M45

[Adreneline]

After numerous trashed attempts at M45 over the past few months (not to mention previous years) I've decided this is probably as good as it's going to get for me.

This is 20x 30s and 20x60s of R, G and B taken with ASI1600 + Canon 200mm, pre and post processed in PI and then colour tweaked in PS followed by a minimal final noise reduction. I took some Ha as well (well you never know!) but that's not in the mix. I tried removing stars with Starnet++ but that didn't help (proabably operator error) and I tried combining both Histogram and Arcsinh stretch to try to get more colour into the stars but most of that seems to have been lost again.

As ever C&C are always welcome. I fear the window of opportunity in this location is pretty much done for this year so it'll be the autumn again before I can have another go.

Thanks for looking.

Adrian

Edited February 3, 2020 by Adreneline
Clarification

[Tommohawk]

Lots of nice nebulosity, so the data seems to be there...  but as you say maybe there should be more colour especially blue. Does your blue stack look to have good detail? Having said this, I've also struggled with processing M42, so probably better advice available from others!

[assouptro]

I like this! 
nice wide wife field view

M42 can be a tricky process and the only advice I can offer is image the blue subs as close to the zenith as possible, that goes for all blue subs on all objects. (Not that I am in anyway an expert! It’s just something I read when trying to research hints whilst trying to image m42 myself)

Thanks for sharing 

Bryan 

[Adreneline]

Thanks for the kind comments Tom and Bryan.

2 hours ago, Tommohawk said:

Does your blue stack look to have good detail?

 

42 minutes ago, assouptro said:

.... is image the blue subs as close to the zenith as possible, that goes for all blue subs on all objects.

I think 'blue' is my nemesis - a bit like OIII. I think I've suffered from 'blue bloat' for all of my AP life and it 's not for lack of trying to achieve optimum focus but I suspect limitations of the optical/filter train. My ED80 is terrible for it - the Samyang 135mm and Canon 200m are not as bad but it is still a problem.

Looking at the subs the only thing that is apparent is that the blue stars look 'bigger' and certainly when I pre-process in APP it always reports higher FWHM figures than R, G and Ha. I did try shrinking the stars in the integrated blue image but I wanted to stretch it as much as reasonably possible to get the nebulosity to come through which resulted in the stars 'growing' again. I've tried to lose the blue halos in the outer lying stars - which on the whole I have - but then that seems to have reduced the blue in the nebulosity and some of the colour in the stars.

I think I need to learn some new skills in PI or PS!

Many thanks again for your helpful comments and insights.

Adrian

[Tommohawk]

1 minute ago, Adreneline said:

Thanks for the kind comments Tom and Bryan.

 

I think 'blue' is my nemesis - a bit like OIII. I think I've suffered from 'blue bloat' for all of my AP life and it 's not for lack of trying to achieve optimum focus but I suspect limitations of the optical/filter train. My ED80 is terrible for it - the Samyang 135mm and Canon 200m are not as bad but it is still a problem.

Looking at the subs the only thing that is apparent is that the blue stars look 'bigger' and certainly when I pre-process in APP it always reports higher FWHM figures than R, G and Ha. I did try shrinking the stars in the integrated blue image but I wanted to stretch it as much as reasonably possible to get the nebulosity to come through which resulted in the stars 'growing' again. I've tried to lose the blue halos in the outer lying stars - which on the whole I have - but then that seems to have reduced the blue in the nebulosity and some of the colour in the stars.

I think I need to learn some new skills in PI or PS!

Many thanks again for your helpful comments and insights.

Adrian

OK that makes a lot of sense and I've just been dealing with the same issue with my ASI1600 - blue bloat! I pretty well fixed it now, but I gave up on M45 for the same reason - I couldn't get any blue without massive star bloat.

The problem is twofold - the optics probably aren't great for blue CA, and the ASI1600 is very sensitive to short wavelength blue. Also some filters transmit much shorter wavelength blues - I fixed mine pretty much 100% by replacing the ZWO blue filter for an Astronomics deep sky (?) one. 

Need to do some domestics now to avoid a domestic, but when I have a mo I'll post some more detail - I did some rough tests. Only thing is I'm using a doublet with a dedicated reducer which may have less CA than yours at the outset.

[Merlin66]

Do you guys refocus for every filter?

A narrow band blue filter with good UV-IR cut should help.

 

[assouptro]

33 minutes ago, Merlin66 said:

Do you guys refocus for every filter?

A narrow band blue filter with good UV-IR cut should help.

 

Good question Merlin 

refocusing between filters is essential! 
I thought my filters were parfocal, they might be, but not with my doublet! 

[Sunshine]

Very nice really, i really like the wide FOV.

[Adreneline]

7 hours ago, assouptro said:

refocusing between filters is essential! 
I thought my filters were parfocal, they might be, but not with my doublet! 

This is certainly my experience although as indicated above I find R, Ha and SII are the same (I'll qualify that below) and B and OIII are the same; G can be a bit tricky but is usually closer to R than it is to B. When doing NB I take Ha and SII together (assuming I want SII) before doing OIII. My experience is that R and G are very similar focus positions but B is always different.

Using my Canon or Samyang I find there are a range of focus positions where the spread of HFD values varies only very slightly so I essentially go for the median position on the focusser (ZWO-EAF). When I use the ED80 (rarely at present) the B is not only in a different position but never gives as low an overall HFD value. (Not sure I've explained that very well but I hope you get the gist).

I've spent the entire evening on one occasion just messing around with focussing, writing down focus position and HFD values and plotting graphs in Excel (how sad am I?) and going round in galactic circles trying to get to a point where I can establish a "best position" for each filter.

Just as a footnote I have tried using the "V-curve" auto focus thingy in SGPro many times and finally got to the point where I could have thrown the whole darn setup in the hedge at the bottom of the garden. What V? W seems be what I get most of the time or a very best a flattened U! I am sure I am using the process incorrectly or got one of the settings wrong but I've given up with it out of pure frustration.

8 hours ago, Merlin66 said:

A narrow band blue filter with good UV-IR cut should help.

This sounds interesting but I'm not sure I know what a NB blue filter is I'm afraid. Can you provide a link by any chance?

6 hours ago, Sunshine said:

Very nice really, i really like the wide FOV.

Thank you Sunshine; I know I lose detail with the 200mm but I too like to see these objects in their surroundings.

Many thanks again to one and all for all the help and valuable comments.

Adrian

[Tommohawk]

Re focusing, yes always refocus between filters, but I still got large blue stars until I switched to Astronomic deep sky blue - you can get them separately. See here - the blue cuts at 420nm.

I just checked and I have some star tests I did with ZWO blue filter compared to Astronomic blue - I'll post later. Unfortunately I only had a short window and tested with Capella which isn't very blue. It make more difference on bluer stars.

[carastro]

I have the same problem with my blue stars, my filters are Baader LRGB.  Blue always produces larger stars and I do them separately as I have to re-focus.  I tend not to bother to stop and re-focus with Red and Green as they are normally the same focus as Luminance.  I check the whole lot before I start imaging and anything not the same focus (I use a Bahtinov mask) I do separately and it always seems to be the blue.  

It makes processing quite difficult as these days I try to shrink the blue stars to match in post-processing, but it doesn't shrink the blue halo sufficiently.  See my latest M42.

Is there a solution? 

Carole 

Edited February 4, 2020 by carastro

[MartinFransson]

1 hour ago, carastro said:

I have the same problem with my blue stars, my filters are Baader LRGB.  Blue always produces larger stars and I do them separately as I have to re-focus.  I tend not to bother to stop and re-focus with Red and Green as they are normally the same focus as Luminance.  I check the whole lot before I start imaging and anything not the same focus (I use a Bahtinov mask) I do separately and it always seems to be the blue.  

It makes processing quite difficult as these days I try to shrink the blue stars to match in post-processing, but it doesn't shrink the blue halo sufficiently.  See my latest M42.

Is there a solution? 

Carole 

Here is another one with the large blue stars. Regardless of what scope or lens I use, blue (and O3) always produce bigger stars. Even with the Astrodon 3nm O3-filter there is a huge difference compared to 3nm Ha. Don´t know if the camera (ZWO 1600 MM-Cool) has a problem with blues or if that´s just the way it is.

Anyway, that M45 looks like it could be good if you collect some nice, tight Lum to go with that RGB. Have you tried stopping the 200mm lens down? It is possible by attaching it to your DSLR selecting for example f/4 and pressing the depth of field preview button and detaching your lens before letting the DOF button go.

[carastro]

Quote

blue (and O3)

Yup Oiii has the same problem.  Maybe it's doublet scopes?

Carole 

[astro mick]

Very nice image indeed.

Mick.

[Tommohawk]

3 hours ago, carastro said:

I have the same problem with my blue stars, my filters are Baader LRGB.  Blue always produces larger stars and I do them separately as I have to re-focus.  I tend not to bother to stop and re-focus with Red and Green as they are normally the same focus as Luminance.  I check the whole lot before I start imaging and anything not the same focus (I use a Bahtinov mask) I do separately and it always seems to be the blue.  

It makes processing quite difficult as these days I try to shrink the blue stars to match in post-processing, but it doesn't shrink the blue halo sufficiently.  See my latest M42.

Is there a solution? 

Carole 

Doublets definitely are compromised for chromatic aberration. In addition the ASI1600MM is very sensitive in the blue end - the window doesn't block much and ZWO filters along with some others are great at passing short wavelengths, down to about 380nm. Smashing if you want lots of near UV, not so good otherwise. Below is transmission for ZWO blue compared to astronomics blue - also shows the QE curve for the ASI1600, and you can see its reaching about 40% of its peak QE even at 380nm. I can confirm these differences do translate into a significant difference in the real world. Trouble is, then the red look a bit big - but its not so noticeable.

Solution #1. Buy an apo and/or use Astronomic filters, cost £££££

Solution #2. Use a newt, eg 130PDS, cost £ (Yes OK you would need a coma corrector and then you get a bit of CA again, but not too bad!!.)

Seriously though, having used newts originally, I've been dabbling with refractors for a while now, and if I could get a fast newt with short FL, say 400mm, I would switch back.

I'll post the star tests later.

 

Edited February 4, 2020 by Tommohawk

[Tommohawk]

PS -  sorry Carole, I know you got rid of your 130PDS …. I was sort of half joking!!

[MartinFransson]

5 hours ago, Tommohawk said:

Re focusing, yes always refocus between filters, but I still got large blue stars until I switched to Astronomic deep sky blue - you can get them separately. See here - the blue cuts at 420nm.

I just checked and I have some star tests I did with ZWO blue filter compared to Astronomic blue - I'll post later. Unfortunately I only had a short window and tested with Capella which isn't very blue. It make more difference on bluer stars.

This is very interesting as I have problems with blue on my 1600MM-Cool and with Zwo filters! Could you also recommend a Lum-filter that does not give the bloated stars that I get with the Zwo Lum?

I would very much like to see star tests. Could you tag me if you start a new post with that?

[Adreneline]

4 hours ago, astro mick said:

Very nice image indeed.

Thanks Mick - glad you like it.

Adrian

Edited February 4, 2020 by Adreneline
Typo

[Adreneline]

4 hours ago, MartinFransson said:

Have you tried stopping the 200mm lens down? It is possible by attaching it to your DSLR selecting for example f/4 and pressing the depth of field preview button and detaching your lens before letting the DOF button go.

No I haven't and I didn't even know you could do that trick with stopping down the lens. I've got to say it's not easy detaching the lens from the ZWO mount, focusser etc. but it might be worth a go. I did try a stop down ring to try to correct star shape problems but it was only a single stop so had little effect. I'm not sure why stopping down might change the CA problem but it's certainly worth a go.

Thank you.

Adrian

[Tommohawk]

4 hours ago, MartinFransson said:

This is very interesting as I have problems with blue on my 1600MM-Cool and with Zwo filters! Could you also recommend a Lum-filter that does not give the bloated stars that I get with the Zwo Lum?

I would very much like to see star tests. Could you tag me if you start a new post with that?

Yes a similar issue applies with the LUM filters - if your scope has any CA these will suffer. Astronomik do a LUM filter (which they refer to as UV/IR blocker) which excludes the top and bottom of the spectrum to help with this. If you have CA go for L3....

….and theres some useful info re this on their page here.

BUT there is a caveat to this. If your RGB aren't parfocal because the scope has different focal points for R G and B, (rather than because the filters themselves aren't parfocal) then the RGB components within the LUM spectrum cant all focus simultaneously. This will obviously impact the quality of the LUM data. One way to know for sure is to check if your RGB filters are parfocal on a scope with no CA (Newt or RC) and if they are parfocal, but when attached to your frac you need to refocus between RG and B, then your frac itself isn't parfocal and your LUM data will likely suffer.

This is a bit beyond the blue bloat issue so best left for another thread I'd suggest!

OK hot off the press here's the Capella  subs - these are unstretched about 12 each colour stacked in APP. If you stretch you'll notice the Astronomik filter version has a light halo - not perfect but better.  Remember this is Capella - bigger difference with Vega or Sirius or any bluer star. I've put all RGB for comparison - hope that helps!

RED:

GREEN:

BLUE ZWO:

BLUE ASTRONOMIK:

[Adreneline]

3 hours ago, Tommohawk said:

This is a bit beyond the blue bloat issue so best left for another thread I'd suggest!

All very interesting and definitely worthy of a separate thread.

My original M45 has taken on a life of its own but all of the contributions have confirmed it’s not just down to poor data, focus or processing - there are more fundamental issues coming into play.

Thanks for all the contributions.

Adrian

Edited February 5, 2020 by Adreneline

[MartinB]

Well I like your M45 as it is Adrian!  They are hot blue stars and very bright for long exposure astroimaging so will bloat and the fact that they are bloated in this image gives a sense of the dynamic range.  Also, living in Derby you are not doubt choked with LP and so you aren't going to be able to pick up a lot of the dust which creates the reflection nebulosity.

Here's my take on blue star bloating.  The sky is blue, that's becaue blue scatters more than the other colours in the visible spectrum so blue is always going to be a bit of a handful.  You do need to carefully check whether your filters are parfocal and if not you will need to focus for each filter.  With a motorised focuser you can work out the focus variation very precisely and incorporate this into your capture software but I doubt this is an option with your Canon lens.  I also use a Canon F2.8 and know how tricky precise focus is to achieve, are you using a bahtinov mask? 

The fact that the ASI 1600 is particularly sensitive to blue isn't really a problem provided you account for this when setting your exposure times.  It is worth spending a bit of time working out your optimum broadband exposure times.  The perfectionist's way is to calibrate using what is known as a G2V star.  This is a catalogue of stars which have the same photometric profile as the sun.  You image using your R Gand B filters and set the exposure of each filter to give the same brightness value.  You then use these exposure ratios on all your images.  So if the brightness level was 30 000 ADUs using 10 seconds for blue, 14 seconds for red and 16 seconds for green you have a 1:1.4:1.6 ratio.  At the start of an image run you work out your ideal exposure for a blue filter based on not having too many saturated stars and then just apply the factors you've calculated.  However a more rough and ready method is to identify the exposure time for each filter which seems to be saturating a similar small number of the brighter stars.  Provided this gives good results all future imaging with the set up used sticks with these ratios.

Try this before buying any new kit!!!  

[Adreneline]

14 minutes ago, MartinB said:

Well I like your M45 as it is Adrian!

Thank you Martin - I'm glad you like it - it's not that I don't like it, it's more a case of could it be better and the answer is usually "Yes"! - but at what cost?

16 minutes ago, MartinB said:

You do need to carefully check whether your filters are parfocal and if not you will need to focus for each filter.

I've done loads of checks and concluded thay are most certainly not parafocal, however, they are not too far away.

One interesting thing I decided to try (probably goes against all perceived wisdom) was a Hoya UV filter on the front of the Canon lens. I tried imaging the Bode region last night just to see what effect it might have on focus positions / star bloating etc. These are my ZWO-EAF positions for all the filters using a Bahtinov mask pointing at Capella in preparation for Bode:

R / Ha / SII 29012

L / G / B    28932

OIII            28892

As you can see OIII is some way off but the others are really not too bad. The ASIair software does not allow automatic refocussing between filters but this at least gives an order for taking the various filters.

The proof of the pudding was how the RGB subs came out on the Bode region and there was virtually nothing between them. APP reported FWHM on all RGB subs of  2.1 - 2.5 which I thought was pretty good for the prevailing seeing last night and far more consistent than my previous experience. No doubt putting a UV cut filter on the front of the lens gives rise to other issues but none were apparent last night.

This is 20 x 120s of unguided RGB pre-processed and integrated in APP and then using PI ChannelCombination and HT. I decided to combine and then stretch as this was more likely to reveal bloating. 

There is minimal cropping so some dodgy stars in the corners.

As nebula season is now pretty much over from this location I shall spend the next few months trying to tease out some of the focus, star shape and exposure problems so I may well give your perfectionist's way a try.

Many thanks for all your advice. I would be interested in your views on the use of the UV filter. I produced new flats with the UV filter fitted but not sure it has completely corrected the gradients - the moon didn't exactly help!

Adrian

 

[Tommohawk]

7 hours ago, MartinB said:

The fact that the ASI 1600 is particularly sensitive to blue isn't really a problem provided you account for this when setting your exposure times.

If the issue were simply that the sensor is exposed to a greater amount of signal with blue light then this would be so - but the issue is partly that the light at the blue end of the spectrum is defocussed, and especially with very short wavelengths. Have a look at this, from Olympus life-science's website:

This probably isn't a typical telescope achromat but it makes the point - light below 450nm wavelength has a huge shift in focal length. A similar example is given at Thor labs here. This illustrates nicely the real problem with light at 380-420nm - its not so much the additional energy, but more about the blur. The same probably applies to luminance filters which transmit at shorter wavelengths when used with achromats.

I certainly agree that you might want to explore all the less expensive options first, but I think you will struggle, as I did, unless you find a way of limiting/blocking the short blue wavelengths.

 

6 hours ago, Adreneline said:

One interesting thing I decided to try (probably goes against all perceived wisdom) was a Hoya UV filter on the front of the Canon lens

That's a great idea …. and I thought exactly the same. But I don't think conventional UV filters block much below 390nm, so unfortunately may not be much help. 

[Adreneline]

33 minutes ago, Tommohawk said:

That's a great idea …. and I thought exactly the same. But I don't think conventional UV filters block much below 390nm, so unfortunately may not be much help. 

I thought it was a relatively inexpensive thing to try - £20 for 72mm filter. First impression is that it has made a difference. As I reported above there was virtually no difference in reported FWHM for R, G or B whereas previous experience (without a UV filter) always gave higher FWHM values on B and the resulting 'blue bloat".

I was also wondering about one of these which claims to "cut out both UV rays below 390nm and IR rays above 700mn resulting in just the light rays in the visible spectrum passing through the filter" but this one is quite expensive.

Adrian