is this Chromatic aberration?

[Dan13]

Hi, ive noticed since ive had my new refractor (starwave 80ED-R FPL53) ive been getting the stars below,especially the bigger brighter ones.

On the whole the scope is really nice and im getting imo some good images, but this is starting to concern me a little and also effecting processes like star net ++ for starless images as the bright ones wont completely be removed.

so is this Chromatic aberration or something else i need to look at ?

many thanks

Dan

[rl]

Difficult to say from a screenshot, but that looks like over-exposure to me. You might have some focussing isssues as well. 

Is the image a crop or the whole frame? What camera and exposure time were you using? What is the target? Were you using any filters?

An ED scope with FPL53 is unlikely to have objectionable chromatic aberration unless there is something seriously wrong with it. 

Some doublets do show minor CA occasionally referred to as blue bloat..but that looks a bit excessive! 

[Dan13]

1 minute ago, rl said:

Difficult to say from a screenshot, but that looks like over-exposure to me. You might have some focussing isssues as well. 

Is the image a crop or the whole frame? What camera and exposure time were you using? What is the target? Were you using any filters?

An ED scope with FPL53 is unlikely to have objectionable chromatic aberration unless there is something seriously wrong with it. 

Some doublets do show minor CA occasionally referred to as blue bloat..but that looks a bit excessive! 

Hi, 

 

1 minute ago, rl said:

You might have some focussing isssues as well

i focus with a B mask as dont have an auto focusser , everything seems ok but thats just to me so could be still out?

1 minute ago, rl said:

2 minutes ago, rl said:

Is the image a crop or the whole frame? What camera and exposure time were you using? What is the target? Were you using any filters?

this is zoomed in on a full image of the cygnus wall this pic here , exposure time was 240 secs (im actually thinking of dropping it a little) cam is an asi 294mc pro @ -10, filter was an optolong L-enhance or L-pro ill have to check

 

 

[Dan13]

For more info this is the final stacked image with just DBE carried out and an STF nothing else. star in question is top left 

[rl]

That's a nice shot seen in context.

Assuming the Bahtinov mask is matched to the scope then the focussing should be spot-on..in which case then yes, you're seeing residual CA, which is a little bit strange because it's not the fastest 80mm doublet out there. The slower ones usually have CA better controlled.  Is yours the original Starwave 80ED? I note they now advertise a Starwave 80ED-R which claims "better colour correction in the blue regions of the spectrum" which probably tells you something.

https://www.altairastro.com/starwave-80ed-r-ed-doublet-refractor-telescope-466-p.asp

 

[Dan13]

29 minutes ago, rl said:

That's a nice shot seen in context.

Assuming the Bahtinov mask is matched to the scope then the focussing should be spot-on..in which case then yes, you're seeing residual CA, which is a little bit strange because it's not the fastest 80mm doublet out there. The slower ones usually have CA better controlled.  Is yours the original Starwave 80ED? I note they now advertise a Starwave 80ED-R which claims "better colour correction in the blue regions of the spectrum" which probably tells you something.

https://www.altairastro.com/starwave-80ed-r-ed-doublet-refractor-telescope-466-p.asp

 

Thank you, yes the Mask is mayched for the scope, they were baught together from Altair.

Its not the original its the new one the 80ED-R that you posted a link too. You think that blue hue is to do with that then? 

Its more the size of the of the stars, I do think there a little bloated .

[rl]

I guess this is the  reason people invented triplets...the colour correction is better, especially for photography.

I'd try halving the sub time just to see if it's some sort of saturation issue on the camera. A luminance filter that cuts off sharply in the violet might help.

[Dan13]

17 minutes ago, rl said:

I guess this is the  reason people invented triplets...the colour correction is better, especially for photography.

I'd try halving the sub time just to see if it's some sort of saturation issue on the camera. A luminance filter that cuts off sharply in the violet might help.

Yea im going to go to maybe 180 secs and see whats thats like for my next image, shame because it handles 240 really nicely bar this issue. I have often read that less exposure on the cooled cams is better which always stumped me because there cooled you would think longer?!

[vlaiv]

I'm not seeing much of chromatic aberration in the image above to be honest.

If you are referring to the flaring of the bright stars - that is something that happens with refractors and can be due to either atmospheric scatter or in refractors to polish of the lenses.

It happens on much more expensive scopes, so nothing to worry about.

Like said - you might have a bit of issues with focusing or maybe seeing was particularly poor on that night. Maybe your guiding was not spot on, what ever the reason - stars are not quite pin point.

If you suspect that you might have some residual color - try using Astronomik L3 or at least L2 as IR/UV cut filter.

Just realized - image is titled drizzle_integration. I don't think that you need to use Drizzle - that might be reason for stars not being pin point.

[rl]

Cooling the camera allows longer exposures because the dark counts drop by half for every 7-10C cooling. But the maximum capacity of the pixels remains unchanged.....if a pixel saturates out at 20,000 captured photons at 20C it will still saturate at 20,000 photons even at -20C. Once a pixel is full that's it....you're only option is to allow in less light. 

[rl]

It's worth keeping this in context; the star in question is Deneb which is 1st magnitude and very blue..the B-V colour index is +0.1 all of which means a lot of blue light. It's a tough test for any doublet system when you're trying to bring out faint nebulosity. 

[Dan13]

11 minutes ago, vlaiv said:

It happens on much more expensive scopes, so nothing to worry about.

I guess in a way thats

 

11 minutes ago, vlaiv said:

Like said - you might have a bit of issues with focusing or maybe seeing was particularly poor on that night. Maybe your guiding was not spot on, what ever the reason - stars are not quite pin point

guiding that evening was between 0.48 and 0.65 for my HEQ5 pro id consider that good so im ruling that out, the focus issue is a pain as with the mask ive never really had too much issue, maybe i do need an auto focusser after all  

 

any reason why not use drizzle? ive always seen good results with this?

 

any idea why starnet wont remove all stars? seems to be only with the big bright blue ones such as the example ive shown?

Edited August 3, 2020 by Dan13

[Dan13]

9 minutes ago, rl said:

Cooling the camera allows longer exposures because the dark counts drop by half for every 7-10C cooling. But the maximum capacity of the pixels remains unchanged.....if a pixel saturates out at 20,000 captured photons at 20C it will still saturate at 20,000 photons even at -20C. Once a pixel is full that's it....you're only option is to allow in less light. 

Makes sense! im going to try 3 mins or less, M31 is my next target so want to keep lower EP time as i dont want to blow the core anyway. yes that star is particularly blue and big i must admit! ill check some of my last targets and see if im getting the same thing.

[vlaiv]

14 minutes ago, Dan13 said:

guiding that evening was between 0.48 and 0.65 for my HEQ5 pro id consider that good so im ruling that out, the focus issue is a pain as with the mask ive never really had too much issue, maybe i do need an auto focusser after all  

0.48 to 0.65 of what units?

0.48" to 0.64" (in arc seconds) is exceptional thing and requires belt modded and tuned HEQ5 mount.

0.48 to 0.65 of pixels (default unit if you don't tell it convert to arc seconds) - well that can be poor, average, good, exceptional - depends on guide camera pixel size and guide scope focal length.

16 minutes ago, Dan13 said:

any reason why not use drizzle? ive always seen good results with this?

Any reason to use drizzle? Drizzle is algorithm developed for cases where you have undersampling and you want to try to recover missing resolution. In 99% of cases with amateur setups, one won't be having undersamping and I don't think that anyone is going to do research grade data reduction that requires restoration of missing resolution.

On the other hand, when done properly, drizzle algorithm reduces your SNR, and why would you like to do that when the name of the game is get the best SNR you can.

Reason why your stars look fatter than they should is because you drizzled - you increased pixel scale without effective resolution gain (you were not under sampled to begin with).

When viewed 1:1 this is what your image looks like:

Let's suppose you did x2 drizzle, that means that your image should look like this:

Ok, now we are talking about nice looking stars rather than having them big and round.

22 minutes ago, Dan13 said:

any idea why starnet wont remove all stars? seems to be only with the big bright blue ones such as the example ive shown?

Starnet++ is neural network platform and as such - it is not 100% effective. It only "knows" how to deal with data that has been "taught" to deal with. If your image is too much different that data set that has been used to teach starnet - it will fail. People having newtonian scopes have issues because starnet++ does not know how to deal with diffraction spikes, for example.

One of reasons why Starnet++ might be failing is drizzle. Try to do regular integration without drizzle and see if that improves things with starnet++

 

[Dan13]

9 minutes ago, vlaiv said:

0.48" to 0.64" (in arc seconds) is exceptional thing and requires belt modded and tuned HEQ5 mount.

apologies i should have stated, yes in Arc seconds. i have belt modded my mount and also tuned with new bearings etc a couple weeks ago.

 

10 minutes ago, vlaiv said:

ny reason to use drizzle? Drizzle is algorithm developed for cases where you have undersampling and you want to try to recover missing resolution. In 99% of cases with amateur setups, one won't be having undersamping and I don't think that anyone is going to do research grade data reduction that requires restoration of missing resolution.

On the other hand, when done properly, drizzle algorithm reduces your SNR, and why would you like to do that when the name of the game is get the best SNR you can.

Reason why your stars look fatter than they should is because you drizzled - you increased pixel scale without effective resolution gain (you were not under sampled to begin with).

I believe i am slightly under sampled depending on the seeing in a given evening, asi 294mc pro and the 80ED-R on astro tools suggests this? ive usually used drizzle due to dithering and killing some walking noise. maybe ill tryu run the stack again without drizzle and see the outcome

 

12 minutes ago, vlaiv said:

Starnet++ is neural network platform and as such - it is not 100% effective. It only "knows" how to deal with data that has been "taught" to deal with. If your image is too much different that data set that has been used to teach starnet - it will fail. People having newtonian scopes have issues because starnet++ does not know how to deal with diffraction spikes, for example.

One of reasons why Starnet++ might be failing is drizzle. Try to do regular integration without drizzle and see if that improves things with starnet++

thats a fair comment and observation, i genuinely believed it was due to how potent the stars were and it just cant ris then, again ill try a stack with same data without drizzle. and yes its 2x drizzle ive been using 

[vlaiv]

2 minutes ago, Dan13 said:

apologies i should have stated, yes in Arc seconds. i have belt modded my mount and also tuned with new bearings etc a couple weeks ago.

Excellent! I also have Heq5 that I tuned and belt modded myself and indeed it runs at about 0.5" RMS when sky plays along (had it once at 0.38" RMS!!).

3 minutes ago, Dan13 said:

I believe i am slightly under sampled depending on the seeing in a given evening, asi 294mc pro and the 80ED-R on astro tools suggests this? ive usually used drizzle due to dithering and killing some walking noise. maybe ill tryu run the stack again without drizzle and see the outcome

Astro tools gives general advice and I don't agree with some of their calculations and recommendations.

One of those being sampling rate. Here is example:

In no universe will any amateur with scope less than 8" benefit from sampling rate of less than 1"/px, but they say 0.67"/px is fine.

For 2-4 FWHM seeing and 80mm scope, even with excellent guiding of 0.5" RMS, star FWHM that you can expect will be 2.71" to 4.4". With those star FWHM, sampling rate should be (FWHM/1.6) in range of 1.7"/px to 2.75"/px and not 0.67"/px-2"/px

You are at lower bound of that so there is a chance that you are slightly oversampled rather than undersampled. General rule is that for 80mm scope you want to be at around 2"/px.

If you wish, we can go into a bit more detail on sampling resolution and FWHM and all of that so I can explain reasoning behind what I've just written but it is a bit technical,

 

[Dan13]

1 minute ago, vlaiv said:

Excellent! I also have Heq5 that I tuned and belt modded myself and indeed it runs at about 0.5" RMS when sky plays along (had it once at 0.38" RMS!!).

there cracking mounts for the price when tuned for sure! i love mine.

 

2 minutes ago, vlaiv said:

n no universe will any amateur with scope less than 8" benefit from sampling rate of less than 1"/px, but they say 0.67"/px is fine.

For 2-4 FWHM seeing and 80mm scope, even with excellent guiding of 0.5" RMS, star FWHM that you can expect will be 2.71" to 4.4". With those star FWHM, sampling rate should be (FWHM/1.6) in range of 1.7"/px to 2.75"/px and not 0.67"/px-2"/px

You are at lower bound of that so there is a chance that you are slightly oversampled rather than undersampled. General rule is that for 80mm scope you want to be at around 2"/px.

If you wish, we can go into a bit more detail on sampling resolution and FWHM and all of that so I can explain reasoning behind what I've just written but it is a bit technical,

def would like some more advise on this if possible that would be very helpful, i think i may have said under instead of over sorry, plus you would need to add my reducer into that equation which is 0.8x bringing  the F/L to 444

 

when i ran psf script in pixinsight i cant remember exactly what my FWHM was but it seemed a little high which made me feel my stars were slightly fat, im just running a new stack now without drizzle

[Ken82]

Looks good to me Dan, I take it your using a colour cam ?

could be a little bit of spherical aberration Giving you a slight halo but nothing to worry about. 

[Dan13]

2 minutes ago, Ken82 said:

Looks good to me Dan, I take it your using a colour cam ?

could be a little bit of spherical aberration Giving you a slight halo but nothing to worry about. 

Thanks Ken, i guess now im getting a lot more comfortable with my set up and astro as a whole im starting to see things that i didnt used to know were there if that makes sense. yes mate im using the asi 294 mc pro cooled cam OSC

[Dan13]

@vlaiv both stacked quickly in DSS using same settings except the one on the left is drizzle x2 and the right no drizzle. i think there identical so may rule this out....

 

 

[vlaiv]

9 minutes ago, Dan13 said:

def would like some more advise on this if possible that would be very helpful, i think i may have said under instead of over sorry, plus you would need to add my reducer into that equation which is 0.8x bringing  the F/L to 444

Ok, I'll be brief and to the point since this is rather technical, but here it goes:

- we can approximate resulting star FWHM from aperture size and corresponding Airy disk, seeing error and guiding error. You need to convert everything to "sigma" of corresponding Gaussian approximation. Guiding error is already given as that. Seeing is converted by dividing with ~2.35482 (that is two times square root of two times log of two - a conversion factor between the two FWHM and sigma for Gaussian) and Airy disk is converted to sigma by conversion factor of 0.42 / 2.44 - this is for Airy disk diameter. You take these three and calculate resulting sigma as square root of sum squares.

Multiply with 2.35482 to convert back to FWHM

- As for sampling rate, it is about Nyquist sampling theorem and star PSF gaussian approximation of certain sigma/FWHM. We can get that with following approximation: Fourier Transform of Gaussian is a Gaussian. We take a frequency of Fourier Transform that has value less than arbitrary threshold - for example 10% and we see what is sampling rate that corresponds to this frequency (twice max frequency).

If we do the math, it turns out that FWHM/1.6 is a good approximation as frequencies beyond that are attenuated more than 90% and hence can be neglected.

[vlaiv]

6 minutes ago, Dan13 said:

@vlaiv both stacked quickly in DSS using same settings except the one on the left is drizzle x2 and the right no drizzle. i think there identical so may rule this out....

They look the same - this means no improvement from drizzle, but they are not the same with respect to starnet++

Left image is zoomed in x2 while right one is zoomed in x4. As far as Starnet++ is concerned - stars are twice as large in left image than in right image.

[Dan13]

8 minutes ago, vlaiv said:

Ok, I'll be brief and to the point since this is rather technical, but here it goes:

- we can approximate resulting star FWHM from aperture size and corresponding Airy disk, seeing error and guiding error. You need to convert everything to "sigma" of corresponding Gaussian approximation. Guiding error is already given as that. Seeing is converted by dividing with ~2.35482 (that is two times square root of two times log of two - a conversion factor between the two FWHM and sigma for Gaussian) and Airy disk is converted to sigma by conversion factor of 0.42 / 2.44 - this is for Airy disk diameter. You take these three and calculate resulting sigma as square root of sum squares.

Multiply with 2.35482 to convert back to FWHM

- As for sampling rate, it is about Nyquist sampling theorem and star PSF gaussian approximation of certain sigma/FWHM. We can get that with following approximation: Fourier Transform of Gaussian is a Gaussian. We take a frequency of Fourier Transform that has value less than arbitrary threshold - for example 10% and we see what is sampling rate that corresponds to this frequency (twice max frequency).

If we do the math, it turns out that FWHM/1.6 is a good approximation as frequencies beyond that are attenuated more than 90% and hence can be neglected.

thank you, this may take me a couple of reads but appreciate the info

[Dan13]

7 minutes ago, vlaiv said:

They look the same - this means no improvement from drizzle, but they are not the same with respect to starnet++

Left image is zoomed in x2 while right one is zoomed in x4. As far as Starnet++ is concerned - stars are twice as large in left image than in right image.

EXCELLENT! you have def pointed out something ive not taken into consideration, so i just ran star net on both, and the "more zoomed" image def keeps more stars then the lesser so thats great news and im guessing i can figure that out from here...

 

question. i use pixinsight to pre process and i add the drizzle files into the image intergration stage, im guessing DSS does this for you if the box (x2 drizzle) is ticked?

[vlaiv]

4 minutes ago, Dan13 said:

thank you, this may take me a couple of reads but appreciate the info

I just realized that there is probably more down to earth way to explain things. For example first part relates how different things that blur image add up.

Here is intuitive way to understand it (and to try it out) - take any image and do gaussian blur with sigma 1.4 for example. Do another round of gaussian blur with sigma 0.8. Resulting image will be the same as if you did just one round of gaussian blur with sigma of not 2.2 (1.4 + 0.8) but rather of ~1.6 which is sqrt(1.4^2 + 0.8^2). So blurs add in quadrature and three main blur types are seeing blur, guiding blur and aperture blur.

In any case, it is a bit complicated stuff, so main point is - you can't really use lower sampling rate than about 1.7"/px when using 80mm of aperture and in most cases you should go for 2"/px - that is if you don't want to over sample.

5 minutes ago, Dan13 said:

uestion. i use pixinsight to pre process and i add the drizzle files into the image intergration stage, im guessing DSS does this for you if the box (x2 drizzle) is ticked?

I honestly don't know. I understand drizzle algorithm. I have my doubts if it works at all and how good it works in general case - but that needs further investigation. I have improvement on original algorithm that should work better if original algorithm works in the first place (yes, I know, it's funny to improve on something that you don't think works in the first place).

However, I don't understand how drizzle is implemented in pixinsight - so I can't comment on that one. I think that DSS implementation is straight forward, but interestingly enough - original algorithm calls for 2 parameters not one so I don't know how ticking x2 translates to two parameters.

Original algorithm asks for - resulting sampling rate and pixel reduction factor. x2 is directly related to resulting sampling rate. It will enlarge image x2 hence it increases sampling rate by factor of x2. However, you don't need to reduce original pixels by factor of x2 - you can reduce it more or less as per original algorithm and I have no idea what selecting x2 (or x3 in DSS) does with this parameter.