Etalon issue?

[Ags]

It's been suggested that my etalon might have some issues.  I don't seem to get even contrast across the disk. Do I need better flats or should I contact my supplier?

Happy with the big sunspot group though!

[vlaiv]

Contrast or brightness?

Brightness variations can be solved with flats, but contrast issues can't.

I don't have experience with Ha solar etalons, but logic tells me that issues with etalon should present themselves as contrast issues - etalon being off band / passing more continuum and so on.

We have to be careful about what we are seeing. Sometimes issues with sharpness are thought of as issues with contrast. In some sense they are - blur is loss of contrast at some frequencies.

However - I don't think that issues with etalon will present them selves as blur.

I see two things (actually three things - I'll talk about third in the end) - variation in brightness and blurring.

Variation in brightness can be easily seen if one really reduces the size of the image:

There are two dark patches - one to the left and one to the bottom. In general - there seems to be lack of uniformity in illumination. I think that this effect can be solved with flats.

There is also issue with sharpness in the image.

If we compare these two patches:

It does not take much to conclude that bottom one seems too blurred out. I have no idea where this effect is coming from. I'm not sure it is down to etalon.

How did you take the image? I ask because there is third thing:

These look like compression artifacts.

It looks like jpeg / mpeg compression after sharpening / wavelets.

This makes me wonder if above blur has something to do with the way image was taken / created.

As far as issues with etalon - this is how I think it would present itself:

Instead of seeing this:

you would see this:

slightly brighter view (more continuum) and "drowned" Ha features that are hard to make out.

 

[Ags]

Thanks Vlaiv, helpful as always!

I am almost sure there is a focus gradient issue which I think is related to my 0.5 reducer. I will take more pictures tomorrow without the reducer to exclude that factor. My flats don't quite work yet, will have another try tomorrow.

[vlaiv]

17 minutes ago, Ags said:

Thanks Vlaiv, helpful as always!

I am almost sure there is a focus gradient issue which I think is related to my 0.5 reducer. I will take more pictures tomorrow without the reducer to exclude that factor. My flats don't quite work yet, will have another try tomorrow.

What is your setup like, what camera are you using?

Is it 485mc?

Maybe you could try EP projection instead of using reducer?

If I'm not mistaken, eyepiece like 18mm ortho should give you full disk view and it would make solar disk about 25 degrees in angular size.

485mc with 12mm C/CS lens should be able to fit whole disk onto sensor.

[Ags]

That's given me something to think about!

The edge of sun artifacts were from AS3! by the way. I needed to place alignment points along the edge.

[Ags]

Maybe it is the etalon. Assuming the weak patch lower right on my first image was caused by the etalon, I tried the relatively obvious solution of positioning the sun off center and avoiding the putative weak patch. I think this result is much better, although you can see a fair bit of vignetting as the Sun is close to the edge of the image circle. Flats should correct this, if I can get flats working - still no luck with that aspect however. My flats don't show the extreme vignetting I can see when positioning the Sun!

I did make other changes, I reversed the reducer (more accurately, de-reversed it) and moved it further away from the sensor, enabling a full disk, but more importantly letting me position the Sun further off axis.

Edited April 21, 2022 by Ags

[Ags]

It was a tuning issue. Making a massive change from -3 to +2 resolved the issue. I still have to try +3 to +5 though...