal seeing.

For this test, you need to use a higher magnification, usually 50 * Lens diameter, that is 120x. If we use a low magnification, the patterns inside and outside the focus tend to be similar.

[Live_Steam_Mad]

Looks like spherical aberration under-correction to me.

"Over-correction is when the concentric rings inside focus are diffuse and not well defined, while those outside focus are sharply defined.

Sharp rings inside focus, together with misty rings outside of focus, shows under-correction."

 

https://www.skyatnig...est-a-telescope

Nope. It is the brightness across the intra- and extra-focalal diffraction disc that is relevant for Spherical Aberration testing, not the contrast or sharpness of the Fresnel rings.

 

When light is hollowed out of the middle within the diffraction disc on one side of focus, and that same light is then added to the middle of the diffraction disc on the other side of focus, then that is Spherical Aberration.

 

When the fresnel rings of the diffraction disc on either side of focus are blurred with respect to the rings on the other side of focus, that is the sign of a Turned Edge.

[Live_Steam_Mad]

The FS-60Q is a fluorite doublet.  It is normal that the pattern will be different on opposite sides of focus.

Interesting. Do you mean that in a Quadruplet which is working at F10 then the diffraction patterns will be different on either side of focus? Where does this information come from? F6 optics are harder to figure than F10. I use Suiter's Star Testing book as a reference, and Aberrator v2.5 and v3 software as my references.
 

Edit: Never mind, I just read Peleuba's post and it appears that you are talking about the chromatic effects due to green, and red+blue light (which makes purple) having different Spherical correction (as well as being focused at different points), blurring the test, an effect called Sphero Chromatism, which is a rather strong effect in a fast Fluorite Doublet.

 

BTW good to have you with us, Peleuba. I value your input.

 

I also agree about the need for testing with a blue-green (510nm for Scotopic vision) or green filter (540nm for Photopic vision), to remove the Spherochromatism. Then we can see how it looks.

 

I am just about to buy an Extender-CQ module for my FS-60, my scope is a rather well corrected example that shows hardly any 1st diffraction ring when tested on a real and bright "white" Star, when tested with a 5mm ep and a 2x barlow. I just assumed that the CQ module was going to be as excellent as my 4 Tak scopes are, but maybe there was a bad batch of CQ modules.

 

Regards.

Edited by Live_Steam_Mad, 08 April 2025 - 09:48 AM.

[Live_Steam_Mad]

Agree.

 

A nearly perfect star test (with artificial star) has been achieved and published for a Tak FS-60 by German optic tester Wolfgang Rohr:

At a focal length of 500mm, that is at F8.3, so is the FC-60 and not the F5.9 FS-60.

[Live_Steam_Mad]

Visually at f/10 I can see 3 clear diffraction rings at 120x, progressively dimmer the further out they are, but I wouldn't describe the airy disk as sharp, it looks fuzzy and doesn't really have an edge. At 40x however nothing really stands out to me as out of the ordinary, but I am not experienced in visual observing, and I've only looked visually through a dob at the moon and planets a handful of times and so have very little experience.

Piotr

Oh dear. Now we are in trouble. At 120x you should have 0.5mm Exit Pupil, so that the enormous majority of the Aberrations are coming from the optics and not from your eyes. 

 

In focus, you should be seeing one rather faint first diffraction ring at 120x (for a Strehl of maybe 0.95+ which I would expect of a typical FS-60, at least in blue-green light, this is because it's objective is the size of a large eyepiece, and they can now be manufactured with 1/40 wave optics, as tested by one of the Astro magazines, a French publication if I remember, where they tested several eyepieces using an Interferometer). 

 

If you are seeing one faint first diffraction ring with just the FS-60 doublet, and then 3 rings with the CQ-module, then something is very wrong with the CQ module or it's configuration. Hopefully you have the correct spacing for the CQ module's 2 element lens (i.e. the spacing between the CQ's doublet and the doublet Objective of the scope), and the CQ's doublet is the correct way around with respect to the scope's Objective, and the lenses didn't get flipped around in the CQ module?

 

I doubt very much that the actual lenses of the CQ module are bad, as they aren't difficult to manufacture to a very high standard.

Edited by Live_Steam_Mad, 08 April 2025 - 07:58 PM.

[Live_Steam_Mad]

I setup before sunset, and the telescope got to cool down for around 30 minutes. I'm still taking pictures at f/5.9 (thought it was f/6 but turns out the scope is slightly faster), meaning that I only have the doublet lens and no other optical elements, no flatteners or reducers. Figured I should mention that.

I've split up the test below into two images, since it was behaving weirdly when I tried uploading a very wide image. Here they are:

intrafocal chart.jpg
extrafocal chart.jpg



Piotr
 

Your FS-60 objective looks to have nearly perfect Spherical correction, no Astigmatism, and a very slight Turned Edge. SA correction around 1/8 wave.

 

Wish I could say the same about your CQ module test (i.e. from the 3 diffraction rings at 120x in focus that you saw).
 

Edited by Live_Steam_Mad, 08 April 2025 - 10:44 AM.

[Live_Steam_Mad]

For this test, you need to use a higher magnification, usually 50 * Lens diameter, that is 120x. If we use a low magnification, the patterns inside and outside the focus tend to be similar.

Correct. Tests with cameras are usually at rather low effective magnifations, unless you use a Barlow and have a very, very small chip. I used a DSLR calculator online and the magnification with a small scope and DSLR is something like 20x, if memory serves.

[Lagrange]

For this test, you need to use a higher magnification, usually 50 * Lens diameter, that is 120x. If we use a low magnification, the patterns inside and outside the focus tend to be similar.

I used a TeleVue 3-6mm Nagler Zoom and magnifications between 100x and 150x.

[Live_Steam_Mad]

I have my own FS-60C (2006 vintage) outside right now and pointed at Dubhe, which is at 72 degrees Altitude, and testing with a 4mm TeleVue Delite. The Star Test even with very little defocus shows almost identical diffraction patterns either side of focus, very close indeed to what Suiter shows in his Star Testing book for an 1/8 wave PTV optic. The scope had maybe 15 minutes of cool down. I'll do another test after it's had like 30 minutes of cool down.

 

At focus I see a perfect Airy Disc and one very faint first diffraction ring.

 

Regards.

Edited by Live_Steam_Mad, 08 April 2025 - 03:13 PM.

[Live_Steam_Mad]

OK so I took the FS-60 up to 177x magnification with the 4mm Delite plus 2x GSO Barlow, looks like 1/8 wave optics, very faint 1st diffraction ring on Polaris. This was after over 1 hour of use, so cool down wasn't an issue. The Airy disc that I see with it is a hard round ball of light.

 

Later on I used my FC-100 from year 1985, it again shows a Star Test that is very similar indeed to Suiter's Star Testing book's illustration for 1/8 wave of Spherical Aberration, again a faint 1st diffraction ring around Polaris using a 4mm Delite and no Barlow, for 200x magnification. Again the Airy disc that I see with my FC-100 at 200x is a hard round ball of light.

 

I tested both scopes with no colored filters, the Star Test was good enough not to need them!

 

I was expecting your Extender-CQ to do similar, not show 3 rings around a bright Star!

 

Regards.

Edited by Live_Steam_Mad, 08 April 2025 - 07:55 PM.