Lee_P

17 hours ago, Padraic M said:

On this point:

I haven't used ASIStudio for stacking so can't say anything about its capabilities. I've used DSS a lot in the past, and currently use Astro Pixel Processor. You can stack multiple sessions with either of these packages, using different sets of calibration frames for each session. With APP, you can even stack different sessions taken with different scopes and cameras! 

A standard technique that imagers on this forum use, is to revisit a target, possibly even years later, and capture more data. If using the same camera, you can probably reuse your dark (or bias) frames, but you will almost certainly need new flats and dark flats. Each stacking session consists of a set of light frames with matching darks/bias and flats/darkflats. The stacking software will apply the correct calibration frames to the correct lights while stacking. If you are using the same calibration frames for everything, you can just add all of the lights to the same session.

As a crude example, take a look at this experiment below. I had a wide-field capture of the Bubble nebula region, taken with a 400mm refractor from last year.  This is a mono Hydrogen-alpha 1.5-hour integration. I then set up a Celestron C8 SCT at 2032mm with the same camera, and centred the Bubble nebula and captured an additional 5.5 hours.

APP registered and stacked the widefield lights and the long FL lights with no issue. You can see that I didn't apply flats as there is vignetting (and some light pollution) but what you can also see is a huge reduction in noise in the central area because of the extra total integration time.

Now with a little crop, rotation and simple curves adjustment in Gimp, I can get a passable Bubble close-up!

That's a great visualisation of the concept, makes it really clear

@Gogleddgazer @rwillett @qisback You might also want to check out this thread I've just spotted, by @powerlord. Sounds like it's a bit simpler, and perhaps cheaper, but also I expect a bit bigger and heavier. Good to have options!

Nah, I just push it up against the telescope's dew shield, nice and flat. That creates a good seal.

2 hours ago, MHaneferd said:

@Lee_P But, if you use flats for severa hours of imaging, the dust particles should stay at same spot over all 20 hours? I am just thinking that if I move the scope in and out for two sessions on the same M31.. the dustparticles will probably move. And how will the flats be intrepeted then in the stacking??

My telescope is a "closed system" so to speak -- easier being a refractor -- so it's not that easy for dust to get in. Also, I don't change anything (like swapping filters, removing or rotating the camera) during a project. So far I've used this Flats method for about a dozen long-integration images, each of them involving data acquisition over many many nights, and have never had any issues. Try it and see if it works for you too?

I've just written up how to make a DIY Flats Panel: http://urbanastrophotography.com/index.php/2021/09/27/diy-flats-panel/

It's very simple, but may be of interest to astroimagers that, like me, have pretty low DIY skills!

Great! I think there's something a bit special about monochrome H-alpha images.

11 minutes ago, Mike73 said:

@Lee_P I've only just noticed your post.

I used to sketch my observations but unfortunately the majority were done through either a 12" or 16" but if I can help in anyway I gladly will. 

I think I sketched around 300 objects in total but definitely completed all the Messiers most of the Caldwell's but the NGC list beat me 😂.

 

Thanks Mike, that thread is a fantastic resource!

Hello and welcome 😃

Nice shot, lots of fine detail there. I use an Askar FRA400 too, great 'scope.

Ooh, interesting question. My approach is just to get as much data as possible, so if I were you I'd be imaging all night, filter or not. Even your pre-midnight skies are better than the best I ever get 😁

Fantastic, especially for your first picture!

On 21/09/2021 at 08:57, licho52 said:

I am using the L-Extreme and it's great, however there are 2 things that come with it, first, to really get star colors it's important to capture an additional broadband stack with color stars to put in place of l-extreme's star field.  Second, some stars are just too bright and their halo is can only be removed by hand using Photoshop or another program like that.  Sadr or Alnitak come to mind.

I essentially throw out all stars that come from l-extreme.  I don't even care so much if there's some frame with elongated stars, they get removed either way.

Putting broadband stars into an L-eXtreme image is something I want to try but haven't gotten round to doing. I'd be interested in seeing any examples you have.

19 hours ago, MHaneferd said:

But, if you do several nights.. How do you manage flats? Do you take a set of flats and darks after each night, or do you do that at the end? 

Darks: I always shoot 120-second subs at -10 deg C, and have a Darks library matching that. So I use the same Darks for every imaging project.
Flats: I take one set of 20 Flats per imaging project (i.e. the same set of Flats covers approx. 600 subs / 20 hours integration time). I've got a DIY Flats panel I use for this: http://urbanastrophotography.com/index.php/2021/09/27/diy-flats-panel/ I take these 20 Flats altogether whenever is convenient; normally before an imaging run one night. This reminds me that I haven't taken the Flats for my current M31 project yet, so I should do that soon!

11 minutes ago, MHaneferd said:

@Lee_P, I think it can be better. It’s what you are saying the longer integration time, the  better signal to noise ratio can be achived. As I had to scrap 50% of the frames, and I suspect the remaining percentage also have som dizzy cloud in them, I will test another go for two minute subs over several hours. Just have to wait for clear sky again.. going to rain for 14 days now 😳

You can always add "just one more night" of data into the stack..! I'm currently imaging M31 as well, and am at about 20 hours. Nothing but rain forecast now, too

@MHaneferd a great result, especially considering it's just the raw stack! (Also, good idea deciding to test various exposure times for yourself and seeing what works best for you!)

3 hours ago, iantaylor2uk said:

Yes - I agree with you that these recommendations are a minimum and you can go longer if you wish - the whole point however is that there is not a lot to be gained by going longer (apart from having less subs). Shorter subs means that if you have to lose any (due to satellites or bad guiding) there is less of an overhead. 

There is a lot to be gained by going longer if the recommended sub length is just a few seconds, but you want to get a total integration time in the tens of hours -- see my comments later about file sizes. I understand the benefits of short subs; I've written about that here. By the way, don't worry about losing subs due to satellite trails, stacking algorithms remove the trails efficiently.

3 hours ago, iantaylor2uk said:

I think you misunderstand though the effects of light polluted and dark skies - if you are in a light polluted area the spreadsheet (and Dr Glover's presentation) would guide you to shorter subs, whereas if you are fortunate enough to be in dark skies, you can use much longer subs. 

I'm not misunderstanding this. If you re-read my previous comment, you'll see I say "The background sky level has a huge influence on the 'optimal exposure' length, which is about 10 seconds for me using an L-eXtreme" and "you trebling your sub length with an L-eXtreme is likely the right thing for you to do as you’ve got darker skies so have got more headroom, so to speak." I'm in agreement with you! 

3 hours ago, iantaylor2uk said:

There are plenty of people around who use short subs (and thousands of frames) - I saw a good M57 where the subs were only 1 sec each: https://www.astrobin.com/345864/?image_list_page=2&end_date=2020-05-02&nc=AnonymousUser&page=3

I used to have a 12" f/4 Newtonian, and I got a good image of M51 using just 10 second exposures (I live on the outskirts of Chester where it is a Bortle 6 sky) - the photo below is a stack of just 180 subs (only 30 mins integration time) - I didn't use a coma corrector so some of the stars at the edges won't look too good: https://photos.app.goo.gl/eDNKesm8ZSNoLCPX9

Sure, I've seen many images like this. They're excellent proofs of concept. But again I reiterate what I've said previously, which is that from heavily light-polluted skies you benefit a lot from long total integration times. Let's say you want to get a total integration of 20 hours just from 1 second subs. That's 72,000 images. Each image from my 2600MC is 50MB. That's about 3.4TB of harddrive storage. Just for the raw subs. OR shoot 120-seconds and then we're talking under 30GB -- and you're still getting most of the benefits you get with shooting ultra-short subs. If I had dark skies and so could get away with lower total integration times, I might lower my sub length, and perhaps get into the realm of "lucky imaging". The same for if I had no restrictions on harddrive space or processing power. But I don't, and find that 120-seconds is a sweet spot for me. That's the sub length I used for most of these shots.

1 hour ago, iantaylor2uk said:

If your background light pollution is high, you should be using shorter subs. I have a 7 year old i3 dual core desktop and I can still  stack 400 subs in deep sky stacker in 30-45 mins or so.

I think we need to be careful about the phrasing here: it would be accurate to say that I could use subs shorter than 120-seconds, but not necessarily that I should.

Keep in mind that total integration time is the most important factor. Especially if you’ve got badly light-polluted skies, you really need to tease out that signal from the noise. I aim for 20 hours per target.

The background sky level has a huge influence on the “optimal exposure” length, which is about 10 seconds for me using an L-eXtreme. That would be 7200 subs. 7200! I could do that but don’t think that I should.

So, I shoot 120-second subs. One 120-second sub is equivalent to 12 10-second ones. And with 120-second subs, I now need 600 of them to make it to 20 hours. Still a lot, but manageable. 120 seconds isn’t some magic number reached at through calculations, but rather a reasonable compromise for me to get the benefits of shooting short subs without filling my harddrive or melting my PC. FYI I have a modern computer and 48GB of RAM. I integrate using PixInsight, which seems to be more resource-intensive than DSS. I generally leave it integrating overnight.

When I got my camera I found Dr Glover’s presentation about CMOS exposure times, and ran through the calculations – it’s only just occurred to me that that’s what the spreadsheet is doing.

I think that we’re both right in our approaches: you trebling your sub length with an L-eXtreme is likely the right thing for you to do as you’ve got darker skies so have got more headroom, so to speak. @MHaneferd is in Bortle 8/9, similar to me, so will likely find that the same kinds of settings as I use will work well.

That’s my understanding of it all anyway 😊

Thanks! I think that my background sky level is so high that 120s for L-eXtreme is way more than three times the optimal exposure when not using any filter anyway, and the limiting factor is storage space and PC processing power. So in effect my current method (120s for everything) is still valid for me given my local sky conditions. If I had much darker skies then it would be a different story.   

Thanks @Budgie1 and @vlaiv, this is all great info and will definitely help me!

Here's my Elephant's Trunk using the same camera and filter, Bortle 8, and also following @Luke Newbould's excellent tutorial. Regarding halos around stars, I fix those as part of my standard workflow: after making a starless version using Starnet++, I then clean up any aberrations -- including the leftover halos -- using the clone tool in Photoshop.

11 minutes ago, iantaylor2uk said:

The L-enhance filter only lets through H alpha, H beta and OIII wavelengths so greatly improves the signal to noise ratio of emission nebula (and is also great at reducing light pollution) but the overall signal is lower so you have to go to longer subs. There is a spreadsheet for ZWO cameras where you can put in details of your telescope (aperture and focal length), sky brightness level, and it tells you min exposure times, also for different filters - you need around 3x longer subs for the L-enhance filter compared to no filter.

I'd be interested in a link to that spreadsheet, if you have it handy. I've always shot 120s regardless of filter (i.e. no filter or L-eXtreme) and I've gotten good results -- happy to change my method if another one is more effective! Not quite sure of the logic though, as you get good signal from a high total integration time rather than individual subs, right? I'm tired, my brain might not be working properly...

@MHaneferd Good idea, I did the same experiment when I first got the camera. That helped me settle on 120s.

That does look like a satellite in your 300s shot; what time was it taken?

I'm in Bortle 8 too. It's a pain, but we can still get good images with long integration times. You'd get there faster from your cabin though!

FYI your 120s sub looks better than my 120s sub:

Hi SGL Hive Mind,

My current imaging project is M31 with my OSC camera. I'm close to 20 hours of RGB data (no filter), and would like to take L-eXtreme data to add in too, to get some glowing red hydrogen. I'm using PixInsight and haven't done this before, so am wondering if anyone can help; either by giving me some tips, a workflow, or link to a tutorial. My guess is that you process the RGB and L-eXtreme data separately and then combine them using some PixelMath wizardry..? I've done a bit of Googling but haven't found quite what I'm after. There are bound to be good resources out there that I'm missing!

Thanks,

-Lee

@MHaneferd Neat test! Remember that it's the total integration time that counts though. In principle your 100 x 120-second integration should be very similar to 200 x 60 seconds, or 40 x 300 seconds. 

1 hour ago, MHaneferd said:

I really liked that article. I did not find anything about the gain at the end, but I got the point of short subs. That makes perfectly sence when the sky have some cluttered clouds flying by.. I experienced that with my 10 mins.subs.

Also loved the Pier!! That is something I will build at my skiing-cabin in the mountains. That was quite awesome!

Thanks! I meant that the reference to 2-min subs was near the end of the article. I don't mention gain as that's not specifically about getting long integration times. A pier is indeed fantastic, I highly recommend them!