The Lazy Astronomer

My initial thought is that it is a stacking artifact caused by the fact that your camera is not tracking the sky. The sidereal rate (the speed at which stars appear to move through the sky) is 15 arcseconds per second. Your camera has 4.1um pixels, which means your imaging scale is 16.9 arcseconds per pixel ("/px) (image scale = pixel size in um / focal length in mm * 206.265), so over the course of your 10 minute total integration time, all the stars will have moved by ~530px in total, which is about 10% of your sensor width or about 15% of your sensor height Eyeballing it, l would say that the lighter area looks to be covering about 10% of the width of the frame. The reason you also see a lighter area along the top is likely because the camera wasn't angled so as to be aligned with the Earth's equitorial plane (so the stars moved diagonally across the frame).

I'll throw my hat in the ring here. Done entirely in Startools 1.8.506, processed as 3 separate images and all layered back together (as l couldn't get the right balance for all 3 elements of the image when processed as a single image). Looking at it now, it may be a little too purple, and I've probably overcooked it a bit with the IFN - I may fiddle around a bit again later. Basic run through: compose as L + synth. L, RGB and Ha to narrowband accent, set sliders accordingly. Binned 50% to better match fwhm to resolution, wipe using only some slight adjustments from the defaults, autodev using different ROIs for each replicate image to try to optimise the stretch for the element of interest. Contrast and hdr modules with mostly default values, sharpening with default values, s.v. decon using 10 or so star samples and the deringing slider turned all the way up (because l really don't like the hard edge decon creates around brighter stars - this seems to soften the edge back out). Colour using the stars as a reference white point, bit of fiddling around with the filter and entropy modules, then NB accent module using galaxy preset, with some slight tweaks to make the Ha regions come forwards a bit more. Then denoise, and faffed around with layers and masks to compose all back together. Finally, flux module to add a bit of extra sharpening.

Hi guys, Thinking of getting myself an IR pass filter for some planetary imaging over the next couple of months, and wondered if anyone had any advice on what might be the best option? My current planetary equipment is a 6" SCT with the asi290mm camera (although l may upgrade the scope to something in the 8 - 10" range for next year). As I'm using a relatively small scope, and my camera is not particularly sensitive at higher wavelengths, l was thinking something like the Astronomik 642 might be a good choice? Any thoughts or alternative suggestions on that? Thanks.

It's a capable mount for sure. Certainly plenty capable for a camera and lens, and would happily carry a small (maybe also medium) refracter. You've also got the option to add the Rowan belt upgrade at a later date to get better performance out of it if/when you decide to get a scope. The only thing l would say though, is Skywatcher mounts do tend to be a bit on the heavy side - no problem if you're only going to be using at home, but if you're planning on travelling out and about, you might want to consider something lighter (e.g. iOptron) (and l say this as a Skywatcher mount owner)

That is incredible for just 30 mins!!

Autostakkert!3 is the software you want - the usual process for planetary imaging is to take thousands of very short exposures (as a video*) and use software to pick the best n% of frames to stack. *note that this only applies if your camera can record full resolution video - most DSLRs can't.

As long as there are at least 8 stars in common between all frames, DSS can rotate them around to match them up no problem. Be aware you will very likely have to crop the stacked image where the different frame orientations don't line up.

If you wanted to download Startools to try (unlimited free trial, btw), make sure you download the 1.8 alpha version - the new deconvolution module is really very good.

2.1"/px is a pretty good sampling rate actually - take the recommendation with a pinch of salt as your maximum resolution (in "/px) is limited by the sky conditions, which sadly for us in the UK (assuming you're in the UK) will rarely support much better than that anyway.* *Note: this statement applies to 'traditional' long-exposure deep sky imaging. The rules change a bit for short-exposure lucky imaging.

Ok, so probably went about this the wrong way, but here's what l got: I have 2 seperate graphs because I my light source was too dim and l realised after a little while I'd be sat there forever doing the higher adu values (I did this all manually, I'm sure there's probably a far more sensible way to this through software, but oh well it's done now...) The first graph shows exposures ranging from 0.1s to 20s, in increments of 0.1s up to 1s, and then increments of 1s thereafter. The second graph is with a brighter light source, exposure times ranging from 2s up to 7.5s in increments of 0.5s. I used the mean adu value as reported in NINA's statistics pane. The second graph stops slightly before the saturation point as when it nears it the curve shape flattens off, but the sensor never fully saturates (there is virtually no difference between a 9s exposure and a 20s exposure - both with mean adus of 65227.xx) My reading of this is that with r2 values of >0.99 on both graphs, l can conclude that the sensor does respond in a linear fashion until just before saturation, at which point something weird begins to happen. Is that fair conclusion to make? If so, then I would also conclude that in an imaging scenario, it should only really be brighter stars which are affected, yes? If I've gone about this completely the wrong way, then more than happy for someone to point me towards a correct way of doing this. Full data table below for anyone interested. Note that from point 30 onwards, the brightness of the light source was increased: ref time (s) mean adu 1 0.1 1798.56 2 0.2 1877.22 3 0.3 1960.96 4 0.4 2056.71 5 0.5 2133.31 6 0.6 2221.36 7 0.7 2296.38 8 0.8 2391.94 9 0.9 2440.63 10 1 2965.45 11 2 3908.2 12 3 4656.95 13 4 5508.67 14 5 6459.62 15 6 7173.07 16 7 8051.98 17 8 8919.21 18 9 9978.88 19 10 10712.89 20 11 11812.54 21 12 12750.33 22 13 13278.09 23 14 14279.82 24 15 15335.5 25 16 16066.85 26 17 17344.15 27 18 17835.57 28 19 19414.6 29 20 21087.09 30 2 22494.96 31 2.5 26243.66 32 3 30755.9 33 3.5 34568.44 34 4 38761.24 35 4.5 43473.7 36 5 46864.11 37 5.5 50186.24 38 6 54370.03 39 6.5 58330.17 40 7 61840 41 7.5 63741.89 42 8 65089.26 43 8.5 65502.59 44 9 65526.98 45 9.5 65527.04 46 10 65527.06 47 15 65527.12 48 20 65527.21

Assuming your filters have a clear aperture of 26mm, then they could be up to 27.5mm from the sensor without causing issues (in theory). Is it possible your sensor is slightly offset such that it doesn't sit in the centre of the filter? Or maybe your filter's clear apertures are <26mm (my ZWO ones are ~24mm, my Astronomik ones are ~27mm, so they do differ).

In terms of equipment compatibility, there are 2 points to consider: the sampling rate and the scopes imaging circle size. Your current sampling rate is 2.3"/px, which is probably about right for typical sky conditions. I tried to look up the imaging circle of your scope but couldn't seem find it, however you sensor diagonal is 22.5mm, so l would be surprised if the scope was not able to fully illuminate the sensor. There does seem to be some hard vignetting though - what size filters do you use, and how far from the sensor are they?

Looks to me like a case of coma. From https://www.lonelyspeck.com/a-practical-guide-to-lens-aberrations-and-the-lonely-speck-aberration-test/: Comatic aberration or just “Coma” is named for its comet like shape for point light sources. Coma occurs when light from a single source entering at the edge of the lens is not projected at the same size as light entering the center of the lens. For this reason it becomes more apparent on point sources of light at the edge of the frame and at low f/numbers. Coma is common on fast (large aperture or low f/number) lenses but can also be reduced by stopping the lens to a higher f/number.

Yes DSS does all the background work for you (the main reason I like it - although it's far from the best stacker, it's much less faff than other software). I doubt there's anything going wrong during the stacking process; due to it's automation, there's really very little you can do wrong in DSS. This leads me to believe it's the flats (or flat darks) themselves which aren't quite right - how do you take them? What does the histogram look like? I had an issue with NB flats a month or so ago and the solution l found was to temperature match all subs, so I now take all lights and calibration frames at -10C. Seems to be working so far 🤞

Do you take flat darks as well? The full image calibration process should be (flat - flat dark)/(light - dark)

Good planetary imaging is sooo dependant on seeing conditions. Don't beat yourself up too much, it's likely it was the conditions holding you back. Is your SCT the standard version or the edge hd?

I think ideally you want a filter as close to the sensor as possible, but this tool can be slightly reverse engineered to give you a maximum distance: https://astronomy.tools/calculators/ccd_filter_size For your ED80 and an APS-C sensor, it looks like you could probably have the filter up to around 200mm from the sensor, so basically you can put it wherever you want. If you experience reflection problems, then you'd want to look at moving the filter closer though.

Yes, I think I solved my problem with the flats by temperature matching everything.

Well, I tried some gain 0 imaging last week thinking I could enjoy those lovely deep wells, but completely overlooked the fact that at gain 0, it's 4e-/ADU, which translates to terrible results. As a side note: I assume in order to get a comparable image to one taken at unity gain, I'd have to expose each sub for 4x longer to record the same ADU value for a given pixel (as it takes 4x as many electrons to bump up to the next ADU value)? Or is it not quite as straightforward as that? Even if it is as straightforward as that, I don't particularly fancy having to increase my imaging time 4-fold, especially with typical UK skies!! So, after that disappointment, I thought about it a bit more, and I'm now erring towards it being a non-issue in real world use, assuming that the issue only presents itself in the upper part of the camera's range. My thinking here is that the vast majority of my data would sit in the lower 1/3 of the ADU range, and it's only really the stars which would be affected at the upper end. So, is there some kind of analysis/experiment I could do to test the sensor response/linearity across it's full range, and what sort of results should l be looking for?

You could just stick with your 3x one for now. It would give you a sampling rate of ~0.11"/px, which is pretty much right on the limit of what's possible, so you'd need excellent conditions to take full advantage of it, but I assume you could just bin/downsample in post processing software (in the same way you can with deep sky images) if the conditions on the night didn't support the very high resolution. The disclaimer here is I've never actually tried software binning planetary images - it may be that it doesn't give you the same benefits as doing it on DSO images... maybe someone cleverer then l can confirm.

To help point you in the right direction: the atmosphere will limit the resolution it's possible to obtain with long-exposure DSO imaging. It's generally accepted that the resolution limit for most people is 1"/px (in reality, it's likely to be quite a bit higher, but let's take the best case of 1"/px for now). To achieve 1"/px with your scope at its native focal length you'd want pixels of ~11um. If you used the 0.7x reducer, then you'd want pixels of ~8um. Smaller pixels than that will just lead to oversampling - that is, you won't be capturing any finer detail (because your resolution is limited by the atmosphere), rather, you'll just be spreading the detail out over more pixels. Also, attempting deep sky imaging as a beginner* with such a long focal length will be PAINFUL. However, all that sampling nonsense above goes out of the window for very short exposure planetary imaging. You are no longer as limited by the atmosphere as you're using exposures of tens of milliseconds rather than several minutes. A general rule of thumb for a decent sampling rate for planetary is to take your focal ratio and divide by between 5 and 7 - this will give you an ideal pixel size in um. Note that high resolution planetary imaging is usually done at very long focal lengths, so ditch the reducer and go for a 2x or 3x Barlow instead. If you used a 2x Barlow, this would give you an f ratio of 20, divide that by 7, and you'd be looking at wanting pixels of ~2.9um. Note also that cooled cameras are not needed for planetary. *I assume you're new to this, apologies if not.

Well, I wasn't quite prepared for 5(!) consecutive nights of clear skies there were in South East UK last week, so after I'd got around 6hrs on the Pelican nebula (which hopefully I will be able to process this week), I was hunting around for something else to shoot and decided to try a new technical challenge and attempt a mosaic. Due to short nights, and as it was just a few days off being a full moon, l decided to stick with Ha only (plus, who doesn't like a lovely little mono Ha image, right?). This is a 4 panel mosaic, 12 x 300s subs per panel (except panel 4, which due to a technical issue, has 1 sub fewer than the others). Stitched in MS ICE, processed with Startools. Binned the composite image x2 to help boost the SNR a bit as only 1 hour per panel wasn't really enough to get a clean looking image at full resolution. Anyway, here it is, presented in a slightly unusual portrait aspect. Thoughts and comments welcome!

Tosh: rubbish, nonsense - see also: codswallop 😉

These mounts certainly are weighty beasts that's for sure! Glad you're pleased with it, looking forward to seeing your results. 👍

Wow. I was pretty pleased with my own attempt at this, but yours just knocks it right out of the park! P.s. How did you find the patience to collect 40 hrs of data!?