Filroden

So the best way to reduce noise is...drum roll...more data. You only have 29 minutes of data using relatively short subs where read noise will have a greater impact. Although the nebula is bright and you have a lot of aperture, you will always have noise, even using the best stacking algorithms etc. Some post-processing noise reduction (using masks to apply the effects more strongly in the weaker signal areas) will help but the reduction is likely to be as noticeable as the noise.

I've never noticed any issues with my flats due to focus changes. On closer inspection, maybe the gradients are all pollution related rather than flats? It's just that each image appears to be vignetting. Here's a crop with the gradient removal and saturation boost. Pre-processed low res JPG isn't the best starting point but it should show you there is colour hidden in there.

I have to disagree. I took a closer look at your jpg of M33. After a quick background gradient removal to remove the worst of the vignetting/light pollution, I only needed to do two boosts to the saturation to see a pale yellow core and lovely blue arms. I think your problem is not the data but the calibration. I can't stress enough how important flats are to an image. I was terrified by them when I started. However, they are actually really simple to take and can be done the next day if you are careful. Here's what I do: - When I tear down the kit for the night I am careful to remove the scope/camera as a single unit (leaving it at its last focused position) and I take it all inside and leave it on the dining room table - The next morning I set up the tripod/mount again and put the scope/camera back on the mount carefully (so as not to disturb any dust) - I point the scope at any bright area of sky (cloudless or uniform cloud both work) - I fold about 4 pieces of white A4 paper of the end of the scope to reduce the sky brightness and act as a natural diffuser (white cotton also works but I like paper as I can fold it above and below the scope so it almost holds itself to the scope) - I then take flats, exposing them until they peak at about 50% of the histogram - typical exposures for me are about 0.1 seconds and I take 25 for each filter - Having captured the flats I bring it all back inside (you could do it outside but why be cold?) - I then take corresponding dark frames of the same exposure lengths as the flats (again, 25 flat-darks should be ok) [Note: for DSLRs you might skip darks and only use bias to subtract] The flats and flat-darks can then be added into your processing software of choice along with your darks and lights Voila. a much better starting point to begin processing.

That is a noticeable improvement in quality! The gradient is gone so the image colours look far more natural. You have some noise creeping in on the upper right of the image. Probably temperature/calibration related. I don't use GIMP but you might want to see if it has a despeckle function like Photoshop which helps remove single pixel noise like that. This hobby is all about finding those improvements. I never stop learning (and hopefully improving). Maybe not as fast as I would like but the weather prevents me from imaging

Nice to see some Southern objects on here! I'd be careful with darks on a DSRL unless it is cooled. You're more likely to add noise rather than remove it. I'm guessing from the target you're imaging from the heights of summer and probably somewhere relatively hot, even at night. DSLR's run quite hot without the addition of air temperature and unless you can exactly match temperatures of your darks to your lights you could make the image work. I'd recommend testing bias frames only. Flats will make a huge difference. You have quite a gradient though not quite centred (it seems to be centred in the SW quadrant). I notice the stars are elongated in that quandrant too. Do you have tilt in the system?

I recommend mid altitudes (45-60) as alt-az mounts find it harder to track closer to the zenith, and the Evo mount doesn't have a generous clearance so you often can't even reach higher altitudes (or I couldn't with my refractor/camera).

I think it’s because NAN would be in the west and at a mid altitude. Almost the perfect location to avoid field rotation. The Evo mount can easily track up to 120 seconds at that focal length so just pick your targets to make best use you have a strong image though I think you still have a gradient. It looks brightest in the centre so my first thought was whether your flats were working but I think it’s probably just stubborn light pollution. I don’t know if there are any gradient removal tools in Gimp or Android PS, but it might help lift the Nebula.

Isn't it great seeing an immediate improvement to the image when you add more data? Just the difference of 1 extra hour can really help the image. I can only say the image quality and ease of processing get better and better with more data! And one tip (it's becoming a mantra for me): there's a lot more colour in your image than you're showing in your processing. I think you may be over exposing your stars so any colour is being washed out. To illustrate, I took you image and applied an "increase star colour" action in Photoshop and then really (over)boosted saturation to show what information is hiding. This isn't the right way to process it. Better not to lose it than to try and recover it!

This is probably the wrong thread to discuss planetary/solar imaging as these are easily accomplished on alt/az mounts (probably even easier than EQ as setup is quicker). This thread is more about achieving longer exposure images, hence the suggestion of tools such as Deep Sky Stacker (alignment and stacking) and StarTools (processing). Registax is a very good piece of software for aligning, stacking and processing planetary images. Nonetheless, I'm glad you shared the image. It's beautiful :)

I forgot to say. As your image seems to have no dust bunnies you could probably still take the flats now and they might last you over a few sessions.

As someone has already suggested, taking flats would probably offer the single biggest improvement to the image. You have some significant vignetting in the image which flats will deal with. I used to stress about taking flats but found they were very easy on a sunny day. Just aim your camera/lens at a part of the sky away from the sun and put 3 or 4 sheets of plain white paper over the lens. Find an exposure that generates a histogram that peaks about 50%. You may need more or less paper to control this. Once you've flattened the images (which can be done in DSS and other stacking software) the light pollution will show a more even gradient (more linear rather than circular) and will be easier to remove. That said, given how much vignetting and light pollution you're fighting, I'm even more impressed with your result

I’m not sure how real the colours are but each of your stars showed very subtle differences in their RGB values but they need to be stretched to extremes to show it. I have an action in Photoshop that increases star colour. I think it takes colour in the halo and maps it back into the white core. I also pushed vibrancy and saturation to make it easier to see. in reality, you’ll get a better result from the original image. I think the magnitude limit is a function of aperture and total integration time so to go deeper you need to stack many more subs.

What I love about this is that you've taken a really sharp widefield image with hand-held (albeit supported). There's even star colour data in the image though you really need to push the processing hard to bring it out. I hit it hard with the star colour hammer (hope you don't mind) to show it's in the image (blues have gone a little purple): With that nice field of view you might want to try some other areas and capture some clusters.

Isn't it great to be imaging again? Looks like you got focus and only a little trailing in the 66 subs you've stacked. I'd say your main issue with the image is, as you've suggested, its green colour. DSS is great for stacking but I would never recommend using it for processing an image. It's too blunt a stick. Even a free package like Gimp will offer far more control over your stretching (small, iterative applications of either levels or curves adjustments to minimise star bloat and protect the brightest areas from over-exposing) and also for colour balancing (again, curve adjustments but in the individual RGB channels). Give the processing another try. Use the autosave file from DSS with no processing, and try something like Gimp or a trial of StarTools, etc. You should find, with some careful stretching, that you can show good star colour across the image.

I have to agree with everyone about this image. It’s one of the most beautiful I’ve seen. You made the right decision to keep the green.

Nice cluster! Did you take any shorter exposures for the core? If not, amazing amount of detail resolved for 6 min exposures. Will be interested to hear how the move to the bigger mount goes and what extra it gives you (other than a bad back!).

Good point. I knew my logic had to be flawed. Back to head scratching!

*** deleted *** Had not measured noise just signal in original post

Ok, here's my logic: A reducer will "squeeze" the same signal into a smaller number of cells, so the signal per cell will increase (assuming you don't exceed the full well capacity). It also squeezes any signal noise into a smaller number of cells, so the signal noise per cell will also increase. As signal noise is proportional to signal, so you've not changed the Signal to Signal Noise Ratio. You do, however, hold instrument noise fixed per cell. Fewer cells means these will reduce compared to signal. So a slight improvement of SNR (more improvement for noisier cameras, less improvement for low read noise/low dark current cameras). Assuming you doubled signal through reduction (not a real example but keeps the maths simple): S / (SN + IN) < (2 x S) / ( (2 x SN) + IN) Now with some made up numbers: If S = 4, SN = 2 and IN = 1 then 4/3 < 8/5 <==== a noticeable improvement So, for a target that fits within the original field of view, you gain a small improvement in SNR by increasing the signal (and signal noise) per cell but maintaining instrument noise. But as instrument noise tends to zero (or if instrument noise is completely swamped by signal) then the reduced SNR will tend towards the original SNR and no real benefit is achieved. You might as well have just used software binning to reduce resolution (which can help mask tracking errors too). If S = 100, SN = 10 and IN = 1, then 100/11 < 200/21 <==== not really a noticeable improvement So for signal (and signal noise) that far exceeds instrument noise, there wouldn't be any benefit in reducing exposure if aperture and pixel sizes remained constant.

I agree that the SNR will increase because you have reduced resolution but your overall image brightness (in the original fov) will be lower if you lower exposure. So SNR will be higher but brightness lower in the area covered by the original fov. So you absolutely should use a reducer if your imaging target is larger than the original fov. You have a choice to make about the reducer if your target fits within the unreduced fov. Do you only want SNR or do you also want brightness. You can push the processing through more aggressive stretching to increase brightness, but then you're also stretching the noise. Does one equal and cancel out the other? I suspect the overall net effect is that whatever gain you make in SNR is lost through lower brightness, so you're back to the same exposure length to achieve the same final quality image and the only parameters that really changed were the fov and resolution. But I'm tired and this sort of logic makes my head hurt!

Just something to add onto Ian's good points above: adding a focal reducer will not make imaging faster. It only increases the field of view (of the same scope) by trading it off with a lower resulting resolution (for the same camera). If you don't need the additional field of view you are better not using the reducer. Or to put it another way - have the reducer available, but only use it when you need the additional field of view.

Sorry for the slow reply. I believe the image was taken "as is" though I cropped the image as I didn't know how to take flats back in those days and the vignetting was severe. That said, my files all say this was created from 30 second images. Unless I was *really* lucky, I don't know how I've achieved such long exposures at such a long focal length. That suggests maybe I did use the reducer after all. I used a T ring and adaptor for the camera. Something like these: https://www.firstlightoptics.com/adapters/flo-125-inch-t-mount-camera-adapter.html https://www.firstlightoptics.com/adapters/t-rings.html So the camera attached to where you'd normally drop in the eyepiece. Without the reducer there was no need to worry about spacing, etc. A 9.25" f10 scope with a DSLR on an AltAz mount is probably the very last place I'd start out imaging DSOs if I knew then what I know today. It's a great visual scope and I did get some really nice lunar and Jovian images but it's not a starter scope for DSO imaging. Although I have recently been thinking of dusting it off for galaxy season this year. Though now I would probably invest first in a new focuser for the visual back so I could lock the mirror and control fine focus, and I would now be mounting it on an AVX so it would be polar aligned and I would be using off axis guiding to help with the long focal length. if the weather was ever going to be nice...

That's a nice image. The core is showing some nice detail and you've got the colours balanced well. I can see a few things that could easily be adjusted for future sessions. You don't mention taking flats. These can have the single biggest impact on the image and remove the vignetting. This allows any remaining gradients to be more easily processed out using software. Your stars also look to be elongated. I can't tell if they are radial, and something to do with the optics, if they are rotational and therefore field rotation, or whether they are caused by camera vibration. I know what I used DSLRs I would include a delay between the mirror flipping up and the exposure being taken to give time for any vibrations to settle. Ideally you will want to lock the mirror up. Otherwise, the next thing is just to keep gathering more data. 20 minutes, even on a target as bright as M42, will still leave you with a lot of noise. Aim to gather about 2 hours of light subs. It's your choice of whether to stay with 15s or push for longer. The former reduces risks of star trails but increases processing time and storage requirements. 2 hours at 15 seconds means 480 subs to integrate!