First LRGB image - strange RGB glows around edges

[Elp]

10 hours ago, BrendanC said:

It seems ZWO cameras prefer longer flats

With my 183mm my flats in LRGB are short around 0.01-0.05 seconds, with a SHO filter maybe 1 second. I use an led tracer pad as my illumination.

Edited March 30, 2022 by Elp

[BrendanC]

So do I - but longer flats is all I have to go by currently as a possible fix for this.

[Varavall]

I do the flats in the same way too, so just as an out there thought. If you hold the filters up to the light, does the colour extend all the way to the edge; no ring of light!

[happy-kat]

With a DSLR a dark flat is the same everything as a flat but with the lens cap on. Darks are the same as the lights but with the lens cap on. I  haven't read yet to check if there are differences to this for an astro camera seeing as in general both are CMOS.

[BrendanC]

5 hours ago, Varavall said:

I do the flats in the same way too, so just as an out there thought. If you hold the filters up to the light, does the colour extend all the way to the edge; no ring of light!

I haven't checked, but they're the ZWO 1.25 inch mounted filters in the ZWO filter wheel, so I would hope there's no issue there.

 

43 minutes ago, happy-kat said:

With a DSLR a dark flat is the same everything as a flat but with the lens cap on. Darks are the same as the lights but with the lens cap on. I  haven't read yet to check if there are differences to this for an astro camera seeing as in general both are CMOS.

Sure, I've used darks, flats and dark flats tons of times with DSLRs and never really had an issue. But it seems there are multiple issues going on with my image, from the duration of my luminance vs RGB subs, to the flats and dark flats exposure time, and maybe even my filters. It could also be that I'm trying to shoot a tough target because it's very low in the sky and near another bright object, so I'm wondering whether I should go for something easier like Bodes. I've also been trying to run before I can walk with gain and offsets, so I'm going to stick with unity gain and offset 21 which is the standard setting.

I've redone all my flats and dark flats - bearing in mind the flats might not be quite aligned any more, it's still worth a punt - and they're calibrating in APP as I type.

I mean, I cannot be the first person to use an ASI1600MM-Cool with ZWO LRGB 1.25" mounted filters and a 130PDS, right? Surely this is possible.

Edited March 30, 2022 by BrendanC

[Elp]

Shooting close to the horizon is probably what's done it. Can't really explain why you've got the bloom in different areas but as an example of an unexplained issue I sometimes have, I setup, point the telescope east towards m3 or m13, there's a lamppost around a fists width or two away from where the scope is pointing. Looking down the length of the scope it's not pointing directly at the glare from the led lamp, the dew shield is fully extended and there's no reflection on the front objective from the led lamppost. My subs however show something entirely different as if it's zoomed and focussed right into the lamp of the lamppost but I suspect it's an internal reflection of the camera sensor and window. So stray light can have odd effects based on angle of incidence/reflection.

[BrendanC]

Very interesting, thank you. I've been advised that there is a bright DSO just off the FOV of my shot which could be affecting things and yes, the object is low in the sky above some streetlamps, which my DSLR probably would have handled but I think the increased sensitivity of the ASI1600 doesn't like it. At all. As for the blooming, well I'm working on a theory that it might have something to do with the filters vignetting and meridian flips, but I need to look into that more. Basically, everything that could have gone wrong, did go wrong!

[symmetal]

Brendan,

Running your fully stacked LRGB images through Startools produced the same results as you had, terrible colour patches which changed dramatically depending on how much cropping was applied, but couldn't be eliminated. The 10 sub samples you posted yesterday were much better in that respect so I think you need to check your subs individually and discard any that look bad before stacking them. You possibly had clouds passing through or haze and moon affecting some of them. Your 10 sample subs you provided just exhibited a small gradient which 'Wipe' corrected well. I loaded your full stacks into ASTAP as a preview and here's a composite of then given similar stretches.

They are very different to each other  and the colour channels have very different complicated gradients and blocky dark artifacts. I think that Startools Wipe just doesn't know what to do as there are no areas where a 'reference' background can be determined for the Wipe to work on. There is no standard scope vignetting and no visible dust spots so I don't think your flats are a problem in that respect though if you used short exposures for the flats the flats probably added some noise which hasn't helped. Given the total integration time the colour channels in particular are very noisy too. The short colour exposures haven't helped here. I think the charts you linked to giving exposure lengths are for luminance exposures only as they seem about right for L. For RGB only roughly a third of the light reaches the sensor compared to L so 3 times longer RGB exposure is required to achieve similar sky background ADU levels.

You used half unity gain too which hasn't helped. Personally I can't see the point of using less than unity gain as you are just throwing away photons. At unity gain (139) one photon received registers 1 ADU (12 bit) on the camera. At half unity (75) two photons need to be received to register 1 ADU, so you are effectively throwing away every other photon received.

At gain 75, offset 21, the 10* RN^2 sky background ADU level to swamp the read noise is 723 ADU (16 bit) as shown below. Your RGB background ADU levels are well below this so read noise is noticeably adding to every sub.

Increasing your RGB exposures by three will bring the background ADU up to between 900 and 1100 depending on the colour which is higher than the 723 ADU on the chart but not enough to worry about for the moment. What bortle skies do you have?

I suggest unity gain (139) and offset 50 for your next outing. Try some test subs using 60 sec L and 180 sec RGB (when there is no moon) and view the image statistics or move your mouse over the sky background in your images and see what the ADU values are. 1290 ADU is the calculated 'magic' value I would use for read noise swamping. If they are closer to 2000 or higher on all channels then you could reduce the exposures but I would always keep the RGB exposures 3 times longer than your L. Once the read noise is effectively swamped by the sky background then there is no difference in SNR between  say 1 exposure of 10 mins, or a stack of 10 exposures of 1 min. The 1 min exposures won't have such clipped stars so the stack will be better.

Your darks look good. Your average value is 1/200 of peak white. Mine are 1/75 of peak white as I use a higher offset so are much 'brighter' than yours. The dark flats and flats look good too. As I mentioned there is no noticeable vignetting or dust spots on your stacks so the flats are doing their job. Just retake then using longer exposures (around 2 seconds) to make their noise profiles match your lights for better calibration.

I think that covers it Brendon. 🙂

Alan

 

[Elp]

@symmetal you mention blocky gradients, looking at your processed images they kind of look like foreground object occlusion. Did you take such images and omit such frames out of the stack @BrendanC?

[symmetal]

44 minutes ago, Elp said:

@symmetal you mention blocky gradients, looking at your processed images they kind of look like foreground object occlusion. Did you take such images and omit such frames out of the stack @BrendanC?

That's a good point. I was wondering why the gradients changed so quickly over certain areas, particularly on the RGB stacks.

Alan

[BrendanC]

Right, well, this is brilliant. I am so grateful. I was freaking out initially, not really knowing where to start.

I think you're both right in all respects.

I should return to unity gain which also means offset 21 according to ZWO recommendations.

My RGB exposures are too short. This is because I didn't notice that the table I was using was based on luminance.

My L full integration time should be around the same as RGB combined.

There's field rotation, and this is a known problem when I do a flip - I think I have cone error, which I've lived with for a while, and just need to fix, but it keeps getting pushed down my 'to do' list

The object was shot low in the sky.

There's another bright DSO just off the FOV.

I need to learn about ADUs more thoroughly, and how to analyse images.

The object was also captured as it should have skimmed a tree, but I'm now wondering whether I misjudged that, and the tree has in fact screwed up a decent proportion of the subs. The problem there is how to get a FITS file viewer that lets me skip through files quickly. I've tried a few and none of them are as fast/responsive as handling CR2 files. I might load them all into DSS and have a look there. Or, I might restack and just use the top 50%, see what difference that makes.

So, all in all, I made a bit of a mess of this! Plus, I probably chose a tough first DSO. I'm going to try Bode's Galaxy, hopefully tonight, which is nice and high, circumpolar, and I've done it before so I get a decent comparison. I might even be able to have a stab at LHaRGB if I can get the LRGB bit to work out.

Again, thank you, really.

Cheers, Brendan

[Elp]

Hope you sort it out, we all have to overcome issues at some point unfortunately.

Regarding a fits viewer, yes it's cumbersome, I'm yet to find one which works just like windows picture viewer does with tifs or jpegs. Your idea about scrolling via DSS is actually a good workaround. Otherwise I usually squint at the tiny jpg previews the zwo generates (doesn't really work that well).

[BrendanC]

Thanks! You've been super helpful. I'm going for Bode's and the Cigar tonight, so hopefully that's going to be a bit easier.

The DSS approach sort of works but FITS is a pain for viewing generally, I've found.

[symmetal]

Brendan,

The latest Ascom drivers by default have offset 50 for all gain settings on the 1600MM-Cool which you can override by clicking the 'Advanced' button. I would recommend using that rather than 21 which was the setting on the original drivers for unity gain. You need an offset value large enough to avoid any black clipped pixels on your images. Normally a bias frame will tell you this as normally the bias will give the lowest ADU values and longer exposures will always have higher ADU than bias frames. On the newer ASI cameras this seems to be the case, and with them having no amp glow, bias can be used as darks. I still take darks on my ASI6200 as habit but I've done comparisons using bias as darks and can't see any difference in the results.

As mentioned, the 1600 and some other earlier cameras internally process short exposures differently. At unity gain, offset 21, a bias frame probably won't show any black clipping but exposures over a second probably will. Hence why the later drivers from ASI have 50 as the default offset for all gain settings. I use 2 sec darks as test frames and found I had to increase the offset to 64 to totally eliminate any black clipped pixels. I believe vlaiv has his ASI1600 offset set to 72 for the same reason.

Take some darks around 2 seconds and examine them for black pixels. Fits Liberator is good for this. The histogram shows the range of ADU values in the image and the latest version of the program will also highlight black and white clipped pixels. If at offset 21 there are no black clipped pixels and the minimum value on the histogram display is at least 50 then by all means use offset 21. 🙂

For previewing fits files quickly I use Nebulosity. It has a nice 'Preview Files' option. The trial version won't let you save images but for previewing that shouldn't matter. I did purchase it many years ago when there were few capture/process programs around but I now mainly use it for previewing fits files. 🙂

Alan

[BrendanC]

Brilliant, again, thank you Alan. I have just been reading up on this (again) and have noticed a lot of recommendations to use 50, so that's kind of nudged me towards actually using it.

Unfortunately I just ran the APT flats tool at offset 21, and everything is set up outside ready to go!

So, I think I'll either re-run at offset 50 (or 64 or maybe even 72 if that's what @vlaiv uses because he's helped me out plenty too in the past) with everything set up in the garden when it's dark enough; or I'll do the shoot at whatever offset I choose, then re-run the tool tomorrow in the closest possible conditions and do my flats then.

Again, I'm so grateful for your help.

Edited March 31, 2022 by BrendanC

[Elp]

Once focussed and setup Ive started doing my flats and darkflats first, that way I can leave it to concentrate on lights.

[vlaiv]

2 hours ago, BrendanC said:

So, I think I'll either re-run at offset 50 (or 64 or maybe even 72 if that's what @vlaiv uses because he's helped me out plenty too in the past) with everything set up in the garden when it's dark enough; or I'll do the shoot at whatever offset I choose, then re-run the tool tomorrow in the closest possible conditions and do my flats then.

Out of interest, in the first link you posted, flat was taken at -15C and flat dark was taken at -5C

There is also significant difference between dates of capture - one was taken on 19th (flat dark) and other on 25th (flat).

Focal length of instrument also differs.

I don't really think flats are to blame here. Maybe there is some sort of light leak from somewhere. Do you have any light near by when taking any of the subs or is it done in complete dark?

[BrendanC]

That's quite simply because I'm trying to get the hang of the camera.

I didn't think the temperature of flats or flat darks mattered?

The dates difference is because I wanted to get the flat darks done ahead of time while I could, before the shoot.

As for the difference in focal length, I have no idea! That is quite bizarre.

There are no light leaks that I'm aware. I live in a Bortle 4 zone, the back garden isn't really brightly lit, just a few houses around. The DSO I was shooting was however quite low in the sky, and I can well imagine there is more LP at that height, in that direction, than I previously thought with my DSLR, which the ASI1600 might pick up more readily. I also thought it was going to skim above some trees but I have a feeling it  might have gone behind them for some of the subs. I'm stacking the top 50% of the subs as I type, so I'll see if that improves things.

Also, I've been told my RGB subs were far too short. This is because I was under the impression all LRGB filters needed the same sub length but I was very wrong! I've had a couple of the good folk on the StarTools forum take a look and they've given me some great advice for next time around - basically, higher gain and offset, longer RGB subs, more L time overall too.

I've taken some more shots of Bodes Galaxy and just need to get my flat darks (or dark flats, whatever we're supposed to call them) and darks sorted, then I'm hoping to see something a bit more presentable. Fingers crossed.

[Elp]

Unless you've got zen like ability, your flats HAVE to be taken at the same time either before or after your lights with the same camera, focus position, camera orientation, temperature, filter orientation and position if you use them, ambient temperature for the scope glass and air. Otherwise it's highly likely they will not match your lights. If you do multiple shoots of the same object, flats have to be taken again for those lights, unless you are absolutely certain nothing of the above has changed. Even new dust/dew settling onto the front objective of the telescope will make a difference to your flats from one session to another.

Edited April 1, 2022 by Elp

[vlaiv]

17 minutes ago, BrendanC said:

I didn't think the temperature of flats or flat darks mattered?

Ideally - you want everything matched between subs and corresponding darks (lights and regular darks, flats and flat darks).

There won't be very huge difference, but it can - especially if you use longer flats (like couple of seconds).

I think there was something odd with subs - it is not due to integration, I don't think it was due to LP - LP creates simple gradients - this has light all over the place. I don't think it is flats - I think your flats are working properly as you have one dust doughnut (r channel) and that is calibrated just fine - can't seem to find it in insanely stretched sub - but I can that strange light gradients.

Here is what I would advise you to do if you want to diagnose this. Take two dark files - with usual settings that you plan to use in future (offset, gain, temperature and exposure length) - take one while camera is on telescope, and do second in dark room with camera covered and placed "face down" on a desk to minimize any change of light leak.

Post those two darks for inspection (or inspect them yourself - do stats on both - compare average and median values and stretch them very hard and see if they look different - also subtract them and look at their difference - ideally it should have 0 mean / median value and be pure noise without any patterns).

[vlaiv]

5 minutes ago, Elp said:

Unless you've got zen like ability, your flats HAVE to be taken at the same time either before or after your lights with the same camera, focus position, camera orientation, temperature, filter orientation and position if you use them, ambient temperature for the scope glass and air. Otherwise it's highly likely they will not match your lights. If you do multiple shoots of the same object, flats have to be taken again for those lights, unless you are absolutely certain nothing of the above has changed. Even new dust/dew settling onto the front objective of the telescope will make a difference to your flats from one session to another.

People with permanent setups and electronic focusers / filter wheels (that are able to repeat position) often reuse flats.

Good thing about flats is that you can do them after the session if you see that something changed - like a bit of dust settled on the optics or moved.

Dew usually won't change flats as it is way out of focus (not being close to focus plane) - and acts as general light block over whole sensor equally.

[BrendanC]

Great, thank you vlaiv, I'll do as you suggest.

[Varavall]

1 hour ago, Elp said:

Unless you've got zen like ability, your flats HAVE to be taken at the same time either before or after your lights with the same camera, focus position, camera orientation, temperature, filter orientation and position if you use them, ambient temperature for the scope glass and air.

Well that sounds good in  theory, but totally impractical in the real World. When I start a run that will last until dawn, the temperature will change all through the night, possibly minute by minute, so it is quite impractical to get everything at the ambient temperature at the time the lights were taken, is it not? Perhaps you have a hermetically sealed observatory with glass roof that permits transmission of all frequencies of light 🤔

[Elp]

6 minutes ago, Varavall said:

Well that sounds good in  theory, but totally impractical in the real World. When I start a run that will last until dawn, the temperature will change all through the night, possibly minute by minute, so it is quite impractical to get everything at the ambient temperature at the time the lights were taken, is it not? Perhaps you have a hermetically sealed observatory with glass roof that permits transmission of all frequencies of light 🤔

No, but it's far easier to take them at the time rather than after the shoot like the next day. The problem you've described, do you think that will be helped by taking flats the next day, its just another variable to introduce and in the hobby of AP you want to minimise variables as much as possible. Taking flats doesn't even take that long.

Edited April 1, 2022 by Elp

[Varavall]

6 minutes ago, Elp said:

No, but it's far easier to take them at the time rather than after the shoot like the next day. The problem you've described, do you think that will be helped by taking flats the next day, its just another variable to introduce and in the hobby of AP you want to minimise variables as much as possible. Taking flats doesn't even take that long.

Sorry but getting up at 5 am and running flats when the temperature has already changed markedly doesn't seem sensible. As my rig is a permanent setup nothing changes in kit orientation, so I'll live with the slight errors that may accrue because of temperature variations.. I can always blow any dust off. Maybe you have a case if its a portable setup.