wimvb

Nice first images @StaceStar. Now just repeat a few dozen times, stack, and you're good. Welcome to the dark side.

Fwiw, here's my linear workflow. Maybe you find something you can use. 1. Crop edges - dynamic crop 2. DBE, using few but large samples. Place samples manually. Sample size 15 pixels or larger, the largest size that fits between stars. Correction method depends on cause of gradient. I always do a test without correction applied to examine the bg model. Depending on how it looks, I adjust the number, placement and size of samples. I always go for a background model that is smooth with no structure, and few colour variations. Adjust tolerance until all samples have a weight in all channels > minimum sample weight (0.75). Check normalise, discard bg model, replace image. Apply correction. 3. Define the largest possible preview in a bg area. Use this for background neutralisation. If there's very little background, I create several small previews and combine them using preview aggregator script. 4. Use the same preview for colour calibration as bg reference. Depending on target, use either with or without structure detection for white reference. If using an aggregated preview, this needs to be made fresh. 4 alt. Use photometric colour calibration with preview for bg neutralisation reference. This procedure gives good colour variation in star fields. ... For stretching, I use Mark Shelley's arcsinh stretch. This is superior to any other stretch I've used regarding star colour. Don't overstretch. A weaker stretch followed by curves transformation usually gives better results than one aggressive stretch. I hope that some of this may be of use.

Lovely image. I'm a bit confused about your process flow, though. 1. Do you separate colour channels of your dslr image? 2. Colour calibration after scnr:green may very well put green back in again. Personally, I always do colour calibration immediately after background neutralisation. Otherwise the neutral background that cc needs, may not be neutral anymore.

I ran mine off a 12 V leisure battery until I got a mains adapter. The synscan manual specifies 7.5 - 12 V. With short the fl and low load, the mount performs very well. As is consistently demonstrated here. The weak points of this mount are the aluminium tripod and the flimsy altitude adjustment bolts.

Nice catch. The star trails you have in the Orion image are actually a good way to see "real" star colour. In your case, they also show that you had excellent sensor alignment.

With just camera and lens (135 mm) I've used mine with no, or only the small counterweight. Unguided up to 7 minutes, with relatively few discarded subs. Runs fine on batteries. But if you have an outdoors mains outlet, I would consider using that. Be prepared to tinker with it, removing backlash and adjusting the gears. With the aluminium tripod, I would consider this a portable setup.

That's "low noise" season in all its splendor (assuming you're into imaging, Les). My main concern isn't so much ice on the scope, as it is cables getting stiff and brittle when it's really cold.

Imo, the asymmetry strongly syggests some kind of tilt. But Adam's advice to put the primary mirror as far into the tube as the screws will allow, makes perfect sense. As this change doesn't involve any butchering, I think it's worth pursuing. If your mount is stable enough, you should also be able to SEE the diffraction spikes in an eyepiece. Especially when viewing a very bright star, such as Vega. Moving the fov such that the star is viewed on one side, centre, and finally on the other side (without refocusing?), may reveal possible tilt.

Regarding the baader cc and its distance to the sensor: When using the cc with my pentax dslr (cc + t2 + dslr), the focus tube went quite far into the light path. When I replaced the pentax with an ASI174 and spacer, the cc distance was initially 1.5 - 2 mm too far out. This affected focus remarkably, and the focus tube wasn't in the light path anymore. Because of the much smaller chip, I didn't have problems with coma. But when I corrected the distance, focus moved back in again. Numberwise: Cc + dslr, focus with tube at about 10 -12 mm from all the way in, no visible coma on aps-c Cc + ASI174 + too long spacer, focus almost all the way out (about 80 %), no visible coma on small chip Cc + ASI174 + better spacer, focus about midway of its travel, no visible coma on small chip. Since the baader cc is supposed to have a best distance of 55 mm ± 1 mm, and focus seems to change substantially with distance, you could just try adding a 1 mm spacer before taking out the hack saw.

Any unevenness like this suggests that there is tilt between the sensor and the focal plane. The SW crayford focuser can actually be collimated. There should be three pairs of screws around the focuser base. These can be used to align the focuser. Here's a video showing how to centre and align the focuser of a SW newtonian (about 25 min in) https://www.youtube.com/watch?v=3LbR1nIx-jw Personally I would do such a job either during the summer, between seasons, or during a cloudy weekend. Btw, my simulations were only for on axis/symmetrical obstructions. I will have a go at simulating off axis effects.

It may partly be a focus issue. When I did the simulations, I noticed that the vertical (unsplit) diffraction spike gets wider when defocused. Eventually it will split in two parallel spikes. The vertical spike looks a bit wider in the first image. So my conclusion is that it's slightly defocused as compared to the other images. Did you refocus between exposures? If not, focus seems to shift across the image plane. If it's not that, I really have no other clue.

I wonder, is it symmetrical around the centre? Are the spikes split to the other side, on the other side of the centre of the image? Just checked your original image. The right hand side looks normal. It's only in the left half of the image.

I've run out of ideas.

That would be the way to do it. Be sure to keep the scope collimated. That's more important than diffraction spikes.

Here's the simulation I mentioned before. An aperture with an obstruction that is slightly shifted upwards and 4 spidervanes. The vertical vanes are precisely vertical, while the "horizontal" vanes connect the 3 (9) o'clock position of the aperture to the 3 (9) o'clock position of the obstruction. The aperture: (Btw, the size of the obstruction shouldn't be a critical parameter here.) The diffraction pattern it produces at focus: And at a very sligth defocus (50 microns according to Maskulator, within the critical focus) (the horizontal spikes are slightly wider, and the vertical X starts to ever so slightly shift upward)

I have the same issue as you, but much less. I've been doing some light diffraction simulations, but can't replicate the proper diffraction pattern. Mainly because any diffraction pattern I get is symmetrical. If you look very closely at the brightest star in your image, you'll notice that opposite of the split spike, the other spike is also split, but much weaker. The closest I get to this is when I simulate a situation where the vanes are not at right angles, i.e. where the secondary mirror is not exactly centered. This gives a diffraction pattern similar to that of a Bahtinov mask. Defocused, the position where the spikes cross, moves away from the central bright spot. It may be worthwhile to test this. I hope this makes sense.

I found that the black point setting (background removal) is very critical. You may need to play around with various settings. Just a thought

At occasions like these I'm glad I use PixInsight. Seriously though, great write up. And thanks again for the PI script.

That's getting really good. To get a significant improvement, you basically need to double the total exposure time ("-ish"). But adding Ha is an entirely new dataset that can lift your image with only a couple of extra hours. And if you want to add Ha in the end any way, why not start now? I would probably go for the modded cam. You have hints of Ha as it is.

And when you get the 2" coma corrector, you replace that last adaptor with the cc. The cc also takes a light pollution filter in its front, should you need one. Btw, make sure you tighten the screws in your focuser when you use that adaptor. You don't want your camera to fall off.

Now, that's a depressing bed time story. Sorry it didn't work out for you. I hope you have better luck tonight. Monday night is supposed to be a clear night out here, according to the Clear Outside app. Probably the same gap in the clouds that's passing your neck of the woods this weekend. But at the same time it predicts 7% chance of rain on a cloudless night, as oppsed to 0% chance on a 100% clouded night. I wonder who programned that app.

Great write up. Glad I have PixInsight; only a few sliders and no layers. Thanks for the script, Mark.

To test this, try the following. Wrap something around the suspected vane and image a star. Repeat with each vane until you find the culprit. Focus on a bright star (like Deneb in Cygnus), and gradually defocus. Eventually you will see the shadows of the vanes. At that point you can hold a finger near each vane at a time and identify them in the view. Either of these methods should give you the position of the twisted vane. Then loosen the screw on the outside of the tube, twist the vane until the diffraction spike looks good, and tighten the screw again. If you tighten as before, this shouldn't throw collimation off. But check collimation of the secondary mirror afterwards anyway. Easiest way to check collimation is to focus on a bright star, then rock focus in and out. The defocused star should be symmetrical and look the same on inside and outside of focus.

Dion from astronomyshed has a youtube video showing how to align a sw focuser. Also, the standard focuser has three pairs of collimation screws on its baseplate. They (should) work the same as those for collimating the primary.

That's a great start. The vignetting is best taken care of by using flats, even if gradient removal tools also can do their share. I think that from here, the next logical step would be to add bias and flat frames. Dark frames with a dslr are debateable. Dithering (= moving the fov slightly between exposures) is more effective.