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Yeah not much to see in narrowband AFAIK but a stunning object and you've done a lovely job of capturing and processing it.

Hi folks, Here is my attempt at imaging Messier 94, The Croc's Eye Galaxy (also known as the Cat's Eye Galaxy but for me this definitely has something of the crocodile about it...), a pretty face on spiral galaxy in the constellation of Canes Venatici, the Hunting Dogs. It lies about 15 million light years from Earth and measures around 50,000 light years in diameter. It's a really interesting object not least because the bright starburst centre is surrounded by a faded outer ring of older stars, part of which shows up as a faint halo in the image below. However, IR and UV images reveal spiral arms in the outer ring which also has star forming regions. Even more interestingly from a scientific point of view, the galaxy seems to have enough luminous material to explain its shape and motion and therefore is thought not to have any "dark matter". It's already been well studied but I have a hunch that M94 has secrets it has yet to give up... Imaging it was a bit tough given the short nights and lack of astronomical darkness but a run of clear nights from 25-31 May meant I was able to capture a bit more than 11 hours of useable data. Kit: CEM60EC; SW Esprit 150; Atik EFW2.2 with Astrodon filters; Atik OAG, QHY5L II with PHD2; Atik 460ex. The data, 45 x 300s in each of R, G, and B, was captured with SGPro and processed in PI and PS. I enjoyed processing it too, now that I am getting a bit more familiar with PI. I am also recalling some of the PS tricks and techniques and more importantly I'm remembering how I like the end result to look so I think I'm pretty happy with this one. There are loads of other galaxies in the star field. PI's annotation script captures a few but close inspection reveals loads of others. Thanks for looking. Clear skies, Ian

That's a very pretty image and I love the colour scheme - a refreshing take that gives it renewed interest. You should be pleased with that.

A thing of beauty John!

+1 for a light panel. Also I try really, really hard to keep everything dust free so that if I use flats (and I have to confess I don't always) they are only correcting for vignetting and not dust-bunnies so I can usually get away with one set of flats for all filters and over multiple imaging sessions. HTH, Ian

Interesting read, thanks for sharing.

Well I don't know how valid these comparisons are but I stacked the same frames in PI (using sigma clipping) and AstroArt 5 (using Sigma average with the default sigma of 3; AstroArt takes a little over a minute to calibrate, register and stack the subs). Measured using sub frame selector: Here are tifs of the two resulting stacks if you want to take a peek. R 45 x 300s AA.tif R 45 x 300s PI.tif

Well I found decon not too bad having read Jon's post - the first time I'd seen a systematic approach to the settings and it works for me - but you're right, it's another thing to wrap your head around. I've had enormous trouble with ABE / DBE clipping the background sky for some reason. Haven't figured it out yet so just not doing it!

It takes me a LOT longer to do the preprocessing in PixInsight than it does in AstroArt (which was my 'goto' until recently). I've yet to do a comparison of the final result to see if it is worth it. Of course there is a script that automates much of it but I am still at the stage where I am trying to really understand PI rather than following recipes. Cheers, Ian

If you fancy trying deconvolution I recommend Jon Rista's tutorial. Never had any success with deconvolution using various software until I read this...

Really it's what ever you find most convenient from a file management point of view and how fed up you get of repeatedly selecting common files (like calibration masters or the file you are aligning to). You can do the same calibration, cosmetic correction, etc on all of your subs if you want but obviously when you get to the stacking process you just stack the ones for each filter separately (unless you are creating a false luminance but that's another matter...) Maybe try different ways and see what suits you? Personally I've settled on the following: All subs together: Calibrate -> Cosmetic Correction Then when I do subframe selection I load up the subs from each filter and when I output files I save each filter to a separate directory. This is so that I can compare like with like and get an acceptable number of subs for each channel (I want similar numbers in each stack so that the noise in each is similar, though of course this depends on the subs themselves). In case it's of use my starting criteria is SNRWeight > 0.72 and Eccentricity < 0.6 but that's partly because the short nights mean I have a lot of subs with poor SNR at the moment... Then I register (star align) the files in each folder to my best R sub. I also save these in their own folder. Then I stack the files in each folder - and here it is definitely quicker to have the files in a separate folder if you are loading drizzle and / or local normalisation files. Now as far as local normalisation goes, I believe you should normalise to the best image from each filter - so you normalise your red subs to the best R image, the green subs to your best G image etc. AFAIK you should NOT normalise all of the subs to, for example, the sub you used to align on. This again is an argument for having your files organised in folders by filter at that point. However, I've abandoned local normalisation. Warren Keller advises to use it with care - you need a very clean reference image, I believe, and I found it does no better than using "additive with scaling" which is one less process. HTH, Ian

Yes - to be clear it is in the expression but for me it doesn't have a strong enough influence on the calculated weighting...

The LV tutorials are carefully put together and are generally excellent but there are some bits where my experimentation and / or other advice has led me to a different process. For example, I am not convinced by the weighting expression used in subframe selection (which in my view - or at least for the images I've been processing lately - doesn't take enough account of eccentricity) so I use Noise Evaluation instead. I've also abandoned local normalisation... Interestingly I've also found sigma clipping to give the best results on stacks of 30+ images...

Yeah you're right Mark there isn't a lot there (though there is some as I didn't calibrate with flats) in fairness so perhaps that is the problem. I guess I need to try another data set from the same set-up. Do you output to PS at all or do all your processing in PI?

Thanks for the reply JimJam. Yes I have the latest edition of the book which I am finding quite useful and seems to correct / contradict some of the popular online tutorials. As I said when replying to Mark I did try ABE immediately after cropping the image but the same thing happens and the background is clipped to zero... Thanks very much. The data is pretty good given that it's not getting astronomically dark (though this is using only around half the subs I captured). I did preprocessing manually. I am really keen to try to understand what I am doing in PI rather than just follow a recipe - it's uphill work as the documentation is, frankly, awful in many places - so I've experimented a lot with different stacking algorithms, subframe weighting, normalisation etc. and have settled on what seems to give the best results for my set-up. Thanks for the replies!

I think it's 7:30pm this time (according to the banner ad...)

Thanks for the suggestion Mark, but that makes no difference. Still get background values of zero...🤒

Hi all. I'm trying patiently to learn how to use PixInsight and posting this question about a problem I'm having with background neutralisation in the hope that someone can shed some light on it and / or it may help others... Basically when I was trying to process my data of the Whale Galaxy I was having no end of trouble stretching the data. Now admittedly I was doing the stretching in PS and I did get slightly better results if I did a histogram transformation in PI but nonetheless... what was happening was ABE (DBE too as it happens) seems to be clipping the black point on the data. This means (I think) that in order to get a natural background colour I was having to stretch a clipped background which resulted in a lot of background noise. It was driving me crazy and it took ages for me to figure out that PI's background normalisation was to blame. I've never struggled so much with basic processing and while I wasn't happy with the result (see the last four images I posted here) I waived the white flag in a fit of exasperation. I don't suffer much vignetting so even without flats if I run the same stacks, sans ABE, through my normal workflow a much better image drops out with no fuss at all (see below). So, OK, I could just dispense with PI's background neutralisation altogether but I am curious: am I missing a step or doing something wrong here? My PI workflow was: calibration with darks only (of the correct length); cosmetic correction using a master dark and auto detect; sub frame selection; registration; stacking using sigma clipping, weighted by noise evaluation; dynamic crop (and I changed camera angle half way through data acquisition so there was a fair bit of cropping); deconvolution, gentle noise reduction using MLT; then ABE (or not) channel combine; (sometimes I'd do photometric colour calibration here but it makes no difference to this issue); export as a TIF for further processing. I've tried doing a linear fit before the channel combination but that too makes no difference. When I open the ABE version in PS the background values are zero. Without ABE they are 3 and that makes a HUGE difference. Any thoughts? Best wishes, Ian Image without ABE (35 x 300s per channel)

Wowsers - just 2 hours 40 minutes? The background looks a bit on the dark side on my laptop (but I don't trust this screen!) and I reckon that's outstanding for under 3 hours at 1700mm f/l.

Well, well, well... what an interesting post. I don't spend as much time on here as I used to but I don't recall ever seeing M101 done in narrowband and it's always interesting to see a new slant on an old friend. I wonder if you are tempted to process this as an Ha / OIII / OIII image or Ha / OIII / Hb (Hb just being a third of the Ha signal). Those ought to give reasonably natural-looking star colours. Really like this though, Mark. Thanks for posting.

Brilliant Gina - you so rarely see the whole of the nebula imaged and as a target I find the witches broom a bit unsatisfactory for some reason but in context this all looks great. I much, much prefer your original process, for what my opinion is worth. I am inspired to try my RedCat on this - it's yet to get first light. Thanks for sharing. Stay safe and well, Ian

This is a lovely image - well done.

4 hours? Good work! I have data for this target (albeit a wider field of view) to process and your image has spurred me on to get on with it!

Really like that Carole - a lot going on but you've processed the data really well. Definitely a keeper!

That's a stunning image, thanks for sharing.