Sky-Watcher secondary mirror defect?

[Jason D]

Don't know the exact percentages but most of the light will be heading towards the central portion of the secondary. I doubt that any of light that hits the eye or the camera is reflected from that area at all. I wouldn't worry too much.

I would not have thought that would be a problem unless the light cone from the primary mirror, at the point where it intersects the secondary, was exactly as large as the area as the secondary mirror, which I doubt that it is. I suspect your secondary will be slightly oversize for the light cone.

The secondaries on both my 250 flextube and the 200PDS are like that . . .

They are both "oversize" and the light cone falls well inside the "defect" making no difference to the view.

IAssuming the scope is collimated, you can probably see the primary reflected within the secondary with a bit of spare around the edge, if you can, then the light cone from the primary is smaller than the secondary at that point.

Your comments on the size of the light cone versus the secondary mirror surface shed some light on the non-importance of this uncoated stripe in terms of reflection into the focuser

Everyone who replied in the above list was only thinking of the star at the center of the FOV. What about the off-center stars?

It is a common misconception to assume the edge of over-sized secondary mirrors is not used. The edge is typically used when observing visually at low/medium magnification and when imaging – otherwise the secondary mirror will have to be way too large especially for fast scopes. See attachment.

With regards to the OP’s secondary mirror, the missing coating is caused by the holder during the coating process as mentioned by some of the replies. It should not have an impact on the view. Light ray diffraction in both directions will most likely miss the eyepiece or CCD because diffractive angle will not be as sharp as reflective angle. It could cause some light scattering but I doubt the scattering will be noticeable especially for a well-flocked OTA.

Jason

EDIT: The attachment is a GIF file. Make sure to click on it to see the animation.

[JamesF]

With regards to the OP’s secondary mirror, the missing coating is caused by the holder during the coating process as mentioned by some of the replies. It should not have an impact on the view. Light ray diffraction in both directions will most likely miss the eyepiece or CCD because diffractive angle will not be as sharp as reflective angle. It could cause some light scattering but I doubt the scattering will be noticeable especially for a well-flocked OTA.

Well, that ought to be possible to prove one way or the other I'd have thought. If the additional diffraction effects disappear if the back of the secondary is made light-proof then it must be light that's passing through the secondary causing them. I assume the unfinished edges of the secondary won't cause as many problems, but I guess it would be sensible to stop light entering the secondary that way as well.

Do newts intended for imaging (say the Quattro models, for instance) also have an uncoated section of secondary?

James

[Jason D]

If the additional diffraction effects disappear if the back of the secondary is made light-proof then it must be light that's passing through the secondary causing them. I assume the unfinished edges of the secondary won't cause as many problems, but I guess it would be sensible to stop light entering the secondary that way as well.

Unless I missed it, I do not recall reading additional diffraction spikes reported by the OP of this thread.

I have not painted or intend to paint the back of my secondary mirror but I do not see any harm doing it assuming the painting process does not accidentally end up scratching the reflective surface of the mirror.

Jason

[JamesF]

Unless I missed it, I do not recall reading additional diffraction spikes reported by the OP of this thread.

I have not painted or intend to paint the back of my secondary mirror but I do not see any harm doing it assuming the painting process does not accidentally end up scratching the reflective surface of the mirror.

I was referring to the additional diffraction spikes that malc-c is getting which, if I have understood properly, were diagnosed by the optical engineer he visited as due to the uncoated section of the mirror as seen in the image of the OPs mirror at the start of this thread.

James

[JamesF]

Everyone who replied in the above list was only thinking of the star at the center of the FOV. What about the off-center stars?

It is a common misconception to assume the edge of over-sized secondary mirrors is not used. The edge is typically used when observing visually at low/medium magnification and when imaging – otherwise the secondary mirror will have to be way too large especially for fast scopes. See attachment.

Specifically with regard to imaging, is this always true, or only true if the sensor is large enough to cover the edges of the field of view? I'm struggling to think it through at the moment. Eg. if you have a secondary that gives a well-illuminated field for a 2" eyepiece with, say, a 35mm field stop and you put, say, an Atik 4000 onto the scope (which I think has square sensor about 21mm across the diagonal?) would you still actually be using the edges of the mirror? Or would you need to be using a much larger sensor?

James

[Jason D]

I was referring to the additional diffraction spikes that malc-c is getting which, if I have understood properly, were diagnosed by the optical engineer he visited as due to the uncoated section of the mirror as seen in the image of the OPs mirror at the start of this thread.

The jury is still out on that one. I have already expressed my high doubts about the analysis of the optical engineer in the other thread. Malcolm can always add a temporary obstacle behind the secondary mirror to cover that uncoated part of the secondary mirror and see if the spike goes away – but the obstacle has to be small enough to only cover the uncoated area.

Specifically with regard to imaging, is this always true, or only true if the sensor is large enough to cover the edges of the field of view? I'm struggling to think it through at the moment. Eg. if you have a secondary that gives a well-illuminated field for a 2" eyepiece with, say, a 35mm field stop and you put, say, an Atik 4000 onto the scope (which I think has square sensor about 21mm across the diagonal?) would you still actually be using the edges of the mirror? Or would you need to be using a much larger sensor?

My 10” F5 reflector will need a 3” secondary mirror (a 30% central obstruction) to provide a 35mm 100% illuminated field – assuming the secondary mirror is perfectly positioned under the focuser.

Jason

Attachment is a GIF file. Click on it to see animation.

[Mycota]

First of all, happy new year people!

I finally had a clearish evening to star test and check the diffraction spikes. The spikes were little better from when the scope came from the seller, however, a clear asymmetry is still seen with one of the diagonal spikes being more prominent. Just like other members, as malc-c for instance. You can see Sirius diffraction spikes from my 550D as you requested malc-c.

When I have more news or more tests done I'll let you know.

Clear skies for 2013!

[Jason D]

Refer to attachment. The spike you are referring to is the one indicated by the yellow arrow, correct? And you believe it is due to the uncoated area indicated by the red arrow, correct?

Is the uncoated area on both sides of the secondary mirror? If it is only on one side, why don't you identify where that uncoated side is located with respect to the spikes by placing your hand in the same direction of the uncoated area and see where the reflection of your hand end up with respect to the main spider vanes spikes.

My point is that if the extra spike happens to be on the opposite side of the uncoated area then that would eliminate it as the culprit.

Jason