FS-60CB diffraction patterns?

[Surfer Chris]

I have just been out for a very quick session with the FS-60 in CB (f/5.9) mode. Pointed the scope at Rigel with a Baader MKIV zoom (and Tak prism diagonal), and thought I would check out the patterns inside and outside of focus. With my FC-100DC, I recall seeing well defined perfectly circular & concentric rings either side of focus. However, with the FC-60 I have recently purchased, I see similar clearly defined concentric rings on one side, but not so on the other. On the other side of focus, I see a perfectly circular uniform coloured disc without any rings clearly visible? Just wondering how this compares with others peoples experience with this scope?

I even swapped the eyepiece over to a 24 Panoptic and got the same result. Stars appear pin-point and sharp with both eyepieces.

[mikeDnight]

Is the scope thermally stable, and are the seeing conditions reasonably stable? If so, and with all other things being equal, it sounds like spherical abberation, which is a little unusual for a Tak. It's possible the lens is warmer than the cell holding it, which may cause this issue. I had a similar problem with a TV NP101 IS some years ago, where one side of focus took forever before it reasonably matched the other. And even then it never matched the Tak symmetry. 

[Surfer Chris]

Hi Mike. Thanks for replying. The scope hadn't been out that long to be honest...probably only 15 minutes or so. So there's a good chance it hadn't fully acclimatised I guess. And the seeing isn't that great here this evening. I would pop it back outside and leave it to stabilise for a while longer, but the clouds have rolled in now. And just checked the forecast, and it's a week of cloud/rain ahead!

Hoping this isn't anything that will be too detrimental to the views through this scope, but not sure how to thoroughly test/evaluate it. Going to have to do some reading up on the subject I guess. Being a Tak, I was kind of expecting it to readily show these clear defined rings either side of focus just like the FC-100DC.

[fireballxl5]

also I wouldn't do a star test using one at low elevation tbh

[jock1958]

My FS-60C / Q doesn’t take that long to acclimatise whether it’s in C or Q modes but I find it does perform better the longer you leave it and that goes for my FC-100DL as well. 
As Mike has alluded to atmospheric conditions do play a part in a reliable star test including viewing the concentric rings, I tend to use Polaris due to its high altitude and relative lack of movement, that’s if you are using an Alt Az mount, also a good high powered eyepiece, I use a 3-6 Nagler zoom or my 5mm Vixen SLV for this. 
 

Edited January 7, 2022 by jock1958

[Fedele]

I think in a doublet that's normal ... but I could be wrong

 

[Highburymark]

Use a much higher power eyepiece. Even a 2.5-3mm ep if you have one. It should make the rings easier to see on the fuzzy side. Not having exactly the same pattern on each side does not automatically mean there’s a problem with the scope. More testing needed - in focus - on different objects.

[Stu]

I wouldn’t expect the FS-60 to be at the same level as an FC100 necessarily as it’s a bit faster and the correction plots look less tight from what I’ve seen.

I’ll give mine a go when I get a chance. The CQ module tightens things up a lot as I understand it so I’ll check with this too.

I did a quick test on my FC76DC with CQ module fitted and it looked pretty good to me, all but identical inside and out. Need to compare again the standard scope without CQ too.

[neil phillips]

I could be wrong but been testing a lot of achromatic refractors lately. Very indistinct patterns on one side is very common. Often its a sign of under correction with mass produced chinese lenses. Though that would be surprising in your case of course.

But here is a example of a early Japanese lens of very good figure F13 77mm clearly there is also a indistinct fuzzy side. But not as bad as no patterns like you describe. The scope may have been dewing over effecting the test a little on this occasion. But the indistinct side will still be indistinct even without that. If your telescope is performing well i doubt its anything to be concerned about. 

Edited January 8, 2022 by neil phillips

[Franklin]

I've been reading Suitor's book "Star-Testing Astronomical Telescopes" and I would highly recommend it. To have absolutely identical diffraction patterns on either side of focus is the goal, but in practise most scopes are not that perfect, the star-test is very sensitive. As long as there are no gross aberrations such as astigmatism, due to mis-alignment of the optics, pinched optics etc I would think that your Takahashi is perfectly fine. After all, if there was a fault with the optics, Takahashi would have spotted it and never let it leave the factory. 

[neil phillips]

32 minutes ago, Franklin said:

I've been reading Suitor's book "Star-Testing Astronomical Telescopes" and I would highly recommend it. To have absolutely identical diffraction patterns on either side of focus is the goal, but in practise most scopes are not that perfect, the star-test is very sensitive. As long as there are no gross aberrations such as astigmatism, due to mis-alignment of the optics, pinched optics etc I would think that your Takahashi is perfectly fine. After all, if there was a fault with the optics, Takahashi would have spotted it and never let it leave the factory. 

Agreed if you look at the patterns above how much difference there is. And that is at F13  F5.9 even at the reduced apeture. Would likely have a larger difference either side of focus i would have thought. Even scopes where the pattern is closer both sides of focus. Will still look different. As the diffraction patterns themselves

display differently either side even with excellent optics

 

[Surfer Chris]

Thanks for the feedback everybody.

Was feeling a little negative about the scope last night. But from the comments above, seems like I maybe just need to focus on 'in focus' performance rather than worry too much about the 'out of focus' patterns. Looking at the two images you have posted Neil, I think both of my discs had a crisper edge to them (if that means anything?). And on my scope, for the side missing the 'distinct' rings, there was in fact perhaps just the faintest trace of rings visible (but they were almost not visible). That faintest trace was probably just the outer ring. I need to repeat the test to check again, but as per comments above, I will wait for better seeing, leave the scope a good 30 minutes to properly stabilise, and use a target higher in the sky. 

I did some daytime testing yesterday, looking across at small branches silhouetted against a bright sky. The trees are fairly close (around 30m or so away). There was a definite green tinge to the branches one side of focus, and a magenta tinge the other. But I believe this is fairly typical of this f/5.9 scope. Interestingly today, I cannot detect any signs of colour at all either side of focus - although it is overcast & raining today and I am looking though a double glazed window (so the sky is not so bright). Yesterday, I also thought I could detect a faint purple tinge around the edge of the view, even when the centre appeared focused and colour free. It was just around the very edge of the view. I put this down to the field curvature that again is typical to using a f/5.9, and am thinking that perhaps my eyes are just quite sensitive to it. Again that was using the Baader MKIV zoom. I should add that it was windy yesterday (as it is today) which means the branches were swaying in the wind (not at all ideal for such a close target focus test). So need to check this out properly when conditions are more stable.

Yesterday evening I did manage to get a brief look at the moon (using the same zoom eyepiece), and actually the image was pretty good. Craters clearly defined and crisp, good contrast (white moon against black sky), no colour visible in craters, and just the slightest hint of yellow around the limb. So that was actually a good result, considering the moon was fairly low at that stage too and the seeing wasn't great (it was around 8pm). Quite impressed with the resolution this 60mm scope was showing.

Although the weather forecast is looing like cloud/rain for most of the following week, it is now showing some potential breaks tonight. So I may be able to get out for a quick look between clouds this evening. But the seeing forecast is not good.

Does anybody have any suggestions for tests I could use to help evaluate the 'in focus' performance of this scope?

[neil phillips]

21 minutes ago, Surfer Chris said:

Thanks for the feedback everybody.

Was feeling a little negative about the scope last night. But from the comments above, seems like I maybe just need to focus on 'in focus' performance rather than worry too much about the 'out of focus' patterns. Looking at the two images you have posted Neil, I think both of my discs had a crisper edge to them (if that means anything?). And on my scope, for the side missing the 'distinct' rings, there was in fact perhaps just the faintest trace of rings visible (but they were almost not visible). That faintest trace was probably just the outer ring. I need to repeat the test to check again, but as per comments above, I will wait for better seeing, leave the scope a good 30 minutes to properly stabilise, and use a target higher in the sky. 

I did some daytime testing yesterday, looking across at small branches silhouetted against a bright sky. The trees are fairly close (around 30m or so away). There was a definite green tinge to the branches one side of focus, and a magenta tinge the other. But I believe this is fairly typical of this f/5.9 scope. Interestingly today, I cannot detect any signs of colour at all either side of focus - although it is overcast & raining today and I am looking though a double glazed window (so the sky is not so bright). Yesterday, I also thought I could detect a faint purple tinge around the edge of the view, even when the centre appeared focused and colour free. It was just around the very edge of the view. I put this down to the field curvature that again is typical to using a f/5.9, and am thinking that perhaps my eyes are just quite sensitive to it. Again that was using the Baader MKIV zoom. I should add that it was windy yesterday (as it is today) which means the branches were swaying in the wind (not at all ideal for such a close target focus test). So need to check this out properly when conditions are more stable.

Yesterday evening I did manage to get a brief look at the moon (using the same zoom eyepiece), and actually the image was pretty good. Craters clearly defined and crisp, good contrast (white moon against black sky), no colour visible in craters, and just the slightest hint of yellow around the limb. So that was actually a good result, considering the moon was fairly low at that stage too and the seeing wasn't great (it was around 8pm). Quite impressed with the resolution this 60mm scope was showing.

Although the weather forecast is looing like cloud/rain for most of the following week, it is now showing some potential breaks tonight. So I may be able to get out for a quick look between clouds this evening. But the seeing forecast is not good.

Does anybody have any suggestions for tests I could use to help evaluate the 'in focus' p ?erformance of this scope?

Yes if your viewing star tests they likely will appear with a crisper edge. A camera tends to highlight and magnify the view more. Making it easier to see any problems or differences i find. If you have a camera you can do the same. Your comments about just seeing the outer ring on the bad side of focus. I know exactly what you mean i have seen the effect myself. I suspect though others will know more, that know these scopes better. Though in your case its highly likely the short F ratio makes the diffraction pattern less distinct. From your other comments it sounds like your optics are Fine. As for  In focus tests,  the first diffraction ring in focus perhaps ? to check collimation Maybe some double star observing to see how well it splits. Others may be able to offer other ideas ?

Edited January 8, 2022 by neil phillips

[John]

I tend to use tight, uneven brightness double stars to see how well a scope is doing. Theta Aurigae is quite a good candidate at the moment. The separation is around 4 arc seconds and the component stars are magnitude 2.6 and 7.2 respectively. The star is quite high in the sky later in the evening as well.

 

[Surfer Chris]

So I figured I would try an indoor artificial star test. Just grabbed a bright LED hand torch, put a bit of foil over the front and used a pin to make a very small hole in the foil. Have positioned the torch in our bathroom, and have a line of sight through the property of around 10-12m. Using this crude set-up, I am getting pretty much identical results to what I saw last night. But I am now able to review using a more critical eye.

So the clearly defined rings are visible inside of focus. And actually, the outer edge of the discs probably are not any sharper than the images you provided Neil. I was hoping & trying to get a picture through my phone but no luck.

Outside of focus, there is an outer ring visible, and in fact looking closely there are other inner rings just visible inside the outer ring. But the inner rings soon bunch up and are very faint - so at a quick glance gives the appearance of a uniform disc without definite ring structure. I think it looks a lot like the first image below which I have lifted from the internet (although my disc is more uniform right in the centre).  

Inside of focus the rings are very defined and clear. The relative difference in contrast of the rings is something like the image below (which was actually taken through a FC-100DF lifted from cloudy nights). But the spacing of the rings makes my weak side look more like the image above, because the rings are bunched up towards the outer edge of the disc. In my discs, the first 4-5 rings that are clearly visible (inside of focus) probably extend only as far as the first 20-25% of the radius depth (whereas below I would say the first 4-5 rings extend for maybe 60% of the radius depth). Although the rings are showing faint outside of focus, I guess they do appear to have the same positioning inside and outside of focus (i.e. both bunched up towards outer edge). 

I had the same effect with both the Baader MkIV Zoom, 4.5mm Morpheus and the 4mm Delite.

Will get the scope out again as soon as possible and try to put it against some test targets.

 

[Surfer Chris]

This is my poor attempt at a sketch of what I see.... 

Its very hard to draw freehand concentric circles!!!

[Andrew_B]

I've got an FS-60CB and the green/magenta colouring is perfectly normal either side of focus but doesn't stop it performing very well and delivering high contrast images in focus. Sometimes when the seeing is bad I use that false colour to focus because it's hard to see the fleeting fine details that would normally be used to judge whether the scope is focused - essentially I'm moving the focus back and forth until green disappears without showing magenta or vice-versa.

On the subject of star testing, I found this quote by Roland Christen about its limitations when testing apos rather than achromats:

Quote

Identical inside/outside rings in an achromat is normal. This is due
to the nature of chromatic aberration, which take all colors, except
green, far away from focus, where they cannot interfere with the
diffraction pattern. In an Apo lens, red, yellow, green and blue are
all very close to focus, but not perfectly, exactly at the same
identical focus. These colors tend to interfere with the perfect
diffraction pattern. Every Apo lens I have ever star tested, even
ones that tested close to 1/20 wave P-V, shows different inside and
outside patterns of interference. This is normal. By contrast, I have
an 8" SCT that shows perfectly identical inside vs. outside
diffraction patterns, but tests only 1/4 wave.

The real test of an optic is not so much how the diffraction pattern
looks outside of focus, rather, how much extraneous junk is floating
around a star when it is in focus. (in my 8" SCT there is quite a lot
of that). In an achromat with perfect optics, there is a lot of
unfocused blue and red light around any object. The brighter that
object is, the more that light interferes with the image. One saving
grace for achromats is a set of filters. It does not "fix" the
chromatic aberration, rather it lets you see the image in one color
in monochrome very sharply.

 

[neil phillips]

1 hour ago, Surfer Chris said:

So I figured I would try an indoor artificial star test. Just grabbed a bright LED hand torch, put a bit of foil over the front and used a pin to make a very small hole in the foil. Have positioned the torch in our bathroom, and have a line of sight through the property of around 10-12m. Using this crude set-up, I am getting pretty much identical results to what I saw last night. But I am now able to review using a more critical eye.

So the clearly defined rings are visible inside of focus. And actually, the outer edge of the discs probably are not any sharper than the images you provided Neil. I was hoping & trying to get a picture through my phone but no luck.

Outside of focus, there is an outer ring visible, and in fact looking closely there are other inner rings just visible inside the outer ring. But the inner rings soon bunch up and are very faint - so at a quick glance gives the appearance of a uniform disc without definite ring structure. I think it looks a lot like the first image below which I have lifted from the internet (although my disc is more uniform right in the centre).  

Inside of focus the rings are very defined and clear. The relative difference in contrast of the rings is something like the image below (which was actually taken through a FC-100DF lifted from cloudy nights). But the spacing of the rings makes my weak side look more like the image above, because the rings are bunched up towards the outer edge of the disc. In my discs, the first 4-5 rings that are clearly visible (inside of focus) probably extend only as far as the first 20-25% of the radius depth (whereas below I would say the first 4-5 rings extend for maybe 60% of the radius depth). Although the rings are showing faint outside of focus, I guess they do appear to have the same positioning inside and outside of focus (i.e. both bunched up towards outer edge). 

I had the same effect with both the Baader MkIV Zoom, 4.5mm Morpheus and the 4mm Delite.

Will get the scope out again as soon as possible and try to put it against some test targets.

 

Great idea on the foil trick to create a artificial Star. Some things that are very useful guides doing star testing lenses, is pinching problems. Astigmatism. And checking for any miscollimation of the lens. From what your describing it sounds to me like your lens is fine. I would imagine other FS60s testing similar to yours. But have no experiance with that. But it certainly seems likely to me.  As Franklin said Quality control at Takahashi is much better than mass produced. So it would be surprising if you had a bad figure or other optical problems. I have never heard of that with Taks I believe you can enjoy your scope with peace of mind. If this post has achieved that. Then its certainly been worthwhile.

Edited January 8, 2022 by neil phillips

[neil phillips]

41 minutes ago, Andrew_B said:

I've got an FS-60CB and the green/magenta colouring is perfectly normal either side of focus but doesn't stop it performing very well and delivering high contrast images in focus. Sometimes when the seeing is bad I use that false colour to focus because it's hard to see the fleeting fine details that would normally be used to judge whether the scope is focused - essentially I'm moving the focus back and forth until green disappears without showing magenta or vice-versa.

On the subject of star testing, I found this quote by Roland Christen about its limitations when testing apos rather than achromats:

 

Not sure what achromats he has seen or tested. But i have never ever once seen any achromat with identical  inside outside rings. I have posted two examples of that here recently in the feild testing these things. And others before that. 

Under or over correction can affect the patterns a lot. And that is something that is normal from mass produced chinese optics.. But anyway i am off. Surfer Chris i think has had hes mind assured so. Its been very worthwhile for him. 

I will let others debate the finer points.