Weird concentric rings with PST and barlow.

[Poita]

If I attempt to use a Barlow with the PST and a DMK21 camera, get these weird concentric rings. Is there any way to be rid of them?

[alocky]

They're Newton's rings. Caused by interference and the extremely monochromatic nature of an H-alpha scope. You can take 'flats' by defocussing when you're pointed at the sun and subtract them.
Cheers,
Andrew.

[Poita]

Might help if I attached a picture. One is defocussed to show the effect more clearly, the other image is without the Barlow in.
The barlow works without a hitch for planetary.

[bojan]

It seems to me those are Newton rings... generated in Barlow.
Get rid of that barlow.

[alocky]

Subtract your defocussed image from your 'lights' and it will go away. Those are particularly severe Newtons rings ( or interference fringes, call them what you like). Hopefully they are originating from the interface between the doublet in your Barlow and its an easy fix.

[Poita]

I'll have to try another Barlow.
Is it an indication of barlow quality, or just a happenstance between the various surfaces?

[bojan]

It is happening between two very close surfaces, not exactly parallel or of the same radius .. this is actually interference pattern, due to multiple reflections

[Poita]

Ah, now I get it.
Thanks for the replies.
I'll try doing a flat, but I think I will not be using this barlow with the PST.

[asimov]

I've got two barlows, & both produce Newton's rings under certain conditions to do with solar imaging. Keep trying barlows (expensive!) Or as Andrew said.

[Poita]

I'll see if I can swap my televue 3x with some people to try out some different models.
I'll give the flats technique a go tomorrow, will help with the hotspot anyway I imagine.
(Will work better than my thong waving technique anyway, I'd wager.)

[alocky]

I use a TV powermate 2.5 with the DMK41 on my Lunt80, and although I can see the rings on the flats they're pretty hard to see in the original image. As far as I know, subtracting the flats is pretty standard practice.
It is also possible that we're actually seeing Haidinger's fringes and they're formed in the blocking filter, but regardless of who we're blaming for them or naming them after, the method for removing them is the same.
Out of interest, This is the way you test a flat against a reference flat -the alternating light and dark bands are the constructive and destructive interference of light of a particularly pure colour - in this case, the H-alpha wavelength. Nice bit of physics in action! Each band represents an increase in physical separation between the two optical surface of odd multiples of 1/2 wavelength of H-alpha.
cheers,
Andrew.