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Martin Meredith

Whale+pup and Whirlpool

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Another night of clear and clouds -- ideal for short live captures! These two are 60s exposures on the usual kit under a half-moon.

First, ngc4631 and its satellite dwarf galaxy ngc4627, known as the whale and pup.


Then, being chased by the clouds the only window happened to be Canes Venatici so I took a first look at M51 with the Lodestar:


Even in a big dob I haven't seen it like that. I dug out some notes from 2011 when M51 hosted a supernova just to check that the bright star at about 5 o'clock wasn't another one :smiley: .

I hate to think what the new monochrome Lodestar would do with this object under moonless skies at a fast f-ratio...

Does anyone know what the almost vertical faint smear near bottom right is? Looks like a faint edge-on galaxy.

I pushed up to 90s to get this noticeably less-noise image


It's beginning to show some oval stars due to running in alt-az mode but nothing to distract from M51 (this isn't AP after all).

Am I the only one getting decent-ish weather at the moment?


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Great images!!  :smiley:

If you want to find out what's in your images upload them here http://nova.astrometry.net/ and it'll list all the known objects in your image.

Thanks Steve. I have used astrometry a few times (it sometimes fails with my images) but I'll give it a go with this one. I'm pretty sure its a real object as I've seen something similar on proper AP images, but at least SkySafari doesn't seem to have it listed.


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Superb stuff Martin - I like the detail coming through on M51. 90 secs is very good for Az-Alt and you must have the AZEQ6GT pretty well balanced as well.

One question - did you capture 10 or so darks before taking these? I see there are some funky colour single pixels so was wondering if they were hot pixels, which I was hoping darks would take care of, or  I might need to look at some bad pixel / filtering on the colour decoding / improving darks.

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Thanks Paul. The hot pixels in the 90s version presumably come from the fact I only collected 60s darks (most of the run was at that exposure). I only captured 5 too (clouds looming is my excuse). My 'normal' workflow is to decide on an exposure for the main type of objects I want to look at and collect some darks at the start of the session, then just let LodestarLive do its stuff. This is probably sub-optimal and I'd appreciate any advice, but it certainly does a good job of cleaning things up during the session. Next time I'll collect 9 or 11 (I read somewhere that odd numbers of darks work best!).

Have you thought about bad pixel maps as an alternative?


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Yep I definitely suspect the fact the dark exposure length didn't match the light exposure length is the prime reason for the odd funky pixels.

The master dark is a median combination of each dark sub you take. As you can add darks whenever you like and the number is not pre-determined at processing time, I use a median estimator rather than a true classic median algorithm. This ideally needs about 10 ish subs to settle on a result that is typically within 1% of the true median.

To that effect, what I tend to do is whilst I am aligning the mount, I have the camera taking darks. In V0.8, it now displays the median master dark, so you can see the dark grow as each sub comes in. What you should find is that the master dark image (you really need to stretch it to see the data) 'settles' after about 7-10 subs, and with each new sub there is very little difference (especially 10+). This should now give a good result in your main images.

I then focus using the focus / align mode (FYI this does not apply darks to the subs, it just displays them) and then hit the first target. I tend to flick back and take a few more darks every 30 mins or so, but I do find that the master dark image changes very little, again confirming it is a 'good dark'.

Bad pixel maps are on the radar for the future, I defiantly think these would be useful for a quick between the clouds style session and you just want to get going rather than faff with darks (however I find getting the darks over with during the mount alignment works out quite well). Also you can save and restore master darks, so if the camera temperature (which is a factor of the ambient) is reasonably the same then it should still work quite well. On the same vein, I think when live stacking is up and running this will also deal with a fair majority of the hot pixels by virtue of drizzle. Most of us aren't using super 10 Micron / Paramount class mounts, so there is movement between each sub. As the stacking will re-align and then combine subs through a statistical process, I would hope the hot pixels effectively get phased out to some extent as the image builds up. Well, thats the theory anyway...

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Thanks for this explanation. Just a further query:

Supposing I want to take a few exposures at say 30s then later some more at 60s. Would the procedure be to take e.g. 10 x 30s darks at the start (and whenever required) and then take 10 x 60s darks just before taking the 60s lights? Would these latter darks effectively overpower the earlier 30s darks in the running median estimator?

As an alternative, if I save the median 30s dark, later do some 60s darks, could I just use the restore option to bring in the whichever master dark I need at any point?


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Hi Martin,

Yep you are correct, the procedure in my head would be to take your darks at 30 secs (and probably save it when you have 10 or so) and then take your 30s image exposures. When you want to switch to 60s, go back to the dark acquisition mode and you can reset it using the reset button on the exposure tab - its the 3rd button on the right along the top with two arrows pointing at each other (next to stop). That resets what you see on the display (back to a pure black frame), but also resets the median dark stack back to the initial all black (i.e. 0) pixels state. You would then take your 60s darks.

You are also correct in that you can take your 30s and 60s darks at the start, and after you have saved them, you can restore them at the appropriate point. You can even add more dark data after restoring a previous dark, so if you go back to one say an hour or so later, its probably good to add a few more dark frames incase the temperature has changed. As a note of interest, when you restore a dark from a file, internally it actually resets your current median dark stack back to default first and then applies the saved data.

The reset button also works on the image acquisition mode - albeit at the moment it will just reset the display back to all black pixels. Once stacking is implemented, this will reset the current live stack, so you will need to hit the reset between different targets. The reset button also resets the dark and light counts (in the applicable mode), and if you are saving the raw FITS, will send the number that is at the end of the filename back to 0. To clarify, the reset button only resets darks when in dark acquisition mode, and light images when in image acquisition mode i.e. it is not a global reset.

This scheme also gives the option (of once live stacking is going) of switching between exposure times for some HDR style video astronomy. You will need to restore the appropriate dark, but provided you don't hit reset when in image acquisition mode, you  will be able to stack a mixture of exposure times.

A handy button! lol!  :grin:

I guess at some point I need to start thinking about a user guide or some tutorial videos on youTube - I suspect this is probably best done after stacking and any other major features are implemented!

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