Purple mark on Tak

[John]

13 minutes ago, mikeDnight said:

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? 

I think the niggling question is why they are showing.

I've never seen newtons rings showing on the objective of a TAK FC100 before, and lets face it, there have been enough pictures of them on the forum over the past 5-6 years or so !

 

[YogSothoth]

5 minutes ago, John said:

I think the niggling question is why they are showing.

I've never seen newtons rings showing on the objective of a TAK FC100 before, and lets face it, there have been enough pictures of them on the forum over the past 5-6 years or so !

 

That is a good point and I’m sure they’ve only recently appeared, although I may be wrong on that. I may email FLO for their opinion, although the thought of sending it back isn’t that appealing, especially as I no longer have the original packaging.

[paulastro]

I can't see Takahashi giving you anything for nothing, not even a Newtons ring.  They are hardly known for supplying anything but the bare minimum with their tube assemblies.

[Peter Drew]

Newton rings are a very sensitive indication of how well the lens is held and centred.  It's one of the first things I look for when assessing one, I would be very pleased with one like yours.     🙂

[YogSothoth]

36 minutes ago, Peter Drew said:

Newton rings are a very sensitive indication of how well the lens is held and centred.  It's one of the first things I look for when assessing one, I would be very pleased with one like yours.     🙂

Thanks for that. Very reassuring 🙂

[John]

1 hour ago, Peter Drew said:

Newton rings are a very sensitive indication of how well the lens is held and centred.  It's one of the first things I look for when assessing one, I would be very pleased with one like yours.     🙂

I've seen them with some Vixen doublets and a TAL doublet that I had but not in Tak FC100 doublets. I put that down to the air space being a touch wider in the FC100 objective design.

I agree that they are not a negative thing to see but just wonder why they are present in just one example of that particular scope 🤔

 

[F15Rules]

When I first saw your photos, my first thought was, "Wow, that scope has a  superbly corrected objective!"..

Maybe yours is an extra special sample of your scope..

If Peter Drew, as an expert Telescope maker of many, many years experience says he would "be very pleased with one like yours", I would take that as a highly credible and reassuring endorsement of your scope..all that remains is for you to go and enjoy it with a big grin on your face!👍😊😁

[Carbon Brush]

My two pennorth from an ill educated source😁

You have two pieces of glass with a very small air gap and very similar curvature.
This means you have the conditions for producing Newtons rings.

Ah but you need monochromatic light not daylight - I used to use a sodium light when measuring.
Yes ordinarily monochrome light is essential. However, we are looking at coated glass. This is intended to let almost all the light through and not reflect.
In practice it will not be 100% ideal - no lens coating will be.
If the coating means a narrow band in the blue/purple part of the spectrum has much more reflection than other wavelengths, you won't notice at the eyepiece.
If your refractor had poor performance at an interesting wavelength like Ha or OIII that would be a different matter.
However, the reflected light, being narrow band, will allow Newtons rings to be seen in reflection.

So nothing at all to bother about. The rings being circular and in the centre of the lens means the glass is mechanically well positioned.
As the rings are in the same place every day, the lenses are not moving. Their colour means reflection is in an unimportant part of the spectrum.

I have seen rings on a refractor. An old doublet, but I can't remember what it was.
By pushing on the lens and squeezing things, the rings would come and go.
Not recommended on an expensive frac unless you are going to ask a grown up to put it back into alignment for you😂

Look through and enjoy.

David.

[Jim L]

Those aren’t rings; these are rings!

Early Vixen made Celestron CO-80

[John]

Here is a relatively recent thread from the CN forum regarding Newton Rings visible in a Tak FC76DS objective. They look almost identical to yours:

Takahashi FC76DS Newton rings - Refractors - Cloudy Nights

The 4th post in the thread mentions a discussion on this topic with an optical technician at Takahashi Europe who provided reassurance that they are not in any way untoward.

 

[Mr Spock]

Don't they charge extra for them? 

[900SL]

Jeremy's been licking the objective. Nothing to worry about there 😉