Is this a star halo, or something else?

[Lee_P]

I'm currently testing an Optolong L-Ultimate filter. Optolong say that this filter doesn't produce any halos, but this was the result I got last night when imaging Vega for 300 seconds.

I'm just after a sense-check: that's a halo, right? Or is it something else? (If I split the image into R G B then the "halo" is present in G and B, but not in R).

For reference, this is the same but with no filter at all. (Apologies for the dust!)

[vlaiv]

10 minutes ago, Lee_P said:

I'm just after a sense-check: that's a halo, right?

Yes it is.

It is of course filter related.

I'm not sure however if we can say it is produced by filter. Using different gear and same filter - it might not be there.

Here is a diagram of what I believe is happening:

Blue line is surface of sensor. Black vertical line is filter and red vertical line is some optical element - probably field flattener / reducer that sits in front of filter (filter sits between that optical element and sensor).

Black converging rays are incoming from the left and they hit filter. Some of the light is reflected of that filter - as it is reflection rather than absorption filter type.  In above diagram it is represented by orange line.

This light should go out in front of the scope - but as it hits rear of flattener / reducer - some of it (very small percentage, like less than 1% - depends on AR coatings applied) is reflected once more towards the filter.

However this time light passes thru filter and ends up on sensor.

How can this happen? Well - there are two things that can cause light to be reflected first time and to pass thru second time.

- Filter is not blocking light 100%. This is normal, no filter is blocking light 100% out of band - but very small amount of it passes. In first contact very small amount of that light passes and is properly focused and majority of light is reflected. In second contact same thing happens - but this time light is out of focus and ends up in halo rather than in star core - and can be seen.

- Second reason why light might come thru in second contact and not in first contacts is angle of incidence. Rear element of flattner is curved and will disperse light rays more. They won't hit filter at same angle as in first contact - and filter QE response curve depends on angle of incidence. Interference filters work the best when light is normal to filter surface, but when light is at an angle - it "sees" dielectric layers as being thicker then they really are (much like sloping armor has more stopping power than vertical one). And this changes dielectric layer thickness compared to wavelength of light - which in turn leads to CWL shift.

In any case - we can say it is due to filter - but not solely fault of the filter. Filter by itself won't produce halo. If you remove field flattener and repeat the experiment - you probably won't see halo, although you'll still be using that filter.

Put in different field flattener and halo might change or disappear.

Even changing F/ratio of the scope (aperture mask) might cause effect to disappear or diminish. Another thing you might try is changing filter position / distance. If you can - if it is 2" filter - put it in front of the field flattener rather than between field flattener and sensor.

 

[Lee_P]

5 minutes ago, vlaiv said:

Yes it is.

It is of course filter related.

I'm not sure however if we can say it is produced by filter. Using different gear and same filter - it might not be there.

Here is a diagram of what I believe is happening:

Blue line is surface of sensor. Black vertical line is filter and red vertical line is some optical element - probably field flattener / reducer that sits in front of filter (filter sits between that optical element and sensor).

Black converging rays are incoming from the left and they hit filter. Some of the light is reflected of that filter - as it is reflection rather than absorption filter type.  In above diagram it is represented by orange line.

This light should go out in front of the scope - but as it hits rear of flattener / reducer - some of it (very small percentage, like less than 1% - depends on AR coatings applied) is reflected once more towards the filter.

However this time light passes thru filter and ends up on sensor.

How can this happen? Well - there are two things that can cause light to be reflected first time and to pass thru second time.

- Filter is not blocking light 100%. This is normal, no filter is blocking light 100% out of band - but very small amount of it passes. In first contact very small amount of that light passes and is properly focused and majority of light is reflected. In second contact same thing happens - but this time light is out of focus and ends up in halo rather than in star core - and can be seen.

- Second reason why light might come thru in second contact and not in first contacts is angle of incidence. Rear element of flattner is curved and will disperse light rays more. They won't hit filter at same angle as in first contact - and filter QE response curve depends on angle of incidence. Interference filters work the best when light is normal to filter surface, but when light is at an angle - it "sees" dielectric layers as being thicker then they really are (much like sloping armor has more stopping power than vertical one). And this changes dielectric layer thickness compared to wavelength of light - which in turn leads to CWL shift.

In any case - we can say it is due to filter - but not solely fault of the filter. Filter by itself won't produce halo. If you remove field flattener and repeat the experiment - you probably won't see halo, although you'll still be using that filter.

Put in different field flattener and halo might change or disappear.

Even changing F/ratio of the scope (aperture mask) might cause effect to disappear or diminish. Another thing you might try is changing filter position / distance. If you can - if it is 2" filter - put it in front of the field flattener rather than between field flattener and sensor.

 

Thanks vlaiv! For reference, I'm using an Askar 130PHQ (so no field flattener for me to remove). The filter is in a drawer as close to the ASI2600MC-Pro as it can be.

[vlaiv]

6 minutes ago, Lee_P said:

I'm using an Askar 130PHQ (so no field flattener for me to remove).

That is quadruplet scope so there is "rear" element that can be cause of reflection, but that is far fetched according to diagram on Askar website as it is very far away.

Maybe try it with some other scope, but if you intend to primarily use it on Askar - then I'd consider returning it (or just seeing what sort of magnitude will make halo visible - weaker stars simply don't produce enough light after all those reflections to show halo).

[CraigT82]

Definitely a halo but looks actually quite attractive to me as far as halos go, though I do like the glare of the unfiltered sub even more.  A 5 min sub of Vega is a hell of a test for any optical element

[Lee_P]

Thanks, maybe 5 mins on Vega was just a bit too harsh a test. I've tried the same test with an Optolong L-eXtreme and got the same halo.

Maybe I should make it a bit shorter. This is the Vega example Optolong give: