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Narrow band filters with an unmodded DSLR


samir_ansari
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Hi everyone,

I have an unmodded 450D and am looking to buy some narrow band filters to eliminate as much light pollution as possible when photographing emission nebulas. However I've read that this will be a problem because of my cameras reduced sensitivity to the red end of the spectrum and that exposures in this band can take 4 times longer than if the camera was modded.

My question is how long is 'long' for say the rossette nebula? Would I be expecting Ha exposures of around 8-11mins, 12-15mins or even more?

Thanks,

Samir

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Not sure about the answer to those questions, but this is what I acheived with my unmodded cam and 5 min exposures on the Rosette. It shows that you can capture the Rosette without specialist filters, although you need a fair number of exposures. OK, it's not as deep as CCD imaging, but at least I captured something.

Edited by lukebl
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Thats a cracking picture Luke! Although the reason I am resorting to narrow band is because I am bang in the center of London where the light pollution is just horrendous.

Thanks! Yes, I am looking at going narrowband too so will watch this thread with interest.

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Samir,

I take it you have already done some imaging and used the subtraction of darks, flats and bias frames and found this still to be unsatisfactory?

In Glasgow I get a fair amount of LP but usually the processing tricks and stacking software takes care of this if you use the darks and flats at least.

I also use a DSLR but with only a CLS filter attached.

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Hi Stuart, yep I have tried subtracting darks but havent gotten round to working with flats just yet.

This was a 3min exposure of the M81 region with a baader light pollution filter and almost directly up in the sky where the light pollution was least visible:

img0002jtt.jpg

And the best I could do with a stack of 20 of these subs was this:

m81v2.jpg

Maybe I need to improve my processing skills but I cant seem to get away from that horrible grain which I assume comes from me having to stretch the curves so drastically to get away from LP

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Ah right, I thought they were only there to reduce the vignetting, what else do they do?

Well, for a start they'll even out the background so hopefully you won't need to stretch the image so much. That's my experience anyway.

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ah ha - I'll definitely have to make sure I do some flats next time then. What is the 'definitive' technique you have found is best for making these? I have heard just wrapping a cloth around a light and taking a photo of that will do but how long should the exposures be, same length as the subs?

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I have had the SW LP filter, Baader Neo LP filter, and Astronomik CLS clip filter.

The only filter that worked for me was the Astronomik CLS. You can even image with the lights from neighbors shining directly on the scope. I have been able to get 15 minute subs with a f5 scope, and 20 mins with f6, before maxxing out the histogram.

Maybe consider it before going to narrowband.

HTH

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Combining flats, darks and bias will help clean up the image a lot. Also try not to go over ISO 800 (you may already restrict to this but going over this will introduce even more noise)

For Flats the conventional wisdom seems to be to take exposures long enough to evenly illuminate the image. If you take 5 minute lights then you will almost certainly end up with an over exposed flat, I would try 30-60 seconds. A lightbox is ideal but any evenly illuminated surface can be used.

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My flats are just quick images (probably about 1/500 sec exposure) taken with white t-shirt over the end of the scope pointed into the sky during the day! Because I have the camera at the same orientation each time I use it, I just use the same flats each time so don't need to take any more.

Edited by lukebl
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My flats are just quick images (probably about 1/500 sec exposure) taken with white t-shirt over the end of the scope pointed into the sky during the day! Because I have the camera at the same orientation each time I use it, I just use the same flats each time so don't need to take any more.

For precise use, the camera needs to be EXACTLY in the same position as the lights, this includes focus.

Otherwise dust on the optical path will not line up and vignetting may be ever so slightly out.

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For precise use, the camera needs to be EXACTLY in the same position as the lights, this includes focus.

Otherwise dust on the optical path will not line up and vignetting may be ever so slightly out.

Yes, I'm aware of that. I guess I'm just a bit lazy, but the focus and position are nearly the same, and it seems to have worked so far! These things aren't always as precise as the textbooks would have you believe.

Edited by lukebl
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The simple solution is to shoot the lights once you are finished imaging before you take off the camera.

If you are lucky to have an Obsy of course and can just leave the camera on all the time then stock lights and darks will do just fine.

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For precise use, the camera needs to be EXACTLY in the same position as the lights, this includes focus.
We are getting a bit away from the point here. The original poster was complaining about the granularity. Flats may help a little with this, but they do not have to be precisely lined-up or in perfect focus to do so. The vignetting and dust spots are a separate issue.

However, there is no magic bullet. LP will destroy your signal-to-noise and make images look grainy - even the best filters cannot restore it to its dark site value.

NigelM

Edited by dph1nm
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I guess it would be better if you could try before you buy. I used to have a complete set of narrowband filters for my Canon EOS and used them in an unfiltered 450D, For me the results were not good enough to continue with them and I sold them on.

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I dont think narrowband filters (especially Ha) will do much for an unmodded camera since a chunk of the emission lines youre trying to capture are more or less blocked by the cameras internal filters. These need to be removed to get the full benefit of NB filters.

Edited by Uranium235
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