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Old 07-07-2017, 09:06 PM
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traveller (Bo)
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HELP: Stacking woes

I am pulling my hair out. Been trying to stack my M16 subs but the end result always seems to have elongated stars, even though the subs themselves looked ok.
I took the subs in three batches. Usually I just process them in one go and they are ok. Tried that elongated stars.
Next, I batch processed the subs into the three batches, then tried to align the FIT files, again, elongated stars.
What am I doing wrong?
I attached the images (batches 1,2 and 3 straight conversion) and then the aligned image of the three files for comparison.

Bo
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  #2  
Old 07-07-2017, 10:15 PM
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barx1963 (Malcolm)
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Bo
What program are you using to stack? I am now doing all that stuff in Pixinsight, getting far more consistent results than with DSS. Did you want me to have a crack at them if you Dropbox the subs and calibration frames?

Malcolm
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Old 07-07-2017, 10:33 PM
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rcheshire (Rowland)
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Bo. If you do decide to dropbox the files, I'll have a go as well.
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Old 08-07-2017, 04:25 PM
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traveller (Bo)
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Thanks Malcolm and Rowland,
Here is the Dropbox link (I hope it works..)
https://www.dropbox.com/sh/zpjembcg5...55o5c0Cja?dl=0
Cheers
Bo
PS: The first 30 images were done after the meridian flip and were the source of the troubles (I managed to stack the remaining 74 frames and got a good result).
I will post a proceed image later!
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Old 09-07-2017, 08:31 AM
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rcheshire (Rowland)
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Bo. No issues stacking. The stars are elongated in each corner - lens distortion. I'm sure someone more knowledgeable can tell you the type of distortion.

Stopping the lens down or using an external aperture mask in front of the objective, should improve things and give a flatter field. Otherwise, with bias dark and flat frames, the central portion is very nice.

Last edited by rcheshire; 10-07-2017 at 05:40 PM.
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Old 09-07-2017, 12:53 PM
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Slawomir (Suavi)
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I agree with Rowland. There is a significant field curvature causing star elongation in the corners, and on top of that there is some tilt in the imaging train as well. Bo, are you using a field corrector? If yes, spacing seems to be off by a few mm (too close or too far).
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Old 09-07-2017, 05:23 PM
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skysurfer
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What's wrong with these pictures ?
They look neat and clean.

But I am not a pixel peeper.
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Old 10-07-2017, 12:35 PM
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Quote:
Originally Posted by skysurfer View Post
What's wrong with these pictures ?
They look neat and clean.

But I am not a pixel peeper.
nothing really, I think the OP is confused. They ALL have the same corner distortion you get in any refractor/camera lens. Its always there in everything, and why people buy field flatteners but still thos are just a hardware cropping solution, the optics are round so the will always have distortion in the corners regardless. Solutions typically restrict the field of view to a rectangle in the middle of the round optics where distortion is minimal (not gone) but then you're also missing a lot more around it.
The registration of the subs looks out in the integrated shot.
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Old 10-07-2017, 12:43 PM
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traveller (Bo)
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Thanks all for your input.
Just to recap and answer some questions:
1. Yes, I use a flattener/reducer. Obviously the distance between the imaging chip and the flattener isn't right, hence the coma at the corners.
2. Yes the individual subs look ok as they are guided images (maybe half a dozen maybe slightly off due to guiding issues).
3. The problem is when the images are stacked and the resultant stack has elongated stars in the central region near the pillars. I am not worried about the corners as they get trimmed off in the final image.
4. I think the problematic images are those done after the meridian flip (when the pillars are upside down). when I stacked the other images before the flip, the stars are sharp in the middle. But if I stack those with the flipped images, the central stars are elongated.
Malcolm managed to stack all the images in PI but I haven't seen the final image to see the central stars.
Anyway, thanks for your comments so far and I will keep you posted re the central star elongation issue.
Bo
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Old 10-07-2017, 01:08 PM
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cant notice any elongation besides corners, cropping might help show, I'm at work and no imagine tools, just web display.

After mirror flip wont the image have a tiny change in focal length as a result so stacking sets would get slight doubling that youre seeing as trailing? isnt this why many cats have mirror locks? plus the vibration from mirror movement.
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Old 10-07-2017, 05:39 PM
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Bo.

Do you have bias and / or darks and / or flats to go with this set?

Last edited by rcheshire; 10-07-2017 at 05:50 PM.
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Old 11-07-2017, 11:30 PM
ErwinL (Erwin)
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Quote:
Originally Posted by sil View Post
After mirror flip wont the image have a tiny change in focal length as a result so stacking sets would get slight doubling that youre seeing as trailing?
In fact there is a slight mismatch in focal length between the two groups of images. The scaling factor is about 0.999.
But even if I don't unlock the scaling in my stacking software, I don't get elongated stars. Therefore, I believe the reason was an inaccurate rotation (in the range of 0.1 degrees) before stacking.

Erwin
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Old 12-07-2017, 10:12 AM
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Octane (Humayun)
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How many images are in your final stack?

I'd be happy to work with your complete data set (lights, bias, darks, flats) if you like.

H
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Old 12-07-2017, 08:37 PM
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traveller (Bo)
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Thanks Rowland and H,
I added the FIT files of Dark, Bias and Flats into the dropbox link.
Cheers
Bo
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Old 13-07-2017, 06:01 PM
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Bo. Do you have 3 of each calibration RAW frames. I seem to be having difficulties with the fit files.
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Old 13-07-2017, 09:34 PM
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Me to: I'm not sure how to exacty apply the FITS calibration frames. Also, the frames seem to introduce some banding noise.
So I have calculated an auto-hotpixel-dark from the lights which removes the hotpixels quite well when subtracted as a raw dark.
Those who can use 16-Bit PNGs as raw darks and like to try it out, can get it here: https://www.dropbox.com/s/xj00blp3l1...k_raw.png?dl=0
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Old 14-07-2017, 01:35 PM
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traveller (Bo)
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The calibration files were auto saved as FIT files, I will see if I can stack the calibration subs into a raw file.
Cheers
Bo
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