wimvb

Great image, Richard. I remember this target from last time you presented it. I like this mosaic. There are a few on the right as well.

Deconvolution is best done on luminance data, so try this: Extract L and do deconvolution on this. Process the RGB image without deconvolution, but with maximum colour punch. Then after stretching and post processing, combine them again with LRGB combination. The easiest way to avoid ringing in bright and overexposed stars is to use a star mask to cover them. If you only see this later on during post processing, you can still repair the star cores. Create a star mask and apply it. Use mlt to remove enough layers so that the ringing is gone. Then sharpen a bit.

Great image. It seems to me that you've lost the beautiful blue colour in the core, in your third version. If you have pixinsight, you can use scnr to green in order to push the dust more towards the red while keeping the blue core.

In the picture which you attached to your post, try to identify what you see. The grey structure inside the red field, extending down, is the side of the secondary mirror. The black crescent on the left, right next to the white circle is the reflection of the tube wall in the secondary mirror. This shows that you have to work with rotating and tilting the secondary first. Forget about the primary until this is fixed. Once this is ok, you can start aligning the primary.

Why stop there? If you add a tilt adjuster, you can tune the filter.

There's no reason why the qhy163 wouldn't be up to the job. If you were to buy an equivalent new setup, the asi294mm would be the way to go. So, compare your deal on the qhy163 with a similar setup, based on the asi294mm (or qhy equivalent of this camera). Personally, I wouldn't use nb filters with a osc camera, and would love to see a shoot out, between a mono + nb, vs an osc + multiband filters.

... if the filter wheel allows. 😉 And if you have the possibility to pick filters, choose two with slightly shifted peaks. 7 nm bandwidth might just become 3 nm. The sky is blue during a full moon, for the same reason it’s blue during the day. A filter that passes only red will give you contrast, but a filter that passes blue won’t. That’s why Ha imaging works during a full moon, but not Oiii. Sii will in theory also work, but because the signal is generally much weaker than Ha, Sii will be affected more by noise.

Sorry, Vlad, maybe I didn't make myself clear. A few percent extra tranmissivity in the pass band of a filter is irrelevant, as you noted. I mean the off band rejection of nb filters. An OD 2 filter will pass 1% of the light it should stop. An OD 4 filter will only pass 0.01%. You can't see this difference on a linear scale in published spectral response curves. But it is, imo, significant if you for example, want to image with a Ha or Sii filter in moon light Exactly, but it shouldn't be neglected in a discussion about nb filters, imo. This link may be of interest http://www.aicccd.com/archive/aic2007/Goldman-AIC2007Talk2.pdf

I'm a bit surprised that in a discussion about nb filters, and especially the quality difference between filters, the concept of OD, or optical density, hasn't come up yet? Does that mean that optical density (the amount of light that is blocked by a filter) is irrelevant? Or is the optical density about the same/large enough for all filters, so that it's a non-issue?

Glad all is ok. I have two silica gel bags taped to the inside of the cap that protects the corrector plate of my MN190. Just to avoid moisture and fungus.

On my belt driven AZEQ6 I could remove a short period wobble by increasing the belt tension, ie by improving the meshing between the motor and the belt. On a wheel driven mount, this would be equivalent to improving the meshing between the motor and the first cogwheel. On an EQ6, you should consider to do a belt modification. This improves smoothness a lot.

If the error is smooth, it will be guided out. Otherwise you probably need to adjust the gear meshing. Have you considered installing a belt drive? This should improve tracking.

Arcsinh stretch was developed for Pixinsight and photoshop by SGL member Mark Shelley, @sharkmelley. If you look in his profile for topics he started, you should find more information. Mind you, you will have to look a few years back.

To bring out the colour, I used arcsinh stretch, which is also available for photoshop. Arcsinh stretch is very good at preserving colour. I increased saturation in the galaxy. Like you've said already, there are strong blue haloes around the stars. I used a halo mask (Blue channel - Green channel) and then desaturated the blue with PI's colour saturation tool. I would suspect that there is an equivalent tool in PS, where you can change the saturation of only one colour. Hope this helps Edit: Btw, how much light pollution do you have? 300 s subs would seem ok, unless you take them at a low ISO setting. If you have light pollution, you need to take more subs to get rid of the noise that comes with LP. More data is always the best noise reduction tool. And the less noise you have, the harder you can push the data.

This is what PixInsight does to your data. There's a magenta/red patch that doesn't look like it should be there, but otherwise there's not much of a problem. But you do have strong blue haloes around your stars that are most likely caused by chromatic aberration.

If I were in your position, I would invest in that Samyang, and maybe a longer fl lens (200 - 300 mm). Add a simple guide setup, perhaps based on a finder guider and a ASI120 camera.

GIMP, PS, Pixinsight, APP, or Startools. GIMP is free, but depending on your ambition, you may want to invest in one of the other tools.

What’s under the cover?

I never use ABE, just because it’s difficult to control. For some reason, my flats correct dustbunnies well enough, but don’t remove all vignetting, and I have to clean up with DBE. It works good enough if you put samples in each corner and along each side (=8), plus from each corner along the diagonal, at most half way in towards the centre (+4). No samples closer to the centre needed. You also don’t have to avoid stars, because the are excluded from the samples. The sample plot shows which pixels will be included in the calculation of the background model. Black means excluded, white means included. Gray is a measure for weight. As long as stars don’t have haloes, they will be excluded. @Jm1973: sorry for derailing this thread. Such is life on SGL, it makes this forum so alive and interesting. Welcome to the club. 😀

Natural skyglow is one origin. Aurora Borealis may be jaw dropping on rare occasions, but mostly it’s a nuisance. The physical process that creates the aurora is always at work, it’s just too weak to be visible by the naked eye, most of the time. I’ve seen its effect through the eye piece of my scope on a dark and cold winter night, when it was invisible otherwise. I don’t want it on my images, though. Normally, I apply DBE after RGB combination, because the gradients I have are easy to remove. But for combined gradients, such as vignetting plus sky glow or light pollution, where the resulting gradient can be complex, it is at times necessary to do DBE on the individual masters before combining them. Also, vignetting has to be divided out, as opposed to additive glow gradients. For OSC images, it is sometimes necessary to split the channels, and apply DBE to each channel. Btw, I don’t use DBE to neutralise the background in my images. I prefer the background neutralization process in PI for that. The math may very well be the same, I just prefer to be in control of this step.

For some reason, the flats aren’t doing their job. Even with only one flat, vignetting should be greatly reduced, albeit at the cost of increased noise. Quality costs. I used Baader a uhc filter before I moved house to my current location, which is much darker than where I lived before. IDAS filters are considered efficient, and colour preserving. But obviously at a price. The very cheapest may also be the lowest in quality, but such an argument doesn’t always hold. A poor filter can give you unwanted reflections, and you will need to do some investigating begore you buy.

Before you stack the sub frames, you need to calibrate them. For the setup you used to take this image, good flats are essential. This is what the gradient in your image looks like. The circular structure is vignetting. But there is also a colour gradient with a red band at the top. Most software can handle one source of an uneven background quite well, but a combined gradient can be a challenge to remove. Especially if you also have different gradients in the three colour channels, which makes it even more of a challenge to remove. Here's what I managed to pull out of your data, before the remains of gradients started to pop up. Nothing too fancy. Arcsinh stretch (available in PS) lifted the colour in your image. after that, curves transformation as Olly already wrote about. (click on the images to see a larger version)

Light pollution is a likely candidate. If you get the histogram so far to the right with only 30 seconds exposures on an unmodded dslr, you probably have a fair amount of light pollution. If this is the case, consider investing in a light pollution filter. To compensate for the added noise, you will need long total integration times, ie lots and lots of data.

In all fairness, it looks to me that the way stars look in this image is determined more by processing than by data capture issues.

A general rule of thumb is that your guiding rms should not exceed half your imaging pixel scale. With 0.4" /p, you'd need 0.2" guiding rms if all other conditions where ideal. But if you have 1" rms, that's 2.5 pixels. To get at least round stars, you need to make sure that a) guide ra rms is equal to dec rms, and b) that deviations in ra and dec are truly random. As @Paul M (sorry, meant @don4l) wrote, seeing will be the most significant limiting factor in how fat stars look in your images, once you have guiding under control. In practice, your guiding rms needs to be significantly better than your seeing. In your case, your imaging scale is working against you. Even with a premium mount on a high mountain top under excellent weather conditions, 0.4"/p is pushing the limit. I think you can safely bin your images 2x2 or even 3x3, and not see a real difference. Finally, to get smaller stars in your images you have to pay attention to them during post processing. Especially during stretching, but you can also reduce them afterwards.