I had a go at imaging this showpiece Northern object on sunday night, with the ED 80, canon 300D, using an astronomik CLS fillter
seeing Robbie and Scott have been pushing the envelope in terms of exposure i tried to go as long as i could..went over an hour in terms of total exposure, but had to dump some frames -must have lost a guide star or something
anyway my polar alignment was bad so had heaps of field rotation over this period. However IRIS was able to rotate and stack the images, and also remove much of the gradient.
Each exposure was around 6 minutes at ISO 800. 48 minutes in total.
Comments welcome....
Last edited by seeker372011; 09-08-2005 at 10:18 PM.
Awesome seeker!!
The longer exposures has reduced you noise a lot! It's interesting to compare Scott & my M20 images. Scott does real long exposures, but only 2 or 3 where as I do many exposures (~30) but for short durations.
The dury is still out, but to me it looks like it doesn't matter haw you get your exposure minutes, but just as long as you get them!! Eg 30x4mins is almost equal to 4x30mins!! Interesting...
By the way how do you like the CLS filter? I am thinking of getting one. Tossing up between that and the Baader UHC-S filter.
Awesome seeker!!
but to me it looks like it doesn't matter haw you get your exposure minutes, but just as long as you get them!! Eg 30x4mins is almost equal to 4x30mins!! Interesting...
By the way how do you like the CLS filter? I am thinking of getting one. Tossing up between that and the Baader UHC-S filter.
I really like the CLS filter but I have had no opportunity to do a direct comparison with the Baader that Scott uses. I chose the Astronomik because the technical spec seeemed to suggest that the cut off profile was the sharpest.it would be nice to do a shoot out with both filters and see what the differences are ..if any. The people at Bintel recommended the Astronomik BTW
I bought the 1.25 inch and maybe it may have been worth getting the 2 inch. However not sure how I would actually use the 2inch..still havent worked out a way to use the 1.25 inch in my 8 inch and still come to focus with the Canon 300d
I have ordered a CLS filter, but Astronomik are out of stock and not shipping for about 3 weeks. Ahh well. Hopefully in time for next New Moon.
I have gone for 2" as that's what my focusser will accept. Thanks for the advice.
Cheers
Hi
Yes great shot there.
I can now say that doing only one very long exposure is NOT the go. Last night while testing my mates Lumicon Coma Corrector I did a single 30 minute ISO 200 shot of the Lagoon, the result was dissapointing. I got inner detail ok but the outer parts were quite noisy and flat looking, so I think that we should take at least 2 preferably more shots at a higher ISO. I may go back to using ISO 1600, and take 4 shots 10 mins each rather than ISO 800, 20 mins like I have done. A mate points out that if something goes wrong, its better to stuff a 10 minute shot than a 20 minute one
As for exposure I try to get the histogram peak about 1/3 to no more than 1/2 the way along from the left. Less than that is too dark, more is getting overexposed.
Scott
Hi Scott,
I'd recommend staying at ISO800, and dropping down to 4 or 5min shots. And do heaps of them 20 or 30. It doesn't seem to matter how long you go!! I know this sounds backwards & I've yet to get my head around it too. It's all about signal to noise ratio. With heaps of short shots the noise is going to be lower, so the signal will be relatively higher, and such you'll be able to "process" more detail. In saying that common sence still prevails. I don't think you'd get very good results from 500x10s shots!...
I think the exposure length minimum is dictated by the black point of the raw image. If you have to move the black point (in Photoshop for example) up alot then you have over-exposed. But if you don't need to move the black point at all then you have underexposed... I notice Scott that you aim for 33%-50% black point adjustment. I go for about 5-10%. Any more I would think is wasted exposure time. But I'm open to correction!
Does this make sense???? Perhaps I'm confusing even myself!!
The advantage with the filters you guys are using is that they essentially improve the black point of the raw image, so you can go a bit longer as an optimum exposure time. I know if I go for 20mins with no filter it's a complete waste of time as it's far too over-exposed. Mind you the detail is still there, but the blackpoint adjustment is crazy!
Phew.. This maybe all a load of rubbish!! It's just my mussings of the last few months.... I'm by no means an expert. Got a massive journey still ahead.
Cheers
I have seen other people say the histogram should be in the 33 % to 50 % region and that is what I have been trying to get. I do have to move the black point a lot in PS, but some of it may be due to the final steps in processing in IRIS.
A consideration with choice of ISO is the dynamic range..apparently this is inversely related to the ISO.
My practical experience has been that a single ISO 1600 shot of say 8 -10 minutes -even with the filter- from a light polluted location -is so noisy as to be unusable.
Somewhere between ISO 200 and 800 and maybe it is dependent on ambient conditions...ISO 800 if you have a really dark site?
On Terry Lovejoy's site someone has done an analysis of the relative merits of varying exposures and ISO settings.
Have to go now but will try to find and post it here
Howdy
yeah I guess its all open to personal choice, and that similar results could be obtained with different settings. I guess what I should consiter is spending one night on just one object and try a series of exposures at different ISO's to see what one works best with my system. Terry goes into signal/noise in detail, I must re-read what is there
Scott
I must admit I have not read Terry Lovejoy's site. Who is he anyway? Link would be good!
My comments are only from my personal experience, so I'm sure there is probably many different and/or better ways to do things. I just found that a 30-50% histogram (if I can call it that) tended to be indicative of too long exposure for my setup. Other setups it may be fine. I guess trial & error will sot out the best method for a setup. It surprises me that a black point at 30-50% is optimal... Hmmm.. There is no useful infomation to the left of the histogram, so why waste it?? Perhaps I am missing something??
Ah well always leaning! Keep 'em coming
Cheers
I did some test a year ago with my D70 to determine the S/N ratio for different ISO settings, and found that any setting over 320 ISO gives the same S/N ratio, but the lower ISO settings give more banding and streaking on dark backgrounds. This is why I use the lowest ISO setting without getting background banding, in praxis this means the lower the f-ratio and the higher the background brightness (light pollution or Milky way), the lower the ISO setting can be.
Under a dark sky and an f-ratio of around 7, I usually end up with 800 ISO.
If for a given setting the brightest part of the object is going to be overexposed, I'll make a few extra short exposures to fill the brightest parts in.
For a given total exposure time shorter exposures will increase the dynamical range and dampen the effects of any stuff-up, but will add extra read-out noise, memory and processing time.
It will depend on the dynamical range of the object, the used hardware and the stability of tracking what determines the best time for the individual exposures, im my case a 10 minute duration works the best.
Consequently my master dark is shot with the same individual exposure duration and ISO setting, thanks to dark frame synthesizing I can use it for different ISO settings and exposure times as well (like my Milky way shot for 30 minutes @ 400 ISO), but still will give the best result when shot under the same circumstances as most exposures in the field.
This is why a camera histogram can tell you a lot, it prevents having banding because of underexposure or burn-out because of overexposure.
Terry is a master in experimenting and knows more about the technicalities of imaging than I do, as well as the use of Iris, which has an almost unlimited amount of processing features if you know where to find it.
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
Is good !
