Finally got the old rattler overhauled inside with new teflon bearings and a full polish inside & a new RA motor from a LXD75 (only $90). The mount will have to do until I save up enough to upgrade to something more capable for astrophotography but with some autoguiding help, it seems to be still ok for widefield.
Tonight I got the guiding much smoother so this was a test run but what exposures could anyone recommend for this lovelly region?
I am using a Meade LXD55 with a canon 40D piggybacked with a 50mm F1.4 and I find F4.0 is good for this lense. ISO is 400 and each frame is 180Sec. I stacked 4 frames with 6 dark frames and 6 flats using Al's VLB and Octane's recommendation for tissue paper on the lense aimed at the PC.
Light pollution is a problem in this shot but I intend to head out to my usual spot soon where I can go for about 5 minutes.
5-10 min at iso 400 should see it starting to really jump out, just make sure you have good noise suppression programs if the ambient temp isn't under 10 deg C good luck!
Thanks Brendan. Will try 5min first as the night ambient temps whilst cool haven't been giving the 40D a fair go yet.
What noise suppressions programs would you recommend (i assume these are PS pluggins?). I know it becomes a real issue with an unmodded SLR and anything that combats noise with this setup would make a difference.
ISO-400 is a good choice. At f/4, 5 minute sub-exposures are plenty. Just take as many as you possibly can. Be sure to take a decent number of dark frames, too. Something that gives you an SNR of 4-5 should be sufficient. That is, betweent 16-25 dark frames. Also, the same for flats and flat darks.
Cooled/modded camera obviously makes a huge difference - but good glass is the starting point.
Don't listen to those low iso 'jessies' ()
IMHO, iso 1600 is the sweet spot on the 40D (best bang for buck point!) - BUT, you must compensate for the noise with loads of subs.
Phil Hart gave a great talk at the Astro Soc of SA stating that point with maths and images a few months ago - if you don't believe me, ask him!
Give it a try on a cold night - what have you got to lose?
I have certainly been inspired by Humayun's images when he was doing the 40D posts last year (and still my favourite is his Orion shot with the 200mm F2.8L) and Doug yes that 200mm lense is a very nice unit. With a cheap and old mount like the LXD55, it doesn't allow the 200mm to really shine but will certainly work on this.
Doug and H thanks very much for your info and that image of Phil's I believe won a David Malin award? For now, I will stick with the 50mm and get the mount tracking nice. I'm almost there so my next step will be to try going out to around 5 minutes for a start.
Couple of questions though:
1. I've seen the more seasoned guys use a method called "dithering" which I understand is a way to slightly vary each frame for improved stacking. Would this be of any benefit with DSLR imaging?
2. A DSLR sensor's temp will always change over the night. Would one recommend taking a light -> dark -> light etc or light -> light ->.. then dark -> dark ->. To me it seems mathematically more accurate to do light-dark-light-dark etc.
Cheers, and thanks. Your work Humayun has kept me to push on to get the best out of my equipment. A G-11 is on my radar once I am happy I have full confidence in my gear & capabilities. I have used a G-11 very recently with the new Gemini 2 (it's a bit tacky compared to Autostar) and compared to the LXD55, I had sweat starting to drip on my credit card
Darrin - your ambition and enthusiasm are admirable but the method you've outlined really doesn't seem too different to just letting the in-camera darks do their work. You take a 5min light - the camera does a temp regulated 5min dark immediately afterwards - whole image takes 10mins.
1. I've seen the more seasoned guys use a method called "dithering" which I understand is a way to slightly vary each frame for improved stacking. Would this be of any benefit with DSLR imaging?
No way an expert on this - but all I do is widefield DSLR/Lens astro-imaging - and I've used the dithering option to stack my subs in Nebulosity when there's been noticeable movement between each sub, and the results aren't too good...seems to result in a strange grid like/ pixelated effect! Perhaps I have to change parameters, but i know that I can achieve better results without the dithering option So, I avoid!
But Doug if one does ICNR, it relies heavily on the fact that the noise in the "dark" shot is very closely matched to the noise in the previous light shot. This means a sensor that is capable of repeatability at the pixel level which I am not sure how true this is.
Whereas if one were to stack a bunch of darks, you are statistically getting closer to the true noise captured statistically in all of your lights stacked?
ICNR is very tempting but I haven't been too excited about its results.
I'll need to research more but my understanding is that stacking is mathematically approaching a truer representation of what should be there (noise and actual light)
I use dithering but only 0.5 of a pixel. I understand about the why, but im like you I am not to sure its doing anything extra. all it does is show up cold pixels to the stacking program or pixels that might be mistaken for signal other wise.
For now focus on capturing and once you look at longer FL's and your guiding is perfect then start researching that area of interest.
I think there's 2 camps here at IIS on this issue - ICNR or seperate darks. The strength of the ICNR case was that the temps of the associated lights and darks were close enough to be a perfect match and therefore the best fit with regard to noise reduction. Trouble was, as stated above , the use of the DSLR is crippled as the dark is subtracted in camera.
I think this is probably a good way to go for shorter exps - but for 5min subs and above a dark library is the go. Just have to ensure a close temperature match.
Thanks very much guys. Just now need to get that opportunity hopefully this weekend. Temperature is always a killer in this type of setup. Will keep you posted. Winter always runs late over here.
Thanks very much guys. Just now need to get that opportunity hopefully this weekend. Temperature is always a killer in this type of setup. Will keep you posted. Winter always runs late over here.
Darrin...
Best of luck mate - I look forward to seeing your results
Doug
In camera noise reduction has a couple of other problems. I'll quote from MaximDL as it sums it up very well -
"Dark subtraction does not work as well when the images have already been converted to color."
"Unfortunately, and perhaps counterintuitively, subtracting a dark frame also adds noise to the image. Every pixel has random read noise, plus the residual dark current noise. This noise does not subtract, but rather adds in a root-sum-square fashion. Therefore simply subtracting one dark frame increases the noise level 41%. The way to get rid of this noise is to average it out of the dark frame by averaging. Every time you quadruple the number of averaged frames, you drop the noise contribution by half."
In camera noise reduction has a couple of other problems. I'll quote from MaximDL as it sums it up very well -
"Dark subtraction does not work as well when the images have already been converted to color."
"Unfortunately, and perhaps counterintuitively, subtracting a dark frame also adds noise to the image. Every pixel has random read noise, plus the residual dark current noise. This noise does not subtract, but rather adds in a root-sum-square fashion. Therefore simply subtracting one dark frame increases the noise level 41%. The way to get rid of this noise is to average it out of the dark frame by averaging. Every time you quadruple the number of averaged frames, you drop the noise contribution by half."
Mark.
Thanks Mark, interesting stats - a darks library or a bunch done throughout the imaging run looks best then.
Doug
good luck!
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