Is this normal? (spherical aberration but only on the brightest stars)

[TheGreenDishcloth]

Greetings!

This is my first post to the group so please bear with me. 

I'm new to astrophotography and still getting to grips with many of the practical aspects and I'm hoping someone can help me with this - I've attached a subframe of the Horsehead nebula (300 secs with RedCat 51, ASI 294 MC Pro cooled camera and Optolong L-Extreme filter) - i.e. no processing done on the image.

While I've been blown away with the pictures I've been able to get, particularly after recently getting hold of the filter, I've noticed something odd while imaging different targets.  I've started to see what appears to be spherical aberration which is really noticeable on some (see "A" and "B" in the photo below) but not all stars (see "C" below) which is kind of odd. The effect on "B" is really noticeable in the processed image (even though I tried to ignore it in the excitement of processing the nebula!). 

I'm not sure if this is down to the formation of dew or if its something more serious with the scope/filter/optical train. I'm cooling the camera down to -20 to get rid of as much noise as possible but I'm wondering if this is the cause of the problem. As of yet, I've not used dew heaters - would these solve the problem?

Any suggestions would be welcome!

Thanks in advance

Tony

 

[newbie alert]

It's a halo from off your filter, is it as close to the sensor as you can get it?

[TheGreenDishcloth]

Thanks for the reply!

Yes, it’s as close as I can get it. I have the filter in Zwo filter drawer. Any reason it’s so pronounced in the bright stars?

 

[newbie alert]

9 hours ago, TheGreenDishcloth said:

9 hours ago, TheGreenDishcloth said:

Thanks for the reply!

Yes, it’s as close as I can get it. I have the filter in Zwo filter drawer. Any reason it’s so pronounced in the bright stars?

 

 

It's probably a reflection off the glass in your sensor, halos are a real nightmare as they're pronounced on some systems and filters and not on others..I had a astronomik HA filter that caused them once , and when I replaced it with the 6nm newer version it doesn't .. there was a thread on a 3nm chroma O111 that caused them the other month.... nightmare!!

Have you tried it without the filter so just in broadband on the same target.. can't get much brighter than Alnitak nr a emission target

[TheGreenDishcloth]

Hmmm, that would be annoying to say the least. I’ve got so much light pollution, the filter is important for most of the targets I’m interested in. The filter does look like the culprit here - I never saw this effect before I started to use it but I also think that I had clean images for a couple of the Cygnus targets before the cold nights in December.

Looks like there are a number of things to experiment with. I’ll post an update.

Tony

[Simon Pepper]

Hi there, I have the same scope, camera and filter and guess what same issue! Glad you mentioned the filter draw though as my l extreme is in the body of the Redcat. I was abut to push the button on a EFW and EAF to get the filter to sit as close to the sensor as possible. This will obviously be pointless now. As the guys have stated and what I have read its the OIII part of the filter that causes this and its a right nightmare! I believe there are a couple of options. Firstly get the l enhance I don't think that has the same issue as the band isn't as narrow, but I am not sure what a side by side comparison would be on the data. There is also a procedure here (may have to google it) https://astrodoc.ca/wp-content/uploads/2017/01/REMOVING-HALOES-FROM-BRIGHT-STARS.pdf I have not tried this yet and I am not sure if it works with a OSC but there must be a way in PI to separate the HA from the OIII think its in channel extraction so should be possible.  If you are not busy now try it out let me know how you get on. One thing to note though Alnitak is a giant and its bound to have some haloing, however I know your pain this happens on most bright start in the FOV! 

[Simon Pepper]

Oh and welcome to the group by the way

[alacant]

On 28/12/2020 at 22:34, TheGreenDishcloth said:

Any suggestions

Hi

Lose the drawer and use a 1.25" filter in the throat of the camera. I think they supply the adapter with the camera.

HTH

[Louis D]

Someday, you may want to upgrade to Astrodon filters.  They tend to halo the least of any filters out there.

[TheGreenDishcloth]

@Simon Pepper

Thanks Simon, that's an interesting workflow you posted (I sometimes wondered why PixInsight included a noise generator!). I'll give the workflow a shot as soon as I get a moment - I would be great to process this out rather than invest in more kit (although @Louis D's pics provide a strong case for an upgrade 😉 )

The main thing is that I know understand that this is a known issue with cheaper OIII filters - being a newbie, I wasn't sure whether it was either something wrong with my kit or something fundamental I was doing wrong in the processing. 

Thanks to everyone who replied!