Filroden

I'm partway through moving house so the scopes are all packed up. Typically tonight looks like it might me clear! Anyway, I doubt I'll be Imaging before July. 

Thats a nice M57 but is there a slight pink/red tint to it? Some of the stars look pink whereas they are probably closer to white? Nonetheless, amazing for 150s of data!

That reducer is giving you a similar field of view as my 80mm. Can't wait to see everyone's efforts on M33 in the coming months. 

3 hours ago, SteveNickolls said:

Hi folks, here is my latest offering taken on the evening of the 14th May 2016 of the Crescent Nebula NGC 6888 in Cygnus. The image was taken using a SkyWatcher Startravel 102mm f/4.9 refractor, Synscan Alt-Az mount and Canon 600D DSLR. Due to the position of the target I was able to employ 60 second exposures. I took 91 sixty second light frames at ISO 1600 and for the first time used stock dark frames (x50) and bias frames (x50). x50 flat frames were taken the following morning. The frames were stacked in DSS and further processed in StarTools. Of the 91 light frames taken DSS was happy to use x67 (just under 74%).

Hope you like the image.

NGC 6888 Crescent Nebula-

 

Beautiful image. Do you think the stock darks and bias had any effect?

I've been quiet for a few days. Although it's been clear here, the moon is getting too bright in the evenings and I've not had the stamina to wait until 2am for it to darken. Also, even though it's often looked clear, there has been a high haze that has reflected more light than usual.

I did take some subs but they were so bright from the moon plus normal background lights that I've not been able to integrate them with earlier subs. However, I did drag myself outside last night at about 2:30 and what a sight! I could almost make out the Milky Way (may have been imagining it but I thought it was there, overhead) and I could definitely make out mag 4.3 stars with direct vision. I was so tempted to set up and take some subs as the North America Nebula was at about 60deg altitude. Antares was quite high and another tempting target, though by 2:30 it was already behind the houses near me. I'm now wondering whether I need to set up and start imaging around 1:30. Though I suspect that now the moon will destroy any attempt other than globulars.

ISO in digital cameras just amplifies the signal and noise. You still capture the same amount of photons at ISO 100 as ISO 1600 ( I think). You can achieve the same amplification in processing  

Exposure time is the important factor. 

There are benefits to higher ISO settings. I think read noise is lower at higher ISO and you also see your image quicker so you can see if you have a good sub. 

12 minutes ago, Stub Mandrel said:

An anyone answer me a question - is the histogram x-scale linear or logarithmic scale?

i.e. if it has say 5 divisions are these:

1          2          3         4        5            -  linear

1          10        100      1000   10000     - log base 10

1          2          4          8       16           - log base 2

My guess is that it's a log base 2, which would fit with things like changing aperture by one f-stop, changing ISO by one step or doubling exposure would move the histogram by one division, this would also explain why a histogram shifted right by a longer exposure doesn't get noticeably wider.

Does anyone know what scale Canon use?

A quick search doesn't give me a clear answer. Your assumption about it being log base 2 looks broadly correct, but based on a test someone did and posted to flickr, it seems the histogram shows more than 5 stops, but with the additional stops bunched at either end of the scale. Not sure me using spaces in this does it correctly but the numbers 0, 4, 8, 16 and 64 line up with the five vertical lines, with the 2 and 32 being halfway between the first set and last set of lines. Note: this is specific to Canon.

0      2      4            8            16    32     64

Hopefully someone much more technical than me has a better answer!

43 minutes ago, SteveNickolls said:

Hi Mike, Thanks for these posts. I can understand some of the article but floundering elsewhere. I took a look on my camera at the RAW exposures I took a few nights ago with the histogram also showing. I found that the histogram, whether for brightness or RGB channels was showing well off from the LH side about 60-70% along not the 10% in the article. Now this was for 50 seconds exposures at ISO 1600. I presume it means the image is being swamped by light pollution (it's bad here) so I should reduce the exposure time or drop the ISO value? Or does it matter? Unfortunately I have no background in photography to draw upon, just learning from mistakes and successes.

Cheers,
Steve

Here's a 90 second exposure at ISO 1600. It's at 80% but it does develop and I have star colours so it hasn't been over exposed. I guess some trial and error is needed to know what your conditions allow. 

1 hour ago, Nigel G said:

wxsatuser,

 To be honest I don't understand what you mean, ( histogram peak on the back of the camera  ) & ( clear gap between histogram tail and histogram origin ) 

At the moment this is foreign to me, but I need to understand.  I think I need to do some research ☺

Nige. 

Here's a screenshot of one of my rates showing the histogram. This 45 second shot shows the peak blue starting around 30% in from the left (left is black and right is white on the scale). You want some gap between the start of your first peak and the left hand as lots a feint details are found here. If you don't expose enough, that detail is never captured. 

Of course, my histogram also shows my light pollution because the red is further right, I.e brighter. Again, you don't want to over expose so you also want a gap at the right. This one is harder to judge because stars can be easily over exposed and end up having no colour. 

So I aim for the peaks to be in the 30-40% from the left. This just happens to be about 45 seconds at ISO 1600 for my garden. 

9 minutes ago, Nigel G said:

Ken. There's a good improvement there, sharper , more details and better colour. 

Nice ☺ I'm going to look into pixlnsight with improvement like that.

Nige.

If you do then take a look at the tutorials here http://www.lightvortexastronomy.com

Back to the pretty pictures...

I've just reprocessed M13 using PixInsight. I'm getting a little faster but it's still a lot slower than using DSS. However, given I'm using the same subs I think the stacked images from PixInsight are much sharper than in DSS. Here's 21 x 45s ISO 1600 using Esprit 80 on the Evo mount and the Canon 60D. Bias and flats used for calibration but tested with no dark; stacked and processed in PixInsight with final tweaks in Photoshop and Lightroom. I think this is better than my original attempt though the difference isn't as pronounced as it was for NGC7000 and M46 from earlier in the week.

BEFORE

AFTER

That puts my mind at rest. I really didn't want to remove backgrounds from individual subs. Thank you

7 minutes ago, nicks90 said:

you are right, the gradient will 'spread' across the bottom of the image and up the sides due to field rotation.

But... how many hours of subs in a single sitting are people taking here? Look at the overall angular rotation of something like Leo over 2 hours by watching it move in stellarium in fast forward... few degrees maybe?

Maybe its my tracking but I notice it frame to frame, i.e. noticeable in minutes, not hours. Thinking about it, I'm exposure limited to 45 seconds for NGC7000 when it was at about 20deg above the horizon and roughly NE (so should show less rotation than something closer to zenith or near the meridian). After somewhere between 60 and 90 seconds I see trails. So I can't imagine how much there is over an hour or two's imaging session.

A quick eyeballing of Hercules between 5pm today and 10pm shows it rotates about 45 degrees in 5 hours.

5 hours ago, The Admiral said:

They are very good scores Nige. I usually find the top score is between 1000 and 2000, with a tail that goes down to single figures, and a handful with no score. Varies of course.

Ian

I never know how much stock to put in the scores other than as a relative measure between subs. For different targets I get very different scores - where there are few stars, the scores are much lower. It's not something I really understand within DSS and only use it as a guide. I've noticed one of the modules in PixInsight that I've used when following the tutorials provides a range of different quality measurements of an image including the size and elongation of stars, and how you can combine these to provide a weighting when stacking the images. My most recent attempt, processing M56, included 5 subs which I'd assessed as only "maybe" quality but I stacked them anyway and PixInsight used them but placed a lower weight against them. I probably should have excluded them but its a balance between what each additional 45 second exposure adds compared to how much it reduces sharpness of the overall stacked image.

There is so much to learn!

I thought I'd try and show what I meant about light pollution gradients with a graphic as I can't explain it well in words. One of the main issues we face as AltAz imagers is field rotation. Our images rotate over time as we track across the sky. The top three "photos" show the same group of 5 stars taken, lets say, 20 minutes apart and show considerable rotation. I've shown an even light pollution rising from the bottom that impacts the bottom part of each photo. So the stars rotate but the light pollution does not.

When you stack these images, the software registers the locations of the five stars and aligns the photos so they stack over each other. However, now the gradient is in a different location. The lower stacked image shows how the program de-rotates the images to align the stars but this now causes the gradient in each image to be rotated the exact opposite angle in each image and this is stacked - pollution being an additive.

It's much easier to remove the simple linear gradient from each image (but very time consuming) than remove the more complex, bow tie, gradient in the stacked image. I wonder how much this impacts on the final quality of our images. 

I decided to reprocess my images of M56 from the other night too with PixInsight to see if it makes a difference. It's not quite a fair competition as I could stack different exposures and ISO setting in the second attempt and I used calibration. However, it does show that I would over clip the blacks in Photoshop (though I've blown the core of M56 in the second image). I don't know if I'm going mad or have I been throwing a lot of data away by using Photoshop badly or am I just creating a false background by overstretching??

BEFORE (15 x 45s ISO 1600 using Esprit 80 on the Evo mount and the Canon 60D. Bro calibration, stacked in DSS and processed in Photoshop with final tweaks in Lightroom

AFTER (15 x 45s ISO 1600 plus 1 x 60s ISO 400 and 7 x 90s ISO 400 using Esprit 80 on the Evo mount and the Canon 60D. Bias, darks and flats used for calibration, stacked and processed in PixInsight with final tweaks in Photoshop and Lightroom)

1 minute ago, SteveNickolls said:

Hi Ken,

The image had WIPE carried out on it, I think the gradient is there because there aren't enough frames to make the data good enough for ST to work adequately and you get blocks of off-colour and gradients appearing. Sometimes you can crop a larger image down and 'fool' ST into thinking the data is better than it is and you can then use modules such as COLOR properly. I put last night down to a learning experience :-)

In DSS field the stacked rotated images take up a 'bow tie' appearance over time on the master image but the software automatically rotates individual frames for you. ST then only has to remove gradients once.

Cheers,
Steve

What I mean is that DSS rotates based on stars located within the frame but the light gradient does not rotate, meaning it would be rotated as DSS de-rotates the stars...if that makes sense. So instead of each frame having a gradient from base to top, assuming an even light pollution from the horizon (but would still hold true for more complex gradients because of nearby lighting) when they get stacked, the gradient gets added diffently from each frame and creates a bow tie gradient which is probably much harder to remove.

1 hour ago, SteveNickolls said:

Here's last nights M29 taken with the SkyWatcher 102mm Startravel Refractor, Synscan Alt-Az mount and Canon 600D DSLR. The image is made from x39 fifty second light frames combined with x50 dark frames, x50 flat frames and x50 bias frames. Frames stacked in DSS and processed using StarTools. The stars of M29 are set in a haze of nebulosity.

Looks like you have a strong gradient from the bottom. How good is the wipe tool in StarTools for removing this?

And that raises my next question: as altaz imagers, we have to deal with rotation. But that also means that when we stack images, any linear gradient that appears in each frame slowly becomes rotated through our stack created a non-linear gradient in the final stack. Does that mean we should be removing gradients from each frame before stacking?

 

Olly Penrice reckons that darks for a dslr are a waste of time because without stabilising the temperature the darks won't be representative of the actual taking conditions. Now I realise that this is somewhat at odds with your findings, but he uses a master bias instead of darks, which would save a lot of time if it could be made to work for us. I'm sure that there is more to it than that and I'll have to get more info, but it may be one route to go.

I think he's also said that he uses Astroart for stacking, which is supposed to be a lot quicker than PI. I downloaded a copy but I've yet to make it work, as you can't save on the trial, and it throws up some odd error messages.

Olly may be right. Although my processor stabilises at 16C, I have no way of knowing if the sensor also stabilises. It's just my assumption that if the processor is heating to that temperature (it rises as it works harder, e.g. during live view) then it will also be heating the internals of the camera to a similar temperature regardless of external factors. I've read about people building coolers for their DSLRs but the bulky nature of the cameras means this is probably very inefficient. As I've said before, I'm not convinced the darks make any difference but given they are easy to take, I've applied them anyway.

I took 4 hours to process that image as I had to learn PixInsight from scratch. However, there are some amazing step-by-step tutorials and I re-ran the process on 7 images of M101 to see if I could remember what I was doing and that took much less time (and I completed more of the steps once I'd figured how to do masks). With practice and the use of batch processing (which I avoided until I knew what the processes did), I think it could be reduced to about an hour's processing for a target. Given I think the image is more than 3 times better for the effort (and I actually enjoy the processing), it's an hour I will enjoy.

I also learnt a new technique with Photoshop which uses the high-pass filter on a desaturated layer, then applied via a mask created by selecting a colour range. I tried it on the North America Nebula selecting the reds/pinks and it did sharpen the nebula, creating a little more "depth".

One question that's had me thinking today: given we are exposure limited, do you think we could image using filters? I'm guessing not except for the very brightest of DSOs like M42.

1 minute ago, The Admiral said:

As you say Ken, very different renditions of the same image, it's hard to believe that they are the same! I think I still prefer your original version using ST, it shows the nebula very well. I suppose purists might argue that the second one is more realistic? I know what you mean about ST, but the more you use it, the more sensitively one can tune it. I had a trial of PI too, and although the workflow is supremely logical there are so many facets to it that I wasn't really prepared to commit. It isn't cheap either. I wish it was possible to directly set the black-point in ST, but on Steve's suggestion I tried the Life module and it does seem to give one a bit of control over that. Else I set it after ST in a final polish in my standard photography application. Do you think you'll migrate to PI?

It definitely has merits over DSS for calibration and stacking but it took 4 hours compared to 20 minutes. I need a few more serious attempts with it to compare results. There is no way I could have achieved the second image with Photoshop. There are batch processes that might speed things up but I first want to know what it's doing within each module.

57 minutes ago, Nigel G said:

I had a go at NGC 7000 last night, a beautiful mild and clear night, went to bed as the morning chorus started 4.20 am

Those are stunning. There is so much to see. 

33 minutes ago, SteveNickolls said:

So do you have dark frames arranged by temperature to use off the shelf Ken?

Cheers,

Steve

 

Yes. For the ZWO I have darks in 5C bands and multiple gains. For the Canon I only keep one set as it seems to settle to a standard temperature after about 30 minutes after aligning. 

The one advantage of British weather is that our night time temperatures are fairly stable. I find my Canon starts at around 26C as I use live view to align and focus, and cools to about 16C where it stays for the entire imaging session regardless of actual outside temperature. That makes it easy for me to build a dark library.

Don't forget you can do your bias and darks during the day and they should be good for any future lights at that ISO and temperature. You can always cool the camera in the fridge or freezer to simulate night temperatures.

Flats can be done at dusk so long as you can see a bright star to focus on. Arcturus is perfect at the moment. 

I do plan on spending more time on the North America Nebula. I only imaged it the other night as a test. It was still very low in the sky. The forecast here suggests no clear nights for a week so I will probably have to wait. But it's going to be my project for a while now I know I can capture it. I'd probably be aiming for 200 more lights. 

Of your two images I do prefer the second even though it shows fewer stars. The background is less noisy and the colours look right. I also like that it gives M57 a little more space. 

Well, I've just spent 4 hours reprocessing my North America Nebula subs. I gave StarTools a try and although I actually managed to create an image I liked, the process still felt like doing surgery with a mallet! So I downloaded a trial copy of PixInsight to see what a real pro piece of software could do. First I had to retake my bias and dark frames so my camera moved to the fridge for a couple of hours. With bias, darks and flats at the ready, I went through the whole calibration, stacking and processing using PixInsight with only a few minor modifications made in Photoshop at the end.

BEFORE (44 x 45 seconds ISO 1600 with no calibration, stacked in DSS and processed in Photoshop)

AFTER (43 x 45 seconds ISO1600 with 28 bias, 85 darks and 40 flats, calibrated, stacked and processed in PixInsight and finished in Photoshop)

Very different images! (And I apologise that they are both low resolution. The final TIFF image for the second version weighed in at over 350Mb.)

I think the biggest differences in processing were around being able to remove the background and controlling the stretching.

6 minutes ago, The Admiral said:

You surprise me Ken, 'cos you got a super image of the North America nebula with the same sort of exposure.

I have no experience using PS, but can I suggest that you have a play with StarTools. May be it's a case of the devil you know, but ST does seem tobe able to reveal faint stuff.

Ian

I should have said. I think I took them with the SCT and only managed 8 or 15 second exposures. By the time my refractor arrived, Orion and his friends had moved to the front of my house where all the street lights are, so I will have to wait until Autumn for round 2!

47 minutes ago, Nigel G said:

As I  have run out of images to share, I was just browsing back in my photos from my beginning in December and January, I took a single image of an open cluster, I can't remember ex time but would guess a 30 - 40s, while trying to enhance the image I kept getting what I  now know as vignetting but didn't at the time, anyway I kept trying and this image kept coming up with quite a deep redish tone,, reading up a bit about the cluster I found it had a different description,  the Rosette neb.

Did I  capture this nebulosity or a bad case of vignetting ?   Here it is with star trails and all ☺ I had forgotten about it. 

I can definitely see structure in the nebulosity but I can't see how much effect vingnetting is having and if there is also a background gradient. Again I'm jealous. I took 45 frames and got some beautiful stars but not a hint of nebulosity. Given its now raining maybe it's time to revisit the vault.