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M33 - 130dps - odd light artifact


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On 20th Dec we finally had a bit of clear skies. Because it was pretty windy I set up the SW 130dps & set about taking subs of M33. It wasn't the target I had in mind, so during set-up I had the DLSR (Canon 600d astromodified) rotated almost 90 deg to my normal orientation... (and so the Galaxy wasn't framed well).

On initial stack of the data, I noticed each of the light frames had a line which looks like some light leakage or reflection, which I haven't seen before. Any ideas what this might be? I had a black shower cap on the primary mirror end of the scope (which blew off half way through the session) & I keep a cover over the camera viewfinder. The artifact is in the same position on all of the images, and thus appears smeared in the stacked composite. (Guiding also wasn't working properly as I failed to redo the calibration after needing to do a meridian flip  after the first 3 subs).

I've managed to remove the artifact in processing using StarNet, but I'd still like to know what the problem was.

Cheers
Ivor

odd_line_single_exp.jpg

odd_line_stack.jpg

m33_full.jpg

m33_crop.jpg

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It could be a diffraction streak. Any straight line or edge in front of the scope can cause this. Even a washing line for example.

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1 hour ago, matt_baker said:

Do you dither?

Yes - between each exposure, though it sometimes times out & on this occasion after the meridian flip PHD stopped guiding in Dec. (It was also the first time out with the guide-scope on the 130dps)

Cheers
Ivor

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44 minutes ago, wimvb said:

It could be a diffraction streak. Any straight line or edge in front of the scope can cause this. Even a washing line for example.

Thanks @wimvb - I guess that is a possibility, although I would then expect it to have moved over the course of the evening relative to the camera image. As far as I can tell it's in the same place on all the subs & only appears smeared on the stacked image, once the subs have been star aligned.

I certainly have problems with low washing lines though & have walked into them in the past!

Cheers
Ivor

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  • 2 weeks later...
On 04/01/2021 at 17:56, Aramcheck said:

On 20th Dec we finally had a bit of clear skies. Because it was pretty windy I set up the SW 130dps & set about taking subs of M33. It wasn't the target I had in mind, so during set-up I had the DLSR (Canon 600d astromodified) rotated almost 90 deg to my normal orientation... (and so the Galaxy wasn't framed well).

On initial stack of the data, I noticed each of the light frames had a line which looks like some light leakage or reflection, which I haven't seen before. Any ideas what this might be? I had a black shower cap on the primary mirror end of the scope (which blew off half way through the session) & I keep a cover over the camera viewfinder. The artifact is in the same position on all of the images, and thus appears smeared in the stacked composite. (Guiding also wasn't working properly as I failed to redo the calibration after needing to do a meridian flip  after the first 3 subs).

I've managed to remove the artifact in processing using StarNet, but I'd still like to know what the problem was.

Cheers
Ivor

odd_line_single_exp.jpg

odd_line_stack.jpg

m33_full.jpg

m33_crop.jpg

Nice M33 but I'd adjust the colour balance, myself. It looks very green as it is.

Olly

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Coming rather late to this, but it appears just possible that the artefact is a reflection in some part of the imaging train from the bright star alpha Tri.

Here's the final image with the artefact image overlayed superimposed on the neighbouring starfield:

714861056_M33overlay.jpg.f6b499c766b6907e7ff3d1102de4b415.jpg

It does seem rather a long way for a diffraction spike, perhaps try a similar exposure with a longer dew shield to see if it goes away?  Or maybe not, given the lack of clear skies at present.

Edited by almcl
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Hi everyone

I posted something similar last year (I'll find the post later) but here it is again this year. The streak is not fixed on every frame and all but disapppears after a meridian flip. Nor does it appear on every target. In fact I think it was this target last year and I haven't seen it since. Even a harsh sigma clip won't shift it entirely.

130pds: 3 minute cr2->fits->jpg frame from eos700d

ss1.jpg.e9ae4f0bff18fe1443c6a4390b5c6283.jpg 

Must admit to liking @almcl's theory of nearby but out of the FOV bright stars as there is also a second fainter streak on the other side of the galaxies to the one which is more prominent. 

ss2.thumb.jpg.7f7b356bbcf8d4fd994c51d3347cc4e3.jpg

Edited by alacant
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Thanks folks! I'm not entirely convinced that it's a stray diffraction spike, given the distance but I think I'll have to wait & see... It's nearly a month now since we had any suitable clear skies! I still suspect that it was something to do with the camera orientation, so will have to have a play around if the problem occurs again. BTW I have flocked the OTA, but haven't painted the outer focus tube black yet.

@ollypenrice - Thanks - I find trying to get the colours look right is always a bit problematic. I use Pixinsight's 'Photometric Colour Calibration' but still have to adjust the R-G-B histograms if I use the ArcSin stretch (my preferred option now on the RGB as I tend to extract & process Luminance separately).

Thanks again everybody.
Cheers
Ivor

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34 minutes ago, Aramcheck said:

 

@ollypenrice - Thanks - I find trying to get the colours look right is always a bit problematic. I use Pixinsight's 'Photometric Colour Calibration' but still have to adjust the R-G-B histograms if I use the ArcSin stretch (my preferred option now on the RGB as I tend to extract & process Luminance separately).

Thanks again everybody.
Cheers
Ivor

I use that, too, if I'm struggling and sometimes it works but sometimes it's right up a gum tree. Maybe my fault. Did you try SCNR green on this? I suspect it would work a treat.

Olly

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On 15/01/2021 at 11:26, alacant said:

 

 

Must admit to liking @almcl's theory of nearby but out of the FOV bright stars as there is also a second fainter streak on the other side of the galaxies to the one which is more prominent. 

 

 

I won't say it's impossible but out-of-shot bright star flare has never produced anything like that for me. What I get is a widening, streaky beam like a light house beam in fog and it starts at the edge of the frame and widens and fades as it enters the image. The fix I've found effective is to aim the mount closer to the star causing the issue. If possible, I get it just in the frame while leaving the problem beam area also in the frame. I shoot a quick set of 'patch-repair' subs in which the flare has disappeared and I use the part overlapping the original image to effect a repair.

Olly

 

 

 

Edited by ollypenrice
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On 16/01/2021 at 17:02, ollypenrice said:

Did you try SCNR green on this?

I did, but I'm not sure whether it helped... I also discarded an earlier version which looked too blue to my eyes.

Cheers
Ivor

M33_SCNR.JPG

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