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Help needed with processing M42


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Hi,

I've just acquired an Optolong L-Pro filter for my DSLR and tried it out on M42 recently. The individual subs looked good with noticeable reduction in sky glow. I took 100x 15 sec exposures (25 minutes in total) at ISO800 and stacked them in DSS. I then processed the stacked file in Affinity Photo using a colour preserving tone stretch and then removed the background gradient as much as I possibly could. 

What do you think is causing the vertical streaking across the image? The only calibration frames used were biases (I didn't take flats and you can see some dust motes on the left hand side).

Thanks in advance for any help.

Steve 

 

M42.jpg

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16 hours ago, Stuart1971 said:

Yes deffo walking noise, Darks would also help a lot, you really need them with a DSLR as it’s uncooled, especially if you don’t or can’t dither…👍🏼

Do I need to be guiding before I can dither? The image above was taken without any guiding. 

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Yes, I often dither without guiding: I use the mount controls to move a little in a random RA and Dec direction between each frame (or between every few frames if I'm feeling more lazy than normal :) ). Very little movement is required, just a few pixels as far as the camera is concerned - just make sure the movements are in varying directions.

Regards, Mike

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17 minutes ago, Steve143 said:

Do I need to be guiding before I can dither? The image above was taken without any guiding. 

That depends on the software used, with NINA there is an inbuilt guider which allows you to dither, you are not guiding but the software thinks you are and sends the dither commands, so that would work perfectly for your situation….

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  • 3 weeks later...

A bit late to the party here. I'm not convinced that what we see here is walking noise. The lines are all the way and vertical, which imo, is a sign of the camera's read pattern. A 15 s exposure time is very short, so the read pattern may very well show. But there is a very easy test that can decide. Flip (blink) between the first and last unaligned subexposures. If the images are offset in the same direction as the lines, then it's walking noise. But if the two images are offset in another direction, then what you see here is read noise. You can also examine the pattern of stretched, stacked bias frames. These show either a vertical or horizontal read pattern.

The dark shadows are caused by dust motes, and you need flats to remove them. Dithering with a dslr camera is highly recommended. This is easiest done with a guiding program, but there are a few ways to achieve dithering without guiding. One way is to do it manually. (Been there, done that, invested in a guiding setup after one season.)

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Like Wim, I immediately thought that this did not resemble walking noise at all. The main reason for this is that it doesn't 'walk.' It's a fixed pattern, most obviously creating vertical banding but also showing horizontal banding on the same approximate pixel scale if you look closely.  For me, walking noise takes the form of erroneous pixels walking across the image, almost certainly at an angle. 

Since this image was taken unguided it must have some inherent dither, how much depending on the polar alignment and the periodic error.  The OP might try taking the first and last images of the run and stacking one on top of the other as they come from the camera. That's to say, stack them by aligning the edges of the images, not by aligning the stars. This will show the extent of the drift over the run and give an idea of the extent of the inherent dither. If it were not quite significant I'd be amazed.

If the DSLR is a Canon, I think Pixinsight has a routine for 'reducing Canon banding' or something like that. I'm not sure because I don't image with a DSLR. I do know that 'Canon banding' has been identified and discussed, though.

Olly

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On 31/03/2022 at 18:21, Steve143 said:

vertical streaking

Hi

The streaking is almost all in the red. The bias subtraction doesn't seem to have worked.  A quick tweak fixes it.

A few other bits and pieces which may also help...

With the filter in place, it would be a good idea to separate and recombine channels anyway.

Flat frames aren't really optional and help ease processing a lot. Dark frames however are likely to make matters worse. We'd also recommend more and longer frames to stack; if your unfiltered frames were 15s, increase that to at least 60s with the filter 

Get the bias sorted out along with trying some of the above and you'll almost certainly find it easier to process.

You may wish to simply subtract the bias without removing a stack full of images. Don't know which camera you used but on our 700ds 2048 works fine and gives clean images. I'll include a screen of where to include that value. Even if you haven't purposely dithered, stack with a modern clipping algorithm anyway.

Cheers and HTH

ss_1.thumb.png.22dfd7b284e126c482f945a1c97be6ff.png

ss_2.thumb.png.5ccf040191d50aa962875cc3ba9806e6.png

ss_4.thumb.png.fb3ae3e3c9f75376d4f53110f31a9779.png

ss_3.thumb.png.1a567b156594920942d62f6b2ca37cab.png

Edited by alacant
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On 18/04/2022 at 19:35, wimvb said:

Flip (blink) between the first and last unaligned subexposures. If the images are offset in the same direction as the lines, then it's walking noise. But if the two images are offset in another direction, then what you see here is read noise.

I checked the first and last subexposures and there is a small difference where the FOV has moved slightly downwards - but not by very much. 

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On 18/04/2022 at 19:35, wimvb said:

Dithering with a dslr camera is highly recommended.

I currently use Backyard EOS to control the camera and I think I can dither with that. Haven't tried it yet but I've installed the ASCOM drivers so I can attach my SW EQM-35 mount to Backyard EOS. 

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6 hours ago, alacant said:

Flat frames aren't really optional and help ease processing a lot.

I did take some flat frames which helped with the dust motes. 

7 hours ago, alacant said:

We'd also recommend more and longer frames to stack; if your unfiltered frames were 15s, increase that to at least 60s with the filter 

I do need more frames but kept the exposures short so as not to blow out the core of M42. 

I will try the offset (my camera is a Canon 70D), but I did stack the original subframes, along with biases and flats, in DSS using the sigma clipping algorithm and then tried processing in Siril. The image below is what I managed to get which looks a little better but still not perfect (still very noisy but the core isn't as blown out as my original effort). I've still got a lot to learn from image acquisition all the way through to image processing, but really appreciate all the help and support from everyone. Thanks.

 

M42 v3.jpg

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The other issue that I've noticed with this image are the elongated stars in the corners. I'm using an Altair Starwave 80 ED-R refractor with the Starwave 0.8 V3 Reducer & Rotator connected to my Canon EOS 70D with the T2 and M48 adaptors. 

Is this issue caused by an incorrect optical train length with my set up from the reducer to the camera sensor?

Or could the grub screws holding the rotator and reducer together be unevenly screwed in so causing a distortion when I screw the unit in the rear of the telescope or when I screw the T2 adaptor on before attaching my camera?

Thanks for any help or suggestions. 

Steve  

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6 minutes ago, Steve143 said:

elongated stars in the corners

Hi

In this case, it is because the corrector is too close to the camera sensor. Easily fixed.

HTH

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1 hour ago, alacant said:

Hi

In this case, it is because the corrector is too close to the camera sensor. Easily fixed.

HTH

Where can I find the correct length needed so I can get the right adaptor to increase the length?

Thanks. 

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15 minutes ago, Steve143 said:

correct length

IIRC, it's 63mm, sensor to shoulder of FF. 

If you have the rotator on the telescope side, my guess is you're 8mm short. A photo may help.

HTH

Edited by alacant
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1 hour ago, alacant said:

IIRC, it's 63mm, sensor to shoulder of FF. 

If you have the rotator on the telescope side, my guess is you're 8mm short. A photo may help.

HTH

Yes, according to the Altair web site it is 63mm. The rotator is on the telescope side and I've attached a couple of photos.

Thanks.

 

Camera setup 1.jpg

Camera setup 2.jpg

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47 minutes ago, Steve143 said:

a couple of photos

Yeah, OK. You're too close. Get a set. Combinations of the 3, 5, 7 and 10mm from here would get you there.
Always good to have spare tubes;)

HTH

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On 20/04/2022 at 20:22, Steve143 said:

I did take some flat frames which helped with the dust motes. 

I do need more frames but kept the exposures short so as not to blow out the core of M42.

To get around the problem of blowing out of the core of M42 I took short exposures but I also took long exposures for the dimmer parts.  For M42 you need both long and short exposures.  They can all be blended so you get the detail in the core and detail plus less noise in the fainter parts.

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