Is overcorrected spherical aberration in spotting scopes bad? (1 Viewer)

mskb · Sep 7, 2019

Hello,

Just playing around with the Kowa 663M + 20-60 I received recently. This is my first time with star tests. I will soon post some pictures once I get those adapters. There were two really big skyline buildings behind our apartment a little more than a couple of miles away, on top of which there are a few bright red lamps. Some switch on and off. I thought I would use those as a point source.

My preliminary observations at 60X max magnification across the various light sources indicate: circular rings, and clean center both inside and outside of focus - indicating potentially low astigmatism & coma. The rings were sharper and brighter outside of focus; the rings were still distinguishable inside of focus, however they appeared less bright, and less sharp. Relating this observation back to the spherical aberration section in: https://starizona.com/tutorial/star-testing-telescope-optics/ , indicates that this could be a potential case of overcorrected spherical aberration?

Is this a bad thing? And under what conditions can I imagine this issue to influence wildlife & potentially some stargazing observations in a bad way?

Again, sorry for not attaching pictures. I will update the thread hopefully soon with those images, and a resolution test.

Cheers,
Kumar

marcsantacurz · Sep 7, 2019

Every scope is going to have some amount of over or under corrected CA. The question is how much?

Was there a big difference between the inside & outside? Could you count the number of different rings? Having photos really helps compare the two. Suiter ("star testing astronomical telescopes") says that 1/4 lamda correction error is very hard to distinguish from better scopes except by very skilled observers under excellent conditions. Could you still count the same number of rings? Inside the focus, how fuzzy did the outer ring get? You said you could still discern the rings, so I'd assume you're under 1/2 lamda.

Although I've tinkered with some astrophotography, I'm no expert on calibration and measurement of scopes.

Marc

jring · Sep 7, 2019

mskb said:
There were two really big skyline buildings behind our apartment a little more than a couple of miles away, on top of which there are a few bright red lamps. Some switch on and off. I thought I would use those as a point source.

My preliminary observations at 60X max magnification across the various light sources indicate: circular rings, and clean center both inside and outside of focus - indicating potentially low astigmatism & coma. The rings were sharper and brighter outside of focus; the rings were still distinguishable inside of focus, however they appeared less bright, and less sharp. Relating this observation back to the spherical aberration section in: https://starizona.com/tutorial/star-testing-telescope-optics/ , indicates that this could be a potential case of overcorrected spherical aberration?

Is this a bad thing? And under what conditions can I imagine this issue to influence wildlife & potentially some stargazing observations in a bad way?

Hi,

using lights on a building is an interesting idea... the question is, if their image is small enough. Vlad gives the following formula for the maximum diameter:

distance in mm / (3000 * objective aperture in mm)

For an assumed distance of 5miles or 8km = 8000m = 8000000mm and 77mm objective aperture we get 35mm or 1.3 inch. Not a lot... and it is quite hard to get the real value...

Otoh having a slightly too large light source will make the diffraction patterns look a bit different than those from real stars, but will still allow you to say sth about the absence or presence of certain aberrations, just the numbers like some image equals to 1/4 lambda p-v of overcorrection might be off - but there will still be overcorrection present.

The distance should be more than far enough to ensure that no SA is introduced by observing a close target.

Seing a bit of spherical aberration is quite common in most refractors - having a quite fast focal ratio (like spotting scopes) doesn't help either. The question is how much you have - I would recommend to repeat with a real star and then compare to Vlads images of 1/4 lambda p-v overcorrection - an instrument with that amount of overcorrection is still diffraction limited which is the usual standard for astronomical optics.
You will certainly not notice it at magnifcations up to 60x

Joachim

Tringa45 · Sep 7, 2019

mskb said:
There were two really big skyline buildings behind our apartment a little more than a couple of miles away, on top of which there are a few bright red lamps. Some switch on and off. I thought I would use those as a point source.

A red light source is probably less than ideal for a star test. I recall seeing some Strehl values for a Takahashi fluorite doublet apo. It was really outstanding across most of the spectrum but rather poor in the red and consequently not good for Mars observation. A corrector, which also extended the focal length, effected a vast improvement in this respect.

John

henry link · Sep 7, 2019

mskb said:
Hello,

Just playing around with the Kowa 663M + 20-60 I received recently. This is my first time with star tests. I will soon post some pictures once I get those adapters. There were two really big skyline buildings behind our apartment a little more than a couple of miles away, on top of which there are a few bright red lamps. Some switch on and off. I thought I would use those as a point source.

My preliminary observations at 60X max magnification across the various light sources indicate: circular rings, and clean center both inside and outside of focus - indicating potentially low astigmatism & coma. The rings were sharper and brighter outside of focus; the rings were still distinguishable inside of focus, however they appeared less bright, and less sharp. Relating this observation back to the spherical aberration section in: https://starizona.com/tutorial/star-testing-telescope-optics/ , indicates that this could be a potential case of overcorrected spherical aberration?

Is this a bad thing? And under what conditions can I imagine this issue to influence wildlife & potentially some stargazing observations in a bad way?

Again, sorry for not attaching pictures. I will update the thread hopefully soon with those images, and a resolution test.

Cheers,
Kumar

Over and undercorrection are equally bad, but first lets make sure we're on the same page. Inside and outside of focus are not necessarily intuitive. Inside of focus means setting the focuser for a longer distance than required to bring an object to good focus and outside of focus means setting the focuser for a closer distance.

How many diffraction rings are you defocusing? 3-5 is good for this kind of test.

I would avoid using a strongly colored light source unless you have a reason to want to know the spherical aberration for only one narrow band of wavelengths. I think the best light source is full spectrum sunlight from a glitter point in a small round shiny object at least 100' from the telescope. That distance will give you a result close enough to infinity and as long as the object is small enough (ball bearing, small Christmas tree ornament, etc.) the glitter point of the sun will be plenty small enough to act as a point source. Place the object as high above the ground as possible and try to have shaded grass between the scope and the object to minimize air turbulence.

IMO 1/8 wave optics look obviously better than 1/4 wave even at relatively low magnification in daylight, perhaps because the eye is at it's best under those conditions. Also keep in mind that all aberrations and defects add together to determine the final wavefront error. A scope with 1/4 wave correction of spherical aberration may also have several other small defects, each of which might be minor taken alone, but added together on top of the spherical aberration could cause the total error to be much worse than 1/4 wave.

Henry

mskb · Sep 8, 2019

Hello Everyone,

Based on Henry Link’s and others' advice, I focused on the sun’s specular highlight arising from an electric pole’s roughly round object ~100ft away. The day was bright and sunny, with some haze observable through the spotter.

What a difference bright day light makes! Defocussing until we get 4 or 5 rings, the rings were bright, sharp and clean circular. I still feel the very center of all the nested circles appears “filled” outside of focus (thanks for correcting my misunderstanding Henry!) compared to a hollow inside of focus center. Perhaps, it is too early to ask you for your guesses on what "wave" pattern this is (or do you have a guess already?). Presenting you with pictures of the diffraction rings will make things much easier moving forward!

With regards to pictures however, Marc and others, I was not successful with the cell phone as the pictures did not deliver the needed resolution. I think I will have to mount a {doubler/our binocular + cell phone combo} behind the eyepiece stably to get a proper picture. These pictures are going to take a few days until I get the needed supplies. If anyone has advice on a cost-effective setup, I would love it! I am currently thinking the following setup:
1. The over-expensive piece of plastic - the Gosky Big type $36 for mounting the cell phone on the scope's eyecup: https://www.amazon.com/gp/product/B07CNL85TT/ref=crt_ewc_title_dp_2?ie=UTF8&psc=1&smid=AKKYSTKSRIIUN
2. Carson binocular adapter at $10 : https://www.amazon.com/gp/product/B00TOSCZUC/ref=crt_ewc_title_dp_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
3. I have some cheap tripods lying around on to which these will be mounted.

Thank you,
Kumar

marcsantacurz · Sep 8, 2019

You can try those cheap cell phone adapters, but i bet you'll end up buying the KOWA one later. It's hard to get the cell phone camera lined up well to get a good circle and flat view. I shelled out for the swarovski one for my ELs and it fits very well and I can get some pretty good shots through it. Very little wobble. I would suspect the kowa one is similar.

I think you want a 50mm adapter for the 9W zoom.

Marc

marcsantacurz · Sep 8, 2019

Oh, and forgot to mention. I use the ProCam app. It has a timer shutter release and also an "anti-shake" shutter release (it waits until the phone stops shaking from you pressing the button). It also lets you shoot raw.

Others use remote shutter release that hookup through the headphone/mic attachment or over bluetooth.

Marc

mskb · Sep 11, 2019

Hello Everyone,

Here are the pictures I promised {Inside of focus, focussed, outside of focus }: https://www.flickr.com/gp/bharathmsk/13o953 .

Subject was an electric pole's ring's highlight was ~100ft, day was partly cloudy, I only observed at times when our sun was out. I mounted an 8X binocular on a Carson TA-6 binocular tripod mount, aligned it behind the TE-9Z + 663M. An iphone6 + Gosky Big Type combo was mounted on one of the binocular barrels. This was the best setup and alignment I could come up with.

Is there anything you can make out from the pictures? Viewing with the eyes present a much clearer picture, than these phone images might suggest. Relative to those inside-of-focus, the outside of focus rings were less bright and less sharp. Some discussions from experts relating these observations to the scope's design/glass elements would be interesting as well.

There was definitely a dependence of spherical aberration noticed from the star test on the closeness of the subjects (like Joachim and others have pointed out in some of their posts on star testing).

Any advice on improving the setup e.g., looking for better light, subject distance, better subjects, alignment of binoculars & scope etc., would be greatly appreciated. I am also not sure where all those specks of dust are coming from. The objective and eyepiece of the scope look clean, and I also used an air-blower with a gentle microfiber cloth wipe.

Cheers,
Kumar

PS: Thanks Marc for taking the time to explain the cheaper options for digiscoping. The over-priced Gosky worked OK for the above cell phone shots. The PA8 is another overpriced kit that we need to get for those neat high resolution videos.

jring · Sep 11, 2019

Hi,

can't say a lot from these images - they don't look very concentric - especially the outside ones. But using the 8x bins is not ideal in my opinion as it introduces other unknowns into the equation - the bins and also possible tilt between the instruments...

As for your last test with the red light - if the distance was a few miles, it will not have introduced SA.
But the red color of your "artificial star" might well have done so as Henry has quite correctly pointed out - optics made for visual observation tends to be optimized for best performance in the green with other colors sometimes not so great. Testing should be done with white light.

Joachim

mskb · Sep 11, 2019

Thanks Joachim. Yes, tilt between the two devices is a real pain, I found out. Otherwise, these experiments would have been a breeze. Any suggestion on how to go about overcoming that problem?

It was harsh white light now, and repeating the experiment I get clear concentric circles both inside and outside of focus. I am super confused.

Aside: Those dirt specs turned out to be from binoculars.

mskb · Sep 11, 2019

If no one is able to judge anything from the posted pictures, I would like to ask: is undercorrected spherical aberration the predominant cause of clear & sharp concentric circles inside of focus, while fainter, less sharp (still) concentric rings outside of focus?

Perhaps the best move forward would be to do those resolution checks.

The reason I asked about a possible discussion on the aforementioned observations with respect to the optical design of the Kowa 663M was that I recalled that this instrument doesn't host aspherics, and is based on an old design.

Of course if Henry / Joachim could find a chance to look through a few 663M s, their findings would be super interesting as well.

jring · Sep 12, 2019

Hi,

since the o.p. has asked for images of a star test of one of my scopes... as he has experienced taking good images of what one sees in a star test is not trivial and I don't have any (not much of a digiscoper or nature photographer anyways... I prefer to enjoy the moment while watching through the scope - be it birds or stars. When I use a camera, it's usually street in a busy city...)

Being able to look through a stack of new scopes and being allowed to choose and buy one might result into a better than usual example - not necessarily a cherry. But I guess you have to be on very friendly terms with an importer/distributor to get that option... a normal store will rarely have more than maybe two examples of each model at hand...

As an example of a star test for a quite good scope, I would recommend the following CN thread:

https://www.cloudynights.com/topic/114680-star-test-of-a-120ed/

I would be quite content if any of my scopes tested like this "cheap chinese optics". Actually my 120ED and its 80mm brother are fairly close.

The Kowa has a bit more undercorrection, but hey, this one is used only at fairly low magnifications up to 100x (when I can be bothered to use the 5mm EP or the extender) and not at double that like the astro scopes...

Joachim

mskb · Sep 12, 2019

I am updating this thread with a rough resolution measurement indoors at 27.5ft using a pocket USAF reference chart from Edmund Optics. The day is dull and overcast, so I have simply used a couple of LED lights to hit the target.

3,3 (sure) and 3,4 (almost sure) elements were resolved at 60X, at a distance of 27.5ft. Calculations with the corresponding LPMM values from EO lead to 2.44'' or 2.17'', which would be the ideal resolution coming out of a ~50mm scope. Close observation of the EO chart with a magnifying lens indicates any group-element combination beyond 3,4 might not be quite reliable, so I will most certainly need to repeat the experiment outdoors at much larger distances.

We might have a perfect lemon in our hands gentlemen or ... not. I will report back.

PS: thank you Joachim for the link on perfect examples.

mooreorless · Sep 12, 2019

The pocket USAF chart that I have is good to the smallest group of elements. I bought this a pretty long time ago. I wonder if they are still that good?

jring · Sep 12, 2019

mskb said:
3,3 (sure) and 3,4 (almost sure) elements were resolved at 60X, at a distance of 27.5ft. Calculations with the corresponding LPMM values from EO lead to 2.44'' or 2.17'', which would be the ideal resolution coming out of a ~50mm scope.

Hi,

I think you need more magnification... like 80 - 100x for 1mm to 0.8mm exit pupil.
Is there an extender available or do you have a doubler or tripler?

Joachim

mskb · Sep 12, 2019

Hi Joachim, I do not have an extender / doubler / tripler. I simply mount another binocular behind it to be a little sure, although in this case, that setup did not work quite well with a ~1mm exit pupil from the scope. I will repeat the experiment when the light is bright and white.

Thanks!

jring · Sep 12, 2019

Hi,

scratch my remark about 80-100x - that was for 80mm scopes - you need 66-83x... so at 60x you're just a bit short...

But still, depending on your eyesight some more magnification might help...

Joachim

Is overcorrected spherical aberration in spotting scopes bad? (1 Viewer)

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