wimvb

The MN190 has a central obstruction that will scatter/diffract some light, there's no way around that. But it doesn't suffer from diffraction spikes, and with the low focuser, no drawtube shadow in the light path.

This thread from down under may be of interest. http://www.iceinspace.com.au/forum/showthread.php?t=140193

That may have been either Jonas Grinde or the late Per Frejvall. Per bought this scope and replaced the focuser, but afaIk, he had trouble collimating the scope afterwards. But since then, Skywatcher have replaced the stock focuser. What I've also learned is that the distance from the corrector plate to the center of the secondary is rather critical. If you remove the focuser, you need to first make sure the scope is collimated. Then you replace the focuser, and align that to the secondary, rather than the other way around. Last saturday (see the Eurovision thread in the Lounge), I recollimated my scope, making sure to center the seconday under the focuser. Cheshire, laser and barlowed laser now tell me that collimation is perfect. But the proof will be in the pudding. And that won't be served until late August.

He has. It's a great scope, but not a newbie item. When I got mine, collimation was off by a mile and a half. And collimating it is a bit harder than an equally fast standard newtonian. But once tamed, it can produce extraordinary results. No CA for one. And unlike a frac + flattener, there are no critical distances behind the focuser. For me, that are two reasons not to add more glass to it. As @Gina wrote, for a shorter fl it's better to use a frac. I've used mine with a finder guider, an ST80 as guide scope, and an oag. The oag adds weight to the stock focuser, which atm is Skywatchers low profile Crayford. It may bring the load outside the focusers comfort zone (2 kg according to Teleskop Service). I'm still considering swapping the stock focuser for a Baader Steeltrack, but I don't like the prospect of having to drill holes in the OTA. Before I make that decision, I'll try to tighten the focuser draw tube. Some (potential) users complain about its weight, but it's not heavier than a frac of comparable size. Regarding Olly's comment about the mechanics, the scope would probably gain more attention if it were made of carbon fibre, and not steel. But as far as I know, the carbon tube would need to be very thick in order to make it stiff enough. And that would necessitate swapping the corrector cell and the primary mirror cell, making the scope more expensive. It's the corrector plate and the mirror that make up most of the ota's 10 kg.

Is your dither working as it should? If you create a movie from your unregistered but calibrated subs, you should see the stars moving in roughly the same pattern as your dither movements. If they still move in a linear pattern, even if that pattern consists of forward and backward steps, your dither doesn't work. (In PixInsight you'd use the Blink tool for that.)

If polar alignment is anything less than perfect, your mount is rotating around another point than the true NP. This results in a drift in RA and DEC, plus field rotation. It is this drift that is used for polar drift alignment, and is the cause for walking noise. That is why walking noise follows the same direction as the offset between stars when you stack. http://wimvberlo.blogspot.com/2016/07/dithering-in-hardware_7.html?m=1 http://wimvberlo.blogspot.com/2016/07/the-effect-of-dithering_21.html?m=1

To manually dither using the hand set (hs): Suppose you have a imaging scale of 2 "/pixel if you want to dither 15 pixels = 30 arcsecs: - Set hs to 1 x sidereal. - Press RA+ button for 2 seconds to dither 15 pixels in RA+. - Same for RA- DEC+ DEC-. - Dither in an outward spiraling pattern. In other words, slew at 1 x sidereal speed for as many seconds as the numerical value of your imaging scale. This will give a 15 pixel dither. I've done it this way until I automated the process; it works. Because you don't change the dither direction in DEC every time, the spiral pattern is less sensitive to backlash than a random dither.

You need to do colour calibration on linear data. It's always the third step in my workflow: Crop DBE Colour calibration

I don't get it. Isn't that the same raw data?

I agree, darks aren't always needed, and they can be reused. Many cmos cameras suffer from amp glow to some degree. In this situation, you do need darks, and they can't be scaled. Peter Shah's results with g2v calibration certainly are stunning.

In pixinsight there's a process button that will extract luminance. You can also use the process channel extraction in either Lab mode or Lch mode, with only L selected. Before I did this I equalised the channels. Set the rgb sliders in colour workspace to 1 (process menu - colorspace) Noise reduction: I tried the sigma thresholding option in multiscale linear transformation, but I may have ended up using the standard noise reduction in MLT with an inverted lightness mask. I would have to check the process history to make sure which I used. I actually used this image to try a few new tricks. My standard noise reduction on L is either TGV denoise or MLT, applied with an inverted L mask. I always increase the background of the L mask to 50%. This way I get noise reduction, and not noise obliteration. Hope this helps.

That's a sound procedure. But I see one practical problem with the use of mono cmos cameras: since you generally can't scale darks, you'd need a master dark for each filter. Of course, as long as you can reuse the darks, this is more of a nuisance than a problem.

That's just what photometric colour calibration in PixInsight does. @gorann: you have this already. No need for ps

Make sure that all the gears are optimised (trade off between smoothness and backlash). Then use a large enough dither step so that the mount moves past the backlash. You can also dither in a spiraling path: ra+ dec+ ra- ra- dec- dec- ra+ ra+ ra+ dec+ dec+ dec+ etc. This sequence traces an outward spiraling dither pattern.

Have you tried cosmetic correction in PixInsight? Then while stacking, you can set an aggressive clipping factor. What exposure time did you use? Your image looks under exposed.