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Here is where theory goes awry. Amp glow does not scale linearly with time. Amp glow is due to an uneven heating of the sensor. A local increase in temperature leads to a local increase in dark current. If the local temperature were constant, amp glow would increase linearly with time. But temperature increases with time, probably until a steady state is reached, and amp glow will increase more than linearly with time. This is the reason why amp glow in ASI cameras can't be calibrated out completely with darks taken with a different exposure time.

Even with the PRO version of the ASI1600, there is amp glow. The only way to calibrate this out is with matching darks. In my case, with the ASI174, "optimised" darks will not completely remove the glow, but matching darks will (temp, time, gain, offset). My procedure is: Lights and darks at the same gain, offset, temperature and exposure time. Flats and dark flats at 0 gain and matching exposure time. Intensity for flats set to 25 000 ADUs. No bias frames. So far I've used a set of flats for each filter, but I must try Olly's method of only using lum flats. That would save some processing time, and disk space.

Yes, a bit small in fov. But paired with a lens rather than a scope, eg a Samyang 135 f/2, this would make a very portable and powerful setup. Or with an ed80 on an eq3 mount. High sensitivity means short exposures, and no guiding. You could improve the resolution a little by drizzling. At 2 Mpixels, the files aren't that big, so even with many short exposures, the stacking procedure shouldn't take that long. On paper, it seems a nice camera that could perform really well if you have access to dark skies. But it should come with a cloud gun.

Nice image of the Horsehead. The gradient is due to light pollution. A filter will make it less, but probably not remove all of it. Gimp doesn't have tools for AP as PS does, but you could try the following crude method. Make sure your image is not clipped. You need to replicate the gradient in some way (that's what DBE in PI, and Gradient Exterminator in PS do), and Gimp has a gradient fill tool. You could try using that. Sample two colours with the eyedropper tool. One becomes the foreground colour, the other the background colour. Create a new empty layer. Use the gradient fill tool to replicate the gradient in your image. Then subtract the two layers. Make sure this doesn't clip the image. You may need to do this in several small iterations, rather than one big swoop. It most likely won't remove the gradient completely, but it may make it less intrusive. I think there should be enough cloudy nights even where you are, to give you time for this kind of experimentation. Good luck. (Btw, I haven't tried this myself. There's always a risk that it won't work at all. )

Same here. I have more opportunities to take pictures OF my setup than WITH my setup, so here's a picture while taking flats after one such rare occasion.

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.