Narrowbands on DSLR - exposures

[obscura]

Have had reasonable success with the DSLR, Canon 550d modded, with just IR/UV filter and a cheapy LP filter.

I acquired 2" HA, O3. and S2 filters early this year and have just got to employing them. (h/w and s/w probs caused delays).

Managed to get a couple of hours on NGC1499 the other night using 5 x 300s exposures each for all three to see how it all went. Each filter was behind my Baader IR/UV cutoff.

The result was a little disappointing in that the images appeared grossly under exposed with just a shadow of the neb visible on Ha though the sky was as black as the ace of spades. DSS would only accept 1 of the 5 in Ha.

Is 300s adequate? Would taking 30+ at 300s improve or do I need to open out to up to 600s exposure? But then, noise?? 

Would appreciate advice.

[perfrej]

Well, your camera already has colour filters on each of its pixels, so adding an Ha, for instance, doesn't get any light at all on the green and blue pixels. All you have left are the red ones and they constitute a fourth of your total pixel count. Add to that that the bandwidth of the filter is a mere 7nm (or 5 or 3 if you want to spend some serious dough) wide and there isn't much light left for the sensor to pick up. Same with S2 which is in the red. The O3 should give a bit better signal as it is in green and there are twice as many of those pixels in your sensor.

To phrase it in simple words: narrow band imaging with a colour sensor is not really a good idea. What you need is a monochrome CCD for those filters to be useful.

Using a DSLR with really long exposures is also a bit of a gamble as the Canon processor does some noise processing of its own (yes, it does) which makes the darks kind of unpredictable. More on that here: http://www.stark-labs.com/craig/resources/Articles-&-Reviews/3CanonDSLRs_ATT09.pdf and here: http://www.stark-labs.com/craig/resources/Articles-&-Reviews/DSLRvsCCD_API.pdf

/per

[obscura]

Must admit that I was beginning to think that since posting. O3 was somewhat better. The very thought of 600s+ exposures wasn't/isn't  an option.

So, I'll sleep on it but sense that the Canon and all my 2" stuff will go up for sale in favour of a sensibly priced mono CCD - SXV9, 314+ and move to LRGB.

Many thanks for for your input and for being straight.

Best Rgds

[perfrej]

Straight may be painful, but it is better in the end. I made some pretty big mistakes in my astro beginnings, but nobody here on SGL really set me straight.

Do keep the 2" filters! You just have to get some noew ones, namely L, R, G and B to complement the Ha, O3 and S2 that you already have

I see before me a vision of the future. It's you, a 29050-based CCD on the back of a Takahashi refractor, all sitting on a 10Micron mount flashing expensive Astrodon 3nm filters. Woah!

/per

[Luke]

I would say, do not give up on the DSLR See the awesome movie below by Mark Hellweg for inspiration.

I have dabbled a bit with a 450D and narrowband though not done any for a while. As far as I remember, I usually took ten minute subs. I think it was either 10 or 20 minutes, probably 10. Without narrowband, I took 5 minutes. I used ISO1600, probably not that it makes that much difference versus 800.

From my efforts, Ha was by far my favourite filter, it generally returned a stronger signal than the OIII and held up better under a bright moon. SII I never managed to get anything useful out of, though I hope one day to get some use out of it.

re: only one in four pixels, that still leaves quite a lot of pixels with the DSLR, you only halve the width and halve the height in pixels, and you still have the advantage of a wide field for those big nebulae. A cooled mono CCD would be better, for sure, especially on the fainter nebulae and for the SII I presume. I guess it depends on budget and expectations. I'm okay with okay images, I feel I am taking holiday snaps, not trying to win astrophotographer of the year. So long as I am happy with them, that's all that matters. That said, 100% I would go CCD if budget was not really an issue (I suspect if I go the cooled CCD route, I would end up wanting a £3K+ mount, a £1.5k+ triplet, a £1.5k+ observatory, on top of my £3.5k CCD, filter wheel and filters - a complete no go for me anyway as I am more into webcam now and would rather have a bigger solar scope!).

If you have the bug and budget to go cooled mono CCD, I think that would be the way to go.

Noise didn't seem to be a huge issue, any more than normal, as the Ha signal seemed pretty well defined on the targets I tried. It gave my DSLR a new lease of life.

Anyways, for the inspiration - taken with a 20Da:

How to make narrowband filter deep sky images with a DSLR

Hope the thoughts help.

[melsky]

It would be interesting to see your result Obsura, attached is what I can get out of my DSLR on the same target through a 12nm clip filter with a very similar total exposure of 22x 480sec.

Mel

[m37]

I chucked an Ha filter in front of my EOS1000D one night just to see what would happen. This is the Rosette Nebula from 2 ten minute subs, guided, no darks, flats or bias. Not properly in focus.

What ISO setting were you using? If you've already got the camera and filters try whacking it up to 800 or 1600 for some 10 minute exposures. Noise will be pretty bad but at least you can see something and take it from there.

[Pav1007]

m37, try to put one light frame to MaximDL and split to tricolor (Color -> Split Tricolor). Use only R channel.

In RGB filteres on chip some of light (with Halpha filter) will be registered by blue and green pixels.

Pawel

[obscura]

Wow! I will do my best to respond to the incredible helpful response on this one. There was I thinking of selling up and taking up a suitable indoor past time. However, I could not think of one so its back to the billion year old photon. 

I have taken some "decent" pics and been more than happy - most of these were with a 350D which isn't renowned for its noiseless capability. Pics were in and around Orion including my favourite- M1. Sorry, I digress. Back to NB.

Seems I should not quit on it and try 30 x 300, 500 and/or ex times 600s using just Ha initially. I standardised on ISO 800 from the beginning and have rarely tried 1600 an above. The NGC1499 shots were all at 300s @ ISO 800 but only 5 off. Exif T was 17C throughout, 14C above air. 

I will lay back on this until New Year when the skies clear again and try it out again I do fancy a CCD but I think its one or the other but not both at this time.

Many thanks to everyone for advice, help and encouragement. All your comments are taken on board.

[m37]

Cheers for the tip Pawel.

I would love a CCD too but, like you, the admittedly inferior results from a DSLR will keep me ticking over till I can afford something better. Decided to upgrade my scope rather than get a better camera.

[obscura]

m37

I too boggle at ccd prices new or s/h. Then Canon sell thousands, hundreds of thousands. CCDs sell far fewer to a more limited market and tend to use CCD rather than CMOS. I s'pose at the end of the day its a question of justification to oneself and the spouse. I haven't managed that to date. To me that is - no chance for the Lady.

melsky

forgot to say that the Ha images of NGC1499 were so under exposed that almost nothing was there. Clue in after thought was that the image appeared heavily clipped - very black sky with a very faint neb show. So dark that couldn't be salvaged with PS. That really is a nice image -I will give that a whirl at 8m and as many as I can get. How many darks? Thanks for that.

Again, thanks to all, my faith in Canon is restored (for the time being). Still fancy a SX H9 mono, its nearly affordable. (Yeah right)

[melsky]

I did shoot some darks for my NGC1499 image but I think the temperature changed to much as I they introduced black spots in the output from DSS, so the image above is just light frames at 800iso, no darks, no flats and no bias frames. I preprocess HA images in Photoshop and split out the red channel to a 16bit tif before loading them into Deep Sky stacker.

Mel

Sent from my iPad using Tapatalk

[obscura]

Ok I'll take that on board re PS pre-process.

Amazing then with 8min subs and no calibration frames. There is hope for me yet.

Cheers

[jackrussell0232]

Ha from a DSLR can be done, but you do need to manage your expectations, as many have said.  

But that said, you should be able to get that target (NGC1499) quite well.  The image below is from a modded 550d (4 x 10 min subs at 1600asa) and an F7 (800/115mm) refractor.

I think the fact that I was using a fairly wide Ha filter (12nm) helps a bit too.

You probably know that getting better at processing is the cheapest way to make progress in this hobby, but I don't think that stops any of us buying new gear.

If you're serious about narrow band imaging, you really do have to go mono CCD. 

In your case though, I do wonder if your UV/IR filter is cutting some of your Ha, as this is considered to be near IR by some.  There really is no need to use any other filter in the path when you're shooting in NB. It's not impossible that this filter is undoing what you've done by modding the camera (ie reducing the amount past 630nm reaching the ccd)

Just a thought,

Good luck,

Jack

[laser_jock99]

Narrowband works to some extent with a modified DSLR- in fact I'm surprised your results were so disappointing?

You could check your cameras narrowband response by trying some widefields with just the 2" filter plonked on the front of a standard 50mm lens.

The image of the constellation of Cygnus below was just 477 seconds with my Fuji DSLR. Seems to have recorded even some very faint nebulosity.

[Sandancer10]

This thread has helped me no end as I was considering a while back buying a Ha filter for my un-modded DSLR.  The only thing that stopped me  was the fact that I use a 2" T-adapter and in the future I want to buy a CCD - I think they are usually 1.25" fitting. I wasn't sure if it was worth buying the Ha filter for really a limited use, so I didn't bother. It didn't occur to me that using it with a DSLR would cut down the amount of photons so drastically. It really is a great help being on this site and getting other peoples views and help via their experiences.

Brenda 

[Russe]

Do keep us updated on how you get on. I as well can't afford a cooled mono CCD (and a wife)!! And the missus is dearer to my heart - and I have abstained from buying NB filters since my 450D is unmodded - but should I get my hands on another 450D next year (or some such stuff), I was thinking about giving it for a mod and then one might consider Ha or the likes.

[sharkmelley]

One other thing to consider (when using NB filters on a DSLR) is that because of the low signal levels, the thermal noise from the dark current will become more noticeable and is likely to be the limiting factor.  This is why mono CCDs are cooled.  That said, I have seen some good NB images from a DSLR but they won't be anywhere near as good as using a cooled mono camera.

Regards,

Mark

[Beardy Bob]

Keep at it. And like Jack said, definitely ditch the UV/IR cut-off filter when you're using narrowband filters, it's not adding anything at all and might even be taking some signal away.

[Gina]

To emphasise what was posted above... Yes, it's amazing what good processing can achieve with what appears to be a pretty useless stack of images.  I think I might go back to some of my DSLR data and see what I can get out of it now that I have learnt so much more about processing.  I stuck with a DSLR for a long time before splashing out on a mono astro CCD camera.  The filter mod makes a huge difference if you want to use an Ha filter.  Then Ha and OIII are certainly possible but SII is so dim anyway that losing 3/4 of it through the Bayer matrix makes it virtually unusable.  Next stage of Peltier cooling really cuts down the noise and allows longer exposures but is a lot more complicated and needs some decent DIY skills and electronics knowledge.  In the depths of winter you may be able to do without it on the brighter DSOs.

[obscura]

Catching up.

I think I am down to hanging in for a clear night and if anyone can help there - that would be good. I needed to get a heads up before the next one which could be before New Year, or not. I think I have what I need and where I am going now and appreciate all the input given in this respect. Oh, the IR/UV cut filter isn't there for the NBs, its for the LP. My filter wheel is a cheapy 4 position. Three are the NBs and the 4th the LP. The IR/UV is there for the LP not the NBs and its therefore up the barrel. It'll probably not reduce too many photons compared with the NBs themselves. I also thought it may be useful in the event of a CCD (yeah right) and RGBs.

As and when I get a decent image, I'll be back.

Again, may thanks to all, your help and advice is highly appreciated and welcome. Its also good to see that the blog has been useful to others.

[Gina]

With NB filters you don't want any other in the light path.