Hmm... lots to learn. I'm getting lots of black (missing) pixels on the ST-8 by the looks. I can only put this down to -30 chip temp on the night (-5 ambient) because I haven't had the sensor that cold before. Any ideas?
Anyway - I haven't tried Ha before, so this is new to me. I have to say that i think I'm going to like it. Lots of fun
Tak FS-102@f6.3, SBIG ST-8i, Astronomics 7nm Ha on G11 guided by FS60-C/QHY5. Quite windy. 4 x 600sec lights and autodarks. No flats at this stage.
EDIT 230709: Tried again - reworked original data and it's smoother.
Last edited by Omaroo; 23-07-2009 at 10:09 AM.
Reason: Re-worked original data
Hmm... lots to learn. I'm getting lots of black (missing) pixels on the ST-8 by the looks. I can only put this down to -30 chip temp on the night (-5 ambient) because I haven't had the sensor that cold before. Any ideas?
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i would have thought the missing pixels is something to do with the dark subtraction.... just a guess
lots of detail but the stars are just a little crunchy or overdone.
Not bad Chris! What image acquisition and calibration software are you using?
Generally, artifacts in your images can be removed by:
a) dark frame subtraction. It's best to take at least 3 dark frames, median combine, then subract that median dark from your lights. Auto darks (at least in CCDSoft) mean only 1 dark is taken and used for all images in a series.
b) hot/cold pixel removal (eg as in CCDStack) after calibrating, registering and normalising your images
c) you could also dither your images by a few pixels in between light frames so when they're median combined artifacts dissappear or are minimised. 3 subs is the minimum for a median combine but 6 or more is best.
d) Doing all three of the above is even better (that's what I do)
Thanks Jeanette Yep - I'm reasonable happy with it because I can see that with a bit of experience with it I'll get there.
Cheers Marcus - thanks for the tips
I've been aquiring via CCDSoft5, and taking autodarks automatically, and as you say, the series utilise the same dark frame. I think I'll take them separately from now on so each light uses its corresponding dark frame immediately. I have a library, but not at this temperature. Ambient was far colder than when I built the library, so I have none suitable.
Quote:
Originally Posted by marc4darkskies
Not bad Chris! What image acquisition and calibration software are you using?
Generally, artifacts in your images can be removed by:
a) dark frame subtraction. It's best to take at least 3 dark frames, median combine, then subract that median dark from your lights. Auto darks (at least in CCDSoft) mean only 1 dark is taken and used for all images in a series.
b) hot/cold pixel removal (eg as in CCDStack) after calibrating, registering and normalising your images
c) you could also dither your images by a few pixels in between light frames so when they're median combined artifacts dissappear or are minimised. 3 subs is the minimum for a median combine but 6 or more is best.
d) Doing all three of the above is even better (that's what I do)
Nice first Ha Chris, Definitely a dark subtraction problem. Lots of nice detail there to work with but as Clive mentioned the stars look a little crunchy. Did you sharpen the image or use deconvolution? Maybe use less.
Wait for a nice cold winters day, setup the camera.
Cool it to the temperature you plan to use. I use -15C almost all the time as there is not much point going colder as the noise reduction is probably not worth the effort of new darks, even then with this method the darks become scaleable.
Take a series of darks at the maximum time you expect to take lights, I haven't yet used 30 minutes, but may in some of the NB stuff I'm into ATM, so mine are 30 minutes. I took a series of 10, which is probably overkill. Now take a series of bias frames.
Go to CCDStack (if you gave it, most other image processing software should have this function anyway), go to calibrate and create master dark and master bias frames, I use the sigma combine method for both, but the clip min/max works as well.
Unfortunately you have to do this for all binning states of the CCD, so it can take a couple of days...
After this has all been done the darks are scaleable for use in CCDStack. One set of darks for everything.
I love auto darks for quick shots, but if you're doing anything serious, then taking real darks is much better..
I have built a library with the ST10XEi at -10°c of darks, 15x 30s, 1, 2, 5, 10 and 20min both bin 1 and 2, and also 15 bias frames both bin 1 and 2. I did this over a couple of cloudy nights, and since then, the images given from the camera have come out much much cleaner...
NABG definitely is a killer sometimes.. especially something like Eta where you have a very prominent star smack bang in the centre of the target...
Great looking image Chris ... You've nailed the focus ... superb ... 1/2 the battle a lot of the time IMO. Even though Ive seen quite a few of this image you've somehow managed to compose it so that it has a uniqueness about it ... Ide say keepem coming. Its amazing the detail you get from this Ha stuff ... amazing enough that I feel my interest level building in it.
Thanks everyone for your great advice and encouragement!
I've re-worked the original data a bit and this rendition doesn't have the hard-edge to it. It's probably better, but I need another night of collection - and this time remember to do a dark after EVERY exposure rather than a single dark for a series. We live we learn I guess! Darks need more attention for sure - I have an extensive library built up, but none at this sort of chip temperature. Stuart - thanks for the advice - I'll have to investigate a calibrated set.
Nice there, Chris....somehow I like the 2nd one better, even though it appears to have a bit more noise in it, the noise "adds" to the ambience of the piccie. The stars look more natural
