Bloaty blue stars - any thoughts on cause?

[Mr Thingy]

I finally managed to get some RGB data on Orion last night, to add to my Ha data. Watching the images come in I noticed that the stars in the blue images were significantly bloated versus R&G.

I've not processed these yet but what could be the reason for this? My scope is a SW 72ED, and I thought that doublets focused red and blue well. Focus was OK. Maybe a filter issue or just a normal phenomenon?

Is there a neat way to reduce these in processing? I use Gimp and normally just basic processing.

I will upload images later, once I've solved a current IT issue!

Thanks for looking.

-Thingy-

[scotty38]

Probably completely different setups and experience but I captured some Orion data last night (image posted in a couple of other threads so won't do so again) but how do yours compare?

I only ask as I did nothing fancy whatsoever mainly because I have no idea how to do anything fancy....

[Simon Pepper]

Hi Thingy,

I noticed the same issue yesterday on the same target, but think this is pretty normal especially on starts like Alnitak! I dont use Gimp, so cant comment. I use PS which has a plug in where you can reduce bright blue halos.

Thanks 

[Mr Thingy]

5 minutes ago, scotty38 said:

Probably completely different setups and experience but I captured some Orion data last night (image posted in a couple of other threads so won't do so again) but how do yours compare?

I only ask as I did nothing fancy whatsoever mainly because I have no idea how to do anything fancy....

😂 Sounds like my approach too. Nothing fancy but I also like images with minimal processing, though I do want to learn how to reduce bloat.

I only have my image for Ha so far but hopefully will process the R,G&B data this evening. This is about 1 hour of data in Ha.

 

[Mr Thingy]

6 minutes ago, Simon Pepper said:

Hi Thingy,

I noticed the same issue yesterday on the same target, but think this is pretty normal especially on starts like Alnitak! I dont use Gimp, so cant comment. I use PS which has a plug in where you can reduce bright blue halos.

Thanks 

I need to see if the same exists in GIMP.

Thanks

[Simon Pepper]

2 minutes ago, Mr Thingy said:

😂 Sounds like my approach too. Nothing fancy but I also like images with minimal processing, though I do want to learn how to reduce bloat.

I only have my image for Ha so far but hopefully will process the R,G&B data this evening. This is about 1 hour of data in Ha.

 

This looks pretty awesome to me! What scope and camera do you use? Thanks

[The Lazy Astronomer]

I had the same problem a while back too - the stars in both blue and luminance channels were bloated compared to red and green. 

After a bit of research, l came to the conclusion the problem was unfocused UV passing through the filters.

Bought an Astronomik L3 filter and problem solved!

[Mr Thingy]

1 minute ago, The Lazy Astronomer said:

I had the same problem a while back too - the stars in both blue and luminance channels were bloated compared to red and green. 

After a bit of research, l came to the conclusion the problem was unfocused UV passing through the filters.

Bought an Astronomik L3 filter and problem solved!

Thanks a lot. I will have to check that out, though I will need to consider where in the image train I can add a filter. 

[scotty38]

Looks pretty good to me that does but I appreciate it's the RGB you're speaking about....

[Mr Thingy]

10 minutes ago, Simon Pepper said:

This looks pretty awesome to me! What scope and camera do you use? Thanks

Thanks! I have a SkyWatcher 72ED 'scope, 0.8 reducer/flattener and a ZWO ASI GT (Mono) camera.

Edited February 19, 2021 by Mr Thingy

[Mr Thingy]

3 minutes ago, scotty38 said:

Looks pretty good to me that does but I appreciate it's the RGB you're speaking about....

Thanks!

The stars look pretty tight in Ha but appreciable less through the blue filter.

I will compare properly later when I process the images.

[The Lazy Astronomer]

7 minutes ago, Mr Thingy said:

Thanks a lot. I will have to check that out, though I will need to consider where in the image train I can add a filter. 

 The other alternative, although more expensive, is to look for a new set of RGB filters which cut at around the 400 - 420nm range in the blue channel. 

[Mr Thingy]

2 minutes ago, The Lazy Astronomer said:

 The other alternative, although more expensive, is to look for a new set of RGB filters which cut at around the 400 - 420nm range in the blue channel. 

Yes, I was wondering about that. I bought some basic ZWO ones but maybe need to throw more money at them.

I was just having a sneaky peak on FLO but then the wife came in... quick! Hide page.

Edited February 19, 2021 by Mr Thingy

[Mr Thingy]

4 minutes ago, The Lazy Astronomer said:

 The other alternative, although more expensive, is to look for a new set of RGB filters which cut at around the 400 - 420nm range in the blue channel. 

Astronomik ones seem to meet that criteria:

https://www.firstlightoptics.com/rgb-filters-filter-sets/astronomik-deep-sky-rgb-filters.html

 

[The Lazy Astronomer]

8 minutes ago, Mr Thingy said:

Astronomik ones seem to meet that criteria:

https://www.firstlightoptics.com/rgb-filters-filter-sets/astronomik-deep-sky-rgb-filters.html

 

I was going to suggest them but didn't want to seem like an Astronomik salesperson! 😁

Will probably replace my ZWO filters with those at some point too. 

[The Lazy Astronomer]

15 minutes ago, Mr Thingy said:

Yes, I was wondering about that. I bought some basic ZWO ones but maybe need to throw more money at them.

I was just having a sneaky peak on FLO but then the wife came in... quick! Hide page.

I suppose there is another slightlyslightly more expensive option: buy a Takahashi!!! 😅

[scotty38]

I'd compare your images with others for the same camera etc on Astrobin before you start buying more hardware that might not be needed.

 

I mean spend money by all means but at least spend it on stuff you do want need 🤣

[Mr Thingy]

4 hours ago, The Lazy Astronomer said:

I suppose there is another slightlyslightly more expensive option: buy a Takahashi!!! 😅

A Tak is way too good for my limited skills. It would be like giving a chimpanzee a boeing 747 to fly 😂

Edited February 19, 2021 by Mr Thingy

[vlaiv]

There are couple of reasons for this and couple of "cures"

- atmosphere distorts shorter wavelengths of light more than longer wavelengths - for the same reason glass bends shorter wavelengths of light stronger.

notice that long wavelengths (red/orange) are bent much less than short wavelengths (blue/violet)

- telescopes tend to be corrected better in red part of spectrum. Lens designers can choose which way doublet curve will "lean" - you can make either red part of spectrum more defocused or blue part of spectrum more defocused. Most opt for blue part of spectrum because of Ha and above fact that shorter wavelengths are bent more.

That is why we often see "violet" halo and not "red" halo in achromat scopes

Even ED doublet have this - and blue part of spectrum will have a bit more defocus then the rest, or rather defocus "range" - you'll put center of blue in focus but rest of filter will be more defocused compared to green and red where you also focus on center of range.

Here is achromat curve - and while it is is much worse than in ED scope - it shows the same pattern - difference in focus between "center line" and "edges" for all three filters.

- third problem is spherochromatism. Fact that spherical aberration depends on wavelength as well. This is much more pronounced with small apertures for imaging where airy disk size is close to seeing limit when we image.

Look at this image:

This is from APO triplet - green lines are straight - no spherical. Red lines are bent outwards and blue bent inwards. One is over corrected and other under corrected for spherical aberration. That blurs things. In ED doublet things are going to be worse than above.

Those are reasons - you probably have combination of them. Now onto the solutions:

1. Since your Ha is very good in sharpness - use that as luminance layer. Eye is much more sensitive to bloat in brightness than in color. Compose HaRGB image to get color you want - and then only use Luminance as brightness. This is easily done in Gimp by pasting stretched Ha as layer over color image and then setting that layer to Luminance (last layer option)

2. As suggested above by @The Lazy Astronomer - get additional filter that will cut off most offending part of blue spectrum 400-420nm range for example. Astronomik L3 is good solution if you have issues with Luminance as well

 

[Mr Thingy]

Many thanks for taking the trouble to explain in so much depth @vlaiv. It helps immensely to understand the source of the problem.

 

[alacant]

2 hours ago, Mr Thingy said:

My scope is a SW 72ED

Hi

Here's our version, also with a 72ed. But yeah, the halos around bright stars are awful compared to that obtained with our reflectors.

Cheers

 

[vlaiv]

3 minutes ago, alacant said:

Hi

Here's our version, also with a 72ed. But yeah, the halos around bright stars are awful compared to that obtained with our reflectors.

Cheers

 

That sort of looks like atmospheric scatter rather than above. Was it hazy on that particular night?

[alacant]

3 hours ago, vlaiv said:

hazy

For such a cheap telescope, we thought the detail was good. Stars rubbish!

@Mr Thingy are your stars worse? Could you post an example? We don't know what to expect from the 72ed.

Cheers

 

Edited February 19, 2021 by alacant

[wavydavy]

I have the same issue using my RGB filters using baarder 1.25". The blue is terrible compared to the red and green.  My camera is an Atik 314L mono,  the blue channel is terrible compared to the red and green. 

[Mr Thingy]

4 hours ago, alacant said:

For such a cheap telescope, we thought the detail was good. Stars rubbish!

@Mr Thingy are your stars worse? Could you post an example? We don't know what to expect from the 72ed.

Cheers

 

I plan to process the images shortly and will share here once done.