Filroden

Not to get too excited but the sky is an unusual colour (blue) and very empty. It's almost like I could be in for a clear night (at least until Midnight). Tempted to capture something new and try for either the Heart or Soul nebulae and really see how dark my skies are. It's premature but I've got the scope set up and bias/flats already being taken/calibrated. Wish me luck!

Yes. I had pretty clear skies. There was a ten minute spell with some very high clouds but otherwise it was clear until dawn when the rains arrived. Not looking good now for the rest of the week.

The background was pretty flat. At ISO800 and 60s I think I was only getting to about 15% on the histogram. I did try 120s and that barely got me over 25% so it must be much darker than my previous house.

Okay, after a long leave of absence moving house I finally found all my telescope boxes and got a clear night to test my new skies. After a very rough start (practically relearning how to set up and align the mount) and being unable to lock my focus (the knob fell off during transit and by the time I realised it was missing I'd already got everything set up and plugged in - so I took my chance and hoped the weight of the camera and any changing temperatures wouldn't affect it much). My new skies look much more polluted - I live much closer to a very industrialised region. I also now have two very bright LED street lights just outside the garden so I had to place the mount right up against a fence to get some shadow. The good news is that it seems almost all the lights in my town are LED and very focused downwards meaning I think the sky is actually darker, even though I can see in full colour in the garden! So here's my first effort of the season (stealing your idea of focusing on something big and bright)... 69 images of M31 ranging between 30s and 60s at ISO800 using the Evo mount, the 80mm refractor and the Canon 60D. I've probably over processed it in Pixinsight (something else I had to relearn from scratch). I was rather surprised I got the result I did. Most of the images showed some trailing, caused more by the mount losing position (I need to recenter my target every 5 to 10 minutes) rather than rotation. Anyway, here's to more clear nights soon!

To get absolute responses yes, but to get comparative measures of cameras probably not.

To test red response wouldn't it be simpler to attach a narrow band that only passes red and photograph a source of infrared such as an electric heating element whose power can be controlled? Then you're just comparing resulting amplitude of the capture in a fixed duration...

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.

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.

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!

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.

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.

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

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.

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)

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.

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?