Hollow stars

[Pitch Black Skies]

Has anyone came across this before?

My stars look hollow, almost like doughnuts.

My first assumption was that the focus was out so I re-ran the EAFs auto-focus feature, same result...

I then manually focused it and the tightest I could get them still left them hollow.

This was the first light of my 1.25'' Antlia 3nm Sii filter on a 294MM Pro and SW130P-DS.

The Ha and Oiii were fine.

Another thing I noticed is that the signal through the Sii filter was only about 10% of what I could achieve through the Oiii and Ha filters. Is that just because the target isn't rich in Sii?

The target was IC 1396 (Elephant's Trunk Nebula)

Light_IC1396_600.0s_Bin2_294MM_S_gain120_20220826-233812_0001.fit

For comparison, here is a Ha and Oiii sub, which are half the exposure length;

Light_IC1396_300.0s_Bin2_294MM_H_gain120_20220814-012109_-10.0C_0001.fit

Light_IC1396_300.0s_Bin2_294MM_O_gain120_20220824-234735_-10.0C_0001.fit

 

[Elp]

I think another thing that can cause doughnuts is a large central obstruction though I never experienced this with a 130pds as the only obstruction is the secondary. Are you to say the stars never became solid between both extremes of focus?

If your filters are all around the same bandpass margin and similar integration time the S2 should be similar strength but slightly less intense compared to the Ha stack on the elephant, though much stronger than o3.

[Antlia_filter]

For IC1396, the SII signal is not rich compared to Ha and OIII

[Pitch Black Skies]

12 minutes ago, Elp said:

Are you to say the stars never became solid between both extremes of focus?

Correct

13 minutes ago, Elp said:

though much stronger than o3.

No, it was quite the opposite actually. As you can see from the attached files the Oiii was much stronger than the Sii, despite Sii being twice the length.

[Pitch Black Skies]

11 minutes ago, Antlia_filter said:

For IC1396, the SII signal is not rich compared to Ha and OIII

Thanks, that's what I suspected.

[Elp]

I'll edit my last statement, the S2 shows a similar shape to the Ha though it's much weaker, it's better to see it for yourself so here's the comparison, HA, S2 and O3 in that order:

[newbie alert]

2 hours ago, Pitch Black Skies said:

Has anyone came across this before?

My stars look hollow, almost like doughnuts.

My first assumption was that the focus was out so I re-ran the EAFs auto-focus feature, same result...

I then manually focused it and the tightest I could get

Just because you run a autofocus doesn't mean that it's in focus.. could mean that your settings aren't right, but if you still have doughnuts in your stars after the manual focus it means that you have a mechanical error, such as collimation, spacing .. lots of people shorten the drawtube of the focuser because it inpeades into the light cone but that usually shows up as bites out of the stars... Be interesting to see the solution on this

[Pitch Black Skies]

37 minutes ago, newbie alert said:

Just because you run a autofocus doesn't mean that it's in focus

Indeed, that's why I then did a manual focus.

I can see some radial distortion in the Ha and Oiii subs, suggesting that the coma corrector is too far from the sensor.

[alacant]

2 hours ago, Pitch Black Skies said:

stars look hollow

Hi

There's quite pronounced astigmatism across the whole field. I'm guessing the Baader cc and/or filters. Could this have fooled the focuser?

Cheers

[Pitch Black Skies]

Thanks @alacant

Good spot, I could see this too but wasn't sure what it was. I was thinking star trailing at first glance but wasn't sure.

These are 3nm Antlia filters, SHO. The coma corrector is Skywatcher Aplanatic.

There wasn't any major changes between each filter change.

[alacant]

4 minutes ago, Pitch Black Skies said:

Skywatcher Aplanatic

So it's not the cc then;)
Do you have an L frame without filter?

 

Edited August 27, 2022 by alacant

[Pitch Black Skies]

1 hour ago, alacant said:

So it's not the cc then;)

I hope not 😆

1 hour ago, alacant said:

Do you have an L frame without filter?

Good question. No I haven't. I do have an L-frame of M31 that a took a couple of weeks ago if it's any help?

Are you trying to see if it's on all of my filters?

I can't really see the astigmatism on the Ha and Oiii that I attached.

[Pitch Black Skies]

Actually Oiii doesn't look great either, but not half as bad as Sii.

The four corners of each filter:

Sii

Ha

Oiii

Edited August 27, 2022 by Pitch Black Skies

[Pitch Black Skies]

Here's a single Sii sub from CCD Inspector if they can help shed a bit more light.

CCD Inspector Sii.fit

CCD Inspector Sii Curvature.fit

[alacant]

1 hour ago, Pitch Black Skies said:

can't really see the astigmatism on the Ha and Oiii that I attached.

Hi

All the three frames display astigmatism. We need a shot without a filter to eliminate the same, although I doubt they're the cause.

Next would be a shot without the cc. Then substitute the secondary. Then remove the mirror clips... etc.

Or simply leave it as it is and correct in software.

Cheers

 

 

 

 

 

 

Edited August 27, 2022 by alacant

[Pitch Black Skies]

18 minutes ago, alacant said:

We need a shot without a filter to eliminate the same, although I doubt they're the cause

You're right, similar problem with my L-filter from a couple of weeks ago.

I have struggled a little bit with tilt in the past. I had put it down to the focuser.

Mirror clips are removed, using your silicone method.

[alacant]

Most of the errors with the silicone are caused by pushing the mirror back into the cell instead of leaving it to cure under gravity. You need at least a 2mm thickness to allow for thermal expansion and contraction. Try reseating the primary. 

If not, next you'll need a secondary of known performance.

HTH