Purple mark on Tak

[YogSothoth]

Hi All. While putting my FC 100 away last night I discovered a small purple/black circle with concentric rings around it. It’s not on the front of the objective and it’s quite noticeable so it must have only recently appeared. I’ve not seen anything like this before on any other scopes. Any ideas as to what it is and what I should do about it? The scope is less than a year old. Many thanks.

[Stuart1971]

Looks just like a collimation concentric star test image…like the one I got on my Zygo test with my scope, that’s really odd….🤔🤔

Edited February 12 by Stuart1971

[dweller25]

That looks like Newtons rings but I don’t see those on my Tak FC100DZ

Edited February 12 by dweller25

[dweller25]

I think this is what you are seeing…

https://www.cloudynights.com/topic/679457-newton-rings/#:~:text=Newton's rings are most commonly,spaces between R2 and R3.

Edited February 12 by dweller25

[johnturley]

27 minutes ago, YogSothoth said:

Hi All. While putting my FC 100 away last night I discovered a small purple/black circle with concentric rings around it. It’s not on the front of the objective and it’s quite noticeable so it must have only recently appeared. I’ve not seen anything like this before on any other scopes. Any ideas as to what it is and what I should do about it? The scope is less than a year old. Many thanks.

I think that they used to be referred to as Newton's Rings, I had something similar with a Vixen 102mm f10 Achromatic Refractor that I used to own, I assumed that it was caused by two of the lens elements touching each other.

I don't see something similar with my Tak 100 DZ .

John 

Edited February 12 by johnturley

[Peter Drew]

I also think it is Newton rings, the mating surfaces have to be quite close to show them and it's possible that thermal equilibrium can make them more or less easy to see.  Try looking at the objective under a monochromatic source like a strip light, this makes the rings easier to see.  I would be more concerned if the rings were significantly off centre or distorted in shape.      🙂

[Stuart1971]

8 minutes ago, Peter Drew said:

I also think it is Newton rings, the mating surfaces have to be quite close to show them and it's possible that thermal equilibrium can make them more or less easy to see.  Try looking at the objective under a monochromatic source like a strip light, this makes the rings easier to see.  I would be more concerned if the rings were significantly off centre or distorted in shape.      🙂

Yea, at least it shows it well collimated….👍🏻

[mikeDnight]

I suppose that if your scope is still under it's 2yr warranty, and if it troubles you, you could contact the vendor and have it replaced, as it should not be there. Does it impact the images at the eyepiece? 

[YogSothoth]

17 minutes ago, dweller25 said:

I think this is what you are seeing…

https://www.cloudynights.com/topic/679457-newton-rings/#:~:text=Newton's rings are most commonly,spaces between R2 and R3.

Thanks for the reply. Looking at some of those pics. It’s exactly what I’m seeing. Maybe it’s always been there and I’ve not noticed before, but it is pretty obvious regardless of what lighting I use to look at it.

[mikeDnight]

Quick thought! May be try very slightly unloosening the retaining ring holding the objective in place. Turn it anticlockwise just to relieve any pressure. It should only lightly hold the lens.

[YogSothoth]

5 minutes ago, mikeDnight said:

I suppose that if your scope is still under it's 2yr warranty, and if it troubles you, you could contact the vendor and have it replaced, as it should not be there. Does it impact the images at the eyepiece? 

No it doesn’t affect the image at all and I get perfect stars through the eyepiece. A star test at high mag also looks very good.

[The Admiral]

Newton's rings will be more evident with light sources having a line spectrum rather than a continuous one. Out of interest, are you using a fluorescent light or is it daylight?

Ian

[YogSothoth]

3 minutes ago, mikeDnight said:

Quick thought! May be try very slightly unloosening the retaining ring holding the objective in place. Turn it anticlockwise just to relieve any pressure. It should only lightly hold the lens.

I get a bit nervous of things like that as I can be a bit crack handed and don’t have a very good track record in that department 🤣

[YogSothoth]

4 minutes ago, The Admiral said:

Newton's rings will be more evident with light sources having a line spectrum rather than a continuous one. Out of interest, are you using a fluorescent light or is it daylight?

Ian

Those photos were taken in daylight with the sun shining directly onto the objective, but I also noticed it last night under an LED light.

[dweller25]

One day you may decide to sell your Tak and those rings may put people off.

I suspect the lens spacing may be different to other FC100’s that do not show the rings.

If that was my telescope I would be talking to the vendor.

Edited February 12 by dweller25

[mikeDnight]

Is there anything at the centre of the rings, as a small particle sandwiched between the elements may create enough pressure to cause the issue? I can't see anything in the images!

[The Admiral]

I assume that the rings remain static as you change the geometry of your viewing? I must admit, if there are any rings, then naively I would have thought they'd be in the centre, which is where you'd expect the lens surfaces to be nearest to each other. Then again, your star tests are OK.

Ian

[Peter Drew]

These Taks are such a worry to their owners!  My strong advice is to leave well alone.  If the telescope gives perfect star images and performance, that's what you pay for.     🙂

[YogSothoth]

1 hour ago, mikeDnight said:

Is there anything at the centre of the rings, as a small particle sandwiched between the elements may create enough pressure to cause the issue? I can't see anything in the images!

No I can’t see anything and other than the rings it all looks pretty clean.

[YogSothoth]

1 hour ago, The Admiral said:

I assume that the rings remain static as you change the geometry of your viewing? I must admit, if there are any rings, then naively I would have thought they'd be in the centre, which is where you'd expect the lens surfaces to be nearest to each other. Then again, your star tests are OK.

Ian

Yes they remain static and they appear to be almost dead centre as far as I can tell, although that doesn’t show up too well in the pics, especially the second one.

[YogSothoth]

1 hour ago, Peter Drew said:

These Taks are such a worry to their owners!  My strong advice is to leave well alone.  If the telescope gives perfect star images and performance, that's what you pay for.     🙂

I’m inclined to agree with you there. It’s more that I’ve never come across this before and was concerned about what might be causing it. It’s a great scope and there’s no chance of me selling it on in the future, so I don’t have to worry about it putting potential buyers off.

[YogSothoth]

1 hour ago, dweller25 said:

One day you may decide to sell your Tak and those rings may put people off.

I suspect the lens spacing may be different to other FC100’s that do not show the rings.

If that was my telescope I would be talking to the vendor.

I love the scope and can’t imagine selling it on in the future, but I take your point about possibly contacting the vendor.

[The Admiral]

5 minutes ago, YogSothoth said:

they appear to be almost dead centre

Well in that case it makes sense. Personally, I wouldn't worry about it.

Ian

[Rusted]

These rings show a difference in radius between two closely spaced lens elements. Provided they are central I'd ignore them, or admire them, according to personal taste. Even if they were off centre this might only be the result of your viewpoint relative to the optical axis.

Newton's rings are used by optical workers to compare radii and to measure the flatness of plane surfaces under monochromatic light.

Monochromatic light used to be easily available under every orange street lamp. Now less likely with updated tech and different spectra.

 

[mikeDnight]

2 hours ago, YogSothoth said:

I love the scope and can’t imagine selling it on in the future, but I take your point about possibly contacting the vendor.

It sounds like your FC100 has won your heart, which I fully understand. DC's are great scopes, and there would always be that niggling question of whether a replacement would be as good?