Ahh! Terrible Stars!

[OldFrozen]

Hello All!

Would anyone be able to look at this image and advise what could be causing the discoloration around (almost) all the stars? User error I am sure (and hope). This was taken with a William Optics Zenithstar 71 with a WO .8x reducer. Camera is a Sony A6000. Mixture of 45 & 60 second subs all @ iso100. Shot in raw (ARW) and converted to TIFF before stacking. This was stacked in DSS (I have also stacked in PI with the same resulting stars). All post work done in PI (still learning this software). Problem with Focus? Defect in the OTA? Bad settings when stacking frames? Spacing between focal reducer and sensor? SOS.

 

I appreciate your help!

 

EDIT: I have attached a copy of the autosave from DSS for reference.

Autosave.fts

 

 

[Tim]

Can you post a link to an unedited, untouched raw file please?

[OldFrozen]

2 minutes ago, Tim said:

Can you post a link to an unedited, untouched raw file please?

Of course! Please see attached & Thank you!

DSC08558.ARW

[wimvb]

What processing did you do in PI? the larger/brighter stars seem overstretched.

[OldFrozen]

 

2 minutes ago, wimvb said:

What processing did you do in PI? the larger/brighter stars seem overstretched.

Nothing too fancy:

AutoDBE, Background Neutralization, Color Correction, SCNR, ATrous Wavelet Transform, Histogram Transform. I did not autostretch with STF when doing the histogram transform as the autostetch was very severe.

 

[Tim]

Just had a quick look. The main problem stems from the out of focus colour elements of the stars, which will be a feature of your scope or a problem with reducer spacing. Telescopes with doublet lenses cant normally bring all three primary colours of light into sharp focus together. The odd colours and shapes you are experiencing are the result of the software trying in vain to make sense of the anomaly.

If you take a raw image, debayer it in PI, then use the channel extraction tool to compare the red green and blue channels you will see what I mean quite clearly.

Doublet telescopes can produce exceptional imaging results, but these are normally the produce of a camera with filters to gather light one wavelength at a time.

What can be done? Bright stars make the chromatic aberration look worse, so try to avoid those at least. By treating the stars separately, and with clever masking and filtering techniques in PI or photoshop you may be able to produce better results by manipulating the data you have to display in a way that is more appealing. 

If you really fancy astrophotgraphy and want to get great results with a colour camera (dont we all  ) then you may want to take a look at the Zenithstar's stablemate, the much more capable  Star 71 II

 https://www.firstlightoptics.com/william-optics/william-optics-star-71-ii-4-element-refractor.html

These telescopes offer better colour correction and a flat field too, by utilising 4 lenses instead of 2.

Hope that helps a bit,

Tim

[OldFrozen]

Tim,

Thank you very much for your well-written response! I suppose I have some decisions to make (processing techniques, target selection/ avoiding bright stars, and possibly selling the OTA). In the meantime, I will continue to enjoy using this doublet .

 

[alacant]

Hi. It looks like chromatic aberration to me, just as you'd find with old refractors. You budgeted a lot not  to have it, or at least not as bad as this! Maybe contact the manufacturer? HTH.

http://www.astrobin.com/148605/

[ollypenrice]

My first thought was the so called 'star eater algorithm' with some Sony cameras. The phrase Googles but I haven't been through the links. 

However, when I ran the raw data through ABE and then gave it a normal stretch in Ps (a log stretch generated by moving the grey point slider to the left) I couldn't see too much wrong with the stars so I suspect that the severe issue with the dark red rings is simply a processing artefact. In the images below I haven't tried to do a proper process, just a stretch to see if I got the same stellar issues, and I didn't.

We don't see perfect colour correction here and, as Tim says, there are better corrected optics available. However, the stark red inner rings seen in your original image clearly don't need to be there. They are certainly not inherent to the data.

Soft stretch:

Hard stretch:

 

By the way, the bright stars to the lower right of M42 are a challenge in any optics. I have always had to give them 'personal attention' even when imaging through Takahashis and a TEC140.

Olly

 

 

[ronin]

May well just be the scope +/- reducer.

The scope is a reasonably fast doublet, and is likely to be performing at it's limit. Minor aspect I know but the ED element is FPL51, FPL53 would have been a little better(if any). I would assume that the reducer also adds something to the problem. After all the scope objective was designed as a stand alone objective, the insertion of additional glass would not have been a factor. Finally the 2 lens of the objective may fall within individual specification but when together may simply may not be a good pairing = sheer chance.

WO have gone away from the fast scopes since the times of the ZS71, their FLT132 triplet is f/7, FLT98 is f/6.3, FLT151 is f/6.6 and the FLT110 is f/7.

So they are now producing FPL53 triplets slower then the FPL51 doublets. And there must be a reason for this.

It sound/reads as a list of problems but each is very minor individually. It is the addition of them all if they are present.

Do you get the same without the reducer?

 

[OldFrozen]

Thank you all for your input!!

Olly! Thank you! I ran the same process on a single ARW and came to the same conclusion.. Now I need to figure out what I am doing wrong to cause the red rings.. I get them in my stacked image using both DSS & PI. Fist thing I am going to do is skip the conversion to TIFF and load the ARW's in to DSS/ PI.

Ronin! I have yet to image without the reducer but I will be sure to try it out this week (weather permitting)!

[geordie85]

When saving the stacked image do you have apply changes or embedded ticked?

[wimvb]

Olly allready showed that your raw frames are ok. When I took the raw frame into RawTherapee, and applied the default debayer/stretch to it, results were bad. Otoh, in PixINsight I get the same result as Olly.

Your autosave is an entirely different beast. Here's a crop with a permanent stretch applied

Even unstretched, the bright stars are saturated. Did DSS apply a stretch??

I also wonder about the cause of the square  star halos. The only time I've seen these structures is when trying to make a star mask in PixInsight when the stars are undersampled. I know that DSS can make star masks, but I wonder if that could be related. Anyway, I would suggest that your restack the image in PI with default settings.

Hope this helps,

[OldFrozen]

9 hours ago, geordie85 said:

When saving the stacked image do you have apply changes or embedded ticked?

embedded is ticked

 

[OldFrozen]

1 hour ago, wimvb said:

Olly allready showed that your raw frames are ok. When I took the raw frame into RawTherapee, and applied the default debayer/stretch to it, results were bad. Otoh, in PixINsight I get the same result as Olly.

Your autosave is an entirely different beast. Here's a crop with a permanent stretch applied

Even unstretched, the bright stars are saturated. Did DSS apply a stretch??

I also wonder about the cause of the square  star halos. The only time I've seen these structures is when trying to make a star mask in PixInsight when the stars are undersampled. I know that DSS can make star masks, but I wonder if that could be related. Anyway, I would suggest that your restack the image in PI with default settings.

Hope this helps,

I do not believe DSS applied a stretch and I am certain I did not stretch the image in DSS.  I didn't apply any masks to the image in PI or DSS (still learning more about masking in PI before attempting).

I will be running a new stack shortly and will post the results!

 

 

Thank you!

[OldFrozen]

Edit: Removal of double post!

[OldFrozen]

Update:

Well, I have attempted to use image integration in PI and it will not accept Sony's .ARW. Below is the output from the console in PI when working on the dark frames.

 

I was really hoping it would accept Sony's raw file into PI natively. I will send the subs through DSS (ver 3.3.4 accepts .ARW) and will post the result along with the settings I used. 

 

* Incremental image integration disabled due to lack of file format support: DSLR_RAW

 

Camera: Sony ILCE-6000

ISO speed: 100

Shutter: 60.0 sec

Aperture: f/0.0

Focal length: 0.0 mm

Embedded ICC profile: no

Number of raw images: 1

Thumb size: 1920 x 1080

Full size: 6048 x 4024

Image size: 6024 x 4024

Output size: 6024 x 4024

Raw colors: 3

Filter pattern: RG/GB

Daylight multipliers: 2.671797 0.921530 1.242426

Camera multipliers: 2820.000000 1024.000000 1732.000000 1024.000000

Invoking: dcraw -4 -o 0 -w -q 1

Decoding Sony ILCE-6000 file (6024x4024 pixels, ISO=100, Exposure=60.00s): done

Loading raw image: done

Computing image statistics: done

MRS noise evaluation: \

** Warning: No convergence in MRS noise evaluation routine - using K-sigma noise estimate./

** Warning: No convergence in MRS noise evaluation routine - using K-sigma noise estimate.done

*** Error: C:/Users/My_Directory_for_images/DSC08725.ARW: Zero or insignificant signal detected (empty image?)

<* failed *>

 

 

 

[OldFrozen]

OK! Better & worse results! Major Vignetting now present but the topic of proper flats is for another time!

> Saved TIF right from DSS New_Arw.TIF

I took the stacked image over to PI. Applied the same tools as mentioned above minus ATrous Wavelet Transform.

New_Arw_ABE.tif

 

 

[OldFrozen]

This auto stf done mid-processing leads me to think that my spacing between the sensor and reducer is too great (blind assumption).

 

 

[ollypenrice]

There's a bit of graphics somewhere which shows the consequences of incorrect spacing. Sara has linked to it in the past. I don't see any sign of such distortions in your image, I must say, but I could easily be wrong. Why do you suspect the spacing?

Olly

[swag72]

As if by magic.......

This is the results if your spacing is too close.......

And this is what happens if your spacing is too much.....

[OldFrozen]

Thank you Olly & Swag! The pattern of the vignetting is what led me to the assumption but I will try to take some better flats next night out. Attached is one that has been STF'd & not cropped to see the total scale of vignetting. There is a flat frame present in this stack.

 

[wimvb]

I think that the dbe process may have gone wrong in your latest processed image. there is a very dark area between the orion neb and running man, as well as to the right of the orion neb. This looks very much like improper sample placement in dbe. Always make sure that samples are placed over background and not nebula or stars. If you find it difficult to avoid neb/stars, you can copy the image, stretch it hard and then use this for placing samples. Create a new instance of dbe in the workspace and apply this to the "real" image.

I rarely use abe for background correction, dbe gives me more control.

[ollypenrice]

Well done Sara!

Vignetting is quite different from field distortion. Vignetting arises from uneven illumination, which is inevitable, and gives a brighter centre and darker corners (on a rectilinear chip.) Incorrect chip spacing gives the stellar distortions shown in Sara's link. Your star shapes look OK to me.

Olly

[wimvb]

8 hours ago, OldFrozen said:

Thank you Olly & Swag! The pattern of the vignetting is what led me to the assumption but I will try to take some better flats next night out. Attached is one that has been STF'd & not cropped to see the total scale of vignetting. There is a flat frame present in this stack.

 

The vertical bands can be removed quite effectively by "CanonBandingReduction", a script in PI. But it is designed to remove horizontal bands, so you'll have to rotate the image 90 deg first.

The three first steps in processing this image, that I would do, are:

1. cropping to remove all stacking edges

2. CanonBandingReduction with the image 90 deg rotated (rotate back afterwards)

3. DBE with careful sample placement, avoiding the middle part with the nebulae completely

 

Good luck,